Regioselectivity in nickel(0) catalyzed cycloadditions of carbon dioxide with diynes

The regioselectivity of Ni(0)-catalyzed cycloadditions of CO2 (1 atm) with various asymmetrical diynes to afford pyrones was explored. The use of 1,3-bis-(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) provided high regioselectivity when one terminal substituent on the diyne was a methyl group and the other was medium or large in size (RL=i-Pr, t-Bu, or TMS). In contrast, the use of a relatively small N-heterocyclic carbene, 1,3-dimesitylimidazol-2-ylidene (IMes), afforded high selectivity only when RL was large (TMS). X-ray crystal analysis of the major isomer indicated that the relatively large RL group was in the 3-position of the pyrone.