Epoxidation of vinyl silane

It is all true. Evidence that a silyl group stabilizes the S 2 transition state comes from the reactions of the epoxides of vinyl silanes. These compounds can be made stereospecifically with one equivalent of a buffered peroxy-acid such as m-CPBA. Epoxidation is as easy as the epoxidation of simple alkenes. You will see in a moment why acid must be avoided. 

By itself, the Brook rearrangement is not very useful but, if the carbanion can do something else other than just get protonated, something useful may happen. We have seen what happens to the epoxides of vinyl silanes. Dihydroxylation of the same alkenes also gives interesting chemistry when the diols are treated with base. 

Many newer methods for generating cyclohexane derivatives from carbohydrates still depend on the intramolecular attack of nucleophilic carbon species at electrophilic centers, and the range of options is now extensive. Thus, the nucleophiles may be carb-anions stabilized by carbonyl, phosphonate, nitro, or dithio groups, and they may bond to carbonyl carbon atoms, or to those that carry appropriate leaving groups or are contained in epoxide rings, or as jj-centers of a,p-unsaturated carbonyl systems. Otherwise, the nucleophilic activity at the 7-centers of allylsilanes or a-positions of vinyl silanes may be used to react with electrophilic carbon atoms. 

The silicon-containing Grignard reagents (27) and (28) are alkylated in high yield in the presence of Cu salts. The conversion of vinyl silanes via the epoxide into silyl enol ethers prompts the recognition of these reagents as examples of the two nucleophilic synthons for acetaldehyde (Scheme 23). 

In Entry 5, the carbanion-stabilizing ability of the sulfonyl group enables lithiation and is then reductively removed after alkylation. The reagent in Entry 6 is prepared by dilithiation of allyl hydrosulfide using n-bulyl lithium. After nucleophilic addition and S-alkylation, a masked aldehyde is present in the form of a vinyl thioether. Entry 7 uses the epoxidation of a vinyl silane to form a 7-hydroxy aldehyde masked as a cyclic acetal. Entries 8 and 9 use nucleophilic cuprate reagents to introduce alkyl groups containing aldehydes masked as acetals. 

The vinyl silane 247 was unmasked to the ketone 250 by epoxidation, subsequent ring opening of the epoxide with HF in pyridine and concurrent cleavage of the THP and TBS protecting group (Scheme 39). The Stork-Jung annulation was completed through the treatment of the diketone 250 with sodium methoxide to mediate the intramolecular aldol con-... 

A catalytic amount of ketone 26 was used to investigate the substrate scope of the asymmetric epoxidation. High enantioselectivities can be obtained for a wide variety of trans- and trisubstituted olefins (Table 3, entries 1 ) [54]. Simple trans-olefins, such as franx-7-tetradecene, can be epoxidized in high yield and enantiomeric excess, indicating that this asymmetric epoxidation is generally suitable for frani-olefms. 2,2-Disubstituted vinyl silanes are epoxidized in high ees (Table 3, entries 5, 6) and enantiomerically enriched 1,1-disubstituted epoxides can be... 

Indeed, Adam observed that, whereas acyclic vinyl silanes 132 undergo smooth epoxidation in excellent yield, the cyclic substrates exhibit varying amounts of allylic oxidation as well. The C-H insertion pathway becomes more prominent when the double bond is deactivated by either steric or electronic influences. Thus, the /-butyl derivative 134 gives as much as 20% of the allylic oxidation product (136) [95TL4991]. 

The product is a vinyl silane as the M Si group is retained in this reaction of the anion. The reaction is stereoselective in favour of the E-alkene as might be expected. The alkene can be epoxidized... 

Addition of R Li to the double bond gives an anion stabilized by two silicon atoms. Peteri-it reaction gives a vinyl silane and epoxidation and hydrolysis (p. 1302) gives the carbonyl compocril . There is no need to concern ourselves with stereochemistry as it all disappears in the final prod- -c The silicon atom is essential in every step except the final tautomerism. 

Lithium derivatives of allyl silanes react in the y-position with alkyl halides, epoxides, and carbonyl compounds. The lithium derivative 110 of allyl silane 109 gives only the y-adduct 111 with ketones.31 Vinyl silanes such as 111 are usually converted into carbonyl compounds via epoxides which rearrange with Lewis acid catalysis and loss of silicon to give protected versions of ketones or aldehydes 112. 

Disubstituted vinyl silanes are epoxidized in high ee s and enantiomerically enriched 1,1-disubstituted epoxides can be obtained via the desilylation of these epoxides e.g., 18). Allylic alcohols and conjugated dienes and enynes are effective substrates (e.g. 19 and 20). ° The epoxidations of enol ethers and enol esters were also studied. The resulting epoxides (e.g., 21) from enol esters can undergo stereoselective rearrangement to give optically active a-acyloxy ketones, (5)-22 or (i )-23, under different acidic conditions. 

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