Thermodynamics, kinetics, and mechanism of (silox)3M(olefin) to (silox)3M(alkylidene) rearrangements (silox = tBu3SiO; M = Nb, Ta)

Olefin complexes (silox)(3)M(ole) (silox = (Bu3SiO)-Bu-t; M = Nb (1-ole), Ta (2-ole); ole = C2H4, C2H3-Me, C2H3Et, C2H3C6H4-p-X (X = OMe, H, CF3), (C2H3Bu)-Bu-t, (C5H6)-C-c, (C6H10)-C-c, (C7H10)-C-c (norbornene)) rearrange to alkylidene isomers (silox)(3)M(alk) (M = Nb (1=alk), Ta (2=alk); alk = CHMe, CHEt, (CHPr)-Pr-n, CHCH(2)C(6)H(4)p-X (X = OMe, H, CF3 (Ta only)), (CHCH2Bu)-Bu-dagger, (C5H8)-C-c, (C6H10)-C-c, (C7H10)-C-c (norbornylidene)). Kinetics and labeling experiments suggest that the rearrangement proceeds via a delta-abstraction on a silox CH bond by the beta-olefin carbon to give (silox)(2)RM(kappa(2)-O,C-(OSiBu2CMe2CH2)-Bu-t) (M = Nb (4-R), Ta (6-R); R = Me, Et, Pr-n, Bu-n, CH2-CH2C6H4-p-X (X = OMe, H, CF3 (Ta only)), (CH2CH2Bu)-Bu-t, (C5H9)-C-c, (C6H11)-C-c, (C7H11)-C-c (norbornyl)). A subsequent a-abstraction by the cylometalated arm of the intermediate on an alpha-CH bond of R generates the alkylidene 1=alk or 2=alk. Equilibrations of 1-ole with ole' to give 1-ole' and ole, and relevant calculations on 1-ole and 2-ole, permit interpretation of all relative ground and transition state energies for the complexes of either metal.