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Indian Journal of Chemistry Vol. 43B, April 2004, pp. 813-838 Advances in Contemporary Research Applications of trivalent and pentavalent tantalum in organic synthesis Srivari Chandrasekhar*",Tokala ~amachandar~ & Tokala Shyamsunder" "Indian Institute of Chemical Technology, Hyderabad 500 007, India ?emple University, Philadelphia, Pennsylvania 19122, USA E-mail: [email protected] Received 1I July 2003; accepted (revised) 7 January 2004 IPC: 1 n t . ~ 1C. ~01 G 35/00 Of the Group B elements, the group V is relatively unexplored especially with reference to Niobium (Nb) and Tantalum (Ta). Interestingly, these two elements have very similar chemical properties but not as similar as Zirconiun~(Zr) and Hafnium (Hf). Oxidation states and stereochemistries of Nb and Ta are shown below (Table I). Nb and Ta have very little cationic behaviour, however they can complex well in all the oxidation states viz., 11, 111, IV and V. In the case of I1 and 111 oxidation states, metal-metal bonds are rather routinely formed. With reference to abundance, Nb is 1012 times more abundant in the earth crust than Ta. The main source being columbite-tantalite series of minerals having general chemical composition (Fe/Mn) ( N b 1 T a ) ~ 0with ~ varying ratios. Both metals are bright, high melting (Nb 2468 "C, Ta 2996 "C) and very resistant to acids. The oxides of these metals are vast', inert and insoluble except in concentrated HF. Accordingly the chemical applications especially in higher oxidation states are rather restricted. However, the penta halides especially chlorides are yellow to purple red solids and easily prepared by direct reaction of metals with excess of halogens.2 The halides are soluble in various organic solvents including diethylether and carbon tetrachloride. The entire penta halides combine with - with amines, NbC15 ofhalide ions to form M X ~ and ten undergoes reduction. The penta halides due to their Lewis acidity are used as catalysts in cyclotrimerizing or lineraly polymerizing acetylenes and in Friedal-Crafts and related reactions to little e ~ t e n t It. ~ is the own experience of the authors that Ta (111 and V) are more user friendly in the laboratory, whereas Nb (I11 and V) are more difficult to handle and also NbC15 was air oxidising after a single use of the bottle despite precautions. Surprisingly. even though the Ta and Nb coordinate complexes are known for a long time, its synthetic applications in organic chemistry has a history of only 10-12 years. Low valent tantalum complexes react with inactivated acetylenic triple bonds and being sterically congested, the only C-C bond formation effected was cyclotrimerisation. However, some contribution in CC bond formation and also Ta in its V oxidation state was effective as Lewis acid catalyst. Keeping aside the elaborate studies carried out on organo-metalllic complexes of Ta and Nb, the present review will describe the synthetic reactions using tantalum chloride as a reagentlcatalyst in both oxidation states V and 111. The literature available on Ta in organic synthesis can be clasified into two major areas. (i) Low valent tantalum (LVT) in combination with acetylenes acting as dianion equivalent (ii) TaCls as a Lewis acid for catalysed reactions. (I) Low valent tantalum (LVT) 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 TaCls and Zn ((reduction of Ta (V) to Ta (111) J . A careful examination of these complexes proved that the two carbons of acetylene act as vicinal d~anionand can add onto electrophiles. Also, the study between Nb-alkyne complexJ ' and Ta-alkyne complex7 l 4 showed that Ta-alkynes complexes are formed more smoothly and reaction\ with o-phthalaldehyde afforded 1-naphthol derivative when terminal acetylene