Cyclopropyl selenides

Lithio-l-cyclopropyl selenides (128) react with aldehydes and ketones to give ) -hydroxycyclopropyl selenides (129) which undergo dehydration and C3 - C4 ring expansion upon treatment with (carboxysulphamoyl)triethylammonium hydroxide inner salt methyl ester (Me02CH S02N Et3) (equation... 

Alkyl aryl and dialkyl selenides, aside from the parent compounds methyl phenyl selenide (Scheme 13, a) and trifluoromethylphenyl methyl selenide (Scheme 13, b), cannot be easily metallated. It was expected that the extra stabilization provided by the cyclopropyl group would be sufficient to permit the metallation of cyclopropyl selenides, but that proved not to be the case. ... 

Cyclopropyl phenyl selenides are inert towards lithium diisopropylamide (LDA) and lithium tetra-methylpiperidide (LiTMP) (Scheme 14, a). Butyllithiums instead react on the selenium atom and pro-duce butyl cyclopropyl selenides and phenyllithium rather than cyclopropyllithiums and butyl phenyl selenides (Scheme 14, b). 

All attempts to metallate cyclopropyl silanes with strong bases such as alkyl-lithiums in THF 94> or sec BuLi and TMEDA in THF 82,98) as well as cyclopropyl selenides with non-nucleophilic bases such as LDA in THF39,94 , or lithium tetramethylpiperidide in THF 35,94) or in THF-HMPT35 (Scheme 12), meet with failure. 

Syntheses of Alkylidene cyclopropanes Via the Selenonium route The selenonium route proved to be more valuable. It has been specifically designed by us to replace the deficient selenoxide route (Scheme 38). It was expected to produce alkylidene cyclopropanes by a mechanism which mimics the selenoxide elimination step but which involves a selenonium ylide in which a carbanion has replaced the oxide. Cyclopropyl selenides are readily transformed to the corresponding selenonium salts on reaction with methyl fluorosulfonate or methyl iodide in the presence of silver tetrafluoroborate in dichloromethane at 20 °C and, as expected, methylseleno derivatives are more reactive than phenyl-seleno analogs. Alkylidene cyclopropanes are, in turn, smoothly prepared on reaction of the selenium salts at 20 °C with potassium tert-butoxide in THF (Scheme 38). Mainly alkyl cyclopropenes form at the beginning of the reaction. They then slowly rearranges, in the basic medium, to the more stable alkylidene cyclopropanes( 6 kcal/mol). In some cases the complete isomerisation requires treatment of the mixture formed in the above reaction with potassium fcrt-butoxide in THF. The reaction seems to occur via a selenonium ylide rather than via a P-elimina-tion reaction promoted by the direct attack of the /crt-butoxide anion on the P-hydrogen of the selenonium salt, since it has been shown in a separate experiment that the reaction does not occur when a diphenylselenonium salt (imable to produce the expected intermediate) is used instead of the phenyl-methyl or dimethyl selenonium analogs. It has also been found that the elimination reaction is the slow step in the process, since styrene oxide is formed if the reaction is performed in the presence of benzaldehyde which traps the ylide intermediately formed... 

The acyl selenide 19 affords the decarbonylated )S-lactam in good yield. A N-hydroxypyridine-2-thione ester 20 is used in the key step to construct the chiral cis-cyclopropane structure in compounds designed as antidopaminergic agents. The observed high cis selectivity is due to the hydrogen abstraction from the sterically demanding (TMSlsSiH, which occurs from the less-hindered side of the intermediate cyclopropyl radical. 

Alkyltin compounds carrying a leaving group at the y-position undergo 1,3-elimination to produce cyclopropanes9. y,<5-Epoxyalkyltin derivatives are converted to cyclopropyl-methyl alcohols under catalysis of a Lewis acid (equation 5)10. y-Hydroxyalkyl selenides... 

Cyclopropropylmethyl radicals.1 Generation of a radical center adjacent to a cyclopropane can result in ring opening of the cyclopropylmethyl system. This reaction can be used to convert allylic cyclohexenols to alkylcyclohexenes, as shown for conversion of 1 into 3 via the phenyl selenide 2. When the cyclopropyl ring is... 

One of the major drawbacks of these reactions is clearly the unavailability of the whole set of alkyl phenyl selenones due to side reactions which occur during their preparations. Methyl, n-alkyl and some cyclopropyl phenyl selenones are available from the corresponding selenides and potassium permanganate, peroxycarboxylic acids or hydrogen peroxide/seleninic acids mixtures, whereas s-alkyl phenyl selenones, 2,2,3,3-tetramediylcyclopropyl selenone and benzyl phenyl selenone are not available in these ways. 

Allylation of a-thio-35), a-seleno-35) and a-silyl- 35,77) cyclopropyllithiums was not very successful35) but addition at —78 °C of 0.5 equivalent of copper (I) iodide-dimethylsulfide complex 35,106, W7> prior to the allylhalide leads 35,106,107) to a very high yield of homoallyl cyclopropyl sulfides or selenides (Scheme 24). Similar observations have been made on cyclobutyl derivatives3S). It is not clear at present whether a cuprate is involved in the process but we have evidence ( Se-NMR) that a new species is transiently being formed, at least in the seleno series. The synthesis of homoallyl cyclopropylsilanes was also reported 78) and involves the allylation of a postulated cuprate formed by the addition of lithium dibutyl cuprate to a-lithiocyclopropylsilane (Scheme 26). 

Syntheses of Alkylidene cyclobutanes Via the Selenoxide Route The selenoxide route which was particularly inefficient with cyclopropyl derivatives in this case proved suitable. The reaction is completely regioselective in the case of P-hydroxy selenides, which produce exclusively the allyl alcohols possessing the endo-cyclic double bond (Scheme 39)s7), whereas a mixture of endo and exo olefins is... 

The synthesis of alkylidene cyclobutanes from l-(-seleno) cyclobutyl carbinols is much more facile than of those bearing a cyclopropyl moiety. It resembles the transformation of other (P-hydroxyalkyl) selenides to olefins 7 9,11,12). 

The first approach to the seleno series taken advantage of the wide availability of 1-(l-seleno)cyclopropyl carbinols and the small tendency (discussed in Section Syntheses of alkylidene cyclopropanes from 3-hydroxy alkyl selenides), of the phenylseleno derivatives to produce alkylidene cyclopropanes on reaction 37), with for example, thionyl chloride/triethylamine mixture (Scheme 51). However, the best results have... 

Table 13. l-[l-(Alkylselanyl)cyclopropyl]alkenes by Dehydration of -Hydroxy Selenides with the Burgess Reagent in Toluene at... 

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