CH-PI INTERACTION AS AN IMPORTANT DRIVING FORCE OF HOST GUEST COMPLEXATION IN APOLAR ORGANIC MEDIA - BINDING OF MONOOLS AND ACETYLATED COMPOUNDS TO RESORCINOL CYCLIC TETRAMER AS STUDIED BY H-1-NMR AND CIRCULAR-DICHROISM SPECTROSCOPY

The CH-pi interaction plays an important role in the complexation involving resorcinol cyclic tetramer 1 in chloroform. Thus, host 1 binds simple monools 4-10 via a cooperation of CH-pi and hydrogen-bonding interactions, where the binding constants at 298 K (up to 13 M-1) increase with increasing chain lengths (from ethyl through propyl to butyl) as well as branching in the aliphatic moiety of the guests. In addition, the largest complexation-induced H-1 NMR upfield shift for a bound guest occurs at the terminal methyl group, which must therefore be deeply incorporated in the aromatic cavity of the host. The complexes derived from chiral secondary alcohols 10-17 including terpenes and steroids exhibit induced circular dichroism as a result of guest-to-host chirality transfer which is mediated by the CH-pi interaction. Borneol (14) having three methyl groups to allow a multipoint CH-pi interaction forms a particularly stable complex (K = 54 M-1). The acetyl group in a guest is significantly complexation-promoting. This is primarily due to enhanced CH-pi interaction involving the polarized C-H bonds of the acetyl group. The binding constants for selected guests are K = 35, 12, 4, and 1 M-1 for 3-oxo-1-butanol (simple alcohol having an acetyl group), bornyl acetate (monoacetate having a bulky and highly branched aliphatic moiety), 2,4-diacetoxypentane (simple diacetate), and cholestane (bulky hydrocarbon), respectively.