Indian Journal of Che mist ry Vo l. 438, April 2004, pp. 8 13-838

Advances in Contemporary Research

Applications of trivalent and pentavalent tantalum in organic synthesis

Sri vari Chandrasekhar*", Tokala Ramachandarb & Tokala Shyamsunder"

"Indian Institu te o f C he mical Technology, Hyderabad 500007, Indi a ~emple Uni versity , Phil ade lphi a, Pennsy lvani a 19 122, USA

E- mail: sriva ri e@ iiet.res. in

Received II Jllly 2003; accepted (revised) 7 Jalluary 2004

IPC: Int.CL? C 01 G 35/00

Of the Group B ele ments, the group V is re lati vely unexplored especially with re ference to Niobium (Nb) and Tantalum (Ta). Interesting ly, these two elements have very s imilar chemica l properties but not as simi ­lar as Zirconium (Zr) and Hafnium (Hf).

Oxidation states and stereoche mistri es of Nb and Ta are shown below (Table I).

Nb and Ta have very little cationic behaviour, however they can complex well in a ll the oxidation states viz., II , III , IV and V. In the case of II and III ox idation states, metal-metal bonds are rather rou­tinely fo rmed. With reference to abundance, Nb is 10-12 times more abundant in the earth crust than Ta. The main source being co lumbite- tantalite series o f minerals hav ing general che mical composItIon (Fe/Mn) (NblTah0 6 with va rying ratios. Both metals are bright, high melting (Nb 2468 DC, Ta 2996 DC) and ve ry res istant to acids.

The ox ides of these metals are vas( inert and in­soluble except in concentrated HF. According ly the chemical applicati ons especiall y in higher oxidation states are rather restricted. However, the penta halides especially chlorides are yellow to purple red solids and eas ily prepared by direct reaction of metal s with excess of halogens.2 The halides are soluble in various organic solvents including di ethylether and carbon tetrachloride. The entire penta halides combine with halide ions to form MX6- and with amines, NbCls o f­ten undergoes reducti on. The penta halides due to their Lewi s acidity are used as catalysts in cyclo­trimeri zing or lineraly po lymeri zing acetylenes and in Friedal-Crafts and related reacti ons to little extent.3 It is the own experience of the authors that Ta (III and V) are more user fri endly in the laboratory, whereas Nb (III and V) are more difficult to handle and also NbCls was air oxidis ing after a sing le use of the bottle

despite precauti ons. Surprising ly, even though the Ta and Nb coordinate compl exes are known for a long ti me, its synthetic applications in organic chemistry has a hi story of only 10- 12 years.

Low valent tantalum complexes react with inacti­vated acetylenic triple bonds and being stericall y con­gested , the only C-C bond formation effected was cyc lotrimeri sation. However, some contributi on in C­C bond formation and also Ta in its V oxidation state was e ffective as Lewi s ac id catalyst. Keeping as ide the elaborate studies carried out on organo-metalll ic complexes of Ta and Nb, the present rev iew will de­scribe the sy nthetic reactions using tantalum chloride as a reagent/catalyst in both ox idation states V and III. The literature available on Ta in o rganic synthes is can be clas ifi ed into two majo r areas. (i) Low valent tanta­lum (L VT) in combination with acetylenes acting as di anion equi va lent (ii ) TaCis as a Lewis acid for cata­lysed reacti ons.

(I) Low valent tantalum (L VT) in combination with acetylenes acting as dianion equivalents

(a) Synthesis of substituted naphthol derivatives Tantalum-alkyne complex can be produced in situ

from acetylenes and combination of TaCis and Z n {(reduction of 1'a (V) to Ta (Ill) \ . A careful examina­ti on of these complexes proved that the two carbons of acetylene act as vi cin al di anion and can add onto e lec trophiles.

Also, the study between Nb-alkyne complex4-6 and

Ta-alkyne complex7-14 showed that Ta-alkynes com­

plexes are fo rmed more smoothly and reactions with o-phthalaldehyde a fforded I-naphthol deri vati ve when terminal acetylene was used (Scheme 1). 15

In case o f internal acetylenes, 2 ,3-di substituted naphtho ls were obtained in good yie lds (Table II).

814 IND IAN J. CHEM., SEC B, APR IL 2004

Table 1- Oxidation states and stereochemistries of niobium and tantalum

Oxidation Coordination Geometry Examples state number

Nb-), Ta-) 5 Tbp M(CO)/ Nb·l, Ta-I 6 Octahedral rM(CO)6r

Nbl, Tal, d4 7 1[ Complex

(CsHs)M(CO)4

7 Distored capped octahedron TaH(COMdiphosh

Nb", Ta", d.l (non- rigid)

NbO 6 Octahedral TaCI2(dmpe)2

Nblll , Ta lll , d2 6 Trigonal pri sm LiNb02 Octahedral Nb2CI9

3- , M2C16(SMe2)3

7 Complex TaCI3(CO)(PMe2PhhEtOH 8 Dodecahedral Ks[Nb(CNh l

Nb IV, Talv, d l 6 Octahedral (NbCI4), TaCI4PY2, MC I 6~ -

7 Distorted pentagonal K}NbF7

bipyramida l 7 Capped octahedron MC14(Pmc.lh 8 TaH4(diphosh

Non rigid in sol uti on Nb(~-dike)4' M2C1 R( Pme3)4, Square anti prism

Dodecahedra l Nb(SCN)4 (dipY)2

1[ Com pl ex K4Nb(CN)82H20 (11-C5HshNbMe2

Nbv, Ta" dO 4 Tetrahedral ScNb04

5 Tbp MCI5(vapour), TaM e5, Nb(N R2h Distorted tetragonal pyramid Nb(NMe2h

6 Octahedral NaMO) (perovskite), NbCls.OPCI", TaCl5·S(CH3h, TaF6-, NbOCI3, M2C1 1O, MCI6-

6 Tri gonal prism fM (S2C6H4)3 r

7 Distorted pentagonal bipyramidal NbO(S2CNEt2h Pentagona l bipyramidal, nuxional S=Ta(S2CNEt2h, Ta(NMe2) (S2CNMe2h,

(S2CNR2h, TaMe) 8 Bicapped trigonal pri sm [Nb(troP)4f

Sqaure anti pri sm Na)TaFx Dodecahedra l Ta(S2CNMe2)4 +

9 1[ Complex (n5 -C5HS)TaH3

R1 (XCHO OH

C(X ' cqR1 II MCls 2,6-lutidine h- CHO -....:::::: -....:::::: R

I h- h- R2 +

I h- h- R2 ..

