Rhodium (II) catalyzed reactions of diazo-carbonyl compounds
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Transcript of Rhodium (II) catalyzed reactions of diazo-carbonyl compounds
Te~~,fronVol 47,No Ml, pi 1765-1808.1991 oo4o-4020/91 $3 oo+ 00
F’madmGreatBmmn Pergmon Ress plc
TETRAHEDRON REPORT NUMBER 287
Rhodium (II) Catalyzed Reacliom of Maze-car bony1 Compounds*
Julian Adams and Denice M. Spero Boehrmger Ingelhetm Pharmaceuttcals Inc
90 East bdgelP.0 Box 368 Ridgefield. CT 06877 U S A
(Recerved 1 October 1990)
Contents
I Introductton ..... 1765
II Cyclopropanauon .......... 1766
III. C-H Insertron ........ 1779
IV Heteroatom-H Insertton ........ 1792
V Ylide Formatton and Reacttvtty ...... 1795
I. Introduction
The focus of thrs report deals with the use of Rh(II) complexes and thetr abrhty to catalyze a
variety of tmportant reacuons Dtazo-carbonyl compounds are tdeal substrates for Rh(II) based
catalysts and react under mrld condtttons to form what 1s probably a Fischer-type carbenotd These
carbenonls have never been observed, due to the hrgh reachvny of the putattve mtermedtate
metal-carbene. Rhodmm (II) acetate dnner 1s the prototypical catalyst for the &azo-transfer process,
although hgand mod&canons have resulted in various tmprovements for specrfic crrcumstances
(vtde mfra)
Thrs revrew wrll cover four rmportant reacttons of Rh(II) carbenes cyclopropanauon,
carbon-hydrogen bond msertton, heteroatom-hydrogen bond msertton, and yhde formatton and
subsequent mactrons. The drversrty of reacttons as welI as the umque bond formmg capabthtres 1s an
rmpressive feature of the rho&urn generated carbenes Free carbenes have long been regarded as
hrghly energehc species which have mtemstmg properttes. though of hmrted value. 111 orgamc
syntheses. Metal-carbenes m general, have generated a wealth of novel and useful reacttons. Thrs
revtew wrIl attempt to demonstrate the scope of the rh~dnun catalyzed carbene reacttons wrth a vtew
to pracucal syntheuc constructton, and brrefly address some of the mechamsnc issues whrch make
the rhodmm based catalysts useful reagents for synthettc chemotry.
1765
1766 J ADAMS and D M. SPERO
Rhodmm acetate dlmer 1s prepared by heating RhC+ 3H,O m acettc acid Other rhodmm
carboxylates may be obtamed by the analogous procedure or by hgand exchange with Rh?(OAc),
Rhodmm (II) carboxylates possess an octahedral D4 symmetry with four bndgmg carboxylates
complexed to the bmuclear rhodnun atoms This has been refemd to as the “lantern” structure The
morgamc chemistry of rhodmm (II) carboxylates has been extensively renewed by Boyer and
Robmson,lb and will not be addressed m this paper
Rhodium Acetate Duner
II. Cyclopropanation
The synthesis of cyclopropanes and the= use m Mg cleavage reactions remams an unportant
area of research m organic chemistry, and several recent reviews have explored these themes 1-4 As
mentioned m the mtroducuon, rhodium II mediated reactions of a-diazo carbonyl compounds may
result 111 ad&tion to unsaturated systems to produce 3-membered nng products The most commonly
used reacuon leads to the formation of cyclopropyl denvatlves I-hstoncally the use of various
copper catalyst.$-’ were prevalent and only the emergence of the group VIII metals m the late 1960’s
and early 1970’s challenged the efficacy of the copper based catalysts 8-12 The pioneermg work of
Belgian chemists Pauhssen, Hubert, Teyss& Anclaux and other collaborators was paramount m
ldenhfymg the rhodium (II) carboxylates as efficient catalysts for carbenoid mediated
cyclopropanauons 13-16
A systematic screening of common transihon metal complexes revealed that Rh(II) species
were the mildest and most efficient catalysts for cyclopropanauon “,I8 AU stable Rh(I1)
carboxylates, most notably Rh,(OAc),, having one coordmation site per metal, form highly reactive
complexes ~th carbenes derived from diazo precursors These carbenolds add rapidly to avadable
carbon -carbon double and mple bonds (and, as will be revlewed, are capable of smgle bond
Rhodmm(II) catalyzed reactions 1767
msemon reactions) By contrast, the tradItional copper metal catalysts which mediate the same
reacclons to varymg degrees, probably do so ma dtierent mechamsms Copper-olefm complexatlon
has been proposed to tiect the cyclopropanatton reaction Rhodmm catalysts Hrlth tbe smgle
avtiable coordmation site do not form complexes ~rlth olefms and this is mferred from the near
random chemo-selectivity observed with Qfferent olefms l’,l* The catalyst selection is crmcal smce
competing processes such as the Wolff rearrangement (seen ~nth sflver based catalysts or C-H
mseruon reachons (observed wth Pd catalysts) are also options for ol-ketocarbenes.19 Rh(II)
catalysis is documented to be very specific for cyclopropanauon
A note on the general reactton conditions is m order Catalyt~ efficiency 1s generally high,
reqmrmg l-3% catalyst (by weight) and best results are obtamed at amblent temperature (solub&y
permmmg) m non-reactive solvents such as methylene chlonde or other halogenated hydrocarbons
Reaction rates are extremely rapid and no intermediate carbenoid complexes have ever been isolated,
unhke the well known tungsten or chromium carbenolds A general correlauon of olefm react~lty
m&cates that the more electron-nch double bonds ~11 react preferentmlly ” Even ketene acetals
react with diazoacetate to give the desired cyclopropyl acetal, but only when Rh(II) was chosen for
catalysis 20 Electron deficient olefins (1 e a$ unsaturated olefms) often foul to generate
cyclopropanes, but rather form pyrazohnes Barrmg stenc factors, CIS olefins react shghtly faster
than traus olefins
scheme 1: General Cyclopropanalion Reaction
R
Coqugated dlenes and trrenes are also reactive, and cyclopropanation at the more
nucleophlhc double bond is predominant. 21-23 By the same token, acetylenes represent good
substrates for Rh(II) medmted carbene additions and lead to highly stramed (and reactive)
cyclopropenes 24 In general the coppe r catalysts foul to produce any product at all As already
mentioned, the more nucleophihc the reactmg substrate the more rapld the reaction This was
demonstrated by Petmiot et al 24 who performed a competition expernnent between 1-hexyne and
1-octene, where the cyclopropene product predominated over the cyclopropane by a ratio of 2 1
Such selectivitles amongst olefins, dlenes and tnenes have been extensively documented m
the hterature 17J8*25*26 Much of the earlier hterature addressed the intermolecular reactions with
respect to regmchemlcal and stereochemlcal aspects of cyclopropanatlon Doyle and co-workers
have conducted the most systematic mvestlgahons ~rlth regards to mechamsm and rhodmm catalyst
compansons lg.27 Doyle’s mlhal efforts confumed the supenotrty of Rh,(OAc)4 as the catalyst of
choice for cyclopropanauon, observmg superror yields under the mild reacuon condlhons
1768 J ADAMS and D M. SPERO
Nevertheless RhZ(OAc), tends to exhlblt shghtly less regloselectWy compared to copper catalysts
The reachon of ethyl dazoacetate and monosubstituted &enes was used to estabhsh a
“metal-carbene regioselechvity m&x “25 Product analysis of the competmg double bonds of the
diene gave results that were mechamstically diagnostic This exercise confiied the tendency of
Rh(II) to form reactive metal-carbene complexes with no coordmation to the reactmg olefin As
mitlally proposed by the Belgian mvesugators electron-nch olefins were more reacuve towards
