Kinetics and mechanism of the insertion of olefins into niobium- and tantalum-hydride bonds: a study of the competition between steric and electronic effects

Abstract

The mechanism of insertion of olefins into transition metal-hydride bonds has been examined for a series of olefin hydride complexes of niobocene, permethylniobocene, tantalocene, and permethyltantalocene. The insertion rates were measured for several meta-substituted styrene complexes of permethylniobocene and found to be comparable to those found for the para-substituted analogues, indicating that the electronic effects are largely inductive in origin. An X-ray crystal structure determination for the parent styrene complex (Pka; a = 14.626 (3) A, b = 18.134 (4) A, c = 18.147 (4) A; a, @, y = 90°) offers an explanation for this observation: the phenyl ring is bent out of resonance with the olefin ?r system by 32'. A comparison of the rates for the entire series of ethylene compounds (where steric effects are minimal) indicates that there are significant electronic effects acting within this family (kNb > k,, and kcp, > kCp*; Cp = (q5:C5H5), Cp* = ($-C5Me5)). In contrast to endo exo Insertion of olefin into the Nb-H bond ( k , ) is reversible, and a lower limit for kl (>lookl) can be estimated (eq 1) from the undetectably small concentrations ('H N M R ) of the alkyl tau-(1) (a) Halpcrn, J.; Okamoto, T.; Zakchariev, A. J. Mol. Catal. 1976, 2, 65-68. (b) Byme, J. W.; Blaser, H. U.; Osborn, J. A. J. Am. Chem. SOC. 1975. 97, 3871-3873. (c) Chaudret, B. N.; Cole-Hamilton, D. J.; Wilkinson, G.

Peculiarities of Ethylene Polymerization Reactions with Heterogeneous Ziegler-Natta Catalysts: Kinetic Analysis

“…The reverse reaction in Equation (4) represents ethylene insertion into the Ti1H bond. This is the "standard" chain initiation reaction; its rate constant, k E i , is usually assumed to be equal to or higher than the propagation rate constant k E p in Equation (6) [29,30] Chain Initiation Reactions Involving Ethylene and a-Olefin…”

On the insertion processes of unsaturated molecules into the Nb–X σ-bond of moieties (Cp′=η5-C5H4SiMe3; X=H, C, P)

“…The insertion of olefins into metal-hydride bonds [16] and its microscopic reverse, b-elimination, are very common steps in industrial catalytic reactions such as olefin isomerization, hydrogenation, and hydroformylation.…”

Synthesis of [Ni(η2-CH2C6H4R-4){PPh(CH2CH2PPh2)2}]+ (R=H, Me or MeO) and protonation reactions with HCl

“…been observed that electron-releasing 4-R-substituents accelerate migration [14], presumably by stabilizing the modest accumulation of positive charge at C b in the transition state for the migration.…”