R2 DME, PhH

25 °C OH

Scheme I

Mechanistically, it is antic ipated that low valent Ta (Nb) produces a three membered ring metal-alkyne complex . In sertion of formyl group into metal carbon bond of the complex followed by second insertion , elimination of metaloxy group and naphtho l is liber­ated after an aqueous work up.

been prepared (without iso lati on) and were reacted with various aldehydes in a one-to-one fashion to yield the corresponding allyl alcohols with exclusive E-geometry (Scheme II) . Interesting ly, in the absence of carbonyl compound and quenching the complex 1 with NaOD-D20 furnished dideuterated cis-olefin 3 in good yields. 16

(b) Synthesis of allyl alcohols A variety of Ta-alkyne complexes 1 and 2 have

T he results of the reac tion reveal s that electronic e ffects play a role in the regioc emistry of the

ADVANCES IN CONTEMPORARY RESEARCH 8 1S

Table I1-Regioseleetive synthesis or 2,3-disubstituted- I -naphthols

Run RI R2 M Timc A ldehydc Product rati o Yield t (hI') Equ iv. (%1

I "CsHW "CsH 11 - Ta 0.5 2 84

2 "CsH 11 - "CsHW Nb 10 2 OH

93

3 Ph Ph Tn 6 3 cxXR1 70

4 -(CH2)10- Ta 0.5 4 ~ .# 70 R2

5 "C6H 1r cC6H W Tn 2 3 0 11

85

6 "C6H IJ- (·C6H

11- Nb \I 3 0):'''''"''

'%. # c~, 1-I 1 1

O?=OC.HU '%. # CCr, 1I 11 84

0 1-1

OH

7 "C7H IS- IBu Ta 4.5 3 ((;(C'H15 7 1

~ # IBu

8 Me Ph 1'3 0.5 3 72

9 "Cr,H 1r Ph Ta 2 3 OH 7 1

ccXR1 : 29 ~RI I 7 1

75 : 25

10 "C6H 1r Ph Nb 12 3 ~ # Ph 55 : -15 ~ # Ph 83

OH

II "C IIIH2 1- H Nb 3 59 01-1

12 Ph H Nb 2 3 crY" I 3 1 '%. #

TaCI 5 , Zn C5H 11 C5H 11 NaOD / D20 C5H1 1 C5H 11 C5H 11

-C5 H11 \d F< • ..

DME, PhH Ta 25 °C, 1 h D D 25 °C, 30 min Ln

3,75% 1

I Ph(CH2),CHO

76% (96%) Scheme I1(a)

816 INDIA N J. CHEM .• SEC B. APRIL 2004

R1 R2 TaCls , Zn THF R3R4C=O R~R3 R~R2 • • +

DME, PhH Pyridine 25°C, 15min R4 25° c, t hr HO R4 OH

A B Scheme II(b)

A. NaOH/H2O

A~ ZR2 R ' ZR2

R' == ZR2 TaCls• Zn. THF R3R4C=O B. H2O

+ \~ R3 ~

DME . PhH Pyridine 25 °C. t hr 25°C. 1 hr R4 OH HO R4

25 °C . t hr A B

Scheme III

Table III - Synthcsis of al lyl ic alcohols rrom acety lcnes and aldchydcs

Run RI R2 R.l

Il -C IOH21 H Il -C~ H 17

2 Il -C IOH21 H -(CH2k 3 Il -CSHI I Il-CSHII Ph

4 Il -CSHI I Il-CSHI I Il-CsH I7

5 Il-CSHII Il-CSHI I c-C6HII

6 Il-CSHII Il-CSH II -(CH2k 7 c-C6HI I Il -C6H 13 Ph(CH2h

8 c-C6H 11 Il-C6H 13 -(CH2k 9 /-B u Il-C7H I5 Ph(CH2h 10 /-B u Il-C7H 1S -(CH2k II Il -C6 H IJ Ph Ph(C H2)2

12 Il -C(,H IJ Ph -(C H2k 13 MC.lSi Il-CIOH21 Ph(CH2)2

14 Me3Si Il-C IOH21 -(CH2k 15 M e3S i Ph Ph(CHzh

16 /-BuMczS i Il-C IIIH21 Ph(CHzh

product. Preference is genera ll y that the acetylenic carbo n hav ing smalle r substituti on adds o nto C =O

(Table III).

(c) Synthesis of (Z)-alkenyl suItides and (E)-3 -hydroxy-I-propenyl methyl sultides

It was observed that tanta lum-alkyne complexes fo rm more readily fro m alkyny l sul fides than the dia l­

kynes. There is a s ide reac tio n of a -chlo rinati on, which could be prevented by the additio n o f pyridine in the reac tion medium (Scheme 111).1 7

As usual quenching of complex in absence o f e lectrophil e (R-C O-R ') resul ted in (Z)-a lkenyl su lfides, whereas the e lec tro phil e presence made the

new C -C bond fo rmatio n f3 to the Lhio g roup preferenti ally yie lding (E)-3-hydroxy- I -propenyl meth yl sul fides. The bulki ness and e lec troni c nature

R4 T ime Yi cld AlB t (hr) (%)

H 0.7 48 >99/< 1

0.7 45 >99/< 1 H 0.5 85 H 0.5 94

H 0.5 75

0.5 87 H 1.5 80 65/35

1.5 83 76/24

H 4.5 67 >99/< 1

4.5 <5 H 73 8311 7

80 79/2 1

H 1.5 77 89/ 1 i

1.5 85 >99/< 1

H 3 7 1 >99/<1 H 3 68 >99/< 1

The bulkiness and e lec troni c nature o f the substituents of a lkynes influences the regiochemis try of the cou­pling reaction (Table IV). 'K

The results have specia l advantage o ver z ircono­cene-(methy lthi o)-I-a lkyne complex in that zircono­cene complex y ie lds as 1: 1 mixture of reg ioisomers, whereas the Ta-complex produced one ste reo iso­meric a lly li c a lcoho l predo minantly.

(d) High oxidation state transition metal carboxy­lates as acylating agents

The abiliti es of meta llocene isobuty rate to ac t as acy lating age nL was demonstrated by Ko lm el al., where in the meta ll ocene complex was prepared by treatment of T aCi s w ith isobutyri c ac id Lo isolate a

ADVANCES IN CONTEMPORARY RESEA RCH 817

Table IV - Reactions of alkynyl sulfides and sulfones with carbonyl compounds by means of a TaCl j-Zn sys tem

Run

2

3

4

5

6

7

8

9

JO

11

12

13

14

RI

II-C IOH21

II-C IOH21 II-C IOH21

II-CIOH21

II-C IOH21

c-C6HII

c-C6HII

Ph

Ph

II-CIOH21

II-C IOH21

II-CIOH21

II-C IOH21

c-C6HI I

ZR2

SMe

SPh

SMe

SMe

SPh

SMe

SMe

SMe

SMe

S02M e

S02Ph

S02M e

S02Me

S02M e

R3

Ph(CH2h Ph(CH1h c-C6H II

-(CH2k -(CH2k

Ph(CH 2h -(CH 2k Ph(CH2)2

-(CH2k

Ph(CH2h Ph(CH2) 2

c-C6HI I

-(CH2k Ph(CH 2h

H

H

H

H

H

H

H

H

H

Ta Cls

15 min

t/h r

0.2

0.5

0.2

0.2

0.5

0.2

0 .2

0.5

0.5

2.5

2 1

2.5

2.5

2.5

Yi eld A/B (%)

