Synthesis of Internal gem-Diborylalkanes by Copper-Catalyzed Double Hydroboration of Conjugated Internal Alkynes

Background: Geminal diboryl reagents have attracted considerable attention due to their unique reactivities. Double hydroboration of alkynes is a convenient and useful method for the synthesis of diborylalkanes. However, the double hydroboration is applied mainly to terminal alkynes. Methods: Ethyl propiolate was treated with B2pin2 and methanol in the presence of various copper catalysts and ligands. The reaction of several internal conjugated alkynes was also investigated under the optimized reaction conditions. Results: Geminal diboryl compounds were obtained in high yields. While 1,10-phenanthroline was more effective for the reaction of methyl propiolate than tri-n-butylphosphine, the divided addition of B2pin2 and the use of tri-n-butylphosphine were necessary for the reaction of several alkynes, for which second hydroboration was slower. Conclusion: We have found that the copper-catalyzed double hydroboration of conjugated internal alkynes was a useful method for the synthesis of internal gem-diborylalkanes.