cyclopropanatlon with the electrophihc ethyl acetate carbenold(Scheme 2)
Scheme 2: Diaxmcetate Addition to Mono-SusbstJtuted Diem%
Doyle and colleagues also undertook a systematic mvestlgauon of the stereoselecuvity m the
ethyl dlazoacetate cyclopropanatlon reaction *s In general the reaction tends towards little If any
select.Wy The trans Isomer predommates to a small degree (l-2.1 trans/c~s) except if a stencally
demanding group or groups mfluence the reaction outcome Trans olefins react to produce
cyclopropanes where the two substltuents derived from the olefm mamtam the trans relationship in
the cyclopropane product The same apphes to CIS substINted olefiis
Scheme 3: Stereuspedflcity of Olefin Substituents in Cyclopropanation
N$HCO,Et +
RI
trans
Rh(IU
R2
A
Rl
CO,Et
tram
R2
“lwR2 ) ma RI N2CHC02Et +
4 CO,Et
CIS CIS
1769 Rhodmm(II) catalyzed reacaons
Doyle correctly pomts out that four vanables conmbute to the resultmg reglo- and
stereochemical course of cyclopropanatlon. the transltlon metal, its associated hgands, the tizo
compound and the olefm The olefm is generally the weakest conmbutor to the stereochemical
outcome
Further mechamshc evaluation by Doyle, usmg kmeuc compebtton expenments and product
analysis, led to the comparison of the stable tungsten metal-carbene, (CO)5WCHPh, for
cyclopropanation, and the rhodmm acetate catalyzed reachon of phenyl dmzomethane.2g Though the
relative reacuvities were much faster in the tungsten catalyzed system (103-104 tnnes) the order of
olefm reactivity remamed the same as for the Rh2(0Ac), case This 1s good evidence for the
assumption of a rhodmm-carbene complex which has been extended to hold true for dlazo-carbonyl
carbenes as well
A plctonal assessment which accounts for the shght preference for the trans-cyclopropane
Isomer m mono- substituted olefins was put forth by Doyle, as shown in Scheme 4 while such an
account is reasonable, cautton must be employed 111 consldenng tis mechanism since none of the
intermediate transition states have ever been observed Nevertheless, as a point of departure, this
analysis 1s consistent with mechamsttc pnnciples and accounts for the experunental observations
Scheme 4: Doyle’s Mechanism of Cyciopropanation 29
H H H
L,M
%
OEt
H R 0
- I R H
SG H COzEt
Followmg the mmal investigations with Rh,(OAc),, Doyle et al proceeded to demonstrate
both the hgand effects on Rh(lI) and the stenc effects of the reachng dlazo carbonyl Both rhodmm
acetamlde. Rh2(NHCOCH3)4 and rhodmm pertluorobutyrate, Rh2(OCOC3F,)4 were found to affect
the trans-selectivity in the reaction of diazo-carbonyls with styrene 3o The results are summarized m
the table below
1770 J. ADAMS and D M. SPERO
Table 1: Ste~ectivity Enhancement in Catalytic Cydopropanation of Styrene
trans/c~s (yield, %)
Catalyst N&HCOOEt NzCHCOOMe(iPr)2 NZCHCONMeZ N$HCON(G’r),
~2(~~,F,), 1.1 (81) 1 1 (83) 15 (67) 12 (51)
Rh2(OAc)4 1.6 (95) 2 4 (95) 2 2 (74) 64 (53)
Rh#HCmH,), 2 1 (89) 4.4 (87) 2 4 (70) 114 (47)
The electron withdrawmg effects of the peffluorobutyrate hgaud adversely mfluences the
trans selectivity while the acetamlde catalyst enhances trans selecuvlty As well, the stenc bulkmess
of the reactmg duuo carbonyl contnbutes to the trans stereochemistry although the yields tend to
Qtmmsh Altematlvely, Callot and Metz were able to effect cis stereoselectlvity in cyclopropauatlon
with the use of bulky 2,4,6-tmuylbenzoate hgands on Rh(II) 31 A detaded account of the
stereoselechvlty m catalyuc cyclopropanatlon reacttons was recently published and includes further
stuQes mvolvmg hgand modification of the catalyst and dlazo carbonyl compounds 32
An mterestmg application of the intermolecular reaction demonstratmg the power of the
Rh(II) catalyzed cyclopropanation is the reaction of ethyl diazoacetate and furan (Scheme 5) With
furan as the solvent, rapid evolution of mtrogen ensued and a mixture of products was obtamed
Wenkert et al analyzed the products 1-4 and put forth an mterestmg mechanistic rationale to account
for their observahons In contrast to Doyle’s hypothesis, two rhodium based metallocycles 5 and 6
(differmg in their regmchemlstry) were postulated to explam the expenmental findings 33
The Merck Frosst Canada group has made use of the furan-diazo carbonyl addition chemistry
to prepare cis-trans dienes m the synthesis of the arachadomc acid metabohtes, hydroxy
eicosatetraenOic acids (HETEs) 34-36
It can, however, be concluded that the mtermolecular cyclopropanatlon reachon 1s relatively
non-selective (by today’s standards) unless particular attenhon is psud to “matchmg” the olefin, the
catalyst and the dlazo carbonyl substrates accordmgly Another potential problem arrses with the
mtermolecular reaction Due to the natme of highly reactive carbenold mtermedlates, dmenzaUon
1s often an accompanymg side reachon Typically the intermolecular reaction has been run using an
excess of the olefin substrate, often as the solvent itself Dlmenzahon can be numm1zed as well by
controllmg the rate of addlhon of the dlazo carbonyl to the reachon mixture, effectively mamtammg
a low concentration of the carbene m solution
Rhudmm(II) catalyzed ITXX~IOIU 1771
scheme 5: Reaction of Ethyl Diazoacetate with Furan
+ N,CHC02Et
CO,Et C02Et
+
C02Et
2
t t
+
COzEt
3
(332
cc
I RhLl
0
6
Et
Scheme 6: Dime&&ion of Carbenoids (a side reaction)
!!
0 WOAc),
R -
0
1772 J ADAMS and D M. SPERO
By contrast the mtramolecular reaction provides greater synthetic uttl~ty due to the geometnc
constramts of formmg the cycle and the entroplc advantages mmnmzmg the dnuenzatlon of the
carbene Several parl~cularly mterestmg demonstrattons of the carbene-olefm addltlons which
underscore the mildness and htgh spectficity of the mtramolecular process are exemphfied below
Dowd performed a cyclopropenauon followed by cyclopropanation to prepare highly stramed
rncyclo [2 1 0 02*5] pentan-3-one 737(Scheme 7)
The first to use cyclopropanauon 111 an intramolecular sense were Stork and FICIIII~* although
at the tune the more efficient Rh(lI) catalysts had not yet been described 38 In many cases the
traditional copper catalysts suffice (albeit 111 lower yield) but reactions sensmve to elevated
temperatures or Lewis acld~c me&a reqmre the use of Rh(II)
Qven the hmlted preparative utity of the mtermolecular machon. many more useful
synthetic consuuchons have been dewsed with the mtramolecula~ process We have attempted m
dus report to bghhght some of the more useful and outstandmg reactions. particularly the cases that
reqlllred the mdd Rh(II) catalysis and faded with other transition metal catalysts
Scheme 7: Fonnation of Highly Strained Cydopropanes
Cl C1CH2ECCHzCl + N&HCOO-t-Bu )
COO-t-Bu
~2WW.4 - c Cl
Followmg the work on the cyclopropanatlon of furan. Padwa3g and Wenkert33 mdependently
mvestlgated the mtramolecular process 111 some detad The mtermeQate cyclopropane was so labile
m the urnmolecular reaction that It was not observed, but its mtermediacy was mferred from the
geometry of the exocychc double bond m the product (see Scheme 8) Thus a general synthesis of
cychc 1,4-d~acyl-2,4-cls,trans butadienes (I e 8) was developed
Rhodmm(II) catalyzed reacuons 1773
Padwa expanded on thts theme usmg benxofuran cychxanons as well as reactron wttb
thtophenes.3g@ In some of these cases the mtermedrate cyclopropane was observed
spectroscopically or even Isolated. The mtramolecular reactron wrth throphenes 1s mterestmg
mechamsucally and contrasts somewhat wrth the bimolecular reactton with Qaxo carbonyls.
