Mechanistic Investigation of the Oxidation of Aromatic Alkenes by Monooxoruthenium(IV). Asymmetric Alkene Epoxidation by Chiral Monooxoruthenium(IV) Complexes

The oxoruthenium(IV) complexes [RuIV(terpy)(6,6′-Cl2-bpy)O](ClO4)2(1a; terpy = 2,2′:6′,2″-terpyridine; 6,6′-Cl2-bpy = 6,6′-dichloro-2,2′-bipyridine), [RuIV(terpy)(tmeda)O](ClO4)2(lb; tmeda = N,N,N′,N′-tetramethylethylenediamine), tRuIV(Cn)(bpy)O](ClO4)2(1c; Cn = l,4,7-trimethyl-l,4,7-triazacyclononane), and [RuIV(PPz*Xbpy)O](ClO4)2(Id; PPz* = 2,6-bis[(4S,7A)-7,8,8-trimethyl4,5,6,7-tetrahydro-4,7-methanoindazol-2-yl] pyridine) are effective for the epoxidation of aromatic alkenes in acetonitrile at ambient conditions. Their reactions with c/s-alkenes such as cis-β-methylstyrene and cis-β-deuteriostyrene afford epoxides nonstereospecifically. The observation of the inverse secondary kinetic isotope effect for the β-d2-styrene oxidations [kH/kD-0.87 (1b), 0.86 (Id)], but not for α-deuteriostyrene (kH/kD= 0.98 for lb and Id), indicates that C-O bond formation is more advanced at the β-carbon atom than at the a carbon, i.e., a stepwise mechanism. The second-order rate constants (k2) for the styrene oxidations are weakly dependent on the E°(RuIV/III) values of the oxoruthenium(IV) complexes, and both electron-withdrawing and -donating para substituents mildly accelerate the oxidation reaction of styrene. These findings discount strongly the intermediaries of an alkene-derived cation radical and a carbocation. A linear free-energy relationship between the second-order rate constants for the para-substituted styrene oxidations and the total substituent effect (TE) parameters has been established: (TE) = +0.43 (R = 0.99) for 1b, +0.50 (R = 0.98) for 1c, and +0.37 (R = 0.99) for Id (Wu, Y.-D.; Wong, C.-L.; Chan, K. W.; Ji, G.-Z.; Jiang, X.-K. J. Org. Chem. 1996, 61, 746). This suggests that the oxidation of aromatic alkenes by oxoruthenium(IV) complexes should proceed via the rate-limiting formation of a benzylic radical intermediate. Oxidation of styrene and cis- and trans-β-methylstyrenes by the chiral oxoruthenium (IV) complex Id attains moderate enantioselectivities, in which the production of cis-epoxide is more enantioselective than the trans counterpart. The ligand dissymmetry of PPz* together with the bipyridine ligand create a "chiral pocket" around the RuIV=O moiety, leading to enantiofacial discrimination through nonbonding interaction. Because the acyclic benzylic radical intermediate would undergo cis-trans isomerization before the second C-O bond formation, the overall product enantioselectivity (% eeobs) cannot be determined exclusively by facial selectivity (eefacial) of the first irreversible C-O bond formation step. The extent of the isomerization, measured by the cis-trans-epoxide selectivity or diastereoselectivity of epoxide ring closure, is an important element in controlling the enantiomeric excess of the epoxides.