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

Abstract

Ta(DIPP){sub 3}Cl{sub 2}(OEt{sub 2}) (DIPP = 2,6-diisopropylphenoxide) can be reduced by two electrons in the presence of the bulky alkynes PhC{triple bond}CPh and Me{sub 3}SiC{triple bond}CMe to provide the pale yellow adducts (DIPP){sub 3}Ta(PhC{triple bond}CPh) (1) and (DIPP){sub 3}Ta(Me{sub 3}SiC{triple bond}CMe) (2). The reduction of Ta(DIPP){sub 3}Cl{sub 2}(OEt{sub 2}) in the presence of smaller internal alkynes (viz. EtC{triple bond}CEt) or the terminal alkynes Me{sub 3}SiC{triple bond}CH or Me{sub 3}CC{triple bond}CH affords metallacyclopentadienes directly. The molecular structure of the PhC{triple bond}CPh adduct 1 is approximately tetrahedral (L-Ta-L angles average 109.4{degree}) and features very short Ta-C{sub alkyne} distances (2.070 (3) and 2.076 (3) {angstrom}, respectively) and an elongated C{triple bond}C' bond (1.346 (5) {angstrom}), which indicate a strongly bound and substantially reduced alkyne ligand. The molecular structure of metallacycle 3 reveals a trigonal bipyramidal geometry (L{sub ax}-Ta-L{sub ax} = 164.9 (3){degree}) with the metallacyclic {alpha} carbons occupying one axial and one equatorial site. The alkyne complex (DIPP){sub 3}Ta(PhC{triple bond}CPh) (1) reacts with MeC{triple bond}CMe, EtC{triple bond}CEt, Me{sub 3}CC{triple bond}CH, Me{sub 3}SiC{triple bond}CH, or PhC{triple bond}CH to afford high yields of the metallacyclization products. The alkyne adduct (DIPP){sub 3}Ta(Me{sub 3}SiC{triple bond}CMe) (2) also engages in metallacyclization chemistry. All of the metallacyclopentadiene complexes canmore » be hydrolyzed with H{sub 2}O/acetone solutions to afford the corresponding 1,3-dienes in essentially quantitative yields.« less