Mechanistic investigation of the oxidation of aromatic alkenes by monooxoruthenium(IV). Asymmetric alkene epoxidation by chiral monooxoruthenium(IV) complexes

'The oxoruthenium(IV) complexes [Ru-IV(terpy)(6,6'-Cl-2-bpy)O](ClO4)(2) (1a; terpy = 2,2':6',2 "-terpyridine; 6,6'-Cl-2-bpy = 6,6'-dichloro-2,2'-bipyridine), [Ru-IV(terpy)(tmeda)O](ClO4)(2) (1b; tmeda = N,N,N',N'-tetramethylethylenediamine), [Ru-IV(Cn)(bpy)O](ClO4)(2) (1c; Cn = 1,4,7-trimethyl-1,4,7-triazacyclononane), and [Ru-IV(PPz*)(bpy)O](ClO4)(2) (1d; PPz* = 2,6-bis[(4S,7R)-7,8,8-trimethyl-4,5,6,7-tetrahydro-4,7-methanoindazol-2-yl]pyridine) are effective for the epoxidation of aromatic alkenes in acetonitrile at ambient conditions. Their reactions with cis-alkenes such as cis-beta-methylstyrene and cis-beta-deuteriostyrene afford epoxides nonstereospecifically. The observation of the inverse secondary kinetic isotope effect for the beta-d(2)-styrene oxidations [k(H)/k(D) = 0.87 (1b), 0.86 (1d)], but not for alpha-deuteriostyrene (k(H)/k(D) = 0.98 for 1b and 1d), indicates that C-O bond formation is more advanced at the beta-carbon atom than at the alpha carbon, i.e., a stepwise mechanism. The second-order rate constants (k(2)) for the styrene oxidations are weakly dependent on the E degrees(Ru-IV/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: rho(TE .) = +0.43 (R = 0.99) for 1b, +0.50 (R = 0.98) for 1c, and +0.37 (R = 0.99) for 1d (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-beta-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 Ru-IV=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 (% ee(obs)) cannot be determined exclusively by facial selectivity (ee(facial)) 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.