Irradiation benzyl alcohol

Rapid loading of cross-linked PS Wang resin (4-(benzyloxy)benzyl alcohol PS) with a selection of /3-ketoesters was shown to reach completion within 1-10 min if microwave irradiation at 170 °C was employed. The conventional thermal method for acetoacetylation of hydroxymethyl-functionalized polystyrene resins takes several hours therefore, microwave heating allowed for... 

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]. 

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]. 

Work by Ono et al. [66] has been specifically directed at ultrasonic control of product-selectivity in electroreductions. Using a lead cathode, in dilute methanolic sulphuric acid, at a constant current of 20 mA cm , Ono electroreduced benzaldehyde under stirred, unstirred and ultrasonic conditions (Fig. 6.17). In an unstirred system, benzyl alcohol (two-electron process) was the major product, while mechanical stirring reversed the position in favour of the hydrodimer (one-electron product). Ultrasonic irradiation from a cleaning bath (100 W, 36 kHz) so strongly favoured the hydrodimer that the alcohol was barely evident (Tab. 6.16). 

Jang and McDow (1997) studied the photodegradation of benzo[a]anthracene in the presence of three common constituents of atmospheric aerosols reported to accelerate benzo [a] anthracene, namely 9,10-anthroquinone, 9-xanthone, and vanillin. The photo-degradation experiments were conducted using a photochemical reactor equipped with a 450-W medium pressure mercury arc lamp and a water bath to maintain the solution temperature at 16 °C. The concentration of benzo [a] anthracene and co-solutes was 10" M. Irradiation experiments were conducted in toluene, benzene, and benzene-c/e- Products identified by GC/MS, FTIR, and NMR included benzo[a]an-thracene-7,12-dione, phthalic acid, phthalic anhydride, 1,2-benzenedicarboxaldehyde, naphtha-lene-2,3-dicarboxylic acid/anhydride, 7,12-dihydrobenzo[a]anthracene, 10-benzyl-10-hydroan-thracen-9-one, benzyl alcohol, and 1,2-diphenylethanol. 

A //-hexane solution containing ///-xylene and spread as a thin film (4 mm) on cold water (10 °C) was irradiated by a mercury medium pressure lamp. In 3 h, 25% of the ///-xylene photooxidized into ///-methylbenzaldehyde, ///-benzyl alcohol, ///-benzoic acid, and m methylacetophenone (Moza and Feicht, 1989). 

Photolytic. A n-hexane solution containing /n-xylene and spread as a thin film (4 mm) on cold water (10 °C) was irradiated by a mercury medium pressure lamp. In 3 h, 18.5% of the p-xylene photooxidized into p-methylbenzaldehyde, p-benzyl alcohol, p-benzoic acid, and p-methylacetophenone (Moza and Feicht, 1989). Glyoxal and methylglyoxal were produced from the photooxidation of p-xylene by OH radicals in air at 25 °C (Tuazon et al., 1986a). The rate constant for the reaction of p-xylene and OH radicals at room temperature was 1.22 x lO " cmVmolecule-sec (Hansen et al., 1975). A rate constant of 7.45 x 10 L/molecule-sec was reported for the reaction of p-xylene with OH radicals in the gas phase (Darnall et al, 1976). Similarly, a room temperature rate constant of 1.41 x 10 " cm /molecule-sec was reported for the vapor-phase reaction of p-xylene with OH radicals (Atkinson, 1985). At 25 °C, a rate constant of 1.29 x lO " cmVmolecule-sec was reported for the same reaction (Ohta and Ohyama, 1985). 

An economical, practical, and environmentally acceptable procedure was elaborated for oxidative deprotection of trimethylsilyl ethers to their corresponding carbonyl compounds. The reaction proceeded in a solventless system, within a short period of time, and yields were good. On irradiation in a conventional microwave for 30 s, trimethylsilyl ether of benzyl alcohol in the presence of mont-morilonite KIO and finely grounded Fe(N03)3 9H2O gave rise to benzaldehyde in 95% yield. The applicability of this method was tested with several aromatic, alicyclic, and aliphatic trimethylsilyl ethers. Duration did not exceed 1 min, and yields were not lower than 80% (Mojtahedi et al. 1999). 

Aromatic ketones 569, benzylic alcohols 570 and 571 as well as alkyl aryl ethers 572 reacted with lithium under ultrasonic irradiation in the presence of catalytic amounts of DTBB (2%) in THF to give, after alkylation with an alkyl iodide at 0°C and final... 

