Oxidation benzyl alcohol to benzaldehyde

Osmium(VI) hydrazido complexes can be generated by electrochemical oxidation of the corresponding osmium(V) hydrazido complexes (Section 5.6.5.3.1). The complex trans-[0s (tpy)(Cl)2(NN(CH2)40)] " (83) is able to oxidize benzyl alcohol to benzaldehyde. It also oxidizes PPhs to PPh30, and R2S to give R2SO the source of O atoms is presumably H2O in the solvent. 

The reactivities of [Ru "(0)(14-TMC)(X)]"+ and its related 15-TMC, 16-TMC, and CRMes coi lexes with organic substrates have also been examined. " " In contrast to polypyridyl Ru =0 species, these macrocyclic Ru =0 complexes are weak oxidants. They oxidize benzyl alcohol to benzaldehyde but do not react with alkenes at room temperature. The lower oxidizing ability of these systems than the polypyridyl systems is due to their lower values. However, [Ru (0)(H20)(N202)](C104)2, which has a higher H value, is able to catalyze the oxidation of norbornylene, styrene, and cyclooctene by PhlO. " ... 

Iodosobenzene diacetate [IBD, PhI(OAc)2] is able to oxidize benzylic alcohols to benzaldehydes when a solid mixture of iodosobenzene diacetate and the alcohol is irradiated with microwaves. Best results are obtained when iodosobenzene diacetate is supported on alumina.118 The use of polymer supported iodosobenzene diacetate (PSDIB) simplifies the work-up in the oxidation of benzylic alcohols to benzaldehydes.119 PSDIB can be employed in the presence of KBr and using water as solvent, resulting in the transformation of secondary alcohols into ketones and primary alcohols into carboxylic acids.117... 

Immobilization of 133 on a polymeric resin shows several advantages over the corresponding monomeric species.199 Oxidation of 133 with sodium periodate affords the corresponding bis-(p ra-methoxyphenyl) tellurone (An2Te02) 134, which presents a peculiar reactivity.198 For example, the tellurone 134 oxidizes benzyl alcohols to benzaldehydes in good yields (Scheme 72), and also converts hydrobenzoin in benzaldehyde in good yield. 

The reagent oxidizes benzyl alcohols to benzaldehydes in over 90% yield. A solution of the oxidant in 50% aqueous acetic acid Is added to the alcohol and the... 

The use of electricity in reactions is clean and, at least in some cases, can produce no waste. Toxic heavy metal ions need not be involved in the reaction. Hazardous or expensive reagents, if needed, can be generated in situ where contact with them will not occur. The actual oxidant is used in catalytic amounts, with its reduced form being reoxidized continuously by the electricity. In this way, 1 mol% of ruthenium(III) chloride can be used in aqueous sodium chloride to oxidize benzyl alcohol to benzaldehyde at 25°C in 80% yield. The benzaldehyde can, in turn, be oxidized to benzoic acid by the same system in 90% yield.289 The actual oxidant is ruthenium tetroxide. Naphthalene can be oxidized to naphthoquinone with 98% selectivity using a small amount of cerium salt in aqueous methanesulfonic acid when the cerium(III) that forms is reoxidized to cerium(IV) electrically.290 Substituted aromatic compounds can be oxidized to the corresponding phenols electrically with a platinum electrode in trifluoroacetic acid, tri-ethylamine, and methylene chloride.291 With ethyl benzoate, the product is a mixture of 44 34 22 o/m/fhhy-... 

Oxidation of alcohols. The complex oxidizes benzyl alcohol to benzaldehyde in yields as high as 97%, but in general primary alcohols such as 1-butanol are oxidized only in low yields. However, secondary alcohols are oxidized to ketones in satisfactory yield (50-707 ). 

