Selenoaldehyde

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. 

Generation of a selenoaldehyde, a selenoketone, and telluroaldehydes by [3,3] sig-matropic rearrangement of allyl alkenyl selenides and tellurides [139]... 

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

Free intermediate thioaldehydes 598 or 602 and the selenoaldehydes 605 and HMDSO 7 are obtained in THF at 0°C on treatment of aliphatic and aromatic aldehydes with bis(trimethylsilyl)thiane 601 or bis(trimethylsilyl)selenide 604 in the presence of traces of butyllithium, while trapping the sensitive intermediate thio- or selenoaldehydes 602 and 605 with cyclopentadiene or cyclohexadiene to furnish mixtures of endo and exo Diels-Alder adducts such as 603 a and 606 a and 603 b and 603 b [148-150], the exo/endo ratio of which can be controlled [150] (Scheme 5.48). Analogous reaction of ketones such as 2-adamantanone or acetylene ketones with MesSiXSiMes 608 (a. X=S (601) b. X=Se (604)) in the presence of... 

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. 

Selenoaldehydes,2 In the presence of 5-10 mole% of butyllithium, this disilyl selenide converts aldehydes into selenoaldehydes with formation of the disiloxane [(CH,)3SiOSi(CH3),]. The active reagent is presumed to be lithium trimethylsilyl-selenide, (CH3)3SiSeLi, which can be regenerated by a Peterson-type elimination (equation I). 

This synthesis was accomplished using selenoaldehyde 5R. Again, cyclocondensation with 6 gave ent 7. Using the same steps as were used for the NeuSAc synthesis, ent 7 was converted to ent 13. From there the steps to the naturally occurring (7R) antipode of KDO merely involved retracing steps followed in the synthesis of the racemate. 

Thioaldehydes stabilized by mesomeric effects include I,11 2,12 and 3.13 Corresponding selenoaldehydes, e.g., 4,11 have also been described. 

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... 

It was later found that some aldehyde complexes of rhenium, [Re(NO) (PPh3) 0=C(Aryl)H (Tj-C5H5)]BF4, are also present in solution as a rapidly interconverting mixture of rf isomers and the if isomer.48,49 With the exception of solvent dependence, similar dependencies of the equilibrium constants on the aryl substituents and on the temperature were observed. The solvent dependence was found to be opposite, which, however, is easily understood when taking into account that the rhenium aldehyde complexes are cationic whereas the chromium and tungsten thio- and selenoaldehyde complexes are neutral. 

In addition to carbon electrophiles and protons, organomelallic electrophiles have also been employed. Thus, the addition of M(CO)s (M = Cr, Mo, W) or Mn(CO)2(TJ5-aH4R) (R = H. Me) to thio- and selenoaldehyde complexes of rhodium provides a convenient route to heterobinuclear p-t) j2-thio- and selenoaldehyde complexes (see Section 1V,B, Eq. (20)].-4 I6fi... 

When cyclic 1,3-dienes were employed, the reactions were found to be exo selective [Eq. (28)]. In contrast, the Diels-Alder reactions of free thio-and selenoaldehydes with cyclopentadiene occur with a preference for the endo isomer.235 Thus, coordination of thio- and selenoaldehydes to a penta-carbonylmetal fragment resulted in a reversal of the endo selectivity. The exo/endo product ratio of [W(CO)5 E = C(Ph)H ] was 7.3 1 (E = S) and 2.6 1 (E = Se), whereas the kinetically controlled exo/endo ratio of the cycloadducts obtained by trapping of the free heterobenzaldehyde with cyclopentadiene was reported to be 1 7236 and 1 4235 237 (E = S) and 1 2.6238 and 1 4239 (E = Se), respectively. 

Although a,/8-unsaturated thio- and selenoaldehyde complexes also seemed to behave as dienophiles in Diels-Alder reactions with cyclopentadiene, the products were too unstable for characterization. Therefore, it remains unknown which double bond (E=C or C=C) was involved in the cycloaddition.64... 

The reaction of the thio- and selenoaldehyde complexes with vinyl ethers provides metal-coordinated substituted thietanes and selenetanes.224,250-252 The cycloadditions are highly regio- and stereoselective. 

Selenoaldehydes.1 The first step in a synthesis of selenoaldehydes involves addition of dimethylphenylsilyllithium2 to aldehydes followed by trapping of the adduct with TsCl. The a-silyl tosylates are converted into a-silyl selenocyanates by reaction with KSeCN catalyzed by 18-crown-6. These products are easily purified, and are converted by fluoride ion into selenoaldehydes, which can be trapped by Diels-Alder cycloadditions (equation I). 

A selenoaldehyde stabilized by a bulky substituent dimerizes upon concentration of the solution yielding the head-to-tail dimerization product 60 in good yield (Scheme 23) <1997T12167>. 

An improved method for the preparation of selenoaldehydes from the reaction of bis(N,N-dimethylcarbamoyl-seleno)methanes with stannic chloride led to formation of a 2H-selenin by in situ trapping with 2,3-dimethyl-l,3-butadiene (cf. Equation 69) <2006HAC125>. 

Selenoketones.2 The synthesis of selenoaldehydes via selenocyanates (13,286) has been extended to a synthesis of selenoketones. Electron-deficient selenoketones are simply prepared by reaction of halides with KSeCN followed by base-induced elimination of HCN (equation I). However, this route fails in the case of precursors... 

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... 

Wittig-type olefinations can also be performed using selenoaldehydes. Phosphorus ylides initially attack the carbon atom of the selenocarbonyl functionality.405 Aromatic selenoketones undergo reductive dimerization with organo-lithium reagents probably via an electron transfer mechanism.406 Also the addition of organolithium reagents takes... 

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... 

Sakurai et al. were successful in reacting even tetramethylsilene (Me2Si=SiMe2) with benzene in a photochemical [2 + 4] cycloaddition at 10 K in an argon matrix [472]. With mentioning the suitability of iminoboranes [473] and selenoaldehydes [474] to serve as dienophiles in hetero Diels-Alder reactions, this enumeration of exemplary, less usual hetero Diels-Alder reactions shall be completed. 

An efficient 1,3-dipolar cycloaddition has been developed to prepare 2,3-dihydro-[l,3,4]selenadiazoles 233 <07TL2302>. The reaction involves selenoaldehyde-anthracene adduct 230 and hydrozonyl chloride 231 in refluxing toluene. The [2+3] cycloaddition of reactive selenoaldehydes 229 (generated from 230 via thermal retro Diels -Alder) and nitrile imine 232 (created via dehydrochlorination of 231 with triethylamine) allows the formation of dihydro-[l,3,4]selenadiazole 233 as a single regiomer. 

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