Palladium(0)-Catalyzed Difunctionalization of 1,3-Dienes: From … · dienes provides a wide...

122

X. Wu, L.-Z. Gong Short ReviewSyn thesis

SYNTHESIS0 0 3 9 - 7 8 8 1 1 4 3 7 - 2 1 0 XGeorg Thieme Verlag Stuttgart · New York2019, 51, 122–134short reviewen

Palladium(0)-Catalyzed Difunctionalization of 1,3-Dienes: From Racemic to EnantioselectiveXiang Wua Liu-Zhu Gong*b,c 0000-0001-6099-827X

a Anhui Province Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, P. R. of China

b Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. of China

c Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, P. R. of [email protected]

Published as part of the 50 Years SYNTHESIS – Golden Anniversary Issue

R

R

PdLnR

Nu

R'-X – Nu

Pd(0)

R'R'

R

R'

Nu

1,2-product 1,4-product

or

Pd(0)

X = Br, I, OTf, ONf, N, N2+

R' = H, N, C

Nu = N, B, C, H, O, Si

Received: 17.10.2018Accepted after revision: 18.10.2018Published online: 15.11.2018DOI: 10.1055/s-0037-1610379; Art ID: ss-2018-z0700-sr

License terms:

Abstract 1,3-Dienes are easily accessible chemicals that participate ina series of reactions acting on the carbon–carbon double bonds. Cata-lytic difunctionalization of 1,3-dienes provides a wide scope of func-tionalized chemicals. Pd(0) catalysts provide a diverse set of principlesfor the creation of asymmetric catalytic reactions, which are initiatedwith the oxidative addition and then undergo insertion reaction withone of double bonds of the 1,3-diene to become a π-allyl palladiumspecies that is reactive toward nucleophilic attack. This review summa-rizes typical advances on the Pd(0)-catalyzed difunctionalization of 1,3-dienes in recent decades, particularly emphasizing the concepts thatenable the switch from a racemic reaction to an enantioselective ver-sion.1 Introduction2 Amination3 Boration4 Carbonation5 Hydrogenation6 Oxygenation7 Silylation8 Conclusion and Outlook

Key words palladium(0), asymmetric catalysis, difunctionalization,1,3-dienes, cascade reaction

1 Introduction

Buta-1,3-diene, which is important industrially as amonomer in the production of synthetic rubber, is pro-duced from steam crackers on a scale of more than 10 mil-lion tons per year worldwide.1 The last several decades havewitnessed the proliferation of fundamentally importantand synthetically significant methods by functionalizing1,3-diene and its derivatives,2 which have been prevalentlyapplied in the natural product synthesis, medicinal chemis-try and materials science.3 The difunctionalization of 1,3-dienes provides a wide spectrum of structurally diverse and

densely functionalized chemicals with great potential in or-ganic synthesis, and has hence been considered a powerfulstrategy in synthetic organic chemistry.4 In the difunction-alization of 1,3-dienes, it is a significant challenge to con-trol the regioselectivity toward 1,2- or 1,4-addition because

Liu-Zhu Gong (left) was born in October 1970 in Henan, China. He graduated from Henan Normal University (1993) and received his Ph.D. (2000) from the Institute of Chemistry, Chinese Academy of Sciences. He was a visiting scholar (Joint Ph.D. graduate student program) at the University of Virginia and an Alexander von Humboldt Research Fellow at the University of Munich (2003–2004). He was appointed an associ-ate professor of Chengdu Institute of Organic Chemistry, Chinese Acad-emy of Sciences in 2000 and was promoted to a full professor in 2001. Since 2006, he has been a full professor of University of Science and Technology of China. He was appointed the Cheung Kong Scholar Pro-fessor of organic chemistry in 2007. His research interests include orga-no-/transition-metal cooperative and relay catalysis, asymmetric multicomponent and cascade reactions, catalytic asymmetric function-alization of allylic C–H bonds, and enantioselective total synthesis of natural products.

Xiang Wu (right) was born in 1982 in Jiangsu, China. He received his B.Sc. degree in chemistry from Nankai University in 2005. He complet-ed his Ph.D. studies in organic chemistry (2005–2010) at the same in-stitution under the supervision of Professor Wei-dong Z. Li. He worked in Suzhou Novartis Pharma Technology Co., Ltd from 2010 to 2012. He was a postdoctoral researcher under the guidance of Professor Liu-Zhu Gong at University of Science and Technology of China (2012–2014) and Professor Gong Chen at the Pennsylvania State University (2014–2015). Since 2015, he has been an associate professor at Hefei Universi-ty of Technology and his research interests are in palladium-catalyzed functionalization of dienes and oxidative asymmetric cycloaddition.

Georg Thieme Verlag Stuttgart · New York — Synthesis 2019, 51, 122–134

http://orcid.org/0000-0001-6099-827X

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of the various coordination and insertion modes conceiv-able for a transition-metal catalyst. In recent years, with thedevelopment of organometallic chemistry, transition-metalcatalyst (palladium, copper, nickel and more) enabled di-functionalization of 1,3-dienes has been reported, fre-quently and continuously. Both Pd(0) and Pd(II) catalystsare able to promote difunctionalization reaction of carbon–carbon double bonds. The palladium(II) coordinated withone of double bonds of 1,3-diene undergoes a nucleopalla-dation with a nucleophile (Nu1

–) in a Wacker type processto generate a π-allyl palladium intermediate A, which canthen undergo a substitution reaction with another nucleo-phile (Nu2

–) to afford either a 1,2- or a 1,4-product, and torelease the Pd(0), which is oxidized into catalytically activePd(II) for the next catalytic cycle (Scheme 1, eq. 1). The pal-ladium(0) complex has also been shown to enable variousdifunctionalization reactions after it undergoes an oxida-tive addition to a high oxidation state compound and a sub-sequent Heck insertion of the 1,3-diene to form a π-allylpalladium species B, which ultimately reacts with a nucleo-phile to generate a 1,2- or 1,4-addition-like product(Scheme 1, eq. 2).

Scheme 1 Pd-catalyzed difunctionalization of 1,3-dienes

Considering that a variety of excellent reviews havesummarized the metal-catalyzed enantioselective difunc-tionalization of the 1,2- or 1,4- positions of 1,3-dienes,4e,4f,5

this short review mainly focuses on highlighting Pd(0)-cat-alyzed difunctionalization of 1,3-dienes. As shown inScheme 2, the Pd(II) intermediate I, generated from an oxi-dative addition reaction of a Pd(0) complex to an R-X, un-dergoes a Heck insertion reaction to give an allylic palladi-um intermediate II, which will be able to undergo isomeri-zation to form a π-allyl palladium intermediate III. The π-allyl palladium species III then participates in an allylic al-kylation reaction with a stabilized carbon nucleophile bydirect back-side attack at one of the allylic terminuses,principally giving rise to either a 1,2-product or a 1,4-prod-uct and releasing Pd(0) (Path A). Alternatively, a transmeta-lation at the palladium of intermediate III gives π-allyl pal-ladium intermediate IV, which then undergoes a reductiveelimination to generate a 1,2-product or a 1,4-product andrelease Pd(0) (Path B).

