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 = 5,6,7-tetrahydro-4,7-methanoindazol-2-yl]pyridine) ar e 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 nonstereos pecifically. 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 a nd 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 wea kly dependent on the E degrees(Ru-IV/III) values of the oxoruthenium(I V) complexes, and both electron-withdrawing and -donating para substit uents mildly accelerate the oxidation reaction of styrene. These findi ngs discount strongly the intermediaries of an alkene-derived cation r adical and a carbocation. A linear free-energy relationship between th e second-order rate constants for the para-substituted styrene oxidati ons and the total substituent effect (TE) parameters has been establis hed: 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 pro ceed via the rate-limiting formation of a benzylic radical intermediat e. Oxidation of styrene and cis- and trans-beta-methylstyrenes by the chiral oxoruthenium-(IV) complex Id attains moderate enantioselectivit ies, in which the production of cis-epoxide is more enantioselective t han the trans counterpart. The ligand dissymmetry of PPz together wit h the bipyridine ligand create a ''chiral pocket'' around the Ru-IV=O moiety, leading to enantiofacial discrimination through nonbonding int eraction. Because the acyclic benzylic radical intermediate would unde rgo cis-trans isomerization before the second C-O bond formation, the overall product enantioselectivity (% ee(obs)) cannot be determined ex clusively 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.