The Phosphaalkene-like Reactivity of Dithienophosphinines

2-Thienyldithienophosphinine (3) has been prepared in three steps starting by the ring expansion of dithienophosphole (4) upon reaction with 2-thiophenecarboxylic acid chloride, triethylamine, and water. DFT calculations indicate that the replacement of the phenyl in 2-phenyldithienophosphinine (2) by the 2-thienyl substituent in 3 significantly reduces the HOMO−LUMO gap. Accordingly, 3 reacts cleanly and quantitatively with 2,3-dimethylbutadiene to give the [2+4] cycloadduct 11, whose structure has been confirmed by X-ray crystal structure analysis, whereas 2 reacts slowly and gives the adduct in only 52% yield, and the analogous 2-phenylphosphaphenanthrene (1) gives only an untractable mixture.