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-1-dtbpy (dtbpy = 4,4'-di-tert-butyl-2,2'-bi-pyridine) 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-disubstutued 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.