was used (Scheme I)." In case of internal acetylenes, 2.3-disubstituted naphthols were obtained in good yields (Table 11). INDIAN J. CHEM., SEC B, APRIL 2004 814 Table IOxidation state ~ b - ' Ta" , ~ b - '7ra-l , Oxidation states and stereochemistries of niobium and tantalum Coordination Geometry number 5 Tbp 6 Octahedral Examples n~omp~ex 7 Distored capped octahedron (non-rigid) 6 6 Octahedral Trigonal prism Octahedral Complex Dodecahedra1 Octahedral Distorted pentagonal bipyramidal Capped octahedron 7 8 6 7 7 8 4 5 NbO TaC12(d~npe)2 LiNb02 N~>CI:-, M2Cl6(SMed; TaC13(CO)(PMe2Yh)3.EtOH Ks[Nb(CN)xI (NbClA), TaC14py2,~ ~ 1 6 ' KWJF~ Non rigid in solution Square antiprism Dodecahedral 6 Tetrahedral T ~ P Distorted tetragonal pyramid Octahedral 6 Trigonal prism 8 Distorted pentagonal bipyrarnidal NbO(S2CNEt2), Pentagonal bipyrarnidal, fluxional S=Ta(S2CNEt2)?,Ta(NMe?) (S2CNMe2)3, (S2CNR212, TaMei Bicapped trigonal prism INb(trop)q 1' Sqaure antiprism NaiTaFx Dodecahedra1 Ta(S2CNMe2)4+ n Complex (n' - C S H S ) T ~ H ~ 9 CHO MCI, ~b(p-dike),,M2CldPme3)4, Nb(SCN), (dipy12 K4Nb(CN)82H20 (q-C5H5)2NbMe2 ScNbO, MCl,(vapour), ?'aMe5. Nb(NR2)5 Nb(NMe2)5 NaMO? (perovskite), NbC15.0PC13, TaC15.S(CH3)2,TaF;, NbOCl;, M2Cllo. MCIL [M(S~C~HI.)~~- - --- OH 2,6-lutidine DME, PhH 25 OC OH Scheme I Mechanistically, it is anticipated that low valent Ta (Nb) produces a three membered ring metal-alkyne complex. Insertion of formyl group into metal carbon bond of the complex followed by second insertion, elimination of metaloxy group and naphthol is liberated after an aqueous work up. (b) Synthesis of allyl alcohols A variety of Ta-alkyne complexes 1 and 2 have been prepared (without isolation) and were reacted with various aldehydes in a one-to-one fashion t o yield the corresponding allyl alcohols with exclusive E-geometry (Scheme 11). Interestingly, in the absence of carbonyl compound and quenching the complex 1 with NaOD-D20 furnished dideuterated cis-olefin 3 in good yields.'6 The results of the reaction reveals that electronic effects play a role in the regiochemistry of rlie 815 ADVANCES IN CONTEMPORARY RESEARCH Table II-Regioseleclive Run R' C5Hl R? M Time t (hr) TaCI5, Zn C5Hl 1 + DME, PhH 25 OC, 30 min synthesis of 2,3-disubstituted-l-naphthols Aldehyde Equiv. Product ratio C5Hvc5H1 >=( NaOD / D20 C5Hi 1 C5Hi I + Ta 25 OC, 1 h 3 D INDIAN J. CHEM.,SEC B. APRIL 2004 R~+R~ THF TaC15, Zn+ DME, PhH 25Oc, t hr R' R~R~C=O R2 b Pyridine 25 OC, 15min OH A B Scheme II(b) A. NaOH/H20 ~1-7~2 TaCI5, Zn TH F DME, PhH 25 OC, t hr Pyridine R3R4c=0 25 OC, t hr 25OC, 1 hr HO A R4 B Scheme I11 Table 111Run RI Synthesis of allylic alcohols from acetylenes and aldehydes R~ R3 product. Preference is generally that the acetylenic carbon having smaller substitution adds onto C=O (Table 111). (c) Synthesis of (Z)-alkenyl sulfides and (E)-3hydroxy-1-propenyl methyl sulfides It was observed that tantalum-alkyne complexes form more readily from alkynyl sulfides than the dialkynes. There is a side reaction of a-chlorination, which could be prevented by the addition of pyridine in the reaction medium (Scheme III)." As usual quenching of complex in absence of electrophile (R-CO-R') resulted in (Z)-alkenyl sulfides, whereas the electrophile presence made the new C-C bond formation to the thio group preferentially yielding (E)-3-hydroxy- 1-propenyl methyl sulfides. The bulkiness and electronic nature R4 Time t (hr) Yield A/B ("/.) The bulkiness and electronic nature of the substituents of alkynes influences the regiochemistry of the coupling reaction (Table IV).I8 The results have special advantage over zirconocene-(methy1thio)-1-alkyne complex in that zirconocene complex yields as 1:l mixture of regioisorners, whereas the Ta-complex produced one stereo isomeric allylic alcohol