257870-19-2Relevant academic research and scientific papers
Thermodynamics, kinetics, and mechanism of (silox)3M(olefin) to (silox)3M(alkylidene) rearrangements (silox = tBu 3SiO; M = Nb, Ta)
Hirsekorn, Kurt F.,Veige, Adam S.,Marshak, Michael P.,Koldobskaya, Yelena,Wolczanski, Peter T.,Cundari, Thomas R.,Lobkovsky, Emil B.
, p. 4809 - 4830 (2007/10/03)
Olefin complexes (silox)3M(ole) (silox = tBu 3SiO; M = Nb (1-ole), Ta (2-ole); ole = C2H4, C2H3-Me, C2H3Et, C2H 3C6H4-p-X (X := OMe, H, CF3), C 2H3tBu, cC5H8, cC6H10, cC7H 10 (norbornene)) rearrange to alkylidene isomers (silox) 3M(alk) (M = Nb (1=alk), Ta (2=alk); alk = CHMe, CHEt, CH nPr, CHCH2C6H4-p-X (X = OMe, H, CF3 (Ta only)), CHCH2tBu, cC 5H8, 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-OStBu2CMe 2CH2) (M = Nb (4-R), Ta (6-R); R = Me, Et, nPr, nBu, CH2-CH2C6H4-p-X (X = OMe, H, CF3 (Ta only)), CH2CH2tBu, cC5H9, cC6H 11, cC7H11 (norbomyl)). 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.
(Silox)3Nb(η2-N,C-4-picoline) as a source of (silox)3Nb: Structure of [(silox)3Nb]2(μ-η2,η 2-C6H6) (silox=tBu3SiO)
Veige, Adam S.,Kleckley, Troy S.,Chamberlin, Rebecca M.,Neithamer, David R.,Lee, Christina E.,Wolczanski, Peter T.,Lobkovsky, Emil B.,Glassey, Wingfied V.
, p. 194 - 203 (2007/10/03)
Electrochemical and chemical reductions of (silox)3NbCl2 (1, -1.42 V versus Ag/Ag+) and (silox)3TaCl2 (2, -2.18 V) generated [(silox)3NbCl2]- (1-) and [(silox)3TaCl2]- (2-). The anions were subject to Cl- loss, leading to (silox)3NbCl (3) and probable disproportionation (Ta). Complex 3 formed reversible solvent adducts (silox)3ClNbL (3-L, L=THF, py). Na/Hg reduction of (silox)3NbCl2 (1) in the presence of 4-picoline afforded (silox)3Nb(η2-N,C-4-NC5H4CH 3) (6), which can be considered a source of '(silox)3Nb', analagous to (silox)3Ta (4). Adducts such as (silox)3Nb(η2-H2CCHR) (R=H, 7; Ph, 8) may be formed upon addition of olefins, and 6 abstracted an oxygen from N2O, NO or ethylene oxide to give (silox)3Nb=O (9) and N2 or C2H4, respectively, which are reactions also observed for 4. 4-Picoline may be abstracted from 6 by 4 to give {(silox)3Nb}2(μ:η2,η 2-C6H6) (11) and (silox)3Ta(η2-N,C-4-NC5H4CH 3) (12). An X-ray structure determination of 11 revealed a disordered bridge that was modeled as a μ:η2,η2-benzene. Extended Hueckel molecular orbital (EHMO) calculations showed that the μ:η2,η2-configuration was energetically better than a μ:η6,η6-alternative, but about the same as a plausible μ:η3,η3-arrangement.
