Phenyl vinyl selenides

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. 

Use of benzeneselen nyl bromide as the electrophile in this reaction provides phenyl vinyl selenides in 70-90% yield. 

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. 

The a-deprotonation of vinylic aryl selenides is strongly dependent on the substituents [4]. For phenyl vinyl selenide, the reaction was successfully carried out with LDA or KDA at -78°C in THF (Scheme 50, reaction 1). When a alkyl group is present, the use of LDA in THF leads to the abstraction of both vinyl and allyl protons. The a-deprotonation was improved using KDA in THF at -78°C (Scheme 50, reaction 2). With two )9,)9 -alkyl groups, the metalation occurred only at an allylic position (Scheme 50, reaction 3). 

Satisfactory deprotonations of l-( )-butenyl and 1-(3-methyl-l-( )-butenyl) selenides in either the phenyl or m-(trifIuoromethyl)phenyl series caimot be achieved with LDA in THF (Scheme 48, compare a, e and g). Even LiTMP does not deprotonate the phenyl vinyl selenides (Scheme 48, a), and complete... 

The reaction of organolithium compounds with phenyl vinyl selenides proceeds by at least three different routes which involve (i) metallation, which leads to 1-phenyIseleno-l-vinylli-thium (Section 2.6.2.2.2) (ii) Se-Li exchange, which produces phenyllithium and a novel vinyl selenide (Scheme 104, or (Hi) addition of the organolithium across the carbon-cartx>n double... 

LDA in THF or in THF-HMPT, LiTMP in THF and KDA in THF have all been successfully used for the metallation of phenyl or m-(trifluoromethyl)phenyl vinyl selenide (Scheme 44). The deprotonation was shown to be reversible with LDA in THF and irreversible with LiTMP when performed in the same solvent. Extension of this reaction to homologous derivatives proved difficult since metallation at the allylic sites often competes. Allylic metallation is particularly favorable with aryl 1-propenyl selenides (Scheme 46) - and, whatever the conditions used (LiTMP or KDA ), with aryl 1-(2-methyl-1-propenyl) selenides (Scheme 47). In the latter case both the (Z)- and the ( )-methyl groups have been metidlated leading to the corresponding a-metalloallyl selenides (Scheme 47). - ... 

Phenyl vinyl selenide has been shown to react in three different ways using different base-solvent systems [86] ... 

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... 

Applications Based on Deprotonation of Vinyl Selenides. Deprotonation (by KDA) of phenyl vinyl selenide and prompt treatmrait with electrophiles such as representatives of the compound classes alkyl halides, aldehydes, enones, and epoxides gives the expected products. Scheme 13 shows a typical example and illustrates the preservation of stereochemistry about the double bond. Vinyl selenides can be hydrolysed to ketones. ... 

Applications Based on Michael Addition to Phenyl Vinyl Selenide. The ct lithio-selenides (51), (52), and (53), which were generated by treating phenyl vinyl selenide with the appropriate organolithium in dimethoxymethane or in ether,... 

Di-(2-hydroxy-2-propylphenyl) selenide gives the selenurane analogue (45a CH, in place of CF H in place of Bu ) of the sulphurane (45a) through a simple procedure (Brz followed by EtjN). Easy access to higher valency states for selenium derivatives may account for some differences in properties of bivalent sulphur compounds compared with their selenium analogues for example, phenyl vinyl selenide PhSeCH=CH2 does not polymerize with benzoyl peroxide, as does the vinyl sulphide, but gives the dibenzoate PhSeCH(OCOPh)CH2(OCOPh) via the selenurane PhSe-(OCOPh)2CH=CH2. ... 

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