Platinum benzyl alcohols

Platinum on charcoal provided a further means for directing the selectivity of the multiphasic hydrodehalogenation reactions. For example, the same reaction in Figure 6.20 conducted using Pt/C instead of Pd/C, yielded selectively the dehalo-genated benzylic alcohol (Figure 6.22)." The same reaction was conducted using... 

In contrast to phenolic hydroxyl, benzylic hydroxyl is replaced by hydrogen very easily. In catalytic hydrogenation of aromatic aldehydes, ketones, acids and esters it is sometimes difficult to prevent the easy hydrogenolysis of the benzylic alcohols which result from the reduction of the above functions. A catalyst suitable for preventing hydrogenolysis of benzylic hydroxyl is platinized charcoal [28], Other catalysts, especially palladium on charcoal [619], palladium hydride [619], nickel [43], Raney nickel [619] and copper chromite [620], promote hydrogenolysis. In the case of chiral alcohols such as 2-phenyl-2-butanol hydrogenolysis took place with inversion over platinum and palladium, and with retention over Raney nickel (optical purities 59-66%) [619]. 

Vinylogs of benzylic alcohols, e.g. cinnamyl alcohol, undergo easy saturation of the double bond by catalytic hydrogenation over platinum, rhodium-platinum and palladium oxides [39] or by reduction with lithium aluminum hydride [609]. In the presence of acids, catalytic hydrogenolysis of the allylic hydroxyl takes place, especially over platinum oxide in acetic acid and hydrochloric acid [39]. 

In this case a platinum anode wich is not sensitive to organic contamination can be used giving a current yield for Cr(VI) of 94%. Electrolyses can be performed in a two-phase medium making an in-cell process possible This method has been applied to the synthesis of benzaldehyde from benzylic alcohol giving 100% current yields at 10 % conversion while the current yield drops to 60 % at high conversions. 

Benzyl alcohol, dehydrogenation of, 279 Bifunctional dicarboxylic acids, 85 Biphasic olefin dimerization, 274-276 Bis(boryl) platinum compounds, 228 Bis(catecholato)diborane(4) compounds, 195, 200-201... 

However, ruthenium, rhodium, and rhodium-platinum catalysts have been found to be highly effective for the selective hydrogenation of these benzyl-oxygen compounds without loss of the oxygen functions. Thus, benzyl alcohol is hydrogenated to cyclohexanemethanol in high yield over ruthenium dioxide with addition of a small amount of acetic acid (eq. 11.35).114... 

Platinum catalysts, which usually tend to cause extensive hydrogenolysis, may be used for the selective hydrogenation of benzyl-type alcohols in the presence of a proper base. For example, benzyl alcohol can be hydrogenated to cyclohexane-methanol almost quantitatively over Adams platinum oxide in ethanol with addition of a small amount of acetic acid (eq. 11.36).160... 

In contrast to benzyl alcohol, a-substituted benzyl alcohols, benzyl ethers, and aryl ketones may be successfully hydrogenated over rhodium and rhodium-platinum catalysts to give the corresponding saturated products in high yields, as shown in eqs. 11.38-11.41. In the hydrogenations shown in eqs. 11.38 and 11.39, no racemization took place D-mandelic acid afforded D-hexahydromandelic acid in 94% yield and meso- and dl-2,3-dicyclohexyl-2,3-butanediol were obtained in 93 and 94% yields, respectively, by hydrogenation of the corresponding diphenyl compounds. 

Hydrogenolysis of the benzyl alcohol occurs readily during the hydrogenation of o- and p- hydroxyphenyl aldehydes and ketones, even over platinum and nickel catalysts. This can be minimized by using an activated Cu-CrO catalyst at 80°-90°C and 165-200 atmospheres of hydrogen. The catalyst was activated by refluxing it in cyclohexanol under nitrogen for four... 

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-... 

There may be cases, however, where pressures below atmospheric are desirable, particularly when the reaction may need to be stopped at some intermediate stage of hydrogenation. Thus, Grignard observed, in hydrogenating at pressures less than atmospheric with nickel, copper, and platinum catalysts, that hydrogenations could be stopped at the first stage. Phenol was reduced to cyclohexenol benzaldehyde to benzyl alcohol benzonitrile to benzaldimine and phenylacetonitrile, at about 220 mm pressure and 200 C, to phenylacetaldimine. 

Platinum and palladium are effective catalysts for alcohol oxidation when used alone however, significant stability and selectivity improvements have been observed on incorporation of a second (usually less active) metal promoter such as Bi, Pb, and Sn [63-65]. These observations are common to aerobic selox of allyhc and benzylic alcohols, as well as polyols such as propylene glycol [64] and glycerol [66]. In the case of Bi, in situ X-ray absorption spectroscopy (XAS) and attenuated total reflection infrared spectroscopy (ATR-IR) indicate that the promoter protects Pt against deactivation by overoxidation and prevents site blocking by, for example, aromatic solvents [67]. 

Platinum.—The Pt—C bond of [Pt(CHDPh)Cl(PPhg)2] is cleaved by /n-chloro-perbenzoic acid to give [ H]benzyl m-chlorobenzoate and [ H]benzyl alcohol with retention of stereochemistry at carbon. This can be accounted for by an addition-elimination mechanism, but provides no support for the involvement of an electron-transfer process. Mechanistic studies of the electrophilic cleavage of Pt—aryl bonds,... 

The same group developed a one-pot microwave-assisted route to Sn02/RG0 composites using benzyl alcohol as a reactant for metal oxide formation and reducing agent for partial reduction of graphene oxide. Platinum nanoparticles... 

Platinum nanostructured networks (PNNs) were prepared via chemical reduction of H2PtCl6 by benzyl alcohol imder microwave irradiation. The networks were micrometer long and consisted of connected secondary nanoparticles, which were actually formed by aggregation of aroimd 3 nm Pt nanocrystals. The formation of the PNN structure was attributed to the collision-induced fusion of Pt nanocrystals due to the cooperative functions of microwave irradiation and benzyl alcohol [150]. Kessler and coworkers reported a one-step synthesis of hydrophilic spherical palladium nanoparticles of imiform size and shape by solvothermal decomposition of Pd(II) acetylacetonate in acetophenone [151]. 

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