Ultrasonic irradiation (25 kHz). Benzyl alcohol was produced in 10% yield. 

Band-gap irradiation of CdS, led to sacrificial 565 photoreduction of Ag+ at the outer surfaces of SUVs via electron exchange between CI6MVi + and CiSMV+i, located at the inner and at the outer surfaces of the vesicles, at the expense of benzyl alcohol as a sacrificial electron donor... 

Table 1 summarizes the changes, during the irradiation times, of the products quantities and the chemical yields. In general, benzaldehyde, benzyl alcohol and benzoic acid were detected as the main products. During experiments 1 and 5, ben zyl alcohol was detected as traces only also in experiment 3, benzoic acid was detected as traces, and that only after prolonged irradiation times. It is inte resting to mention here, that, in all experiments, traces of 2-, 3-, and 4- ere sols were also detected. In addition, under the experimental conditions 2, tra ces of benzyl benzoate ( 2 ) and o-benzyl benzoic acid ( 3 ) were detected. 

It is interesting to mention, that from results reported by Fujihira et al. (ref. 6), after UV-irradiation up 2 h, only benzaldehyde was detected, when using pu re TiO. These observations are in accordance with our results, below 3 h of irradiation, for experiments 1 and 2 in which TiO, were used as photocatalyst in all of our experiments the benzyl alcohol only appeared after prolonged irra diation times, at least above 3 h. 

On the other hand, the heterogeneous photocatalytic oxidation of toluene, in aqueous suspensions of TiO, has been studied with some details, by Fujihira et al. (refs.4-5). These authors have reported that, in aqueous media, up to 2 h of UV-irradiation, benzaldehyde was one of the main products benzyl alcohol was de tected as traces and benzoic acid was not detected (refs.4-5). However, in our experimental conditions using neat-liquid toluene, not only benzaldehyde but both benzyl alcohol and benzoic acid were detected as main products. 

According to our results, the formation of benzyl alcohol, generated from the photooxidation of benzyl radicals (ref.4) could be associated to the presen ce of water. It is of primary importance to be considered that in aqueous sys terns, due to solubility reasons, benzyl alcohol must be easier solved than ben zaldehyde then benzyl alcohol, in the aqueous phase could be photocatalytically destroyed by a drastic photooxidation to C02 and water. In fact, the photocata lytic oxidation of the aromatic ring to CO under UV-irradiation in aqueous emulsions of TiO, have been observed by Izumi et al. (refs. 14-15). It is worthy... 

In an attempt to couple halobenzaldehydes with amines, A1203 was pre-absorbed with the substituted benzaldehydes and imidazole or piperidine as a base and irradiated with microwaves. However, the corresponding benzylic alcohols and benzoic acids were unexpectedly obtained by the Cannizzaro route (Scheme 4.20). The products of Cannizzaro reactions were also obtained as the main products, when microwave-assisted condensation reactions of benzaldehydes with vinyl acetate using barium hydroxide as the catalyst were attempted40. 

Aromatic aldehydes can also be converted selectively into the corresponding benzylic alcohols in a crossed Cannizzaro reaction, if NaOH is used as a base. Similarly, the reaction is performed under solvent-free conditions by mixing the aldehyde with the base and an excess of paraformaldehyde and irradiating with microwave for 20—25 s. An alternative protocol uses 40% formalin solution and basic alumina to obtain comparable yields. The thermal reaction in refluxing methanol was found to require 12 h, providing considerably lower yields of the benzylic alcohols (Scheme 4.22)42. 

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... 

In a typical experiment, benzaldehyde (106 mg, 1 mmol) was added to the finely powdered paraformaldehyde (60 mg, 2 mmol). To this mixture, powdered barium hydroxide octahydrate (631 mg, 2 mmol) was added in a glass test tube and the reaction mixture was placed in an alumina bath (neutral alumina 125 g, mesh 150, Aldrich bath 5.7 cm diameter) inside a household microwave oven and irradiated for the specified time at its full power of 900 W intermittently or heated in an oil bath at 100-110 °C. On completion of the reaction, as indicated by TLC (hexane-EtOAc, 4 1, v/v), the reaction mixture was neutralized with dilute HC1 and the product extracted into ethyl acetate. The combined organic extracts were dried over anhydrous sodium sulfate and the solvent removed under reduced pressure. The pure benzyl alcohol (99 mg, 91%), however, is obtained by extracting the reaction mixture with ethyl acetate prior to neutralization and subsequent removal of the solvent under reduced pressure. 

