Reaction rate Reductive-elimination reactions

A. Gillie and J. K. Stille, J. Am. Chem. Sac.. 1980, 102, 4933. Rates of other reductive elimination reactions are also reported in this reference. 

In general, in the case of an S 2 C-0 reductive elimination reaction with the rate limiting step a i (Fig. 4), the presence of a different nucleophilic species in the reaction mixture in sufficient concentrations can lead to elimination of the product R-Y which results from an interception of the intermediate 4 by Y (4) ... 

As both nucleophiles, Z and Y , can participate in a reductive elimination reaction, a mixture of reaction products, R-Z and R-Y, forms. Such behavior is observed in the Shilov reaction (Fig. 2). Formation of a mixture of methanol and methyl chloride is considered in Shilov chemistry as a result of occurrence of two concurrent S 2 processes with water and chloride anion as competing nucleophiles [18, 19]. The rate of the formation of chloromethane in aqueous solutions of [MePt Clsl in this reaction was shown to be first order in chloride ion concentration. 

Many factors control the rates of reductive elimination reactions (Equation 8.4). In many cases, the effects of the steric and electronic properties of the metal complex on the rate of reductive elimination are the opposite of the effects of these properties on the rate of oxidative addition because the effects originate from thermodynamic factors. The steric and electronic properties of the metal ttiat thermodynamically favor oxidative addition must thermodynamically disfavor the opposite reductive elimination reaction. 

On the other hand, the nature of the ancillary L ligands is believed to have an important effect on the rate of the reductive elimination reaction. For example, the... 

Recent tlieotetical studies of reductive elimination ftoni Me- Cu-S in tlie presence of BF suggest tliat reaction rate of tlie conjugate addition can inctease if one of tlie Me groups is detadied ftoni tlie coppetflll) to bind witli a hot on atom rsdienie I0.I2)[I29],... 

The mechanism of carbometallation has been explored computationally.77 The reaction consists of an oxidative addition to the triple bond forming a cyclic Cu(m) intermediate. The rate-determining step is reductive elimination to form a vinyl magnesium (or zinc) reagent, which then undergoes transmetallation to the alkenyl-copper product. 

Reductive elimination, though usually not a rate-limiting step, may seriously affect the reaction outcome, particularly in those cases when side reactions may occur in the intermediate complex, e.g., Pd hydride elimination in the case when alkyl groups are involved, leading to the loss of regioselectivity. 

There are distinct advantages of these solvent-free procedures in instances where catalytic amounts of reagents or supported agents are used since they provide reduction or elimination of solvents, thus preventing pollution at source . Although not delineated completely, the reaction rate enhancements achieved in these methods may be ascribable to nonthermal effects. The rationalization of microwave effects and mechanistic considerations are discussed in detail elsewhere in this book [25, 193]. A dramatic increase in the number of publications [23c], patents [194—203], a growing interest from pharmaceutical industry, with special emphasis on combinatorial chemistry, and development of newer microwave systems bodes well for micro-wave-enhanced chemical syntheses. 

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