Scheme 2 Putative catalytic cycle for the Pd(0)-catalyzed difunctional-ization of 1,3-dienes

2 Amination

2.1 Three-Component Arylation or Vinylation/Ami-nation

The first example of Pd(0)-catalyzed three-componentdifunctionalization of 1,3-dienes was reported by Heck’sgroup in 1978.6 They initially planned to synthesize conju-gated dienes from bromobenzene or 2-bromopropene withisoprene by palladium-catalyzed arylation; unexpectedly,regiospecific 1,4-difunctionalized allylic amines 1 and 2were obtained when a large excess of secondary amine(piperidine or morpholine) was employed in the reaction(Scheme 3, eqs. 1 and 2).6 In contrast to acyclic dienes, 1,3-cyclohexadiene provided both 1,2- and 1,4-products(Scheme 3, eqs. 3 and 4).7,8

Scheme 3 Three-component arylation or vinylation/amination

RR

PdLnR

Nu2

Nu1– Nu2

–

Pd(II)

Nu1Nu1

R

Nu1

Nu2

1,2-Product 1,4-Product

or

RR

PdLnR

Nu

R'-X – Nu

Pd(0)

R'R'

R

R'

Nu

1,2-Product 1,4-Product

or

(1)

(2)

Pd(0)[O]

Pd(0)

A

B

R X +Pd(0)

Nu-R'

Nu

RR'

X

LnPd0

insertion

PdR

LnPd

R

π-allylformation

Nu-

PdII

RLn

Ln X

oxidative addition

R'

R X

X

R'R'

Nu

LnPd

RR'

1,2-productor

1,4-product

Nu-

X-

1,2-productor

1,4-productI

IIIII

IV

(Path B)

(Path A)

Br

NH

N

157%

Pd(OAc)2 (1 mol%)P(o-tol)3 (2 mol%)

100 °C, 48 h

Pd(OAc)2 (1 mol%)PPh3 (2 mol%)

100 °C, 16 h N

51% 2%

N

NH

O

240%

Br


100 °C, 24 h N

35% 15%

N

+ +

N

O

+ +

Pd(OAc)2 (1 mol%)P(o-tol)3 (2 mol%)

100 °C, 45 h

Br

NH

+ + +

NH

Br+ + +

(10 mmol)(300 mmol)

(10 mmol) (300 mmol)

(1)

(2)

(3)

(4)


124


Moreover, Dieck and co-workers also found that a broadscope of amines such as ethyl amine, diethyl amine, n-bu-tylamine, tert-butylamine and pyrrolidine could work asnucleophiles to participate in the arylation/amination andto yield 1,2- and 1,4-products (Scheme 4).9

Scheme 4 Amine nucleophiles for the three-component arylamination

2.2 Arylation/Intramolecular Amination

At the same time, Dieck’s group reported a cascade ary-lation and intramolecular amination reaction of o-iodoani-line with isoprene and 1,3-cyclohexadiene, to generate 2-isopropenyl-2,3-dihydroindole 3 (72%) and la,3,4,4a-tetra-hydrocarbazole 4 (70%), respectively (Scheme 5).9

Scheme 5 Arylation/intramolecular amination cascade

Larock and co-workers then developed an even more ef-ficient heteroannulation reaction of 1,3-dienes with 2-io-dophenyltosyl amide, leading to dihydroindole 5 and tetra-hydrocarbazole 6 in higher yields (Scheme 6).10 2-Iodoben-zylic tosyl amides also turned out to be excellent substratesto furnish six-membered nitrogeneous heterocycles 7.

Scheme 6 Arylation/intramolecular amination cascade to generate di-hydroindole, tetrahydrocarbazole and six-membered ring nitrogen het-erocycles

Inspired by Dieck9 and Larock’s10 pioneering work, Hanand co-workers described the first Pd(0)-catalyzed enantio-selective heteroannulation of 1,3-dienes with 2-iodoani-lines (Scheme 7).11 Chiral indolines 8 were obtained in up to83% yield and with fairly good enantioselectivities of up to87% ee. The employment of a BINOL-derived phosphora-midite ligand L1 bearing electron-withdrawing substitu-ents is the key to delivering high enantioselectivity.

Scheme 7 Enantioselective cascade arylation/intramolecular amina-tion reaction to access chiral indolines assisted by chiral BINOL-derived phosphoramidite ligand

In 1999, Helmchen reported an enantioselective tandemHeck/intramolecular allylic amination reaction using aminogroup tethered 1,3-dienes and aryltriflates as substrates(Scheme 8).12 Chiral PHOX ligand L2 allowed the reaction togive chiral piperidine 10 with 80% ee. Compared to thearyliodides, aryltriflates 9 gave higher enantioselectivities,but required prolonged reaction time of 10 days.

Scheme 8 Enantioselective arylation/intramolecular amination for the synthesis of chiral piperidine

2.3 Intramolecular Arylation or Vinylation/Amina-tion

In 1989, Grigg and co-workers found that dienamidegroup tethered phenyliodide 11 could undergo a Pd(0)-cat-alyzed intramolecular 5-exo-trig cyclization on a proximatediene functionality to generate π-allyl-palladium species,which was subsequently captured by secondary amines, in-cluding morpholine, piperidine or 1,2,3,4-tetrahydroiso-quinoline, giving 1,4-products 12 in 40–60% yield (Scheme9).13

In 1993, Shibasaki’s group reported a Pd/BINAP-cata-lyzed intramolecular asymmetric Heck reaction/allylic ami-nation reaction (Scheme 10).14 Under the catalysis of a chi-

I Pd(OAc)2 (1 mol%)PPh3 (2 mol%)

