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](ClO 4 ) 2 ( 1a; terpy = 2,2‘:6‘,2‘ ‘-terpyridine; 6,6‘-Cl 2 -bpy = 6,6‘-dichloro-2,2‘-bipyridine), [Ru IV (terpy)(tmeda)O](ClO 4 ) 2 ( 1b; tmeda = N, N, N ‘, N ‘-tetramethylethylenediamine), [Ru IV (Cn)(bpy)O](ClO 4 ) 2 ( 1c; Cn = 1,4,7-trimethyl-1,4,7-triazacyclononane), and [Ru IV (PPz*)(bpy)O](ClO 4 ) 2 ( 1d; PPz* = 2,6-bis[(4 S,7 R )-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 -β-methylstyrene and cis -β-deuteriostyrene afford epoxides nonstereospecifically. The observation of the inverse secondary kinetic isotope effect for the β- d 2 -styrene oxidations [ k H / k D = 0.87 ( 1b ), 0.86 ( 1d )], but not for α-deuteriostyrene ( k H / k D = 0.98 for 1b and 1d ), indicates that C−O bond formation is more advanced at the β-carbon atom than at the α carbon, i.e., a stepwise mechanism. The second-order rate constants ( k 2 ) for the styrene oxidations are weakly dependent on the E °(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: ρ 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 -β-methylstyrenes by the chiral oxoruthenium(IV) complex 1d 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.