A-selenocarbonyl compounds

JJ Reactivity of 1,1-bis(seleno)- -alkyI metals and a-selenocarbonyl compounds with enals and enones... 

Hi Reactivity of enolates derived from a-selenocarbonyl compounds... 

Although some reactions, such as the transformation of -hydroxyalkyl selenides to -haloalkyl selenides (Scheme 161, b) or to vinyl selenides, enones (Scheme 161, 0. a,a-dihalocyclopropanes (Scheme 162, f) or p-hydroxyalkyl halides (Scheme 161, h Scheme 162, g), have bwn occasionally described or found only with specific types of -hydroxyalkyl selenides, especially those having a strained ring [e.g. their transformation to allyl selenides (Scheme 163, b), 1-selenocyclobutenes (Scheme 163, c) and cyclobutanones (Scheme 163, f),i2.i59,i60.i63] odiers are far more general. This is particularly the case of eir reductions to alcohols (Scheme 161, a Scheme 162, a Scheme 163, a Scheme 164, a Scheme 165, a) - or alkenes (Scheme 161, c Scheme 162, c Scheme 163, d Scheme 164, c Scheme 165, a Scheme 166, c), 89,i94,239 transformation to allyl alcohols (Scheme 161, e Scheme 162, b Scheme 164, b Scheme 166, b), - epoxides (Scheme 161, g Scheme 162, d Scheme 163, e Scheme 164, d Scheme 165, b Scheme 166, d) and rearranged carbonyl compounds (Scheme 162, e Scheme 164, e Scheme 165, a, c Scheme 166, e), - as well as oxidation to a-selenocarbonyl compounds (Scheme 161, d). 2 2.248-25i... 

Scheme 10.15 Michael addition of a-selenocarbonyl compounds to nitroalkenes.

Cations [(RR C=Se)2I]+ In the case of A-methylbenzothiazole-2(3//)-selone (mbts), one equivalent of iodine provides a molecular adduct in dichloromethane solution 50 crystallisation, however, leads to a ionic compound of the type [(RR C Se)2I]+I3, 43 This compound was the first one reported in the literature featuring a two coordinated iodine(I) complex with two donor molecules containing selenocarbonyl groups. In the solid state two molecules of mbts are linearly coordinating a central I+ ion to give a slightly... 

Alkynyl anions can be reacted with elemental selenium to form alkynylselenolates 203. These alkynylselenolates 203 have been used as key intermediates for the synthesis of a variety of selenocarbonyl compounds. First reactions have already been carried out earlier,384 and some later improvements widened the scope of this reaction including the synthesis of selenoesters, selenothioesters, and selenoamides of type 204 (Scheme 60).385,386 Even a direct... 

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). 

The selenium counterparts of enolates, eneselenolate ions, can be generated by deprotonation of the corresponding selenocarbonyl compounds such as selenoamides427,428 or selenoesters.403,429 They can either be trapped with trimethylsilyl chloride or react with a variety of substrates to form a range of different products. 

The synthesis of metal complexes of type 213 can be performed by reacting metal-carbene complexes with selenium sources such as alkyneselenolates 203.430 Also the stability of unstable selenocarbonyl compounds such as selenoaldehydes can be enhanced by coordination to metal carbonyls and the reactivity of such complexes has been studied. Complex 216 can react with methylthiohexyne and the product is a different complex 217 with the selenium atom still coordinating to the metal carbonyl fragment (Scheme 66).431... 

Studies on the chemistry of selenocarbonyl compounds have been increasingly reported din ing the past decade. Initially, the focus had been mainly on the development of new synthetic methods for the preparation of selenocarbonyl compoimds. The various achievements have allowed the evaluation of a wide range of applications. X-ray molecular structures of some derivatives have been disclosed. Selenocarbonyl compoimds that are relatively stable but still retain high reactivity have exhibited unique reactions. The present review outlines the characteristic features of selenocarbonyl compounds. Their syntheses and reactions are classified on the basis of reaction patterns rather than the substituents adjacent to selenocarbonyl group. Several types of metal complexes having selenocarbonyl groups are described as well. 

X-ray molecular structure analyses of a wide range of selenocarbonyl compounds, in particular the compounds in category C in Scheme 1, have been studied (Scheme 3) [5,8b, 20-27]. Representative bond lengths of C = Se are shown in Scheme 3. As for selenoketones 21a, b [5], the bond lengths are less than 1.80 A. In the nitrogen-substituted selenocarbonyl compounds. 

A variety of synthetic methods for preparing selenocarbonyl compounds have been developed, and recent reviews disclosed details of each compound in Scheme 1 [2b, 6, 8c, 9,11]. Accordingly, the synthetic methods are classified based on the reaction patterns rather than the types of compounds in this review. In particular, attention has been paid to the step of the formation of the carbon-selenium bond. The selenium atom has generally been introduced electrophilically or nucleophilically to the organic molecules. Additionally, heterocumulenes involving selenium atoms have been used as a starting material. 

The reactivity and reaction patterns of selenocarbonyl compounds vary considerably from the compounds in categories A-C in Scheme 1. The substituents adjacent to the selenocarbonyl group attenuate the polarity of the compounds. As a result, less polar selenoaldehydes and selenoketones behave as olefins, and the carbon selenium double bond of heteroatom-substituted selenocarbonyl compounds becomes more polar. In this section, a variety of transformations of selenocarbonyl compounds have been classified based on the reagents reacted with selenocarbonyl compounds. 

Essentially all of the work on [4 + 2] cycloadditions of selenocarbonyl compounds has been reported within the past five years.Krafft and coworkers have developed a novel method for producing seleno-aldehydes and -ketones, and have investigated in some detail the Diels-Alder chemistry of these species.Alkyl- and aryl-substituted selenocarbonyl compounds could be formed from silyl seleno-cyanates (198) (equation 105). As is the case with thioaldehydes, selenoaldehydes react with cy-clopentadiene to afford predominantly endo cycloadducts. This stereochemical preference has also been observed by Segi et using a different method for generating selenoaldehydes. 

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