100 °C, 48 h+ + NuH

Nu = EtNH2, Et2NH n-BuNH2, t-BuNH2 pyrrolidine

Nu

+

Nu

I

NH2

NH

NH


125–130 °C39 h

120–125 °C24 h

372%

470%

NHTs

I

C4H9

Pd(OAc)2 (5 mol%)Na2CO3, n-Bu4NCl

DMF, 100 °C, 1 d

N

Ts

C4H9

NHTs

I

Pd(dba)2 (5 mol%)Na2CO3, n-Bu4NCl

DMF, 100 °C, 2 d

N

Ts

IC4H9

Pd(OAc)2 (5 mol%)PPh3 (5 mol%)Et3N, n-Bu4NCl

DMF, 80 °C, 2 d

NHTs N

C4H9

Ts

+

+

+

H

H

584%

687%

781%

NH2

I R1

R2

O

OP N

Ar

Ar

O2N

L1 Ar = 3,5-(CF3)2C6H3

Pd(OAc)2 (5 mol%)L1 (15 mol%)

KHCO3, DME, 80 °Cthen AcCl, Et3N N

Ac

R1R2

+R R

8up to 87% ee

up to 83% yield

NHBn

PPh2N

O

Ph

OTf

L2

NBn

1080% ee, 47% yield

+

9


DMF, 100 °C, 10 d


125


ral complex formed in situ from Pd(OAc)2 and (S)-BINAP,prochiral alkenyl triflate 13 and benzylamine underwent avinylamination to give a bicyclic product 14 with three con-tinuous chiral centers in 76% yield and with 81% ee.

Scheme 10 Enantioselective intramolecular vinylation/amination

The Pd-catalyzed intramolecular arylamination hasbeen applied in the total synthesis of a natural product byOverman and co-workers (Scheme 11).15 The catalyticasymmetric Heck cyclization/allylic amination reaction of(2Z)-2,4-hexadienamide tethered diketopiperazine precur-sor 15 in the presence of Pd2(dba)3 and (S)-BINAP producedpentacyclic products 16 and 17 in 6:1 ratio and 28% com-bined yield. Interestingly, when ligand (R)-BINAP was used,a 1:6 diastereomeric mixture of pentacyclic products 16and 17 was obtained with similar efficiency. However, theuse of tri-o-tolylphosphine as the ligand enabled (2E)-2,4-hexadienamide18 to give a 1:1 mixture of pentacyclic prod-ucts 19 and 20, attributed to the anti-capture of the initiallyproduced η3-allylpalladium intermediate. Removal of theSEM group from the product 19 provided optically pure (–)-spirotryprostatin B. Notably, the other three stereoisomerscould also be obtained by following a similar procedure.

2.4 Aminomethylamination

To expand the application of the aminal activation con-cept,16 Huang and co-workers recently described a highlyenantioselective aminomethylamination reaction of 1,3-dienes with aminals enabled by a chiral palladium complexof BINOL-derived chiral diphosphinite L3 (Scheme 12).17

The reaction proceeded through a cascade reaction se-quence of C–N bond activation (P1), aminomethylation(P2), and asymmetric allylic amination reaction (P3), givingsynthetically useful chiral 1,3-diamines 21 with high regio-and enantioselectivity (Scheme 12).

2.5 Diamination

Chiral vicinal diamine is a structural motif prevalentlyfound in numerous biological compounds and appears to bea core structural element of chiral auxiliaries and ligandsthat have been widely applied in asymmetric synthesis.18

Metal-mediated or catalyzed diamination of olefins consti-tutes one of the most efficient approaches to access theskeleton.19

In 2007, Shi and co-workers reported that Pd(PPh3)4could catalyze the diamination of a variety of conjugateddienes using di-tert-butyldiaziridinone 23 as nitrogensource to give the racemic imidazolidinones 24 in high

Scheme 9 Racemic intramolecular arylation/amination

N

Bn

O

IP(OAc)2 (5 mol%)PPh3 (10 mol%)

MeCN, 80 °C, 3 h

NBn

O

NH

NH

= OHN HNHN

1240–60%

N

+

11

Me

OTfNHBnH

Me

1481% ee, 76% yield

13

Pd(OAc)2 (5 mol%)(S)-BINAP (6.3 mol%)

NH2Bn

DMSO, 20 °C, 2 h

Scheme 11 Enantioselective intramolecular arylamination for the total synthesis (–)-spirotryprostatin B

N

SEM

O

NH

N

I

H

O

O

N

N

N

O

O

O

H

SEM

N

N

N

O

O

O

H

SEM

N

SEM

O

NH

N

I

H

O

O

N

N

N

O

O

O

H

SEM

N

N

N

O

O

O

H

SEM

Pd2(dba)3·CHCl3BINAP

PMP, DMA

100 °C

(S)-BINAP 6:1 (28% yield)(R)-BINAP 1:6 (26% yield)

+

Pd2(dba)3·CHCl3P(o-tol)3, KOAc

THF, 70 °C

+

1:1 (72% yield)

N

N

N

O

O

O

H

H

spirotryprostatin B

15

18

16 17

19 20

Scheme 12 Asymmetric intermolecular aminomethylamination of 1,3-dienes with an aminal

R1[Pd(allyl)Cl]2 (2.5 mol%)

AgClO4 (5.5 mol%)L3 (5.5 mol%)

DCM, 10 °C, 48 h R1

NR2

NR2

R2N NR221

up to 99% eeup to 93% yield

R2

R2

OPAr2

OPAr2

L3R2 = 2,4,6-(i-Pr)3C6H2

Ar = 4-FC6H4

+

asymmetricallylic amination

(P3)

Pd(0)

R1

Pd

NR2

NR2

R2NCH2Pd+

R1 NR2

R2N–

R1

NR2

NR2

R2N–

Ln X

21C–N bondactivation

(P1)

aminomethylation(P2)


126


yields (Scheme 13).20 In this reaction, the palladium com-plex first undergoes an oxidative addition to the N−N bondof diaziridine to form a diamido Pd(II) species Int-1, whichthen reacts with the 1,3-diene to give a π-allyl Pd speciesInt-2 through a migratory insertion to the double bond anda subsequent reductive elimination to give diaminationproduct 24 (Scheme 13).21,22 Among these elementary reac-tions, the migratory insertion of the double bond of 1,3-di-ene to the diamido Pd(II) intermediate Int-1 builds up theinitial stereogenic center and the reductive elimination ofπ-allyl Pd species Int-2 leads to another one. Both eventsinvolve the palladium complex. Thus, the enantioselectiveversion could in principal be accessed by exploiting chiralphosphine ligands.21 Shi and co-workers found that a palla-dium complex adorned with tetramethylpiperidine-derivedand binol-based phosphoramidite ligand L4 enabled asym-metric diamination of 1,3-dienes to furnish the correspond-ing products 25 in good yields and with high levels of regio-, diastereo-, and enantioselectivity (Scheme 14). Notably,the diamination takes place predominantly at the internaldouble bond of the 1,3-dienes.23

Scheme 13 Diamination of 1,3-dienes

Shi further found that N-heterocyclic carbine (NHC)-Pd(0) complexes were also able to efficiently promote thediamination of 1,3-dienes with di-tert-butyldiaziridinone23 (Scheme 15).24 Moreover, the chiral NHC-Pd(0) complex[Pd1]Cl was found to be more catalytically active than oth-er tested ligands for the diamination.25

Scheme 15 NHC-Pd(0)-catalyzed asymmetric diamination

Di-tert-butylthiadiaziridine 1,1-dioxide 26 is also an ac-tive substrate to undergo Pd-catalyzed diamination of 1,3-dienes. Optically active cyclic sulfamides 27 were manufac-tured in up to 98% yield and with up to 93% ee from the re-action of 1,3-dienes with 26 enabled by palladium catalystgenerated from Pd2(dba)3 and chiral phosphoramidite L5(Scheme 16).

