Conformationally Controlled Intramolecular Charge Transfer Complexes

• Published in: Journal of organic chemistry 2002-10, Vol.67 (20), p.6938-6943
• Main Authors: Kost, Daniel, Peor, Na'ama, Sod-Moriah, Gali, Sharabi, Yifat, Durocher, David T, Raban, Morton
• Format: Article
• Language: English
• Subjects: Alicyclic compounds; Alicyclic compounds, terpenoids, prostaglandins, steroids; Chemistry; Exact sciences and technology; Organic chemistry; Preparations and properties
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo020164t

Trans-1-acceptor-2-donor-substituted cyclohexanes (1), as well as their 4- (or 5-)methyl-substituted homologues (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 interconversion of the cyclohexane ring. The CT absorptions have been measured and the equilibrium constants for the ring reversal have been determined by UV/vis spectroscopy at 298 K, as well as by NMR spectroscopy at two temperatures:  at 183 K, by direct comparison of signals due to the two chair conformations, and at 300 K, by comparison of calculated and measured widths of the α-proton signals. 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 were characterized by NOE interaction between the methyl and an α-proton cis to it.