Catalyst, additive and counterion effects on the efficiency and enantioselectivity of copper-catalysed C–H insertion reactions of α-diazosulfones

Asymmetric copper-catalysed intramolecular C–H insertion reactions of α-diazosulfones in the presence of various group I salts are reported leading to substantial variation in reaction efficiencies and enantioselectivities. The borate additives NaBARF and KBARF were found to be the most effective additives for permitting highly enantioselective syntheses with short reactions times and high efficiency. Significantly, direct evidence of the critical role of the additive in enantioselective carbenoid reactions has been secured.