Influence of peripheral groups on the physical and chemical behavior of cinchona alkaloids.

While cinchona alkaloids play a key role in many applications, from medicine to catalysis, there is not yet a complete understanding of the reasons for their unique chemical behavior. Past studies have identified the chiral pocket formed by the two main constituting moieties of the cinchona, the quinoline and quinuclidine rings, as the main factor determining their physiological and enatioselective reactivity. That explanation, however, does not account for the differences observed among similar cinchona alkaloids. Here we show that subtle changes in the position of the substituent groups outside the central chiral pocket explain the disparities observed in basic physicochemical properties between pairs of near-enantiomers (quinine vs quinidine, cinchonidine vs cinchonine) such as crystal structure, solubility, and adsorption equilibrium. Both energetic and entropic factors need to be considered to fully account for the trends observed.

Effect of protonation on the conformation of cinchonidine.

The protonation of cinchona alkaloids in solution leads to the severe restriction of their internal rotational degrees of freedom and to the locking of the molecule around a specific conformation held in place by a bridging counterion of the acid used for protonation. For HF, direct interactions were detected by NMR between the fluorine anion and not only with the acidic hydroxo group but also with non-acidic hydrogen atoms in the quinoline ring.

Dibromo[5,10,15,20-tetrakis(4-methoxyphenyl)porphyrinato-kappa 4N]platinum(IV) chloroform acetonitrile solvate.

In the title compound, [PtBr(2)(C(48)H(36)N(4)O(4))].0.896CHCl(3).0.569CH(3)CN, the centrosymmetric metal complex is octahedral, with the porphyrin ring essentially planar and the Br atoms occupying axial positions. The two independent methoxyphenyl substituents are tilted at angles of 89.0 and 67.0 degrees with respect to the porphyrin plane. The Pt-N distances are 2.035 (4) and 2.036 (4) A and the Pt-Br distance is 2.4666 (6) A. Chloroform and acetonitrile solvent molecules, exhibiting substantial disorder, occupy positions between the porphyrin molecules. This is the first crystal and molecular structure of a Pt(IV)-porphyrin complex to be reported.