Elimination and addition reactions. Part 28. Nucleophilic addition–displacement reactions with allenic sulphonium salts

Abstract

Addition of enolate ions stabilised by conjugative groups to allenic sulphonium salts has been investigated. Addition of a cyano-stabilised enolate leads to a cyano-furan but heterocyclic products are not obtained when the stabilising group is dimethylsulphonio or nitro. In each case, alternative reactions which do not involve the allenic sulphonium salt are preferred. ¹³C Labelling studies show that in the formation of furans where the cyclisation step can involve direct substitution (S_N) or addition-elimination (S_N′), the former is exclusively preferred.

Addition of bidentate sulphur nucleophiles in the presence of proton donors gives mixtures of cyclic and acyclic products which result from bis-addition or addition–displacement sequences.

Addition of cyanide and arenesulphinate ions to allenic sulphonium salts gives initial adducts which are activated towards further addition, and eventual displacement of the sulphonium group yields products derived from incorporation of three of the nucleophilic species into the original allene chain. ¹³C Labelling and structural studies of the arenesulphinate reactions show that S_N′ reactions occur very rapidly and lead to equilibration of the α and γ termini of the allene systems.