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We have explored the reactions of 2-(3-oxo-1-aryl-3-phenylpropyl)cyclohexanone (1–3) with hydrogen selenide in situ in conditions of acid catalysis, and synthesized new 2-aryl-4-phenyl-5,6,7,8-tetrahydro-4H-selenochromenes (4–6).

Oxygenated and reduced selenium and tellurium compounds, particularly thoese of practical importance have been extensively investigated especially during the last few decades. Selenium(IV) oxide, selenoxides, selenones, seleninic and peroxyseleninic acids, selenenamides and seleninamides are involved in oxyfunctionalization of unsaturated hydrocarbons, their derivatives, alkylarenes, sulfides and amines, carbon-oxygen and carbon-carbon bond cleavage, ring closure and ring transformations, syn-elimination, sigmatropic [2,3]-rearrangement and other, chemo- regio- and stereocontrolled reactions. The chemistry of oxygenated tellurium compounds has been less extensively explored than their selenium counterparts;, nevertheless some of them have been successfully used for oxyfunctionalization of different groups of organic compounds while telluroxide elimination has gained more attention in the last years becoming a well-established method for the synthesis of olefins. Hydrogen selenide and telluride, alkali selenides and tellurides, selenoles and telluroles are the most common reduced derivative of these elements. Their properties are expressed in many reactions such as reduction and deoxygenation of different groups of organic compounds alkenes, alkynes, ethers, carbonyl compounds, esters, lactones, oxiranes, nitrogen, sulfur and halogen compounds, as well as dealkylation of tertiary amines. The method for preparation of the different oxygenated and reduced selenium and tellurium compounds are described, their reactions are reviewed, their usefulness in synthesis is indicated and more representative mechanisms are discussed. Keywords: organoselenium compounds; organotellurium compounds; oxidation; reduction; selenides; selenium dioxide; tellurides; tellurium dioxide

α-Diketones can be partially reduced to the corresponding α-hydroxyketones by sodium hydrogen selenide in the presence of acetic acid, under mild conditions and in high yields.

A convenient method for the synthesis of 6-selenopurine ribosides by nucleophilic displacement of chloride by sodium hydrogen selenide from the corresponding 6-chloropurine ribosides is described. This method provides a useful improvement in yield while significantly simplifying the literature procedures.

Selenium reacts with carbon monoxide and amines under mild conditions to give ammonium carbamoselenoates, which were then converted to corresponding ureas by aminolysis upon oxidation with molecular oxygen. Under the controlled conditions, the reaction proceeds with a catalytic amount of selenium, and its turnover number reached ca. 104. Successful applications of this SeKO system include not only the synthetic reactions of a variety of carbamates, carbonates, and sulfur or selenium containing compounds but also metallurgical refinement of selenium and several important industrial processes as exemplified by isocyanate synthesis, water-gas shift reaction, and separation of carbon monoxide. The basis of these reactions is the facile in situ generation of carbonyl selenide, which possesses a potent reactivity toward a variety of nucleophiles. THE DISCOVERY OF SELENIUM CATALYZED CARBONYLATION WITH CARBON M O N O X I D E Water and carbon dioxide are very stable compounds, however, if (one of) the oxygen atom(s) in these molecules is replaced by one of the homologous chalcogen elements, the stabilities of these compounds decrease and their reactivities increase in the same order (Table 1). Hydrogen selenide is stable at ambient temperature like hydrogen sulfide but it is easily oxidized with molecular oxygen. Carbonyl selenide and hydrogen telluride are not stable at ambient temperature and gradually decompose to give elemental selenium or tellurium. Carbonyl telluride has not yet been identified. TABLE 1. Comparison of physical constants of chalcogen elements. X 0 S Se Te electronegativity (Pauling) 3.5 2.5 2.4 2.1 bond energy (Kcal/mol), H-X of H2X 119 91 73 64 bond length (A), H-X 0.96 1.35 1.46 1.69 dissociation constant (pKa) of H2X 14.0 7.0 3.7 2.6 bond energy (Kcal/mol), C-X of Et2X 86.5 69.7 59.9 (c60) abond energy (Kcallmol, calculation value), CH2=X 93 56 bond length (A), C-X 1.42 1.82 1.96 2.12 Carbonyl selenide is an especially attractive compound because it involves a Se-C double bond consisted of a weak 4pa-2px interaction, which facilitates the attack of nucleophiles at the carbon atom of carbonyl selenide (eq. 1). This prompted us to examine the reaction of carbonyl selenide with nucleophiles like