Qllespre and Potter have reported a stable yltde mtermedtate wtth dmxomalonate, 9, for whtch an
X-ray crystal smtcture was determmed 41 ‘llus complex does not react further to produce the
cyclopropanated throphene The authors ranonahzed this result by statmg that the two electron
wrthdrawmg esters render the yhde too stable for rearrangement. They proposed that the
dtaxoacetate addrtton to throphene may also proceed via a less stable (and not observed) yhde prtor
to cyclopropanatton Further dtscusston mgardmg the correlauon between yhde formauon and
cyclopropanatron may be found by readmg the work of Doyle 42
Scheme 8: Intramokulac Diazoketone Reaction with FUIWI
N2
-
Scheme 9: Thiophene-Malonate Ylide
I Et0 c ’
C- 2 ‘C02Et
1774 J ADAMS and D M. SPERO
In a S~ULU vem we have exammed the mtramolecular reactron on the relatively nucleophlllc
enol ether olefin m dlhydropyrans. 43 The oxa-mcychc ketone products, 10. were found to be useful
mtermediates for the synthesis of small and medmm rmg carbocycles. The sequential carbon-carbon
and oxygen-carbon bond cleavages afforded carbocychc ketones, 11, web syn-tiubsututlon The
hmit of rmg size formation was addressed ws a ws competmg C-H msertlon (Scheme 10) This
synthehc approach was employed m the total syntheses of natural products eucalyptoP and
P-chamigrene45
Scheme 10: Intramolecular Addition of Diazoketones on Dihydropyrans
Rh,OAc,
W 032)
H+/HX
X
0 X -72 HhY
Y
(CH$ - OH
n
0 ti (CH2)
n
0
1L
n= O-3
The addmon of alkyl or arylhthlum or Gngnard reagents to the carbonyl of the simplest
oxa-mcychc ketone (n=O), 10, led to carbmols, 12, which upon Lewis acid treatment (TiCI,)
provided a novel entry to cyclohexacbenes or eventually meta-subsmuted aromatic compounds 46
Another novel nng construction owing to Rh2(OAc), catalyzed cyclopropanation was
described by McKervey and used to prepare fused cycloheptatnene compounds 47 Once again, the
use of Rh(I1) 1s cntical since copper catalysis or photochemlcal methods foul The general scheme
involves the converSion of aryl prop1on1c acids to cycloheptatnenes, 13, v1a 1ntermdate fused
cyclopropanes Tetralones, 14, could be obtained by treating the cycloheptamenes with
mfluoroacetic acid 47 Doyle apphed the same reaction pathway to prepare azablcyclo [5 3 0]
decatnenones48(Scheme 12)
1775 Rhodmm(II) catalyzed rcacuons
scheme 11: Synthesis of Cyclohexadienes and m-Substituted
Aromatic Compounds
Scheme 12: Synthesis of Fused Cycloheptatrienes and Tetralones
/ OQ \ t 1 Piers et al made use of an elegant intermolecular addmon of dtazoacetate to a cyclopentane
denvative, 15, to set up a dlvmyl cyclopropane rearranagement, which afforded a unique bndged
tr~cychc construction, 17, en route to the natural product quadrone 4g Here the cyclopropanaclon
occured with high exo selectivity and the stereochemical preference was dctated by the 5,5 fused
cyclopentane substrate, 15 (Scheme 13)
One particularly novel and elegant apphcatlon of a carbenold-olefin addmon is the reachon
of the rhodmm-vmyl carbenold with dienes resultmg m a formal 3 + 4 cycloaddihon This work,
pioneered by Davies, adds a new dimension to the cyclopropanation repertoue An example of tlus
process IS the addition of dlethyl4-dlazo-2-pentendloate to furan (Scheme 14) 5o Evidence was
presented favormg a cyclopropanauon-dlvmylcyclopropane (or Cope) rearrangement based on the
endo selectivity for the carbenold addition ”
On the same theme, Davies also apphed the same methodology m an mtramolecular sense to
prepare fused 5.7 membered carbocycles and heterocycles (Scheme 15) 52+53 Once agam the
remarkable endo selectivity of the cyclopropanation accounts for the facile tandem process to afford
the observed products (I e 18)
1776 J ADAMS~II~D M SPERO
Scheme 13: Divmyl Cyclopropane Rearrangement
O’IBDMS OTBDMS s 5
8
\
I
N,CHCOOEt
) KY
C02Et
u2(OAc)4
OTBDMS
heat b
Scheme 14: Formal 3 + 4 Cycloaddition of a Vinyl Carbene to Furan
RhLn
6cLf
I CO,Et -
C02Et
Scheme 15: IntramokeuIar Tandem Cyclopropanation/Cope Reactions
Rhodmm(ll) catalyzed reactions
Another clever apphcauon by Davies allows for the synthesis of substituted furans.”
Recogmtion that &eto cyclopropenes could be formed under the nuld Rh(II) concfitions, usmg
dnuomalonates and keto-este~. led to the observed furan products. This reaction IS formally related
to a vmyl cyclopropane rearrangement. The polarizalon of the zw~tter~omc keto-olefin mtermtite,
19, serves to accelerate this process leadmg to furans (Scheme 16) 54
Seheme 16: Cydopropenation and Synthesis of Furans
R RhLn Y
II +
x
-
H 0 X
-
R 0
W
X+
Y
-
R
0 v ;_ x 19
Y I
1777
Recent examples of mtramolecular processes leadmg to unusually stramed rmg systems are
noted below (Scheme 17) 55,56 These examples further demonstrate the power of dus chemM.ry
This account IS by no means an exhaustive catalogue of all known cyclopropanat.Ions, but rather an
instructive hterature survey and cnt~al assessment of the potential for useful synthetic construchon
Chermsts 111 the future will surely wlmess further developments and apphcation of this chemistry
Scheme 17: Highly Strained Cydopropane Riq@*
0 0
cc) N2
0 0
fi@Ac),
[3 3 l]-Propellane-2,8-Qone
Rh2(oAc)4
Cyclopropanoblcyclo [3 2 11 octanone
1778 J. ADAMS and D M. SPERO
The chemistry described so far has been restncted to dmzo carbonyl ad&uons to olefms. The
extension of the Rh(II) based carbenoid ad&tions has been extended to other electron de&lent
carbenes. Druley and O’Bannon have recently made use of ethyl mtrohazoacetate and
mtrodmzomethane to form cyclopropanate alkenes 575s Once agam Rh2(OAc)., was successfully
employed where thermal or photochemical methods faded
Another mterestmg development was the report of the rhodmm catalyzed reaction of
dmzomalonate with mtnles to form oxazoles Formally one could consider a dipolar ad&hon as
proposed by Helqmst et.al.5g Nevertheless, It 1s temptmg to speculate that a 3-membered unme
species, 20, may also be possible. sunliar to the Davies synthesis of furans (Scheme 18). Precedent
for an unmo awrrdme mtermedate exists from the previous work of Hubert et aL60
Scheme 18: Preparation of Oxazoks from Dlazomalonate Addition to Nitriles
MeOqOMe + R-CN . .
