Selenide, methyl phenyl metallation

The presence of a group able to delocalize the charge from the caibanionic center favors the synthesis of the carbanion whatever the method used, whereas substitution of the carbanionic center by alkyl groups disfavors it. Thus, whereas methyl phenyl and dimethyl selenide cannot be metallated wiA LDA at -78 °C or 0 C, phenylseleno and methylseleno acetates and propionates lead, on reaction with the same base at -78 °C, to the corresponding organometallics (Scheme 10). - ... 

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. ... 

Bu"Li, which usually performs the Se-Li exchange in selenides, effects the metallation of some functionalized selenides. This is p cularly the case of phenylselenomethyl phosphonates and phosphonium salts (Scheme 28, c). b6.w >p g t),g methyl phenyl selenide (Scheme 13, a... 

The nature of the substituents located at the 2-, 4-, and 6-positions of 1,3-dise-lenanes favors either C-Se bond cleavage or a-deprotonation. For = H, Me, Ph, in the 2-position, the action of n-BuLi, PhLi or MeLi afforded C-Se cleavage with formation of j8-hydroxyalkyl-(3-butylselanylpropyl)selenides (60-85%) after reaction with benzaldehyde or acetone [16] (Scheme 17, reaction 1). With two methyl groups in cfs-4,6-positions, the metalation by an alkyllithium is strongly dependent on the nature of the C-2 substituent. From a czs-C-2-methyl-substituted derivative, metalation requires the use of s-BuLi, no reaction being observed with n-BuLi or t-BuLi (Scheme 17, reaction 2). For cis- or trans-2-phenyl-l,3-disele-nane, metalation can be performed with MeLi or PhLi and an unexpected C-Se... 

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). - ... 

Although unable to metaUate selenides, dialkyl amides are sufficiently strong to metallate phenylselenoacetals as well as methyl and phenyl selenoorthoesters. They are also able to metallate relenoxides and selenones Finally selenoacetals are readily available from carbonyl... 

---