Imidazolidine Hydride Donors in Palladium-Catalyzed Alkyne Hydroarylation.

Aldehyde-derived imidazolidines participate as hydride donors in intramolecular reductive Heck-type reactions. N , N '-Diphenylimidazolidines prepared from ortho -alkynyl benzaldehydes underwent regio- and stereoselective palladium-catalyzed hydroarylation followed by formal 1,5-hydride transfer and reductive elimination to afford substituted alkenes and imidazolium moieties, the latter conveniently converted in situ to ring-opened benzanilides to simplify product isolation. Internal alkynes were converted to trisubstituted alkenes via a syn hydroarylation process, while a terminal alkyne was converted to a cis alkene via a formal trans hydroarylation reaction. Benzanilide products could be converted to carboxylic acid derivatives under basic conditions, resulting in the net conversion of alkynyl aldehydes to alkenyl carboxylic acids. A styrene derivative with an attached N , N '-dimethylbenzimidazoline hydride donor was also found to undergo an analogous hydroarylation/benzimidazoline oxidation to give a diarylethane product.