Dehydrogenation of alcohols via Ru catalyst
(Rated as: excellent)
The following article has been retrieved from Tetrahedron Letters. It explains the hydrogenation of alcohols via a Ru catalyst (Robinson catalyst), where the R2-CH-OH is oxidized to R2-C=O. It might be interesting to note the possibilities for phenylethanol (Table 1, entry 3). And for secondary alcohols, also read the conclusion section. I admit, Ru is certainly not the cheapest chemical substance to find, but that doesn't mean it is not an interesting method
Tetrahedron Letters 44 (2003) 1507-1509
Highly sustainable catalytic dehydrogenation of alcohols with evolution of hydrogen gas
GBWL Ligthart, RH Meijer, MPJ Donners, J Meuldijk, JAJM Vekemans, LA Hulshof
Eindhoven University of Technology, Laboratory of Macromolecular and Organic Chemistry, PO Box 513, 5600MB Eindhoven, The Netherlands
Received 25 November 2002; revised 4 December 2002; accepted 4 December 2002
Abstract - The catalytic dehydrogenation of alcohols into aldehydes and ketones in the absence of H-acceptors was studied with several transition metal catalysts in order to develop a large-scale procedure. Applying Ru(OCOCF3)2(CO)(PPh3)2, the so called Robinson catalyst, several secondary alcohols could be dehydrogenated with high selectivity into the corresponding ketones in relatively short reaction times. Highly effective atom utilization could be realized avoiding solvents and giving hydrogen gas as the sole by-product. However, in contrast to Robinson’s work the catalytic dehydrogenation of primary alcohols appeared to be problematic due to decarbonylation with concomitant catalyst deactivation and aldol condensation under the strong acid or basic conditions applied.
Article - The development of catalytic oxidations under mild reaction conditions is highly appealing. The fine chemical industry invests strong effort in the development of sustainable technologies, based on environmental acceptability. The criteria include high atom efficiency, formation of little inorganic waste and selective synthesis of the desired products. Usually, salts represent the main waste.1,2 Moreover, the reactions are often performed in environmentally unfriendly solvents, typically chlorinated hydrocarbons.3 Here we wish to report our initial results with an environmentally benign catalytic oxidation methodology using transition metal catalysts.4 Homogeneous systems capable of catalyzing these reactions are rare. Inspired by the work of Robinson et al.5 in 1975, the feasibility of the dehydrogenation of primary and secondary alcohols was studied with respect to avoiding H-acceptors and developing a large-scale synthesis using their Ru(OCOCF3)2(CO)(PPh3)2 catalyst. Although application of the catalyst is known to produce hydrogen gas, to our surprise no systematic and quantitative study with respect to the fate of the alcohols was presented,4–7 and to the best of our knowledge such information was not reported elsewhere. Instead, the specific activity of the Robinson catalyst in hydrogen production resulting from dehydrogenation of the primary alcohols ethanol, n-propanol and n-butanol was disputed.7
Compared to classical oxidations, the catalytic oxidation accompanied by elimination of hydrogen gas will enhance the atom efficiency from ~50% up to ~98%. The catalytic dehydrogenation of secondary alcohols described in this paper can schematically be represented by Eq. (1).
The applied Ru(OCOCF3)2(CO)(PPh3)2 catalyst could be prepared in situ from commercially available chemicals, as reported by Robinson et al.4–6 In addition to their work with primary alcohols such as ethanol, n-propanol to n-hexanol and benzyl alcohol and secondary alcohols such as isopropanol and cyclohexanol, we found that Ru(OCOCF3)2(CO)(PPh3)2 also catalyzes selective hydrogen elimination from long chain aliphatic secondary alcohols. Ketones were produced in high yields and in relatively short reaction times at 130°C using 1 mol% of catalyst with respect to the substrate. In the absence of solvents only 12–18 equiv. (with respect to the catalyst) of CF3CO2H were required. All reaction mixtures, started as a bright yellow solution, turned dark red after a few minutes and changed again to bright yellow at the end of the reaction. The catalyst could be recycled three times without significant loss in catalytic activity.
