Date of Award

Winter 2022

Project Type

Dissertation

Program or Major

Chemistry

Degree Name

Doctor of Philosophy

First Advisor

Richard Johnson

Second Advisor

Gonghu Li

Third Advisor

Margaret Greenslade

Abstract

The [n]triangulenes (n = number of rings on a side) are a triangle-shaped class of polycyclic aromatic compounds that have been of interest for almost 100 years. Triangulenes are expected to be high-spin species with unusual electronic structures and with potential applications in molecular electronics. The first examples of simple triangulenes have been synthesized very recently by reactions on a surface and were characterized by scanning electron microscopy. Our research group has reported the solution phase synthesis and polymerization of [3]triangulene. This thesis describes approaches toward an efficient scalable synthesis of the little-known [4]triangulene ring system. Our synthetic plan, which takes full advantage of three-fold symmetry, has been to synthesize substituted 1,3,5-triphenyl benzenes by one-step trimerization of acetophenone derivatives. Strategic installation of three vinyl or ethynyl groups would be followed by three-fold photochemical or electrophilic intramolecular cyclization to give precursors to [4]triangulenes. Significant progress has been made toward this goal. Triflic acid or SiCl4 mediated trimerization of halogen-substituted acetophenones provided an efficient multigram synthesis of 2,2',2''-trishalogenated-1,3,5-triphenylbenzenes (X = Br or I). Triple Sonogashira or Heck coupling was then used to install vinyl or ethynyl groups in high yield. Cyclization reactions with model vinyl or ethynyl-substituted biphenyls were carried out successfully by photochemistry or electrochemistry, resulting in phenanthrenes or dihydrophenanthrenes. Despite these successes, numerous attempts failed to demonstrate the desired triple-cyclization reactions to form [4]triangulene precursors. Complex mixtures of starting material and products were observed in each case. This failure may be due to the structure following alternate cyclization modes beyond the initial phenanthrene formation or to polymerization. In the course of this work, a general photochemical cyclization of 2-ethynylbiphenyls to phenanthrenes was developed. This also provides a new route to [5]helicene and pyrene from 2-ethynyl-1,1'-binaphthyl and 2,2’-bis-ethynyl biphenyl, respectively. Methanol addition products during irradiation of 2-ethynyl biphenyl suggest that the photocyclization might proceed through a carbene intermediate. The mechanism was studied by deuterium labeling. Photocyclization of 2-ethynyl-1,1′-biphenyl-d5 showed that deuterium from the aromatic ring is retained in the product, with migration from the aromatic ring to C1 of the original ethyne group. The mechanism is unclear but is unlikely to proceed through a strained cyclic allene. Further mechanistic and computational studies are needed for this reaction.

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