Sulfides, allyl benzyl selectivity

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). 

Oxidation of primary and secondary alcohols. In contrast to dimethyl sulfide-NCS (4, 88-90), which oxidizes both primary and secondary alcohols, diisopropyl sulfide-NCS can effect selective oxidation of these substrates. At 0° it oxidizes only primary alcohols to aldehydes, but at —78° it oxidizes only secondary alcohols to ketones. However, allylic or benzylic alcohols are oxidized at either temperature. 

An interesting example of this type of chemoselective oxidation has been reported with the reagent mixture derived irom (Uisopropyl sulfide and )V-chlorosuccinimide. This reagent will oxidize selectively a primary alcohol to an aldehyde at 0 C. Surprisingly, this same reagent at -78 C will oxidize selectively a secondary alcohol to the corresponding ketone (Scheme 2). Allylic and benzylic alcohols are oxidized at both temperatures. 

Selective conversiou of allylic and benzylic alcohols into halides It was noted above that allylic or benzylic alcohols are not oxidized by the N-chlorosuccinimide-dimethyl sulfide reagent but instead are converted into chlorides. Corey et al. now report that this procedure can be used to form allylic and benzylic chlorides in high yield if the sulfoxonium intermediate corresponding to (3, above) is allowed to decompose in methylene chloride without addition of a tertiary amine. For example, the allylic or benzylic alcohol is treated with the complex of N-chlorosuccinimide-dimethyl sulfide as above in methylene chloride and then allowed to stand for 4 hr. at - 25°. In the case of benzhydroi [(CfcH5)2CHOH], benzhydryl chloride can be obtained in > 95% yield. 

Susceptible positions of organic compounds can be directly acyloxylated " by ferf-butyl peroxyesters, the most frequently used being acetic and benzoic (R = Me or Ph). " The reaction requires a catalyst (cuprous ion is the actual catalyst, but a trace is all that is necessary, and such traces are usually present in cupric compounds, so that these are often used) and without it is not selective. Susceptible positions are similar to those in 14-6 benzylic, allylic, and the a position of ethers and sulfides. Terminal alkenes are substituted almost entirely in the 3 position, that is, with only a small amount of allylic rearrangement, but internal alkenes generally give mixtures containing a large amount of allylic-shift product. If the reaction with alkenes is carried out in an excess of... 

As already mentioned, a-metalloallyl vinyl sulfides and their 2-ethoxy analogs have been selectively alkylated, benzylated and allylated at their a-site, then the resulting compounds have been thermally rearranged to y,8-unsaturated aldehydes or y-oxo aldehydes on heating in aqueous DME (Scheme 21). 

Full details for preparation of this tetravalent iodine compound are now available. Since explosions have been reported in the preparation, the directions should be followed with care. Moreover, the reagent is sensitive to moisture, which converts it into iodosylbenzoic acid, also reported as explosive. The periodinane oxidizes primary and secondary alcohols rapidly and efficiently to carbonyl compounds without further oxidation to acids. Benzylic and allylic alcohols can be selectively oxidized. It does not react with sulfides or vinyl ethers. The iodine-containing by-product can be removed by hydrolysis to 2-iodosylbenzoic acid. 

The silyl group and ArS groups are effective for the oxidation of the benzylic position. The anodic oxidation of benzylic sulfides in the presence of allylsilanes takes place smoothly, giving rise to selective C-S bond cleavage and introduction of an allyl group on the benzylic carbon Eq. 11 [15]. 

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