PHENYL-TO-OXO MIGRATION IN AN ELECTROPHILIC RHENIUM(VII) DIOXO COMPLEX

Reaction of (HBpz(3))ReO(Ph)(OTf) with oxygen atom donors leads to oxi dation of the phenyl group [HBpz(3) = hydrotris(1-pyrazolyl)borate, OT f = triflate, OSO2CF3]. Reaction with Me(2)SO gives the adduct [(HBpz( 3))ReO(Ph)(OSMe(2))]OTf, which undergoes phenyl-to-oxo migration at 25 degrees C to give the phenoxide complex [(HBpz(3))ReO(OPh)(OSMe(2))]O Tf and Me(2)S. The Me(2)SO adduct readily and reversibly loses Me(2)S (k = 2.9(4) s(-1) at 25 degrees C) as indicated by isotope exchange re actions and magnetization transfer. The Me(2)SO adduct also slowly oxi dizes Me(2)SO to Me(2)SO(2). These reactions all proceed via an interm ediate rhenium(VII) dioxo complex, [(HBpz(3))ReO2(Ph)]OTf. This dioxo complex can be observed at low temperature on reaction of (HBpz(3))ReO (Ph)(OTf) with pyridine N-oxide. It rearranges at 0 degrees C by pheny l-to-oxo migration to give phenoxide products and the catecholate comp lex (HBpz(3))ReO(O2C6H4). The kinetics of this migration have been mea sured (Delta H-double dagger = 14.8(7) kcal/mol, Delta S-double dagger = -20.5(25) eu). From these data and the activation parameters for re actions of [(HBpz(3))ReO(Ph)(OSMe(2))]OTf, a detailed free energy surf ace for the reactions of [(HBpz(3))ReO2(Ph)]OTf is constructed. The di oxo complex reacts very rapidly with Me(2)S (1.7 x 10(5) M(-1) s(-1)) with essentially no enthalpic barrier, consistent with the action of a highly electrophilic oxo ligand. Electrophilicity of the oxo groups i s suggested to be a critical factor in facilitating the phenyl-to-oxo migration. A general explanation for the relative ease of various orga nometallic migration reactions, based on analogies with organic [1,2]- shifts, is presented.