Catalytic oxidations in carbon dioxide-based reaction media, including novel CO2-expanded phases

Environmentally benign oxidations with dense CO2 (either near-critical, ncC O(2) or supercritical carbon dioxide, scCO(2)) as solvent media have been r eceiving increased attention during the last decade. This paper reviews cat alytic oxidations in dense CO2 with emphasis on reported homogeneous system s in scCO(2) most of which involve transition metal catalysts and dioxygen or organic peroxides as oxidant. Based on recent work in our laboratory, we offer some perspective and provide examples to demonstrate that scCO(2) ca n be adapted to a broader range of homogeneous oxidations including those t hat utilize CH3ReO3 as catalyst and t-BuOOH as terminal oxidant, and the ox idation of substituted phenols by dioxygen using Co(salen) complex as catal yst. The advantages of using scCO(2) include the total replacement of organ ic solvents with environmentally benign CO2 the complete miscibility of the oxidants such as O-2 in scCO(2) eliminating interphase transport limitatio ns, and the resistance of CO2 to oxidation. However, the scCO(2)-based oxid ation has limitations including low reaction rates, inadequate solubilities of a number of transition metal catalysts in CO2 necessitating high proces s pressures on the order of hundreds of bars, and the lack of pressure-tuna bility of the dielectric constant of the reaction medium. We present a new process developed in our laboratory in which the conventional solvent mediu m is only partially replaced by dense CO2. We term this a CO2-expanded solv ent medium, which offers several advantages as follows: solvent replacement with dense CO2 by up to 80 mol%, representing a substantial reduction in s olvent usage; maintenance of the solubilities of the catalyst and substrate in the reaction mixture while enhancing the miscibility of dioxygen therei n; lower process pressures on the order of tens of bars; and pressure-tunab le dielectric constants making it possible to realize an optimum reaction m edium between scCO(2) and neat solvent limits. We distinguish CO2-expanded phases from the traditional concept of a 'co-solvent' for a CO2 based syste m in the following way. To produce a CO2 expanded organic solvent medium, w e start with the organic solvent and increase its volume by the addition of CO2, whereas relatively small amounts of 'co-solvents' have traditionally been added to dense CO2 phases to improve solubilities of certain compounds . We present examples that show enhanced oxidation rates compared to either n eat organic solvent or scCO(2) for organic substrates (alkenes and phenols) in CO2-expanded media using dioxygen and metal complexes of both Schiff ba se and porphyrin ligands. Further, the selectivity toward desired products (alkenes to epoxides; phenols to quinones) is also improved over either nea t solvents or scCO(2). The CO2-expanded solvents thus offer excellent poten tial for exploitation in catalytic oxidations. (C) 2001 Elsevier Science B. V. All rights reserved.