| Abstract
| - Trans-1-acceptor-2-donor-substituted cyclohexanes (1), as well as their 4- (or 5-)methyl-substitutedhomologues (2), have been prepared and are shown to form intramolecular charge-transfer (donor−acceptor) complexes. These weak complexes are turned on and off by the chair−chair interconversionof the cyclohexane ring. The CT absorptions have been measured and the equilibrium constantsfor the ring reversal have been determined by UV/vis spectroscopy at 298 K, as well as by NMRspectroscopy at two temperatures: at 183 K, by direct comparison of signals due to the two chairconformations, and at 300 K, by comparison of calculated and measured widths of the α-protonsignals. The Gibbs free energies assigned to the donor−acceptor interactions range between 0 and−1 kcal mol-1. A crystal structure of one of the complexes (1b) confirms the intramolecular donor−acceptor alignment and interaction. The regioisomers of the methyl-substituted complexes werecharacterized by NOE interaction between the methyl and an α-proton cis to it.
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