Selenophenes ring-opening

The synthesis and chemistry of an 7] -selenophene osmium complex 50 has been studied <19990M1559>. Protonation and electrophilic substitution with acetaldehyde diethyl acetal occurred at C-2. Methylation of complex 50 with methyl triflate gave 51 which upon treatment with tetrabutylammonium borohydride (TBAB) led to the selenophene ring-opened complex 52 (Scheme 5). 

By comparison with the corresponding thiophenes, the tendency to ring opening is greater in the selenophene series. One of the first examples was the unexpected observation that 2,2, 5,5 -tetramethyl-3,3 -biselenienyl was... 

Substitutional ring openings seem to be more unusual (Eqs. 22-24, Scheme 5). The reactions of 2,5-dimethoxyselenophene with butyl- and phenyllithium have been examined.85 In view of the formation of dibutyl-and diphenylselenide, respectively, together with some dienes, a logical reaction sequence could be formulated in which the first step is an attack on the selenium atom by the lithium reagent. Nonfused selenophenes are also cleaved in a substitution-type reaction. 

Experimental precautions are required for NMR measurements in this group because the adducts rapidly undergo ring opening. The NMR spectra of the selenophene adducts in solution are similar to those of the corresponding thiophene adducts. The H-NMR data for adducts 142-145, as obtained from dinitro and cyanonitroselenophene derivatives, are reported in Table XXI. It is of interest that, unlike 2-cyano-4-nitrothiophene, its selenophene analog is converted to the anionic a-adduct by nucleophilic attachment at the unsubstituted C-5 position without any competition from a reaction at the CN side chain.169... 

Five-membered heterocycles also capable to ring-opening and recyclisation processes by nucleophile action. Well known examples include the interconversions of pyrrole, furan, thiophene and selenophene rings e.g. furan and ammonia over a suitable catalyst can give pyrrole (82KGS418), (80DOK1144). 

Selenienyllithium, for the most part, undergoes a completely similar series of transformations (75CS(7)lll) because of the great tendency of 3-selenienyllithium to ring open, it is necessary, however, to work at low temperatures (-70 °C). 3-Selenienyllithium can be converted into 3-formylselenophene by reaction with iV,N-dimethylformamide, and 3-acetylselenophene by reaction with N,N-dimethylacetamide <75CS(7)lll). These latter conversions illustrate the utility of 3-selenienyllithium since it will be recalled that acylation of selenophene occurs preferentially in the 2-position. 

The 2,3-dihydrobenzo[6]selenophene (113) yields the oxide (114) on treatment with ozone. The oxide may be ring opened by treatment with sodium hydride and the product of the ring opening can be alkylated by reaction with benzyl bromide. Thermal rearrangement of the oxide yields a 15 85 mixture of compounds (115) and (116) (Scheme 15) (76JOC2503). 

Selenophene 1,1-dioxides have been found to be more thermally labile than the corresponding thiophene 1,1-dioxides. While dimerization is often observed in the thermal decomposition of the latter, the neat thermolysis of selenophene 1,1-dioxides leads primarily to ring-opened products. For example, thermolysis of 2,3,4,5-tetraphenyl-selenophene 1,1-dioxide 5 gave a variety of a ring-opened products (Equation 1) <1998H(48)61>, whereas thermolysis of compound 5 in toluene leads to the formation of 2,3,4,5-tetraphenylfuran. 

Se-Heterocycles as precursors of selenoaldehydes 88YGK1149. Ring-opening of heteroaromatic anions derived from selenophene and tellurophene 87AHC(41)41. 

Selenium-77 NMR is more useful than the H technique for studying isoselenazoles because the selenium signals can be spread over 1000 ppm. Being attached to a nitrogen, the selenium of an isoselenazole is further downfield than in a selenophene. Much can be learned of the selenium atom of a benzisoselenazol-3-one for example, the ring-opened diselenide of (3b) resonates at 453.6 ppm. Upon cyclization to (3b) and oxidation to (11a) it shifts to 888 ppm and 1142 ppm, respectively (Table 4). Making use of this, a H and Se NMR study of the mode of action of ebselen was reported and a mechanism for the in vivo catalytic reduction of hydroperoxides was proposed <87BSB757>. 

Determination of the relative kinetic acidities of the 2- and 3-protons in thiophene (H/D exchange experiments) shows the 2-proton to be 500 times more acidic than the 3-proton [127]. However, when a sufficient amount of HMPT is added to a solution of 2-lithiothiophene, ring opening to LiC=C—CH=CH—SLi occurs, which means that under these conditions exchange between Li and the 3-proton can take place [128]. Selenophene [129] and tellurophene [130] have been lithiated with satisfactory results. We have the impression however, that a more careful control of the reaction conditions is required, and bases with a more moderate reactivity, such as EtLi LiBr in Et20, seem to give better results, especially in the case of tellurophene [9]. 

Although C-S bond cleavage reactions between thiophenes and osmium clusters have not been observed, selenophene and tellurophene undergo ring opening reactions with [Os3(CO)io(NCMe)2] to give complexes 28a and 28b (Scheme T). " It is likely that the sulfur-extrusion reactions of iron and ruthenium carbonyl clusters with thiophenes proceed via ring-opened intermediates of this type. 

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