73 >99/< 1

85 77/23

74 >99/< 1

77 >99/< 1

75 85/ 15

68 >99/< 1

70 >99/< 1

64 >99/< 1

54 >99/< 1

54 54/46

43 28/72

46 65/35

62 >99/< 1

59 39/6 1

0

yNH~PH 6

Scheme IV

CI Cl Cl CI

~ /O~//CI,\~O" -< C Ta, Ta , / CH H'/ I '/ I" o CI CI CI 0

Figure 1

coordi nated complex (Figure 1)19, which on exposure to benyzl amine yielded the N-benzyl isobutyramide 6 in less than 15 min utes .2o The same experiment with other metallocenes such as Ti and Zr y ielded the cor­responding 6 albeit at a very s low rate compared to Ta (Scheme IV) . Thi s may be due to high positive charge on the metal.

(e) Synthesis of (E)-a , /3-unsaturatcd amidcs from reaction of tantalum-alkyne complexes with isocy­anates

Reaction of metal -a lkyne complexes 7 , 8 and 9 is

well studied with few meta ls.21-24 (E)-a, !3-unsaturated amides can be stereoselecti ve l y synthes ized2s ,26 by the reaction between low va lent Ta-alkyne complex and isocyanates. In a typica l experime nt, the reactio n was

performed between 6-dodecyne, tanta lum (Ill) and PhNCO to generate (E)-N-pheny l-2-pe nty l-2-octen­amide 10 in 80% y ie ld [Scheme V(a)]. The hi gh yields are obtained subject to filtration of Ta-alkyne complex from reaction mixture and later isocyanate was added . Quenching the reaction with D20/NaOD yie lded deute ro amides, which are hitherto unaccess i­ble or difficult to make.

Again, as in the case of additio n to carbony l group, the subst itution pattern of acetylene plays a majo r ro le in the reg io chemistry , Only in the case of methy lthi o substituted alkyne, the regioisomer A was produced exclusively because of the e lec tronic nature of the substituent (Table V).

({) Chlorination of alcohols using TaCls Selecti ve and a lmost complete convers ion of

cyclohexanol 11 to chloro cyclohexane 12 (96%) was achi eved whereas other meta l ha lides name ly WCl r" did not g ive a good conversion and NbCls gave only 70% conversion after prolo nged reacti on hours.

Thus, this is one of the rare d-b lock e lements whi ch is comparabl e with ha lides of p-block e lements (S02C1 2, PCls) in conversion of 2° alcohol to the re­spec tive chloridcs.27

-34 Thus TaCis proved to be a

8 18 INDIAN J. CHEM., SEC B, APRIL 2004

TaCls, Zn . DME, PhH

25 0 C, 1 h

R1 R2

LnTa~NPh o

8

Sch eme V(a)

TaCls, Zn . DME, PhH 25 °C, t hr 25 0 C, 1 h

A B

Scheme V(b)

Table V - Rcactions betwccn alkynes and isocynates by means or TaCis and Zn

Run RI R2 RJ R.1 NCO/cquiv. Time/hI' Yield A/B

I II-CsHI I II -CsH II Ph

2 II-CsHII II-CsHII Bu

3 II-CsHI I II-CsHII I-B u

4 II-CsH II II-CsHII McJSi

5 II-CsHII II-CsH II Ts

6 c-C6H II II-C6H IJ Ph

7 c-C6H I I II-C6H IJ Bu

8 Ph II-C(,I-11.1 Ph

9 I~ h II-C6HI.1 Bu

10 M e.1Si II-C IOH21 Ph

II Me.1 Si II-C IOH21 Bu

12 MeS II-C IOH21 Ph

13 MeS II-C IOH21 Bu

bette r halogenating agent compared to NbCls (Scheme VI) .35

(g) Carbocations of alkyl benzenes with TaCls-CH2CI2

TaCls-CH2CI2 performed C-C and C-H bond cleav­age in 1,3,5-triisopropyl benzene 13 to y ie ld stable indane cation 14 as shown in ( Scheme VII).

The transformation generally demands super­ac ids36-3<J or very acidic meta l ha lides viz., TiCI4, ZrCI4 and HfCI 4.

40.4 1 It is rathe r interesting to note that even

though 'Ta ' in oxidation state V is almost non­metallic and not a strong acidic metal halide it is able

1.2 3

1.2 3

1.2 20

1.2 20

1.2 20

2.0 3

2.0 3

4.0 3

4 .0 3

4.0 3

1.2 3

2.0 0.2

2.0 0.2

(%)

80

72

79

63

48

69

62

51

74

60

33

58

90

55/45

73/27

60/40

72/28

84/16

82/ 18

>99/<1

>99/< 1

to perform such a transformation albeit in moderate yields (50%).42

(h) Allylic amine synthesis via tanta lum-alkyne complexes

Nucleophi lic addition of organo meta ll ic compounds to carbon-nitrogen double bonds constitute an important method for the synthesis of amines.43-49 Tantalum-alkyne complex 15 adds onto N,N-dimethy lhydrazones 16 at e levated temperatures

(80°C) to yield ally lic hydrazine in rather low yields. Attempt to add activation such as Me3AI, Me3Ga helped to improve the yields. The regiochemistry of the ally lic hydrazine depends o n the bulkiness of

ADVANCES IN CONTEMPORARY RESEARCH 819

Table VI - Rcacti ons bctwccn tantal uill -alkync complex and hydrazoncs in thc prescnce of Me)A I

Run

I

2 3 4 5 6 7 8

OH

C 11

A 13

R1 -

RI R2 R' tl/hr

II-CsHII II-CsHII Ph(CH2h 2 II-CsH II II-CsH II c-C6H II 2 II-CsH II II-CsH II (E)-PrCH=CH 2 II-CsHII II -CsH II (cyc lohexanone) 2 c-C6HII II -C6HIJ Ph(CH2)2 3 Ph II-C6H 13 Ph(CH2)2 4

Mc)Si II-C IOH2S Ph(CH2h 3 MeS II-C IOH2S Ph(CH2h 0.2

CI

TaCI 6 (96%)

12 CI

NbCls 6 (70%)

12 CI

WCI 6 (50%)

12 Scheme VI

TaCls

CH2CI2

14 Scheme VII

TaCls, Zn R1 R1

R2 \=I • DME, PhH Ta

55 °C, t1 / h Ln

15

R1 R2

>-R3 +

HN I NMe2

A

t2/hr Yicld AlB Unreactcd Recov. (%) alkene (%) Hydrazone

(%)

16 80 12 31 16 71 21 24 16 32 26 48 16 5 45 62 16 64 5 1/49 10 23 16 76 44/56 9 22 16 57 80/20 21 33 26 69 >99< 1 0 33

substitutions o n the acetylenes as ex pec ted . E lectro ni c effects are a lso observed in regiochemi stry when Me3S i and - SMe substitutions (Run 7 and 8, Table VI) are pl aced o n acety lene. T hi s reaction was restricted to a ldehyde hydrazones (Scheme VIII).50

Interesting ly, insertion of isocyanide into tanta lu m­alkyne complex was achieved to synthes ize 2,3,4-tri substituted N-amino pyrrox 17 albeit in only 17% yie ld (Scheme IX).