Scheme 16 Enantioselective diamination with di-tert-butylthiadiaziri-dine 1,1-dioxide

2.6 Hydroamination

Hydroamination refers to the direct addition of aminesto unsaturated hydrocarbons, leading to amines.26 1,3-Dienes and primary or secondary amines could undergohydroamination smoothly in the presence of Pd(0) and ap-propriate ligands, in which η3-C3H5 Pd(II) complex are usedwidely as catalyst precursors. In the catalytic cycle (Scheme17), an amine attacks the original η3-C3H5 Pd salt to gener-ate an ammonium salt 29 and Pd(0). Oxidative protonationwith the ammonium salt then forms a transient Pd-H Int-3.Diene migratory insertion to the Pd-H intermediate initiallyleads to a Pd-σ-allyl Int-4, which may isomerize into π-allylintermediate Int-5. The subsequent attack by the aminegenerates a Pd0-allylic ammonium complex Int-6, which re-leases the product 28 and regenerates Pd(0).

In 2001, the Hartwig lab showed that aryamines couldbe added to cyclohexene in the presence of [Pd(η3-C3H5)Cl]2and Trost ligand L6 to give chiral 1,4-products 30 with up to95% ee (Scheme 18).27

Cationic η3-C3H5 palladium complexes [Pd2]OTf, pre-pared by the treatment of [Pd(η3-C3H5)Cl]2 with 1,2-diaryl-3,4-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cy-

R3 R1

R2

Pd(PPh3)4(10 mol%)

C6D6, 65 °C0.25–5 h

NN

R3 R2

R1+N N

O O

2423

R3 R1

R2

N

NPd R1

R2

tBu

tBu

NN

R3 R2

tButBu

R3

R1

Int-1

Int-2

24

LnPd

PdNN

O

tBu tBu

Ln

O

O

22

Scheme 14 Enantioselective diamination for the synthesis of chiral im-idazolidinones assisted by a tetramethylpiperidine-derived phosphorus amidite ligand

R +N N

O Pd2(dba)3 (5 mol%)L4 (22 mol%)

C6D6, 65 °C, 12 hNN

O

R

25up to 95% ee

up to 95% yieldO

O

P N

L4

23

R2 +N N

O [Pd1]Cl (5 mol%)NaOtAm (15 mol%)

THF, 65 °C, 12 h

NN

O

R2 R1

up to 78% eeup to 97% yield

R1

N N

Pd

Ph PhEt

Et Et

Et

Cl

[Pd1]Cl

23

R +N N

S

Pd2(dba)3 (2.5 mol%)L5 (10 mol%)

toluene, 65 °C, 3 hN

SN

R

27up to 93% ee

up to 98% yield

O

O

P N

Ph

L5

O O O O

Ph

26


127


clobutenes and AgOTf in CH2Cl2, rendered the hydroamina-tion of 1,3-cyclohexadiene with aniline at room tempera-ture to give the corresponding 1,2-addition products 31 inhigh yields (Scheme 19).28 The use of diphosphinidenecy-clobutene ligand with sp2-hybridized phosphorus atomshaving strong-acceptor ability is critical for the catalytic ac-tivity.

Scheme 19 [Pd2]OTf-catalyzed racemic hydroamination

Beller and co-workers reported a 1,4-hydroaminationacyclic and cyclic dienes catalyzed by Pd(cod)Cl2 in combi-nation with a bidentate phosphorus ligand DPEphos L7(Scheme 20).29 The reaction proceeds in good yields andwith high regioselectivity.

Scheme 20 Racemic 1,4-hydroamination of acyclic or cyclic dienes

In 2017, Malcolmson et al. established an enantioselec-tive hydroamination of aliphatic amines with acyclic 1,3-dienes, generating chiral allylic amines 32 in up to 94% ee(Scheme 21).30 Chiral PHOX ligand L8 involving an electron-deficient phosphine not only shows high reactivity in thetransformation but also plays a special role in achievinghigh site and enantioselectivity for the 1,2-addition prod-uct. Notably, more electron-rich substituents on the dienehave a dramatic effect on the formation of the 1,2-product.

Scheme 21 Enantioselective hydroamination of aliphatic amines with acyclic 1,3-dienes

Very recently, the same group reported a highly enan-tio- and regioselective Pd(0)-catalyzed hydroamination of1,4-disubstituted acyclic internal 1,3-dienes, which areconsidered even more challenging substrates (Scheme22).31 A variety of secondary aliphatic amines, indoline, and

Scheme 17 Catalytic cycle for the hydroamination of 1,3-dienes

NH

R2R1

R

Pd(η3-C3H5)X

R

NR2R1

+

R1R2NHLn

LnPd0R1R2C3H5NH+X–

LnPdII

H

X

R

PdLn X

RHR1R2NH

RH

NHR1R2

LnPd0

RH

NR1R2

X LnPd

RH

Pd(0)

L H

X–

29

Int-3

Int-4

Int-5

Int-6

28

28

+

Scheme 18 Enantioselective hydroamination with symmetrical cyclic dienes

ArNH2

ArHN

NH HNOO

P PPh2Ph2

L6

30up to 95% ee

up to 87% yield

+

[Pd(η3-C3H5)Cl]2 (5 mol%)L6 (11 mol%)

THF, r.t., 120 h

P

P

Ar

Ar

MeO

MeO

Pd

Ar = 2,4,6-tri-tert-butylphenyl

[Pd2]

+ PhNH2

[Pd2]OTf

toluene, r.t., 5 hNHPh

3189%

R1

+ R2

NH2Pd(cod)Cl2 (5 mol%)

