Reduction benzaldehyde

Catalysts and Catalyst Concentration. The most active catalyst for benzaldehyde reduction appears to be rhodium [Rh6(C0)i6 precursor], but iron [as Fe3(C0)i2] and ruthenium [as Ru3(C0)12] were also examined. The results of these experiments are shown in Table 1. Consistent with earlier results (12), the rhodium catalyst is by far the most active of the metals investigated and the ruthenium catalyst has almost zero activity. The latter is consistent with the fact that ruthenium produces only aldehydes during hydroformylation. Note that a synergistic effect of mixed metals does not appear to be present in aldehyde reduction as contrasted with the noticeable effects observed for the water-gas shift reaction (WGSR) and related reactions (13). 

KOH Concentration Studies. The effect of KOH concentration on benzaldehyde reduction was examined, and the results are shown in Figure 2 along with our previous results for ruthenium catalyzed hydroformylation (12). 

Table I. Benzaldehyde Reduction with Various Catalysts...

Figure 3 shows the results of varying the CO pressure. The maximum activity appears to lie near 600 psi for benzaldehyde reduction. Figure 3 is an attempt to elucidate an apparent reaction order with respect to the arithmetically averaged CO pressure. At pressures less than 400 psi, the order is nearly first order. The situation at higher pressures is not clear however, it is reasonable to speculate that the dominant aspects of the kinetics shift at these pressures. The data suggest the shift is to zero-order dependance. 

Figure 4. Time varying benzaldehyde reduction T = 94°C (O) purge every 1 h,...

The effects of changes in KOH concentration on catalyst activity for benzaldehyde reduction are complex. Figure 2 compares the present work with KOH concentration studies for ruthenium catalyzed hydroformylation ... 

Much more spectacular were the selectivity variations in the case of 3-nitro-benzaldehyde reduction (Table 3). Depending on the hydrogen donor used 3-nitro-benzyl alcohol (methanol, 450°C) or 3-aminobenzaldehyde (i-propanol, 450°C) were the main reaction products. 

Rogic, M. M. Conformational Analysis and the Transition State in Asymmetric Reductions with Boranes Based on (+)-a-Pinene. 1. Benzaldehyde Reduction with Alpine Borane and Other B-Alkyl-9-borabicyclo[3.3.1]nonanes. ASemiempirical Study. J. Org. Chem. 1996, 61, 1341-1346. 

The main absorption band of benzoquinones appears around 260 nm in nonpolar solvents and at 280 nm iu water. Extinction coefficients are 1.3-1.5 x 10 M Upon reduction to hydroquinones, a four times smaller band at 290 nm is found. The most important property of quinones and related molecules is the relative stability of their one-electron reduction products, the semiquinone radicals. The parent compound 1,4-benzoquinone is reduced by FeCl, ascorbic acid, and many other reductants to the semiquinone anion radical which becomes protonated in aqueous media (pk = 5.1). Comparisons of the benzaldehyde reduction potential with some of the model quinones given below show that carbonyl anion radicals are much stronger reductants than semiquinone radicals and that ortho- and para-benzoquinones themselves are even relatively strong oxidants comparable to iron(III) ions in water (Table 7.2.1). This is presumably caused by the repulsive interactions between two electropositive keto oxygen atms, which are separated only by a carbon-carbon double bond. When this positive charge can be distributed into neighboring n systems, the oxidation potential drops significantly (Lenaz, 1985). 

Doherty AP, Brooks CA (2004) Electrosynthesis in room-temperature ionic liquids benzaldehyde reduction. Electrochim Acta 49 3821-3826... 

Since the rates of the individual steps were measured at very different temperatures, measured activation parameters were used to estimate rate constants at the temperature of a given catalytic reaction [69]. The rate of dissociation of bridging hydride 1-tol at 60°C kj) was obtained by measuring the rate of reaction with PPh3 to give 2-tol and phosphine complex C3 the rate was first order in [1-tol] and independent of [PPha]. The rate of benzaldehyde reduction by 2-tol ( 3) had been measured below 0°C, and the rate of loss of H2 from 2-tol had been measured at 90°C. The rates of reaction of reactive intermediate A with H2 ( -2) and with 2-tol... 

Kaskel et al. [145] also studied the catalytic action of MIL-101 in the cyanosilylation reaction. The coordinated water molecules are easily detached, thereby a coordination unsaturation at the Cr(III) sites is created. Owing to the fact that the Lewis acidity of the Cr(III) sites is much higher than that of Cu(II), MIL-101 turned out to be much more active than HKUST-1 in this reaction. In addition, the catalytic sites of MIL-101 are resistant to the side reaction of benzaldehyde reduction, unlike HKUST-1 sites. 

Table 1. Catalytic results for benzaldehyde reduction over Cai.xMgxtTii.xLixXlMxFsx perovskites (pre-reduction 2h/350°C/H2 Treaction=310°C).

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