I. The synthesis and spectroscopy of a series of 1,3-diaryl-1,3-cyclopentadiyls and 1,4-diarylbicyclo[2.1.0]pentanes. II. Progress toward the synthesis of non-Kekulé naphthalene, a series of tetramethyleneethanes, and bi(cyclobutadienyl)
1,4-Diarylbicyco[2.1.0]pentanes 6-XY were prepared from the corresponding diazenes by thermolysis. Variable temperature NMR reveals that para substituents have only small effects on the free energies of activation at coalescence in the bridge-flip reaction. The effects correlate with the Hammett par...
| Summary: | 1,4-Diarylbicyco[2.1.0]pentanes 6-XY were prepared from the corresponding diazenes by thermolysis. Variable temperature NMR reveals that para substituents have only small effects on the free energies of activation at coalescence in the bridge-flip reaction. The effects correlate with the Hammett parameters derived from the thermolysis of para-substituted dibenzyl mercurials, σj˙. The reduced magnitude of substituent effects observed in these compounds was attributed to the incomplete formation of radical character at the transition state and to its cumyl radical nature.
1,3-Diaryl-I,3-cyclopentadiyls 7-XY prepared by low-temperature photolysis of the same diazenes have triplet grounds states. The zfs parameter |E/hc| is constant for the series, indicating para substituents cause no substantial change in molecular symmetry. The zfs parameter |D/hc| correlates extremely well with the general Hammett parameter for para groups, σp˙, which allows the separation of observed |D/hc| values for bromine-containing
members of the series into contributions from spin-orbit coupling and from inductive and resonance effects of the substituents. Spin-orbit coupling effects of the
bromines on ID/hcI are small relative to the system's hyperfine couplings and estimated spin-orbit effects based on interactions without heavy atoms.
Phenylsulfonyl-containing members of this series undergo significant unimolecular decay at 77 K. The relative decay rates correlate well with a decrease in the exchange
energy (as implied by smaller |D/hc| values), which decreases the singlet-triplet gap.
Linear free-energy relationships for these systems depend on both substituents present. A sum of the two individual substituent constants was used because each radical
unit contributes equally to the observed properties. This is the first reported correlation of biradical properties with para-substituent effects determined in other systems.
Significant progress toward the syntheses of non-Kekulé naphthalene, a series of tetramethyleneethanes, and bi(cyc1obutadienyl) was made. The reactivity of precursors to these compounds has been explored, and routes for future efforts have been charted.
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