METHYL TRANSFERS .14. NUCLEOPHILIC CATALYSIS OF NUCLEOPHILIC-SUBSTITUTION

Nucleophiles X- can catalyze the substitution Nu- + RY → NuR + Y- by adding the faster pathway X- + RY → XR + Y- followed by Nu- + XR → RNu + X-. New examples include catalysis by I- of the exchange of methyl between two dialkyl sulfides and the transfer of methyl from an arsonium salt to a phosphine. TThe individual reactions are separately studied, and some equilibrium information is presented. Iodide is ineffective in the transfer of methyl between two phosphines, which is not detected with or without iodide. The Marcus equation treatment of this catalysis is shown to require that the identity transfer of R between two X- groups be far faster than that for transfer of R between two Nu- groups. Nucleophiles other than I- are discussed. The possibility that some “supernucleophiles” may have fast identity rates is discussed, and literature evidence that this is indeed the case is presented. Stereochemical studies using chiral methyl derivatives have shown that vitamin B12 does provide a nucleophilic catalysis to methyl transfer in living systems. Thus, the apparently superfluous participation of B12 in some biological methyl transfers is explained. © 1990, American Chemical Society. All rights reserved.