With funding from the Chemical Structure, Dynamics & Mechanisms B Program of the Chemistry Division, Professor Miguel A. Garcia-Garibay of the Department of Chemistry and Biochemistry at the University of California-Los Angeles is establishing new synthetic strategies to transform simple chemicals into more complex, high value substances, by taking advantage of reactions that occur under the high order and rigidity of the crystalline state. Their strategy relies on the selection of structures that are known to form highly reactive intermediates when activated by the absorption of ultraviolet light. By executing those reactions in the crystalline state, instead of doing them in liquids, they are able to obtain nearly pure solid products with no need for solvents, catalysts or additional reagents. This project is in the SusChEM portfolio as the reactions do not use solvent and require no energy for the separation and removal of byproducts. This project provides an excellent multidisciplinary training ground for several students. Graduate students and summer undergraduates from three partnering Historically Black Colleges and Universities institutions: Howard University, Clark Atlanta University, and Spellman College, are involved.
Having implemented robust strategies to engineer reactions in crystals by selecting stable molecules that generate highly reactive intermediates, this project addresses the solid state photodecarbonylation of hexasubstituted ketones and the photodenitrogenation of triazolines, pyrazolines, and selected aromatic azides. Radical pairs, biradicals, and nitrenes are generated in the crystalline state in highly exothermic processes from the excited state of the reactants, and their product formation and kinetics are analyzed in detail. In the solid state, radical-radical combination reactions with retention of stereochemistry, and selective nitrene insertions occur. Photochemical reactions are carried out in well-characterized crystalline phases and their transformations from the solid phase of the reactant to the solid phase of the product are analyzed by thermal analysis and spectroscopic means. This characterization is done in order to determine the corresponding phase diagrams and the conditions required for clean and efficient reactions. Detailed reaction mechanisms are explored by taking advantage of structural information available from single crystals X-ray diffraction and kinetic measurements obtained from laser flash photolysis measurements using nanocrystals suspended in water.