Nitriles react with sodium hydrogen selenide, pyridine and hydrochloric acid in ethanol to give primary aryl and alkyl selenocarboxamides. The alkyl selenocarboxamides are converted into selenazoles by reaction with phenacyl bromide

The triplet 1,4-biradical intermediates formed in the photochemical cycloaddition reaction between 2-cyclohexenone and the alkenes cyclopentene and ethoxyethene have been trapped quantitatively with hydrogen selenide. The structures of the biradicals reveal that they are formed by bonding of both the 2-position and the 3-position of the enone to one terminus of the alkene; in the case of ethoxyethene it is exclusively the less substituted terminus of the alkene that becomes bonded to the enone in the biradical intermediates. The yields of the trapped biradicals indicate that the regiochemistry of the photocycloaddition reaction is determined by the manner in which the biradicals partition between fragmentation to give ground state starting materials and closure to give product, and not by their relative rates of formation. The results suggest that the mechanistic factors governing the photocycloaddition chemistry of cyclohexenone are similar to those controlling the photochemistry of cyclopentenones.

ABSTRAC Aryl nitriles react smoothly with sodium hydrogen selenide, itself prepared in situ from selenium and sodium borohydride, in ethanol to give the corresponding primary selenoamides in moderate to good yield.

The treatment of hexopyranosyl bromides, also activated at C6 (Br, OTs, OMs), with H2S/HCONMe2 under basic conditions gives rise to 1,6-dideoxy 1,6-epithio sugars. One such sugar has been further transformed into the synthetically useful 3,4-anhydro-1,6-dideoxy-1,6-epithio-β-D-galactose. The treatment of this epoxide with sodium azide and with cyclohexylamine is described. An analogous treatment of one doubly activated hexopyranosyl bromide with sodium hydrogen selenide has led to a novel 1,6-dideoxy 1,6-episeleno sugar which displayed interesting n.m.r. spectra. Finally, in an attempt to prepare 1,6-dideoxy 1,6-epidithio sugars, a tetraalkylammonium tetrathiomolybdate reagent was found to be the reagent of choice for converting doubly activated hexopyranosyl bromides into 1,6-dideoxy 1,6-epithio sugars.

Selenium is an essential trace element and a toxicant for animals. Selenocystine, a selenium-containing amino acid, is one of the chemical forms in which selenium exists in food. This review summarized recent studies on the toxicity mechanism of selenocystine in experimental animals. Hepatotoxicity is caused by repeated oral administration of selenocystine. Selenocystine is metabolized by reduced glutathione and/or glutathione reductase to hydrogen selenide via selenocysteine-glutathione selenenyl sulfide. The hydrogen selenide is a key intermediate in the selenium methylation metabolism of inorganic and organic selenium compounds. Accumulation of the hydrogen selenide resulting from inhibition of the selenium methylation metabolism, detoxification metabolic pathway of selenium, is found in animals following repeated administration of a toxic dose of selenocystine. The excess of the hydrogen selenide produced by inhibition of the selenium methylation metabolism contributes to the hepatotoxicity caused by selenocystine.

Both the direct and sensitized photolyses of [2-(1,2-propadienyl)-substituted alkylidene]malonates gave mainly intramolecular [2+2] cycloadducts. On the other hand, the direct irradiation of methyl (E)-4,4-dimethyl-2,5,6-heptatrienoate (3a) competitively underwent E–Z geometric isomerization and intramolecular [2+2] cycloaddition, though prolonged irradiation led to [2+2] cycloadducts as main products. For methyl (E)-4,4,7-trimethyl-2,5,6-octatrienoate (3b), the intramolecular [2+2] cycloaddition occurred under both direct and sensitized irradiation and the E–Z geometric isomerization scarcely occurred. In these allenyl(vinyl)methanes under direct irradiation, the 1,2-propadienyl-substituted esters gave a 1,2-propadienylcyclopropane or (2-ethenylidenecyclopropyl)acetate as a minor product. When 3b was irradiated in the presence of hydrogen selenide in toluene, cyclopentenes were formed. The formation of the cyclopentenes indicated that the biradical intermediate is a cyclopentane-1,3-diyl radical. By MO c...

