Selenoxides ketones

Both the selenoxide and sulfoxide " reactions have been used in a method for the conversion of ketones, aldehydes, and carboxylic esters to their a, P-unsaturated derivatives (illustrated for the selenoxide). 

Selenoxides are even more reactive than sulfoxides toward (3-elimination. In fact, many selenoxides react spontaneously when generated at room temperature. Synthetic procedures based on selenoxide eliminations usually involve synthesis of the corresponding selenide followed by oxidation and in situ elimination. We have already discussed examples of these procedures in Section 4.3.2, where the conversion of ketones and esters to their a, (3-unsaturated derivatives is considered. Selenides can... 

Epoxyketone 60 has also been prepared by hydroxyselenation of 4-acetyl-1-methylcyclohexene with phenylselenium chloride and water, oxidation of the selenide to selenoxide with buffered aqueous oxone, and elimination of the se-lenoxide in the same pot to provide the epoxide [80]. Control of the conditions was essential to prevent epimerization of the ketone. This route has little to recommend it given the expense and toxicity of the reagents, the moderate yield, and the problems with epimerization. 

It is interesting to discern the different migratory aptitudes displayed by spirocyclic ketones on exposure to hydrogen peroxide [252] (Eq. 226,227). Although participation of the selenium atom to direct the attack of the peroxide has been formulated, the results are also consistent with an electronic explanation. Thus, the rapid formation of selenoxide renders the spirocyclic center acceptor-like through polarity alternation, and the migration of the methylene group becomes more favorable. 

Utilizing selenoxide syn elimination as the key step, a two-step procedure has been developed for the conversion of ketones to enones.22 Cyclobutyl phenyl ketone (52) and 3-phenylcyclobu-tanone (55) were both transformed via the phenyl selenides to the corresponding 1-cyclobutenyl phenyl ketone and 3-phenyIcyclobut-2-en-l-one in 74 and 26% overall yield, respectively.22... 

Reaction with ketones. The reagent reacts with ketones in ether to introduce a C6H5SeCl2 group into the a-position. The products undergo selenoxide elimination to give a,0-enones they are reduced by thiourea to a-phenylseleno ketones. 

Iodine-catalysed hydroperoxidation of cyclic and acyclic ketones with aqueous hydrogen peroxide in acetonitrile is an efficient and eco-friendly method for the synthesis of gem -dihydroperoxides and the reaction is conducted in a neutral medium with a readily available low-cost oxidant and catalyst.218 Aryl benzyl selenoxides, particularly benzyl 3,5-bis(trifluoromethyl)phenyl selenoxide, are excellent catalysts for the epoxidation of alkenes and Baeyer-Villiger oxidation of aldehydes and ketones with hydrogen peroxide.219 Efficient, eco-friendly, and selective oxidation of secondary alcohols is achieved with hydrogen peroxide using aqueous hydrogen bromide as a catalyst. Other peroxides such as i-butyl hydroperoxide (TBHP), sodium... 

This Chapter contains reactions which prepare the oxides of nitrogen, sulfur, and selenium. Included are N-oxides, nitroso, nitro compounds, nitrile oxides, sulfoxides, selenoxides, and sulfones. Oximes are found in Sections 60A (Protection of Aldehydes) and 180A (Protection of Ketones). Preparation of sulfonic acid derivatives are found in Chapter Two and the preparation of sulfonates in Chapter Ten. 

Selenenyl halides are relatively stable, though moisture sensitive, compounds that are generally prepared by the reactions shown in Scheme 7 and behave as electrophihc selenium species. " They react with ketones and aldehydes via their enols or enolates to afford a-seleno derivatives (e.g. (17) in equation 11). Similar a-selenenylations of /3-dicarbonyl compounds, esters, and lactones can be performed, although the latter two types of compounds require prior formation of their enolates. Moreover, the a-selenenylation of anions stabilized by nitrile, nifro, sulfone, or various types of phosphorus substituents has also been reported (equation 12). In many such cases, the selenenylation step is followed by oxidation to the selenoxide and spontaneous syn elimination to provide a convenient method for the preparation of the corresponding a ,/3-unsaturated compound (e.g. 18 in equation 11). Enones react with benzeneselenenyl chloride (PhSeCl) and pyridine to afford a-phenylselenoenones (equation 13). 

Enantioselective Selenoxide Elimination Producing Optically Active Allenes and or,]3-Unsaturated Ketones... 

Thus, when cyclohexyl selenides 1, prepared from the corresponding 4-sub-stituted cyclohexanone via the selenoketals, were oxidized with various Davis and Sharpless oxidants, the chiral alkyl aryl 4-substituted cyclohexylidenemethyl ketones were obtained in excellent chemical yields with high enantiomeric excesses. Typical results are summarized in Table 4. In this asymmetric induction, of the substrate and the chiral oxidant employed were revealed to show a remarkable effect upon the enantioselectivity of the product. The use of a methyl moiety as instead of a phenyl moiety gave a higher ee value, probably due to the steric difference between the two groups bonded to the selenium atom of the substrate. The results indicate that the titanium complex of the Sharpless oxidant may promote the racemization of the chiral selenoxide intermediate by acting as a Lewis acid catalyst, whereas the racemization in the case of the Davis oxidant, which is aprotic in nature, is slow. 

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