Synthesis and properties of dimethyltetradehydrothia-[19]-, -[21]-, -[23]-, and -[25]-annulene. Diatropic and paratropic analogues of thiophene

Abstract

The title thia-annulenes (9), (Il), ( 14), and ( 16) have been synthesized by Wittig reactions of (2€,4Z)-5-methylhepta-2,4-dien-6-ynal (6) and its vinylogous aldehydes ( 7 ) , (12) with bis-[ (triphenylphosphonio)methyl] sulphide dibromide (5), followed by intramolecular oxidative coupling of the resulting acyclic sulphides. The properties of the thia-annulenes are discussed on the basis of 'H NMR and electronic spectra and compared with those of their lower homologues.Although diatropicity or paratropicity in monocyclic conjugated systems reduces with increasing ring size,' a monocyclic 30-membered annulene, tetradehydro[30]annulene, has been reported to show diatropicity.2 The degree of dia-or paratropicity of an annulenone or a heteroannulene has been found to be much smaller than that of the corresponding carbocyclic annulene; similarly, benzene, the parent compound of annulenes, is more diatropic than tropone or heteroaromatics such as pyridine, pyrrole, furan, and thiophene. Thus, the limiting ring size to show dia-or para-tropicity is predicted to be smaller for annulenone and heteroannulene systems than the 30-membered ring observed in the carbocyclic annulene system. However, the alternation of the tropic nature between [4n + 2 3 ~-and [4n]~-electron systems, arising from polarization of carbonyl group of dimethyl-or trimethyltetradehydroannulenones of type (l), has been confirmed from 13to 25-membered ring In 1975, Sondheimer and co-workers reported the synthesis of diatropic dimethyltetradehydrothia-[ 131-and -[ 171-annulene [(2)6 and (3)' respectively] as well as that of paratropic 2.22, and 2.10 in 3 : 4 : 3 ratio.

Synthesis and Relative Diatropicity of a Remarkably Aromatic Thia[13]annulene

“…In Kato's annulene 5 , the annulene form is stable, but diatropicity conclusions based on coupling constant data suggested the compound had little aromaticity. Sondheimer's thia[13]annulene 6 , and Ojima's series 7 ( n = 2−4) and 8 ( n = 2−4, m = n + 1) appear to be diatropic, though not strongly. In these cases, despite their elegant syntheses, it is difficult to calibrate the aromaticity because of anisotropy effects, conformational mobility, or just plain lack of a good comparison parent annulene, particularly one with the same geometry.…”