N2
-
One remiumng area to explore 111 cyclopropanatlon chemistry 1s the prospect for asymmetnc
catalysis Some examples are already described usmg a copper based catalyst, but the enantiomenc
‘l-64 excesses (ee) observed have been relatively modest We have attempted to prepare choral Rh(I1)
catalysts usmg ammo acids and other choral acids but so far have been unsuccessful in obtammg
asymmetnc mductlon 65 At the hme of this wntmg we became aware of the work of Stephen Martm
(U of Texas) and Micahel Doyle (Tnmty Umverslty) who described the general reachon shown
below (Scheme 19) +% This encouraging result bodes well for future advances m thus field and may
provide addmonal useful avenues for the Rh(I1) based catalyuc cyclopropanation chemistry
Scheme 19: Asymmetric Induction Using a Chiral Rh(II) Catalyst
95% ee
Rhodmm(II) catalyzed reactions 1779
III. C-H Insetion:
Free carbenes have long been known to be capable of msertmg mto C-H bonds, although both
low yields and lack of chemical control have mitigated agamst rehable and useful synthetic
applications 67 With the advent of Rh(I1) carboxylates, the carbenouls generated from a-duuo
carbonyl compounds were found to be effective m C-H msetion under extremely mild conditions.
Belgian chemists Noels et al reported the Rh(II) catalyzed C-H msertton of ethyl diazoacetate
usmg a vartety of alkane hydrocarbon solvents, at ambient tempertahu$js (Scheme 20) WMe the
overall yields were generally good, no useful regloselectivlty was seen Some preference for
secondary C-H bonds was observed with Rhz(OAc),, and mcreasmg the size of the carboxylate
hgand on rhodium (1 e 9-tnptycene carboxylate) enhanced the ratio of prunary C-H bond mser&on
Nevertheless, the intermolecular reaction has no real preparative value since mixtures of mmers are
formed 6g*70
Scheme 20: Intermolecular C-H Insertion of Hydrocarbon Solvents
0 I-WI) CH2C02Et
) \ OEt alkane
mixture of products
(2OY > 1°y)
The reahzation by Taber71 and Wenkert72 mdependently that the utihty of the carbene-metal
catalyzed C-H insetion process could be exploited in an mtramolecular sense was demonstrated in
the preparation of cyclopentanone denvatives Both groups found that the cychzation of carbenoids
denved from dlazo ketones and keto esters preferenttally formed 5-membered carbocycles This
process had been previously described ~nth copper catalysts, but once agam Rh(II) species proved to
be more selective, higher yieldmg and proceeded at amblent temperature73(Scheme 21)
The preference for 5-membered nng formahon 1s not absolute, and 1s strongly influenced by
the local stenc and electronic environment of the reactmg carbenoid Interestmgly, Taber et al
1780 J ADOS and D M. SPERO
Scheme 21: Intramolecular C-H Insertion to Cyclopentanones
Taber
Wenkert
N2
n
Rh,(OAch
published a synthesis of pentalenolactone, 21, whereby the mcychc skeleton was assembled through
a rhodmm-carbene medlated cychzatlon to form a 5-membered nng 74 By contrast, the Cane group
also constructed pentalenolactone, 21, but the key cychzauon step mvolved a C-H msemon to form a
6-membered lactone In the Cane approach, the posslblhty for 5-membered formauon exists, but the
6-membered nng 1s formed since It 1s the more stencally accessible optson ‘Ihe two successful yet
&ffenng synthetic constructions underscore the versatlhty of the intramolecular C-H msertlon
reaction (Scheme 22)
Taber and Ruckle conducted a systematic study of this reaction and concluded that the order
of reactlvlty for C-H insertion In an ahphatlc hydrocarbon was methme > methylene > methyl, which
1s consistent with the observations of the intermolecular Ractlon 76 Furthermore, It was determined
that allyhc and benzyhc C-H bond insertions were disfavored Taber ratlonaltzed that alkyl groups
are mducuvely electron donating and increase the electron density of the C-H bond, thus making tt
1781 Rho&urn(U) catalyzed Eacoons
scheme 22: Syntheses of Pentdenobctone
Taber
~co2M!EL & 0 0
Cane
COzMe
more susceptible to the electrophlhc Rh(I1) carbenold The mechamstic imphcattons of tlus have
been exploited by others and will be described later m thts chapter @de mfra)
Though little 1s known about the mechamsm for the rhodmm-carbenoid C-H mseNon
reaction, Taber offered a plausible hypothesis which accounts for preferential S-membered rmg
formation76 (Scheme 23) The fiit step reqmres the formation of a Rscher-type metal-carbene
complex, 22, at a vacant coordination site on rhodmm (a well accepted assumpaon based on the
cyclopropanatlon mechamsm) In the next step, it 1s unknown whether or not the C-H msertlon 1s a
concerted 3-bond process (mtermedlate 23) Also unknown, 1s the role of the carboxylate hgands on
the metal In any case, a hydrogen atom ends up on the rhodium@9 and the final step reqmres
P-hydnde transfer from the metal to regenerate the catalyst and afford the observed cyclopentanone
1782 J ADAMS~~~D M
Scheme 23: Mechanism of Aliphatic C-H Insertion
23 2.4 + &(OAc),
Taber et al. also detemuned that C-H insemon proceeded with retention of stereochemMy
at the reactmg carbon center and made use of this knowledege to synthesize (+)-a-cuparenone”
(Scheme 24) This enanttoselectlve process represents a general strategy for constructmg cycles
contammg quatemary centers
A further development by the Taber group demonstrated the trans selectivity of 3,4 dlalkyl
cyclopentanone subsutuents ‘* This IS sigmticant since it indicates a highly ordered transitton state
geometry whereby unfavorable ster~c mterachons are mmunized(Scheme 25)
Taber’s cyclopentanone synthesis employed a diazoketo-ester precursor, whereby the ester
functionality could be removed by hydrolysis and decarboxylation following cyclization This
Rhodmm(II) catalyzed reactions 1783
Scheme 24: Synthesis of (+)-a-Cuparenone
InserUon at C-H ~th Retention
a-cuparenone
Scheme 25: Synthesis of Trans-3,4 Dialkyl Cyclopentanones
Hb 0 0
P -3
R \
R’
provided an opportunity to explore Qsposable chrral amullarres m place of snnple esters to achieve
enanuoselective carbocycllzatlon7g (Scheme 26) This was indeed reahz.ed by the Taber group as
they prepared a naphthalene substituted (+)-camphor denvahve. 25 Excellent QastereoselectlWy
(> 80%) was observed m the C-H msertlon reaction These results suggested an ordered chaN&e
transition state whereby the bulky naphthalene moiety of the the chnal ester effectively shielded one
face of the pendant cham of the substrate The assumptton that the carbenoid ester exists m an
extended conformation duects the mode of cychzatton towards Ha This apphcatlon provides a
convenient synthesis of chmtl cyclopentanones
1784 J ADAMS and D. M SPERO
Scheme 26: Asymmetric Induction Using a Cl&al AtdIary
0 0
R
WW
OR*
‘“s R
2s dlastereoselecttvlty > 80%
Other examples of 5-membered rmg products are described m the hterature so*81 In general
polycychc systems tend to be well suited for the C-H msefion process and the stereochemical
outcome IS usually predxtable The synthesis of [4 4.5 51 fenestrane, 26, by Agosta et al illustrates
dus p~rnt!*-~ (Scheme 27) The constramts imposed by stram m the rmg system led to the predxted
result.
