Regioselectivity of Intramolecular Rhodium-Catalyzed C–H Insertion Reactions of α-Aryl-α-diazocarboxylates: Influence of the Aryl Substituent

It was found that α-aryl-α-diazocarboxylates react with variable regioselectivity in intramolecular rhodium-catalyzed C–H insertion reactions. 3-(Trialkoxysilyl)propyl esters underwent a clean cis-γ-lactone formation (62–69%) if the aryl rest was a phenyl group while the analogous α-(2-bromophenyl)-α-diazocarboxylates produced the respective β-lactones. In general β-lactone formation was shown to be the only detectable C–H insertion pathway for the latter substrate class irrespective of the ester substituent. The β-lactone products (eight examples) were obtained in yields of 41–68% with a diastereomeric ratio (dr) of 75:25 to 96:4 in favor of the trans diastereoisomer. The 2-bromo substituent could be displaced by an aryl group in a subsequent Suzuki cross-coupling reaction.