Thesis advisor: Marc L. Snapper / Intramolecular [2+2] photocycloadditions have been studied: (i) Previous studies in our group have demonstrated that there is a regiochemical dependence of intramolecular [2+2] photocycloadditions on the solvent employed. In order to understand the generality of hydrogen bonding effects of photochemical reactions of β-hydroxyl enones, we examined the photochemical outcome of enones with both acyclic and cyclic olefin tethers. (ii) An application of an intramolecular [2+2] photocycloaddition/ fragmentation strategy to form 5-8-5 ring systems has been demonstrated toward the synthesis of the natural product cycloaraneosene. / Thesis (PhD) — Boston College, 2008. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

The use of principles of supramolecular chemistry to direct reactions in the crystalline state has emerged as a reliable means to facilitate highly selective reactions in a solvent-free environment. In this context, we have utilized the fixed spatial arrangements of molecules in a crystal lattice to promote intermolecular [2+2] photodimerizations. We have shown how principles of supramolecular chemistry can be used to preorganize olefins to undergo intermolecular [2+2] photodimerizations in the solid state. Explorations into fluorinated and partially fluorinated olefinic substrates has allowed for development of underutilized supramolecular interactions to direct such chemical reactions. We reported interactions between fluorinated molecules in inorganic complexes and organic solids that supported intermolecular [2+2] photodimerizations as well as cross-reactions between olefinic species. Analogous to the hydrogen bond templates commonly utilized by our group to engineer reactive solids, we expanded the methodology by developing templates that direct reactions via halogen bonding. We report on the use of a halogen-bond-acceptor template to direct the formation of a small molecule lined with I- and F-atoms. We demonstrate the olefins in the solid undergo a [2+2] photodimerization with UV radiation in a stereoselective and quantitative manner. Our work provides the first use of halogen bonds to direct the formation of a small-molecule lined with halogen atoms in a solid. We reveal the halogenated product provides access to a unique family of host-guest materials that exhibit an extremely rare form of self-inclusion. We describe the ability of halogen bonds to undergo bending deformations. There are currently great interests in halogen bonds as related to applications to organic synthesis, supramolecular chemistry, and materials science. While halogen bonds are often compared to hydrogen bonds, the wealth of knowledge concerning the fundamentals and behavior of halogen bonds has admittedly experienced less time to develop. In our work on single-crystal reactions and transformations, we have discovered the ability of halogen bonds based on rigid interacting partners to bend and deform. We provide a quantitative comparison of the bending to the literature to demonstrate how the geometric landscape of the bending compares to known data. We report a single-crystal-to-single-crystal transformation of a cocrystalline material involving a fluorinated, olefinic compound. The olefins adopt a well-defined, reactive geometry mediated by halogen-bonding templates. We exploit metal-organic complexes to fix spatial arrangements of molecules in crystal lattices to promote intermolecular [2+2] photodimerizations. Perfluorophenyl-perfluorophenyl (C6F5···C6F5) interactions have recently emerged as a means to control supramolecular architectures and frameworks. However, it was not clear whether perfluorophenyl-perfluorophenyl interactions can be integrated in [2+2] photodimerizations in solids. We describe the significant effect fluorine exercised on the structure and photoreactivity of an octafluoro stilbene derivative in the solid state. Specifically, we report the olefin, trans-1,2-bis(2,3,5,6-tetrafluorophenyl)ethylene, self-assembles via C-H···F and perfluorophenyl-perfluorophenyl interactions for a stereoselective and quantitative topochemical photodimerization. Lastly, principles of crystal engineering and supramolecular chemistry are exploited to design and achieve a supramolecular protecting group strategy for the solid-state synthesis of a cyclobutane—a cyclobutane that supports a rare hydrogen-bonded self-catenating framework. The application of a hydrogen bonding coformer to separate a trans-isomer of a pyrimidyl olefin directly from a Wittig reaction is demonstrated. The coformer supports a head-to-head [2+2] photodimerization in the solid state. The work demonstrated here describes our explorations into the supramolecular interactions of organic fluorine, and how such interactions can be used to facilitate the synthesis of small molecules via [2+2] photodimerizations in solids—alone and in accompaniment of other crystal engineering strategies.

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