Design,Synthesis,and Conformational Dynamics of a Gated Molecular Basket

摘要

We have developed a synthesis and examined the conformational behavior and recognition properties of dynamic molecular containers 1-3.As follows from the ~1H NMR dilution,diffusion NMR,and vapor pressure osmometry measurements,compound 1 has a low affinity for intermolecular aggregation and is mostly present in monomeric form in dilute chloroform solutions.Inspecting the O-H chemical shift resonances of 1,3,and model compound 4 as a function of temperature afforded the DELTA delta/DELTA T coefficients of 17.0,17.3,and 4.7 ppb K~(-1),respectively.In combination with the results from variable temperature ~1H NMR and IR measurements,the existence of conformers of 1 and 3 in equilibrium,each having a different extent of hydrogen bonding,was confirmed.Molecular mechanics calculations suggested 1_a as the most favorable conformation,with three additional conformers,1_b,1_c,and 1_d,populating local energy minima.Further optimization of each of the four conformers using semiempirical PM3 and ab initio (HF/6-31G*)methods allowed a determination of their relative free energies and the corresponding Boltzmann population distributions which were heavily weighted toward 1_a.A computed composite IR spectrum of a fraction-weighted mixture of the conformers of 1 reproduced the experimentally observed IR spectrum in its structural features,leading to a conclusion that conformer 1_a indeed dominates the equilibrium.The egg-shaped cavity of 1 (136.6 A~3)is complementary in size,shape,and electrostatic potential to chloroform (74.9 A~3).A single-crystal X-ray study of 2 revealed a disordered chloroform molecule positioned inside the cavitand along its C_3 axis.