Substituent Effects in the Substrate

1. Hydrogen Generation from Alcohols by Homogeneous Catalysts Tarn C. Johnson, David J. Morris and Martin Wills Department of Chemistry, The University of Warwick, Coventry, CV4 7AL, UK Mechanism of Alcohol Oxidation Catalyst Selection In considering the mechanism of hydrogen gas production from alcohols, two discrete steps can be envisaged; in the first step the active catalyst removes two hydrogen atoms from the alcoholic substrate, resulting in a metal hydride complex and a carbonyl compound, the second step being the release of hydrogen gas to regenerate the active catalyst (Figure 4). HDelivery Introduction The ruthenium-catalysed oxidation of alcohols with subsequent elimination of hydrogen gas has been investigated.[1] As an alternative, the use of a hydrogen acceptor has an accelerating effect on the reaction and allows the generation of low molecular weight alcohols such as methanol, ethanol and iso-propanol which could have applications in fuel cells. The selection of an efficient hydrogen-transfer catalyst (Figure 1) allows this process to be carried out in high yields. As fossil fuel reserves are progressively depleted, the development of a clean energy supply that is sustainable and can meet the rising global demand for energy is rapidly becoming one of the greatest challenges of the 21st century. A promising solution to this problem is the use of hydrogen as an energy vector. This has the advantage of a significant reduction in greenhouse gas emissions as hydrogen can be either combusted or converted directly into electricity via fuel cells, liberating water as the sole by-product. For this reason the catalytic oxidation of primary and secondary alcohols to yield molecular hydrogen is desirable (Scheme 1). Figure 4. Mechanism of alcohol oxidation Scheme 1. The oxidation of alcohols with release of hydrogen gas. A BCD Scheme 3. Conditions; 3 mmol A, 3 mmol B, 0.5 mol % [RuCl2(p-cymene)]2, 8 mol % PPh3, 15 mol % LiOH.H2O, toluene, 110 °C, 15 h. Alcohol oxidation reactions with subsequent release of hydrogen gas occur slowly and require raised temperatures. By performing such reactions in the presence of a hydrogen acceptor the hydrogen transfer behaviour of the catalyst was probed. The reaction shown in Scheme 3 was carried out with monitoring by NMR spectroscopy after 1, 2, 3, 6, 9 and 15 h and the conversions are plotted in the graph shown in Figure 5. The conversions are high after the Figure 1. The oxidation of phenylethanol by ruthenium catalysts in the presence of acetone. The application of acetaldehyde or formaldehyde (via paraformaldehyde) as a hydrogen acceptor in conjunction with catalyst 3 results in essentially quantitative conversion of phenylethanol to acetophenone and the formation of ethanol or methanol respectively. Figure 5. The conversion of A to C and B to D. first hour which indicates a fast equilibration process between species A, B, C and D. This shows that the first mechanistic step, the removal of hydrogen, is fast. The slow rise of the blue line and decline of the red line as the reaction progresses show a decrease in the quantities of alcohols A and D as hydrogen is gradually lost from the system. This shows that the second mechanistic step, the elimination of hydrogen gas, is slow and therefore, the rate-determining step. R = 1 2 34 Cyclone Catalysts R = Substituent Effects in the Substrate Δ Scheme 2. The simultaneous oxidation of three alcohols with release of hydrogen gas. Figure 6. The Shvo catalyst and the mechanism by which it oxidises alcohols. Catalysts bearing a cyclopentadienone ligand, so-called ‘cyclone catalysts’ are very effective for hydrogen transfer reactions. The Shvo catalyst (Figure 6) splits into two catalytically active species which take part in hydrogen transfer through a bifunctional mechanism utilising a pendant oxygen atom.[2] An analogous iron complex has also been shown by Casey et al.[3] to be an efficient catalyst for ketone reductions. Current work focuses on the synthesis of novel iron cyclone catalysts (Figure 7) with higher activities that operate at lower temperatures. The oxidation of phenylethanol and two derivatives bearing increasingly electron-donating substituents indicates a trend whereby more electron-rich alcohols are oxidised more readily. A competition experiment in which three alcohols are oxidised simultaneously in one reaction vessel is illustrated in Scheme 2 and the results in Figure 2. This demonstrates the importance of substituent effects in the catalysis. Conclusions The oxidation of alcohols by the method presented is very substrate sensitive, with more electon-rich alcohols being oxidised more readily. The rate-determining step in this process is the release of hydrogen gas from the catalyst. For this reason high temperatures and long reaction times are necessary to achieve high conversions. The use of a hydrogen acceptor results in a dramatic rate increase and leaves scope for the transfer of hydrogen from complex alcohols to form more simple ones for use in fuel cell applications. Figure 7. Complexes under investigation Figure 3. The oxidation of three alcohols with the release of hydrogen gas with a less efficient catalyst. This trend is emphasized by the use of a less efficient hydrogen-transfer catalyst. By removing the additive PPh3 from the catalytic system used in the competition experiment, a less efficient active catalyst is generated. This results in lower conversions with the notable exception that the most electon-rich alcohol, p-methoxyphenylethanol, is still oxidised in a moderate yield (Figure 3). Acknowledgements. We thank the EPSRC (via SUPERGEN IV) for funding of this project. Figure 2. The simultaneous oxidation of three alcohols with the release of hydrogen gas. References. [1] Tarn C. Johnson, David J. Morris and Martin Wills, Chem. Soc. Rev. 2010, 39, 81-88.[2] R. Karvembu, R. Prabhakaran and K. Natarajan, Coord. Chem. Rev., 2005, 249, 911-918. [3] C.P. Casey and H. Guan, J. Am. Chem. Soc., 2007, 129, 5816-5817.