It may be infe rred that externa l Lew is ac id add ition is essenti al to promote the additio n of tanta lum-alkyne complex to imines.

(i) Selective reduction of acetylene to olefin with Z­stereoselectivity using L VT

Selecti ve reduction of acety lenes to o lefins with precise cis geometry is an important task.51 Cata lyti c hydrogenatio n g ives cis-o lefin (some time react ion does not stop at o lefin stage and proceeds to

,NMe2 N

THF R3)l~6 NaOH / H20 ..

R1 R2

R3-<

NH I NMe2

B SchemeVIII

820 INDIAN J. CHEM ., SEC B, APRIL 2004

TaCls, Zn •

DME, PhH

25 °C, 30 min

CSH11 CSH1 1

hCpr N I NMe2

17 Scheme IX

TaCls, Zn . DME, PhH 25°C, t

Scheme X

saturation) whereas metal/NH3 red uction genera ll y yields the E-o lefin . Low vale nt tantalum produced from TaCls-Zn which is much faster than analogues NbCls-Zn for complexatio n with acety lene after basic hydro lys is with H20/(NaOH) furni shed the protonated o lefin in good yields with more than 99% Z selec tivity (Scheme X) .S2

It is observed that in the case of NbCls, HMPA as add iti ve is des ired which is carcinogen ic and not ava il able on many catalogues. Thus 'Ta ' proved to be ;) superi or metal over ' Nb ' in thi s transformation . En­tri es 10 and II (Table VII) are inert to a poss ibl e cy­c li zation despite possess ing an add iti onal o lefini c sys­tem thus proving of 'Ta' is different from low va lent 'Zr ' wherein an intramolecul ar cycli sation with con­comitant reduction of triple bond is observed (Table VII). s3.s5

(j) Synthesis of substituted furans from tantalum­alkyne complexes

Although insertion of isocyanide into Ta-alky ne was recogni zed as ealry as 1974,s6 the process has not been utili zed in organic synthes is. Jnsertion of isocya­nide into the tanta lum-carbon bond was successful ly achieved for the synthes is of highly substituted furans 18 (Scheme XI and Table VIII).

Thus a variety of 2,3,4-tri substituted furans werc prepared by the treatment of vario us tantalum-alkyne complexes with the a ldehydes follo 'vved by addition of an isocyanide in DME-PhH-THF (1: 1:1 ). The general approach is shown in below.s7

(k) Preparation of (E)-allylic amines by reaction be­tween tantalum-alkyne complex with metalloimines

There are two typical approaches for the prepara­tion of amines from acety lenes and imine derivat ives.

Table VII - RcduClions o f alkyncs 10 (Z)-a lkcncs by means a TaC ls-Zn systcm

En try RI R2 t / hI' Yield (%) Z/E

I Il -CIOH21 H 0.3 39

2 Il -CJOH21 H I 52

3 Ph H I 68

4 Il -CSHI I Il-CSHI I 0.5 85 >99/< 1

5 -(CH2)J(]" 0.3 69 >99/< 1

6 Ph Il -C6H 1J 0.5 85 >99/< 1

7 c-C(,H I I Il-C6H 13 4 80 >99/< 1

S /-Bu Il-C7H 1S 4.5 82 >99/< 1

9 M cJS i Il-C IOH21 2 79 >89/11

10 Bu CH 2=CI'I(Q-12h 0.6 8 1 >99/< 1

I I Il -CJ(}H21 CH2=CH(CH2).j 0.6 82 >99/< 1

2. Il-C IOH21 HO(CH 2).j 0.5 80 >991< 1

ADVANCES IN CONTEMPORARY RESEARCH 821

DME:PhH:THF

+ + ArNe 1 : 1: 1

Scheme XI

Table VIII-Synthesis of 2,3,4-lrisubslitulcd furans

Run RI R2 R3 Ar t I h,. Yield (%) AlB

II-C IOH21 H n-C)H7 2,6-Mc2(C6H) 0.5 8 >991<1

2 II -CsHII II-CsHIl n-C8H 17 2,6-Mc2(C6H3) 0.5 66

3 II -CsHII II -CsHII II-CKH17 [M c3CCH2CMc2NC] 0.5 28

4 c-C6HII c-C6HI3 c-CsHI I 2,6-Mc2(C6H3) 2 55 69/3 1

5 t-Bu II-C7H1S II-C3H7 2,6-Mc2(C6H3) 5 40 98/2

6 MC3Si II-C IOH21 II-C3H7 2,6-Mc2(C6H3) 2 57 94/6

7 Mc)Si Ph II-C)H7 2,6-Mc2(C6H3) 3.5 42 >991<1

8 t-BuMe2Si Il-C iO H21 II-C3H7 2,6-Me2(C6H3) 3.5 54 >991< 1

In one approach as reported by Buchwalds8.s9 and Liv­inghouse60

, the reaction between ziroconocene-imine complex and acetylene would lead to allylic amines . Alternatively, Ta-alkyne complexes can add onto N, N-dimethyl hydrazones of aldehydes in the presence of Me3AI to yield (£)-allylic hydrazines, which in principle can be reduced to amines.

Since both the approaches are restricted to imines of aldehydes, the methods cannot be used for primary hydrazine/amine synthesis having 3° carbon due to the steric factors. 61 It is observed that lithioiminel29] as C=N component62 which is brought close to the Ta­alkyne complex by ligand exchange.63.64 Alkaline work-up as usual to cleave tantalum complex fur­nished trisubstituted allyl primary amines. In a typical experiment addition of nonane nitrile 19 to an ether solution of MeLi (in situ to generate lithioimine), fol­lowed by treatment with low valent tantalum-(6-dodccyne) complex 20 gave allylic amine 21, which was acetylated for characterisation (Scheme XII).6s

(I) Low valent Ta mediated Reformatsky reaction

Reformatsky reaction is a powerful protocol for the synthesis of [3-hydroxy esters . Generally this reaction is performed at elevated temperatures using Zn metal.66 However, Sm (II) iodide has been used as one electron reducing agent in this reaction which is per-

formed at low temperature. TaCls-Zn was found to be an efficient combination for the Reformatsk)1 reaction at 0 DC (Table IX)67. Typically to a suspension of low valent tantalum prepared from TaCis (3 mmol) and Zn (4.5 mg atom) in THF (15 mL) at room temperature was added a-bromopropionate 23(1 mmol) and car­bonyl compound 24 (1 mmol) at ODC under argon at­mosphere. After 2 h of stirring at ODC and usual work­up produced [3-hydroxy ester 25 (Scheme XIII and Table IX)

(m) Insertion of carbon-carbon double bonds into tantalum-alkyne complexes

Not only tantalum-alkyne complexes add onto C=O and C=N but can also be inserted into simple al­kenes.68 This has a great potential, as this protocol enables one to create a new C-C bond between inter­nal acetylenes and terminal olefins.