L7 (5 mol%)

toluene, 100 °C, 10 h

R1HN

R2

40–95% yield

O

PPPhPh PhPh

L7

NH

R2R1

R

[Pd(η3-C3H5)Cl]2 (2.5 mol%)L8 (5 mol%)

AgBF4 (6 mol%)

CH2Cl2, 0–60 °C, 3–20 h R

NR2R1

PAr2N

O

tBu

Ar = 3,5-(CF3)2C6H3

+

3294% ee

L8

Scheme 22 Enantioselective hydroamination of 1,4-disubstituted acy-clic internal 1,3-dienes

NH

R2R1

R3

R3

NR2R1

R4

[Pd3]BArF4 (5 mol%)

Et3N

hexanes–Et2O (1:1), 22 °C R4

PAr2N

O

tBu

Ar = 3,5-(CF3)2C6H3

PdBArF

4

33up to 97% ee

up to 78% yield up to 98:2 rr

+

[Pd3]BArF4


128


primary anilines undergo the asymmetric 1,2-hydroamina-tion reaction with a diverse spectrum of aryl/alkyldisubsti-tuted dienes as well as sterically differentiated alkyl/alkyl-disubstituted dienes, generating allylic amines 33 bearingvarious α-alkyl substituents in up to 78% yield, with >98:2rr, and 97% ee.

3 Boration

The boration of 1,3-dienes has received a great deal ofattention because it generates a diverse range of alkyl boro-nates, which are important intermediates and buildingblocks in synthetic organic chemistry.32 Fe,33 Cu34 or Ir35-catalyzed boration of 1,3-dienes has been investigated in-tensively by several groups; however, the Pd(0)-catalyzedvariants are relatively rare.

3.1 Hydroboration

The preparation of allylic boronates 36 from a 1,4-hyd-roboration of 1,3-dienes was initially reported by Suzuki’sgroup in 1989.36 Under the catalysis of Pd(PPh3)4, the hyd-roboration of buta-1,3-diene, isoprene, myrcene or 2,3-di-methylbuta-1,3-diene with catecholborane (1,3,2-benzodi-oxaborole) 35 proceeds smoothly to provide 2-[(Z)-2-alkyl-2-butenyl]-1,3,2-benzodioxaboroles 36a–d with very highregio- and stereoselectivity, which are able to undergo car-bonyl allylation with benzaldehyde to produce homoallylicalcohols 37 in high yields and diastereoselectivities(Scheme 23).

Scheme 23 Hydroborations of 1,3-dienes

3.2 Arylboration

Recently, Gong and co-workers reported a stereo- andregioselective multicomponent carbonyl allylation reactionof buta-1,3-dienes, aryldiazonium tetrafluoroborates, andaldehydes in the presence of octaphenyl-2,2′-bi(1,3,2-diox-aborolane) [B2(Pin)2], enabled by the combined catalysis ofpalladium acetate and chiral anion phase transfer, favoringthe assembly of chiral Z-configured homoallylic alcohols 38in high yields and with excellent levels of enantioselectivity(Scheme 24).37 The key chiral allylboronate intermediateInt-7, which then undergoes the asymmetric allylboryla-

tion of aldehydes to give homoallylic alcohols, is initiallygenerated from the arylborylation of a 1,3-diene with anaryldiazonium tetrafluoroborates and B2(Pin)2 rendered bythe palladium and chiral anion phase-transfer combinedcatalysis.

Scheme 24 Enantioselective alkynylboration with buta-1,3-dienes and alkynyl bromides

3.2 Alkynylboration

To extend the scope of the palladium and chiral anionphase-transfer combined catalysis for the difunctionaliza-tion of 1,3-dienes, Gong and co-workers established a mul-ticomponent carbonyl allylation reaction of buta-1,3-dienes, alkynyl bromides, and aldehydes with octaphenyl-2,2′-bi(1,3,2-dioxaborolane) (Scheme 25).38 The alkynylpalladium phosphate Int-8 generated in situ from the me-tathesis reaction of a chiral silver phosphate and thealkynyl palladium bromide turns out to be a key intermedi-ate that controls the stereoselectivity of chiral allylboronateintermediate Int-9.

Scheme 25 Enantioselective alkynylboration with buta-1,3-dienes and alkynyl bromides

4 Carbonation

In addition to heteroatom nucleophiles, stabilized car-banions such as –CH(CN)2, –CH(CN)CO2R or –CH(CO2R)2 havebeen widely employed in the Pd-catalyzed carbonation of1,3-dienes.

R R'

OHB

O R R'

BO

O

34a: R = H, R' = H34b: R = H, R' = Me34c: R = H, R' = CH2CH2CH=CC(Me)234d: R = Me, R' = Me'

36a–d

35 PhCHOR'

R'

Ph

OH

37

Pd(PPh3)4 (1.5 mol%)benzene

R

+ Ar N2XH R'

O+

Pd(OAc)2/CPANaHCO3, toluene

R' R

HO

B BO

O O

OPhPhPh

Ph

PhPh

PhPh

O

OP

O

OH

38up to 99% yield

>20:1 Z/E, 94% ee

Ar

CPA

iPr

iPr iPr

iPr iPr

iPr

RAr

B

*

OO

Ph PhPhPh

Int-7

Pd(OAc)2/CPAAgAg2CO3, toluene

B BO

O O

OPhPhPh

Ph

PhPh

PhPh

R'

+ +

R

Br

H R''

O R''

R'

OH

R

Ar

Ar

O

O

PO

O

Ar = 2,4,6-(Cy)3C6H2

Ag

CPAAg

up to 93% ee

PdCPA*

Ln

RInt-8

R'

B

*

OO

Ph PhPhPh

Int-9

R


129


4.1 Vinyl or Arylation/Alkylation

In 1983, Dieck and co-workers reported the first vinylal-kylation reaction of 1,3-dienes with dimethyl sodio-malonate and 1-bromo-2-methylpropene catalyzed by pal-ladium complex formed from Pd(OAc)2 and PPh3, to give thecorresponding 1,4-selective product 39, albeit in moderateyield (Scheme 26).9

Scheme 26 Vinylalkylation with 1-bromo-2-methylpropene

In 1987, Takahashi and co-workers described a Pd-cata-lyzed three-component arylalkylation of buta-1,3-dienewith aryliodide and malononitrile or methyl cyanoacetate,allowing for the generation of the corresponding 1,4-prod-ucts 40 and 41 with iodobenzene and buta-1,3-diene inmoderate yields (Scheme 27).39

Scheme 27 Arylalkylation with iodobenzene

4.2 Intramolecular Arylation or Vinylation/Alkyla-tion

Grigg and co-workers demonstrated that the sodio-malononitrile could attack the π-allyl-palladium species,which is catalytically generated from an intramolecular 5-exo-trig cyclization on a proximate diene mediated with Pdcomplex, to afford the corresponding regiospecific 1,4-product 42 in 60% yield (Scheme 28).13

Scheme 28 Intramolecular arylalkylation

By using BINAP as a chiral ligand, Shibasaki establishedan intramolecular asymmetric Heck insertion and allylic al-kylation cascade reaction (Scheme 29).40 An optically activefunctionalized bicyclo[3.3.0]octane 43 could be feasibly ac-

cessed by this reaction and was used as a chiral buildingblock for the first catalytic asymmetric total synthesis of(–)-∆9(12)-capnellene. Interestingly, the addition of sodiumbromide improved the enantioselectivity without erosionof the chemical yield in all cases by preventing counteran-ion exchange between the triflate anion and the enolate an-ion by coordination with sodium enolate.

