Seleno Acids

Various seleno acids (9.663) and their salts are known. They are generally less easy to isolate than their thio analogues. Although there is limited experimental data, acid strengths appear to lie in the order  

R2PSeSeH R2PSSeH R2PSSH Representative methods of preparation are 

Et2P(Se)Cl + 2NaSeH Et2P(Se)SeNa + H2Se + NaCl 

Tautomeric equilibria (9.670) presumably exist with acids of types (9.640b,c), and with phospho-roselenoic acids such as (9.671). 

In the case of phosphoroselenothioic acids three forms are possible in principle. 

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The situation is clearly different for a-seleno ketones (Schemes 34-36), ° " " a-seleno esters (Schemes 10 and 37),56.74,75.ii2.ii3 selenolactones (Scheme a-seleno acids (Scheme 39)56,ns gjjjj... 

Anhydro-4-hydroxy-2,3,5-triphenyl-l,3-selenazolium hydroxide (65) is prepared by a multistep reaction selenobenzanilide (63) reacts with a -bromophenylacetic acid to give the a-seleno acid (64) this acid readily cyplizes to the mesoionic selenazole (65), which upon reaction with DMAD affords an unstable adduct which by extrusion of selenium gives the pyridone diester (66 Scheme 23) (75CC617). 

Selenocarboxylate salts are not generally available due to the instability of the corresponding seleno-acids. An indiret method to... 

The early experimental efforts to separate enantiomeric forms of simple derivatives of phosphono-thioic or -selenoic acids concentrated on the fractional crystallization of quinine, brucine or strychnine salts of the acid monoesters 69 (Z = S or Se) the early work in this area has been summarized It was not long before this tedious procedure, which did not always provide the best results, was superseded by a modification devised by Boter and Platenberg which utlized enantiomers of 1-phenylethylamine. Essentially, (+)-l-phenylethylamine (0.5 mol) is added to the acid monoester in ether and the (+),(+)-salt is allowed to crystallize out, although this does not generally occur quantitatively. The excess... 

Detailed studies on the use of H2O2 in the phosphonothioic(and selenoic) and phos-phinothioic (and selenoic) acid series (in addition to experiments with derivatives of phos-phorothioic and phosphoroselenoic acids) have been carried out, also by Polish workers. It appears that the stereochemical course of oxidative desulphurization (or deselenation) is the same for all three series of esters (retention of configuration) but differs from the reaction for phosphine sulphides (net inversion), whilst the course of the reaction for phosphine... 

Quaternization of selenides with methanesulphonyl fluoride or methyl iodide-silver tetrafluoroborate, followed by elimination with potassium t-butoxide ki THF or DMSO at room temperature, offers an alternative procedure to selenox-ide elimination. An advantage appears to be the formation of the volatile side product dimethyl selenide, rather than the selenenic, seleninic, or selenoic acids... 

The Sn2 cleavage of esters and lactones is achieved with sodium phenyl selenide generated by the reaction between sodium hydride and benzene selenol, instead of the more usual sodium borohydride reduction of diphenyldiselenide. When generated under the former conditions the phenyl selenide anion is a potent nucleophile and good yields of seleno-acids are obtained from a wide range of esters and lactones (Scheme 3). ... 

Replacement of Oxygen by Other Chalcogens. Acids derived from oxoacids by replacement of oxygen by sulfur are called thioacids, and the number of replacements are given by prefixes di-, tri-, and so on. The affixes seleno- and telluro- are used analogously. 

The presence of many nonprotein amino acids has been reported in various living metaboUtes, such as in antibiotics, some other microbial products, and in nonproteinaceous substances of animals and plants (7). Plant amino acids (8) and seleno amino acids (9) have been reviewed. 

Selenocysteine was identified in 1976 (57) in a protein produced by Clostridium stricklandii, and it is thought to be the form in which selenium is incorporated, stoichiometricaHy, into proteins. Studies with rats show that over 80% of the dietary selenium given them is incorporated into proteins, thus selenocysteine takes on metaboHc importance. Selenoproteins having known enzymatic activities contain selenocysteine at the active sites. Two other forms of metabohc selenium are recognized methylated selenium compounds are synthesized for excretion, and selenium is incorporated into some transfer ribonucleic acids (tRNAs) in cultured cells (58). Some of the more important seleno-compounds are Hsted in Table 4. Examples of simple ring compounds are shown in Eigure 4. 

Selen-verbindung, /. selenium compound, -wasserstoff, m, hydrogen selenide. -was-serstoffsaure, /. hydroselenic acid, -wismut-glanz, m. guanajuatite. -zelle, /. selenium cell, -zyanid, n. selenium cyanide seleno cyanate. -zyankalium, n. potassium selenocyanate. 

Carboxyethyl 1 -seleno-p-D-xylopyranoside or 3-(P-D-xylopyranosylseleno)propanoic acid... 

Fujiwara S-i, Kambe N (2005) Thio-, Seleno-, and Telluro-Carboxylic Acid Esters. 251 87-MO... 

Kato S, Niyomura O (2005) Group 1-17 Element (Except Carbon) Derivatives of Thio-, Seleno- and Telluro-Carboxylic Acids. 251 19-85 Kato S, see Niyomura O (2005) 251 1-12... 

Thus it has so far proved possible to isolate stable derivatives of monomeric metaphosphoric acid and of metathio- and metaselenophosphoric acid, which, understandably, generally bear tert-butyl and/or trimethylsilyl substituents u. Specifically, we know aminobisiminophosphoranes (3, Z = NR2, X = Y = NR)2,3,4), aminoiminothio (or seleno)phosphoranes (5, Z = NR2, X = NR, Y = S or Se)5), and aminoiminomethylenephosphoranes (1, R = NR2, X = NR)6>. Conspicuously, no stable phosphorus(V) three-coordinate compounds have been synthesized with oxygen as divalent ligand. 

The results of studies of the acid-catalyzed hydration of oxygen-, sulfur-, seleno-and nitrogen-substituted alkenes and the relevance of this work to partitioning of the corresponding carbocation intermediates (Chart 1) between deprotonation and nucleophile addition was reviewed in 1986.70. We present here a brief summary of this earlier review, along with additional discussion of recent literature. 

The naphthalene-derivatized seleno-ether macrocycles dinaphtho-[16]aneSe4> naphtho-[8]aneSe2 and naphtho-[12]aneSe3 have also been prepared. Hydrolysis of dinaphtho-[16]aneSe4 with sulfuric acid leads to formation of the ring contracted species naphtho-[8]aneSe2.103,104... 

Use of diamines in the preparation of chalcogenofcamidophosphonic acids gives heterocyclic products for example, reaction of l,l -diaminoferrocene with RPC12 (R = tBu, Ph) and elemental sulfur or selenium gives the thio/ seleno-feamidophosphonic acid 33 (Equation 48).61... 

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