Laboratory of Organic Chemistry
William C. Agosta
We are interested in the structures and properties of molecules, their chemical reactions, synthetic transformations, and interactions with other molecules. We work particularly with reactions brought about by light and with the transient intermediates formed in such reactions, as well as on the chemistry of fenestranes and other strained small-ring compounds.
Our long-standing interest in alkyl propargyl biradicals recently led to discovery of a new reaction. Irradiation of conjugated acetylenic α-diketones in the presence of tetra-methylethylene furnishes tetrasubstituted furans in high yields. We are now studying the mechanism and scope of this novel [3 + 2] cycloaddition-cyclization process. This new reaction grew out of earlier exploration of the reaction of alkenes with 2-(1-alkynyl)-2-cycloalken-1-ones in collaboration with Professor Paul Margaretha (University of Hamburg). We are also attempting to extend the new reaction of α-diketones to related systems such as α-imino α,β-acetylenic ketones and cross-conjugated alkynenones. Synthesis of ketones for these investigations is under way.
Another area of interest is the photochemistry of nitrogen heteroaromatics. There is little information characterizing excited states and their photochemical properties for these systems, owing in part to the large number and complex interactions of the available states. Earlier we found that, in contrast to abstraction of hydrogen by carbonyl oxygen, there is little effect of carbon-hydrogen bond strength on the rate of hydrogen abstraction by nitrogen. Such abstractions are biologically important through photochemical reactions of nucleic acids and flavins. We have completed a study of the effect of nΠ* triplet energy and heteroaromatic system on the selectivity of abstraction by nitrogen. We are also probing the excited states of nitrogen heteroaromatic ketones that mediate hydrogen abstraction by both oxygen and nitrogen. The behavior of these states is sensitive to the nΠ* triplet energy of the parent heterocycle, which we can control through substitution in the aromatic ring. An important unsettled question is whether these abstractions by two different atoms in the same molecule emanate from two equilibrating nΠ* triplet states or one triplet state with mixed character. We recently discovered that abstractions by oxygen and nitrogen in 4-trifluoromethyl-2-isovalerylpyridine emanate from two different triplets. These states are differentially quenched by piperylene and differentially populated by a series of sensitizers of increasing triplet energy. The theoretically interesting problem here is the electronic nature of the triplet excited state(s) responsible for photochemical reactivity.