Furthermore, no loss in catalytic activity was observed when the catalyst came in contact with water. The turnover number could well exceed 200 in the case of the oxidation of 2-octanol.8 The results are depicted in Table 1. Very low concentrations of H2 in boiling alcohol as a result of immediate removal ensure that the reverse reaction, i.e. hydrogenation of the ketone or aldehyde, does not play a significant role.
The dehydrogenation of secondary alcohols in the absence of a H-acceptor to produce ketones can be rationalized with the catalytic cycle for the production of hydrogen gas from simple alcohols like ethanol or 2,3-butanediol as proposed by Dobson and Robinson. These authors stated that the catalysis is promoted by the addition of small amounts of free acid (CF3CO2H) and is inhibited by aldehyde or ketone and large amounts of acid.6c However, in our hands, in situ preparation from 1 mol% RuH2(CO)(PPh3)3 and 12–18 molar equiv. (with respect to the catalyst) of CF3CO2H yielded the highest selectivities in the dehydrogenation of secondary alcohols. To obtain mechanistic information, a ruthenium complex formed during the dehydrogenation of 2-octanol with 18 molar equiv. of CF3CO2H was isolated after distillation of the reaction mixture. The mass (m/z) of 877 obtained from MALDI-TOF measurements was assigned to Ru(OCOCF3)2(CO)(PPh3)2 thus proving the statement of Robinson et al.6
Four acids of varying strength were investigated in the dehydrogenation of 2-decanol and 2-octanol using RuH2(CO)(PPh3)3 as the catalyst precursor. The experiments were performed in the presence of 1 mol% RuH2(CO)(PPh3)3, with different stoichiometries of acid compared to the catalyst but always at 130°C in the absence of solvent. All experiments were stopped after 5 h. The results of these experiments are collected in Table 2.
Due to the high volatility of CF3CO2H (bp 72°C)—it evaporates from the reaction mixture—acids with higher boiling points were investigated. Small amounts of p-toluenesulfonic acid (p-TsOH) as relatively strong acid and varying amounts of trichloroacetic acid and acetic acid as weaker acids were tested as well. With an increasing amount of acid the dehydrogenation rate increases but the selectivity to the corresponding ketone decreases; presumably due to acid-catalyzed aldol condensation of the ketone. In contrast, the selectivity and rate of dehydrogenation improve with increasing amounts of the less strong acid CF3CO2H indicating a plateau value for acid amounts larger than 12 equiv.
When even less strong acids are used, hydrogen gas is released from the metal center at a lower rate and consequently the overall dehydrogenation is slower. This observation can be rationalized by a more limited ability to protonate the metal hydride compared to CF3CO2H and p-TsOH.
In contrast to the claims of Dobson and Robinson, low conversions and selectivities were observed when primary alcohols were dehydrogenated. The aldehydes probably decarbonylate and produce CO that poisons the catalyst as was demonstrated by Geoffroy and Pierantozzi.10 Furthermore, the strong acidic conditions employed are known to catalyze aldol condensation and dehydration. Milder reaction conditions have to be found to render the oxidation of primary alcohols successful. These milder reaction conditions can probably be achieved by varying the nature of either the phosphine or carboxylato ligands.
Conclusions - An environmentally benign dehydrogenation methodology using transition metal catalysts like Ru(OCOCF3)2(CO)(PPh3)2 has been developed. The criteria include high atom efficiency as well as formation of little inorganic and organic waste. Several secondary alcohols have been selectively dehydrogenated into the corresponding ketones in relatively short reaction times and in the absence of solvents. Furthermore, the catalysts used in these reactions could be prepared in situ from commercially available chemicals. The catalyst could be recycled three times without significant loss in catalytic activity. As the best acid for the dehydrogenation CF3CO2H has been selected.
For secondary alcohols the intrinsic catalytic activity will be studied further by fine-tuning the ligands of the Robinson catalyst.
Tables and equations
Table 1. Ru(OCOCF3)2(CO)(PPh3)2-catalyzed hydrogen elimination from secondary alcohols at 130°C