Synthesis of 2-substituted pentacyclo[6.3.0.02,6.03,10.05,9]undecanes (D3-trishomocubanes) and a study of their 13C 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.