The Radical Anions of [2.2]Paracyclophane‐1,9‐diene and Some of its Derivatives. An Unexpected Conformational Interconversion

Abstract

Dedicated to Prof. Virgil Boekelheide on the occasion of his 65th birthday (27.XII.84) ~The radical anions of [2.2]paracyclophane-1,9-diene (2) and its 1,10,12,13,15,16-hexadeuterio derivative 2-D6, as well as those of 4,5,7,8-tetramethyl[2.2]paracyclophane-l,9-diene (3) and its 12,13,15,16-tetradeuterio derivative 3-D4, have been studied by ESR spectroscopy. The coupling constants for 2: at 178 K are 0.422 mT for four equivalent olefinic protons and 0.046 and 0.020 mT, each for a set of four equivalent aromatic protons. This hyperfine pattern is consistent with either benzene ring bearing two pairs of equivalent protons and it points to a lowering of the anticipated D,, symmetry. The ESR spectra of 2: are strongly temperature dependent, due to modulation of the two coupling constants of 0.046 and 0.020 mT; these have opposite signs and average to 0.013 mT at 273 K. The experimental findings are interpreted in terms of a transition state of D2h symmetry, 33 kJ/mol above two interconverting equivalent conformations of lower symmetry. Several pieces of evidence suggest that this symmetry is D,, i.e., the benzene rings in 2: are twisted in opposite directions about the vertical axis. Temperature dependence of the ESR spectra, resulting from modulation of the hyperfine interactions with the aromatic protons, is also observed for 2-D6; and 37. In the case of 3', the olefinic protons are, as expected, only equivalent in pairs, the pertinent coupling constants being 0.560 and 0.325 mT. Upon standing at low temperatures, 2; and 3: gradually convert into the radical anions of [2.2]paracyclophane (1) and its 4,5,7,8-tetramethyl derivative, respectively. At higher temperatures, cleavage of one bridging chain in 2: also occurs, with the formation of the radical anion of (E)-4,4'-dimethylstilbene (7). Both reactions of 27 must involve the transient radical anion of [2.2]paracyclophane-l-ene (4) as proved by the observation of the spectra of 1 : and 7: with 4 as the starting material.Introduction. -Replacement of the single by the double bonds in the two bridging alkyl chains of [2.2]paracyclophane (l), to yield the 1,9-diene 2, has only a small effect on the geometry and electronic structure. As shown by the X-ray analyses of 1 [I] and 2 [2],

The Radical Anions and Trianions of 1,8‐Diphenylnaphthalene and of Some of its Derivatives Containing a Cyclophane Substructure

“…In fact, the two largest coupling constants assigned to protons in the naphthalene moiety of 3-(0.464 and 0.243 mT) and 4: (0.451 and 0.273 mT) are very similar to the analogous values for 1-(0.459 and 0.261 mT) and 2-(0.456 and 0.250 mT). The additional coupling constant of a substantial magnitude observed for 4-(0.196 mT) must arise from the two olefinic protons in the etheno bridge (a value of 0.422 mT was found for the corresponding protons in the radical anion of [2.2]paracyclophane-1,9-diene [6]). A more pronounced effect of bridging the p-C-atoms in 1 is expected for 337 and 43T, since the spin population in 13' resides to a large extent in the benzene rings.…”

“…Such a conversion is brought about by a formal uptake of two protons and two electrons and must be attributed to traces of protic impurities in the s o h . It is fully analogous to formation of the radical anion of [2,2]paracyclophane from that of its I-monoene or 1,9-diene [6].…”