Oxidation of Alcohols to Aldehydes and Ketones. Hyper-valent iodine compounds have powerful oxidizing capabilities. However, IBX possesses different properties than many similar oxidants, such as the related analogs iodoxybenzene and m-iodoxybenzoic acid. Until recently, the major application of IBX was its use in DMSO for the oxidation of primary alcohols to aldehydes at room temperature, without the danger of over-oxidation to carboxylic acids. The related iodo-oxy reagents oxidize benzyl alcohols to benzaldehydes at elevated temperatures in benzene (80 °C, 5-10 h) or in acetic acid (rt, 24 h), while IBX oxidizes the same compounds in 15 min (or less) at room temperature. IBX is equally effective for the oxidation of secondary alcohols to ketones under analogous conditions. Even sterically hindered alcohols are readily oxidized. Bomeol undergoes smooth oxida-... 

In neutral aqueous solution at elevated temperatures, sodium dichromate oxidizes benzyl alcohol to benzaldehyde. The mechanism involves formation of a chromate ester and the unfavourable entropy of activation may be due to formation of a cyclic transition state (3) in which the carbonyl group is developed, while a substantial isotope effect (Ath/Atd = 9.4 0.9) indicates transfer of a proton. The oxidation of benzaldehyde to benzoic acid is substantially slower and involves a preequilibrium step in which the carbonyl is hydrated. A similar cyclic transition state is proposed, however. 

Two phenoxyl radical complexes [Cu (2 )N03] and [Zn (2 )N03] oxidize benzyl alcohol to benzaldehyde and have been studied as models for the enzyme galactose oxidase (GO). GO contains a dipeptide unit (3) in which a tyrosine residue is covalently bound to an adjacent cysteine residue and which is similar to (2), the tyrosyl radical in (3) also being bound to the Cu centre (see Figure 1). Second-order kinetics were observed with respect to [Zn°(2 )N03]+ and there was no evidence of redox reaction at the zinc site, suggesting that a dimeric form of the complex is active however, the reaction of [Cu H2 )N03]+ with benzyl alcohol is first order in the metal complex and [Cu (2H)]+ is identified as a product, suggesting a formal 2e /2H+ mechanism in which the monomeric form coordinates the alcohol in the manner believed to operate for G0. 2... 

Hydrated cerium(IV) periodate, CeHI06-4H20, was obtained as a yellow solid by reaction between (NH4)2[Ce(N03)6] and H5IO6 in aqueous solutions at low pH (Levason and Ol-droyd, 1996). This compound, which is insoluble in water, is able to oxidize benzyl alcohol to benzaldehyde in dichloromethane with tetramethylammonium periodate as co-oxidant, but the yields are only moderate (Griffith et al., 1996). 

Under microwave irradiation and applying MCM-41-immobilized nano-iron oxide higher activity is observed [148]. In this case also, primary aliphatic alcohols could be oxidized. The TON for the selective oxidation of 1-octanol to 1-octanal reached to 46 with 99% selectivity. Hou and coworkers reported in 2006 an iron coordination polymer [Fe(fcz)2Cl2]-2CH30H with fez = l-(2,4-difluorophenyl)-l,l-bis[(l//-l,2,4-triazol-l-yl)methyl]ethanol which catalyzed the oxidation of benzyl alcohol to benzaldehyde with hydrogen peroxide as oxidant in 87% yield and up to 100% selectivity [149]. An alternative approach is based on the use of heteropoly acids, whereby the incorporation of vanadium and iron into a molybdo-phosphoric acid catalyst led to high yields for the oxidation of various alcohols (up to 94%) with molecular oxygen [150]. 

The oxidation of benzyl alcohol to benzaldehyde is carried out for elucidating mass transfer effects in a mini trickle-bed reactor [58]. 

Oxidation of Aicohois Investigated in Micro Reactors Cas/iiquid reaction 23 [CL 23] Oxidation of benzyl alcohol to benzaldehyde... 

Figure 5.5 Example of drastic activity enhancements. Aerobic oxidation of benzyl alcohol to benzaldehyde in scC02 over TPAP entrapped in aged ( , ) and fresh ( ) 75% methyl-modified silica matrix. (Reproduced from Adv. Fund. Mater., with permission.)...

