Benzylic alcohols, selective oxidation

Ferrate salts have been used under phase-transfer catalytic conditions for the oxidation of alcohols. Selective oxidation of allylic and benzylic alcohols to the corresponding aldehydes occurs under mild conditions [4],... 

Primary and secondary alcohols were selectively oxidized to the corresponding aldehydes and ketones, respectively, by using Oxone in the presence of a catalytic amount of TEMPO (2,2,6,6-tetramethyl-l-oxypiperidinyl). This reaction has been proved to be a highly selective and efficient oxidation reaction, where a catalytic amount of TEMPO plays an important role. Thus TBDMS protected benzyl alcohols were oxidized selectively to benzaldehydes in 81% yield, without affecting the TBDMS moiety. 

Allylic and benzylic alcohols were oxidized to aldehydes or ketones with BnPhsPHSOs in refluxing CHsCN. The yield increased in the presence of bismuth chloride in a catalytic amount. Selective oxidation of various alcohols under solvent free conditions was also reported Interestingly, benzyl alcohols were oxidized selectively to benzaldehydes in very high yield (95-100%) when reacted with BnPhsPHSOs (1.2 eq.) and AICI3 (1 eq.) in the presence of an equimolar amount of 2-phenethyl alcohol, diphenyl carbinol or methyl phenyl sulfide (equation 72). 

Such regioselectivities are unique and suggest that redox pillared clays may have broad scope and utility as selective, heterogeneous catalysts for liquid phase oxidations. Indeed, V-PILC also catalyzes the oxidation of benzyl alcohol (to a mixture of benzoic acid and benzylbenzoate) whilst a-methyl benzylalcohol is left completely untouched.71 Similarly, p-substituted benzyl alcohols are oxidized whilst o-substituted benzyl alcohols are inert.71... 

Oxidation of primary and secondary alcohols. In contrast to dimethyl sulfide-NCS (4, 88-90), which oxidizes both primary and secondary alcohols, diisopropyl sulfide-NCS can effect selective oxidation of these substrates. At 0° it oxidizes only primary alcohols to aldehydes, but at —78° it oxidizes only secondary alcohols to ketones. However, allylic or benzylic alcohols are oxidized at either temperature. 

Many chromium oxidants suffer from problems of stability, light sensitivity or acidity, but tetrakis(py-ridine)silver dichromate is stable, nonphotosensitive, nonhygroscopic and a neutral oxidant It can be used to oxidize allylic and benzylic alcohols selectively in benzene. Unfortunately, it cannot be used in chlorinated solvents because it decomposes in these solvents. 

An interesting example of this type of chemoselective oxidation has been reported with the reagent mixture derived irom (Uisopropyl sulfide and )V-chlorosuccinimide. This reagent will oxidize selectively a primary alcohol to an aldehyde at 0 C. Surprisingly, this same reagent at -78 C will oxidize selectively a secondary alcohol to the corresponding ketone (Scheme 2). Allylic and benzylic alcohols are oxidized at both temperatures. 

In contrast to the lack of selectivity observed in the TS-1 catalyzed oxidation of 3-penten-2-ol (1) (Eqn. 21.5), the oxidation of 1 with tert-butyl hydroperoxide (TBHP) over Cr-PILC gave the unsaturated ketone, 3, in 82% yield (Eqn. 21.13)42 while the oxidation of 1 over a vanadium pillared montmorillonite (V-PILC) gave the epoxy alcohol, 2, in 94% yield.43 V-PILC, however, does promote the oxidation of primary benzyl alcohols to the acids with tert-butyl hydroperoxide. This reaction exhibits shape selectivity in that para-substituted benzyl alcohols are oxidized while the ortho- and meta- substituted species are essentially inert (Eqn. 21.14).44... 

In continuation of this study. Farmer and Welton investigated the selective oxidation of alcohols to aldehydes and ketones by TRAP in conjunction with eithCT A-methylmorpholine-N -oxide or molecular oxygen as oxidants [40]. In the case of molecular oxygen (as oxidant), Cu(I) and a diamine ligand must also be added. Under the optimized conditions, benzylic alcohols were oxidized in good to excellent yields, whereas aliphatic alcohols required longer reaction time and gave poor yield. The reaction products were easily removed from the reaction mixture by extraction with diethyl ether. 

Figure 22.1. Toluene conversion and selectivity to partial oxidation products. Reaction conditions 160°C, 0.1 MPap02, 20 ml toluene, 0.8 g of catalyst (1 wt% AuPd/C prepared by sol-immobilisation with 1 1.85 Au/Pd ratio), toluene/metal molar ratio of 3,250 and reaction time 110 h. Key o conversion, selectivity to benzyl alcohol, selectivity to benzaldehyde, selectivity to benzoic acid, selectivity to benzyl benzoate.

Selective oxidation of a,p-unsatutrated (allylic, benzylic, acetylenic) alcohols. 

Oxidizing toluene to benzaldehyde is a catalyzed reaction in which a selective catalyst limits further oxidation to benzoic acid. In the first step, benzyl alcohol is formed and then oxidized to benzaldehyde. Further oxidation produces benzoic acid ... 

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...

Similarly, Dakka and Sasson (ref. 26) showed that benzylic alcohols could be selectively oxidized to the corresponding aromatic aldehydes using HBr/H202 as the oxidant (Fig. 23). The reaction was not successful with electron-rich aromatics which underwent competing nuclear bromination. 

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]. 

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

In earlier work, Bhaumik and Kumar (1995) have reported that the use of two liquid phases in the oxidation of hydrophobic organic substances with aqueous H2O2 using titanium silicate as the catalyst not only enhances the rate of oxidation but also improves selectivity for species like toluene, anisole, and benzyl alcohol. For a single liquid phase acetonitrile was u.sed a solvent. The solid-liquid system gives high ortho selectivity. Thus, in the case of anisole the ratios of o to p for. solid-liquid and solid-liquid-liquid system were 2.22 1 and 0.35 1, respectively. 

Generally, primary aliphatic alcohols are oxidized to their respective aldehydes, secondary aliphatic and aromatic alcohols to the corresponding ketones, and allyl and benzyl alcohols to their carboxylic acid or carboxylate ions. For instance, 2-propanol, acetaldehyde, and methyl-benzoate ions are oxidized quantitatively to acetone, acetate, and terephtalate ion respectively, while toluene is converted into benzoate ion with an 86% yield. Controlling the number of coulombs passed through the solution allows oxidation in good yield of benzyl alcohol to its aldehyde. For diols,502 some excellent selectivity has been reached by changing the experimental conditions such as pH, number of coulombs, and temperature. 

Manganese dioxide (Mn02) supported on silica provides an expeditious and high-yield route to carbonyl compounds. Benzyl alcohols are selectively oxidized to carbonyl compounds by use of 35 % Mn02 doped silica under MW irradiation conditions (Scheme 6.28) [96]. 

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