Selenides, aryl 1- metallation

Competing reactions to addition to a-hetero-substituted alkenes are metallation of vinylic or allylic protons (see 5.5.2.3.2) and cleavage of the carbon-hetero-element bond (see 5.5.2.2.1.). a-Metallations occur with vinylic chlorides, fluorides and ethers no addition of RLi occurs with vinyl fluorides, chlorides or ethers. For vinylic sulfides and selenides, whether metallation or addition (or even carbon-heterobond cleavage) occurs depends on the conditions and reagents. Addition of RLi (R = Et or n-Bu, not Ph) occurs to aryl vinyl sulfides ... 

Metallation of aryl selenides and selenoacetals.1 This base (KDA) is the most efficient for metallation of these substrates. 

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). 

These bases, however, are not strong enough to metallate dialkyl and alkyl aryl selenides. Amylso-dium and alkyllithiums, " which have proved particularly useful for the metallation of the corresponding sulfides and phosphines, are not useful for the metallation of dialkyl or alkyl aryl selenides owing to their tendency to react on the selenium atom and to produce a novel selenide and a novel orga-nometallic (Scheme 2). This latter reaction is easier when carried out in THF than in ether (Scheme... 

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. ... 

Benzyl selenides exhibit a similar pattern of leactivi. Thus, although the parent aryl benzyl selenides are successfiilly metallated with LDA at -78 their higher homologs, as well as benzyl mediyl... 

Metalloamides are suitable for the synthesis of 1-metallovinyl aryl selenides (Scheme 44) and 1-metalloallenyl phenyl selenides (Scheme 45) from the corresponding vinyl or allenyl selenides. The synthesis of the latter organometallics is even more conveniently achieved by metallation of l-phe-nylseleno-l-propyne or by a two-step, one-pot sequence of reactions fiom 2-chloroallyl phenyl selenide (Scheme 45). 

Related results have been described from pyridyl alkenyl selenides which, contrary to odier aryl alkenyl selenides (see below)," are easily metallated at the vinyl site with LDA (Scheme 48, compare i and j with f and h). The ready deprotonation and stereoselective methylation of these selertides have been ascribed to the presence of the nitrogen atom which can reduce the electron density of the double bond and also chelate with the lithium counterion." "... 

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). - ... 

Oxidations. Alkene formation from alkyl aryl selenides via oxidation is cleaner when the aryl group is ort/io-substituted (vs. para-substitution), for example, with a nitro group. In the presence of a phase-transfer catalyst, oxidation of aldehydes to carboxylic acids with H2O2 is successfully carried out without an organic solvent, halide, or metal ion. 

Lithiated allylic sulfoxides may be a-alkylated and the resulting products subjected to [2,3]-sigmatropic rearrangement induced by a thiophile to give allylic alcohols (eq 43). In contrast, alkenyl aryl sulfoxides produce a-lithiated species which are alkylated with Mel or PhCHO in good yields (eq 44). LDA has also been used to metalate allylic and propargylic selenides as weU as aryl vinyl selenides. ... 

---