Using pentafluorophenyl as a Lewis acid to stabilize a cis secondary amideconformation

[GRAPHICS] N-(Pyrimidin-2-yl)pentafluorobenzamide adopts a cis amide bond in the solid state with a pyrimidyl nitrogen pointing toward the center of the perfluor ophenyl ring. In solution, the compound is a mixture of cis and trans rotam ers, The conformational equilibrium is strongly solvent dependent, and the cis rotamer is entropically favored. It is proposed that the entropic drivi ng force is decreased solvation of the two aryl groups when in the cis conf ormation.