Cleavage carbon-selenium bonds

Reductive Cleavage of Carbon-Phosphorus, Carbon-Sulfur, and Carbon-Selenium Bonds... 

Besides the use of electrophilic or nucleophilic selenium reagents in synthesis, selenium has another advantage. It can stabilize adjacent carbanions or carbocations. While the deprotonation of selenides is difficult, the corresponding diselenoacetals can be used as efficient precursors through the cleavage of one of the carbon-selenium bonds. The chemistry of selenium-stabilized carbanions and carbocations is outlined in Section 9.11.2.3. 

The homolytic cleavage of the carbon-selenium bond provides an easy access to radicals, which can undergo various subsequent reactions. The plethora of radical chemistry is accessible using selenium-containing compounds and examples of free-radical reactions are described in Section 9.11.2.4. 

Fig. 41. Molecular structure of [ (CO)5Fe2(jU.-Se2)2C(Ph)C(H)(jU.-dppe)] (129). [Reprinted with kind permission from Mathur, P. Hossain, M. M. Mahon, M. F. Carbon-selenium bond cleavage in the double butterfly complex [ (CO)6Fe2(jU.-Se)2)2(C(Ph)-C(H) ]. J Organomet. Chem. 1994, 471, 185. Copyright 1994 Elsevier Science S. A., P.O. Box 564, 1001 Lausanne, Switzerland.]...

First, the homolytic cleavage of carbon-selenium bonds and selenium-selenium bonds is an easy process that can be initiated thermally or photochemi-cally. Sun lamp irradiation is usually sufficient to initiate a chain reaction involving organoselenium derivatives. 

The reaction occurs quite instantaneously at —78 °C with n-BuLi86) or tert-BuLi 35,99 jn tHF, or with /m-BuLi in ether87. The availability of selenocyclo-propyllithiyms in the last solvent is particularly important since, for example, their nucleophilicity towards carbonyl compounds is enhanced under these conditions 871 (vide infra). However in this solvent sec- or tert-BuLi must be used 87) in place of -BuLi in order to obtain quantitative cleavage of the carbon-selenium bond. For example, l,l-bis(methylseleno)cyclopropane is recovered unchanged after addition of n-BuLi in ether at —78 °C or —40 DC. However, l,l-bis(phenylseleno)-cyclopropane is more reactive since, under these conditions, 35% of 1-lithio-l-phenylseleno cyclopropane is produced ". ... 

The cleavage of the carbon-selenium bond has also been used 36) for the synthesis of 1-lithio-l-vinyl cyclopropanes from 1-seleno-l-vinyl cyclopropanes (Scheme 17). 

Other important cycloelimination procedures correspond to an elimination of H2O from cyclic ketones. Thus, the a-hydrogen atoms of semicarbazones of cyclic ketones are removed by oxidative cyclization with thionyl chloride or selenium dioxide (Scheme 8-7). The 1,2,3-thiadiazoles (71) or 1,2,3-selenadiazoles (72) which result from these reactions can be cleaved in a second step to yield cyclic alkynes (Scheme 8-8) [28]. Several fragmentation conditions are known, among them thermal decomposition and base-induced cleavage. The mechanism of these reactions has been studied in detaU [29]. It has been noted that the crucial step is the cleavage of the carbon-sulfur or carbon-selenium bond, as in this step the geometrical strain is introduced into the system. Clearly, due to the weakness of the C —Se... 

It is interesting to note that the latter result is exceptional since l,l-bis(phenyl-seleno)cyclopropane is the only selenoacetal derived from ketones to be at least partially cleaved under these conditions and even the homologous cyclobutyl derivative is inert under these conditions. This may be due to the extra stabilization introduced by the cyclopropyl ring. The case of 2-decy 1-1,1-bis(methylseleno)cyclo-propane merits further comment. It is difficult to assess whether the cleavage of the carbon-selenium bond occurs on the methylseleno moiety cis or tram to the alkyl group, since this organometallic leads to a mixture of the two possible stereoisomers on further reaction with electrophiles (Scheme 16). 

Addition Reactions with Formation of Carbon-Nitrogen Bonds Addition Reactions with Formation of Carbon-Sulfiir or Carbon-Selenium Bonds Addition Reactions with Formation of Carbon-Halogen Bonds Cleavage Reactions... 

Scheme 8.23. Reduction of hydroxymethylcyclohexane, as an example of a primary alcohol, by the replacement of the hydroxyl with an arylselenium and the subsequent cleavage of the carbon-selenium bond with tri-n-butyltin hydride.

Another systematic study of the radical cyclization was reported by Burke and coworkers [54]. Homolytic cleavage of the carbon-selenium bond in 1.47 provided tetrahydropyran 1.49 as a single diastereomer (Scheme 1.9). The stereochemical outcome of this reaction was explained by the equatoriaUy oriented substituents, which leads to the conformational preference as shown in transition state 1.48. 

The reactions of selenocarbonyl compounds with electrophiles are also well-established procedures. Alkylations or acylations of the selenium atom of selenoamides409 or selenoureas410 are known. Selenonium salts are formed initially they can then be converted into diselenides, selenazoles, or cyclic selenides depending on their structure. Reactions of selenocarbonyls with bromine and iodine have also been widely exploited. Selenocarbonates, sele-nothiocarbonates,411-415 and selenoureas416-418 can be employed, the reaction of 209 with 1 equiv. bromine led to the hypervalent 10-Se-3 complex 210, whereas an excess of bromine gave rise to a cleavage of the carbon-selenium double bond and formation of product 211 (Scheme 64). 

Selenol esters are expected to be a more reactive species than the corresponding thiol esters or 0x0 esters due to the comparatively weak bonding between carbon and selenium. The ability of the selenol esters to serve as active acyl transfer agents has been readily demonstrated by the easy conversion of the selenol ester (34 equation 15) to its corresponding acid (35), ester (36) or amide (37). This acyl-selenium bond cleavage has also been promoted by Cu and Cu" salts. - The isopropylidene derivative (38 equation 16) of ribofuranosylacetate has been converted to a lactone (40) in good yield via the selenol ester (39). 

Less interest has been shown, however, in the photoreactions of organoselenium compounds. Radical additions of diphenyl diselenide to 1,1-dimethylallene to give the l-(phenylseleno-methyl)vinyl selenide (193) and of diethyl (2-phenylseleno)-propanedioate to alkenes to give the adducts (194) have been reported. In contrast, carbon-selenium heterolytic bond cleavage is preferred on irradiation in the presence of 1,4-dicyanonaphthal-ene in this way, for example, the phenylselenylcyclohexane (195) is converted into the ether (196) by irradiation in methanol. 

---