Scheme 27: Synthesis of [4.4.5&j Fenestrane
1 I’WOAc), D
2 reduce
Formal aromatic C-H mseruon is well documented As discussed in the cyclopropanation
section, It may not be possible to dlstmguish between dmxt C-H mseruon and apparent C-H
1785 Rhodmm(II) catalyzed reactions
msemon ansmg from an unobserved cyclopropyl mtermedlate which then rearranges to the final
product. Shown below are several examples of aromatic msetion to form mdanones. 27.85
naphtbalenes. 2t3.% fused pyrroles. 29, ~‘UXJ and 1.3 dthydrothlophene 2.2 dioxtdes, 30891~ (Scheme
28) The versat&y of dus process allows for convement preparation of a vanety of 5-membered
fused heterocycles Perhaps the most unexpected example was reported by Matsumo and co-workers
m the cychzahon of 2-dmzo-4-(4-mdolyl)-3-oxobutanoic esters, 31” Catalysis by Rhz(OAc)d gave
exclusively the 5-membered fused mdole, 32, whde reaction with Pd(OAc), afforded the
6-membered product, 33, presumably via a cationic mechamsm (Scheme 29)
The early work of Taber suggested that some stereoelectromc control exists m the C-H
msemon process The more electron nch C-H bond tends to be kmeucally more reactive This
phenomenon was also observed in our laboratory, when we first noticed the susceptlbllity towards
insetion of C-H bonds a to ether oxygens Our first expenence with this reaction mvolved the
trans-annular cycbzation of a dmzoketone appended to a tetrahydropyrau rmg, 34. which produced a
smgle product of mserhon at the 6-posttlon ether C-H, 35, and no mseruon at the ahphatlc
methylene This construction led to the total synthesis of the natural insect attractant
endo-1,3-dunethyl-2,9 dloxabicyclo[3 3 llnonane, 36 g2
Further studies in our group indicated that this selectlvlty for ether C-H bonds could be
applied to the mtermolecular reaction of ethyl dmzoacetate with ether solvents.g3 The
intermolecular reaction IS mechamsfically mstrucUve, but suffers from the same dtsadvantages
discussed previously, and holds little synthetic mterest Alternatively the mtramolecular cychzatlon
of dmzoketones denved from 2-hydroxy carboxyhc acids provided a general route to furanones. 37
(Scheme 31) The S-membered nngs were favored m the cychzauon as witnessed by the reaction of
dlazoketone, 38,m which both the 5membered and 6-membered nngs (39 and 40) are possible as
both methylenes are flanked by ether oxygens Furthermore, when we studied the cychzation of the
dlazoketone, denved from 6-benzyloxy pentanolc acid, 41, the carbon analogue, the 6-membered
nng, 42, was formed (albeit in low yield) owmg to the ether oxygen activation, and no
cyclopentanone, 43, was observed (Scheme 32)
While we were pursumg the phenomenon of ether C-H msemon, the fmdmgs of Stork and
Nakatam described a complementary C-H msetion reachon under stereoelectromc controlg4
(Scheme 33) They observed that the mducuve effects of electron withdrawing esters disfavored
C-H msertlon at methylenes a or even p to the ester As a result they were able to direct C-H
insertton at an unactivated methylene of a proximal ahphatlc cham to generate cyclopentanones, 44,
m a selective manner
1786 3 ADAMS andD M. SPERO
Scheme 28: Fused Aromatic Rings
R
o+ :I 0’N2
0
N2
R
/
05; I
0
\
22
OH
28
a 0
\ N
C02R
30 C02R
Rhoclmm(II) catalyzed reactions
Scheme 29: Cyclization of Indoles
1787
Scheme 30: TIIUW-AMUI~W Cyclization
a 0
0
34 35
Scheme 31: Synthesis of 2(H)-3-Furanones
-) @ 0 0 i
36
0 0
Rh2(OAd,
R CH2C12
R
(3-8 1 CldtranS~
1788 J ADAMS andD M. sPF!RO
Scheme 32: Intramolecular Cydization Ether Competition
(+bmuscarme
OBn
!?!J
OBn
Rhodmm(II) catalyzed reactions 1789
Scheme 33: Stereoelectronic Effect of Electron Withdrawing Groups
COCHN,
A (cH2&C0 Me!=% 2
n =1,2 44
45 46 2:l 47
The report of Stork and Nakatam was slgmficant to us smce we had observed that the
cycltzahon of a the dlazoketone denved from Mosher’s acid (2-methoxy-2-Wluoromethyl
phenylacettc acid) 45, led predommantly to aromatic C-H product, 46, mseNon with the moor
product, 47, artsmg from C-H insetion at the methyl ether Thts could be explamed by the mducuve
electron wtthdrawmg effect of the tnfluoromethyl group which serves to partmlly cancel the ether
acttvatton (Scheme 33)
Further studies m the preparation of unsymmetrical 2,5- dtsubstituted furanones led to the
observatton that the reachon favored the CIS dtsubstttuted Isomer g5 This appears to be the result of
kmettc control and an ordered transthon state mvolvmg the metalcarbene complex Base catalyzed
equihbratton of the product furanones provtded a thermodynamic mixture wherein the c~~/trans rauo
IS essentially 1 1 An attempt to rahonahze the results was put forth in a mechamsttc hypothesis
(Scheme 34) The proposal rehes on Taber’s ongmal mechamsm which calls for hydrogen transfer
to rhodium The electron donatmg ether oxygen faclhtates thts process, and parttctpates 111 the
transmon state by chelating the rhodium m such a way as to mmtmlze substltuent mteracttons
1790 J ADAMS~~~D M SPERO
Scheme 34: F’roposed Mechanism for Furanone synthesis
Me
In order to tighhght the useful apphcahon of our findmgs, both the furanone constructton and
the cls-selecttvlty of the C-H msertlon mediated by RhZ(OAc), were explolted m a total synthesis of
the natural product (+)-muscarmeg5 (Scheme 32)
A related C-H msertton process in which the carbene was drrected a to nitrogen was
observed by Rugglerr and co-workers m the preparation of ergohne dertva0vesg6 (Scheme 35) In
their case Cu212 was used as the catalyst to generate the carbene Here Rh,(OAc), falled to catalyze
the reachon because the metal forms strong complexes with basic mtrogens and renders the catalyst
inactive This represents a hmltahon for Rh(I1) catalyzed processes Nevertheless, the mechanism
proposed is slmllar to our hypothesis for ether activated C-H insetion, and the electron donatmg
capacity of the nitrogen lone pau directs the mode of cychzatlon Although mixtures of isomers
were obtamed, 48 and 49, mcludmg some cychzahon at an unactlvated C-H bond (9%), this reachon
further emphasizes the stereoelectronic effect of heteroatom achvabon of the C-H bond
Doyle recently described a very effective C-H insetion to prepare p-lactams, 51, from
dmzo-gketoamldes, 50, in which the nitrogen of the amide bore a bulky t-butyl or branched alkyl
subshtuent g7 The usual S-membered nng products were seen using the catalyst m dlchloromethane
at ambient temperature However, m benzene at reflux a different reachon prevaded and p-lactam,
51, was obtamed Doyle argued that the selectlvlty m the reachon is not of an electromc nature, but
1791 Rhodmm(II) catalyzed xeactlons
