Dehydrogenation, secondary benzylic alcohol

Specific dehydrogenation of secondary benzylic alcohols (Table 10.39)... 

Although a suitable acceptor for the transfer dehydrogenation of benzylic alcohols has not yet been found, under the present conditions the low conversion of benzylic alcohols is only an apparent drawback. Indeed, it has a positive side as it allows us to fine-tune the system s selectivity. This makes the catalytic system unique among all the others known, operating both under aerobic and anaerobic conditions, that preferentially oxidize benzylic alcohols with respect to nonacti-vated secondary ones. 

Using a commercially available AC ( 50wt%), various secondary benzylic alcohols were dehydrogenated aerobically to the corresponding ketones [62, 63]. 

This C - H activation event is reversible, and is required to achieve catalytic turnover [62], A series of alcohols, mostly secondary benzylic examples, have been oxidized using this catalyst. The catalytic activity does not match that of the Ir examples described above, but it has been used in several tandem reactions that feature both dehydrogenation and hydrogenation steps to achieve interesting transformations. One example is a tandem alcohol oxidation/Wittig reaction/alkene hydrogenation sequence (Scheme 9) [61,62],... 

Alcohol dehydrogenases catalyse the oxidation of primary and secondary alcohols to aldehydes and ketones, respectively. Typical primary alcohols acting as substrate are ethanol, benzylic alcohol, phenylethanol, geraniol and retinol. Methanol is a poor substrate and is only slowly oxidized at high concentrations. Secondary alcohols are more difficult to oxidize, while tertiary alcohols are resistant towards dehydrogenation. This explains in part the greater... 

Triphenylsilyl ethers are typically prepared by the reaction of the alcohol with triphenylsilyl chloride (mp 92-94 °C) and imidazole in DMF at room temperature. The dehydrogenative silylation of alcohols can be accomplished with as little as 2 mol% of the commercial Lewis acid tris(pentaf1uorophenyl)borane and a silane such as triphenylsilane or triethylsilane [Scheme 4.98]. Primary, secondary, tertiary and phenolic hydroxyls participate whereas alkenes, alkynes, alkyl halides, nitro compounds, methyl and benzyl ethers, esters and lactones are inert under the conditions. The stability of ether functions depends on the substrate. Thus, tetrahydrofurans appear to be inert whereas epoxides undergo ring cleavage. 1,2- and 1,3-Diols can also be converted to their silylene counterparts as illustrated by the conversion 983 98.4. Hindered silanes such as tri-... 

Mechanistic studies revealed that the reaction does not involve ester intermediacy and proceed via a hemiaminal-type mechanism (Scheme 5). When the benzyl benzoate and benzylamine were reacted under similar experimental conditions of the amidation reactions, either in the presence or absence of catalyst 5, no amide formation took place and the starting materials were recovered. These studies preclude the involvement of a hemiacetal pathway in the reactions. Further, the dehydrogenation of hemiaminal prevails over the anticipated water elimination to result in an imine, which upon H2 addition would provide the secmidary amine. Secondary amines were observed only in trace amounts when linear alcohols were used. 

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