The kinetics and main pathways for the liquid-phase oxidation of cyclohexene by air in the presence of cobalt, chromium, iron, and nickel µ3-oxotrimetallohexapivalates

Abstract

The liquid-phase oxidation of cyclohexene (CH) by atmospheric oxygen is of considerable practical interest in connection with the broad synthetic possibilities of its oxidation products -the epoxide (OCH), 3-hydroperoxide (HP), cyclohexen-3-ol (CHol), cyclohexen-3-one (CHone), etc. [1,2]. At the same time CH is a convenient (model) compound for investigation of the main pathways and the selectivity in both direct and conjugate oxidation of compounds with an activated CH bond in relation to composition of the catalytic system and the structure of the metal-complex catalyst.In a continuation of our researches on the catalytic characteristics of the polynuclear pivalates of 3d metals [3, 4] we examined the oxidation of CH in the presence of a series of homo-and heteronuclear complexes with the CoM 2 O core (M = Co, Cr, Fe), viz.: [Co 3 OPiv 6 (g-Pic) 3 ]Piv (I), CoCr 2 OPiv 6 ·3Ppy (II), and CoFe 2 OPiv 6 ·3Ppy (III). For comparison the pivalate NiCr 2 OPiv 6 ·3Ppy (IV), which exhibited catalytic characteristics close to its cobalt analog in the decomposition of the hydroperoxide of dibenzyl ether [5] (II), was included in this series. The synthesis and structure of the investigated pivalates were described in [6,7]. 900040-5760/07/4302-0090

Effect of the nuclearity and ligand environment of cobalt on the catalytic characteristics of its pivalate complexes on the liquid-phase oxidation of cyclohexene by oxygen

“…As seen from Table 1, the general pattern of the oxidation of cyclohexene in the presence of complexes of Co(II) I-III, VII-X and Co(III) V is similar to that observed in [2] -the main products are HP, CHone, and CHol. The complexes IV and VI are inactive under the investigated conditions (Table 1), while the conversion of cyclohexene after 3 h in the presence of the remaining complexes amounts to between 7 and 90%.…”

“…Here it is impossible to rule out the disproportionation of two C 6 H 9 OO × radicals and also two molecules of the hydroperoxide (scheme). Since one of the likely paths for the formation of the epoxide is not only direct oxidation of the double bond of cyclohexene by the complex Cat×O 2 (e.g., see [2]) but also its epoxidation by the hydroperoxide (scheme), the latter can then be transformed into the alcohol, which is then oxidized to the corresponding ketone. Thus, the cyclohexenol, present in the oxidate in "trace" quantities (equivalent to the content of the Note.…”

“…epoxide, see Table 1), can then not only be oxidized to cyclohexenone but can also promote dehydration of the hydroperoxide to the latter (see the scheme) similar to that described in [2]. Let us consider the effect of the composition of the complexes on their relative activity in the oxidation of cyclohexene to the hydroperoxide -the main intermediate participating in all the alternative pathways of the reaction under discussion.…”

“…It can be supposed that the presence of the two coordination-unsaturated centers does ensure the possibility of additional coordination (activation) at these centers both of the initial substrates (O 2 , cyclohexene) and of the intermediates such as the hydroperoxide and the cyclohexenol (see [2]). …”

“…Here the ACA also increases, indicating the possible formation of complex-intermediates (transition states), the "active centers" of which (like the active part of metalloenzymes [13]) contain two or more metal atoms (e. g., see [2]). …”