Reactions of Catecholborane with Wilkinson's Catalyst: Implications for Transition Metal-Catalyzed Hydroborations of Alkenes

Reactions of catecholborane (HBO₂C₆H₄) with RhCl(PPh₃)₃ (1) yield a variety of products depending on the B/Rh ratio, solvent, and temperature. Of particular relevance to catalyzed alkene hydroboration is degradation of HBO₂C₆H₄ to B₂(O₂C₆H₄)₃/‘BH₃’ and the dihydride RhH₂Cl(PPh₃)₃ (3). The molecular structure of 3, determined by X-ray diffraction, has meridional phosphine ligands and cis hydrides. Catalyst systems formed from in situ addition of PPh₃ to [Rh(μ-Cl)(COD)]₂ (COD = 1,5-cyclooctadiene) are fundamentally different from Wilkinson's catalyst; RhCl(COD)(PPh₃) forms initially, but the reaction of this with PPh₃ is slow. Monitoring catalyzed hydroborations using Wilkinson's catalyst and catecholborane by multinuclear NMR spectroscopy, prior to oxidative workup, showed that alkylboranes were formed with some sterically hindered alkenes. With 2-methylbut-2-ene (24), for example, we observed significant quantities of disiamylborane, (CHMeCHMe₂)₂, formed via addition of ‘BH₃’ to 24. When excess PPh₃ was added to the catalyst system, however, the desired alkylboronate ester was formed in high yield. Partial oxidation of RhCl(PPh₃)₃ had a significant effect on product (and D-label) distributions. Detailed investigations of catalyzed additions of DBO₂C₆H₄ to allylic silyl ethers CH₂=C(Me)CRR′(OSi^tBuMe₂) (R, R′ = H, Me) demonstrated that deuterium incorporation at the carbon bonded to boron in the primary alcohol product occurs only with freshly prepared Wilkinson's catalyst or when excess PPh₃ is added to the oxidized catalyst. With freshly prepared Wilkinson's catalyst, addition of H₂ (or D₂) to these substrates is a significant competing reaction and appreciable catalytic formation of vinylboronate esters is also observed. The latter presumably arise via insertion of alkene into a Rh-B bond, followed by β-hydride elimination. Subsequent in situ addition of H₂ (DH or D₂) to these vinylboronate esters provides an alternative explanation to α-deuterium incorporation into the resulting primary alcohols.