Steric modulations in the reversible dimerizations of phenalenyl radicals via unusually weak carbon-centered pi- and sigma-bonds.
[reaction: see text] Spontaneous self-associations of various tricyclic phenalenyl radicals lead reversibly to either pi- or sigma-dimers, depending on alkyl-substitution patterns at the alpha- and beta-positions. Thus, the sterically encumbered all-beta-substituted tri-tert-butylphenalenyl radical (2*) affords only the long-bonded pi-dimer in dichloromethane solutions, under conditions in which the parent phenalenyl radical (1*) leads to only the sigma-dimer. Further encumbrances of 1* with a pair of alpha, beta- or beta, beta- tert-butyl substituents and additional methyl and ethyl groups (as in sterically hindered phenalenyl radicals 3* - 6*) do not inhibit sigma-dimerization. ESR spectroscopy is successfully employed to monitor the formation of both diamagnetic (2-electron) dimers; and UV-vis spectroscopy specifically identifies the pi-dimer by its intense near-IR band. The different temperature-dependent spectral (ESR and UV-vis) behaviors of these phenalenyl radicals allow the quantitative evaluation of the bond enthalpy of 12 +/- 2 kcal mol(-1) for sigma-dimers, in which the unusually low value has been theoretically accounted for by the large loss of phenalenyl (aromatic) pi-resonance energy attendant upon such bond formation.
Zaitsev
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V
Department of Chemistry, University of Houston, Houston, Texas 77204-5003, USA.
Rosokha
S V
SV
Head-Gordon
M
M
Kochi
J K
JK