Phenylethanol, from acetophenone

The catalytic hydrosilylation of acetophenone or t-butyl methyl ketone with diethyl-, methylphenyl- or diphenylsilane in the presence of rhodium(I) catalysts containing (R,i )-(-l-)- or (S,S)-(—)-170, followed by acid cleavage of the intermediate silyl ethers, affords the respective alcohols with optical yields of 10—42% . The synthesis of (ff)-(-l-)-l-phenylethanol from acetophenone and diethylsilane in conjunction with the catalyst derived from (S,S)-( —)-170 was the most effective reaction (equation 28). In... 

The coproduct 1-phenylethanol from the epoxidation reactor, along with acetophenone from the hydroperoxide reactor, is dehydrated to styrene in a vapor-phase reaction over a catalyst of siUca gel (184) or titanium dioxide (170,185) at 250—280°C and atmospheric pressure. This product is then distilled to recover purified styrene and to separate water and high boiling organics for disposal. Unreacted 1-phenylethanol is recycled to the dehydrator. 

The HOPG (highly oriented pyrolytic graphite) carbon electrode chemically modified with (5[-phenylalanine at the basal surface led to 2% ee in the reduction of 4-acetylpyridine [377]. A cathode modified with a chiral poly(pyrrole) reduced 4-methylbenzophenone or acetophenone in DMF/LiBr and phenol as proton donor to 1-phenylethanol with up to 17% ee [382]. Alkyl aryl ketones have been reduced to the corresponding alcohols at a Hg cathode in DMF/water in the presence of (1R,2S)-A,A-dimethylephedrinium tetrafluorobo-rate (DET), producing (5 )-l-phenylethanol with 55% ee from acetophenone. Cyclovoltammetry supports an enantioselective protonation of the intermediate (PhCOH(CH3)) [383]. 

As an added benefit, the immobilization resulted in an enhanced half-life of the biocatalyst at elevated temperatures and during storage [12], and thus in an overall improvement of technical applicability. With the entrapped ADH, (Rj-phenylethanol was synthesized from acetophenone with a productivity of... 

From the results obtained for the oxidation of 1-phenylethanol to acetophenone, they have found that the system using IL-NHPI (10 mol%)-Co(OAc)2-02 in the ionic liquid [bmim][PFJ was reusable, hi the same system, various types of carbinols were transformed into the corresponding aldehydes and/or ketones in good yields. 

In addition to the above mentioned products, traces of benzaldehyde are detected. The amount of benzaldehyde increases in the course of the reaction and is higher over TS-2 catalyst. It is probably formed after a C-C bond break in the side chain of 1-phenylethanol or acetophenone. Another by-product - ethylbenzoquinone, is observed in the experiments without solvent and in ethanol under standard conditions. The ethers derived from 1-phenylethanol and the solvent - methanol or ethanol, are also found. 

Toluene is oxidized by purified naphthalene dioxygenase from Pseudomonas sp. strain NCIB 9816-4 to benzyl alcohol and ben-zaldehyde, and ethylbenzene to (S)-l-phenylethanol and acetophenone. Whereas the initial reactions involve monooxygenation, oxidation to the aldehyde and ketone are dioxygen-dependent (Lee and Gibson 1996). 

Chemoselective oxidation of a secondary alcohol moiety can be also performed with ruthenium catalysts with phenylindenyl Hgand. The selective oxidation of 1-phenylethanol to acetophenone from a mixture of phenylethanol isomers, without oxidizing the other isomer, can then be achieved (Scheme 29). In general, only the secondary alcohol moieties are oxidized and the catalyst can be used for the chemical separation of isomers or specific oxidation of highly functionalized molecules. The OKR of unactivated racemic alcohols with dioxygen of air as the hydrogen acceptor was effectively performed at room temperature with [(aqua)Ru(salen)] com-... 

Acetophenone was reduced in methanol to 1-phenylethanol at 20 °C by boro-hydride moieties coupled to a porous polymer resin [3], In principle, four hydrogen atoms can be released from the borohydride the reactivity, however, decreases with each hydrogen atom lost Experimentally it was shown that the first two atoms mainly contribute and to the reduction the other two remain on the polymer site. 