predominantly. (d) High oxidation state transition metal carboxylates as acylating agents The abilities of metallocene ~sobutyratelo ' ~ c t '14 acylating agent was demonstrated by Koln~ct ~ r l . wherein the metallocene complex was prepared b) treatment of TaCls with isobutqric 'ic~d to 14olate 1' 817 ADVANCES IN CONTEMPORARY RESEARCH Table IV -Reactions Run of alkynyl sulfides and sulfones with carbonyl co~npoundsby means of a TaCI,-Zn system R' ZR~ R~ R~ t/h r Yield A/B (a) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 11-C10H21 11-c10H21 n-CloH21 11-C10H21 n-CloH21 c-ChH I I c - C ~ HI I Ph Ph n-CloH21 n-CloH21 n-CloH21 n-CloH21 c-C6H~I Ph(CH212 Ph(CH212 c - C ~ HI I -(CHz)s-(cHz)8Ph(CH2)z -(CH2)5Ph(CH2)2 -(CHz)5Ph(CH2)2 Ph(CH212 c-C~H II -(CH2)5Ph(CHz)z SMe SPh SMe SMe SPh SMe SMe SMe SMe S02Me S02Ph S02Me S02Me S02Me 0.2 0.5 0.2 0.2 0.5 0.2 0.2 0.5 0.5 2.5 21 2.5 2.5 2.5 H H H H H H H H H TaCI, 73 85 74 77 75 68 70 64 54 54 43 46 62 59 + 15 min 5 Scheme IV -< Figure 1 coordinated complex (Figure I)'', which on exposure to benyzl amine yielded the N-benzyl isobutyramide 6 in less than 15 minutes." The same experiment with other metallocenes such as Ti and Zr yielded the corresponding 6 albeit at a very slow rate compared to Ta (Scheme IV). This may be due to high positive charge on the metal. (e) Synthesis of (E)-a, P-unsaturated amides from reaction of tantalum-alkyne complexes with isocyanates Reaction of metal-alkyne complexes 7, 8 and 9 is well studied with few metals."-'"E)-a, P-unsaturated amides can be stereoselectively synthesized25926 by the reaction between low valent Ta-alkyne complex and isocyanates. In a typical experiment, the reaction was performed between 6-dodecyne, tantalum (111) and PhNCO to generate (E)-N-phenyl-2-pentyl-2-octenamide 1 0 in 80% yield [Scheme V(a)]. The lijgh yields are obtained subject to filtration of Ta-alkyne complex from reaction mixture and later isocyanate was added. Quenching the reaction with DIO/NaOD yielded deutero amides, which are hitherto unacce5i1ble or difficult to make. Again, as in the case of addition to carbonyi group. the substitution pattern of acetylene plays a majol- [-ole in the regio chemistry. Only in the case of methylthio substituted alkyne, the regioisomer A was produced exclusively because of the electronic nature of the substituent (Table V). (f) Chlorination of alcohols using TaClj Selective and ,ilmost complete convcriioii o f cyclohexanol 11 to chloro cyclohexane 12 (96V) was achieved whereas other metal halides naniel) WC16. did not give a good conversion and NbCi? 2aL.c. only 70% conversion after prolonged reaction hours. Thus, this is one of the rare cl-block elements ~ v h i c l ~ is comparable with halides of p-bloch eieinznts (S02C12. PClr) in conversion of 2" alcohol a, the respective chloride^.'^-'^ Thus TaClS proved :o he :i 1 1 1 INDIAN J. CHEM., SEC B, APRIL 2004 -- TaCI5, Zn R+ l R2 DME, PhH 25 OC, 1 h PhN=C=O NaOD / D 2 0 2 5 0 ~ NaOH / H 2 0 * RqN, (H)D 0 10 Scheme V(a) R-1 TaC15, i n R2 R~N=C=O NaOH 1 H 2 0 R1< DME, PhH 25 OC, 1 h 25 OC, t hr + * w 250C, 1 hr 0 NHR~ 6 A Scheme V(b) Table V -Reactions between alkynes and isocynates by means of TaCIS and Zn Run R1 Timelhr n-CsH1 I n-CsH I I n-CsH1 I n-CsH I I n-CsH1 I c - C ~ HI I c-ChH I I Ph Fh Me3% Me3Si MeS MeS 3 3 Yield (%) 1 2 3 4 5 6 7 8 9 10 I1 12 13 better halogenating (Scheme VI).