Phenanthrenes [8]Circulenes and Dibenzo[n]helicenes?Syntheses Structural Analyses and Properties

Abstract

This dissertation presents recent efforts to synthesize various important polyarenes such as phenanthrene derivatives persubstituted [8]circulenes and their fragments (63 and 93-96) and dibenzo[n]helicenes (123 129 and 135) under palladium-catalyzed conditions using haloarenes as starting materials Alkyne is a useful building block and synthon in the extension of an aromatic system The palladium-catalyzed annulations of 2-halobiphenyls with alkynes efficiently yielded phenanthrene derivatives These newly investigated palladium-catalyzed conditions have a large scope and high functional group tolerance but the regioselectivity in the formation of a highly substituted phenanthrene from an unsymmetrical biphenyl and an unsymmetrical alkyne is generally highly complex and its major product cannot be easily predicted The aforementioned synthetic method for preparing phenanthrene was used as the key step in the generation of persubstituted [8]circulenes In the presence of palladium catalysts the reaction of tetraiodo-substituted 2 3 6 7 10 11 14 15-octamethoxytetraphenylene (61c) and 2 3 6 7 10 11 14 15-octamethyltetraphenylene (61d) with excess diarylethynes efficiently generated octamethyl- and octamethoxy-substituted octaaryl[8]circulenes (63b 63c and 63d) respectively which are the first reported examples of the [8]circulene family X-ray crystallography revealed that persubstituted [8]circulene is a saddle-shaped molecule with an [8]radialene character The saddle-to-saddle inversions of persubstituted [8]circulene via a pseudorotation mechanism were experimentally and theoretically verified and the energy barrier was determined to be 20 7 kcal/mol Persubstituted [8]circulene and its nonplanar fragments (93-96) were obtained by palladium-catalyzed benzannulations of tetraiodotetraphenylene 61d with insufficient di(4-anisyl)ethyne The structures of a full series of benzannulated tetraphenylenes were unambiguously verified by X-ray crystallography and they changed from a saddle shape (tetraphenylene) through a twisted saddle (monoannulated and para-diannulated products 93 and 94) to a saddle (ortho-diannulated and triannulated products and persubstituted [8]circulene 95 96 and 63j) again The strain energy of a compound is proportional to the benzannulation number and persubstituted [8]circulene is the most strained in this series The dynamic behaviors of these compounds were studied both experimentally and theoretically Using variable-temperature NMR techniques barriers to coalescence of the 4-anisyl signals were determined to be in the range from 16 4 to 20 8 kcal/mol These dynamic behaviors correspond to the bending and the rotation of an aryl substituent as a kind of "gateway" to the pseudorotation of persubstituted [8]circulene The photophysical and electrochemical properties of the studied compounds depend strongly on the extent of efficient conjugation The successive benzannulations red-shift both the absorption and the emission bands and reduce the first oxidation potential Dibenzo[n]helicenes (123 129 and 135) were prepared by the cyclization of dihalo-substituted polyarenes and the key step comprised the palladium-catalyzed C-H activation and subsequent C-C bond formation Methyl or methoxy groups were selectively installed at various positions yielding a series compounds whose structure-property relationship could be investigated Structural analyses based on X-ray crystallographic data revealed that 17 18-dimethyl-substituted dibenzo[5]helicenes (123a and 123d) and 19 20-dimethyl-substituted dibenzo-[6]helicene (129) have more twisted structures than the corresponding parent [n]helicenes or dibenzo[n]helicenes The steric hindrances that are generated by two additionally fused benzene rings and two methyl groups at the C17 and C18 positions do not critically affect the racemization barrier The photophysical and electrochemical properties of dibenzo[n]helicenes do not strongly depend on the conjugation of the molecular backbone of the molecule but they are dominantly affected by the number and positions of substituents

Date of Award	2016 Jan 30
Original language	English
Supervisor	Yao-Ting Wu (Supervisor)