Synthesis of fluoroladderenes, semi-fluorinated polyacetylene, and a metal-free cyclobutadiene reagent
Keywords: Chemistry.

Ladderanes are some of the most unusual, yet stable hydrocarbons. First discovered in natural phospholipids, their linearly-fused cyclobutane ring systems have challenged chemists to devise new methods for their chemical synthesis. Prior work in the Burns lab at Stanford prepared some of these natural ladderane phospholipids, and, in collaboration with Professor Yan Xia, showed that ladderanes can be polymerized into force-responsive polymers. These polymers would 'unzip' when acted on by a force into long conjugated polyenes. This type of polymer mechanochemistry differs from traditional ball-milling, since the applied forces are directional. Applying directional forces modifies the potential energy surface, and can yield chemical products that are not achievable using heat alone. In this thesis, we apply a mechanochemical unzipping approach to synthesize a previously inaccessible material: fluorinated polyacetylene. Although fluorinated polyacetylene has long been studied computationally, no chemical synthesis existed. We set out to prepare a semi-fluorinated polyacetylene from the polymerization of a fluoroladderene monomer. Fluoroladderenes are ladderene scaffolds containing fluorine substituents, and we develop their synthesis from a marvelous photochemical cascade reaction with hexafluorobenzene. These chapters discuss the synthesis of fluoroladderenes and investigate methods to polymerize them into fluoroladderene polymers. We then perform the mechanochemical unzipping into semi-fluorinated polyacetylene and characterize its properties. Surprisingly, it displays an iridescent gold color when dried to a solid. Thin films of it are semiconducting, and show improved air-stability compared with polyacetylene. Not only is this work the first synthesis of a fluorinated polyacetylene, it showcases an example of polymer mechanochemistry in target-oriented synthesis. There has perhaps been no other molecule which has cultivated and sustained such an interest from both theoretical and experimental chemists as cyclobutadiene. We became interested in using cyclobutadiene as a reagent since it produces ladderenes from a [4+2] cycloaddition with olefins. The state-of-the-art method for generating cyclobutadiene is by release from a stable complex with iron. However, this method requires a multi-step synthesis and the use of toxic, heavy-metal reagents. We set out to discover an alternative cyclobutadiene reagent. In this thesis, we develop a metal-free cyclobutadiene reagent: diethyldiazabicyclohexene dicarboxylate. Inspired by a report from Masamune and coworkers, we investigate the class of diazabicyclohexenes as potential cyclobutadiene reagents. We carry out an optimization effort and develop a procedure that liberates free cyclobutadiene, and then study its in situ [4+2] cycloaddition with olefins to yield ladderenes. Our procedure for generating cyclobutadiene benefits from being very convenient and mild. It is performed entirely at room temperature and is tolerant of air, water and light. We then develop a synthesis of this reagent in two steps on gram-scale