The 4-Selenanols (2,3) and 4-selenanones (4) were obtained by the reaction of 1,4-pentadien-3-ones (1) with sodium hydrogen selenide under different conditions. The fused 1,2,3-selenadiazoles (6) and 1,2,3-thiadiazoles (7) were prepared from 4 on oxidative cyclization with SeO2 and Hurd–Mori reaction with SOCl2.

The regioselective and stereospecific intramolecular ring closure reactions of o-ethynylbenzyl tellurols 5A and o-ethynylbenzyl selenols 5B, which were readily generated by the reaction of the o-ethynylbenzyl bromides 4 with sodium hydrogen telluride (NaHTe) or sodium hydrogen selenide (NaHSe), produced the isotellurochromenes 6A and isoselenochromenes 6B together with (Z)-1-methylidene-2-telluraindans 7A and (Z)-1-methylidene-2-selenaindans 7B, respectively. The obtained isochromenes 6A and 6B were transformed into the corresponding 2-benzotelluropyrylium tetrafluoroborates 9A and 2-benzoselenopyrylium tetrafluoroborates 9B by treatment with triphenylcarbenium tetrafluoroborate (Ph3C+BF4−) in excellent yields, respectively. An X-ray structural analysis of the tert-butyl derivatives 9Ac and 9Bc is also described.

An improved practical synthetic method was developed for the preparation of selenocarboxylates of amino acids through the reaction of the corresponding activated esters with sodium hydrogen selenide in alcoholic or aqueous medium. The protected α-amino selenocarboxylates reacted readily with sulfonyl azide to form N-(α-aminoacyl)sulfonamides in high yields. The commonly used protecting groups in amino acid and peptide chemistries are well tolerated under these reaction conditions. No protecting groups are needed for the side chains of Arg, Met, Ser, Tyr, and Trp.

The reduction reaction of selenate ions in an aqueous solution by TiO 2 photocatalyst is investigated. The conduction band of TiO 2 is much more positive than the standard redox potential of the reduction reaction of selenium to selenide. However, the selenate ions are reduced to hydrogen selenide via amorphous selenium, by the TiO 2 photocatalyst. After all selenate ions and selenite ions are reduced to amorphous selenium, no chemicals in the solution can capture the electrons from the TiO 2 photocatalyst. Under this condition, photogenerated electrons accumulate in the TiO 2 particles, and they raise the conduction band of the TiO 2 All the experimental results can be explained by this mechanism.

Carbon–carbon double bond of α,β-unsaturated carbonyl compounds can be reduced selectively with hydrogen selenide, generated in situ by the reaction of elemental selenium with carbon monoxide and water in the presence of strongly basic tertiary amine.

The complexes [Pd2Cl2{µ-PPh2CH(R)PPh2}2], where R = H or Me, react with H2Se or elemental Se to yield the bridging-selenide derivatives [Pd2Cl2(µ-Se){µ-PPh2CH(R)PPh2}2]; these, on treatment with ButOOH, yield the corresponding µ-SeO (dimetallic selenoxide) species.

The title compound was prepared by the reaction of sodium hydrogen selenide with 4-chloropent-3-en-2-one. The compound appears unstable with respect to higher oligomers of the SeC (CH3)CHC(CH3)O ( seacac ) unit. X-ray crystallography reveals the unusual structure in which a central 'complexed' selenium atom is coordinated to the two selenium atoms of the seacac ligands which are in their normal cis conformations. The oxygen atoms are uncoordinated and as far removed from the coordination environment of the central selenium atom as possible. Crystals of Se( seacac )2 are orthorhombic with space group P21212, and cell dimension a 9.761(2), b 14.828(3) and c 4.6447(5)Ǻ.