Photochemical fixation of supercritical carbon dioxide: the production of a carboxylic acid from a polyaromatic hydrocarbon

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Abstract

We present our initial results on the use of supercritical carbon dioxide as both a reactant and as a reaction medium to photo-chemically produce carboxylic acids from polycyclic aromatic hydrocarbons via radical anion intermediates. In the present work, we have found that photolysis of anthracene in supercritical carbon dioxide produces up to 57% yield of 9,10-dihydroanthracene-9-carboxylic acid in the presence of appropriate electron and hydrogen donors. This is in stark contrast to the lack of carboxylic acid formation in conventional non-polar aprotic solvents. Additionally, we have found no evidence that under reaction conditions similar to those necessary for radical anion fixation of carbon dioxide that carbon-centered free radicals produce carboxylic acids.

Introduction

Currently, there is great interest in the use of supercritical carbon dioxide (SC CO2) as a more environmentally responsible alternative to conventional organic solvents for use in chemical production and materials processing (Noyori, 1999, Jessop and Leitner, 1999). There is also significant interest in the use of CO2 as an inexpensive environmentally friendly C1 feedstock (Jessop et al., 1995, Fujita, 1996). That is, to use CO2 as a carbon source for the production of valuable chemical commodities.

In this report, we present our initial results of the use of SC CO2 as both a reactant and as a reaction medium for the production of carboxylic acids. Specifically, we have photochemically synthesized a carboxylic acid from an aromatic hydrocarbon via production of an aromatic radical anion produced from a photo-induced electron transfer reaction. We present a comparison of the percent yield of carboxylic acid production under SC CO2 conditions to those produced in conventional solvents.

There are two major impediments to using CO2 as a chemical reagent. One is that CO2 is only reactive towards a few chemical substrates. Second, when using CO2 as a reagent in conventional liquids, the concentration of CO2 is limited by its solubility in the reaction medium. We have overcome the second limitation by using CO2 as the reaction medium. However, for the specific case of using CO2 as a C1 feedstock, to overcome the first difficulty requires the use of an extremely reactive (nucleophilic) substrate. The most well known example of such a reactive reagent is the Grignard reagent, which is essentially a carbanion, that will nucleophilically attack CO2 to produce carboxylate, which may subsequently produce carboxylic acid [Eq. (1)]. However, CO2 fixation of the Grignard reagents is, as mentioned, dependent on CO2 solubility in conventional solvents.

Another potential ‘reagent’ that may react with CO2 to produce carboxylic acids are carbon-centered free radicals [Eq. (2)]. When using CO2 as a solvent, it may be possible to have carbon-centered free radicals react with CO2 to produce carbonyloxyl radicals (RCO2radical dot), which in the presence of a hydrogen atom donor, may then result in carboxylic acid production [Eq. (2)]. It is well known that carboxylation of carbon-centered free radicals is not favored under normal reaction conditions, e.g. in organic solvents. In fact, alkyl and aryl carbonyloxyl radicals are known to decarboxylate on the picosecond to nanosecond time-scale, respectively (Chateauneuf et al., 1987, Chateauneuf et al., 1988a, Chateauneuf et al., 1988b).RMgX+CO2RCO2+H+RCO2HR+CO2RCO2+R-HRCO2HR+CO2RCO2+R-HRCO2H

It is interesting to note, however, that when CO2 or SC CO2 is used as the solvent, Le Chatelier's principle would suggest a shift in the equilibrium for the first portion of Eq. (2). This could favor the carbonyloxyl radical, and if there was a sufficient amount of hydrogen atom donor present, carboxylic acid could possibly be formed (Hadida et al., 1997). We have explored this possibility by the generation of either methyl, phenyl or benzyl free radicals in SC CO2 in the presence of a hydrogen atom donor, and as of yet, we have not been able to detect carboxylic acid as a product. However, we have been successful in the generation of carboxylic acids when we have generated radical anions in SC CO2 in the presence of hydrogen donors, according to Eq. (3). In this report, we present carboxylic acid yields obtained under a variety of reaction conditions, and describe the reaction mechanism involved and the high-pressure methodologies used.

Section snippets

Materials

Anthracene (99%), Aldrich (Milwaukee, WI), was re-crystallized with 90% ethanol and 10% water mixed solvent. Acetonitrile (HPLC grade, UV cut off 190 nm); cyclohexane; 1,4-cyclohexadiene; 2-propanol; N,N-dimethylaniline (DMA); Dimethylformamide (DMF), acetic acid, benzoic acid, benzylcarboxylic acid and dibenzylketone were also purchased from Aldrich (Milwaukee, WI) and used as received. Carbon dioxide (SFC grade, 99.99%) was obtained from Air Products (Allentown, PA) and used without further

Background

In an initial communication in 1975 and then in a full paper in 1986 Tazuke and Ozawa reported some of the first examples of photofixation of CO2 in a non-biological system (Tazuke and Ozawa, 1975, Tazuke et al., 1986). Specifically, they reported the first examples of reductive photocarboxylation of aromatic hydrocarbons, e.g. anthracene, phenanthrene, pyrene, to generate carboxylic acids. Photocarboxylation was observed upon irradiation of the aromatic hydrocarbons (A) in an aprotic polar

Conclusions

We have demonstrated that it is possible to generate an aromatic radical anion via a photoinduced electron transfer mechanism in SC CO2. It is also possible to trap the aromatic radical anion with appropriate hydrogen donors (cyclohexane, 1,4-CHD and 2-propanol) to produce a carboxylic acid product, i.e. CO2 fixation. Therefore, SC CO2 can be effectively used as both an environmentally benign solvent and act as an environmentally friendly C1 feedstock. Alternatively, under comparable

Acknowledgements

This work was initially supported by funds from the Faculty Research and Creative Support Fund, Western Michigan University, and later by the donors of the Petroleum Research Fund, administrated by the American Chemical Society.

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