In a typical experiment low valent tantalum-6-dodecyne complex with lithium-3-buten-l-olate 26 in DME-benzene-THF (I: I: 1) at 25 DC for 3 hr gave two alkylated alkanols 27 and 28 in 98:2 ratio as regio iso­mers (Scheme XIV). Prior coordination of the tanta­lum-alkyne complex with the hydroxyl group of the homo allyl alcohol facilitates both insertions as well as regio chemistry (Table X). Anchimeric assistance of phenols and amines is also a notewOlthy feature. 69-76

822 INDIAN 1. CHEM., SEC B, APRIL 2004

TaCI5• Zn •

DME. PhH

25 DC, 30 min

C5H~:~ H2N n-CaH1 7

21 22

A: M = Li additive = none 50 DC 4h

25 DC 20h

= Me3A1 25 DC 4h

B: M = Mg additive = none 50 DC 6h

C: M = A1Me2 additive = none 50 DC 6h

Scheme XII

Table IX - The reactions of ethy l a-bromopropionate with ketones and alde­hydes in the presence of the L VT reagent

Entry Carbonyl tl hr Solvent T loe Yield (%) of compound (3-hydrox y-esters

1 Cyclohexanone 2.0 THF r.l. 67

2 Cyc lohexanone 2.0 Benzene r.l. Manyproducts

3 Cyc lohexanone 2.0 Et20 r.l.

4 Cyclohexanone 2.0 CH2C12 r.l. 0

5 Cyc lohexanone 2.0 MeCN r.l. 44

6 Cyc lohexanone 2.0 DMF r.l. Manyproducts

7 Cyclohexanone 2.0 DMF r.l. 78

8 Cyc lohexanone 2.0 THF -78 79

9 Cyclohexanonc 2.0 THF Renux

10 Cyclohexanone 2.0 THF 0 Manyproducts

II Cyclohexanone 12 THF 0 40

12 Cyc lopentanone 2.5 THF 0 59

13 Acetophenone 2.0 THF 0 81( 17: 13)

14 Benzophenone 2.0 THF 0 62

15 Octan-2-one 2.0 THF 0 75(3:2)

16 Cyclohexenone 0.5 THF 0 16(3: I)

17 D-Camphor 3.0 THF 0 19

18 Benza ldehyde 1.0 THF 0 75(rhreo:elyrhro=4:3)

19 Cyc lohexy laldehyde 2.0 THF 0 52(rhreo:erylhro=6:5)

20 Octanal 1.5 THF 0 60(rhreo:eryrhro=4:3)

additive

67%

43%

85%

65'Yo

75%

ADVANCES IN CONTEM PORARY RESEARCH 823

CH3 R 1 LVT R'Y ~OR >=0 R 2 OR Br I + R 2

0 CH3

23 24 25

Scheme XIII

TaCis, Zn •

DME, PhH

~OLi 26

25 °C, 2 h

~OLi

CSH11

•

CSH11 C5H~C5H" +

OH OH

27 28

OH Scheme XIV(a)

R1..... ~ J y r:---1 n 'R4

BuLi NaOH

R2

R

R OH

+ R4 R4

R1 R2 R1 R2

A B

Scheme XIV (b)

Paralle l to the above observation , Takai et ai., ob­served that a ll yl alcohols and amines cou ld be in­serted into low valent tantalum-alkyne complexes. 77

N,N-dimethy lallylamine was found to react with the tantalum complex at 55 DC and 1, 4-diene 29, a fo rmal cis-hydroall yJation product of 6-dodecyne 30, was obtained (Scheme XV) .

Mechanistically, insertion of an o lefin ic bond of al­lyl alcohol into the tantalum-carbon bond of the com-

plex 31 form two oxatantalabicyclo heptene interme­diates 32 and 33. Deoxygenative ring opening of oxatantacyclo butane ring of intermediate 32 followed by hydro lysis provides the product 34/34a .

(n) Selective cyclotrimerisation of L VT -a lkyne Low valent tanta lum-a lkyne (internal) complex re­

act with terminal diynes 35 in THF to give tetrasubsti­tuted benzene derivatives 36 (Scheme XVI)78

824 INDI AN J. CHEM. , SEC B, APRIL 2004

TaGI 5 , Zn 1

~X • .. •

DME, PhH

29 30

~X' X2 t / 1/ h 29 30 °C

a: ~NMe2 NMe2 55 20 54% Not detected

b: ~Ou OH 25 30 65% 6%

c: N :u ~NH 25 4 86% Not detected

R - R TaGI5, Zn •

~OLi

[

R R

\=I -- OTlaLn111

-LiGI V DME, PhH J--31

34 32

Scheme XV

When acetylenic nitrile 37 was used, pyridine ring 38 was formed in 73% yield (Scheme XVII).

Functionalised acetylenes viz., TMS-acetylene and methy lthio substituted alkynes also behaved well (Table XI).

(II) TaCls as a Lewis acid for catalysed reactions

Differential behaviour of TaCls and NbCls in Sakurai Reaction

An unusual reaction was observed, when NbCl5

was used instead of TiCl4 in the Sakurai Reaction79

ADVANCES IN CONTEMPORARY RESEARCH 825

Table X - Reactions between a tantalum-6-dodecyne complex and olefinic alcohols

Run R' R2 R3 R4 n Tempi Timel Yield AlB °C hr (%)

I H H H H I 25 2 80 98/2

2 H H H H 2 25 2 72 94/6

3 H H H H 3 50 20 4 (85/1 5)

4 n-C6H13 H H H 50 20 24 >991< 1

5 H n-C6H13 H H 50 20 17 98/2

6 H H n-C6H13 H 50 20 4 >991< 1

7 H H H n-CsH" 25 3 82 97/3

Run olefin Major product Tempi Timel Yi eldl AlB °C hr %

R OH OH

8 ~ 25 3 84 98/2

R

R OH

9 ~ 25 3 83 >991< 1

R ~

OH OMe R

10 ~ 25 3 85 >991<1

R

;:0 R~ I I 25 3 86 >991< 1

-::? ~ R ~ II

12 HN~ R~ ~

I HN 50 3 70 >991<1

R

Table XI - Formation of benzene derivatives from acetylenes and diynes

Run R' R2 R3 -(CH2)n- Timel Yield hr (%)

