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

Abstract

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 (1b; tmeda = N,N,N‘,N‘-tetramethylethylenediamine), [RuIV(Cn)(bpy)O](ClO4)2 (1c; Cn = 1,4,7-trimethyl-1,4,7-triazacyclononane), and [RuIV(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-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°(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: ρTE • = +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 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.

Hydrocarbon Oxidation Catalyzed by [Ru(TDL)(XY)Z] Complexes (TDL = Tridentate Ligand; XY = Bidentate Ligand and Z = H2O or Halide)

“…19) product enantioselectivity also suggestively [19] takes place by the formation of the opposite enantiomeric epoxide via rotation/collapse pathway (Scheme 4), and hence, the stereoselectivity of the epoxide ring-closure is also important in controlling the overall product enantioselectivity [19].…”

“…It was further observed [19] that in the case of styrene oxidation (i.e., the only asymmetric center to be created), the rotation/collapse pathway tends to reduce the enantioselectivity by generating the opposite enantiomeric epoxide (Scheme 4).…”

“…However, the reaction of [Ru IV (PPz*) (bipy))O] with aryl alkenes induced a moderate enantioselectivity in acetonitrile (Table 4). A facial selectivity through side-on approach that brings about the observed enantioselectivity had been proposed by the authors [19] by considering that the Ru=O axis is orthogonal to the molecular plane of PPz* but collinear to 'bipy' in [Ru IV (PPz*)(bipy))O]; the oxo-moiety is thus sterically encumbered and only one face of the Ru=O site is exposed for the olefinic approach (Scheme 3). For instance, when (?…”

“…Additional support in favor of the above argument comes from the linear Hammett plot constructed by plotting the relative reactivity (k rel = k pss /k s ; where k pss and k s represent secondorder rate constants for the oxidation of para-substituted styrene and styrene, respectively) against the TE values (total substituent effect parameter) [23]. In the context of asymmetric epoxidation, rutheniumoxo complexes containing chiral ligands 'cbpy' and 'PPz*' that afford enantioselective epoxidation of unfunctionalized olefins was first reported by Che and co-workers [17,19,26]. The results of stoichiometric oxidation of styrene by [Ru IV (cbpy)(Me 3 tacn))O] resulted in only 9% ee [17].…”

Ruthenium complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane for atom and group transfer reactions

“…In this review, we focus on the Me 3 tacn complexes of ruthenium bearing terminal ruthenium-ligand multiple bonds including ruthenium-oxo and -imido species and their oxygen [16,17,33,34,[45][46][47] and nitrogen atom/group transfer reactions, together with related reactions catalyzed by ruthenium Me 3 tacn complexes, including epoxidation of alkenes [17,48,49], oxidation of alkanes [17,48], alcohols [50,51], aldehydes [51], and arenes [52], oxidative cleavage of C C, C C, and C-C bonds [51], cisdihydroxylation of alkenes [53], and amination of saturated C-H bonds [54]. Also included here are some of our unpublished results on the DFT calculations on reactive Ru O and Ru NTs (Ts = ptoluenesulfonyl) complexes supported by Me 3 tacn, and on C-N bond formation reactions catalyzed by [Ru(Me 3 tacn)(NH 3 ) 3 ] 2+ , and electrochemical oxidation of Ru(III)-NH 2 R to the putative [Ru V NR] 2+ from the [Ru(Me 3 tacn)(H 2 L)] 2+ (H 3 L = ␣-(1-amino-1-methylethyl)-2-pyridinemethanol) complex [55].…”