Modes of xanthine complexation to dirhodium tetrakis[(R)-alpha-methoxy-alpha(trifluoromethyl)-phenylacetate] in solution and in the solid state

It is shown by IR and NMR studies that the xanthines 1-5 prefer a side-on c omplexation to the chiral dirhodium tetrakis [(R)-alpha -methoxy-alpha-(tri fluoromethyl)phenylacetate] (Rh*) in solution whereas carbonyl groups are i nvolved in the solid state. For 6, at least the carbonyl group C-6 contribu tes to complexation in solution as well. Alternating strands of 6 and Rh* e xist in the solid state as revealed by X-ray diffraction analysis described in detail. The determination of enantiomeric excess of the chiral xanthine 6 can easily be accomplished by the "dirhodium method" (H-1 and C-13 NMR i n the presence of Rh*).