Selenides, from carboxylic acids

Example 2, in addition to oxyselenide formation, carbo- and heteroseleno cycliza-tion, A -PSP can be used to generate selenides from alcohols and selenol esters from carboxylic acids, respectively, in the presence of a stoichiometric amount of n-Bu,P. ... 

Selenocyanates produce selenols or diselenides upon either reduction (e g. with sodium borohydride) or hydrolysis (see Scheme 1). They undergo displacement of the cyanide ion by various nucleophiles and add to alkenes in a maimer similar to selenenyl halides (see equation 14), except that catalysis with Lewis acids is required in the case of unactivated alkenes. The selenocyanates are also popular reagents for the preparation of selenides from alcohols, and (8) from carboxylic acids, as indicated in Scheme 3. 

Isonitriles have been prepared from carboxylic acids via selenocarbamates. When a leaving group is present at the /3-position of the selenium atom, the homolysis of the C-Se bond is followed by expulsion of the -substituent. Glycal formation from azi-dosugar selenides on treatment with BuaSnH occurs in excellent yields. 

Organolithiums. Allylic and benzylic alcohols undergo deoxygenative lithiation by treatment of their lithium alkoxides or phenyldimethylsilyl ethers with LDTBB. Alkyl phenyl selenides are also cleaved to give organolithium species that can react with aldehydes and allyl bromide. Some special alkyllithiums have been prepared from (2-pyridylthio)alkanes, which are available from carboxylic acids. ... 

Diphenyl selenide di-p-carboxylic acid, Se(C6H4.C02H)2, is the principal constituent of the mother-liquors from the preceding compound its melting-point varies from 312° 313° C. to 315° 316° C. 

Selenolate 77, prepared from diselenides with sodium borohydride, is also not reactive enough for reactions with esters or lactones. Only under higher reaction temperatures, lactones 90 can be ring opened with selenolates to the corresponding carboxylic acids 91.173,174 The selenium moiety can then be used for further manipulations. Using similar reaction conditions, esters 92 can be converted to the corresponding carboxylic acids 93 and selenides 94 (Scheme 21).149... 

Acyl radicals are very useful synthetic intermediates. Their preparation is not simple since the corresponding halides are highly electrophilic and cannot be used as radical precursors. Organocobalt compounds were proposed as suitable source of acyl radicals [44]. However, the use of acyl selenides proved to be more general [45, 46]. These radical precursors can be efficiently prepared from the corresponding carboxylic acids and esters [47]. Acyl phenyl selenides should be preferred, when possible, relative to acyl methyl selenides due to the consumption of two equivalents of tin hydride with this last system (Scheme 1) [4]. Acyl selenides have found many applications in tandem radical additions to alkenes. Examples of intermole-cular [Eq. (18)] [48,49] and intramolecular reactions [Eq. (19)] [50a] are reported. The enoyl selenide 68 give the unsaturated acyl radicals 69. This intermediate... 

The decarboxylation of carboxylic acid via thiohydroxamate derivatives (often called Barton esters ) is an efficient procedure for the generation of radicals. When this reaction is performed in the presence of diphenyl diselenide, the corresponding selenides are obtained in excellent yield [Eq. (42)] [104]. For example, this reaction has been used for the preparation of AT,Se-acetals from a-aminoacids [105,106]. 

Ebselen derivatives (3) can be prepared by any of three routes from ortAo-selenobenzoic acid derivatives (Scheme 22) <90MI 307-02 91MI 307-01). Introduction of diselenide by conversion of the ortho-amino carboxylic acid to the diazo derivative followed by addition of sodium selenide results in a Sandmeyer reaction. The diselenide is converted to the selenenyl chloride (38) and ring closure... 

Oxidations. Alkene formation from alkyl aryl selenides via oxidation is cleaner when the aryl group is ort/io-substituted (vs. para-substitution), for example, with a nitro group. In the presence of a phase-transfer catalyst, oxidation of aldehydes to carboxylic acids with H2O2 is successfully carried out without an organic solvent, halide, or metal ion. 

Vinyl selenides (3), prepared from selenoketals (2), are cleaved by hydrogen peroxide to carboxylic acids or ketones (Scheme 3), thus effecting an overall degradation of ketones to carboxylic acids with the loss of the carbonyl group. In a similar vein, alkaline hydrogen peroxide in aqueous methanol has been used for the scission of a-ketols R C0C(0H)R R, resulting in the formation of acids... 

Sodium hydrogen selenide Carboxylic acid selenoamides from iminochlorides... 

O-acetyl-jS-D-glycopyranosides from 0-, m- and p-02T C H 0 a-nitro esters from methyl nitroacetate, and alkyl phenyl selenides from PhSe". Similarly arenesulfonyl fluorides or isothiocyanates can be produced from arenesulfonyl chlorides and F" or SCN", and arene-carboxylic acid cyanides and isothiocyanates from arenecarboxylic acid chlorides and CN" or SCN ... 

Monocyclic Selenophens.—Selenophen has been prepared from butadiene and selenium dioxide. Selenophen-3-malonic esters have been prepared by condensation of l,4-dichlorobut-3-en-2-one with malonic esters followed by reaction with sodium selenide and cyclization. 2-(2-Selenienyl)-3,4-dicarbeth-oxyfuran has been prepared analogously to the thiophen derivative.The transient absorption spectra in the flash photolysis of selenophen have been studied. "" Butyl-lithium and phenyl-lithium attack 2,5-dimethoxyselenophen at the selenium atom, giving dibutyl selenide and diphenyl selenide, respectively, together with some dienes. " Quantitative studies of the acid- and base-catalysed hydrolyses of ethyl selenophen-2-carboxylate, of the reactions of selenophen-2-sulphonyl chloride and of 2-(chloromethyI)selenophen with aniline, and of selenophen-2-carboxaldehyde with aniline and with a phosphorus ylide have been carried out. The results were rationalized by correlation analyses with polar or electrophilic constants of heterocycles, depending on the electron demand of the side-chain. ""... 

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