~' agent 20 20 20 3 3 3 3 3 3 compared to NbCIS (g) Carbocations of alkyl benzenes with TaC15-CH2CI2 TaCIS-CH2CI2performed C-C and C-E-I bond cleavage in 1,3,5-triisopropyl benzene 13 to yield stable indane cation 14 as shown in ( Scheme VIJ ). The transformation generally demands superor very acidic metal halides viz., TiCl,, ZrCI, and ~ f c l ~ . " . "It is rather interesting to note that even though 'Ta' in oxidation state V is almost nonmetallic and not a strong acidic metal halide it is able acids3h-39 80 72 79 63 38 69 62 51 0.2 74 60 33 58 0.2 90 to perform such a transformation albeit in moderate yields ( 5 0 % ) . ~ ~ (h) Allylic arnine synthesis via tantalum-alkyne complexes Nucleophilic addition of organo ~netallic compounds to carbon-nitrogen double bonds constitute an important method for the synthexis of amines."-" Tantalum-alkyne complex 15 adds onto N,N-dimethylhydrazones 16 at elevated temperatures (80°C) to yield allylic hydrazine in rather low yields. Attempt to add activation such as Me3AI. Me;Ga helped to improve the yields. The regiocher:listsy of the allylic hydrazine depends on the b~:lkiness of ADVANCES IN CONTEMPORARY RESEARCH Table VI -Reactions between tantalum-alkyne colnplex and hydrazones in the presence of MelAl Run 1 2 3 4 5 6 7 8 R~ R~ RI II-C~H~~ l1-C5Hl1 II-C~H~ I I - C ~ IH ~ c-C6HII Ph Me3Si MeS Ph(CH212 C - ~I ~ ~ I (E)-PrCH=CH (cyclohexanone) Ph(CH2I2 Ph(CH2)2 Ph(CH2)? Ph(CH212 tl/hr t2/hr Yield (%I 2 2 2 2 3 4 3 0.2 16 16 16 16 16 16 16 26 80 71 32 5 64 76 57 69 A/B Unreacted alkene (%) Reco~, Hydrazone (%I substitutions on the acetylenes as expected. Electronic effects are also observed in regiochemistry \\lien Me& and -SMe substitutions (Run 7 and 8. Table VI) are placed on acetylene. This reaction ~vas restricted to aldehyde hydrazones (Scheme VIII)."' (96%) 12 NbCI, Interestingly, insertion of isocyanide into tantalumalkyne complex was achieved to synthesize 2.3,4trisubstituted N-amino pyrrox 17 albeit in only 17% yield (Scheme IX). (70%) 11 It may be inferred that external Lewis acid addition is essential to promote the addition of tantalum-alkq ne complex to imines. (50%) 12 Scheme VI (i) Selective reduction of acetylene to olefin with Zstereoselectivity using LVT 14 13 Scheme VII R' = 2 Tzicl5,.Z DME, PhH 55 O C , t' 1 h Selective reduction of acetylenes to olefins \s.ith precise cis geornetry is an important task." Catalytic hydrogenation gives cis-olefin (some time reaction does not stop at olefin stage and proceeds t o 'yl1 15 TliF + Me3AI ,NMe2 N 1 1 6 R3 H * 55 OC, t2 / h NaOH I H,O * 25 O C , 1 h INDIAN J. CHEM., SEC B, APRIL 2004 C5HI C 5 H ~1 DME, PhH 25 OC, 30 min NaOH 1 H 2 0 17 Scheme IX R1 Z I R~ TaCI5, Zn NaOH I H 2 0 * R' R2 HHH DME, PhH 25 OC, t Scheme X saturation) whereas metal/NH3 reduction generally yields the E-olefin. Low valent tantalum produced from TaC15-Zn which is much faster than analogues NbC15-Zn for complexation with acetylene after basic hydrolysis with H20/(NaOH) furnished the protonated olefin in good yields with more than 99% Z selectivity (Scheme x).~' It is observed that in the case of NbC15, HMPA as additive is desired which is carcinogenic and not available on many catalogues. Thus 'Ta' proved to be a superior metal over 'Nb' in this transformation. Entries 10 and 11 (Table VII) are inert to a possible cyclization despite possessing an additional olefinic system thus proving of 'Ta' is different from low valent 'Zr' wherein an intramolecular cyclisation with concomitant reduction of triple bond is observed (Table V I I ) . ~ ~ . ~ ~ Cj) Synthesis of substituted furans from tantalumalkyne complexes Although insertion of isocyanide into Ta-alkyne ~ process has not was recognized as ealry as 1 9 7 4 , ~the been utilized in organic synthesis. Insertion of isocyanide into the tantalum-carbon bond was successfully achieved for the synthesis of highly substituted furans 18 (Scheme X I and Table VIII). Thus a variety of 2,3,4-trisubstituted furans were prepared by the treatment of various tantalum-alkyne complexes with the aldehydes followed by addition of an isocyanide in DME-PhH-THF (1: 1:1). The general approach is shown in below.57 (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 