Catalytically Generated Meerwein's Salt-Type Oxonium Ions for Friedel-Crafts C(sp2)-H Methylation with Methanol.

Abstract

A catalytic protocol for a Friedel-Crafts-type direct C(sp2)-H methylation of various arenes with methanol is disclosed. The reaction is initiated by counteranion-stabilized silylium or arenium ions, which form Meerwein's salt-like oxonium ions with methanol as the active methylating agents. The silylated methyloxonium ions are stronger electrophiles than their protonated congeners, allowing the Friedel-Crafts alkylation to proceed more efficiently and at a lower reaction temperature. The regeneration of these superelectrophiles within the catalytic cycle is accomplished by the addition of a tetraorganosilane additive, i.e., trimethyl(phenyl)silane or tetraethylsilane, that releases a silylium ion through protodesilylation by the Brønsted acidic Wheland intermediate, thereby acting as a productive "proton-into-silylium ion" generator. By this method, even the C-H methylation of electronically deactivated aryl halides with methanol is achieved. The protocol is also applicable to nonactivated primary as well as π-activated benzylic alcohols. Dialkyl ethers are also competent alkylating agents in the presence of the quaternary phenylsilane additive.