Stereoelectronic effects of substituent groups in the solid state. Crystal chemistry of some cubanecarboxylic and phenylpropiolic acids

A series of 4-substituted cubanecarboxylic acids and phenylpropiolic acids have been studied with the aim of elucidating steric and electronic factors exerted by the 4-substituent in the formation of the dimer, or alternatively, the rare syn-anti catemer patterns in their respective crystal structures. It is shown that catemer formation depends critically on the ability of a proximal C-H group to form a supportive C-H...O bond. In turn, this means that the C-H group must be sufficiently activated toward hydrogen bond formation. Such activation is inherent to the cubyl group but must be present additionally from a suitable electron withdrawing group in the phenylpropiolic acids. In any event, while C-H activation is necessary for catemer formation it is not sufficient. The substituent group that is present in the 4-position must also be sufficiently bulky so as to form a close packed array that is compatible with the catemer geometry. These trends are justified by the crystal structures of the 12 acids in the two families wherein the 4-substituent group is H, F, Cl, Br, I, and CH3. Our results indicate that electronic and steric effects of functional groups may be distinguished in the solid state, in that the formation of either a dimer or catemer may be rationalized on the basis of these effects.