Synthesis of 2-substituted pentacyclo[6.3.0.02,6.03,10.05,9]undecanes (D3-trishomocubanes) and a study of their13C spin–lattice relaxation times in solution

Abstract

D 3 -Trishomocubane (THC, 1a) and four 2-substituted derivatives (THC-X, X = CO2H, CO2Me, CH2OH, C(O)NH2, 1b–e, respectively) have been synthesized. 13C NMR spin–lattice relaxation times (T1) and nuclear Overhauser enhancements of various skeletal carbon atoms in each of these five cage molecules have been obtained experimentally. In each case the results thereby obtained were used to determine the molecular perpendicular and parallel reorientational diffusion coefficients (D⊥ and D∥) in CHCl3 solution as a function of temperature. Molecular tumbling rates (D⊥) in all of the derivatives are markedly longer than in the parent hydrocarbon. A comparison of their relative magnitudes provides convincing evidence for the formation of (i) long-lived hydrogen-bonded dimers in THC-CO2H and (ii) more transient hydrogen-bonded association complexes in THC-C(O)NH2. In contrast to the perpendicular diffusion coefficients, values of D∥ in THC-CO2H, THC-CO2Me and THC-C(O)NH2 are equal to the corresponding values in the parent compound, thereby indicating the absence of any significant barrier to rotation about the C(2)–X bond in these 2-substituted THCs. A slower spinning rate is observed in THC-CH2OH, which is attributed to the existence of a steric barrier to internal rotation in this compound.