SYNTHESIS OF 2-SUBSTITUTED PENTACYCLO[6.3.0.0(2,6).0(3,10).0(5,9)]UNDECANES (D3-TRISHOMOCUBANES) AND A STUDY OF THEIR C-13 SPIN-LATTICE RELAXATION-TIMES IN SOLUTION

D3-Trishomocubane (THC, 1a) and four 2-substituted derivatives (THC-X, X = CO2H, CO2Me, CH2OH, C(O)NH2, 1b-e, respectively) have been synthe sized. C-13 NMR spin-lattice relaxation times (T1) and nuclear Overhau ser enhancements of various skeletal carbon atoms in each of these fiv e cage molecules have been obtained experimentally. In each case the r esults thereby obtained were used to determine the molecular perpendic ular and parallel reorientational diffusion coefficients (D(perpendicu lar-to) and D(parallel-to)) in CHCl3 solution as a function of tempera ture. Molecular tumbling rates (D(perpendicular-to)) in all of the der ivatives are markedly longer than in the parent hydrocarbon. A compari son 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( parallel-to) in THC-CO2H, THC-CO2Me and THC-C(O)NH2 are equal to the c orresponding values in the parent compound, thereby indicating the abs ence of any significant barrier to rotation about the C(2)-X bond in t hese 2-substituted THCs. A slower spinning rate is observed in THC-CH2 OH, which is attributed to the existence of a steric barrier to intern al rotation in this compound.