Selenophens

Selenophens.— The preparation of selenophen by the action of sodium hydrogen selenide on bis(trimethylsilyl)butadiyne, Me3SiC=C—C=CSiMe3, has been described. The reaction of dimethyl acetylenedicarboxylate with the potassium salt PhC=CSe yields the ester (270), together with a little of the diselenole (271). Treatment of dibenzylideneacetone with selenium tetrabromide gives compound (272), which is converted into the benzoselenophen (273 R = COCH=CHPh) in the presence of pyridine.The ester (273 R = C02Et) has been obtained by cyclization of ethyl o-methylselenocinnamate (274) by means of bromine and pyridine. 

The action of ethyl-lithium on 2,3,5-tribromoselenophen results in cleavage and condensation to yield compound (275). Triplet irradiation of a mixture of 3-acetoxybenzo[f ]selenophen and tran5-l,2-dichloroethylene gave the adducts (276) and (277).  

Marechal, L. Christiaens, M. Renson, P. Jacquignon, and A. Croisy, Bull. Soc. chim. France, 1977, 157. 

Tran Quang Minh, L. Christiaens, P. Grandclaudon, and A. Lablache-Combier, Tetrahedron, 1977, 33, 2225. 

Monocyclic Selenophens.—Some comparative work on selenophens and thiophens has been mentioned previously in this Report. Further applications of the Fiesselmann reaction have appeared. 2-t-Butyl- and 2-phenyl-selenophen were obtained using this route. Addition of hydrogen selenide to diacetylenic ketones led to (273).  

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.  

Condensed Selenophens.—The reaction of phenylacetylenesulphonamide with selenium dioxide and hydrogen bromide gave 3-bromo-2-benzo[i ]selenophen-sulphonamide. Heating of (165) with selenium gave (201). The reaction of 2,3-dimethylselenophen-5-thiol with methyl y-chloroacetoacetate, followed by cyclization with PPA and hydrolysis, gave 4,5-dimethyl-3-selenolo[2,3- ]- 

Monocyclic Selenophens.—A review article, covering research in selenophen chemistry carried out during the 1960 s, mainly at Moscow State University, has recently been published.  

The dipole moments of some simple halogeno-, cyano-, and formyl-selenophens have been measured and the conformations of these compounds have been discussed. Through a study of the n.m.r. spectrum of selenophen in lyotropic mesophase, the ratios of the interproton distances were calculated from the direct couplings and found to be in good agreement with the corresponding values calculated from microwave data.  

spectra of a number of mono- and di-substituted selenophens have been analysed and the chemical shifts discussed in additivity terms. An A%ray analysis of racemic 4,4 -dicarboxy-2,2, 5,5 -tetramethyl-3-3, -biselenienyl has been carried out. Dipole moments and u.v. spectra of chalcone analogues containing a selenophen ring have been studied. 

SindelaF, J. MetySovd, and M. Protiva, Coll. Czech. Chem. Comm., 1971, 36, 3404. 

Although some 3-selenienyl-lithium derivatives ring-open even at — 70 they are still very useful for the synthesis of selenophen 

Monocyclic Selenophens.—Ring-closures. The Fiesselmann reaction has been applied to the synthesis of selenophens. The reaction of (500) with sodium selenide and ethyl bromoacetate thus gave (501). ° Also the Gewald reaction is successful cyclohexanone reacts with ethyl cyanoacetate and selenium in the presence of base to give (502). 2,5-Diarylselenophens are obtained, together with small amounts of the 2,4-isomer, on heating 4-aryl-1,2,3-selenadiazoles. Upon pyrolysis of (503) at 250 C the selenophen derivative (504) is formed.  

Gronowitz, A. Konar, and V. P. Litvinov, Izv. Akad. Nauk SSSR, Ser. Khim., 1981, 1363. 

Substituent effects in 2-substituted selenolo[3,2-Z ]selenophens have been studied by H, and Se n.m.r. spectroscopy good linear correlations between these shifts and those of thieno[3,2- ]thiophens were observed. The H, and Se n.m.r spectra of some selenolo[3,2-c] thiophens have also been studied. CNDO/2 calculations have been carried out on selenophen and on [2,3-b] [3,2-Z ] and [3,4-b] -fused selenophens.  

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In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

Fig. 4. Simple organic ring compounds containing selenium selenophene [288-05-1] (1), selenolo[2,3- ]selenopliene [250-85-1] (2), seleno[3,2- ]selenopliene...

With its sextet of 7T electrons, thiophene possesses the typical aromatic character of benzene and other similarly related heterocycles. Decreasing orders of aromaticity have been suggested to reflect the strength of this aromatic character benzene > thiophene > pyrrole > furan (9) and benzene > thiophene > selenophene > teUurophene > fuian (10). 

The much less sophisticated PPP approximation has been shown to well reproduce the electronic spectral features not only of the monocyclic furan, pyrrole, thiophene, selenophene and tellurophene but also many of the benzo fused derivatives as well (79MI30101, 68JPC3975, 68MI30100). 

Pyrrole Furan Thiophene Selenophene Tellurophene Cyclopentadiene... 

The direct /c-3,h-3 coupling constants decrease regularly along the series O > NH > S > Se > Te, whereas the /c-z.h-z values show a peculiar behavior with the value for selenophene... 

In summary, most of the presently available criteria point to an order of decreasing aromaticity of benzene > thiophene > selenophene pyrrole > tellurophene > furan. 

Substituent Pyrrole Percent of syn form (51) Furan Thiophene Selenophene Tellurophene Phase... 

Mercapto derivatives of furan, thiophene, selenophene (77ACS(B)198) and pyrrole (72AJC985) all exist predominantly in the thiol form. 2-Mercaptobenzothiophene is also a thiol (70JCS(C)243i) whereas 2-mercaptoindole is mainly indoline-2-thione (89) (69CPB550). The finely balanced nature of this system is indicated by the fact that a 3-aryl, but not a 3-alkyl, substituent will stabilize the 2-thiol form, whereas for 3-aryl-fV-methyl derivatives the 2-thione tautomer is preferred (71CC836). 

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