trans-Alkylative Cyclization of Alkynals and Alkynones

Significance

This is the first example of palladium/monophosphine-catalyzed trans-addition coupling of alkynes, carbonyls and organoboronic reagents. While the cis-addition counterpart has been reported, this equally important version has remained unattainable. Both ketone and aldehyde functionality are tolerated with the former requiring longer reaction times, but not suffering lower yields. Aryl boronic acids give high yields of products including substituted and heteroaryl groups. The reaction also works with trialkylboranes, which importantly do not undergo competitive β-hydride elimination. The authors also discovered that internal alkynes were not reactive with Pd(PPh₃)₄, but that the use of the more electron-rich Pd(PCy₃)₄ gave a mixture of 5- and 6-membered ring products.