Hybrid hydride elimination

The Heck reaction is a synthetically powerful reaction wherein a carbon-carbon bond is formed between two sp hybridized carbon atoms. The syn nature of the addition of vinyl-aryl palladium species to carbon-carbon double bond precludes a syn p-hydride elimination for cyclic olefins. As a result, a new chiral center is formed. Shibasaki and Vogl provided a comprehensive review of this subject in 1999 440 Overman and Donde reviewed the intramolecular version of this reaction in 2000. Pfaltz and co-authors specifically reviewed this reaction using PhosOx ligands. ... 

The -alkylmetal complexes discussed above are undoubtedly stable because they are coordinatively saturated and, lacking a vacant coordination site, are unable to undergo a facile /S-hybride elimination reaction. Since 8-hydride elimination is the most common way for > -alkylmetal complexes to decompose, coordinatively unsaturated complexes are often considerably less stable. 

Although the Heck reaction mainly involves the formation of bonds between sp -hybridized carbon atoms, there are asymmetric versions. One approach is to capture the chirality of the insertion intermediate by ensuring the p-hydride elimination either occurs away from the site of the original alkene (Scheme 5.35), or is pre-empted by a different step such as another insertion in a tandem process. Another approach is to provide a symmetrical substrate with two enantiotopic alkenes (Scheme 5.36), an approach also used in metathesis chemistry (Section 8.3.6). In all of these reactions, the source of chirality is from the employment of chiral ligands for the palladium catalyst, often chelating bisphosphines. 

The elimination step itself is the reverse of Cossee-type insertion into an Al—H bond. This insertion is much easier than in the Al—C bond, presumably because of the lack of directionality of the hydride Is orbital involved in the reaction (compared to the sp hybrid of an alkyl group). Nevertheless, -elimination has a rather high activation energy because the initial product, a terminal aluminium hydride, is very unfavourable. This initial product can... 

The shortest Ge—H bond yet found in an organogermane occurs in the recently reported complex hydride [(CH3)3Si]2CH 2(OC2H5)GeH (118). The shortness of the Ge—H bond (1.46 A) must, at least in part, arise from the hybridization demands of the electronegative ethoxy group. Distortions in the tetrahedral environment about the germanium (Table I) arise from the steric requirements of the bis(trimethyl-silylmethyl) moieties, while the authors attribute to the same source the stability of this compound with respect to the reductive elimination of alcohol normally observed (127). 

Reactivity from the naphthyl hydride ruthenium complex is believed to occur via initial reductive elimination of naphthalene followed by oxidative addition of a C-H bond. These activation processes (requiring reductive elimination) occur at temperatures of 150°C in alkane or arene solvents, depending on the desired product ". This complex shows general C-H activation behavior with sp, sp, and sp hybridized C-H bonds . ... 

Let us now examine how substituent effects in reactants influence the rates of nucleophilic additions to carbonyl groups. The most common mechanism for substitution reactions at carbon centers is by an addition-elimination mechanism. The adduct formed by the nucleophilic addition step is tetrahedral and has sp hybridization. This adduct may be the product (as in hydride reduction) or an intermediate (as in nucleophilic substitution). For carboxylic acid derivatives, all of the steps can be reversible, but often one direction will be strongly favored by product stability. The addition step can be acid-catalyzed or base-catalyzed or can occur without specific catalysis. In protic solvents, proton transfer reactions can be an integral part of the mechanism. Solvent molecules, the nucleophile, and the carbonyl compound can interact in a concerted addition reaction that includes proton transfer. The overall rate of reaction depends on the reactivity of the nucleophile and the position of the equilibria involving intermediates. We therefore have to consider how the substituent might affect the energy of the tetrahedral intermediate. 

---