The Electronic Excited States of Small Ring Compounds: Heterobicyclopentanes

Intermediates (trimethylenes) formed upon bond cleavage of three-membered rings are usually too unstable for direct characterization. They may. however. he stabilized by substitution and by incorporation into a cyclic system where the ring closed form is strained and conrotatory closure of the trimethylene is inhibited. These principles have been applied to two heterobicyclo[2.l .O]pentane systems. The trimethylene intermediates resulting from irradiation of substituted 2,4,6,8,9-pentazatricyclo[5.2.02' 6]dec-8-ene-3,5-diones (X a—d) are triplets (XI) and are stable at low temperature. Some reactions of the ring closed form (XII) demonstrate that the central bond is weak. Ring inversion of (XII) occurs at low temperature. Derivatives of 5-oxabicyclo[2.l.O]pentane (XIII) are thermoand photochromic (direct and sensitized irradiation). the result of cleavage of the central bond. The carbonyl ylide (XIV) formed is relatively stable and reverts back to the oxide with a half-life equal to 8 minutes in benzene solution at 22 . Dipolarophiles react readily with the carbonyl ylide to give cycloaddition products. The trimethylene singlet and triplet are related to the corresponding three-membered ring by heterolytic and homolytic bond cleavaget. Consider the general case with combinations and permutations of carbon and heteroatomst. Many of the possible trimethylene singlets are stable, particularly if atom h hears an unshared pair of electrons and is thus capable of delocalizing the positive charge. When this is the case, irradiation of the trimethylene singlet frequently leads to formation of the three-membered ring2. Few examples of triplet trimethylenes have been characterized: however. they have frequently been proposed as intermediates in photochemical and thermal reactions of three-membered rings3. * Address correspondence to the Photochemistry Unit, Department of Chemistry, University of Western Ontario, London, Ontario, Canada. The term trimethylene is used here in a generic sense. There are two classes of trimethylene: (1) a singlet species where the two electrons interact with one another and have the opposite spin (e.g. a 1,3-dipole); (2) a triplet species where the two electrons interact and have the same spin. When the two electrons are only three atoms apart it is unlikely that the interaction between the two can be negligible so that the quantization of each is independent of the other (i.e. a biradical).