GENERATION OF BICYCLO[3.2.0]HEPT-6-ENE-2,4-DIYL RADICAL CATIONS BY CHEMICAL ELECTRON TRANSFER (CET) WITH TRISARYLAMINIUM SALTS AND INTRAMOLECULAR CYCLOBUTENE TRAPPING AS AN ALTERNATIVE ENTRY TO THE QUADRICYCLANE-NORBORNADIENE VALENCE ISOMERS

摘要

For the first time, chemical electron transfer (CET) studies have been conducted for the tricyclo[3.2.0.0(2,4)]-hept-6-enes 2 and the quadricyclanes 3 in solution. The 2,4-dimethyl-sustituted bicyclo[3.2.0]hept-6-ene-2,4-diyl radical cation 2a(.+), generated by trisarylaminium salt oxidation of 2a, is intramolecularly trapped by the juxtaposed cyclobutenyl double bond to afford the quadricyclane radical cation 3a(.+) in addition to the expected 1,2-methyl migration to the bicyclo[3.2.0]hepta-2,6-diene radical cation 6a(.+). Radical cation 3a(.+) leads to the norbornadiene 4a by valence isomerization and the bicyclo[3.2.0]hepta-2,6-diene 5a by skeletal rearrangement. For comparison, oxidation of quadricyclane 3a yields exclusively norbornadiene 4a. Whereas the 2,4-diphenyl-substituted bicycle[3.2.0]hept-6-ene-2,4-diyl radical cation 2b(.+) derived from 2b is intramolecularly trapped by the juxtaposed cyclobutenyl double bond to afford norbornadiene 4b and bicyclo[3.2.0]hepta-2,6-diene 5b through the quadricyclane radical cation 3b(.+), the quadricyclane 3b yields on oxidation also 5b besides 4b. These experimental facts are rationalized in terms of distinct radical cation structures, namely the pi complex (oxidation of the lateral cyclopropane bond) and trimethylene (oxidation of the internal cyclopropane bond). Their preferences are dictated by the substrate structure, i.e., tricycloheptene 2 versus quadricyclane 3 as well as by the substitution type (phenyl versus methyl) and is corroborated by AM1 calculations. [References: 58]