1,3-Steric Induction in Intermolecular Radical Reactions Mediated by a Co2(CO)6-Metal Core

Diastereoselectivity of propargyl coupling reactions can be controlled by using the bulkiness of a gamma-substituent as a stereochemical tool. This 1,3-steric induction was observed with gamma-t-Bu and gamma-Me3Si groups, both favoring a d,l-configuration of the head-to-head coupling products, 3,4-disubstituted 1,5-alkadiynes (d,l- 95-100%). X-ray crystallography analysis suggests that the most favorable orientation of converging propargyl radicals is the one in which the bulky gamma-substituents are positioned anti to each other. Overall, the synthetic strategy of employing a Me3Si auxiliary group involves five steps and affords, with 28-33% overall yields, pure d,l-3,4-diaryl-1,5-hexadiynes, otherwise hardly accessible.