Cross-coupling reactions of d2 tantalum alkyne complexes: Selective 1,3-diene syntheses and their relevance to alkyne cyclization chemistry

Article abstract of DOI:10.1021/om00047a038

Ta(DIPP)₃Cl₂(OEt₂) (DIPP = 2,6-diisopropylphenoxide) can be reduced by two electrons in the presence of the bulky alkynes PhC≡CPh and Me₃SiC≡CMe to provide the pale yellow adducts (DIPP)₃Ta-(PhC≡CPh) (1) and (DIPP)₃Ta(Me₃SiC≡CMe) (2). The reduction of Ta(DIPP)₃Cl₂(OEt₂) in the presence of smaller internal alkynes (viz. EtC≡CEt) or the terminal alkynes Me₃SiC≡CH or Me₃CC≡CH affords the metallacyclopentadienes (DIPP)₃Ta(CEt=CEtCEt=CEt) (3) or (DIPP)₃Ta(CR=CHCR=CH) (4, R = SiMe₃; 5, R = CMe₃) directly. The molecular structure of the PhC≡CPh adduct 1 is approximately tetrahedral (L-Ta-L angles average 109.4°) and features very short Ta-C_alkyne distances (2.070 (3) and 2.076 (3) A?, respectively) and an elongated "C≡C" bond (1.346 (5) A?, which indicate a strongly bound and substantially reduced alkyne ligand. The molecular structure of metallacycle 3 reveals a trigonal bipyramidal geometry (L_ax-Ta-L_ax = 164.9 (3)°) with the metallacyclic α carbons occupying one axial and one equatorial site. The alkyne complex (DIPP)₃Ta(PhC≡CPh) (1) reacts with MeC≡CMe, EtC≡CEt, Me₃CC≡CH, Me₃SiC≡CH, or PhC≡CH to afford high yields of the metallacyclization products (DIPP)₃Ta(CPh=CPhCMe=CMe) (6), (DIPP)₃Ta(CPh=CPhCEt=CEt) (7), (DIPP)₃Ta-(CPh=CPhCH=CCMe₃) (8), (DIPP)₃Ta(CPh=CPhCH=CSiMe₃) (9), and (DIPP)₃Ta-(CPh=CPhCH=CPh) (10), respectively, while 1,7-octadiyne HC≡C(CH₂)₄C≡CH reacts with 2 equiv of (DIPP)₃Ta(PhC≡CPh) to provide the unusual bimetallic complex (DIPP)₃Ta(CPh=CPhCH=C(CH₂) ₄-C=CHCPh=PhC)Ta(DIPP)₃ (11). The alkyne adduct (DIPP)₃Ta(Me₃SiC≡CMe) (2) also engages in metallacyclization chemistry as it reacts with MeC≡CMe to afford (DIPP)₃Ta(CMe=CSiMe₃CMe=CMe) (12), with PhC≡CH to provide (DIPP)₃Ta(CSiMe₃=CMeCPh=CH) (13), and with Me₃CC≡CH to afford (DIPP)₃Ta(CCMe₃=CHCSiMe₃=CMe) (14). All of the metallacyclopentadiene complexes can be hydrolyzed with H₂O/acetone solutions to afford the corresponding 1,3-dienes in essentially quantitative yields. However, iodination of metallacycles 6, 7, and 13 does not yield the expected 1,4-diiodo-1,3-dienes, but rather the ring-opened monoiodinated butadienyl compounds (DIPP)₃(I)TaCPh=CPhCMe=CMe(I) (15), (DIPP)₃(I)TaCPh=CPhCEt=CEt(I) (16), and (DIPP)₃(I)TaCSiMe₃=CMeCPh=CH(I) (17), respectively. The subsequent hydrolysis of compounds 15-17 provides the corresponding 1-iodo-1,3-dienes.