Scheme 29 Enantioselective intramolecular arylalkylation for the total synthesis of (–)-∆9(12)-capnellene

4.3 Arylation/Intramolecular Alkylation

In parallel with the development of heteroannulation of1,3-dienes,10 Larock and co-workers also accomplished anintramolecular carboannulation of 1,3-dienes with aryl io-dides to give indanes 44 and tetralins 45 in high yields(Scheme 30).41 In addition to malonate-type nucleophiles,other carbon nucleophiles α to an ester, a ketone or a ni-trone functionality were also tolerated, as exemplified by46–48, to afford the corresponding products in good yields.Nevertheless, a palladium-catalyzed annulation of 1,4-dienes using ortho-functionally substituted aryl halideswas also developed by the same group.42

Scheme 30 Arylation/intramolecular alkylation for the synthesis of in-dane and tetralin

The enantioselective carboannulation of 1,3-dienes andaryl iodides was very recently established by Gong and co-workers. The use of chiral palladium complex of BINOL-

BrNaCH(CO2Me)2 CH(CO2Me)2


DMF135–140 °C, 4 h

+ +

3922% yield

CH2(CN)2Pd(PPh3)2Cl2 (1 mol%)

NaH, THF, 70 °C, 67 hI

Ph

CNCN

CH2(CN)CO2MePd(PPh3)2Cl2 (1 mol%)

NaH, THF, 70 °C, 45 h

Ph

CNCO2Me

+40

62% yield

4142% yield

NBn

O

I

P(OAc)2 (5 mol%)PPh3 (10 mol%)

MeCN, 80 °C

NBn

O

NaCH(CN)2

CN

NC

+

4260% yield

OTf

H

4387% ee, 77% yield

TBDPSO

CO2Et

CO2EtNa

EtO2C CO2Et

OTBDPS

H H

H

(–)-capnellene

+

[Pd(allyl)Cl]2 (2.5 mol%)(S)-BINAP (6.3 mol%)

NaBr, DMSO, r.t.

CO2Et

II

O

I

NO2

CO2Et

CO2Et

I

EtO2C CO2Et

I

CO2Et

CO2Et

4592% yield

CO2Et

CO2Et

4482% yield

+

+

46 47 48


Na2CO3, n-Bu4NCl

DMF, 60 °C, 8 d


Na2CO3, n-Bu4NCl

DMF, 60 °C, 12 d


130


based phosphoramidite ligand L9 allowed the reaction toprovide optically active indanes 49 in high yields and withexcellent enantiomeric excesses (Scheme 31).43

Scheme 31 Enantioselective carboannulation of 1,3-dienes and aryl iodides by using a BINOL-based phosphoramidite ligand

4.4 Three-Component Arylation, Vinylation or Al-kylation

In 2011, Sigman and co-workers reported a three-com-ponent coupling reaction of vinyl triflates and boronic acidswith terminal 1,3-dienes catalyzed by palladium to give1,2-vinylarylation product 50 (Scheme 32).44 The Pd-π-allylintermediate int-10 tends to undergoing transmetalationwith a boronic acid derivative rather than β-hydride elimi-nation, after reductive elimination to give the products 50.

Scheme 32 Three-component vinylarylation

In 2015, the same group reported a three-componentcoupling of isoprene, an alkenyl triflate, and styrenylboron-ic acid to produce skipped polyenes from simple chemicalfeedstocks (Scheme 33).45,46 However, complex isomericproduct mixtures 51–53 were always obtained because ofthe difficult-to-control migratory insertion of isoprene intoa Pd-alkenyl bond, while a good site selectivity of 1,4-addi-

tion (for the generation of 53) can be achieved by using eas-ily accessible pyrox ligand L10.

Subsequently, Sigman and co-workers reported an in-termolecular 1,2-diarylation reaction of 1,3-dienes witharyldiazonium salts and aryl boronic acids, allowing the in-stallation of two different aryl groups (Scheme 34).47 In thepresence of a chiral bicyclo[2.2.2]octadiene ligand L11, agood enantiomeric excess was obtained, albeit in rather lowyield.

In 2015, Gong and co-workers successfully established ahighly enantioselective three-component coupling of 1,3-dienes with aryl iodines and stabilized carbon nucleophiles(sodium dialkyl malonates) (Scheme 35).48 A H8-BINOL-based phosphoramidite L12 turned out to be the most ef-fective chiral ligand, which not only provides high catalyticactivity, but is also able to efficiently control the regio- andstereoselectivity.

Scheme 35 Enantioselective three-component coupling of 1,3-dienes with aryl iodines and sodium dialkyl malonates

CO2R'

CO2R'

I

+ Ar

CO2R'R'O2CPd(OAc)2 (5 mol%)

L9 (10 mol%)

DIPEA, DME60 °C, 72 h

R RAr

O

OP N

Ar

Ar

O2N

L9 Ar = 3,5-(CF3)2C6H3

49up to 98% yield up to >99% ee

R1

TfO

R2

R3 ArB(OH)2

Pd2(dba)3, KF

DMA, 55 °C, 12 h R1

Ar R2

R3+

50

Pd0Ln

Ln Pd

R1

Int-10

50TfO

R2

R3

oxidative addition

LnPd

R2

R3

R1

R1

PdLn

R2

R3

R2

R3

ArB(OH)2

Ln Pd

R1 R2

R3

Ar

R1

Ar R2

R3

Heckinsertion

transmetallation

reductiveelimination

OTf

TfOOTf

Scheme 33 Three-component coupling of isoprene, an alkenyl triflate, and styrenylboronic acid for the synthesis of skipped polyenes