acetylenes and imine derivatives. Table VII -Reductions of alkynes to (Z)-alkcncs by means a TaC1,-Zn system Entry Yield (%) 39 52 68 85 69 85 80 82 79 81 82 80 ZIE ADVANCES IN CONTEMPORARY RESEARCH Scheme XI Table VIII-Synthesis of 2,3,4-trisubstituted furans Run t l hr 1 0.5 8 2 0.5 66 3 0.5 28 4 2 55 5 5 40 6 2 57 7 3.5 42 8 3.5 54 In one approach as reported by ~ u c h w a l and d ~ Liv~~~ inghouse60, 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 Me3A1 to yield (E)-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 hydrazinelamine synthesis having 3" carbon due to ~~' the steric factor^.^' It is observed that l i t h i ~ i m i n e ' as C=N component62 which is brought close to the Taalkyne complex by ligand exchange.63264Alkaline 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-(6dodecyne) complex 20 gave allylic amine 21, which was acetylated for characterisation (Scheme X I I ) . ~ ~ (1) Low valent T a mediated Reformatsky reaction R~formatskyreaction is a powerful protocol for the synthesis of P-hydroxy esters. Generally this reaction is performed at elevated temperatures using Zn However, Sm (11) iodide has been used as one electron reducing agent in this reaction which is per- Yield (70) ~ formed at low temperature. TaC15-Znwas found to be an efficient combination for the Rqformatsky reaction at 0 "C (Table 1x1~'.Typically to a suspension of low valent tantalum prepared from TaC15 (3 mmol) and Zn (4.5 mg atom) in THF (15 mL,) at room temperature was added a-bromopropionate 23(1 mmol) and carbony1 compound 24 (1 mmol) at 0°C under argon atmosphere. After 2 h of stirring at 0°C and usual workup produced P-hydroxy ester 25 (Scheme XI11 and Table IX) (m) Insertion of carbon-carbon double bonds into tantalum-alkyne complexes Not only tantalum-alkyile con~plexesadd onto C=O and C=N but can also be inserted into sirnplz alk e n e ~ This . ~ ~ has a great potential, as this protocol enables one to create a new C-C bond between internal acetylenes and terminal oiefin~ In a typical experiment Ion valent tantalum-hdodecyne complex with lithium-3-buten-1-oiate 26 in DME-benzene-THF (1: 1: 1) at 3-5 "C for 3 hr gave t w ~ alkylated alkanols 27 and 28 in 38:2 ratio as regio isomers (Scheme XIV). Prior coordination of the tantdlum-alkyne complex with the nyciroxyl group of the homo allyl alcohol facilitates both i~lsertionsas well a:, regio chemistry (Table X). Anchimeric assistance of. phenols and amines is also a noteworthy feature.""?" I INDIAN J. CHEM., SEC B, APRIL 2004 822 I I I I I I I I I + n-C8Hl,CN 19 Tact5, Zn + C5H1 1+C5H1 1 MeM [c5Hxc5H1] 1 THF 1 = DME, PhH 25 OC, 30 min ~1~1 n-C8H7~ additive I I I I I Ac20 1 Et3N NaOH / H20 25 OC, 1 h * CH2CI2 A: M = Li C 5 H ~ ~C 5 H ~ 1 %Me AcHN addi.tive = none n-C8H17 50 OC 25 OC 6: M = Mg 4h 67% 20h 43% additive = none 50 OC 6h 65% C: M = AIMe2 additive = none 50 OC 6h 75% Scheme XI1 Table IX -The Entry reactions of ethyl a-bromopropionate with ketones and aldehydes in the presence of the LVT reagent Carbonyl compound tl hr Solvent Cyclohexanone Cyclohexanone Cyclohexanone Cyclohexanone Cyclohexanone Cyclohexanone Cyclohexanone Cyclohexanone Cyclohexanone Cyclohexanone Cyclohexanone Cyclopentanone Acetophenone Benzophenone Octan-2-one Cyclohexenone D-Camphor Benzaldehyde Cyclohexylaldehyde Octanal 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 12 2.5 2.0 2.0 2.0 0.5 3.0 I .O 2.0 THF Benzene Et,O CH2C12 MeCN DMF DMF THF THF THF THF THF THF THF THF THF THF THF THF THF 1.5 T / "C Yield (56)of b-hydroxy-ester\ 67 Manyproductc 0 44 Manyproducts 78 79 Manyproductc 40 59 81(17: 13) 62 751:3:2) 16(3: 1) 19 75(tl~reo:c~r-!