Of benzylic alcohols to aldehydes

Oxidation. This reagent is prepared by reaction of CAN with K2C03 (2 equiv.) to form Ce(C03)2, which is then treated with trifluoromethanesulfonic acid (4 equiv.). This oxidant is effective for oxidation of benzylic alcohols to aldehydes (72-92% yield), and of alkylarenes to aldehydes or ketones (65-70% yield). 

Oxidations. Aromatic aldehydes are obtained by oxidation catalyzed with a Mn(IV) complex. A procedure for oxidation of alcohols which is organic solvent free and halide free employs 30% H Oj, Na2W04 dihydrate, and a quaternary ammonium hydrogensulfate. Under such conditions secondary alcohols are oxidized 4-5 times faster than primary alcohols. In toluene the conversion of benzylic alcohols to aldehydes or acids (depending on quantities of HjOj) is accomplished. A similar system is also effective for epoxidation of alkenes. Terminal epoxides are obtained in reactions mediated by a Mn(II) complex or Mg-Al-O-t-Bu hydrotalcite. The last catalyst is capable of inducing epoxide formation from other alkenes and enones. 

Jiang N, Ragauskas AJ (2005) TEMPO-catalyzed oxidation of benzylic alcohols to aldehydes with the HjO /HBr/ionic liquid [bmim][PFJ system. Tetrahedron Lett 46 3323-3326... 

Chang SU, Cho JH, Lee JC (2008) Efficient oxidation of benzylic alcohols to aldehydes and ketones in ionic liquid using N-chlorosuccinimide/AlClj-6HjO. Bull Korean Chem Soc 29 27-28... 

Zr compounds are also useful as Lewis acids for oxidation and reduction reactions. Cp2ZrH2 or Cp2Zr(0 Pr)2 catalyze the Meerwein-Ponndorf-Verley-type reduction and Oppenauer-type oxidation simultaneously in the presence of an allylic alcohol and benzaldehyde (Scheme 40).170 Zr(C)1 Bu)4 in the presence of excess l-(4-dimethylaminophenyl) ethanol is also an effective catalyst for the Meerwein-Ponndorf-Verley-type reduction.1 1 Similarly, Zr(0R)4 catalyze Oppenauer-type oxidation from benzylic alcohols to aldehydes or ketones in the presence of hydroperoxide.172,173... 

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. 

Much emphasis has been placed on the selectivity of quaternary ammonium borohydrides in their reduction of aldehydes and ketones [18-20]. Predictably, steric factors are important, as are mesomeric electronic effects in the case of 4-substituted benzaldehydes. However, comparison of the relative merits of the use of tetraethyl-ammonium, or tetra-n-butylammonium borohydride in dichloromethane, and of sodium borohydride in isopropanol, has shown that, in the competitive reduction of benzaldehyde and acetophenone, each system preferentially reduces the aldehyde and that the ratio of benzyl alcohol to 1-phenylethanol is invariably ca. 4 1 [18-20], Thus, the only advantage in the use of the ammonium salts would appear to facilitate the use of non-hydroxylic solvents. In all reductions, the use of the more lipophilic tetra-n-butylammonium salt is to be preferred and the only advantage in using the tetraethylammonium salt is its ready removal from the reaction mixture by dissolution in water. 

The conversion of primary alcohols to aldehydes was achieved by using a biphasic system consisting of water and a nonpolar organic solvent such as petroleum ether. Under these conditions, benzylic and allylic alcohols were oxidized with high selectivity to the aldehydes (for example, (16) to (17) in Scheme 4), while aliphatic alcohols were converted to aldehydes with poor selectivity [17]. 

In this transformation, manganese(IV) oxide oxidizes allylic or benzylic alcohols to aldehydes followed by nucleophilic attack of the in situ formed triazolinyli-dene carbene (Scheme 41). The authors suggest the formation of an acyl anion equivalent LX is slow in MeOH compared to oxidation to allow for an activated carboxylate LXII. 

