CHOLIC-ACID INCLUSION-COMPOUNDS WITH AROMATIC KETONE GUESTS - STRUCTURE AND REACTION-KINETICS

The crystal structure of the inclusion compound formed with host choli c acid and guest propiophenone indicates different packing from that s een in other CA.aromatic guest compounds. Space group P2(1), a = 16.79 0(1), b = 7.928(5), c = 12.262(3) Angstrom, beta = 114.25(2)degrees, Z = 2, D-c = 1.211 g.cm(-3), R = 0.0515 for 2145 independent reflection s. The kinetics of decomposition of the inclusion compounds of the inc lusion compounds of CA with acetophenone and propiophenone are present ed and mechanism and rate of decomposition compared and contrasted. Th e steroidal bile acid (3 alpha, 7 alpha, 12 alpha-trihydroxy-5 beta-ch olan-24-oic acid), commonly known as cholic acid (CA), forms inclusion compounds with a wide range of guest molecules. The crystal structure s of the inclusion compounds with water(1,2,3) and alcohol guests(4,5, 6) exhibit extensive host-host and host-guest hydrogen bonding. The in clusion compound of CA with acetone and three water molecules is compr ised of hydrogen bonded bilayers of host and guest which pack together resulting in a close packed structure(7). All water molecules and hos t and guest hydroxyl and carbonyl groups are involved in intermolecula r hydrogen bonds of which there are ten per asymmetric unit. Apart fro m the above examples and an interesting structure of CA with m-fluoro- aniline, which exhibits an unusual 1:2 host:guest ratio and peculiar p acking arrangements, most CA inclusion compounds form tubulate clathra te type structures. Puckered host steroid bilayers pack together such that channels are formed in which the guest molecules are trapped with out benefit of short range host-guest interactions. Bilayers result fr om the head to tail and alpha-face to alpha-face hydrogen bonding of t he host while adjacent bilayers are bound by van der Waals forces. Gue st molecules included in such fashion encompass lactones,(9,10) benzen e(11) and benzene derivatives,(12,13) aliphatic ketones(14) and esters (15). We have in the past considered the phases occurring during therm al decomposition of various CA inclusion compounds(7) and the differen t packing modes and subtle guest responsive changes in host conformati on and packing occurring(15). More recently we have undertaken study o f the rates and mechanisms of decomposition of CA tubulate clathrates with substituted aromatic guests(16) as well as investigated the solid state reactivity of CA and its O(28) methyl ester, methyl cholate (MC )(17). We now present the crystal structure of the 1:1 inclusion compo und formed by CA with propiophenone (CAPR) and consider the complex th ermal decomposition modes of this com pound and that formed by CA with acetophenone(13) (CAACET) both with respect to rate and mechanism of reaction. Kinetic parameters are derived and presented for comparison.