# Low Temperature Chemoselective Hydrogenation of Aldehydes over a Magnetic Pd Catalyst
## 1 Introduction  
Catalytic hydrogenation reactions are one of the most important processes, which are key routes for the production of numerous bulk products and intermediates in the chemical industry [1]. In particular, the chemoselective reduction of aldehydes in the presence of reducible functional groups under mild conditions has been recognized as a major challenge [2; 3; 4; 5]. Hence, there as been a growing focus on the purity of the hydrogenation of aldehydes in recent years. The catalytic hydrogenation of aldehydes can be performed either using hydrogen or other hydrogen donors such as organic silanes [6; 7; 8] and alcohols [9]. Transfer hydrogenation of carbonyl compounds by organic silanes can be performed under very mild conditions in the presence of homogenous catalysts, but the use of organic silanes was neither cheap nor green due to the high cost of the organic silanes and the lower atom efficiency [7]. Alcohols especially _iso_-propanol had also been reported to be used for the transfer hydrogenation of carbonyl compounds [9]. However, these catalysts were usually not recyclable and required harsh reaction conditions [10; 11]. Nonetheless, molecular hydrogen is a lower cost, much more atom-economic and cleaner reducing agent compared to the silanes. Therefore, the reduction of aldehydes with H\({}_{2}\) represented one of the most efficient and atom-economical transformations. Both heterogeneous and homogeneous catalysts had been used for the reduction of aldehydes with molecular hydrogen. Generally, reactions with homogeneous catalysts could be performed underrelative mild conditions, as the homogeneous catalysts moved freely in the reaction solution [12; 13; 14]. Of particular note was that several kinds of homogeneous base metal catalysts had been discovered to be active for the hydrogenation of aldehydes under mild conditions. For example, [PERSON] and co-workers demonstrated that manganese complexes could promote the aromatic carbonyl compounds into the corresponding alcohols under mild conditions (60 \({}^{\circ}\)C and 10 bar H\({}_{2}\) pressure) [14]. However, this method required the use of additional additives such as \(t\)-BuONa, which is sensitive to the air and moisture. More importantly, it was difficult to recycle and reuse the homogeneous catalysts [15; 16]. To overcome the associated drawbacks of homogeneous catalysts [17; 18; 19; 20], great effort has been devoted to the design of heterogeneous catalysts for the reduction of carbonyl groups at low temperature. Several kinds of metal catalysts have been reported to be active for the hydrogenation of carbonyl compounds into alcohols at low temperatures or hydrogen pressure. For example, [PERSON] and co-workers reported palladium nanoparticles (Pd-BNP) stabilized by a binaphthyl-backbone can be efficiently used for the chemoselective reduction of aldehydes in the presence of hydrogen at room temperature in water [17]. To further recycle the heterogeneous catalyst easily, a graphene-modified Ru catalyst was also reported to be active for the reduction of carbonyl groups into hydroxyl groups at a low temperature of 20 \({}^{\circ}\)C and 40 bar H\({}_{2}\)[18]. Acknowledging these important achievements, however, the range of chemoselective heterogeneous hydrogenation of aldehydes using molecular hydrogen in the presence of reducible functional groups was still quite limited [20], which restricted their application. For example, the Pd-BNP [17] could not reduce aliphatic dehydes and heteroaromatic aldehydes, while graphene-modified Ru [18] could not selectively reduce aldehydes from ketone. Indeed, to the best of our knowledge, there are no heterogeneous catalysts that can selectively hydrogenation of aldehydes bearing various reducible functional groups including halogen, ketone, ester, carbon-carbon double bond, amine, nitro, or cyano group at low temperature yet.