In the same year, Wang and Stack (211) reported the first truely catalytic system. They discovered that Complexes E and F shown in Fig. 28 can be oxidized by 1 equiv of tris(4-bromophenyl)aminiumhexachloroantimonate, (TPA)SbCl6, at -0.7 V Fc+/Fc to the corresponding (phenoxyl)copper(II) species, which in turn were shown to stoichiometrically convert 1 equiv of benzyl alcohol to benzaldehyde. In the presence of 20 equiv of (TPA)SbCl6 and base, Complex E displays catalytic behavior (-10 turnovers). 

Deviations from equation (57) have also been used to demonstrate that tunnelling is important in the enzyme-catalysed oxidation of benzyl alcohol to benzaldehyde by NAD+ and yeast alcohol dehydrogenase (YADH) (reaction (60)) (Cha et al., 1989 Klinman, 1991). 

Table 40 The primary and secondary deuterium-tritium and hydrogen-tritium KIEs for the oxidation of benzyl alcohol to benzaldehyde with NAD+ and yeast alcohol dehydrogenase at 25°C.a...

Fig. 6 Illustration from Chin and Klinman. Increased catalytic activity of horse-liver alcohol dehydrogenase in the oxidation of benzyl alcohol to benzaldehyde by NAD, measured by cat/ M (ordinate), correlates with the Swain-Schaad exponent for the -secondary isotope effect (abscissa), for which values above about four are indicators of tunneling. This is a direct test of the hypothesis that tunneling in the action of this enzyme contributes to catalysis. As the rate increases by over two orders of magnitude and then levels off, the anomalous Swain-Schaad exponents also increase and then level off. Reproduced from Ref. 28 with the permission of the American Chemical Society.

Cl /Cl 0 In the presence of polymer-supported phase transfer catalyst and redox mediator Cl /ClO, the oxidation of benzyl alcohol to benzaldehyde or benzoic acid was achieved [39-41]. 

Furthermore, the mediator has been used for the bond cleavage of benzyl ethers, the oxidation of benzyl alcohol to benzaldehyde, the oxidation of toluene derivatives to benzoic acid esters, and the oxidation of aliphatic ethers [47]. 

Hydrated RuO is often used to generate RuO, [RuO ] or [RuO ] using cooxidants such as periodate or bromate. There are many examples in this and subsequent chapters of the use of RuOj.nH O as starting material with co-oxidants such as Na(IO ) for organic oxidations. Surprisingly, RuO was found to be inactive as an oxidation catalyst as RuOj.nHjO/NMO/acetone or DMF [647]. Oxidation of benzyl alcohol to benzaldehyde was effected with RuO or RuCljA Bu bOCl/aq. H O / CHjCy60°C [648]. 

Ru(0)(Rj-dppi)(tpy)] (R3-dppi=R3-3,6-bis(6-chloropyrid-2-yl)pyridazine with R=H, Me, Cl) is probably the species generated by cyclic voltammetric oxidation of [Ru(H30)(R2-dppi)(tpy)] ". The reagents [Ru(0)(Rj-dppi)(tpy)]Vwater pH 7 electrochemically oxidised benzyl alcohol to benzaldehyde. Second-order rate constants were determined and a mechanism proposed which involves pre-association of the substrate with the Ru(lV) complex [679],... 

Typical examples are listed in Table 2.1. A few oxidations are effected by RuO but in general it is too powerful an oxidant for this purpose. The system RuCyaq. NaCl-CCy Pt anode oxidised benzyl alcohol to benzaldehyde and benzoic acid and p-anisaldehyde to p-anisic acid [24], and a wide range of primary alcohols and aldehydes were converted to carboxylic acids, secondary alcohols to ketones, l, -diols to lactones and keto acids from RuOj/aq. NaCl pH 4/Na(H3PO )/Pt electrodes (Tables 2.1-2.4). The system [RuO ] "/aq. K3(S303)/Adogen /CH3Cl3 oxidised benzyhc alcohols to aldehydes [30]. The oxidation catalyst TPAP (( Pr N)[RuO ]) (cf. 1.3.4) is extremely useful as an oxidant of primary alcohols to aldehydes and secondary alcohols to ketones without... 

SCHEME 116. Oxidation of benzylic alcohols to benzaldehydes and benzoic acids with Na2W04/... 

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