I II-CsH" n-CsH" H -(CH2)4- 4 82

2 II-C6H'3 n-C6H" H -(CH2)4- 4 80

3 II-C6H13 II-C6H" H -(CH2k 6 76

4 II-CsI-l" n-CsH" H -(CH2)r 4 74

5 II-CsH" II-CsH" H -(CI-I2)2- 4 75

6 II -CsH" II-CsH" II -CI-I2OCH2- 4 76

7 II-CsI-I" n-CsI-I, , 1-1 -CH2N(CH2Ph)CI-I2_ 4 57

8 II-CsI-I" n-CsI-I 11 II-C6 I-I 13 -(CI-I2)4- 4 78

9 II -CsI-I" n-CsI-I 1i Et -(CI-I2h OCl-Ir 4 71

10 Me3Si II -C 101-12 I 1-1 -(CH2h- 4 6 1

II Me3Si n-C IOH21 H -CH2OCI-Ir 4 56

12 MeS n-CIOH21 H -(CH2)r 6 69 13 MeS II-C IOI-I2 I H -(CH2k 6 64

826 INDIAN J. CHEM., SEC B, APRIL 2004

o C5Hll - C

5H1 1 TaC/5, Zn,

DME, PhH

THF

Pyridine

35

Mechanism

Scheme XVI

TaCI5, Zn THF .. DME, PhH Pyridine

Scheme XVII(a)

N. ~ IU 37

36, 82%

38,73%.

T CI Z THF R3----==--(CH2ln - NaOH R):;l ~] _a_=5,_n... ______ .. _________ -I.~ ------i.~ I ~H2ln

DME, PhH Pyridine 50°C, t hr H20 R2 ;""'-

50°C, 2hr 25°C, 0.5 hr 25°C, 1 hr ~3

o

Ph~H 39

Scheme XVII(b)

r.t., 30 min 40

R~ Scheme XVIII

Ph~

between a ldehyde 39 and ally l trimethyl silane 40 (Scheme XVIII).8o It was visualised that niobium alkoxide species is a leaving group and cyclopropana­li on is forced via homoallyl group participation. As

the intermediacy of long lived carbonium ions is unlikely , it is assumed that the cations are trapped by a ch loride ion so that the cyclopropyl methyl ch lo ride 41 is formed to yield the unexpected product.

ADVANCES IN CONTEMPORARY RESEARC H 827

R- CHO + ~SiMe3 TaCls OAc

R~ R = Ph, naphthyl, cyclohexyl

Scheme XIX

o + ~~2 +

OHC 40

endo exo

Scheme XX

Interestingly, when TaCls was used in the reaction, in presence of AC20, a one-step conversion followed by acetylation is observed (Scheme XIX).81

(b) TaCIs as a catalyst in Diels-Alder reaction In fact the first application of TaCls as Lewis acid

was dl~ monstrated in a Diels-Alder reaction. A com­parison between NbCls and TaCls for Lewis acid cat a­lysed Diels-Alder reaction was attempted and found that NbCls catalysed the reaction better between cyclopentadiene 42 and methacrolein .82

However, replacing the chloride ligands with chiral ligands (preferably bidentate) was attempted and found that, even though enantioselective Diels-Alder reaction could not be achieved, endoselective Diels­Alder to an extent of 90% (Scheme XX). However, the ee's were in the range of 10-40%. Thus, the nio­bium tantalum complexes with bidentate ligands have a good potential (Table XII and XIII).

(c) Synthesis of imides from anhydrides catalysed by TaCIs

In depth studies on the Lewis acid catalysed reac­tions of TaCls revealed that TaCls adsorbed on sili ­cagel acts as a better catalyst compared with the TaCls alone. This is in agreement with the observations made by Howarth et at., who also noticed that when the chloride ligands were replaced with other oxo­ligands, the efficiency of catalyst was enhanced . In a typical experiment, equimolar phthalic anhydride 41 and benzyl amine were adsorbed on silicagel, ad­mixed with 10 mol % of TaCls-silicagel and exposed to microwave irradiation for 5 min, furnished N-

° C;~R TaCls - Si02 •

MW, R-NH2

42

R = N-benzyl amine Scheme XXI

Table XII - Results for the Diels·Alder reac ti on betwccn cyclopentadie ne and crotonaldehyde or methacro le in in lhc presence o f niobium complexes o f so me bi deillaa te li gands

Bidentate Dienophile Methacrolein ligand Croto n aldehyde

I L-Phe nyl alaninc 18%; 85: 15 42 %; 25: 75

2. L-Alanine 27%; 95: 5 44%; 7: 93

3. L-Le ucine 10%; 93 : 7 27%; 13: 87

4. L-Isolcucinc 20%; 94: 6 40%; 8: 92

5. L-Tryptophan 10%; 84: 16 57%; 22: 78

6. L-Valine 5%; 95: 5 13%; 13: 87

7. Diethy l L-Iartrate 9%; 93 : 7 6 1 %; 6:94; 25%

8. Diisopropy l L- 16%; 94: 6 52%; 3:97; 40% tartrate

benyzl phthalimide 42 in 92% isolated yield (Scheme XXI). When optically active R-(+)-a-methyl benzy l amine was lIsed, the product was obtained without any racemisation (Table XIV).83

The same reaction could also be accompli shed on solid support such as Merrifield resin (Scheme XXII and Table XV).84

Solid supported amjno acid 43 wherein the resin part was connected through ester linkage, when

828 INDI AN 1. CHEM., SEC B, APRIL 2004

Table XIII- Results for the Dicls-A lder react ion between cyc lopentadiene and crotonaldehyde or methac-role in in the presence of tanta lum complexes of some bidentaate li gands.

Bidentate ligand Dienophile Crotonaldehyde Methac role in

I Diethy I-L-tartratc 42%; 90: 10 78%; 6:94; 7% 2. Diiosopropy l-L-tartratc 24%; 95: 5 55%; 6: 94 3. (+ )-2,3-0- lsopropylidcnc-L-thrcitol 14%; 99: I 49%; 16: 84 4. (- )-2,3-0- lsopropyliden I , I ,4,4- tetraphenyl-L-thrc ito l 72%; 86: 14 50%; 9: 91 5. (- )-2,3-0-l sopropy lidcn I , 1,4,4-tetra(2-naphthyl)-L-threitol 42%; 99: I 75%; 10: 90

Table XIV -Synthesis of N-alkyl and N-aryl imides

Entry Anhydridc Ami ne Amide Time Yie ld (min) (%)

0 0

I. ~o H2N~Ph ~N\Ph 5 92

0 0

0 0

2. ~o H2NI ~Nh 5 90

0

0 0

~o (:H3 ~N-t 3. 5 88 H2N~Ph Ph

0 0

0 0

4. ~o H2N~Ph ~N\ 5 82

Ph

0 0

0 0

5. ~o H2N- Ph qN-~ 6 79

0 0

0 0

~o (:H3 ~ FH

3 6. I N-\ 5 78

H2N~Ph Ph

0 0

--Coll ld

ADVANCES IN CONTEMPORARY RESEARCH 829

Table XIV - SynLhesis of N-alky l and N-ary l imides-Col1ld

EnLry Anhydride Amine Amide Time Yield (min) (%)

0 0

qo H2N/'...Ph qN\ 6 80

Ph

7.

