Diheteropentalene

A similar protocol applied to 2,5-biscarboxymethyl-3,4-bis bromomethyl thiophene gives rise to a thiophene-tellurophene diannellated compound A, which is the first tellurium-containing diheteropentalene. It cannot be isolated since it gives a dimeric product B and the reduced compound... 

Proton nuclear magnetic resonance (NMR) data of parent A,B-diheteropentalenes have been reviewed <1984CHEC(4)1037, 1984CHEC(6)1027, 1996CHEC-II(7)1>. 

Diheteropentalene systems are very popular substrates for the investigation of cycioaddition reactions due to their diene character. 

The l,3,3,3a,8a-tetrahydro-8-oxa-2-thia- 205, l,3,3,3a,8a-tetrahydro-2,8-dioxa- 206, and 3,3a,8,8a-tetrahydro-12/-2-thia-8-aza-cyclopenta[ ]indene-2,2-dioxide 207 were prepared by Michael addition or radical cyclization from the corresponding sulfones 202-204 and brominated-dehydrobrominated (in the case of 207, after tosylation) to produce 1,5-diheteropentalene systems 208-210, respectively <1997TL5315>. 

Generally, cycloadditions represent powerful reactions for construction of heterocycles. Tandem intramolecular Diels-Alder/retro-Diels-Alder reaction sequences were applied in the syntheses of many A,B-diheteropentalenes <1996GHEC-II(7)1>. Gribble and co-workers <1998SL1061> reported new syntheses of pyrrolo[3,4-, ]indoles 426, benzo[4,5]furo[2,3-f]pyrroles 429, and benzo[4,5]thieno[2,3-4pyrroles 430 using the 1,3-dipolar cycloaddition... 

The first diheteropentalene bearing the tellurolo[3,4-f]thiophene framework, for example, 470, has been generated from 464 using tellurium metal in the presence of sodium iodide in 1,2-dimethoxyethane (DME). It appears to be stable in solution and adds to DMAD across the 4,6-positions in the tellurophene part. The intermediate 471 loses tellurium and collapses to a 1,3,5,6-tetramethoxycarbonylbenzo[f]thiophene derivate 472 (Scheme 58) <20020L1193>. 

The heterocycles (la)-(ld) are heteropentalenes they have a central C—C bond, and are isoelectronic with the 107r pentalene dianion (2). They are formally derived by the fusion of two of the well-known simple heterocycles including furan, pyrrole, thiophene and selenophene. Considering the two rings as rings A and B, this entire group of five-five fused heterocycles with one heteroatom in each can be conveniently referred to as A,B-diheteropentalenes. 

Table 1 H NMR Chemical Shifts and Coupling Constants of Some A,B-Diheteropentalenes... 

The UV spectral data of various A,B-diheteropentalenes are presented in Table 2. The systems containing only sulfur and/or selenium show more absorption maxima in their UV spectra than those molecules possessing one or two nitrogen atoms. The UV spectrum of the quinonoid thieno[3,4-6]thiophene is in marked contrast to the UV spectra displayed by thieno-[2,3-6]- and -[3,2-6]-thiophenes (67TL761). The analogous, known... 

There has been a great deal of interest in recent years in photoelectron spectroscopy (PES) of the isomeric A,B-diheteropentalenes containing sulfur, selenium and nitrogen. This technique has offered for the first time direct experimental evaluation of the ionization energies of the valence electrons. The sulfur- and selenium-containing heteroaromatics appear to be the class most investigated by PES. The ionization potentials of various A,B-diheteropentalenes, are presented in Table 3. 

---