HOST-GUEST COMPLEXATION .58. GUEST RELEASE AND CAPTURE BY HEMICARCERANDS INTRODUCES THE PHENOMENON OF CONSTRICTIVE BINDING

Two rigidly hemispherical cavitands containing three out of four possible phenolic hydroxyl groups regularly spaced on their rims were shell closed by their reactions with three molecules of CH2BrCl and K2CO3 in (CH3)2NCHO, (CH3)2NCOCH3, or (CH3)2SO to give hemicarceplexes 1.(CH3)2NCHO (20%), 1.(CH3)2NCOCH3 (42%), or 1.(CH3)2SO (51%), respectively. The crystal structure of 1.(CH3)2NCHO.2CH3CN.2CHCl3 shows the (CH3)2NCHO guest to be firmly lodged within the cavity, the other molecules acting as solvates. The guest's carbonyl group is pointed toward the portal connecting the inner and outer phases. Upon heating in solvents too large to occupy the cavity, these complexes released their guests to give free hemicarcerand 1. The t1/2 values for decomplexation in 1,2,4-Cl3C6H3 at 140-degrees-C for 1.(CH3)2NCHO and 1.(CH3)2NCOCH3 were 14 and 34 h, respectively, and for 1.(CH3)2SO at 195-degrees-C, 24 h. When 1 was dissolved in the presence of small molecules such as CH3CN, CS2, CH2Cl2, or CH2Br2, new hemicarcerands 1.CH3CN, 1.CS2, 1.CH2Cl2, and 1.CH2Br2, respectively, were formed, isolated, and characterized. Complexes 1.O2, 1.N2, 1.H2O, and 1.Xe in CDCl3 solution were prepared and detected by the effect of guest on the H-1 NMR signals of the host. Only 1.Xe was kinetically stable and subject to isolation and characterization. The association constant of 1 with N2 in CDCl3 at 22-degrees-C was estimated to be approximately 180 M-1, with O2 to be approximately 44 M-1, and with Xe to be approximately 200 M-1. The t1/2 value for 1.Xe dissociating in CDCl3 at 22-degrees-C was 47 h. Formation of 1.C6H6, 1.(CH2)4O, and 1.C5H5N required days of refluxing 1 in guest as solvent, and proceeded to 80, 100, and 100% completion, respectively. Heating 1 in a 5:1 solution of C6H5Cl-Et2NH at 65-degrees-C (4 h) gave a 70% conversion to 1.Et2NH. Heating 1 in 4:1 solutions of C6H5Cl-CH3(CH2)3NH2 at 105-degrees-C (24 h) gave a 90% conversion to 1.CH3(CH2)3NH2. No hemicarceplexes were formed when 1 was heated at reflux in CHCl3, C6H5Cl, C6H5CH3, (CH3)2CHC6H5, or 1,2,4-Cl3C6H3. The H-1 NMR signals of incarcerated guests were moved upfield by 1-4 ppm while the Xe-129 signal of 1.Xe was moved upfield 101 ppm compared to free guest dissolved in CDCl3. We suggest the term constrictive binding to describe the steric forces that must be overcome for decomplexation of hemicarceplexes whose guest cross sectional sizes exceed those of the host's portals.