STUDIES ON THE INSERTION REACTIONS OF ACTIVATED ALKYNES INTO NB-H BONDS IN HYDRIDE-NIOBOCENE COMPLEXES - X-RAY CRYSTAL-STRUCTURES OF (5)H(4)SIME(3))(2)(H)[ETA(2)-RO(2)C(H)=C(H)CO(2)R] (R=ME OR (T)BU)

The hydride isocyanide complexes, Nb(eta(5)-C(5)H(4)SiMe(3))(2)(H)(CNR ), R=xylyl=2,6-dimethylphenyl; Cy (cyclohexyl); (t)Bu; as well as the hydride carbonyl complex Nb(eta(5)-C(5)H(4)SiMe(3))(2)(H)(CO) react wi th several activated alkynes to afford the corresponding alkenyl isocy anide complexes Nb(eta(5)-C(5)H(4)SiMe(3))(2)(CNR)(C(R')=CH(R '')), R= xylyl, R'=R ''=CO(2)Me 1; R=Xylyl, R'=R ''=CO(2)(t)Bu 2; R=Xylyl, R'=C O(2)Me, R ''=H 3; R=Xylyl, R'=CO(2)Me, R ''=Me 4; R=Cy, R'=R ''=CO(2)M e 5; R=Cy, R'=R ''=CO(2)(t)Bu 6; R=Cy, R'=CO(2)Me, R ''=Me 7; R=(t)Bu, R'=R ''=CO(2)Me 8; (t)Bu, R'=R ''=CO(2)(t)Bu 9 and alkenyl-carbonyl c omplexes, Nb(eta(5)-C(5)H(4)SiMe(3))(2)(CO)(C(R')=CH(R '')), R'=R ''=C O(2)Me 10; R'=R ''=CO(2)(t)Bu 11; R'=CO(2)Me, R ''=H 12. The reaction of hydride isocyanide complexes with activated alkynes gives cis-inser tion resulting in the formation of (E)-alkenyl products. However, the hydride carbonyl derivative undergoes insertion to give, under kinetic control, a mixture of (E)- and (Z)-alkenyl isomers. Finally, the trih ydride niobocene complex Nb(eta(5)-C(5)H(4)SiMe(3))(2)(H)(3), reacts w ith esters RO(2)CC=CCO(2)R' or MeO(2)CC=CH to give the hydride olefin derivatives, )H(4)SiMe(3))(2)(H)[eta(2)-RO(2)C(H)=C(H)CO(2)R'], R=R'=M e 13; R=R'=(t)Bu 14; and a(5)-C(5)H(4)SiMe(3))(2)(H)[eta(2)-MeO(2)C(H) =CH2] 15, probably as the result of a stereospecific trans-insertion. The different complexes have been characterized by spectroscopic metho ds. In addition, the structure of 13 and 14 were determined by single crystal X-ray diffraction.