Selenoaldehyde compounds

Selenoaldehydes 104, like thioaldehydes, have also been generated in situ from acetals and then directly trapped with dienes, thus offering a useful one-pot procedure for preparing cyclic seleno-compounds [103,104], The construction of a carbon-selenium double bond was achieved by reacting acetal derivatives with dimethylaluminum selenide (Equation 2.30). Cycloadditions of seleno aldehydes occur even at 0 °C. In these reactions, however, the carbon-selenium bond formed by the nucleophilic attack of the electronegative selenium atom in 105 to the aluminum-coordinated acetal carbon, may require a high reaction temperature [103], The cycloaddition with cyclopentadiene preferentially gave the kinetically favorable endo isomer. 

Conversion of Carbonyl Compounds into Thio- and Selenoaldehydes and Ketones... 

On transforming the cyclopentenone 1431 into the trimethylsilyloxy diene 1432 the ensuing Diels-Alder cyclization gives rise to 69% tricychc compound 1433 [10] (Scheme 9.8). For Diels-Alder-reactions of thioaldehydes, selenoaldehydes, or unsaturated nitroso compounds with cyclopentadiene, see the reactions of 602 to 603 and of 605 to 606 in Scheme 5.48 and of 1092 to 1093 in Scheme 7.43. For Diels-Alder-reactions of silyloxyazadienes such as 510 with maleic anhydride to give 511, see Scheme 5.29. 

The stabilization of these heteroaldehydes and -ketones by coordination to transition metals is the subject of this review. Many problems connected with the high reactivity of these heterocarbonyl compounds can be circumvented by using their transition metal complexes. The chemistry of organo-sulfur and organoselenium transition metal complexes in more general terms30,31 and some aspects of thio- and selenoaldehydes and -ketones as... 

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 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... 

The treatment of vinyl anions with elemental selenium also leads to seleno-carbonyl compounds via eneselenolate anions (Eqs. 4-6). Deprotonation of l,3-selenothio-2-thione with EDA followed by the reaction with elemental selenium proceeds accompanied with the rearrangement of vinylic skeleton to form l,3-dithio-2-selone derivatives (Eq. 4) [38]. The vinyl anions from oxa-zolines and imidazoles were treated with elemental selenium to give the oxazo-line selone and imidazoline selone after the aqueous workup (Eq. 5) [39]. The reaction of vinyl magnesium halides with elemental selenium and allyl bromide gives allyl vinyl selenides that undergo a seleno-Claisen rearrangement to generate y,6-unsaturated selenoaldehydes (Eq. 6) [40]. 

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. 

Ab initio quantum chemical calculations concluded that HN=S is more stable than the isomeric structure HS=N and that thermal isomerization between the two species is unlikely (86IC4221 81PS325). It is now clear, both from experiment and from theory, that election-donating substituents attached to nitrogen stabilize the RN=S system, while electron-withdrawing substituents detabilize these compounds. This is the same effect observed for substituents attached to the carbon of thioaldehydes and selenoaldehydes. 

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. 

Se chemical shifts span a wide range of ca. 3300 ppm the extremes are marked by selenoaldehydes, some molybdenum selenides, and cationic heterocycles (up to S = 2434) at the high-frequency end and bridging selenium (p) in tungsten complexes (3 = —900) at the low-frequency end. A detailed discussion of electronic influences on Se chemical shifts and its variation in different classes of compounds has been published. In the following, some general characteristic tendencies are collected. 

Telluroaldehydes have been generated and trapped for the first time by the reaction of benzylidenetriphenylphosphorane with "activated" tellurium (a method analogous to that previously used to prepare selenoaldehydes) (Scheme 22).71 A wide range of reactive ylides have been converted into the adducts (118) by reaction with borane.72 On heating, (118) rearrange to triphenylphosphine-monoalkylborane adducts (119) which undergo the expected hydroboration reactions with alkenes. A new route to phosphaalkenes (121) is available from the reaction of phosphinomethylenetriphenylphosphoranes (120) with Lewis acids.73 In the case of (120, R2=NPr 2) the compounds (121) can be isolated and in one case an X-ray structure was obtained. However, similar reactions of (120, r2=Bu ) lead to the dimers (122). 

Selenocarbonyl compounds have been virtually ignored as hetero-dienophiles. Recently, Krafft and Meinke have found a novel method to generate various selenoaldehydes (Scheme 5-XXII).59 These compounds are reactive dienophiles and can be trapped in situ with cyclopentadiene to give Diels-Alder adducts. As can be seen in the scheme, endo adducts predominate in these cycloadditions. Similar results were found in thioaldehyde additions to cyclopentadiene (cf. Scheme 5-IV). 

The dimerization of a,/3-unsaturated selenoaldehydes (selenals) and selenoketones (selones) is known to proceed in a head-to-head fashion (see Schemes 21 and 22). This transformation has been studied at the density functional theory (DET) level using selenoacrolein 4 as a model (Scheme 1) <1999JOC1565>. These calculations indicate that even though both types of dimerization have very low energy barriers (0.9-2.8 kcal mol ), head-to-head dimerization is thermodynamically favored over head-to tail addition by about 14kcalmol . The optimized geometries of the product diselenanes 5 and 6 have also been determined. The rw-compounds prefer the half-chair conformation while the /ra r-isomers prefer boat-like conformations. 

Synthesis.—No report concerned with the preparation and identification of stable thioaldehydes has appeared however, pyrrolo(2,l- >lthiazole-7- (1) and -5-carbo-selenoaldehydes (2)—(4) have been synthesized by the selenoformylation of pyrrolo[2,l-6]thiazoles and characterized by their H n.m.r. and u.v. spectra. With the exception of (4), these selenoaldehydes are stable, green, crystalline compounds. 

The carbonyl compound can also contain additional functionality. Thus, treatment of an a,fi- poxy ketone with excess lithium reagent (1) provides the allyl alcohol (2) (eq 2). The use of an a-phenyl selenoaldehyde as electrophile allows either an allyl selenide or a /3-silyl aldehyde to be obtained, depending upon the reaction conditions used with the hydroxysilane (eq 3). With a,/8-unsaturated ketones, the lithium reagent (1) adds in the 1,2-sense the Grignard analog can provide 1,4-addition. The cuprate derived from (1) undergoes the expected reactions for this class of compounds, such as 1,4-addition. ... 

Formylation of Aromatic Rings. The Vilsmeier reagent attacks electron-rich aromatic systems to form aryl-methyleneiminium ions which liberate a formylated aromatic compound upon hydrolysis (eq 2). Thio- and selenoaldehydes can be prepared by hydrolysis in the presence of Sodium Hydrogen Sulfide or Sodium Hydrogen Selenide. A wide range of aromatic systems can be formylated in this fashion, including benzene derivatives, polyaromatic hydrocarbons (eq 3), and azulene. Substitution occurs at relatively electron-rich positions. 

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