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

Abstract

Environmentally benign oxidations with dense CO₂ (either near-critical, ncCO₂, or supercritical carbon dioxide, scCO₂) as solvent media have been receiving increased attention during the last decade. This paper reviews catalytic oxidations in dense CO₂ with emphasis on reported homogeneous systems in scCO₂, 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 scCO₂ can be adapted to a broader range of homogeneous oxidations including those that utilize CH₃ReO₃ as catalyst and t-BuOOH as terminal oxidant, and the oxidation of substituted phenols by dioxygen using Co(salen) complex as catalyst. The advantages of using scCO₂ include the total replacement of organic solvents with environmentally benign CO₂, the complete miscibility of the oxidants such as O₂ in scCO₂ eliminating interphase transport limitations, and the resistance of CO₂ to oxidation. However, the scCO₂-based oxidation has limitations including low reaction rates, inadequate solubilities of a number of transition metal catalysts in CO₂ necessitating high process pressures on the order of hundreds of bars, and the lack of pressure-tunability of the dielectric constant of the reaction medium. We present a new process developed in our laboratory in which the conventional solvent medium is only partially replaced by dense CO₂. We term this a CO₂-expanded solvent medium, which offers several advantages as follows: solvent replacement with dense CO₂ by up to 80 mol%, representing a substantial reduction in solvent usage; maintenance of the solubilities of the catalyst and substrate in the reaction mixture while enhancing the miscibility of dioxygen therein; lower process pressures on the order of tens of bars; and pressure-tunable dielectric constants making it possible to realize an optimum reaction medium between scCO₂ and neat solvent limits. We distinguish CO₂-expanded phases from the traditional concept of a ‘co-solvent’ for a CO₂ based system in the following way. To produce a CO₂ expanded organic solvent medium, we 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 neat organic solvent or scCO₂ for organic substrates (alkenes and phenols) in CO₂-expanded media using dioxygen and metal complexes of both Schiff base and porphyrin ligands. Further, the selectivity toward desired products (alkenes to epoxides; phenols to quinones) is also improved over either neat solvents or scCO₂. The CO₂-expanded solvents thus offer excellent potential for exploitation in catalytic oxidations.

Introduction

Supercritical and near-critical dense carbon dioxide phases have been heralded as environmentally benign solvents of great promise in many chemical applications, including the long used preparation of materials for decaffeinated drinks, the chemical process industry, and, more recently, to replace the conventional organic solvents used in the cleaning of garments. The low toxicity and limited reactivity of CO₂ make it suitable for use around foods and other consumer goods, and its low cost supports its use in very broad ranges of applications. Reviews of reactions in sc media are provided elsewhere [1], [1](a), [1](b), [1](c), [1](d), [1](e), [1](f), [1](g).

This review is focused on homogeneous catalytic oxidations in dense CO₂ media. Historically, emphasis has rested on scCO₂, and the many advantages of that medium over traditional solvents are first discussed. Attention is then directed to the still greater advantages of CO₂ expanded organic solvents; these are new media that were first introduced for oxidation reactions in our laboratories. These extremely variable solvent systems retain solubility characteristics of the two combined solvents and still enjoy the environmental advantages of more familiar dense CO₂ media. A CO₂-expanded organic solvent medium is produced by increasing the volume of an organic solvent through the addition of relatively large amounts of CO₂, whereas the ‘cosolvent’ concept has traditionally been referred to the addition of relatively small amounts of organic solvent to dense CO₂ phases to improve the solubilities of certain compounds. A thumbnail sketch of known homogeneous transition metal catalyst systems is then offered from the viewpoint of investigators concerned with their study in dense CO₂ media. Preferred terminal oxidants are recognized and some well known four-component catalyst systems are mentioned to illustrate those considerations that must be brought along when the newer media are to be used. At this point, the reported catalytic studies of reactions are summarized, with heterogeneous examples described first, followed by the more recently opened subject of homogeneous transition metal catalyst systems in dense CO₂ media. Most of those earlier studies employ scCO₂. The authors then turn to their perspective on the subject and present their contribution to the field, including a number of systems explored for the first time and discovery of the potential of chemical reactions in CO₂-expanded organic solvents. Suggestions for future research complete the review.