One of the most versatile methods for adding a carbon-carbon bond to an aromatic ring is Friedel-Crafts alkylation, in which a Lewis acid (such as AlCl3) catalyzes the addition of an electron-deficient s- or p-bonded compound. However, most variants of this venerable reaction are not stereoselective when adding more complex unsaturated species, such as a,b-unsaturated aldehydes.
Paras and MacMillan now show that reaction of pyrroles with a,b-unsaturated aldehydes in the presence of a chiral amine (a benzyl imidazolidinone) and acid co-catalysts leads to stereospecific addition (typically 90% enantiomeric excess) to the C=C bond. This pathway is favored over the conventional acid-catalyzed pathway that would add to the C=O bond, and the benzyl group helps direct the pyrrole to adding to the si-face. The reaction is quite tolerant of substituents on the pyrrole ring as well as the N-alkyl group, and of various terminal groups on the aldehyde. — PDS
J. Am. Chem. Soc., in press.