Selenides, vinyl metallation

Synthesis of a-Selenoalkyl Metals by Addition of Organometallics to Vinyl Selenides, Vinyl Selenoxides and Vinyl Selenones... 

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 ... 

Cross-coupling reactions of vinyl zirconocenes of general structure 41, mainly using group 10 metal catalysts, can be smoothly effected to give a variety of vinyl selenide-con-taining materials, which are amenable to further elaboration through nickel-catalyzed... 

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). 

Z)-vinyl selenides after hydrolysis of the metallic intermediates (Scheme 56, reaction 2). 

Synthesis of a-selenovinyl metals (1 -seleno-1-alkenyl metals) by metallation of vinyl selenides 2.6.2.23 Synthesis of a-metalloalkyl selenoxides, selenones and selenonium salts... 

Phenylselenomethyl phosphonates and (phenylseleno)chloromethane have been metallated by NaH and Bu OK, respectively (Scheme 28, a and b Scheme 29). The former organometallics react with carbonyl compounds and dirratly produce vinyl selenides (Scheme 28, a and whereas the second decomposes to phenylselenomethylene which adds stereospecifically to alkenes and leads to phenyl-selenocyclopropanes in reasonably good yields (Scheme 29). 

Synthesis of a-selenovinyl metah (I-seteno-l-olkenyl metals) by metaUation of vinyl selenides... 

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). 

Increasing the substitution of the alkyl group on the 3-carbon of 1-alkenyl phenyl selenides again favors metaliation at the vinylic site (Scheme 48). > KDA in THF has proved to be, without contest, the most successful reagent for this purpose. The reaction takes place at low temperature (-78 C) and the 1-phenylselenoalkenyl metal, produced in almost quantitative yield, retains its stereochemistry (Scheme 48, b-d). 

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." "... 

J Synthesis of a>Selenoalkyl Metals by Addition of Organometalllcs to Vinyl Selenides,... 

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... 

The addition reaction is particularly favored when carried out in ether or DME (Scheme 104, b Scheme 105). Addition takes place between 0 °C and 20 C, under conditions at which the a-phenylsele-noalkyllithium isomerizes rapidly to the corresponding aryllithium (Scheme lOb). - So, further reaction with electrophiles, for example, must be carried out as soon as possible (compare Scheme 106 with Scheme 105). The reaction takes another course when performed in THF since 1-phenylseleno-l-vinyllithium and phenyllithium are produced from, respectively, the metallation and C—Se bond cleavage of the vinyl selenide (Scheme 104, a). Interestingly, the temperature has an important effect on the course of this reaction, since in THF the Se-U exchange becomes predominant if die reaction is carried out at low temperature (-78 C instead of 0 C). ... 

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). - ... 

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