The vast majority of organic reactions occur via the pairwise transfer of electrons. By comparison, organic reactions that proceed via single electron transfer processes remain relatively unexplored to date. Prof. Dinnocenzo and his group are actively probing the chemistry of organic ion radicals — the products of single electron transfer.
One interesting aspect of ion radical chemistry results from their remarkably high reactivity. For example, the rearrangement of vinylcyclopropanes to cyclopentenes (e.g. 1-›2) is normally a sluggish reaction requiring high temperature, as expected based upon the Woodward-Hoffmann rules of orbital symmetry. However, we have discovered that one-electron oxidation can accelerate the rearrangement by as much as 10 20 ! Other examples of "forbidden" pericyclic reactions that become "experimentally allowed" upon one-electron oxidation await study.
Another ion radical project involves nucleophilic substitutions on cation radicals. In contrast to conventional even-electron SN2 reactions, odd-electron SN2 reactions substitutions are dramatically faster — in fact, they are the fastest SN2 reactions discovered to date. Despite this high reactivity, the reactions can be highly selective. For example, we have shown that they occur with complete inversion of configuration. Most remarkably, we have found that odd-electron SN2 reactions show an unusual preference for substitution at hindered carbon atoms! This opens up new opportunities for their use in synthesis.
A second research theme involves an interdisciplinary, collaborative project with colleagues in Optics and at Eastman Kodak to investigate the fundamental chemical, photochemical, photophysical, polymer, and optical properties of a new class of photoresponsive polymers that undergo electron transfer-initiated ion radical chain isomerization reactions upon absorption of light. In addition to the novel amplification feature of these materials, the materials are attractive because they provide a flexible platform that can be used to design polymeric materials that respond to light by transforming any of a number of optical properties, including refractive index, absorption, fluorescence, nonlinear optical susceptibilities, as well as their spectral dependencies.
A wide variety of experimental methodologies are employed in our research, including small molecule and polymer synthesis, all of the common forms of spectroscopic characterization, stereo-chemical experiments, isotopic labeling studies, kinetic analyses (picosecond and nanosecond), photochemical techniques, fluorimetry, and time-resolved single photon counting techniques. Experimental work is frequently supplemented by state-of-the-art quantum chemical calculations.
