Vinylic C--H borylation of cyclic vinyl ethers with bis(pinacolato)diboron catalyzed by an Iridium(I)-dtbpy complex.

Borylation of the vinylic C--H bond of 1,4-dioxene, 2,3-dihydrofuran, 3,4-dihydro-2H-pyran and their gamma-substituted analogs was carried out in the presence of bis(pinacolato)diboron (B(2)pin(2)) and a catalytic amount of Ir(I)-dtbpy (dtbpy=4,4'-di-tert-butyl-2,2'-bipyridine) complex. The two boron atoms in B(2)pin(2) participated in the coupling, thus giving two equivalents of the coupling product from one equivalent of B(2)pin(2). The borylation of 1,4-dioxene in hexane resulted in 81 % yield at room temperature. The borylation of 2,3-dihydrofurans at 80 degrees C in octane suffered from low regioselectivity, and gave a mixture of alpha- and beta-coupling products even for hindered gamma-disubstituted analogs, but gamma-substituted analogs of 3,4-dihydro-2H-pyran achieved high alpha-selectivity, giving single coupling products. This protocol was applied to the syntheses of a key precursor of vineomycinone B2 methyl ester and other C-substituted D-glucals by borylation of protected D-glucals with B(2)pin(2) to give alpha-boryl glucal followed by cross-coupling with haloarenes, benzyl bromide, and allyl bromide. A catalytic cycle that involves the oxidative addition of sp(2) C--H bond to iridium(III)-trisboryl intermediate as the rate-determining step has been proposed.