Acylation of N-p-toluenesulfonylpyrrole under Friedel–Crafts conditions: evidence for organoaluminum intermediates

The Friedel–Crafts acylation of N-p-toluenesulfonylpyrrole under Friedel–Crafts conditions has been reinvestigated. Evidence is presented in support of the hypothesis that when AlCl3 is used as the Lewis acid, acylation proceeds via reaction of an organoaluminum intermediate with the acyl halide. This leads to the production of the 3-acyl derivative as the major product. With weaker Lewis acids (EtAlCl2, Et2AlCl) or less than 1 equiv of AlCl3 the relative amount of 2-acyl product is increased. A mechanistic rationalization is presented to explain these data.