Oxidation of benzylic alcohol

The only industrially important processes for the manufacturing of synthetic benzaldehyde involve the hydrolysis of benzal chloride [98-87-3] and the air oxidation of toluene. The hydrolysis of benzal chloride, which is produced by the side-chain chlorination of toluene, is the older of the two processes. It is no longer utilized ia the United States. Other processes, including the oxidation of benzyl alcohol, the reduction of benzoyl chloride, and the reaction of carbon monoxide and benzene, have been utilized ia the past, but they no longer have any iadustrial appHcation. 

We have also found that BTMA Br3 can be used as a reagent for the oxidation of benzyl alcohols to benzoic acids. That is, the reaction of benzyl alcohols with 2-equiv. of BTMA Br3 in an aq. alkaline solution at room temperature or at 70°C afforded benzoic acids in good yields. Thus, we could selectively obtain the oxidation products, benzaldehydes and benzoic acids, from benzyl alcohols by using a stoichiometric amount of BTMA Br3 (Fig. 24) (ref. 32). 

Fig. 24. Selective oxidation of benzyl alcohols with BTMA Br3...

In the field of nonheme iron complexes, Miinck, Collins, and Kinoshita reported the oxidation of benzylic alcohols via stable p-oxo-bridged diiron(IV) TAME complexes, which are formed by the reaction of iron-28 complexes with molecular oxygen (Scheme 23) [142]. 

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]. 

An analogous study of the oxidation of benzyl alcohol by permanganate in aqueous perchloric acid yielded a rate law... 

These are generally oxidised with the same kinetics as saturated alcohols but more rapidly. In addition to the example of allyl alcohol - discussed previously (p. 377), several reports exist of oxidations of benzylic alcohols. The importance of a 1 1 complex K = 0.8 0.2 at 25 °C, [HCIO4] = 0.525 M) in the Ce(IV) oxidation is clear from agreement of spectroscopic and kinetic data . For a... 

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

Especially the favorable mass transfer of micro reactors is seen to be advantageous for the oxidation of benzyl alcohol [58]. As one key to this property, the setting and knowledge on flow patterns are mentioned. Owing to the special type of microreactor used, mixing in a mini trickle bed (gas/liquid flows over a packed particle bed) and creation of large specific interfaces are special aspects of the reactor concept. In addition, temperature can be controlled easily and heat transfer is large, as the whole micro-reactor construction acts as a heat sink. 

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

Seddon, K.R. Stark, A. (2002) Selective Catalytic Oxidation of Benzyl Alcohol and Alkylbenzenes in Ionic Liquids. Green Chemistry, 4(2), 119-123. 

The pyridine-catalysed lead tetraacetate oxidation of benzyl alcohols shows a first-order dependence in Pb(OAc)4, pyridine and benzyl alcohol concentration. An even larger primary hydrogen kinetic isotope effect of 5.26 and a Hammett p value of —1.7 led Baneijee and Shanker187 to propose that benzaldehyde is formed by the two concurrent pathways shown in Schemes 40 and 41. Scheme 40 describes the hydride transfer mechanism consistent with the negative p value. In the slow step of the reaction, labilization of the Pb—O bond resulting from the coordination of pyridine occurs as the Ca—H bond is broken. The loss of Pb(OAc)2 completes the reaction with transfer of +OAc to an anion. 

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.)...

OxidationThe rate of oxidation of alcohols with 1 can be markedly increased by addition of pyridine. In fact 1 in combination with pyridine is an excellent reagent for oxidation of 1,4-hydroquinones to the quinones. When used in the absence of pyridine, 1 can effect selective oxidation of benzylic alcohols in the presence of a secondary alcohol. 

Figure 32. Proposed mechanism for the aerobic oxidation of benzyl alcohol by Complex E. [Adapted from (212).]...

Izumi and Urabe [105] found first that POM compounds could be entrapped strongly on active carbons. The supported POMs catalyzed etherization of ferf-butanol and n-butanol, esterification of acetic acid with ethanol, alkylation of benzene, and dehydration of 2-propanol [105], In 1991, Neumann and Levin [108] reported the oxidation of benzylic alcohols and amines catalyzed by the neutral salt of Na5[PV2Mo10O40] impregnated on active carbon. Benzyl alcohols were oxidized efficiently to the corresponding benzaldehydes without overoxidation ... 

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...

Preferential oxidation of benzylic alcohols with Fremy s salt... 

Experiments with the oxidation of benzyl alcohol by NAD, catalyzed by yeast alcohol dehydrogenase, yielded =. 12 —. 16 (standard... 

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.

Other examples of conversion using a nickel oxide electrode are the oxidation of benzyl alcohols. A nickel foam electrode was used for oxidation [18] in this case and the mechanism was studied by... 

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]. 

---