M11O2 doped silica (1.25 g, 5 mmol of M11O2 on silica gel, Selecto Scientific, 230-400 mesh with large surface area of 600 m2 g ) is thoroughly mixed with benzyl alcohol la (108 mg, 1 mmol) and the material is placed in an alumina bath inside the microwave oven and irradiated for 20 s. Upon completion of the reaction, monitored on TLC (hexane-AcOEt, 10 1), the product is extracted into methylene chloride, solvent removed and the residue passed through a bed of silica gel (4 cm) to afford exclusively benzaldehyde 2a. The overoxidation to carboxylic acid is not observed. The same reaction could be completed in 2 h at a comparable temperature of 55 °C in an oil bath. 

The oxidation of benzyl alcohol la to benzaldehyde 2a is representative of the general procedure employed. Benzyl alcohol la (0.108 g, 1 mmol) and IBD (0.355 g, 1.1 mmol) doped on neutral alumina (1 g) are mixed thoroughly on a vortex mixer. The reaction mixture is placed in an alumina bath inside an unmodified household microwave oven and irradiated for a period of 1 min. On completion of the reaction, followed by TLC examination (hexane-AcOEt, 9 1, v/v), the product is extracted into dichloromethane and is neutralized with aqueous sodium bicarbonate solution. The dichloromethane layer is separated, dried over magnesium sulfate, filtered, and the crude product thus obtained is purified by column chromatography to afford pure benzaldehyde 2a in 94% yield. Alternatively, the crude products are charged on a silica gel column that provides io-dobenzene on elution with hexane followed by pure carbonyl compounds in solvent system (hexane-ethyl acetate, 9 1, v/v). 

Irradiation of 2,2-dimethyl chromene through Pyrex using a 550-W Hanovia lamp initiates a retro 4 + 2 reaction to form the extended quinone methide 4, which reacts with methanol to form a pair of methyl ethers (Scheme 6A).18 Flash photolysis of coniferyl alcohol 5 generates the quinone methide 6 (Scheme 6B) by elimination of hydroxide ion from the excited-state reaction intermediate.19 The kinetics for the thermal reactions of 6 in water were characterized,20 but not the reaction products. These were assumed to be the starting alcohol 5 from 1,8-addition of water to 6 and the benzylic alcohol from 1,6-addition of water (Scheme 6). A second quinone methide has been proposed to form as a central intermediate in the biosynthesis of several neolignans,21a and chemical synthesis of neolignans has been achieved... 

Brunow and Sivonen (34) obtained similar results on a lignin model system, ethylguaiacylcarbinol. This compound did not undergo any oxidation when irradiated with near-uv light until p-methoxyacetophenone, a triplet sensitizer, was added to the solution. As the molar ratio of p-methoxyacetophenone to ethylguaiacylcarbinol was increased to 0.32 the rate of photooxidation of this benzyl alcohol increased. This result is consistent with the mechanism of light-induced yellowing shown in Scheme 3. 

These reactions are illustrated with the fragmentation of two isomeric aminoalco-hols and one aminodiol, which have been observed upon thioindigo sensitized irradiation in benzene solution [203]. Under these conditions, the (p-N,N-dimethyl-aminobenzyl) benzyl alcohol (24) suffers fragmentation with apparent internal disproportionation the benzyl alcohol function is oxidized, whereas the p-aminobenzyl function is reduced, giving rise to iVJV-dimethyl-p-toluidine and benzaldehyde. The acceptor is involved only as a sensitizer and, hence, remains unchanged. 

Ultrasound activation.1 Commercial Mn02 is a weak oxidant, but can be markedly activated by ultrasonic irradiation and agitation in the case of oxidation of allylic or benzylic alcohols. 

Marci et al. [233] concluded that the presence of O2, a catalyst and irradiation was required for toluene degradation. They found that the intermediate breakdown products were benzyl alcohol, benzoic acid and benzaldehyde. However, the benzaldehyde was only generated in small quantities and large amounts of benzoic acid were found adsorbed onto the catalyst surface. With the two forms of Ti(>2 used in this study, Degussa P25 and Merck, it was observed that the P25 material deactivated even in the presence of water vapour. 

As expected, no photodegradation of benzyl alcohol or phenol was observed in oxygenafed solution under irradiation but without catalyst. In the presence of cafalysf buf without irradiation no oxidation of benzyl alcohol or phenol was defecfed but a small adsorption of benzyl alcohol was measured while for phenol it was negligible. 

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