Rhodium complexes containing arylspiroborates derived from 3,5-di-tert-butylcatechol and their use in catalyzed hydroborations

Geier, Michael J. and Geier, Stephen J. and Halcovitch, Nathan R. and Vogels, Christopher M. and Decken, Andreas and Westcott, Stephen A. (2013) Rhodium complexes containing arylspiroborates derived from 3,5-di-tert-butylcatechol and their use in catalyzed hydroborations. Polyhedron, 52. pp. 1181-1189. ISSN 0277-5387

Abstract

Tl(acac) reacts quantitatively with B2butcat3 (butcat = 3,5-di-tert-butylcatecholato) to give butcatB(acac) and Tl(Bbutcat 2) (1), the latter of which is soluble in common organic solvents such as THF and dichloromethane. Addition of 1 to a mixture of [RhCl(coe) 2]2 (coe = cis-cyclooctene) and diphosphine in THF gave the thallium bridged dinuclear complexes [(P2)Rh(μ-Cl) 2(μ-Tl)Rh(P2)][Bbutcat2] (2-5, P2 is dppm = 1,1′-bis(diphenylphosphino)methane, dppe = 1,2- bis(diphenylphosphino)ethane, dppp = 1,3-bis(diphenylphosphino)propane and dppb = 1,4-bis(diphenylphosphino)butane) as the only new rhodium containing species. Unsymmetrical binding of the thallium atom is observed in some cases owing to a secondary interaction with the phenyl rings of the phosphine ligands. Mononuclear cationic species of the type [(P2)Rh(NCCH 3)2][Bbutcat2] (6-9) were formed when reactions were carried out in acetonitrile, where preferential binding of the solvent was observed over the bulky arylspiroborate ligand. The zwitterionic species Rh(η6-butcatBbutcat)(P2), 11 and 12, could be generated via a different route, by addition of B2butcat3 to Rh(acac)(P2), but only for the sterically-constrained diphosphines dppm and dppe, respectively. While all new complexes effectively catalyzed the addition of catecholborane to 4-vinylanisole and α-methylstyrene, product distributions suffered from a competing dehydrogenative borylation pathway.