Selenides, disubstituted, oxidation

This regioselectivity is practically not influenced by the nature of subsituent R. 3,5-Disubstituted isoxazolines are the sole or main products in [3 + 2] cycloaddition reactions of nitrile oxides with various monosubstituted ethylenes such as allylbenzene (99), methyl acrylate (105), acrylonitrile (105, 168), vinyl acetate (168) and diethyl vinylphosphonate (169). This is also the case for phenyl vinyl selenide (170), though subsequent oxidation—elimination leads to 3-substituted isoxazoles in a one-pot, two-step transformation. 1,1-Disubstituted ethylenes such as 2-methylene-1 -phenyl-1,3-butanedione, 2-methylene-1,3-diphenyl- 1,3-propa-nedione, 2-methylene-3-oxo-3-phenylpropanoates (171), 2-methylene-1,3-dichlo-ropropane, 2-methylenepropane-l,3-diol (172) and l,l-bis(diethoxyphosphoryl) ethylene (173) give the corresponding 3-R-5,5-disubstituted 4,5-dihydrooxazoles. 

A related radical transformation can be used to prepare 2,2-disubstituted selenanes. Treatment of the alcohol 124 with oxalyl chloride followed by addition of 2-mercaptopyridine iV-oxide and heating afforded the selenane 125 in moderate yield (Scheme 11) <2001JOC6286>. The required alcohol precursor for this reaction can be readily prepared from the bromo ester and in ////-generated sodium benzyl selenide, followed by Grignard addition. 

Disubstituted alkenes. Under controlled conditions to prevent a-deprotonation or C—Se cleavage (DME or ether, 0°) alkyllithium reagents (with the exception of methyllithium) add to phenyl vinyl selenide to give a-lithioalkyl phenyl selenides (2), which can be trapped with electrophiles to give 3. On oxidation, these products form alkenes (4) by elimination of benzeneselenenic acid. The reagent thus functions as CH=CH. 

As a second example of chemical properties we have considered peak oxidation potentials of 4,4 -disubstituted diphenyl selenides and diphenyl tellurides. We consider these quantities to be chemical properties because no covalent bonds have been formed or broken. The best regression equations obtained were for the selenides ... 

As with amine oxides and sulfoxides, acyclic 1,2-disubstituted alkenes are usually obtained with the ( )-stereochemistry, although the formation of a,(3-unsaturated nitriles is reported to give a mixture of ( )- and (Z)-isomers. For cyclic alkenes, the stereochemistry of double bond formation depends upon ring size. However, it can be affected by conformational factors, e.g. cyclododecyl phenyl selenide gives a mixture of cis- and fra/is-cyclododecenes on oxidative elimination (equation 38) but only the (El-isomer (101) was obtained from the acetoxycyclododecyl selenide (100 equation 39). ... 

Alkylation and other electrophilic reactions of vinyl sulphides and selenides following of-metallation include a number of useful synthetic procedures. Addition of an alkyl-lithium to phenyl vinyl selenide gives the cc-lithio- -alkylated selenide, which, on further alkylation and oxidative deselenation, gives the 1,2-disubstituted ethylene (H2C=CHSePh - R CHjCHLiSePh - R CH2CHR SePh - R CH=CHR ), thus permitting the vinyl selenide to be... 

Raucher views vinyl phenyl selenide as a CH=CH synthon, on the basis of its reaction with alkyl-lithium reagents under strictly controlled conditions. Trapping of the resulting a-lithioalkyl phenyl selenide with electrophiles, followed by oxidative elimination of phenylselenenic acid, offers a route to disubstituted olefins (Scheme 23). ... 

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