Synthesis and chemistry of highly distorted polycyclic aromatic hydrocarbons

Mannion, Michael R. (1999) Synthesis and chemistry of highly distorted polycyclic aromatic hydrocarbons. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

A significant feature of smaller cyclophanes and buckminsterfullerenes is the presence of nonplanar aromatic rings. Such compounds are of considerable interest due to both the synthetic challenge they pose and to their unusual conformational, spectroscopic, and chemical behavior. A great deal of work has focussed on determining the extent to which an aromatic ring can be distorted from planarity while remaining isolable under ambient conditions. Although this question has been examined in detail for isolated benzene rings (for example, through investigations of [n]paracyclophanes), analogous studies of polycyclic aromatic hydrocarbon (PAH) frameworks have never been pursued. -- Here the first systematic examination of the distortion from planarity of a PAH moiety is reported. The synthesis of a number of [n](2,7)pyrenophanes from [3.3]dithiacyclophane precursors is described. Some physical, spectroscopic, and chemical properties of these molecules are also described, and a number of X-ray structures of markedly nonplanar aromatic moieties are reported. From this data, it is concluded that the end-to-end bend of the most strained pyrenophane prepared is greater than the average end-to-end bend of the pyrene moiety. However, POAV analysis of the pyramidalization of pyrenophane sp² carbon atoms reveals markedly lower pyramidalizations than are observed in D₅h C₇₀. Attempts at the functionalization of [n](2,7)pyrenophanes in the hope of using them as precursors for larger nonplanar PAHs were made. However, suitable conditions for functionalization of pyrenophanes were not found. A synthetic approach to a C₂-chiral 1,6-[n]pyrenophane is also described. -- An attempted synthesis of a derivative of the buckybowl pinakene using a tandem Bergman cycloaromatization/free radical conjugate addition is presented. -- The experimental work is preceded by reviews of the literature concerning the concept of aromaticity, nonplanar aromatic molecules (especially [n]paracyclophanes) and fullerene fragments.

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