This heme-dependent enzyme [EC 1.11.1.14], also known as diarylpropane peroxidase, diarylpropane oxygenase, and ligninase I, catalyzes the reaction of 1,2-bis(3,4-dimethoxyphenyl)propane-l,3-diol with hydrogen peroxide to produce veratraldehyde, l-(3,4-dimeth-ylphenyl)ethane-l,2-diol, and four water molecules. The enzyme brings about the oxidative cleavage of C—C bonds in a number of model compounds and also oxidizes benzyl alcohols to aldehydes or ketones. 

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... 

Taylor and Flood could show that polystyrene-bound phenylselenic acid in the presence of TBHP can catalyze the oxidation of benzylic alcohols to ketones or aldehydes in a biphasic system (polymer-TBHP/alcohol in CCI4) in good yields (69-100%) (Scheme 117) °. No overoxidation of aldehydes to carboxylic acids was observed and unactivated allylic alcohols or aliphatic alcohols were unreactive under these conditions. In 1999, Berkessel and Sklorz presented a manganese-catalyzed method for the oxidation of primary and secondary alcohols to the corresponding carboxylic acids and ketones (Scheme 118). The authors employed the Mn-tmtacn complex (Mn/168a) in the presence of sodium ascorbate as very efficient cocatalyst and 30% H2O2 as oxidant to oxidize 1-butanol to butyric acid and 2-pentanol to 2-pentanone in yields of 90% and 97%, respectively. This catalytic system shows very good catalytic activity, as can be seen from the fact that for the oxidation of 2-pentanol as little as 0.03% of the catalyst is necessary to obtain the ketone in excellent yield. 

Scheme 12 Electrocatalytic oxidation of benzylic alcohol to benzylic aldehyde using [Cr" (0H2)PWii039] as redox catalyst taken from Ref 8).

Basic alumina (13 g) was added to a mixture of methyl 4-formylbenzoate Id (1.00 g, 6 mmol) and acetophenone 2 (0.48 g, 4 mmol) at room temperature. (When the reactants were solid, a minimum amount (2x3 mL) of dichloro-metliane was used to dissolve them prior to the addition of the alumina.) The reaction mixture was then agitated at room temperature for 2.5 h using a Fisher vortex mixer. The product was extracted into dichloromethane (5x15 mL). Removal of the solvent, under reduced pressure, yielded the solid product. Further purification (removal of traces of benzyl alcohol and aldehyde) was carried out by recrystallization from a petroleum ether-ether mixture to afford l-phenyl-3-[4-(carbomethoxy)phenyl]-2-propen-1-one 3d (4-carbomethoxychalcone), mp 119— 120 °C (81%). 

A catalytic system, based on TEMPO and Cu(II), has been developed for the selective oxidation of primary alcohols to aldehydes under very mild conditions. Cu(II) is generated in situ by oxidation of elemental copper and chelated by means of 2,2 -bipyridine. The reaction is dependent on pH. New insights into the currently accepted mechanism have been discussed.76 Allylic and benzylic alcohols are selectively oxidized with trimethylamine N-oxide in the presence of cyclohexa-1,3-dieneiron carbonyl.77... 

Catalysis by TEMPO has the advantage of being general for oxidation of both benzylic or non benzylic alcohols to aldehydes, whereas catalysis by PINO, although limited to the synthesis of aromatic aldehydes, has the advantage that the radical is generated in situ from the less expensive N-hydroxyphthalimide, which can be more easily recovered and recycled. 

Our own work in the area of aerobic oxidations was inspired by the exquisite research performed on the structure and reactivity of the binuclear copper proteins (7), hemocyanin and tyrosinase, and by the seminal contribution of Riviere and Jallabert (8). These two authors have shown that the simple copper complex CuCl - Phen (Phen = 1,10-phenanthroline) promoted the aerobic oxidation of benzylic alcohols to the corresponding aromatic aldehydes and ketones (Fig. 2). 

The oxidation of primary alcohols with K2Cr207 in aqueous solution to nothing but the aldehyde, (i.e., without further oxidation to the carboxylic acid) is possible only if a volatile aldehyde results and is distilled off as it is formed. This is the only way to prevent the further oxidation of the aldehyde in the (aqueous) reaction mixture. Selective oxidations of primary alcohols to aldehydes with the Jones reagent succeed only for allylic and benzylic alcohols. Otherwise, the Jones reagent directly converts alcohols into carboxylic acids (see above). 

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