OTfR1 (HO)2B

R21

23

4

L10

O

N

N

Pd2(dba)3•CHCl3 (3 mol%)L10 (7 mol%)

Na2CO3, DMF, 45 °C, 16 h+ +

R1

R24

1+ R1

1

R2

4R1

R21

4+

F3C

514,1-addition

521,2-addition

531,4-addition

Scheme 34 Asymmetric 1,2-diarylation

Ph

Pd2(dba)3 (5 mol%)L11 (12 mol%)

Ph-N2BF4, Ph-B(OH)2

NaHCO3, tAmylOH–8 °C, 12 h

Ph

Ph

Ph

iPr

OH

10% yield83% ee

H

L11

Ar1

Ar1

+

+

CO2RRO2C

NaCH(CO2R)2

Ar2-I


MTBE, 80 °C, 72 h

O

OP N

Ar

Ar

O2N

Ar = 3,5-(CF3)2C6H3

up to 93% yieldup to 98% ee

Ar2

L12


131


4.5 Others

Yoshida and Ihara reported a cascade insertion–ring ex-pansion reaction of 1,3-dienylcyclobutanols with aryl io-dides to generate (Z)-2-(3-aryl-1-propenyl)cyclopenta-nones 54 in a stereospecific manner (Scheme 36).49 In thereaction, an arylpalladium complex formed from aryl io-dide with palladium(0) undergoes a Heck insertion reactionwith 1,3-dienyl moiety to give an allylic palladium interme-diate Int-11. The Int-11 reacts with a base to form a zwitte-rionic π-allylpalladium intermediate Int-12, which subse-quently undergoes a ring rearrangement to furnish a ring-expanded product 54 and regenerate the palladium(0) cata-lyst.

Scheme 36 Cascade insertion-ring expansion reaction of 1,3-dienylcy-clobutanols with aryl iodides for the synthesis of (Z)-2-(3-aryl-1-prope-nyl)cyclopentanones

Recently, Luan and co-workers described a Pd-catalyzeddearomatization reaction of phenol-derived biaryls with1,3-dienes to generate spirocyclic compounds 55 in goodyields and with excellent chemo- and regioselectivity(Scheme 37).50 The reaction proceeds through a reaction se-quence of oxidative addition (P4, Scheme 38) to the C–Ibond, regioselective olefin insertion (P5), and allylativedearomatization (P6).

Preliminary studies on the enantioselective version re-vealed that chiral phosphoramidite ligand ent-L5 could al-low the reaction to yield spirocyclic compounds with goodenantioselectivities (Scheme 38).50

In a continuation of the asymmetric hydroamination of1,3-dienes,30,31 Malcolmson and co-workers recently de-scribed a highly efficient and enantioselective intermolecu-lar addition of activated C-pronucleophiles to acyclic 1,3-dienes enabled by Pd catalysts ([Pd4]BF4 or [Pd5]BF4) bearing electronically deficient phosphines (Scheme 39).51 The

1,2-difunctionalized products 56 could be obtained in up to96% yield and 95% ee.

OH

R1 R2

ArI

Pd2(dba)3•CHCl3 (5 mol%)P(o-tol)3 (20 mol%)

Ag2CO3, toluene

45 °C, 2–13 h

O

R1

H

R2Ar

54up to 98% yield

+

Pd(0)ArI

ArPdI

OH

R1 R2

OH

R1 R2

Pd

ArI

O–

R1 R2

Pd

ArI

Pd+

O

R1 R2Ar

1/2 Ag2CO3

AgI1/2 H2O, CO2

Int-11

Int-12

54

H

Scheme 37 Dearomatization reaction of phenol-derived biaryls with 1,3-dienes for the synthesis of spirocyclic compounds

I

OH

R1

R2R'

Pd(OAc)2 (5 mol%)P(2-furyl)3 (6 mol%)

K2CO3

MeCN, 90 °C, 18 h

R1

O

R2

R'

+

55

Pd(0) I

OH

Pd

OH

I

Ln

OH

Ph

PdLn I

O

Ph

Ln IPd

H

B

O

Ph

oxidativeaddition

(P4)

olefin insertion(P5)

allylativedearomatization

(P6)

Ph

Scheme 38 Preliminary studies on asymmetric reaction

I

OH

R2Ph

Pd2(dba)2 (2.5 mol%)L5 (6 mol%)

K2CO3

DME, 65 °C, 18 h

O

R2

Ph

+*

*

ent-L5

O

Ph

O

Ph

33% yield70% ee

76% yield80% ee2.5:1 dr

O

O

P N

Ph

Ph

Scheme 39 Enantioselective hydroalkylation

EWG2R2

R1

[Pd4]BF4 or [Pd5]BF4(5 mol%)

Et3N

CH2Cl2, 22 °C, 2–20 h

PAr2N

O

tBu

[Pd4]BF4 Ar = 3,5-(CF3)2C6H3[Pd5]BF4 Ar = 4-CF3C6H4

EWG1

Pd

R1EWG1

EWG1

R2

56up to 96% yieldup to 97.5:2.5 er

BF4

+


132


5 Hydrogenation

Sigman and co-workers recently reported the only ex-ample to date of regio- and stereoselective 1,2-vinylhydro-genation of terminal 1,3-dienes with enol triflates/nonaf-lates in the presence of sodium formate (Scheme 40).52

Trapping of the π-allyl intermediate generated from the ini-tial migratory insertion of the diene with a hydride sourceallows access to structurally complex and syntheticallychallenging stereodefined (E)- and (Z)-tri- and tetrasubsti-tuted alkene building blocks 57.

Scheme 40 1,2-Vinylhydrogenation of (E)- and (Z)-tri- and tetrasubsti-tuted alkene

6 Oxygenation

Larock and co-workers created a Pd-catalyzed oxyannu-lation of 1,3-dienes with o-iodophenol substrates to give di-hydrobenzofuran products (Scheme 41).10 Cyclohexa-1,3-diene, 1-butyl-1,3-butadiene and 2-methylbuta-1,3-dieneunderwent facile intramolecular oxyannulation to deliverthe corresponding dihydrobenzofurans 58–60 in moderateyields (Scheme 41, eqs. 1–3). The reaction of o-iodophenoland isoprene affords compound 60 in reasonable yield(Scheme 41, eq. 3), although a minor amount of a regioiso-mer is observed. Phenols bearing electron-withdrawing

groups such as aldehydes and ketones generally give higheryields. Particularly, o-iodobenzyl alcohol can be employedto form isochroman derivative 62 (Scheme 41, eq. 4).

