Date: March 10, 2005
Creator: Hirsekorn, Kurt F.; Veige, Adam S.; Marshak, Michael P.; Koldobskaya, Yelena; Wolczanski, Peter T.; Cundari, Thomas R., 1964- et al.
Description: This article discusses thermodynamics, kinetics, and mechanisms. 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), C₂H3tBu, 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)), CHCH2tBu, cC5H8, cC6H10, cC7H10 (norbornylidene)). Kinetics and labeling experiments suggest that the rearrangement proceeds via a ơ-abstraction on a silox CH bond by the β-olefin carbon to give (silox)2RM(κ2-O,C-OSitBu2CMe2CH2) (M = Nb (4-R), Ta (6-R); R = Me, Et, nPr, nBu, CH2-CH2C6H4-p-X (X = OMe, H, CF3 (Ta only)), CH2CH2tBu, 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.
Contributing Partner: UNT College of Arts and Sciences