Oxidative coupling involving carbanions

E. Oxidative Coupling 19-32 Coupling Involving Carbanions De-hydro,chloro-coupling... 

Work in the mid-1970s demonstrated that the vitamin K-dependent step in prothrombin synthesis was the conversion of glutamyl residues to y-carboxyglutamyl residues. Subsequent studies more cleady defined the role of vitamin K in this conversion and have led to the current theory that the vitamin K-dependent carboxylation reaction is essentially a two-step process which first involves generation of a carbanion at the y-position of the glutamyl (Gla) residue. This event is coupled with the epoxidation of the reduced form of vitamin K and in a subsequent step, the carbanion is carboxylated (77—80). Studies have provided thermochemical confirmation for the mechanism of vitamin K and have shown the oxidation of vitamin KH2 (15) can produce a base of sufficient strength to deprotonate the y-position of the glutamate (81—83). 

In contrast to the transition metals, where there is often a change in oxidation level at the metal during the reaction, there is usually no change in oxidation level for boron, silicon, and tin compounds. The synthetically important reactions of these three groups of compounds involve transfer of a carbon substituent with one (radical equivalent) or two (carbanion equivalent) electrons to a reactive carbon center. Here we focus on the nonradical reactions and deal with radical reactions in Chapter 10. We have already introduced one important aspect of boron and tin chemistry in the transmetallation reactions involved in Pd-catalyzed cross-coupling reactions, discussed... 

The anodic oxidation of a-sulfonyl carbanions is shown to involve a coupling reaction between the electrogenerated radical and the parent anionic species. The relevance of this mechanism to transition metal oxidations of ot-sulfonyl carbanions is examined [224d]. 

Transmethylation by Cobalamins.— Two mechanisms of methyl transfer from cobalamins are found, a homolytic process with metals that have reduction potentials less than 0.50 V and an electrophilic (carbanion heterolytic) process for metals with potentials greater than 0.85 V. The PF /ii couple is 0.76 V and both oxidation states are required for methylation. A mechanism is proposed in which [Ptcy " binds through the propionamide and acetamide side-chains of the corrin thereby promoting formation of the base on form and activating the Co— C bond. The methylation step involves [PtCIe] " either in a redox switch where two electrons are abstracted from bound Pt followed by methyl transfer to the then bound Pt v or by direct electrophilic attack by [PtClel " ... 

Many C(sp )—C(sp ) coupling reactions not transition metal-mediated involve the two electron oxidation of ethers or amines to oxocarbenium ions or iminium ions. These are then attacked in an ionic mechanism by in i dM-formed enolates or other carbanion nucleophiles to generate product. As such, they will not be covered in this review. 

Three important concurrent reactions involving electron transfer may occur in ATRP (Figure 8.22 E is the redox potential of the couple shown in parentheses) (i) disproportionation of the ATRP (usually Cu -based) activator, (ii) oxidation or reduction of organic radicals to carbocations or carbanions, respectively, and (iii) formation of organometallic species via a reaction between radicals and the (usually lower oxidation state) complexes catalyzing the ATRP process. 

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