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.