Meerwein–Ponndorf–Verley-type Reduction over a Metal-free TiO2 Photocatalyst in Alcohol: Chemoselective Hydrogenation of Chlorobenzaldehyde to Chlorobenzyl Alcohol

Abstract

p-Chlorobenzaldehyde was almost quantitatively and chemoselectively reduced to p-chlorobenzyl alcohol in alcoholic suspensions of a metal-free titanium(IV) oxide photocatalyst under hydrogen-free conditions. Since alcohols act as electron donors and hydrogen sources, this reaction can be regarded as a heterogeneous Meerwein–Ponndorf–Verley-type reduction over a photocatalyst. Excellent chemoselectivity was also observed in the intermolecular competitive reduction of benzaldehyde (BA) and chlorobenzene (CB), i.e., only BA was reduced to benzyl alcohol, with CB not being reduced.

Heterogeneous Meerwein‐Ponndorf‐Verley–type Reduction of Aromatic Aldehydes Having Other Reducible Functional Groups over a TiO2 Photocatalyst

“…As shown in Figure , the values of MB close to unity and the high yield of CBAO indicate that only hydrogenation of the carbonyl group to a hydroxy group occurred and that no decomposition of CBAD or CBAO, no excessive hydrogenation of the hydroxy group to a methyl group and no hydrogenation of the aromatic ring occurred. In the previous paper, we found that acetone was formed with a stoichiometry of 0.96 in the photocatalytic MPV‐type reaction in 2‐propanol . According to Scheme , acetaldehyde would be formed as the product of ethanol oxidation, although we did not analyze acetaldehyde in this study.…”

“…However, there is little information on chemoselective reduction of carbonyl compounds having other functional groups over a photocatalyst because they focused on the mechanism in the reduction (hydrogenation) of acetophenones. On the other hand, we briefly reported that almost complete chemoselectivity toward the carbonyl group was achieved in reduction of p ‐chlorobenzaldehyde (CBAD) to p ‐chlorobenzyl alcohol (CBAO) in alcohol suspensions of metal‐free TiO 2 photocatalyst, i. e., only the carbonyl group was reduced, with the chloro group being preserved . When 2‐propanol was used as a solvent, acetone was formed with almost the same yield of CBAO, indicating that a heterogeneous MPV‐type redox reaction actually occurred over the TiO 2 photocatalyst (Scheme ).…”

Selective Transfer Hydrogenation of Cinnamaldehyde with Alcohol on Amorphous TiO2 Catalysts: Competitive Adsorption between H2O and Carbonyl

“…Highly selective hydrogenation of CAL to COL was achieved in the alkaline isopropanol solution using Au‐supported SiC and Au@TiO 2 nanoparticles as photocatalysts, respectively . Kominami et al . reported that p‐chlorobenzaldehyde was almost quantitatively and chemoselectively reduced to p‐chlorobenzyl alcohol in alcoholic suspensions of metal‐free titanium(IV) oxide photocatalyst under hydrogen‐free conditions.…”

“…Recently, several heterogeneous catalytic Meerwein‐Ponndorf‐Verley (MPV)‐type reductions have been reported to hydrogenate the unsaturated organic compounds in alcohols without high‐pressure hydrogen . Comparing to the traditional MPV reaction, the photocatalytic MPV system is a green pathway to realize the hydrogen transfer reaction . Highly selective hydrogenation of CAL to COL was achieved in the alkaline isopropanol solution using Au‐supported SiC and Au@TiO 2 nanoparticles as photocatalysts, respectively .…”

Bromine Substitution of Organic Modifiers Fixed on a Titanium(IV) Oxide Photocatalyst: A New Strategy Accelerating Visible Light‐Induced Hydrogen‐Free Hydrogenation of Furfural to Furfuryl Alcohol

“…We have reported H 2 -free and metal-free hydrogenation of FAL to FOL in alcohol suspensions of a TiO 2 photocatalyst having a large surface area under atmospheric pressure at room temperature. [14] Since TiO 2 is chemically stable and shows high photocatalytic performance under irradiation of UV light, [15] TiO 2 has been used as photocatalytic materials for various reactions (Figure 1(a)). However, UV light accounts for only about 5 % of total solar energy, and the development of a photocatalyst that works under visible light is therefore important for better utilization of solar energy.…”