Bimetallic Base Has Proton-Pulling Power

Prying the hydrogen atom away from an alkyl or vinyl C–H bond can take considerable effort. Now, a research team led by Robert E. Mulvey of Scotland’s University of Strathclyde has developed a bimetallic base that’s capable of pulling hydrogens off the cyclic ethers tetrahydrofuran (THF) and tetrahydropyran, as well as ethene (Science 2009, 326, 706). The inherent difficulty in deprotonating THF with a strong base, such as an organolithium reagent, is that the cyclic ether tends to decompose via spontaneous ring opening. Mulvey and coworkers reasoned they could overcome this problem by combining sodium with zinc in the same reagent (shown). Their base is a mixed complex of three compounds in which zinc bonds to a THF carbon and sodium bonds to THF’s oxygen. The metals form a stabilizing five-membered ring that includes a nitrogen bridge courtesy of 2,2,6,6-tetramethyl­piperidide. Swapping sodium for potassium renders the base capable of a rare ethene deprotonation. In terms of reactivity, the researchers were able to use the “zincated” THF complex as a nucleophile in reactions with benzoyl chloride. They plan to continue testing the bimetallic bases with other electrophiles.