Stereochemical Requirements for Bishomoaromaticity

Abstract

<p> To investigate the stereochemical requirements for bishomoaromaticity it was necessary to develop a synthetic scheme that would give suitable precursors in which the stereochemistry could be unambiguously assigned. Arguments are presented in this theses to suggest that, if bishomoaromaticity is to be detected when the two methylene bridges are trans with respect to each other, the 1,4-bishomotropylium system is more favourably oriented for cyclic delocalization than the 1,3-bishomotropylium system. </p> <p> By reacting benzotropone ethylene ketal with phenyl mercuric trichloromethane, followed by removal of the chlorines and protecting group, it is possible to synthesize the trans-4,5-benzo-2,3:6,7-bishomotropone. The cis-4,5-benzo-2,3:6,7-bishomotroponewas synthesized by the action of dimethyloxosulfonium methylide on benzotropone. The stereochemistry in the cis and trans isomers was unambiguously established by examining the nmr spectra of the derived alcohols. </p> <p> To determine the effect of the benzene ring in these homoaromatic systems it was necessary to synthesize 4,5-benzo-2,3-homotropone. By reacting benzotropone with dimethyloxosulfonium methylide it was possible to obtain a high yield of 4,5-benzo-2,3-homotropone. </p> <p> The low temperature nmr spectra of protonated 4,5-benzo-2,3-homotropone and derived alcohol clearly showed that the benzene ring did not decrease the homoaromatic nature of these systems. The hydroxy substituent, however, had a marked effect on the homoaromatic nature. </p> <p> The low temperature nmr spectra of the protonated cis and trans-4,5-benzo-2,3:6,7-bishomotropones showed that the trans hydroxy cation could be best interpreted as a cyclopropyl carbinyl delocalized system and the cis hydroxy cation as a bishomoaromatic species. </p> <P> The low temperature nmr spectra of the trans-4,5-benzo-2,3:6,7bishomotropylium cation also supported the cyclopropyl carbinyl delocalization in this system. In contrast to the hydroxy-substituted system, however, the unsubstituted system was not equilibrating between the two possible equivalent boat conformations. This was attributed to the hydroxy substituent effect on the transition state of the boat-boat equilibrium. </p> <p> In conclusion, arguments are presented that would suggest that the trans-1,3-bishomotropylium cation is not a bishomoaromatic cation as previously reported, but that is can be best represented as a cyclopropylcarbinyl delocalized system. </p>