Dialkyl selenides, reaction with

Reduction of elemental selenium with Sml2 leads to the selective formation of Se and Se depending on the reaction stoichiometry, similar to the reduction with NaBH4 and hydrazine (Sects. 2.2.2 and 2.2.3). These inorganic dianions react with alkyl bromides to produce dialkyl selenides and diselenides, respectively, in excellent yields (Scheme 15) [17a]. 

Yanada et al. synthesized a new selenide exchange resin 78 by treatment of borohydride exchange resin (BER) with selenium. The subsequent reaction with alkyl halides or tosylates gives dialkyl selenides 79 selectively in a high yield (Scheme 50) [95]. 

Catalytic reactions have the advantage over the methods discussed so far in that the chiral catalyst need not be added in stoichiometric amounts, but only in very small quantities, which is important if not only the metal (very often a precious one) but also the chiral ligand are expensive. Among the ferrocenes, phosphines are by far the most important catalysts for stereoselective reactions, and are covered in Chapter 2 of this book. We will therefore focus here mainly on the catalytic applications of chiral ferrocenes not containing phosphine groups. Only recently, some progress has been made with such compounds, mainly with sulfides and selenides, and with amino alcohols in the side chain (for this topic, see Chapter 3 on the addition of dialkyl zinc to aldehydes). 

Direct cyclization to the 1,3-diselenolylium cation takes place when a -formyl-7V,A(-dialkyl-diselenocarbamate is treated with acid subsequent reaction with hydrogen selenide gives the parent... 

Other nucleophiles, which give substitution with inversion in their reaction with ( )-l-alkenyliodonium salts, include dialkyl sulfide and selenide," phosphorose-lenoates, and sulfonates (Scheme 18). Amides also react with E)- -alkenyliodonium salts and give a Z enol formate (Scheme 19). " The mechanism of these reactions, involving very weakly basic or non-basic nucleophiles, may well be in-plane SnVo- attack. Moderately basic nucleophiles such as amines and carboxylates, even dichloroacetate, give -elimination. 

Dialkyl diselenides. A convenient access to RSe-SeR from RBr is by reaction with KSeCN in aq DMF in the presence of K2CO,. Alkyl selenocyanates are hydrolyzed in situ and the resulting alkyl selenide ions undergo aerial oxidation. 

Dialkyl selenides. The reaction of trialkylboranes with an equimolar amount of selenium dioxide suspended in THF leads to dialkyl selenides as the major... 

Dialkyl selemdes dialkyi diselenides. Reaction of selenium with 2.1 equiv. of this hydride in THF gives a suspension of milky white lithium selenide. On addition of alkyl halides dialkyl selenides are formed in 60-95% yield (equation I). 

Oxidation of -hydroxy selenides.s p-Hydroxy selenides (1), obtained by reaction of a dialkyl ketone with phenylselenenylrnethyllithium, are oxidized by... 

Alkyl selenides are most conveniently prepared by the dialkylation of Na2Se or by the monoalkylation of selenolates. Common routes to aryl derivatives include the reaction of a selenolate with a diazonium salt, the SrnI reactions described earlier, and the reaction of aryUithiums with aryl selenocyanates (ArSeCN). A different approach employs the reaction of alcohols with aryl selenocyanates or N-(phenylseleno)phthalimide (see Section 6) in the presence of tri-n-butylphosphine. Similar conditions can also be used to convert carboxylic acids to selenoesters (8). These reactions are illustrated in Scheme 3. 

The reaction involving dichlorocarbene also provides a wide range of epoxides, " including terminal. a.a- and a,p-dialkyl-substituted and tiialkyl-substituted compounds, from a large variety of 3-hy-drcxyalkyl methyl and phenyl selenides (Scheme 162, d). The thallous ethoxide reaction, although it takes place more slowly (especially with phenylseleno derivatives) than under phase transfer catalysis condirions, " has to be in several instances preferred since it avoids the concomitant formation of... 

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