1-Phenylethane-1,2-diol

In 2008, Que and coworkers reported an asymmetric version of the dihydroxylation with a new type of ligands bearing bipyrrolidine as the chiral backbone [71]. The corresponding iron(II) complex showed general activity in the dihydroxylation of various olefins using H202- Satisfactory results are obtained with aliphatic as well as with aromatic olefins. For example, dihydroxylation of styrene gave styrene oxide and 1-phenylethane-1,2-diol in <1% and 65% yield, respectively (Scheme 10). 

The solvent-free 0-silylation reaction can also be accomplished selectively. For example, when l-methyl-l-phenylethane-l,2-diol (39) was treated with 27 at 60 °C for 5 h, its primary hydroxyl group was silylated to give 40 in 70% yield. 

The secondary benzylic alcohol l-phenylethan-l,2-diol requires 20 hours of treatment at room temperature to produce a 64% yield of 2-phenylethanol (Eq. 43).137 Under the same conditions, methyl mandelate fails to undergo reduction, presumably because of the greater carbocation-destabilizing effect of a neighboring carboalkoxy compared to a hydroxymethyl group (Eq. 43).137... 

Recently, Schaumann et al. 153,154 an(j Bienz et tf/.155,156 have developed dependable routes for the resolution of racemic functionalized organosilanes with Si-centered chirality using chiral auxiliaries, such as binaphthol (BINOL), 2-aminobutanol, and phenylethane-l,2-diol (Scheme 2). For instance, the successive reaction of BINOL with butyllithium and the chiral triorganochlorosilanes RPhMeSiCl (R = /-Pr, -Bu, /-Bu) affords the BINOL monosilyl ethers 9-11, which can be resolved into the pure enantiomers (A)-9-ll and (7 )-9-11, respectively. Reduction with LiAlFF produces the enantiomerically pure triorgano-H-silanes (A)- and (R)-RPhMeSiH (12, R = /-Pr 13, -Bu 14, /-Bu), respectively (Scheme 2). Tamao et al. have used chiral amines to prepare optically active organosilanes.157... 

Table 9.1). The rate of dehydrobromination from the intermediate bromoalkenes follows the pattern 2-bromoalkenes > Z-l-bromoalkenes > E- -bromoalkenes the corresponding chloro derivatives react more slowly. For optimum yield, the reaction temperature should be <100°C to reduce decomposition of the catalyst, and the concentration of base should be kept low to prevent isomerization of the resulting alkynes. [3-Elimination of HBr from 1,2-dibromo-1 -phenylethane can be controlled to yield 1-bromo-l-phenylethene in 83% yield [15]. The addition of alcohols and diols have a co-catalytic effect on the elimination reaction, as the alkoxide anions are transferred more effectively than the hydroxide ions into the organic phase [13]. 

The base-catalyzed hydration of 2-phenyloxirane involves nucleophilic attack preferentially at C(3) (0-C(3) cleavage), but with only partial regio-selectivity. Acid-catalyzed hydration is mainly by 0-C(2) cleavage. The hydration of 2-phenyloxirane catalyzed by epoxide hydrolase is characterized by its very high regioselectivity for the less-hindered, unsubstituted C(3) [175] [176], involving retention of configuration at C(2). In other words, (R)-and (5)-2-phenyloxirane are metabolized to (/ )- and (S)-l-phenylethane-l,2-diol (10.118), respectively. Substrate enantioselectivity was also character-... 

For instance, styrene oxide was resolved by whole cells of Aspergillus niger and Beauveria bassiana via two different pathways showing matching enantio- and regioselectivities with excellent results (Scheme 8). Combination of the two biocatalysts employing a deracemization process in a single reactor led to R) phenylethane-l,2-diol as the sole product in 98% ee and 85% isolated yield [58]. 

The (R)-(+)-chiral 1,4-diol 17b was easily prepared from L-(+)-dimethyl tartrate. Dimethyl tartrate was converted to the corresponding phenylethylidene derivative by treatment with 1,1-dimethoxy-l-phenylethane and cat. p-toluenesulfonic acid in refluxing benzene, followed by conversion to the diol 17b with excess phenylmagnesium bromide. The diol was purified by column chromatography on silica gel (hexane ethyl acetate =5 1), and recrystalization from a mixture of hexane and 2-propanol. 

