CYCLIZATION REACTIONS OF OMEGA-TOSYLOXY-1-ALKYNYLBORATES AND OMEGA-TOSYLOXY-1-ALKENYLBORATES AND THEIR OMEGA-HALO ANALOGS

The reaction of trialkylboranes with omega-tosyloxy-1-lithio-1-alkynes can induce transfer of an alkyl group from the boron atom to the alkynyl carbon atom with concomitant formation of four- through six-membered carbocycles via intramolecular displacement of the omega-tosyloxy group. The stereoselectivity of the reaction, however, is low (anti/syn congruent-to 1.6-1.7). The corresponding reaction of omega-halo- or omega-tosyloxy-1-alkenylborates also gives exocyclic alkenes via 1,2-migration-cyclization followed by dehydroboration. In the cases of cyclopropanation, cyclopropylcarbinyl-to-homopropargyl rearrangement rather than dehydroboration takes place. Diphenylzirconocene reacts similarly with 6-lithio-5-hexynyl tosylate to give phenylmethylenecyclopentane in 45% yield On the other hand, attempts to induce a similar migration with phenyl derivatives of Y, V, Cr, and Mn have led to < 5-10% yields of the same cyclization product.