DOI: 10.1002/adsc.200700014 Fast and Chemoselective Transfer Hydrogenation of Aldehydes Catalyzed by a Terdentate CNN Ruthenium Complex [RuClACHTUNGTRENNUNG(CNN)ACHTUNGTRENNUNG(dppb)] Walter Baratta, a, * Katia Siega, a and Pierluigi Rigo a a Dipartimento di Scienze e Tecnologie Chimiche, Università di Udine, Via Cotonificio 108, 33100 Udine, Italy Fax:(+ 39)-0432-558-803; e-mail: inorg@dstc.uniud.it Received:January8,2007 Abstract: Aromatic, aliphatic and a,b-unsaturated aldehydes are quickly, quantitatively and chemose- lectively reduced to primary alcohols with 2-propa- nol using 0.05–0.01 mol% of the terdentate CNN ruthenium complex RuClACHTUNGTRENNUNG(CNN)ACHTUNGTRENNUNG(dppb) (1) [HCNN = 6-(4’-methylphenyl)-2-pyridylmethylam- ine; dppb = Ph 2 PACHTUNGTRENNUNG(CH 2 ) 4 PPh 2 ] in the presence of po- tassium carbonate (K 2 CO 3 ; 1–10 mol%) as a weak base,affordingTOFvaluesupto5.010 5 h À1 . Keywords: M-NH bifunctional effect; phosphanes; pyridine; ruthenium; transfer hydrogenation Catalytic transfer hydrogenation of carbonyl com- pounds mediated by transition metal complexes has received increasing attention as a possible synthetic route for the production of a wide range of alcohols. Excellent results have been obtained in the asymmet- ric transfer hydrogenation of ketones to optical active secondary alcohols, which are an important class of intermediates in the fine chemical industry, using 2- propanol or formic acid derivatives as hydrogen sour- ces. [1] It is worth noting that, because of the higher redox potentials of aldehydes compared to ketones, the equilibrium of the transfer hydrogenation reaction of aldehydes with 2-propanol is more shifted toward the products, compared to the corresponding transfer hydrogenation of ketones. [2] By contrast, reduction of aldehydes to primary alcohols via transfer hydrogena- tion as well as the control of the chemoselectivity of this reaction are considered rather difficult process- es. [3] The difficulty for the catalytic reduction of alde- hydes resides in the side reactions that may occur during the catalytic reaction, usually performed in basic media. As a matter of fact, the hydrogens of the a-CH group are susceptible to deprotonation and can lead to aldol condensation. Furthermore, during catal- ysis aldehydes may also undergo decarbonylation re- actions, [4] which may result in deactivation of the cata- lysts through coordination of carbon monoxide. Therefore, in order to suppress these side reactions, weak basic conditions and very short reaction time are prerequisites to achieve efficient aldehyde reduc- tion. One of the most active catalyst has been de- scribed by Crabtree and co-workers using an iridium NHC complex that catalyzes the reduction of alde- hydes in 2-propanol with TOF values up to 3000h À1 . [4b] More recently, Xiao and co-workers re- ported the most efficient system prepared by reacting [Cp*IrClACHTUNGTRENNUNG(m-Cl)] 2 with diamines and leading to TOF valuesupto130,000h À1 (calculated on the conversion after 5 min) for the reduction of benzaldehyde with sodium formate in the aqueous phase. [5] Despite the catalysts of choice for transfer hydrogenation of ke- tones being ruthenium-based derivatives, only few catalytic systems with this metal have been described for the transfer hydrogenation of aldehydes, namely some ruthenium phosphane complexes, [3a,c,6] and chiral arene-aminoderivatives. [3g,7] In our recent study on catalytic transfer hydrogena- tion reactions, we have found that ruthenium phos- phane complexes containing 2-(aminomethyl)pyridine (ampy) are highly efficient catalysts for the reduction of ketones, [8] with the derivatives of general formula RuCl 2 (PP)ACHTUNGTRENNUNG(ampy) (PP = chiral diphosphane) [8d] afford- ing ee values up to 94%. An extremely active system has subsequently been prepared using 6-(4’-methyl- phenyl)-2-pyridylmethylamine (HCNN), leading to the terdentate complex RuClACHTUNGTRENNUNG(CNN)ACHTUNGTRENNUNG(dppb) (1) [dppb = Ph 2 PACHTUNGTRENNUNG(CH 2 ) 4 PPh 2 ] which catalyzes the reduc- tion of ketones with 2-propanol at low ruthenium loadings (0.05–0.001 mol%), in a very short reaction time and with high TOF values (10 6 h À1 ) (Figure1). [9] In this paper we describe the use of complex 1 asa highly efficient catalyst for the chemoselective re- duction of aldehydes with 2-propanol as hydrogen donor. When a 0.1M solution of benzaldehyde is refluxed in 2-propanol with 1 (0.05 mol%) and K 2 CO 3 (1 mol%), quantitative formation of benzyl alcohol has been observed within 1 min, leading to a high turn- Adv. Synth. Catal. 2007, 349,1633–1636 #2007Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim 1633 COMMUNICATIONS