Scheme 35: C-H Insertion a to Nitrogen
48 91.9 49
rather that the C-H bonds m closest proxnnlty to the reachve rhodmm-carbene center led to the
preferred p- and not y-lactam products T~LS suggests a very stable anude conformation which 1s
governed by the bulky subsutuent on mtrogen (Scheme 36)
scheme 36: Synthesis of p-Lactams
0
In contrast when Doyle studied the analogous reachon using duuoesters and diazoketo-esters
no p-lactone formation was observed, but rather the normal S-membered nng leadmg to y-lactone
construcuon previuled g8 Interestmgly, hgand effects on rhodium led to dramatic regmchemlcal
preferences A competmon for C-H mscmon of dlazoester, 52, yielded product mixtures ( 53 and 54)
usmg Rh,(OAc), or Rh2(OCOC3F,)4 (Scheme 37) However when rhodium acetamide,
Rh2(NHCOCH,),, was employed as the catalyst the reaction was totally regloselecttve. These
results suggest, as m the cyclopropanauon chemistry, that the hgands on rhodmm play an Important
role in the C-H insetion process
1792 J ADAW and D M SPERO
scheme 37: Synthesis of y-L&ones
52 53 54
m2L4
~2(oAc>4 53 47
fi2(NI’=OCH,), >99 <l
The C-H mseztton chemistry of rhodmm medated carbines 1s stdl Elaclvely new and has
only received lmuted attention over the past decade It remams to be seen what future combmauons
of catalyst selection and stereoelectromc control m the reactmg substrate WIU reveal m establishmg
reglo-, stereo-, and eventually enanfio-specific control m the carbene C-H mserClon reacuon
IV. Heteroatom-H Insertion
Smce the pioneermg work of Yates on the copper-catalyzed decomposltton of dmzoketones
111 alcohols and phenol,W the mseNon of carbenes and carbenoids into hydroxyhc bonds has been
extensively mvestigated Teyssit? reported rhodium-catalyzed msetion of ethyl dmzoacetate mto the
hydroxyhc bond of sunple alcohols, 100,lol as well as unsaturated alcohols.102 In the case of ethylemc
and acetylemc alcohols there is a preference for O-H msettton over cyclopropanauon,102 and
examples a~. shown m Scheme 38
Scheme 38: O-H Insertion of Unsaturated Alcohols
N2CHC02Me
HX CH20H - HGCH20CH2C02Me + cyclopropanauon
Rh,(OAch 60% 12%
N2CHC02Et
OH - ~0cH2cC12Et + cyclopropanation
&(OAc), 68% 14%
Rhodmm(II) catalyzed reactions 1793
The mtermolecular O-H msemon of rhodmm carbenolds has been used to convert &ok to
&oxanes.lo3 I)lols treated ~nth ethyldiazoacetate and catalyuc rhodtum (II) pivalate afforded a mono
O-H msemon product Subsequent acid catalyzed lactonizauon followed by reduction yielded a
dloxane (Scheme 39)
Scheme 39: Synthesis of Dioxanes
R
x
OH N2CHCOzEt R OH 1. H+ )
x *
R OH Rh(II)pivalate R OCH$OzEt 2. reduce
The mtramolecular O-H msemon of rhodium carbenotds has been exploited to make fivelo
through eight membered oxygen contammg nngs lo5 Moody explored the rhodmm carbenold
cychzahons as a general route to oxygen, sulfur, and mtrogen contatmng nngs as shown m Scheme
40 lo5
Scheme 40: Synthesis of Lactones, Thiolactones, and Lactams
X= 0, S, N
Moderately good yields were obtamed for the formation of SIX and seven membered rmgs
The yield of cychzation to eight membered rtngs was lower due to compettng C-H msemon. When
X = sulfur, cychc SIX and seven membered thmethers were obtamed albelt 111 low yteld (30-35%) 105b
When X = tert-butyloxycarbonyl (Boc) or pivaloyl protected tutrogen only C-H msemon was
observed to afford cyclopentanones,‘05bV1e mstead of seven or eight membered rmgs The authors
propose that the fatlure of N-H msemon was probably due to the fact that the Boc or
plvaloyl-protected mtrogen 1s too hmdered and non-nucleoptic to mtercept the electrophlltc
rhodium carbenold However, the formatton of four, five and SIX membered rmgs by rhodmm
carbenoid msemon mto N-H bonds has been reported and is a &able process 104J06
1794 J ADAUS and D. M SPERO
An unportant syntheuc apphcatlon of the N-H msertton reactton was the con&u&on of
p-lactam antlblohcs. The earbest version of this reacaon was reported by workers at Merck Sharp &
Dohme. where dmzoketone, 55, was cychzed to the oxepenam, 56, with catalyhc rhodmm (II)
acetate’” (Scheme 41)
Scheme 41 Synthesis of Oxepenams
Bncom~[~)-co2Bn m2(oAc)4+
0
BncoNHzQ
0 t?02Bn
55 ss
Merck has also apphed the rhtium (II) catalyzed N-H mscruon towards the synthesis of
carbapenem rmg systems, and found thus method preferable to photochemical methods lo8
Photolyhc decomposhon of the dmzoketones gave Wolff rearranged products m addition to the
desmd N-H mseruon A rhodmm (II) catalyzed mtramolecular N-H mser&on was used as a key step
111 the total synthesis of the carbapenem tienamycm and IS even used in the commercial
manufacturmg of the druglOg~llO(Scheme 42)
Scheme 42: Synthesis of lhienamydn
wlo2B; NB~ +-3-&- SO-V2Nf-4+
H 2
co,
Thlenamycin
Synthetic approaches to l,Z&azehdmones have been mvestlgated by Moody and Pearson 111
Aza-P-lactams can be prepared m high yield from the Rh@) catalyzed decomposmon of dlazo
hydravdes (Scheme 43)
Scheme 43: Synthesis of Aza-p-LactamS
Rh,(OAc), z
Z
0
1795 Rhodmm(II) catalyzed reactions
The msemon reaction of carbenolds mto SI-H bonds to form carbon-s&on bonds IS a
synthehcally useful procedure Doyle has recently pubhshed a general procedure for the formation
of a-nlyl carbonyl compounds by the rhodmm (II) catalyzed decomposihon of duizoketones in the
presence of organos&mes112 (Scheme 44) In general the yields are high and thts methodology
offers an alternative to enolate amon displacement of chlonde from chlorosrlanes
Scheme 44: Preparation of a-Silyl Carbonyl Compounds
WOAc), R- L R
7 7. n R$H - 1
N2 =3
V. Ylide Formation and Reactivity
The reaction of carbenes with heteroatoms to form ylides has been known for many years ‘13
Recently there has been renewed mterest m these reactions due to the synthetic utity of the reactive
yhde In the following sectton the reactions of rhodmm (II) acetate denved carbenolds with oxygen,
sulfur, and nitrogen is reviewed
a) Carbonyl Yhde Formatton
The reaction of an a-ketocarbenoid with a lone pour of electrons on a carbonyl group
generates a carbonyl ylide Recently synthetic chemists have made use of the carbonyl yhde as a
l,f-dipole, trapping the reactive species with olefins, llSa acetylenes, carbonyls and hetero-multtple
bonded dlpolarophiles The Padwa group has been a major contnbutor to this area of research and
some of this work 1s reviewed below
Padwa et al have constructed a number of mtereshng nng systems using a carbonyl ybde
[2,3] cycloaddltion strategy 117 For example, oxapolycychc lactones can be synthesized by the
rhodium (ll) acetate catalyzed reaction of 1-acyl-1-dtazoacetates 114 The mtramolecular
