Intermolecular Hydroacylation of Cyclopropenes

Significance

Following up on their recently reported intramolecular hydroacylation of electron-neutral olefins (J. Am. Chem. Soc. 2009, 131, 14190), the Glorius group reports an intermolecular version of this reaction. In the presence of the triazolium salt 1 derived NHC, aromatic aldehydes 2 reacted with cyclopropenes 3 to give the desired hydroacylation products 4 in high yields and generally high diastereoselectivities. Mechanistic studies support a scenario in which the Breslow intermediate reacts kinetically favored but reversible with another equivalent of 2 to form the benzoin product. The hydroacylation, which is suggested to be a concerted process, proceeds slower but therefore irreversibly, eventually leading to high yields of the products 4.