~i/1ro=4:3) 57,(tl1reo:er~.rli1'0=6:5) 6O(tlzreo:or~rllro=4:3) + ADVANCES IN CONTEMPORARY RESEARCH 23 25 24 Scheme XI11 C5H1 C5H1 1 TaCI,, Zn m 26 O L i DME, PhH 25 OC, 2 h - C Y ~ H I I NaOH / H 2 0* - + C5H7C5Hll OH Scheme XIV(a) [w BuLi R=n-C,Hll R] *- NaOH Hzo A Scheme XIV(b) Parallel to the above observation, Takai et al., observed that ally1 alcohols and amines could be inserted into low valent tantalum-alkyne complexes.77 N,N-dimethylallylamine was found to react with the tantalum complex at 55 "C and 1, 4-diene 29, a formal cis-hydroallylation product of 6-dodecyne 30, was obtained (Scheme XV). Mechanistically, insertion of an olefinic bond of allyl alcohol into the tantalum-carbon bond of the com- plex 31 form two oxatantalab~cycloheptene intermdiates 32 and 33. Deoxygenative ring openlng of oxatantacyclo butane ring of interl-nediate 32 follo\teci by hydrolysis providci the product 34134a. (n) Selective cyclotrin~erisationof IA\.'T-alk\.lle Low valcnt tantalum-alh~ne (inte~nal)complex I c act with terminal dlynes 35 in THF to y v e t r ~ i a b u b i r ~ tuted b e n ~ e n ed e n v a t i ~e5 36 (Scheme XVI) " INDIAN J. CHEM., SEC B, APRIL 2004 C5H1 C5H1 1 TaCI5, Zn ex NaOH i b DME, PhH a: fiNMe2 NMe2 m N H 55 20 54% Not detected 25 4 86% Not detected Scheme XV When acetylenic nitrile 37 was used, pyridine ring 38 was formed in 73% yield (Scheme XVII). Functionalised acetylenes viz., TMS-acetylene and methylthio substituted alkynes also behaved well (Table XI). (11) TaC15 as a Lewis acid for catalysed reactions Differential behaviour of TaClj and NbCl5 in Sakurai Reaction An unusual reaction was observed, when NbC15 was used instead of TiClj in the Sakurai ~eaction" INDIAN J. CHEM., SEC B, APRIL 2004 C5Hl C5Hl 1 TaC15, Zn THF 35 - r DME, PhH Pyridine 50 OC NaOH / H 2 0 36,82% Scheme XVI C5H1 C5H1I - TaCI5, Zn THF DME, PhH Pyridine * - NaOH 1 ~ ~ 0 0 38,73% Scheme XVII(a) RI-R~ -TaCI5, Zn THF DME, PhH Pyridine R3-(cH2)n- NaOH 50°C, t hr "20 R-, Scheme XVII(b) Mechanism 4 - Me3SiCI +741 Scheme XVIII between aldehyde 39 and ally1 trimethyl silane 40 (Scheme XVIII).~' It was visualised that niobium alkoxide species is a leaving group and cyclopropanation is forced via homoallyl group participation. As the intermediacy of long lived carbon~um run5 I \ unlikely, it is assumed that the cations are trapped b j a chloride ion so that the cyclopropyl methyl chlonde 41 is formed to yield the unexpected product. ADVANCES IN CONTEMPORARY RESEARCH R- CHO + e S i M e 3 R = Ph, naphthyl, cyclohexyl Scheme XIX exo R' = Me, R2 = H Scheme XX Interestingly, when TaC15 was used in the reaction, in presence of AczO, a one-step conversion followed by acetylation is observed (Scheme XIX).~' (b) TaCl5 as a catalyst in Diels-Alder reaction In fact the first application of TaC15 as Lewis acid was di~monstratedin a Diels-Alder reaction. A comparison between NbC15 and TaC15 for Lewis acid catalysed Diels-Alder reaction was attempted and found that NbC15 catalysed the reaction better between cyclopentadiene 42 and m e t h a c r o ~ e i n . ~ ~ 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 XI1 and XIII). (c) Synthesis of imides from anhydrides catalysed by TaC15 In depth studies on the Lewis acid catalysed reactions of TaCl5 revealed that TaC15 adsorbed on silicage1 acts as a better catalyst compared with the TaC15 alone. This is in agreement with the observations made by Howarth et al., 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 TaC15-silicagel and exposed to microwave irradiation for 5 min, furnished N- TaCI, - SiO, MW, R-NH2 cfiR 41 42 R = N-benzyl amine Scheme XXI -- Table XI1 - Results for the Diels-Alder reaction bet\vee~l cyclopentadiene and crotonaldehyde or melhacrolein in tlic presence of niobiuin complexes of some bidentaate ligands Bidentate ligand 1 L-Phenylalanine 2. L-Alanine 3. L-Leucine 4. L-Isoleucine 5. L-Tryptophan 6. L-Valine 7 . Diethyl L-tartrate 8. Diisopropyl Ltartrate Dienophile Crotonaldehyde Methacrolei~~ 18%; 85: 15 37%: 95: 5 10%: 9-3: 7 20%: 94: 6 10%: 84: 16 5 % : 95: 5 9%; 93: 7 16%: 94: 6 43-%: 75: 75 44%: 7: 93 77%: 13: 87 40%; 8: 92 57%. 