THEORETICAL AND EXPERIMENTAL ASSESSMENT OF THE VIABILITY OF 1,4,6,9-SPIRO(4.4)NONATETRAYL AS A REACTIVE INTERMEDIATE (BIRADICALS, SPIROCONJUGATION, TETRARADICALS).

摘要

Qualitative molecular orbital (MO) theory predicts that 1,4,6,9-spiro{lcub}4.4{rcub}nonatetrayl (7) should be stabilized via spiroconjugative interaction of the four radical p orbitals. In addition to this thermodynamic stabilization, energetic barriers are predicted for closure to either of the closed shell forms.; The electronic structure of 7 has been investigated using ab initio electronic structure theory. The spiro-conjugative interaction of the four radical centers is evidenced by a large orbital splitting. However, spiroconjugation does not confer upon the structure the electronic properties of a biradical, contrary to qualitative MO considerations. Structure 7 possesses the six, low-lying (covalent) states that characterize a tetraradical. Spiroconjugation does strongly influence the relative energies of these six states, and does lead to a small but significant stabilization of the molecule. Possible modes of ring closure and closed shell isomers of 7 are also discussed.; Direct photolysis of spiro{lcub}bis(2,3-diazabicyclo{lcub}2.2.1{rcub}hept-2-ene)-7,7'{rcub} (17) leads to loss of a single equivalent of N(,2) and ring closure of the resulting biradical to 2,3-diazabicyclo{lcub}2.2.1{rcub}hept-2-ene-7,5'-spirobicyclo{lcub}2.1.0{rcub}pentane (19). Generation of the triplet biradical by sensitized photolysis results in a competition between ring closure to 19 and a 1,2-alkyl shift to 8,9-diazatricyclo{lcub}5.2.2.0('2,6){rcub}undeca-2,8-diene (23). While direct photolysis and thermolysis of 19 yield primarily ring closure product, sensitized photolysis leads to a series of biradical-to-biradical rearrangements that ultimately produce 2,3-divinyl-cyclopentene (24). Deuterium labeling studies indicate competing mechanistic pathways for this reaction. Rationalization of the label distribution requires one of two unprecedented processes: frontside radical attack on a C-C bond or intermediacy of 1,4,6,9-spiro{lcub}4.4{rcub}nonatetrayl.