Photoinduced electron-transfer reactions. The radical cations of methano-bridged paracyclophanes

Abstract

The radical cations 4 and 6 of 1,2:9,10-bismethano[2.2]paracyclophane (3) and of 10,l l-methano[3.2]paracyclophane (5) have been generated by electron transfer to photoexcited (triplet) chloranil. 'H and "C CIDNP effects observed during these reactions allow an insight into the structure of these species. The polarization patterns indicate that charge and spin are delocalized into the benzylic cyclopropane positions, Le., that the tertiary-tertiary cyclopropane bonds are broken or weakened. The different enhancement of the secondary cyclopropane protons is ascribed to different dihedral angles with the singly occupied orbital at the benzylic carbon. The findings that only the benzylic cyclopropane carbons show I3C polarization and that the 'H polarization in this position is relatively weak are interpreted as evidence that these carbon atoms are pyramidal in the radical ion.

Radical cations of strained ring hydrocarbons. Cyclopropane derivatives

“…Pyramidal carbon radicals are reasonably rare; most examples are derived from strained ring systems (25)(26)(27)(28). For the radical cation discussed here, the change in the angle, +, and the adoption of a pyramidal structure apparently relieve some of the strain in the carbon skeleton, yet maintain a reasonable degree of overlap between the benzylic carbons and the benzene rings (24 appears less strained than 17 yet more restricted than diphenylcyclopropane. The ClDNP effects observed for this hydrocarbon once again show different intensities for the two geminal cyclopropane protons.…”

“…The relative magnitude of the CIDNP effects and the chemical shifts of the corresponding protons are fully consistent with this interpretation. ' The endo proton, which in the diamagnetic molecule lies in the shielding cone of the aromatic moieties, has in the radical the smaller dihedral angle €Ic, with the benzylic 7~ orbital and, therefore, the greater hyperfine coupling (24). The comparably weak enhancement of the benzylic proton requires an additional explanation.…”

“…6). Obviously, a large fraction of the strain inherent in 17 is relieved by substituting a trimethylene bridge for one of the cyclopropane moieties (24).…”

“…The analogy with a related ClIH1, system suggests the former spin density pattern. Benzocycloheptatriene (24) gives rise to a CIDNP spectrum in which essentially all aromatic/olefinic protons show enhanced absorption and in which the benzylic/allylic doublet shows strong emission (Fig. 7).…”

“…J. Chern. 61, 1027(1983.A series of radical cations has been generated from various cyclopropane derivatives by photoinitiated electron transfcr to chloranil. Nuclcar spin polarization effects observed during these reactions have been interpreted as evidence for the structure of these intermediates.…”