# Low Temperature Chemoselective Hydrogenation of Aldehydes over a Magnetic Pd Catalyst
## 3 Results and Discussion
### Substrate Scope of the Reduction of Aldehydes to Alcohols  
After achieving the best reaction conditions, thescope of the hydrogenation of various aldehydes wasinvestigated and the results were summarized in Table 2. To our delib \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Entry** & **Catalyst** & **Solvent** & \
\begin{tabular}{c} **Con. of Benzyl** \\ **Aldehyde(\%)** \\ \end{tabular} &
\begin{tabular}{c} **Sel. of Benzyl** \\ **Alcohol(\%)** \\ \end{tabular} \\ \hline
1 & \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)@HAP-Pd & Hexane & 17.2 & 90.1 \\
2 & \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)@HAP-Pd & THF & 34.2 & 90.9 \\
3 & \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)@HAP-Pd & \(i\)-PrOH & 56.9 & 99.0 \\
4 & \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)@HAP-Pd & EtOH & 95.6 & 96.5 \\
5 & \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)@HAP-Pd & H\({}_{2}\)O & 98.7 & 96.4 \\
6b & \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)@HAP-Pd & H\({}_{2}\)O & 100 & \(>\)99 \\
7c & Pd-BNP & H\({}_{2}\)O & 90d  \\ \hline \hline \end{tabular}  
* \end{table} 
Table 1: Results of the hydrogenation of benzyl aldehyde in different solvents a.  
Figure 1: Results of the reduction of benzaldehyde at different hydrogen pressure. Reaction conditions: benzaldehyde (1 mmol), 2 wt.%-Fe\({}_{2}\)O\({}_{3}\)@HAP-Pd catalyst (10 mg), H\({}_{2}\)O (10 mL), 25°C, 1 h.  
kinds of labile groups on the aromatic ringwere well tolerated and gave the corresponding alcohols with moderate to excellent yields in 1-21 h at room temperature. It wasinterestingly to note that the reaction rate was accelerated in presence of electron-withdrawing groups such as -CF\({}_{3}\), -F, -Cl, -Br, -NO\({}_{2}\), and -CN (Entries 1-6, Table 2). 4-(Trifluoromethyl) benzaldehyde, 4-fluorobenzaldehyde, and 4-chlorobenzaldehyde were converted to the corresponding alcohol smoothly in 4 h with nearly quantitative yields; however, deformation occurred when 4-bromobenzaldehyde used as substrate (Entry 4, Table 2). Interestingly, our catalyst couldselectivelyreduce aldehydes, while other carbonyl compounds such like ester or ketone groups were well preserved (Entries 7-8, Table 2). Lower reaction rates were observed for substrates with electron-donating substituted groups than those with electron-withdrawing substituents (Entries 10-14 Table 2 vs. Entries 1-8 Table 2). Longer reaction times (5-21 h) were needed to get good to excellent yields of corresponding alcohols. These results suggested that the electron state of other functional groups controls the activation state of the intermediate in this reaction. The reaction rate increasing with the electron-withdrawing groups indicated that electron went towards the aromatic ring in intermediates [30]. Moreover, all fused-ring aromatic aldehyde (2-naphthaldehyde), the heterocyclic aldehyde (4-pyridinescarboxaldehyde), aliphatic aldehydes (heptanal), and \(\alpha\),\(\beta\)-unsaturated aldehydes (cinnamaldehyde), which were reported unreactive by [PERSON] [17], could be reduced to corresponding alcohols with good to excellent selectivity by our method(Entries 15-19, Table 2).This means that our methodhada broad scope for aldehydes. It was worth noting that our method providedan economical and environmentally-friendly protocol for the hydrogenation of aldehydes into alcohols, because of the use of cheap and green solvent of water, and the mild reaction conditions. Our method could selectively hydrogenation of aldehydes bearing all kinds of reducible functional groups suchas ketone, halogen, nitro, amine, or cyano groups with moderate to excellent yield, which expanded the scope of substrates for this kind of reaction and could be very useful in organic synthesis. To get more insights into the catalytic reduction of aldehydes over the \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)@HAP-Pd catalyst, the reaction rates were studied in detail using the Hammett equation as an empirical rule.