Hydrogenation of benzophenone

During the hydrogenation of benzophenone to benzhydrol, the undesired side-product, diphenylmethane, is obtained by hydrogenolysis of the C-O bond (Scheme 4.25). 

As documented throughout this handbook, the diversity of reaction patterns of transition-metal complexes leads to a remarkably rich chemistry, with a tremendous mechanistic diversity in the details of how H2 is added to unsaturated substrates. Over forty years ago, Walling and Bollyky reported a catalytic hydrogenation of benzophenone that required no transition metal at all They found that the C=0 bond of benzophenone can be catalytically hydrogenated using KOtBu as a base [88], but harsh conditions (200°C, 100 bar H2) were used (Eq. (49)). Ber-kessel et al. recently examined details of this reaction and provided evidence that it was first order in ketone, first order in hydrogen, and first order in base [89]. 

An alternative variation to this catalyst, Lrans- RuCl3j P(Q,I l4-4-CI l () (b (H2NCH2CH2NH2)] and KOtBu in isopropanol, is excellent for the selective hydrogenation of benzophenones (Scheme 15.8) [53]. 

Table 15.11 Hydrogenation of benzophenones with trans-[RuCI2 P(C6H4-4-CH3)3 2(H2NCH2CH2NH2)] and KOtBu in /PrOH under 8 bar H2 at 28-35 °C.

The transfer hydrogenation of aromatic ketones, which is typically catalyzed by ruthenium half-sandwich complexes using, e.g., formic acid as hydrogen source, was chosen as another model system. After applying an appropriate TSA molecule as template, i. e., a ruthenium phosphinato complex, the resulting MIP catalyzed the hydrogenation of benzophenone approximately twice as effectively as the CP [116]. 

Table 6.10 Hydrogenation of benzophenone Initial reaction rate (r,) selectivity to products at 100% conversion. Results for the chemical reduction with NaBH4 are included for comparison. DPM diphenylmethanol, CHPK cyclohexyl phenyl ketone, CHPM cyclohexylphenylmethanol, DCHK dicyclohexyl ketone and DCHM dicyclohexylmethanol. (Reproduced from Reference [34].)...

T. Yokozawa, and R. Noyori, Selective hydrogenation of benzophenones to benzhydrols. Asymmetric synthesis of unsymmetrical diarylmethanols, Org. Lett. 2000a, 2, 659-662. 

In a patent, lead-poisoned palladium catalyst was claimed to be effective for hydrogenation of benzophenone to benzhydrol at 115°C and 0.34 MPa H2.115 Kumbhar and Rajadhyaksha hydrogenated benzophenone to benzhydrol in 98.4% selectivity at 88% conversion over Ni-Fe (75 25) on Ti02 using methanol-10% water as solvent and NaOH (0.1 wt% of benzophenone) as additive at 135°C and 5.9 MPa H2.116... 

Not surprisingly, these rhodium and iridium carbene complexes were tested for their catalytic behaviour in the transfer hydrogenation of benzophenone and acetophenone (M +3), the hydrosilylation of alkynes (M +1) and also the catalytic cyclisation of acetylenic carboxylic acids (M +1). Hydrogenation works better for iridium than rhodium and for aromatic than for aliphatic ketones [40,43,44]. The iridium(I) complex is the first iridium catalyst showing activity for the cyclisation of acetylenic carboxylic acids [40]. The results for the hydrosilylation reactions were very moderate. 

Liquid phase catalytic hydrogenation of benzophenone Role of metal support interaction, bimetallic catalysts, solvents and additives... 

Liquid-phase hydrogenation of benzophenone was studied over well characterised supported monometalic Ni and bimetallic Ni-Cu and Ni-Fe (75 25, composition by mass) catalysts. Interesting changes in selectivity were obtained over the bimetallic catalysts. Different strategies such as use of Ti02 as a support, change in solvent and use of NaOH were tested to improve the activity and selectivity to benzhydrol. A Ni-Fe(75 25)/Ti02 catalyst combined with NaOH and methanohwater as solvent was found to be the most efficient system to selectively obtain benzhydrol. 

Hydrogenation of benzophenone (0.065 mol) (Loba Chemie, A.R. grade) was carried out in a 100 ml Parr autoclave using appropriate solvents (35.5 ml,... 

Hydrogenation of benzophenone was first studied using methanol as solvent. A typical concentration profile for the reaction over Ni/Si02 catalyst is shown in Figure 1. 

Comparison of various catalysts for hydrogenation of benzophenone in presence of methanol... 

An cxcellenl catalytic performance in liquid phase hydrogenation of benzophenone to ben/hydrol was accomplished with Ni/titania cogeled aerogel catalyst (153. The heat preireatinent conditions liad profound effect on tlie textural and structural properties of the resulting aerogels and their catalytic perfonnance. 

Figure 13 Transfer hydrogenation of benzophenone catalyzed by a ruthenium containing MIP.

Bawane SP, Sawant SB. (2003) Kinetics of liquid-phase catalytic hydrogenation of benzophenone to benzhydrol. Org. Proc. Res. Dev., 7 769-773. 

Figure 5. Transfer hydrogenation of benzophenone catalysed by chitosan-Pd/C...

In 2000, Noyori et al. investigated enantioselective hydrogenation of benzophenones 88 to benzhydrols in the... 

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