Ru(0)-Catalyzed Regioselective Synthesis of Borylated-1,4- and -1,5-Diene Building Blocks

A new methodology for preparation of borylated-1,4- and -1,5-diene building blocks is established. Ru(0)-catalyzed cross-dimerization of (1E,3E)-penta-1,3-dien-1-ylboronic acid pinacolate ester (2b) with but-3-en-2-one (3a) selectively gives a borylated-1,4-diene product, (3E,6E)-5-methyl-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)octa-3,6-dien-2-one, in 76% yield. A similar treatment of (E)-penta-1,3-diene (2d) with 2-vinyl-2,3-dihydro-1H-naphtho[1,8-de][1,3,2]diazaborinine (vinyl B(dan)) (3i) also gives a borylated-1,4-diene product. These cross-dimerizations give mono-borylated-1,4-dienes, yet cross-dimerizations using (1E,3E)-penta-1,3-dien-1-ylboronic acid pinacolate ester (2b) with vinyl B(dan) (3i) produce a diborylated-1,5-diene. Selective formation of 1,6-diborylated-1,5-dienes is unprecedented, and a series of diborylated-1,5-dienes can be used as building blocks for Suzuki–Miyaura cross-coupling reactions. As an application of the present method, the formal synthesis of rac-bongkrekic acid, a strong inhibitor of the adenine nucleotide translocator, has been achieved.