Selenocarbonyl compounds

Synthesis.—K recent paper by Reid appears to be the only one reporting 

Roussel, R. Gallo, M. Chanon, and J. Metzger, Bull. Soc. chim. France, 1971, 1902. 

The reaction of carbon diselenide with active-methylene compounds in the presence of a base has been methodically studied by Jensen and Henriksen, who found that the reaction course, resulting in the formation 

The reactions of carbon diselenide with hydrazines and with alcohols in the presence of potassium chloroacetate and a suitable base, yielding stable diselenocarbazic acids (334) and [(alkoxyselenocarbonyl)seleno]acetic acids (335), respectively, were studied in the same laboratory. Compounds of the type (336) have been prepared analogously to (335), using a thiol instead of the alcohol, and O-alkyl selenocarbazates (337) have been synthesized by the action of hydrazines on (335).  

The formation of 2-alkylidene-l,3-diselenoles by the photolysis of 1,2,3-selenadiazoles suggested to the investigators the intermediacy of selenoketens. The tetramerization of bis(trifluoromethyl)thioketen (100), which was found to take place only in the presence of selenium, was considered by Raasch to depend on the transient existence of the seleno-carbonyl intermediate (338). The preparation of transition-metal complexes 

---

The structural parameters describing the geometry of di-/inter-halogens binding to chalcogen-containing molecules (thioethers, selenoethers, thiocar-bonyl, and selenocarbonyl compounds) are defined in Figure 1. 

Conversion of thio- and selenocarbonyl compounds into their oxo analogues... 

Usually these heteroaldehydes and heteroketones are generated in solution in the presence of suitable substrates and immediately trapped. The synthesis, properties, and reactivity of thioaldehydes and thioketones have been the subject of several reviews.1-5 The chemistry of selenocarbonyl compounds has likewise been reviewed in recent years.6-9... 

C=0 —> C=Se.2 The reagent reacts with enones and several masked carbonyl compounds to form selenocarbonyl compounds. 

Selenoaldehydes and selenoketones are selenocarbonyl compounds which are usually unstable. Only the attachment of bulky groups next to the selenocarbonyl moiety has enabled their isolation, and selenoaldehydes357 as well as selenoketones have been described recently. More stable are selenoesters and selenoamides, and different... 

The ability of carbanions to react with elemental selenium can be advantageously used for the synthesis of selenocarbonyl compounds. For example, sulfur ylides 201 (E = +SMe2) have been reacted with elemental selenium to generate the corresponding selenocarbonyl compounds 202 (Scheme 59).374,375 But Staudinger selenylation also has been applied to the synthesis of selenoketones 202 from phosphorus ylides 201 (E = +PPh3), which have been trapped by dienes in hetero-Diels-Alder reactions.376-383... 

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. 

Selenocarbonyl compounds can form different metal complexes and some general structures are shown in Figure 9. 

Figure 9 Metal complexes with selenocarbonyl compounds.

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... 

During the last decade, many novel synthetic procedures for the synthesis of selenocarbonyl componds have been established avoiding the use of toxic hydrogen selenide. The utilization of selenocarbonyl compounds in biological chemistry as well as in the development of new synthetic reactions based on chiral selenocarbonyl compounds has been rather unexplored despite the fact that slight modifications of the structures of selenocarbonyl compounds are expected to highly attenuate the reactivity and sensitivity of these compounds. 

The relatively poor 2p-4p r-overlap of carbon-selenium double bonds makes selenocarbonyl groups less stable than the corresponding thiocarbonyl and carbonyl analogs. Until the mid-1970s, only those selenocarbonyl compounds where the bond order of the C=Se bond was lowered by coordination or resonance were known. However, the isolation of two deep blue, thermally stable selones (46) and (47) proved that such... 

The higher reactivity of organoselenium derivatives can also be observed in the various reactions of selenocarbonyl compounds which are described in detail by T. Murai and S. Kato in Chap. 7. 

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. 

The selenocarbonyl compounds can be classified into three categories on the basis of the substituents adjacent to the carbon atom of the selenocarbonyl group (Scheme 1). 

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. 

Elemental selenium can react with carbanions smoothly. The syntheses of selenocarbonyl compounds using elemental selenium are shown in Eqs. (1-13). 

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. 

---