Studies aimed at the elucidation of reaction mechanisms have been performed by many groups, notably by those of Backvall [28]. In test reactions, typically enantiopure 1-phenylethanol labeled with deuterium at the 1-position (8) is used. The compound is racemized with acetophenone (9) under the influence of the catalyst and after complete racemization of the alcohol, the deuterium content of the racemic alcohol is determined. If deuterium transfer proceeds from the a-carbon atom of the donor to the carbonyl carbon atom of the acceptor the deuterium is retained, but if it is transferred to the oxygen atom of the acceptor it is lost due to subsequent exchange with alcohols in the reaction mixture (Scheme 20.4). 

The reduction of dialkylketones and alkylaryl ketones is also conveniently accomplished using chiral oxazaborolidines, a methodology which emerged from relative obscurity in the late 1980s. The type of borane complex (based on (,V)-diphenyl prolinol)[39] responsible for the reductions is depicted below (10). Reduction of acetophenone with this complex gives (/ )-1 -phenylethanol in 90-95% yield (95-99% ee) [40]. Whilst previously used modified hydrides such as BiNAL-H (11), which were used in stoichiometric quantities, are generally unsatisfactory for the reduction of dialkylketones, oxazaborolidines... 

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. 

In a time course study on the conversion of ( )-l-phenylethanol 13 (X=H), formation of acetophenone was observed to a maximum of around 20% during the conversion of (S)- to (R)- alcohol which occurred over 24 h to give (R)-13 in 96 % yield, 99% e.e. The effect of ring substitution on the efficiency of the dera-cemization was notable. While para substituents (Cl, OMe, Me) gave good results, ortho derivatives could not be deracemized and the biocatalyst showed little activity towards meta substituted compounds. On addition of allyl alcohol, improvements in e.e. were obtained, particularly for the conversion of l-(m-methylphenyl)ethanol (from 21 to 94% e.e.). However these improvements did appear to be at the expense of yield (89% diminished to 55%). The authors sug-... 

Table 6.13 Racemic hydrogenation of acetophenone" initial reaction rate rf, selectivity to products at 100% conversion. Results for the chemical reduction with NaBH4 are included for comparison. PE 1-phenylethanol, CHMK cyclohexyl methyl ketone and CHE 1-cyclohexylethanol. (Reproduced from Reference [34])...

Certain alkaloids are able to effect asymmetric induction during a reduction process at a mercury cathode even when present in low concentration in an aqueous alcohol acetate buffer. Asymmetric induction under these conditions was first observed [39] during the conversion of 4-methylcoumarin to 4-methyl-3,4-dihydro-coumariit (sec page 60). Induction results because a layer of alkaloid is strongly adsorbed on the electrode surface thus permitting transfer of a proton to a carban-ion intermediate m an asymmetric environment. Up to 16% asymmetric induction has been achieved in 1-phenylethanol recovered from reduction of acetophenone in a buffer of pH 4.8 containing a low concentration of quinidine. lire pinacol formed simultaneously shows no optical activity. However quinidine is itself reduced at the potential employed so that the actual catalyst for the asymmetric process is not defined [34,40],... 

Based on the catalytic activity of aluminum alkoxides in the Meerwein-Ponndorf-Verley-Oppenauer reaction, Berkessel et al. envisioned that aluminum complexes can act as alcohol racemization catalysts [32]. Aluminum alkoxide complexes generated from a 1 1 mixture of AlMes and a bidentate ligand such as binol or 2,2 -biphenol were effective catalysts for alcohol racemization. At room temperature, 10mol% of the aluminum catalyst racemized 1-phenylethanol completely within 3h in the presence of 0.5 equiv. of acetophenone. The aluminum catalysts were... 

Acetophenone can be hydrogenated catalytically to 1-phenylethanol. It is obtained as a byproduct in the Hock phenol synthesis and is purified from the high-boiling residue by distillation. The quantitites obtained from this source satisfy the present demand. 

In the course of irradiation of acetophenone in the presence of 1-phenylethanol, the actual quantum yields for pinacol formation do not exceed 50%, but rise to 71% when PhCH(OD)Me is used for photoreduction of acetophenone in acetonitrile683,684. A conclusion has been reached from this inverse DIE that half the reaction of triplet acetophenone with 1-phenylethanol involves abstraction of an OH hydrogen followed by disproportionation of the initial radical pair back to reactants. A transfer of an O-bonded hydrogen to a triplet ketone is taking place (equation 318) besides the abstraction of hydrogen from... 

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