cycloaddltlon of a mixed dlazomalonate ester, 57, with a suitably positioned oletin affords the
tr~ychc lactone, 58 (Scheme 45) In order for cychzation to occur, the dlazoacetate must have an
electron withdrawing group on the carbon bearmg the dlazo moiety In similar systems when the
electron withdrawing group 1s replaced by hydrogen, no cycloaddmon occurs, presumably because
the mtermedlate rhodium carbenoid has dlmmlshed electroptihcclty and therefore does not react with
the carbonyl to form the yhde I14
1796 J ADAMS and D M. SPERO
Scheme 45: Intramolecular Carbonyl Ylide Fommtion and 2,3 Cycloaddition
The same group also prepared unsaturated &alkoxyacyl-cll-&azoacetophenones, 59, and
mvestlgated rhodmm (II) acetate catalyzed cychzatlons Compound, 59, was treated with catalytic
rhodium (II) acetate and cycloadduct, 61, was isolated (Scheme 46) Expernnental support for the
formation of the mtermebate carbonyl yhde, 60, was provided by trappmg with dlmethylacetylene
dlcarboxylate @MAD) As seen m Scheme 46, the yhde mtermedlate IS suitably disposed for the 2
+ 3 cycloaddmon process
Scheme 46: Cyclization of Diazacetophenone Derived Ylides
COCHN, ~2(oAc), b
C02(CH2),CH=CH2
In the first two examples (Schemes 45 and 46) the tandem cychzatlon-cycloaddmon
sequence occurred with SIX membered rmg cat-bony1 yhdes Also reported are sumlar sequences
with five and seven membered rmg carbonyl yhdes ‘16 When cyclopropyl substituted diazoketone,
62, was treated with rhodium (II) acetate, the intermediate five membered rmg ylide was trapped
with DMAD to afford, 63116 (Scheme 47) Methyl propiolate, N-phenylmaleumde, ethyl
cyanoformatc and methyl propargyl ether were also used as 1.3~d~polarophlles
Scheme 47: Cyclization of Mam+Diketones with Acetylenes
0 0 0
CHN, Rh,(OAc),
0
R 37 R
I = CO,Me CO&H3
Me
62 63
1797 RhodNm(II) catalyzed reactions
The rhodmm (II) catalyzed reaction of a-dtazoketones ~th nelghbormg oxune ethers also
has been mveshgated 118 Cychzatton to the azomethme yhde occurs only d the oxnne ether exists N
such a conformation that the lone pau of electrons on Ntrogen am accessible to the carbenold The
rhodium (II) octanoate catalyzed reactton of 64 and DMAD afforded the azomethme yhde delrved
cycloadduct, 65 11* Sumlarly 66 could be converted to 67 (Scheme 48)
Scheme 48: Oxhne Ylides and 1,3 Cydoaddition
V-XzWOCHN2 WII) b
DMAD
65
OMe
+ ,OMe
Rh@I) ) q; DMAD. &zMe
0 0
b) Sulfonium Yltdes
The reaction of carbenes with the lone p;tlr of electrons on sulfur to form sulfomum ylldes 1s
finding mcreasmg ut~hty in orgaNc synthesis. ‘lg. 12’S 12’ KametaN has demonstrated the synthesis of
penicillin denvatlves utiizmg sulforuum yhdes as Ntermedlates Carbon can be introduced at the
C-4 poshon of azefidmones by employing a rhodNN-catalyzed decomposlhon reacuon of
ol-diazoNalonate120 with 4_phenylthloazetidmones, 68 lZ (Scheme 49) The formation of tbe C-4
carbon substituted azehdmone, 70, occurs through the yhde mtermehate, 69 Oxygen funcuonabty
can be mtroduced at C-4 of the azet&none by treatment of 68 wltb a-dnuoacetoacetate. In both
cases, the substltuents at C-3 and C-4 end up trans
As an extension of this work, the reaction of penicillm, 73, with the rhodmm carbenold of
p-mtrobenzyl a-dlazoacetoacetate afforded eight-membered oxa-derrvauves, 741Z (Scheme 50)
1798 J ADAMS andD M SPERO
Scheme 49: C-4 Substitution of Azetidinones
COZR
N2 <
COMe L R3
Rl
-P@ N 0 ‘R2
21
The approach of the carbene is from the less hindered p-face of the pemcillm bicyclic system The
conversion of 4-thloxo-2-azebdmones into 4-alkyhdme-2-azet.&nones has also been reported 134
A novel synthesis of 3,4-dlhydro- 1,3-thlazm-4(2H)-ones. 77, also made use of a carbene
addition-rmg expansion sequence ‘2.1 The rhodmm-catalyzed reacuon of dmzo compounds with
2-subsututed tsothmzol-3(2H)-ones, 75, afforded an intermediate sulfomum yltde. 76, which
rearranged to the six membered nng, 77 (Scheme 5 1)
Ando and co-workers have mvestlgated the reaction of cyclic disulfides with carbenes to
afford 1 ,Z&thlanes, or desulfurtzatlon products if the dlsulfide and the carbene are stertcally
hindered 125 In most cases the authors used CuCl to catalyze carbene formation, however, some
examples employed rhodmm (II) acetate and the results dtd not depend on the method of carbene
generation In general, the yields for the desulfurrzation reacttons are approximately 30% The
yields for the S-S mseNon products are typically greater than 65%
Rhodmm(II) catalyzed reactions 1799
Scheme 50: Ring Expansion of Penidllimi
H
COzR
Me C02R
N2
Rh20W,
Scheme 51: Synthesis of DihydIxAhhzinones
Me
R R +
It 1s well known that sulfomum yhdes are isolable when stabtized by two electrons
wlthdrawmg groups 121*126 Stable cychc sulfomum yhdes are less well known although some have
been prepared by the treatment of cychc sulfomum salts ~th base. 127 Stable cyclic sulfomum yhdes
formed via a carbene route have been reported by DavieslB and MoodylB Moody demonstrated
that 111 the presence of catalec rhodmm (II) acetate the dmzosulfide, 78, gave the stable yhde, 79, m
24% yield (Scheme 52). Further heating m toluene effected a Stevens type [ 1,2]-rearrangement to
the thlapyrone, 80
Thlophenes react with dlazomalonates under rhodium (II) acetate catalysis to give
thlophenmm methylides 130 ImtAly, copper catalysis was mvesttgated as the means for genemtmg
the carbene but these reachons were lmprachcally slow, typtcally takmg eight days at reflux with
incomplete reaction 130 In contrast the rhodium (II) acetate reactions are complete m 18h at room
1800 J ADAMS~~~D M SPERO
Scheme 52: Sulfonium Ylides and Stevens Reanangement
0 m@Ad,
S I
C02Et
\Pll
28
0
C02Et
Ph
!B
temperature Otto Meth-Cohn found that 2.5&chlorothtophenes reacted ~rlth Qazoketones to yield
yhdes which reatiy underwent thermal rearrangement to give oxathmcmes 131
Intramolecular versions of tiophemum yhde formation have also been reported m the
hterature 132 Rhodmm (II) acetate catalyzed decompostuon of the Qazo compound, 81. gave
cychzed 83 as the maJor product, presumably arrsmg from a Stevens rearrangement of yhde,
82133(Scheme 53) In addmon, the C-H mseruon product, 84, was Isolated 11125% yield
c) Oxomum Ylides
Oxonmm yhdes have been postulated as mtermedtates when diazo compounds decompose m
the presence of 2-phenyl-l,3-&oxolsne,135 styrene o~ldes,‘~~ allyhc ethers,13’ ahphatlc ethers,138.13g
allyhc acetals,lN oxetanes141 and furans 142 Carbenoids react wtth the oxygen of epoludes143 and
sulfoxldes144 effectmg transfer of oxygen to the carbene center Oxomum yhdes have also been
generated by deprotonatton’” and desdylauon 145 of oxomum ions and they are mvolved m the
