Oxidation of hydrocarbons. 12. Kinetics and mechanism of the oxidation of trans-cinnamic acid by ruthenate and perruthenate ions

Abstract

, 1085 (1984). Recent theoretical calculations by RappC and Goddard have pointed to the importance of spectator 0x0 groups in stabilizing intermediates produced during thc reduction of high-valent transition metal oxides; intermediates which contain triply-bonded metal 0x0 groups are predicted to be unusually stable. In an attempt to test these conclusions experimentally, the rates of oxidation of cinnamate ion by ruthenate and permthenate ions have been studied. Since the most likely intermediate formed from permthenate ion (but not ruthenate ion) would contain a stabilizing, triply-bonded spectator 0x0 group, it would be expected to react faster if the calculations are correct. In agreement with these predictions, it was found that the activation energy (AG*) for the ruthenate reaction was about 7 kcal/mol higher than for the corresponding permthenate reaction. DONALD G. LEE et STUART HELLIWELL. Can. J . Chem. 62, 1085 (1984) Les recents calculs thCoriques de RappC et Goddard ont mis en Cvidence I'importance des groupes 0x0 spectateurs dans la stabilisation dcs intermediaires obtenus lors de la riduction d'oxydes de mCtaux de transition de valence ClevCe; d'apres ces calculs, les intermCdiaires qui contiennent les groupes 0x0 triplement liCs au metal seront particulierement stables. Dans le but de verifier expCrimentalement cette hypothkse, on a CtudiC les vitesses d'oxydation de I'ion cinnamate par les ions ruthenate et permthCnate. Puisque l'intermtdiaire le plus communtment form6 a partir de I'ion permthenate (mais non B partir de I'ion ruthhate) contiendrait le groupe spectateur 0x0 triplement liC, on pourrait s'attendre 21 ce qu'il soit plus rkactif si les calculs sont corrects. En accord avec ces prkvisions, on a trouvC que I'tnergie d'activation (AG*) de la rCaction du ruthhate est d'environ 7 kcal/mol plus ClevCe que celle de la rCaction correspondante du permthCnate.[Traduit par le journal] Introduction Transition metal oxides and oxyanions have been used extensively as oxidants in both inorganic and organic chemistry (1). The driving force in most such reactions is generally considered to be a lowering of the oxidation number of the oxidant, which may formally be considered as a flow of electrons into the d orbitals of the metal. Since many of the high-valent do, d l , and d2 oxidants are capable of accepting several electrons before reaching a stable oxidation state, these reactions usually proceed in several steps with the formation of a number of intermediates of greater or lesser stability. The reactions are consequently of interest from both a practical and a theoretical point of view.RappC and Goddard (2) have recently described results, obtained from a theoretical study, that are extremely useful for predicting which intermediates are most likely to be formed during the course of several of these reactions. By use of ab initio methods they found that the metal-0x0 bonds present substantially influenced the stability of an intermediate. Metal-0x0 groups which have triple bond character were f...

Metallaoxetanes as intermediate in oxygen-transfer reactions - reality or fiction?

“…The differentiation between route A and B in scheme 20 can be rationalized from the introduction of the spectator oxo bond. 69,70,147 The preference of either C or D can to some degree be traced to the bond energies of the resulting oxometal or carbenemetal species, respectively. 73,74 In the case of titanium route C will be favored, as the bond energy of Ti=0 is larger than the Ti=CH2 bond energy; this is probably the driving force in the "Tebbe-like" reactions.…”

“…141 It was proposed that the alkene-permanganate complex 74 was first formed, followed by a rate-determining nucleophilic attack by one of the oxygen atoms in the permanganate ion at the carbon atom nearest the aromatic ring initiated by a slipping motion of the alkene along the C=C bond, producing the metallaoxetane 75 (Scheme 14). The metallaoxetane then rearranges to the cyclic manganese(V) ester 76.141 The idea with the slipping motion of the alkene toward one of the oxygens and subsequent formation of the metallaoxetane has been used by Lee The spectator oxo bond introduced by Rappe and Goddard to be able to stabilize intermediates persuaded Lee et al 147 to investigate the oxidation of trans-cinnamic acid by ruthenate (Ru042~) and perruthenate (Ru04"). The purpose of using ruthenate and perruthenate was the following: it was assumed that the metaUaoxetane intermediates were formed in rapid equilibrium reactions and that the perruthenate ion would react more rapidly with the carbon-carbon double bond to give the cyclic ruthenium(V) diester 77 (reaction 17) because 77 is stabilized by the spectator oxo group.147 However, the similar reaction with ruthenate would not occur so readily (reaction 18), because of the formation of the cyclic ruthenium(IV) diester intermediate 78 could only be achieved by loss of the stability associated with the presence of the spectator oxo group in the metaUaoxetane 79.147 It was concluded that the experimental observation was in agreement with the predictions made by Rappe and Goddard.69 In a study of the mechanism of cyclization of tripeptides by the enzyme isopeniciUn N synthase (IPNS) a ferraoxetane 81 formed by a [27t + 2ir] cycloaddition of the oxo-iron species to the double bond has been postulated (reaction It was found that the reaction rate for the oxidation of cinnematic ion with perruthenate ion was 5 x 10® times faster than the corresponding ruthenate reaction.…”

“…The purpose of using ruthenate and perruthenate was the following: it was assumed that the metaUaoxetane intermediates were formed in rapid equilibrium reactions and that the perruthenate ion would react more rapidly with the carbon-carbon double bond to give the cyclic ruthenium(V) diester 77 (reaction 17) because 77 is stabilized by the spectator oxo group.147 However, the similar reaction with ruthenate would not occur so readily (reaction 18), because of the formation of the cyclic ruthenium(IV) diester intermediate 78 could only be achieved by loss of the stability associated with the presence of the spectator oxo group in the metaUaoxetane 79.147 It was concluded that the experimental observation was in agreement with the predictions made by Rappe and Goddard.69 In a study of the mechanism of cyclization of tripeptides by the enzyme isopeniciUn N synthase (IPNS) a ferraoxetane 81 formed by a [27t + 2ir] cycloaddition of the oxo-iron species to the double bond has been postulated (reaction It was found that the reaction rate for the oxidation of cinnematic ion with perruthenate ion was 5 x 10® times faster than the corresponding ruthenate reaction. 147 The slow step in reaction 18 is the conversion of the metaUaoxetane to the ruthenium(IV) diester as ruthenium changes from a d2 to a d4 state, compared to the previous steps which also are fast. The activation entropy for the ruthenate reaction is very similar to that for the oxidation of the cinnamate ion by permanganate, whereas the activation enthalpy is larger.…”

Kinetics and mechanism of the oxidation of alcohols by ruthenate and perruthenate ions

“…-k -, [1] Because the values of the observed rate constant depend directly on the magnitude of k l (which would, in turn, be reduced by deuterium substitution at the a-hydrogen) a primary isotope effect results. The negative entropy of activation, AS* = -23 2 1 eu, observed for the oxidation of 2-propanol by ruthenate, indicates that the rate-limiting step, in common with most second order reactions, involves an increase in organization of the transition state as compared to the ground state.…”