LAUSR

A cyclometalated rhodium complex has been found to show excellent activity in dehydrogenation of methanol at room temperature. This feature allows for the catalytic transfer hydrogenation of various aldehydes with methanol as both hydrogen source and solvent under very mild conditions. A hydroxy functionality on the ligand is shown to be critical for this unusual activity. Reduction of carbonyl compounds is an essential synthetic method for obtaining valuable alcohol products in both academia and industry. Transfer hydrogenation (TH) has attracted a great deal of attention in this area due to its simplicity, safety and low cost. Compared with common hydrogenation, easily accessible hydrogen sources are used in TH instead of hazardous H2. Conventionally, isopropanol and formic acid are used as the hydrogen donor in TH. In contrast, the simplest, cheapest alcohol, methanol, has been much less employed in TH, although it could serve as an excellent hydrogen source in catalytic reactions. Methanol is a promising hydrogen source and could be the base for a future economy, owing to its high hydrogen storage capacity of 12.5% (wt). Methanol, easily derived from oil, coal and biomass, is one of the cheapest, most easily available and most easy-to-handle chemicals. However, as a primary alcohol, methanol is generally considered thermodynamically unfavourable for the generation of H2 or metal hydrides via dehydrogenation (Scheme 1), although low-temperature dehydrogenation of methanol has recently been reported. Dehydrogenation of isopropanol to acetone is considerably more favourable than that of methanol to formaldehyde, and thus the latter requires a higher energy input. To date, only a few reports have appeared that describe the use of methanol as hydrogen source in catalytic TH; however, none of the thermal catalytic reactions could be performed around room temperature. Thus, the Maitlis group early reported catalytic reduction of ketone by ruthenium and rhodium complexes with methanol at 150 °C. Chen and co-workers demonstrated the selective TH of biomass-based furfural and 5-hydroxymethylfurfural with methanol on a copper catalyst at over 200 °C, and the García group reported Ni-catalyzed alkylation and TH of α,β-unsaturated enones with methanol at 180 °C. Using an iridium Nheterocyclic carbene complex in the presence of 5 equivalents of a base, the Crabtree group demonstrated TH of aromatic ketones and imines with methanol at 120 °C. More recently, Li and co-workers accomplished a much lower-temperature TH of ketones with methanol catalyzed by an iridium-bipyridonate complex, which proceeded at 66 °C. As far as we are aware, there appear to be only two examples of room temperature TH reactions with methanol. In one example, the TH is driven by light, while in the other the reaction is enabled by an enzyme. As indicated by the thermodynamics and literature, lowtemperature TH with methanol is challenging. If the substrates are ketones or aldehydes, the challenge can be more significant, as the product, a secondary or primary alcohol, is expected to be dehydrogenated more easily than methanol. In our continued study of TH reactions, we have found a cyclometalated rhodium complex that catalyzes, remarkably, TH of aldehydes around room temperature. In 2018, our group reported a cyclometalated Cp*Rh(III) complex, a rhodacycle, for catalytic TH of α,β-unsaturated ketones and aldehydes with methanol at 90 °C. Unexpectedly, our further studies showed that the rhodacycle could effect the TH reaction at even room temperature. Given the significance of transferring hydrogen from methanol at room temperature and the easy accessibility of various rhodacycles, we have undertaken further studies to examine the potential of such rhodium complexes for low-temperature TH of carbonyl compounds. Herein, we report the rhodacycle-catalyzed TH of aldehydes with methanol as hydrogen source under ambient reaction conditions. We initially explored the catalytic TH of 4-bromobenzaldehyde by rhodium complexes using MeOH as both hydrogen donor and solvent. As shown in Table 1, 4-bromobenzyl alcohol was obtained in almost quantitative yield (97%) in the presence of 1 and Na2CO3 at 30 °C in 1 h (Table 1, entry 1); the yield decreased only slightly (83%, entry 2) on lowering the temperature to 25 °C, which can be attributed to a reduction in reaction rate at the lower temperature. A similar yield of alcohol product (91%) was obtained for the model reaction in ethanol [a] Z. Chen, G. Chen, A. H. Aboo, Dr. J. Iggo, Prof. J. Xiao Department of Chemistry University of Liverpool Liverpool, L69 7ZD (UK) E-mail: [email protected] Supporting information for this article is available on the WWW under https://doi.org/10.1002/ajoc.202000241 Scheme 1. Selected thermodynamic data for alcohol dehydrogenation. Communication DOI: 10.1002/ajoc.202000241 1 Asian J. Org. Chem. 2020, 9, 1–6 © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! �� Wiley VCH Dienstag, 30.06.202