Selenoaldehydes, generation, reactions

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. 

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. 

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 selenoaldehydes and selenoketones generated in situ by a wide range of methods can generally be trapped with dienes [30, 31, 56, 57, 60-63]. Cyclo-pentadiene has often been used, and the hetero-Diels-Alder reaction proceeds to give endo-adducts as major products. The regioselectivity of the reaction... 

The addition of alcohols to N-alkylidene selenoamides generated in situ leads to N-alkoxyalkyl selenoamides [84]. The substitution reaction of alkoxy groups of selenoesters [RC(Se)OR ] with aluminum thiolates or selenolates proceeds smoothly to give the corresponding esters [RC(Se)SR, RC(Se)SeR ] [7a, 85], whereas the addition of thiols to selenoaldehydes takes place at the selenium atom to afford selenenesulfides (RCH2SeSR ) [86]. 

The reaction of aromatic and aliphatic aldehydes with (Me2Al)2Se in the presence of anthracene gives the [4+2] cycloadducts 83. Thermally generated selenoaldehydes from 83 react with 5-ethoxyoxazoles to afford 3-selenazolines 84 as a mixture of diastereomers (Scheme 18) <2005PS1045>. 

Cycloaddition reactions of selenoaldehydes and their generation via retro Die-ls-Alder reaction of substituted 2-selenabicyclo[2.2.1]heptene derivatives, cycloadducts of selenoaldehydes with anthracene as a stable sourcese of selenoaldehydes 03YGK661. 

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). 

Until relatively recently 3f/-l,2-diselenines were not well represented in the chemical literature. Dimerization of in situ generated a,/3-unsaturated selenals (selenoaldehydes) 92 or selones (selenoketones) 94 affords 3,4-dihydro-l,2-diselenins 93 and 95 (Schemes 21 and 22) <1992TL3515, 1999JOC1565>. These [4-1-2] cyclization reactions are... 

The reaction of an aldehyde with sodium hydrogen selenide and an amine hydrochloride generates a selenoaldehyde, which is reduced in situ to a symmetric diselenide by sodium borohydride. ... 

A mechanistic proposal includes addition of MesSiSeLi to aldehydes, followed by elimination of MesSiOLi to generate selenoaldehydes, which undergo Diels-Alder reaction with dienes. Aldehydes bearing a conjugate diene unit provide intramolecular Diels-Alder adducts (eq 7). ... 

Addition of PhMc2SiLi to aldehydes and subsequent 0-sulfo-nylation of the generated alkoxides with /7-toluenesulfonyl chloride affords a-silyl tosylates in good yields. Conversion of these intermediates into a-silyl selenocyanates followed by fluoride-induced cyanide elimination reaction permitted the synthesis and study of reactivity of previously unknown selenoaldehydes (eq A) ... 

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