32: 7X l i ' % : 13: X7 6l(;,t,6:94:25(; 52Ik: .?:97: 40c; - benyzl phthalimide 42 in 92% isolated yield (Scheme XXI). When optically active R-(+)-@-methyl benzy l amine was used, the product was obtained u,ithout any racemisation (Table XIV).'" The same reaction could also be accomplished o n solid support such as Merrifield resin (Scheme XXII and Table X V ) . ' ~ Solid supported amino acid 43 wherein thc 1-chin part was connected through ester linkage. ~ i l i e n INDIAN J. CHEM., SEC B, APRIL 2004 828 Table XI11 -Results for the Diels-Alder reaction between cyclopentddiene and crotonaldehyde or methacrolein in the presence of tantalum complexes of some bidentaate ligands. Bidentate ligand 1 2. 3. 4. 5. Dienophile Crotonaldehyde Diethyl-L-tartrate Diiosopropyl-L-tartrate 42%; 90: 10 24%; 95:5 (+)-2,3-0-Isopropylidene-L-threitol 14%; 99: 1 (-)-2,3-0-Isopropyliden 1,1,4,4-tetraphenyl-L-threitol 72%; 86: 14 (-)-2,3-0-Isopropyliden 1,1,4,4-tetra(2-naphthy1)-L-threitol 42%; 99: 1 Table XIV -Synthesis Entry Anhydride Amine Methacrolein 78%; 6:94;7% 55%; 6:94 49%; 16:84 50%; 9:91 75%; 10:90 of N-alkyl and N-aryl imides Amide Time (min) Yield ($6) 829 ADVANCES IN CONTEMPORARY RESEARCH Table XIV -Synthesis of N-alkyl and N-aryl imides-Xontd Entry Anhydride Amine Ti me (min) Amide Yield (%I 1. TaCI5 - Si02 MW, 5 rnin 2. TFA in CH2C12 43 Scheme XXII treated with phthalic anhydride 41 in presence of TaCls-silicagel, after reaction and cleavage of bound resin yielded the phthalimido acids 44 in good yields. R- TaCI, - Si02 EtSH (d) Use of TaCIs-silicagel as Lewis acid for protection of organic functional groups SEt OH OTHP TaCI, - S102 R+ Highly oxophylic TaC15 and TaCls-silicagel has also been utilized as a catalyst in protective group introduction namely -0THP formation and thioacetalisation of aldehydes (Scheme X X I I I ) . ~ Series ~ of alcohols both lo, 2" and 3" alcohols converted to tetrahydropyranyl ethers in good yields. The yields were higher when Ta-silicagel was used instead of CHO R2 DHP + R+R' R R = A I ~ ~R', I , R~ = H or A I ~ ~ I Scheme XXlII TaCIS alone even at higher concentration of TaClj (Table XVI). ' . t*:~ INDIAN J. CHEM.. S E C B, APRIL 2001 830 Table XV-Synthesis Entry Polymer-bound ,ilnlnc of N-alkyl imides o n solid support in solid htate Anhyd~~de Table XVI-Thioacetalisatiori Substrate Entry ph- Ph 4. 5 TBDMSOC -HO - and tetruhycil-opyr-;i~~yi~~~~or~ lieaction condition CHO CHO 5 X B 1 I si '\ 311111 1 ") T a D ~ , ~ , l s o / \ / - ~ ~ ~ s ' -- c 111 1 ADVANCES IN CONTEMPORARY RESEARCH Table XVI-Thioacetalisation Entry Substrate and tetrahydropyranylatiodorltd Reaction condition - C 0 Product (time) % 7 \ \ P 11. 12. B ~ n O 83 1 ~ O H n 0THP C C P ~ 0, OTHP -7 ~ O ( 1 0 min) 71 P ( 5 min) 88 0 0 0--\ \ T H Bnon ( 6 ~nin) 86 A.TaCI5-silicagel, ethanethiol, CH2C12; B.TaC1,-silicagel, propanedithiol, CH2CI2; C.TaC1,-silicagel, dihydropyran, CH2CI2. TaCI, + b R2 solvent, r.t Scheme XXIV Thioketalisation of aldehydes in presence of ketones is an added advantage of this catalyst. The aldehydes in absence of propanedithiol underwent selective cyclo trimerisation (Scheme XXIV). Which helps in separating aldehydes from ketones if they have close boiling points. However, a side reaction observed was self-aldolisation. Changing the solvent could alter the ratios of self-aldolisation vs trimerisation. Different ratios of products were obtained in DME, ether, CH2C12and in the absence of solvent (Table XVII, Table V). (e) TaC15-silicagel catalysed Prim reaction Pri~lsreaction, wherein a new C-C bond formation is achieved with simultaneous functionalisation of 1.3 carbons as diol is generally performed with mineral acids and clay besides few Lewis a ~ i d s . ~ " ~ ~ u ~ Si02 proved to be a powerful catalytic system for P r i m reaction between olefin and paraformaldehyde (Scheme XXV, Table X V I I I ) . ~ ~ INDIAN J. CHEM., SEC B, APRIL 2004 832 Table XVII-Tri~nerisationand/oraldolisation Entry 5. Aldehyde Solvent Reactiontim(hr) la Ether 4 la CH2C12 4 la DME 4 Neat 2 f i CHO Trimer Aldolproduct(%) (%I 2a 2c(O) 6. 