Dimethyl Acetylenedicarboxylate and Phospholes: A Variety of Reaction Pathways

Correcting an earlier report, the reaction of 1‐phenyl‐3,4‐dimethylphosphole ( 1a ) with an excess amount of dimethyl acetylenedicarboxylate (DMAD) at room temperature affords the stable cyclopentadienylidenephosphorane 3a resulting from unexpected deoxygenation of initially formed [phosphole + 2DMAD] adduct 2a by the starting phosphole. The identity of 3a was established by X‐ray crystal structure analysis. DFT calculations on a model [1+2] adduct similar to 2a shows a bent allenic structure and an ambident reactivity for the terminal carbon of the (DMAD) 2 chain. Cyclopentadienylidenephosphorane 3b is also obtained with 1‐benzylphosphole 1b . With 1‐stannylphosphole 1c , a 1:3 adduct with a seven‐membered ring (i.e., 5 ) is obtained. It was also characterized by X‐ray crystal structure analysis. Finally, with 1‐benzyl‐2‐benzoylphosphole 1d , the P lone pair has lost its nucleophilicity and a [4+2] cycloaddition takes place with the dienol tautomer of the unsaturated ketone to give phosphindole 6 , also characterized by X‐ray.