Thermodynamics, Kinetics, and Mechanism of (silox)3M(olefin) to (silox)3M(alkylidene) Rearrangements (silox = tBu3SiO; M = Nb, Ta)

Abstract

Olefin complexes (silox)₃M(ole) (silox = ^tBu₃SiO; M = Nb (1-ole), Ta (2-ole); ole = C₂H₄, C₂H₃Me, C₂H₃Et, C₂H₃C₆H₄-p-X (X = OMe, H, CF₃), C₂H₃^tBu, ^cC₅H₈, ^cC₆H₁₀, ^cC₇H₁₀ (norbornene)) rearrange to alkylidene isomers (silox)₃M(alk) (M = Nb (1 alk), Ta (2 alk); alk = CHMe, CHEt, CHⁿPr, CHCH₂C₆H₄-p-X (X = OMe, H, CF₃ (Ta only)), CHCH₂^tBu, ^cC₅H₈, ^cC₆H₁₀, ^cC₇H₁₀ (norbornylidene)). Kinetics and labeling experiments suggest that the rearrangement proceeds via a δ-abstraction on a silox CH bond by the β-olefin carbon to give (silox)₂RM(κ²-O,C-OSi^tBu₂CMe₂CH₂) (M = Nb (4-R), Ta (6-R); R = Me, Et, ⁿPr, ⁿBu, CH₂CH₂C₆H₄-p-X (X = OMe, H, CF₃ (Ta only)), CH₂CH₂^tBu, ^cC₅H₉, ^cC₆H₁₁, ^cC₇H₁₁ (norbornyl)). A subsequent α-abstraction by the cylometalated “arm” of the intermediate on an α-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.