Recently, Han and co-workers realized an asymmetricversion of the Pd-catalyzed difunctionalization between o-iodobenzyl alcohol and arylbutadienes (Scheme 42).11 Un-der similar conditions in the synthesis of chiral indolines,11

chiral isochromans 63a–d could be obtained with high en-antioselectivities and moderate yields.

Scheme 42 Enantioselective arylation/intramolecular oxygenation to generate chiral isochromans

In 2005, Yeh and co-workers reported a palladium-cata-lyzed difunctionalization of 7-hydroxy-1,3-dienes with arylbromides (Scheme 43).53 The reaction proceeded throughdifferent paths depending on the structure of the sub-strates. With cyclic 7-hydroxy-1,3-dienes 64, the insertionof a C–C double bond into the Pd–O bond of the initiallyformed Pd(Ar)(OR)-olefin complex Int-13 is predominantand results in the formation of 1,4-alkoxyarylation product66. In contrast, the reaction of acyclic 7-hydroxy-1,3-dienes

Ar X

R1

R2

Pd2(dba)3•CHCl3HCO2Na

DMA, r.t., N2, 16 h Ar

H R1

R2

X = OTf or ONfR3 R3

+

57up to 86% yield

Scheme 41 Arylation/intramolecular oxygenation to generate dihy-drobenzofuran and isochroman derivatives

OH

I

C4H9

Pd(OAc)2 (5 mol%)NaOAc, nBu4NCl

DMF, 100 °C, 1 d

O

C4H9

OH

I

Pd(dba)2 (5 mol%)Na2CO3, nBu4NCl

DMF, 100 °C, 1 d

O

IC4H9

Pd(OAc)2 (5 mol%)PPh3 (5 mol%)KOAc, nBu4NCl

DMF, 80 °C, 1 d

OH O

C4H9

+

+

+

H

H

5975%

5844%

E/Z = 16:156% yield

OH

I

Pd(OAc)2 (5 mol%)Na2CO3, nBu4NCl

DMF, 100 °C, 1 d

O

+

O

7:151% yield

60 60'

+

(1)

(2)

(3)

61 62

(4)

IAr

Ar

O

OH+

61 63


KHCO3, DME, 80 °C

O O

O O

F

OMe

63a36% yield, 80% ee

63b50% yield, 79% ee

63c46% yield, 80% ee

63d76% yield, 71% ee

Scheme 43 Difunctionalization of 7-hydroxy-1,3-dienes with aryl bro-mides

OHArBr

Pd(PPh3)4 (2 mol%)dpe-phos (2 mol%)

tBuONa, THF65 °C, 2 h

O

Ar

66 n = 1

O

Ar

67 n = 0

+ +

LnPd(Ar)Br

NaOtBu

O

Pd

Ar

O

PdLn

Ar

O

PdAr

Ln

O

Ar

O

Pd

Ar

O

PdLn

Ar

O

Ar

Int-13n = 1

n = 0

( )n

OH

( )n

Ln

Ln

64 n = 165 n = 0

64 n = 165 n = 0

66

67

Ar

O

PdLnor

Int-14

Int-15

Int-16 Int-16'


133


65 proceeded through the insertion of the double bond intoeither the Pd–C or the Pd–O bond of the Pd(Ar)–(OR)–olefinintermediate Int-14 to afford 1,2-oxyarylation products 67after reductive elimination. The difference in the formationof alkoxyarylation products (1,4- vs. 1,2-alkoxyarylation)between cyclic and acyclic substrates actually arises be-cause the η1-η3-η1 allylic isomerization may be faster in thecyclic intermediate Int-15 than the acyclic intermediateInt-16 or Int-16′ for steric reasons.

To synthesize capnellenol, a catalytic asymmetric cas-cade Heck reaction and allylic esterification was accom-plished by Shibasaki.14,54 Various ligands and solvents werescreened to reveal that the chiral palladium complex of (S)-BINAP delivered the best results in dimethyl sulfoxide (DM-SO) (Scheme 44).

Scheme 44 Enantioselective intramolecular vinyloxygenation

7 Silylation

Optically active allylsilanes are useful reagents in stereo-selective organic synthesis, because they are able to partici-pate in asymmetric carbonyl or imine allylations with high-ly efficient chirality transfer.55 Increasing attention hasbeen directed toward their asymmetric catalytic synthesis.Among the methods to access chiral allylsilanes, the palla-dium-catalyzed asymmetric hydrosilylation of 1,3-dieneshas unique advantages, for example, using readily accessi-ble starting materials.5 However, no breakthrough had beenachieved in this field until recently.56 The chiral monoden-tate phosphine L13 with a binaphthyl moiety was identi-fied as the most efficient ligand for the asymmetric hydro-silylation of cyclic 1,3-dienes whereas the planar chiral fer-rocenylmonophosphine L14 with two ferrocenyl moietiesturned out to be an efficient ligand for the reaction involv-ing linear 1,3-dienes (Scheme 45).5

Scheme 45 Asymmetric hydrosilylation of 1,3-dienes

8 Conclusion and Outlook

In the past forty years, the palladium(0)-catalyzed di-functionalization reactions of 1,3-dienes have made signifi-cant progress, culminating in a diverse range of transforma-tions that provide efficient way to assemble densely func-tionalized molecules from readily available substances.Abundant availability of chiral ligands for the palladium(0)catalysis has enabled switching the racemic reaction to anenantioselective version. Nevertheless, the stereochemicalcontrol remains a formidable challenge in the difunctional-ization of 1,3-dienes, as indicated by the fact that many re-actions are still not enantioselective. In addition, 1,3-dienecomponents in these known processes are limited to arylsubstituted or terminal dienes. Either alkyl substituted orinternal acyclic dienes have rarely been reaction compo-nents in the asymmetric difunctionalization. Moreover, ef-ficient control of regioselectivity is another big deal in suchtransformations. Therefore, new concepts, proper chiral li-gands designed for Pd catalysis, and the development ofnew transformations for building up structural complexitywill be future focuses in the difunctionalization of 1,3-dienes.

Funding Information

We are grateful for financial support from NSFC (21672197,21672049).National Natural Science Foundation of China (21672197)National Natural Science Foundation of China (2167204)

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OTfOAcH

80% ee89% yield

Pd(OAc)2 (1.7 mol%)(S)-BINAP (2.1 mol%)

nBu4NOAc

DMSO, 20 °C, 2.5 h

( )n

R

HSiCl3

Pd/L*

( )n

SiCl3

R

SiCl3

*

*

L* =Ph2P

OMe Fe

FePOMe

ArMeO

L13 L14


134


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Palladium(0)-Catalyzed Difunctionalization of 1,3-Dienes: From … · dienes provides a wide...

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