Theoretical and Experimental Assessment of the Viability of 1,4,6,9-Spiro[4.4]Nonatetrayl as a Reactive Intermediate
<p>Qualitative molecular orbital (MO) theory predicts that 1,4,6,9-spiro[4.4]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 clos...
| Summary: | <p>Qualitative molecular orbital (MO) theory predicts that 1,4,6,9-spiro[4.4]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.</p>
<p>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.</p>
<p>Direct photolysis of spiro[bis(2,3-diazabicyclo[2.2.1]-hept-2-ene)-7,7'] (17) leads to loss of a single equivalent of N<sub>2</sub> and ring closure of the resulting biradical to 2,3-diazabicyclo[2.2.11hept-2-ene-7,5'-spirobicyclo[2.1.0]-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 ,9-diazatricyclo-[5.2.2.0<sup>2,6</sup>)]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[4.4]nonatetrayl.</p> |
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