Generation and tandem reactions of 1-alkenyl-1,1-heterobimetallics: Practical and versatile reagents for organic synthesis

A practical and straightforward method for generation of versatile 1-alkenyl-1,1-heterobimetallic intermediates and their application to construction of functionalized building blocks are disclosed. Beginning with readily available air-stable 1-alkynyl-1-boronate esters, hydroboration with dicyclohexylborane generates 1-alkenyl-1,1-diboro species. In situ transmetallation with dialkylzinc reagents furnishes 1-alkenyl-1,1-heterobimetallic intermediates. Direct treatment with aldehydes followed by workup allows isolation of B(pin)substituted allylic alcohols in 70-95% yield. The B(pin)-substituted allylic alcohols react with NBS to afford (E)-alpha,beta-unsaturated aldehydes in 51-77% yield via a semipinacol-type rearrangement. In situ treatment of 1-alkenyl-1, 1-heterobimetallic intermediates with aldehydes followed by TBHP oxidation enables the preparation of a-hydroxy ketones. Under optimized conditions, addition of 1-alkenyl-1,1-heterobimetallic intermediates to a variety of protected alpha- and beta-hydroxy aldehydes proceeds with good to excellent control over diastereoselectivity to furnish differentially protected dihydroxy ketones. The 1-alkenyl-1,1-heterobimetallic intermediates have also been employed in tandem aldehyde addition/Suzuki cross-coupling reactions to provide densely functionalized allylic alcohols in good to excellent yields.