Bi(III) Catalysed O-acylative cleavage of 2,5-dimethyltetrahydrofuran: a substrate dependent borderline mechanism

The Bi(III) catalysed O-acylative cleavage of cis- and trans-2,5-dimethyltetrahydrofuran with AcCl, BzCl or i-PrCOCl is stereochemically consistent with the operation of a concerted process (ANDN), which proceeds via a stabilised carbocation or ‘loose’ SN2 transition state. However, the O-acylative cleavage of cis-2,5-dimethyltetrahydrofuran with sterically demanding electrophiles such as t-BuCOCl, appears to be stereochemically consistent with the alternative SN1 (DN+AN) pathway. The apparent merging of mechanistic pathways is rationalised by the participation of a strained acyloxy cation.