0 0

0 0

8. qo H2N- Ph qN-~ 6 75

0 0

0 0

qo H2NJ cfr G 78

0

9.

0 0

~o /'... QN\ 7 74 H2N Ph Ph

10.

0 0

00 1. TaGls - Si02

C~OOH C,)OO-o MW, S min

+ 2. TFA in GH2GI2

41 43 44 Scheme XXII

treated wi th phthalic anhydride 41 in presence of TaCls-silicagel, after reaction and cleavage of bound resin yielded the phthalimido ac ids 44 in good yields .

(d) Use of TaCls-silicagel as Lewis acid for protec­tion of organic functional groups

Highly oxophylic TaCis and TaCls-silicagel has also been uti li zed as a catalyst in protective group in troduction namely - OTHP format ion and thio­acctalisation of a ldehydes (Scheme XXIII).8s Series of alcohols both 1°, 2° and 3° alcohols converted to tetrahydropyranyl ethers in good y ie lds. The y ie lds were hi gher when Ta-s ilicagel was used instead of

R-CHO TaGls - Si02

EtSH

OH

R+ R2 TaGls - Si0 2 R1 DHP

R = Alkyl , R\ R2 = H or Alkyl

Scheme XXIII

SEt

R-< SEt

OTHP

R~R2 R1

TaCis a lo ne even at higher concentration of TaCis Cfable XVI).

830 INDIAN J. CI-I EM., SEC B, APRIL 2004

Table XV- Synthes is of N-alk yl imides on solid support in soli d state

Entry Polymer-bound amine Anhydri de Imide Time Yield (min) (%)

0 0

0

ex > cc?ICH2bCOOH I. ~O~NH2 5 65

0 0

0 0

0 qo cc?ICH2bCOOH 2. ~O~NH2 5 65

0 0

0 0

0 ~o CC/'ICH2bCOOH 3. ~O~NH2 7 60

0 0

0 0 0 exi° eX>yCOOH 4. /'-O~NH, 5 72

0 0

Table XVI- Thioacetali sation and tetrahydropyranylation

Entry Substrate Reacti on Product Yield condition (ti me) (0/0 )

I. < )-coo A O-t' (2 min) 98

- SEt

~CHO SEt

2. A Ph~SEt (5 min) 92 Ph

3. Ph ~CHO B Ph~:J (4 min) 93

~CHO SEt

4. A Ph~SEt (5 min) 96 Ph

5. TBDMSO~"CHO B s:) TBDMSO~S (8 min) 89

-Col/td

ADVANCES IN CONTEMPORARY RESEARCH 83 1

Table XVI-Thioacetali sa ti on and tetrahydropyranylat ion-Col/(d

Entry Substrate Reacti on Product Yi eld condition (time) (%)

0

6. Ph~CHO

7. Ph~OH

8. o-0H

9. ~

~~ / 0

OH ° 0-\-

\

10.

I I. Ph~OH

12. Bn0V=V0TI-IP

A.TaCis-silicagel, ethanethiol , CH2C I ~;

B.TaCls-silicagel, propaned ithiol, CH 2C1 2;

C.TaCls-silicagel, dihydropyran, CH2C1 2.

TaCls

solvent, r.t

A

C

C

C

C

C

C

0

Ph~S) (5 min) 9 1

S

Ph~Ol11P (4 min ) 98

o-0l11P (5 min) 94

~P ( 10 min) 76

~~ / ~IIP ° ( 10 min) 7 1

0-\-\

Ph~Ol11P (5 min ) 88

Bn0V=V0H (6 min) 86

+

Scheme XXIV

Thioketalisation of aldehydes in presence of ke­tones is an added advantage of thi s catalyst. The alde­hydes in absence of propanedithiol underwent selec­ti ve cyclo trimerisation (Scheme XXIV).

Which helps in separating aldehydes from ketones if they have close boi ling points. However, a side re­action observed was self-a ldoli sation . Changing the solvent could alter the ratios of self-aldoli sation vs

trimeri sation . Different ratios of products were ob­tai ned in DME, ether, CH2Cl2 and in the absence of

solvent Cfable XVII, Table V).

(e) TaCls-silicagel catalysed Prins reaction

Prins reaction, wherein a new C-C bond formati on is achi eved with simultaneous functiona lisation of 1,3 carbons as diol is generally performed with mineral ac ids and clay bes ides few Lewis acids.86

.87 TaCl s-

Si02 proved to be a powerful catalytic system fo r Prins reaction between o lefi n and parafonnaldehyde (Scheme XXV, Table XVIII).xS

832 INDIAN 1. CHEM. , SEC B, APR IL 2004

Table X V I 1- Tri meri sa li onanclloraldol isalion

Enlry Aldchydc Solvcnl Rcacl ion- Trimer AldolproduCI('1o) Ovcrall lim(hr) (%) yield(%)

fPh

l:0 ~CHO o 0 Ph I I. Ph Neal 2 ~O~Ph 82 '"

13 'Ph

Ph Ic(O)

I b( I 00)

2. l a Elher 4 I b(SO) I c(SO) 76

3. la CH 2C I2 4 I b(20) I c(80) 72

4. l a DME 4 I b( IS) I c(8S) SO

S. ~CHO

Neal 2 o~ l:0 81

2a ~o~ "' ........ 2c(0)

2b( 100)

6. 2a Elhcr 4 2b(40) 2c(60) 69

7. 2a CH2C1 2 S 2b(SO) 2c(SO) 7S

8. 2a DME 6 2c(SO) 2c(SO) 70

Ph~CHO X 9. Neal 2 YoJ....y 86

3a Ph Ph

3c( 100)

Table XVIII- Prins react ion calalysed by TaCls- Si02

Entry SubSlranlS ProduCI Reaction time & Yield (%) Microwave Conventional irradiation heating

0/'-..0

a () ~ 3 min (90) 12 hr (80)

ul 0/'-..0

b 7 3 min (88) 10 hr (S2)