7. 8. 2a 2a Ether CH2CI2 4 5 2b( 100) 2b(40) 2b(50) 2a DME 6 2c(50) 'f" 9. Ph A CHO Neat 2 3a Ph Table XVIII-Prins Entry Substrants Ph reaction catalysed by TaCI5-Si02 Product Reaction time & Yield (%) Microwave Conventional irradiation heating Overall yield(%) 5-33 ADVANCES IN CONTEMPORARY RESEARCH Table XVIII-Prins Entry reaction catalysed by TaC15-SiO&otltcl Subslrants Product Reaction lime & Yield (9) Microwave Conventional irradiation heating 4 rnin ( 8 5 ) 12 hr (78, 5 min (80) 13 hs (70) C1 02N MW / 3-5 min / TaC15-Si02(or) R H Dioxane, A, 10-13 h / TaC15-Si02 R' = H or CH3, R2 = H or CH3 or C6H5 Scheine XXV k2 INDIAN J. CHEM., SEC B, APRIL 2004 834 (f) Multicomponent coupling catalyzed by TaC15SiOz a-Amino phosphonates were synthesized in one pot by coupling three fragments namely carbonyl compouhd, amine and diethyl phosphite in a one-toone fashion catalysed by 10 mol% TaC1,-SiO, (Scheme XXVI). In a typical experiment, ptolualdehyde, aniline and diethyl phosphite and stored in CH2C12 in presence of 16 mol% TaCls-Si02 and after 22 h, clean formation of a-amino phosphonate was observed (Entry 1, Table XIX). The reaction is performed at room temperature.89 This is unlike lanthanide triflates, which are also known to catalyse the addition of phosphonate to imines that require reflux temperatures.90 (g) Kinetic resolution of 2' alcohols via acetylation Acetylation of alcohols was also reported by treat- 6, "4'2 R'. I + + OEt ment of alcohols with Ac20 in presence of TaC15-SiO? (Scheme XXVII) . Keeping the size of Ta in view, it was anticipated that complexation with chiral ligands should give a bulky chiral catalyst which enables efficient kinetic resolution. Two ligands were prepared using TaCli and TADDOL (A)la,a-diphenyl prolinol (B) and used as catalysts in acetylation of 2" alcohols by allowing the reaction to only 50% alcohol will be enantiomerically enriched (Scheme XXVIII)." Unexpectedly, however at best only 40% ee of chiral alcohol was recovered (Table XX). (h) Cleavage of epoxides Symmetrical epoxides are opened with aryl amines in presence of TaC15-Si02 (Scheme XXIX). Interestingly, the protocol is restricted to aryl amines as external nucleophiles whereas aliphatic amines were innert." This reaction has a potential to desymmetrise meso epoxides to chiral aminols (Table XXI). TaCI5 - Si02 (10 mol%) - HO-P; OEt 9 - CH2CI2, 18-24 h, r.t NH R = Alkyl, R' = H, Alkyl R~ = H, NO2, OMe, OH, F Scheme XXVI OH TaCI, - SiO, (10 mol%) OAc Scheme XXVII OH TaCI, - Chiral ligand OH b Ac20, CH2CI2 n = 1,2, 4 Scheme XXIX 'AR, R Scheme XXVIII OAc + R~.-4R2 835 ADVANCES IN CONTEMPORARY RESEARCH Table XIX-Tantalum Entry 1. (V) chloride silica gel catalysed one-pot synthesis of a-amino phosphonates from aldehydes, ketones and amines Aldehydelketone H 3 c oc H o Amine Time(hr) Yield(%) INDIAN J. CHEM., SEC B, APRIL 2004 836 TableXX-Enantioselective Entry Substrate Ratio of TaCI5 and ligand in mole % acylation of 2"-alcohols Rccoveredalcohol Convcrsion (96) c. c (% i Table XXI-TaCI5-Silica gcl catalyscd epoxide opening with aromatic amines Entry Aminoalcohols Yield (%) Conclusion In conclusion, this review attempted to cover various aspects of organic chemistry of Ta and its complexes for various transformations. 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