Selenoxides, alkylation alkenes

Sulfonesand sulfonic esters can also be alkylated in the a position if strong enough bases are used. Alkylation at the a position of selenoxides allows the formation of alkenes, since selenoxides easily undergo elimination (17-11). 

Selenoxide elimination. The yield of alkenes from alky) phenyl selenoxides and alkyl methyl selenoxides under usual conditions (30% H2O2, O3, Oa) tends to be rather low because of formation of the original selenide. Much higher yields are obtained if the selenides are oxidized with /-butyl hydroperoxide (4 equiv.) in the presence of basic alumina (8 equiv.) in THF at 55°. No epoxida-tion is observed under these conditions. A less satisfactory method is ozonization in CH2CI2 in the presence of 1-3 equiv. of triethylamine. ... 

Sulfides and selenides both stabilize an a-carbanion (see Section 1.1.5.2) and alkylation followed by elimination provides a route to substituted alkenes. Some examples of selenoxide eliminations are given in Schemes 2.24—2.26. Like the other syn eliminations described in this section, the regioselectivity of selenoxide elimination can be poor. Elimination normally takes place preferentially towards a conjugating (3-substituent or away from an electronegative p-substituent. This latter facet allows a good method for converting epoxides into allylic alcohols. 

In another development, Krief has shown that selenoxides (especially aryl/alkyl selenoxides) are capable of replacing amine-oxides as the reoxidant in the dihydroxylation reaction. An asymmetric process based on (DHQD)2PHAL (named SeOAD) was also developed and this gave good to excellent ee s with a range of alkenes." ... 

Arylthallium bis(trifluoroacetate)s are converted by successive treatment with KF and BF3 into aryl fluorides.Thallium(iii) nitrate (TTN) readily oxidizes dialkyl sulphides and selenides to the corresponding sulphoxides or selenoxides, and 2-(alkylthio)-l-arylethanones (37) into compounds (38) in methanolic solution.In a modification of the TTN oxidative conversion of aryl alkyl ketones into arylacetic acids, enol ethers derived from the ketones are used instead of the ketones themselves. This reduces the formation of side products. Cyclic aralkyl ketones (39) may be ring-expanded and alkylated to give compounds (40) via treatment of their Wittig-derived alkenes with TTN/ an extrapolation of the basic reaction discovered previously. 

Second, among the newer methods developed to effect the elimination to produce the least substituted alkene is one that involves converting the alcohol to a selenium derivative. In this procedure, a primary alcohol is treated with o-nitrophenyl sele-nocyanate in a suitable solvent such as (THF, oxacyclopentane) in the presence of a phosphine (such as tri-n-butylphosphine [(CH3CH2CH2CH2)3P]) to produce the primary alkyl selenide (Scheme 8.74). Then, in a second step, the primary alkyl o-nitrophenyl selenide is oxidized with hydrogen peroxide to yield the corresponding selenoxide, which readily undergoes elimination to the alkene. Scheme 8.74 shows the application of the sequence of reactions described above to cyclohexylmethanol and the resulting formation of the exo-methylenecyclohexane. 

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