Phenylacetaldehyde can be obtained in high yield by vapor-phase isomerization of styrene oxide, for example, with alkali-treated silica-alumina [147]. Another process starts from phenylethane-l,2-diol, which can be converted into phenylacetaldehyde in high yield. The reaction is performed in the vapor phase in the presence of an acidic silica alumina catalyst [148]. 

To a flask containing 6.9 gm (0.05 mole) of phenylethane-l,2-diol in 25 ml of chloroform is added dropwise at room temperature 5.95 gm (0.05 mole) of thionyl chloride. After the initial reaction in which hydrogen chloride is evolved, the solution is refluxed for 1 hr. The solution is cooled, washed with water, then with 2 % sodium bicarbonate solution, dried, and distilled to afford 5.7 gm (62%) b.p. 62°-64°C (0.15 mm), n 4-6 1.5421. 

Stilbene is also converted into the epoxide and the preparation of the lsO-isomcr 3 in high isotopic purity can also be performed, while 1,1-diphenylethene is transformed to 1.1-di-phenylethane-l,2-diol (4),8 and (T)-stilbene to a mixture of benzophenone9 and //wo-2-fluoro-1.2-diphenylethanol. Fluoro-substituted alkenes can also be converted with a hypofluorous acid/acetonitrile mixture into epoxides 5 in high yield.10... 

Because organobismuth(V) compounds have found considerable use as oxidizing agents, the oxidizing ability of methyl di-l-naphthylbismuthinate [124066-66-6], C21H17Bi02, was investigated. Benzoin yields benzil, naphthalene, and metallic bismuth hydrazobenzene yields azobenzene, and 1,1,2,2-tetraphenylethanediol yields benzophenone. 1,2-Diphenyl-1,2-ethanedione dihydrazone gives diphenylacetylene in 50% yield. Cyclohexane-1,2-diol and 1-phenylethane-l,2-diol, however, were unaffected. 

Other 1,2-diols have also been examined. Phenylethane-l,2-diol has been oxidised using a colloid-derived Au/C catalyst (dAu = 7nm) to mandelic acid (C6HsCH(0H)C02H), an important pharmaceutical intermediate.84,85... 

One phenyl substituted 1,2-diol has been studied in detail, viz. phenylethane-... 

C-p position would probably be nearly equal to that of (+)-phenylethane-1,2-diol. The value of the molecular optical rotation differences suggested that p,3,4-tri-hydroxyphenethyl moiety is a racemate. Consequently, the structure of suspensa-side (28) was established as DL- p,3,4-trihydroxyphenethyl-O-a-L-rhamnopyrano-syl-( 1 —>6)-4-0-caffeoyl-p-D-glucopyranoside. 

Using phenylethane-l,2-diol as a substrate, however, a new selectivity scenario appeared owing to a strong induction effect of the phenyl group. In fact, according to Scheme 13.4, two abundant by-products, namely benzoate and phenylglyoxylate, were detected beside the expected mandelate, working in the presence of NaOH. 

Scheme 13.4 Reaction products detected during the oxidation of phenylethane-1,2-diol (P) with Au/C catalyst. [P] = 0.4M, P/Au = 500, P/NaOH = l, T=70°C.

Scheme 13.5 Reaction pathway of phenylethane-1,2-diol oxidation in basic solution.

Scheme 13.6 Reaction pathway of phenylethane-l,2-diol oxidation at pH 7.

Table 13.5 Optimization of phenylethane-1,2-diol (RED) oxidation for production of mandelate with Au/C catalyst.

Naemura, K. Ogasahara. K. Hirose, K. Tobe. Y. Preparation of homochiral azophenolic crown ethers containing l-phenylethane-1.2-diol and 2,4-dimethyl-3-oxapentane-l,5-diol as a chiral subunit Enantiomer recognition behaviour towards chiral 2-aminoetlianol derivatives. Tetrahedron Asymmetry 1997. 8 (1). 19-22. 

Cyclohexanone oxime dissolved in ca. 2 M NaOH containing NaBH4, and refluxed 2 hrs. cyclohexanol. Y 87%. - Similarly via the Na-salt of the boric acid diester a-Hydroximinoacetophenone l-phenylethane-l,2-diol. Y 72-78%. -Oximes of conjugated carbonyl compds., such as cinnamaldehyde or acetophenone, do not react. F. e. s. K. H. Bell, Australian J. Chem. 23, 1415 (1970). 

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