R3O+BF4-: Meerwein's Salt

Abstract

(A) Thioethers can be quantitatively transformed into their corresponding sulfonium salts, which display increased leaving group ability properties. In the depicted example below, deprotonation of a ketosulfonium salt leads to the sulfonium ylide, which then undergoes a highly diastereoselective epoxy-annulation. In this case epoxidation of a corresponding alkene with MCPBA gives only a 3:1 mixture of diastereomers.6 (Equation Presented) (B) O-Alkylation of amides7 and S-alkylation of thioamides8 leads to iminoesters which are much more reactive than their corresponding amide towards nucleophiles. As an example Meldrum's acid undergoes a condensation reaction with an iminoester species. In Danishefsky's total synthesis of indolizomycin, O-alkylation of a vinylogous amide was sucessfully achieved. The iminium species obtained was then reduced by NaBH4. A McCluskey type fragmentation led to the nine-membered ring of the natural compound.9 (Equation Presented) (C) Carbonic acids can be esterified with Meerwein's salt in aqueous media in the presence of proton acceptors. Under these conditions O-alkylation of amides doesn't occur.10 In non-protic media without addition of base, only O-alkylation is observed and the carboxyl group is not esterified.11 (Equation Presented) (D) One important method to form Fisher carbene complexes utilizes the methylating properties of Me3O+BF4-. They are made by one-pot reactions of suitable nucleophiles (e.g. alkyl, aryl, alkynyllithium, or lithiumdialkylamide) with chromiumhexacarbonyl and subsequent O-alkylation with Meerwein's salt, furnishing alkoxy-carbene complexes. In the depicted example a Fisher carbene complex undergoes a unique benzannulation reaction known as the 'Doetz reaction'.12 (Equation Presented).