---Collld

Entry

c

d

e

g

h

ADVANCES IN CONTEMPORARY RESEARCH

Table XVIII- Prins reacti on catalysed by TaCI.,- SiO::-Colird

Substrants Product

O~O

V oY O~O

ff ff O~O

If CI ~ I CIT O~O

If 0 2N ~ I 0 2NT

O~O

~ DY

~OMe

o

.R\ /R2 J==\ MW /3-5 min / TaCls-Si02 (or)

R H Dioxane, 6.,10-13 h lTaCls-Si02

R 1 = H or CH3 , R2 = H or CH3 or CsHs

Scheme XXV

Reacti on time & Yie ld (%) Microwave Conventional irrad iati on heating

3 min (86) 10 hr (80)

3 min (88) 10 hr (80)

4 min (85) 12 hr (78)

5 min (80) 13 hr (70)

3 min (90) 10 hr (80)

4 min (85) 13 hr (78)

4 min (78) 13 hr (70)

833

834 INDIAN J. CHEM. , SEC B, APRIL 2004

(0 Multicomponent coupling catalyzed by TaCis-Si02

ex-Amino phosphonates were synthesized in one pot by coupling three fragments namely carbony l compou'nd, amine and diethyl phosphite in a one-to­one fashion catalysed by 10 mo l% TaCls-Si01

(Scheme XXVI). In a typical ex periment, p­lol ualdehyde, aniline and di ethyl phosphite and stored in CH1C11 in presence of 10 mol% TaCls-Si01 and after 22 h, clean formation of ex-amino phosphonate was observed (Entry I, Table XIX). The reacti on is

. X9 performed at room temperature.

Th is is unlike lanthanide triflates, which are also known to catalyse the addition of phosphonate to im-. I . fl ~ lI1es t "lat require re ux temperatures .

(g) Kinetic resolution of 2° alcohols via acetylation

Acety lat ion of alcohols was also reported by treat-

OEt /

HO-R \

+ OEt

R = Alkyl , R1 = H, Alkyl

R2 = H, N02, OMe, OH, F

ment of alcohols with AC20 in presence of TaCls-Si01

(Scheme XXVII).

Keeping the size of Ta in view, it was antic ipated that complexation with chiral ligands should give a bulky chiral catalyst which enables efficient kinetic resolution . Two li gands were prepared using TaCis and TADDOL (A)/ex,ex-diphenyl prolinol (B) and used as catalysts in acety lation of 2" alcohols by al­lowing the reaction to only 50% alcohol will be enan­tiomerically enriched (Scheme XXVIII).91 Unexpect­ed ly, however at best onl y 40% ee of chi ra l alcohol was recovered (Table XX).

(h) Cleavage of epoxides

Symmetrical epoxides are opened with aryl amines in presence of TaCis-Si02 (Scheme XXIX). Interest­ingly, the protocol is restricted to aryl amines as ex­ternal nucleophiles whereas aliphatic amines were innert. 92 This reaction has a potenti al to desy mmetrise lIleso epox ides to chiral aminols (Table XXI).

TaGI5 - Si0 2 (10 mol%)

GH2GI2, 18-24 h, r.t

~2

Q NH

RipO(OEtb

Scheme XXVI

n = 1,2, 4

TaGI5 - Si0 2 (10 mol%)

AC20 , GH2GI2

Scheme XXVII

TaGI5 - Ghiral ligand

Scheme XXVIII

..

TaGI5 - Si02 (10 mol%) •

Scheme XXIX

OH

~-o-V"'NH R ~ j

ADVANCES IN CONTEMPORARY RESEARCH 835

Table XIX-Tantalum (V) chloride silica gel catalysed one- pot synthes is of a-amino phosphonates from aldehydes, ketones and amines

Entry Aldehyde/ketone Amine Time(hr) Yield(%)

I. H3C-o-CHO H2N-{ ) 22 92

2. Meo-Q-CHO H2N-< ) 19 88

3. Meo-Q-CHO I-hN-Q-OMe 18 94

4. < t-CHO H2N-o-OMe 18 93

5. < t-CHO H2N--\ ) 20 90

6. < (-CHO H2N--\ ) 24 84

OH

7. (OCHO H2N-o-OH 24 81

o #

8. H3C-o-CHO H2N-oMeo 18 93

9. H3C-D-CHO H2N-D-F 18 94

10. CJ-o CHO H2N-o-OMe 20 92

11 . CJ-S CHO I-hN-o-F 20 93

12. < (-CHO H2N--) ) 24 87

N~ HO

CI

13. < t CHO H2N-D-OMe 18 88

CI

836

Entry

2

3

4

5

INDIAN J. CHEM. , SEC B, APRIL 2004

TableXX- Enantiose lcct i vc acylation of 2°-alcohols

Substrate

OH

gA OH

00 OH

~CI

Rat io o f TaCls and ligand in mo le %

15:15(A)

5:5(B)

10: IO(A)

5: 10(A)

10: 10(A)

Recoveredalcoho l

OH

gA QH

00 OH

~CI

Conversio n c (%) c

(%)

60 25

50 16

50 18

55 40

70 < 10

Table XXI- TaCls-S ili ca gcl catalysed c pox ide opcning with Conclusion

Entry

I 2 3 4 5 6 7

8 9 10 II 12 13 14

15 16

aromatic amincs

Aminoa lcoho ls

R=H ,Ar=Ph R=H ,Ar=C6H4-p-OCl-l , R=H ,Ar=Cc,H4-o-C H3

R= H,A r=C6H4-o-COCHJ

R=H,Ar=C6H.-I'-C I R=H ,Ar=C6H4-p-Br R=Et,Ar=C6Hs

a OH

", NRAr

R=H,Ar=Ph R=H ,Ar=Cr,H4-jJ-OCH3

R=H ,Ar=Cr, H.-o-C H3

R=H ,Ar=Cc, H.-o-COCHJ

R=H,Ar=Cc, H.-I'-CI R=H ,Ar=C6H.-I'-Br R=Et,Ar=C6Hs

O ....... OH

"" N R.o\ r

R=H,Ar=Ph R= H,Ar=C6H4-I'-OCH)

Yi cld (%)

85 82 79 78 75 77 73

83 80 76 76 74 75 72

80 78

In conclusion, this review attempted to cover various aspects of organic chemistry of Ta and its complexes for various transformations. Eventhough in asymmetric transformations not vary high selectivities are obtained, modifications in ligands and experi­mental protocols will allow great improvements. Also the new C-C bond forming reactions and synthesis of substituted heterocycles which are hitherto not possible by other metals Catalysis would be a g reat advantage.

Acknowledgement TS thanks CSIR New Delhi , for financial

assistance.

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3 10 I. 2 Masuda T , Takahashi T & Hi gashimura ,J Chelll Soc Chelll

CO IllIl1/./I1 , 1982, 1297. 3 Sommer J, NOllv J Chilli, 6, 1982,3. 4 Hartling J B Jr & Pederscn S F, J AII1 Chelll Soc, III , 1989,

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