NUCLEOPHILE PROMOTED DEGRADATION OF CATECHOLBORANE - CONSEQUENCES FOR TRANSITION METAL-CATALYZED HYDROBORATIONS

Reactions of tertiary phosphines or phosphinorhodium complexes with catecholborane (HB(cat)) give boron-substituent redistribution products. While sterically compact phosphines (PMe3, PEt3, PMe2Ph) react with 1 equiv of HBcat to give [(PR3)2BH2]+[B(cat)2]-, bulkier phosphines (PPh3, PPri3, PCY3) afford phosphine-boranes H3B.PR3 and tris(catecholato)diboron, B2(cat)3. Reactions of HB(cat) with coordinatively saturated [RhH(DPPP)2] (DPPP = 1,3-bis(diphenylphosphino)propane) and [RhH(PMe3)4] afforded cationic species [RhH2(DPPP)2]+[B(cat)2]-and [RhH2(PMe3)4]+[B(cat)2]- respectively, both of which are active catalyst precursors for addition of HB(cat) to 4-vinylanisole. These catalyzed hydroborations, however, gave significant amounts of BH3-derived products, arising from Rh-mediated HB(cat) degradation. The only catalyst precursors examined which did not lead to degradation of HB(cat) were rhodium(I) chloride complexes containing basic monodentate phosphines. Molecular structures of B2(cat)3 (3), [(PEt3)2BH2] [B(cat)2] (4b), and [RhH2(PMe3)4] [B(cat)2] (9) were determined by single crystal X-ray diffraction. Colorless crystals of 3 are monoclinic, P2(1)/c (No. 14), with four molecules per unit cell of dimensions a = 12.922(5) angstrom, b = 12.245(2) angstrom, c = 10.784(8) angstrom, and beta = 109.03-degrees. Colorless crystals of 4b are monoclinic, P2(1)/n (No. 14), with eight molecules per unit cell of dimensions a = 17.453(6) angstrom, b = 11.414(l) angstrom, c = 26.666(9) angstrom, and beta = 95.17(2)-degrees. Orange crystals of 9 are trigonal, P3(2)21 (No. 154), with three molecules per unit cell of dimensions a = 13.222(1) angstrom, b = 13.222(l) angstrom, and c = 17.294(6) angstrom.