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′(OSitBuMe2) (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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry