Expanded product, plus kinetic and mechanistic, studies of polyoxoanion-based cyclohexene oxidation catalysis:: the detection of ∼70 products at higher conversion leading to a simple, product-based test for the presence of olefin autoxidation

First, catalyst variation and solvent optimization studies are reported for polyoxoanion-supported transition-metal pre-catalysts, novel complexes such as [(1,5-COD)Ir(I).P(2)W(15)Nb(3)O(62)](8-), [(C(6)H(6))Ru(II).P(2)W(15)Nb(3)O(62)](7-) and [(OC)(3)Re(I).P(2)W(15)Nb(3)O(62)](8-), catalyzing cyclohexene oxidation at low (less than or equal to25%) conversion with O(2) to, predominatly, four main autoxidation products: 2-cyclohexen-1-yl hydroperoxide, 2-cyclohexen-1-one, 2-cyclohexen-1-ol, and cyclohexene oxide, all at a mass balance of greater than or equal to80-92% under the low conversion conditions. Next, radical-chain initiator, inhibitor, and other kinetic (rate law) evidence for the reaction behaving, at lower conversions, as classical Haber-Weiss autoxidation was obtained. Those studies lead, in turn, to the study of cyclohexene autoxidation at higher conversions, notably the identification of ca. 70 gas chromatography (GC)-detectable products. This was followed by identification of 27 of those ca. 70 products, the first reported detailed identification of more than 8-10 products of cyclohexene autoxidation. These results led, in turn, to the five main findings of this study: (i) product and kinetic-overall compelling-evidence that the main reaction is free-radical-chain autoxidation; (ii) the first detection of ca. 70 cyclohexene autoxidation products, followed by the identification of 27 of those products-this is not trivial given that complete product studies are the required first step of rigorous mechanistic work; (iii) plausible arrow-pushing mechanisms to many of the observed products using known radical chemistry-previously unavailable schemes; (iv) the observation of chlorinated hydrocarbons among the products, results which require the precedented participation in the oxidation catalysis by CH(2)Cl(2) solvent-derived, .CHCl(2) radicals, and perhaps most significantly (v) the development of a relatively simple and quick, yet definitive, GC and GC-MS fingerprint method for detecting autoxidation catalysis using the prototype olefin, cyclohexene. Such product studies should prove to be a useful tool in the continuing problem of detecting, or ruling out, classical autoxidation in attempts to develop new oxidation chemistry using Q(2) as a highly desirable terminal oxidant. (C) 2002 Elsevier Science BX All rights reserved.