Rhodium complexes hydrogen-bonded acceptors

In the past, this field has been dominated by ruthenium, rhodium and iridium catalysts with extraordinary activities and furthermore superior enantioselectivities however, some investigations were carried out with iron catalysts. Early efforts were reported on the successful use of hydridocarbonyliron complexes HFcm(CO) as reducing reagent for a, P-unsaturated carbonyl compounds, dienes and C=N double bonds, albeit complexes were used in stoichiometric amounts [7]. The first catalytic approach was presented by Marko et al. on the reduction of acetone in the presence of Fe3(CO)12 or Fe(CO)5 [8]. In this reaction, the hydrogen is delivered by water under more drastic reaction conditions (100 bar, 100 °C). Addition of NEt3 as co-catalyst was necessary to obtain reasonable yields. The authors assumed a reaction of Fe(CO)5 with hydroxide ions to yield H Fe(CO)4 with liberation of carbon dioxide since basic conditions are present and exclude the formation of molecular hydrogen via the water gas shift reaction. H Fe(CO)4 is believed to be the active catalyst, which transfers the hydride to the acceptor. The catalyst presented displayed activity in the reduction of several ketones and aldehydes (Scheme 4.1) [9]. 

Dehydrogenation of alcohols to aldehyde or ketone allows subsequent bond construction steps which would not be possible for the parent alcohols. Hence, a variety of iridium, rhodium or ruthenium phosphine, pincer and related complexes, that are efficient catalysts for the dehydrogenation of alcohols, can potentially be appHed for the related hydrogen-transfer reactions, thus leading to new added-value compounds. The hydrogen atoms transfer to a sacrificial hydrogen acceptor, such as a carbonyl compound or an olefin which is reduced to the corresponding alcohol or alkane. 

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