Carbinols selective

With 1-2 mol% of 64b, racemic mixtures of aryl-alkyl carbinols 86 [103], propargylic [104] and allylic alcohol [105] 88 and 87, respectively, were resolved (Fig. 43). The best selectivities were attained for aryl-alkyl-carbinols 86, where the unreacted isomer was obtained with excellent ees after 55% conversion, while propargyl alcohols 88 required clearly higher conversions for high ees in the remaining starting material [106]. 

Schreiber et al.47 have described a mathematical model that combines enantiotopic group and diastereotopic face selectivity. They applied the model to a class of examples of epoxidation using several divinyl carbinols as substrates to predict the asymmetric formation of products with enhanced ee (Scheme 4-28). 

Benzyl alcohols Aryl alkyl carbinols (11) can be oxidized to ketones (12) by the direct electrochemical method (Eq. 4) since they possess their oxidation potentials at around 2.0 V versus SCE (saturated calomel electrode) however, cleavage products decrease the selectivity [14]. 

Allylic and benzylic alcohols were oxidized to aldehydes or ketones with BnPhsPHSOs in refluxing CHsCN. The yield increased in the presence of bismuth chloride in a catalytic amount. Selective oxidation of various alcohols under solvent free conditions was also reported Interestingly, benzyl alcohols were oxidized selectively to benzaldehydes in very high yield (95-100%) when reacted with BnPhsPHSOs (1.2 eq.) and AICI3 (1 eq.) in the presence of an equimolar amount of 2-phenethyl alcohol, diphenyl carbinol or methyl phenyl sulfide (equation 72). 

The reactions were conducted in the liquid phase at conditions described in the experimental section. Test reactions were conducted to establish that the reactions were kinetically limited. In cases where the rate of reaction was >5 mmoP(g min), the selectivity to 6-PPD was >97% and the yield of 6-PPD was >96%. Hence, the rate of hydrogen uptake was taken to be directly proportional to the formation of 6-PPD. This rate calculated at constant temperature and conversion was normalized to the amount of catalyst used and is shown in column 6 of Tablel. The two cases where Pt/S ratio was high (Run 3 4), hydrogenation of the ketone (MIBK) to the alcohol, methyl isobutyl carbinol (MIBC), was observed. In cases where the Pt/S ratio was low (Run 5 6), significant amounts of the imine was detected in the GC. 

For chiral-subsdtuted alcohols, e.g., 1/2, this inherent enantioface selectivity is modified by the diastereofacial influence of the carbinol center, which leads to the preferred formation of the erythro-cpoxy alcohol. Thus, for each DIPT enantiomer, one may distinguish between a matched (kinetically fast) and a mismatched (kinetically slow) diastereomer. After 50% conversion of the starting material 1/2, the products are 1 and 4 in case and 2 and 5 in case ... 

The resulting ketone (21-4) is then condensed with methylmagnesium bromide to give the corresponding methyl-carbinol the highly unreactive nature of the carbonyl group at position 11 is further illustrated by the selective reaction at the 6 position. 

The oxidation of various alcohols by the poly(e-carbobenzoxy-L-lysine)-Cu complex was studied by Welch et aL127. The polymer catalyst showed selectivity in oxidation by virtually excluding alcohols of bulky structure such as diisopropyl and diisobutyl carbinol while admitting simple alcohols arch as n-butyl, iso-butyl, and sec-butyl. It was suggested from structural studies that the selectivity of the polymer catalyst resulted from the highly complex geometry of the molecular cage formed by the helix and the amino acid side chain around the coordinated Cu. The... 

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