Selenocyanates synthesis

Olefin synthesis from the 5yn-oxidative elimination of o-nitrophenyl selenides, which may be prepared using o-nitrophenyl selenocyanate and BU3P, among other methods. 

The absence of any derivatives of selenophene or tellurophene from the chemical catalogues makes their laboratory synthesis an unavoidable point of departure for any study of their chemistry or other properties. As the range of potential organic selenium or tellurium precursors is also extremely restricted, there has been particular emphasis on routes starting from simple inorganic species such as the elements themselves, the dioxides, the halides, the dihydrides and, in the case of selenium, the selenocyanate anion. 

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). 

I - Alkyny lphenyl selenides. Benzeneselenenyl bromide and silver nitrite (1 equiv.) rcncl to form a reagent (presumably C6H5SeN02) that converts 1-alkynes into 1-nlkynyl phenyl selenides (equation I).1 The same reaction has been effected in generally lower yields with phenyl selenocyanate catalyzed by Cu(I).2 The products tire useful intermediates for regio- and stereoselective synthesis of vinyl selenides. 

A. Krief, A.-M. Laval, Ortho-Nitrophenyl Selenocyanate, a Valuable Reagent in Organic Synthesis Application to One of the Most Powerful Routes to Terminal Olefins from Primary-Alcohols (The Grieco-Sharpless Olefination Reaction) and to the Regioselective Isomerization of Allyl Alcohols), ... 

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... 

A. Krief and A.-M. Laval, u-Nitrophenyl selenocyanate, a valuable reagent in organic synthesis Application to one of the most powerful routes to terminal olefins from primary-alcohols (the Grieco-Sharpless olefination reaction) and to the regioselective isomerization of allyl alcohols, Bull. Soc. Chim. Fr. 1997,13, 869-874. 

In a number of reports, Zingaro and co-workers35-42 described several transformations in seleno sugars that involved the application of such selenium reagents as selenourea and potassium selenocyanate in this synthesis of diselenides 43, 46, and 49. The first approach involves the SN2 displace-... 

Reactions leading to the formation of alkenic or a,/3-unsaturated bonds are very important in synthesis. This type of functionality is synthetically versatile, and of wide utility in the carbohydrate field. The use of sulfur reagents for conversion of sugar epoxides on vicinal disulfonates into al-kenes is well established97a 97b, and subsequent work with selenium reagents provided essentially comparable results. Thus, treatment of methyl 2,3-anhydro-4,6-0-benzylidene-a-D-mannopyranoside (219) with potassium selenocyanate in aqueous 2-methoxymethanol afforded methyl 4,6-0-benzylidene-2,3-dideoxy-a-D-ery/firo-hex-2-enopyranoside (220).27 Similarly, treatment of 5,6-anhydro-l,2-0-isopropylidene-a-D-glucofuranose (221) with potassium selenocyanate in methanol at room temperature... 

Another useful system is o-nitrophenyl selenocyanate and tributylphos-phine (Bu3P), developed by Grieco and co-workers,104-106 which has been employed in the synthesis of intermediates leading to the ionophone antibiotic calcimycin.103... 

Several types of electrophilic selenium species have been immobilized on solid supports, for more convenient application to organic synthesis. These include selenenyl halides, " selenocyanates, and selenosulfonates. ... 

Synthesis from isoselenocyanate Synthesis from potassium selenocyanate... 

Selenocyanates (RSeCN), which are employed for the synthesis of selenides and selenoesters, are usually derived from selenenyl halides and cyanide ion (or silyl cyanate) [38 k, 40]. Alkylation of potassium selenolate (KSeCN) with alkyl halides is an alternative means of preparation of aliphatic selenocyanates (Scheme 15.13)... 

Methods of synthesis of seleninic acids are based on oxidation of diselenides (or selenocyanates) with HNOj [38k, 42[ or H2O2 [23a, 42d, 43[. Seleninic anhydrides are prepared by dehydration of seleninic acids [42 e, 44[ or by oxidation of diselenides with ozone (Scheme 15.14) [44, 45]. 

Block, E., Birringer, M., Jiang, W., Nakahodo, T., Thompson, H. J., et al. Allium chemistry synthesis, natural occurence, biological activity, and chemistry of Se-alk(en)ylselenocysteines and their y-glutamyl derivatives and oxidation products. J Agric Food Chem 2001, 49, 458 470. Das, A., Desai, D., Pittman, B., Amin, S., El-Bayoumy, K. Comparison of the chemopreventive efficacies of l,4-phenylenebis(methylene)selenocyanate and selenium-enriched yeast on 4-(methylnitrosamino)-l-(3-pyridyl)-l-butanone induced lung tumorigenesis in A/J mouse. Nutr Cancer 2003, 46, 179-185. 

Olefin synthesis. The preparation of olefins by decomposition of alkyl phenyl selenoxides (Diphenyl diselenide, 5, 272-276) is very useful, except in the case of primary alkyl phenyl selenoxides, which usually give low yields of terminal olefins on decomposition. However, the presence of electron-withdrawing substituents on the benzene ring increases both the rate of elimination and the yield of olefins. In instances where use of diphenyl diselenide results in low yields. Sharpless and Young recommend use of o-nitrophenyl selenocyanate (1) or 4,4 -dichlorodiphenyl diselenide, both of which are converted into the corresponding ArSe Na" reagents on reduction with sodium borohydride in ethanol. 

Paulmier and coworkers, starting from ortho-nltro-substituted thienyl- and selenienylthiocyanates and selenocyanates, have worked out convenient methods for the synthesis of thienot2,3-dlthiazole and selenolot2,3-d]thiazoles. From the same starting materials, thienot2,3-elthiazines were also prepared. The same group also... 

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