Reactions with Singlet Oxygen and Supercritical Fluids

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This Proposal aims to develop a greener approach to the use of a highly reactive form of oxygen (singlet oxygen, 1O2) for reactions involving compounds of interest to the pharmaceutical and low-tonnage fine chemicals industries by exploiting several different properties of high pressure CO2 (scCO2) to address and overcome problems in existing 1O2 methodologies. The Proposal has been developed with the active participation of industrial partners, AstraZeneca and Thomas Swan & Co Ltd, both of whom have a record of effective collaboration with our research group.1O2 has a history of cleaner chemical synthesis which long predates the birth of Green Chemistry in the 1990s for example, with the German chemist Schenck's post-war synthesis of the medicinal compound ascaridole, reputedly with spinach leaves as the photosensitizer! From a green standpoint 1O2 has several attractions: (i) 1O2 has the potential to promote chemically useful transformations and (ii) 1O2 can be generated photochemically, sometimes even using sunlight, and only requires visible light rather than UV, thus avoiding problems of inadvertent generation of ozone. (iii) On grounds of both safety and atom efficiency, the photochemical generation of 1O2 is preferable to current thermal routes. 1O2 is additionally attractive for pharmaceutical process chemistry which has a pressing need for transformations that combine reduction in waste with complex chemistry. In particular, our Proposal targets two aspects of Green chemistry, formation of allylic alcohols, and safe use of O2 in non-chlorinated solvents, recently highlighted as being of specific importance to the pharmaceutical industry. Furthermore, the low tonnages in most pharmaceutical processes mean that the degree of scale-up required is less than in many sectors of the chemical industry.Our aim is to demonstrate the feasibility of continuous photochemical reactions of 1O2 in solvents expanded with CO2, so-called Gas-Expanded liquids, GXLs. The key will be to exploit the biphasic nature of the GXLs to maximize the efficiency not only of the reaction itself but also of the separation of the product and photosensitizer. Our strategy is to build a relatively small high pressure falling-film reactor, FFR, where the liquid flows down a plate thereby giving more precise control over reaction conditions. The design has been chosen to give us flexibility in the way that the reactor is configured (e.g. concurrent or counter-current flow, etc). Combining this flexibility with the modular nature of the high-pressure equipment available in Nottingham will allow us to modify our approach rapidly in response to our experimental results. A particular advantage of the design is that, in most cases, it will allow us to carry out control experiments in the absence of CO2 so that we can establish the precise effect of CO2 on a given reaction. Our chemical programme focuses on four classes of reactions chosen in consultation with our partners, specifically for their relevance to pharmaceutical process chemistry.