zeohte catalyzed conversion of methanol to ethylene146
Johnson has deslgned ahphatic, alkoxysubshtuted hazoketones and ut&zed theu rhodunn
(II) acetate decomposlhons to yteld cycloalkanones 147 The presence of an mtermebte oxomum
yltde was proposed An example of tis work is shown m Scheme 54 Diazoketone, 85, was
decomposed m the presence of rhodium (II) acetate to form cyclobutanone, 87, and cyclooctenone,
89 When the methyl subshtuted diazoketone, 86, was SubJected to the reactton condmons the
resultmg cyclobutane, 88, was formed 111 high tiasteroselectivlty (97 3) with the CIS vlcmal methyl
groups of the cyclobutanone nng as the maJor diastereolsomer
Rhodmm(II) catalyzed xeactlons 1801
scheme 53: Thiophenlum YIldes
0
sl
I- C-H insertion
CO,Me MeOzC 0
P
Pumng148 has reported results on the mtramolecular generation of allybc oxomum yhdes and
tbelr subsequent [2,3]-szgmatroplc rearrangement to @ve five-, six-, and eight- membered oxygen
heterocycles In a generalized scheme dmxoketone. 90, was treated with rhodium 0 acetate to form
au allyhc oxomum yhde, 91, which underwent a [2.31-slgmatroplc rearrangement to form an oxygen
contimng nng, 92 (Scheme 55)
In the majortty of examples the htghest ytelds were obtamed when R=C02R1 Altemattve
pathways avalable to the carbenold (e g , C-H msertlon, cyclopropanation and dlmenzauon) were
muumlzed by the kmeuc preference for five-membered nng formation Furanones are formed 111
very good yields (93-W. whereas pyranone formation occurs m modest yield (95-W) due to
competmg C-H msemon (Scheme 56)
Padwa has also observed that for the formation of SIX-membered rmgs, C-H msertlon can be
competitive wtth yhde formation and subsequent [2,3] sigmatroplc rearrangement 149 In ad&tion he
1802 J ADAMS andD M SPERO
Scheme 54: J.ntmmokcdar Cyclhation of Oxonium Ylides
a R=H
@i R=Me
\ Oil0 f , a R=H
@ R=Me
Scheme 55: Allylie Oxonimn YUde
0
@ R=H
noted that the replacement of ally1 substituted oxygen with sulfur resulted m pnmanly yhde dewed
products Moody has also studied the [2,3]-rearrangements of S-ally1 sulfomum yhdes to form
six-membered nngs 12g
Rhodmm(II) catalyzed n?acnons 1803
Scheme 56: Synthesis of Furanones and Pyrmms
0 0
C02Me
w
d) Intermolecular Reactions of Carbenes with Allyhc Hetero Compounds
Intermolecular reacttons of rhodmm carbenolds with allyhc hetero compounds ate Hndely
recogmzed The [2,3]-sigmatropic rearrangement of S-ally1 sulfomum ylldes. generated by the
reaction of a dmzoketone and an ally1 alkyl sulfide, has been stu&ed by Takano lx) In a typxal
procedure allylphenylsulfide, 97, and dlazo compound, 98. were heated to reflux m toluene in the
presence of rhodmm (II) acetate to form the intermediate sulfomum yhde, 99, which underwent 2,3
sigmatropic rearrangement to form 100 (Scheme 57)
Scheme 57: Interrnokcular Ylide Formation with Allylic Sulfides
Ry&x Y
G31 R -
-R G
SPh
x Y
J ADAMS andD M SPERO
With simple alkyl ally1 sulphldes, [2,3]-slgmatropic rearrangement was the maJor pathway
However with more complex systems, such as 2-vmyl derrvatlves of 1.3~d~tiuane and 1,Zditiolane.
ehmmatton reactions of the mtermediate yhdes was compettuve with [2.3]-sigmatroplc
rearrangement 140
Ally1 acetals underwent yhde generauon m rhodmm (II) acetate catalyzed reactions with
dmzoesters to form 25Qalkoxy44kenoates by 12.31~slgmatroplc rearrangement. 140 Competmg
cyclopropanation. and 111 some cases Stevens rearrangements, occurred The product dlstnbutlons
were dependent on the catalyst, dlazo compound, acetal and temperature
Metal catalyzed decomposition of dlazoesten m the presence of ally1 hahdes can afford
halomum yhdes, but cyclopropanatlon IS competitive with yhde generauon and Earrangement
Compeutton between cyclopropanation and yhde generation can be mampulated by varymg the
nucleophihclty of halogen m the reactant ally1 halide. Reactions with ally1 to&de resulted solely 111
the product from [2,31-slgmatroplc rearrangement, whereas cyclopropanation occured predommantly
with ally1 chlorrde 150bz1 Catalyst selection was also Important: Cu catalysis afforded more
complex reacuon mixtures due to C-Br bond cleavage Another method of mcreasmg the relattve
yield of yhde denved product was through the use of rhodmm (II) perfluorobutyrate 151
In summary, the chemistrypf yhde formation resultmg from Rh(II) catalyzed dtazo-carbonyl
decomposition IS very nch and dtverse. allowmg for many novel synthettc appllcahons
Nevertheless, optunum utdity of these processes reqmres analysis of the reacting substrates, the
catalyst employed and the reaction conditions, smce the reacuon pathways of yhde mtermedmtes are
numerous
Rhodmm(II) catalyzed reactions 1805
(a) Wong, H.N.C.; Hon, M., Tse, C., Yip, Y.; Tanko, J ; Hudhcky, T. them Rev (1989). 89, 165 (b) Boyer, E.B.; Robmson, S.D. Coordmatwn Chtmstry Rev (1983) 50.109. Ed. Lever, A B P., Elsemer, Amsterdam-oxford-New York Brookhart, M., Studabaker, W B Chem Rev (1987) 87.411 Maas. G Topics zn Current Chemstry. Vol 137, Sprmger-Verlag, Berlm Hadelberg, 1987 Doyle, M P them Rev. (1986),86,919. Knmse, W “Carbene Chemistry”. 2nd ed., Academtc Press, New York, 1971. ~116 Marchand. A.P.. MacBrockway. N Chem Rev (1974) 74.431. Jones Jr., M.; Moss, R A., Eds., “Carbenes”, Vol. 1. Whey, New York, 1973 Armstrong, R K .Z. Org Chem (1966) 31.618 McBee. E T , Calundann, G.W ; Hodgm, T J Org. Chem (1966) 31,426O. Werner, H , &chards, J H. J Am. Cheer. Sot. (1968) 90,4976. Montam, I , Yammamoto. Y., Homsb, H .Z Chem Sot , Chem Commun (1969) 1457 K&mam, K., Hlyama, T , No&, H. Tetrahedron Lett (1974) 153 1 Pauhssen, R , Hubert, A J ; Teyssl P. Tetrahedron Letz (1972) 1465 ;;;ssen, R , Rennlmger H., Hayez E , Hubert, A J , Teyssd, P Tetrahedron I&. (1973)
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J ADAMS~~~D M SPERO
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1807 Rhcdmm(II) catalyzed reactions
91 92 93
98 99 100
101 102
103 104 105
106 107 108 109
110
111
112 113
114 115 116 117 118 119 120 121 122 123 124 125 126
127
128 Davies, H &I L ; Cnsco, L.V.T Tetr&edrdn Lett (i987) 28; 37 1.’ 129 Moody, C J , Taylor, R J Tetrahedron Lett (1988) 29,6005
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(b) Moody, C J , Pearson,
aza-analogues of B_lactam antlbtottcs see For an attempted approach to the
(1984) 49,113 Taylor, E C ; Davies, H M L J. Org Chem.
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1808 J ADOS andD M SPERO
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139 140 141
142
143
:z
146
147 148 149 150
151
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Doyle, M P. Act Chem. Res.(1986) 19.348