ATTEMPTED SYNTHESIS OF [14]ANNULENO[14]ANNULENES AND CONFORMATIONAL STUDY OF FLOURENE-BASED CYCLOPHANES

Abstract

The synthesis of 13b, 13c, 16d, 16e-tetramethyl-13b, 13c, 16d, 16e-tetrahydropyreno[4,5-e]pyrene, a novel [14]annuleno[14]annulene, was attempted using selected tetraarylethanes as precursors. The meso and racemic stereoisomers of two 1, 1,2,2-tetraarylethanes have been separated by chromatography. All stereo isomers are believed to exhibit the anti conformation at the central bridge. The individual aryl ring, however, enjoys unrestricted rotation. Assignments of these stereoisomers on the basis of their 1H NMR spectral data and results from theoretical calculations are inconclusive. The racemic stereoisomers of one tetraarylethane underwent an intramolecular cyclization to afford a 10, 11-diaryl-2-thia[3.2]cyclophane. Its meso stereoisomer, however, resulted in dimerization and/or polymerization. These results help confirm the assignment of the respective racemic and meso stereoisomers of the tetraarylethanes. They also serve as chemical evidence to indicate that the gauche population of a 1, 1,2,2-tetraarylethane is insignificant relative to that of the corresponding anti conformer. The 10, 11-diaryl-2-thia[3.2]cyclophane prepared was successfully transformed into the corresponding diaryl[2.2]- cyclophanene via a Stevens rearrangement-Hofmann elimination sequence. Various attempts to introduce appropriate functions on the diary! rings of the thiacyclophane or the cyclophanene, however, failed due to thermal bond cleavages in these molecules under the employed reaction conditions. Oxidation of the cyclophanene to the corresponding 4,5-diaryl-10b, 10c-dihydropyrene was also unsuccessful due to steric factors. The synthesis of 1, 16-dehydro-13b, 13c, 16d, 16e-tetramethyl-13b, 13c, 16d, 16e-tetrahydropyreno[4,5-e]pyrene was investigated using a fluorene-benzophenone-based dithia[3.1.3.0]cyclophane as a precursor. The desired dithiacyclophane has been successfully synthesized but ring contraction reactions attempted failed to afford the corresponding cyclophane or cyclophanediene. Intramolecular reactions to bridge the central carbon functions to form a cyclophanocyclophane were also unsuccessful. Based on results from theoretical calculations and 1H NMR studies, the dithiacyclophane was shown to exhibit a conformation with tilted T-shaped orientations between the corresponding pairs of aryl rings in the molecule. Such a conformational preference in fact does not favour a bond formation between the central carbon functions. Bifluorenylidene was formed when 9-mercaptofluorene was treated with a base. The reaction was found to proceed only in the presence of oxygen. The mechanism is believed to involve an oxidative dimerization of sulphide ions followed by intramolecular oxidative coupling of fluorene anions to form a 1,2- dithiethane derivative. Elimination of sulphur then afforded bifluorenylidene. 1, 1'-Ethano-9,9'bifluorenyl was formed by a regioselective dimerization of a novel o-xylylene derivative. The reaction is shown to be kinetically controlled and non-concerted. Its two rigid conformers represent respectively the first observed examples of an antibifluorenyl derivative and a twist dibenzo1,5-cyclooctadiene derivative. A semiempirical molecular orbital PM3 calculation supported the observed results. Dynamic 1H NMR studies indicated an interconversion process between the two conformers at higher temperatures involving a conformational barrier estimated at 65.2 kJ mol-1. A fluorene-biphenyl-based dithia[3.0.3.0]cyclophane was synthesized. This has been successfully converted to the corresponding [2.0.2.0]cyclophane by photochemical desulphurization. Detailed 1H NMR analysis shows that both these cyclophanes exhibit a conformational behaviour similar to that of the fluorene-benzophenone-based dithia[3.1.3.0]cyclophane mentioned earlier. The larger ring size and the more flexible benzophenone moiety in the dithia- [3.1.3.0]cyclophane allows unrestricted "flipping" of the two rings in the benzophenone unit. With the more rigid biphenyl moiety in the dithia[3.0.3.0]- cyclophane, pseudorotation about the aryl-aryl bond in the biphenyl unit is restricted. Protons of one of the internal methyl groups in this molecule exhibit the highest degree of intrusion into the ?-cloud of a benzene ring among a series of related dithiacyclophanes. Going from the dithia[3.0.3.0]cyclophane to the corresponding [2.0.2.0]cyclophane places the biphenyl and fluorene units closer to each other. The unfavourable steric and ?-? interactions change the conformational preference from a tilted T-shaped arrangement to a tilted anti,syn-type conformation.