MOLECULAR RECOGNITION .15. MOLECULAR RECOGNITION AND STEREOSELECTIVITY - GEOMETRICAL REQUIREMENTS FOR THE MULTIPLE HYDROGEN-BONDING INTERACTION OF DIOLS WITH A MULTIDENTATE POLYHYDROXY MACROCYCLE

Resorcinol-dodecanal cyclotetramer 1 in CDCl3 forms hydrogen-bonded, 1/1 complexes with cyclohexanediols as well as with 2,4-pentane- and 2,5-hexanediol as their open-chain analogues and cyclohexanol and cis- and trans-4-tert-butylcyclohexanol. The affinities to 1 of cyclic diols (K = (1.1-10) X 10(2) M-1 at 25-degrees-C) are significantly larger than those of open-chain diols (36-43 M-1) and monools (8-11 M-1). Those of regio- and stereoisomers of cyclohexanediol depend on the configuration (axial-equatorial > diequatorial) and relative positions (1,4 >> 1,2 > 1,3) of the two OH groups involved and decrease in the order cis-1,4 (K = 1.04 X 10(3)) > cis-1,2 (2.664 X 10(2)) > trans-1,3 (1.81 X 10(2)) > trans-1,4 (1.29 X 10(2)) > cis-1,3 (1.24 X 10(2)) > trans-1,2 (1.06 X 10(2) M-1); the stereoselectivities are thus cis-1,4/trans-1,4 = 8.0, cis-1,2/trans-1,2 = 2.5, and trans-1,3/cis-1,3 = 1.5. The selectivities in the diol binding are discussed in terms of multiple hydrogen bonding of diol and 1. The relatively large binding constant (K) for cis-1,4-diol with one axial and one equatorial OH group is attributed to an effective and simultaneous two-point hydrogen bonding of the two OH groups with two adjacent binding sites of 1 as a multidentate host.