Crystal Chemistry of Some (Alkoxyohenyl)propiolic Acids. The Role of Oxygen and Hydrogen Atoms in Determining Stack Structures of Planar Aromatic Compounds

Article abstract of DOI:10.1021/ja00195a040

Crystal structures of planar oxygenated aromatic compounds are determined by an interplay of C...H interactions which steer to herringbone structures, characterized by adjacent inclined molecules, and C-H...O interactions, which being lateral, steer to stacked-sheet structures with short axes of 3.8-4.2 Angstroem.The number of hydrogen and oxygen atoms in the molecule seems to determine which of these two preferences is exercised, a smaller number of hydrogens and a greater number of oxygens leading to the stack structure.Accordingly, if an alkoxycinnamic acid has a stack structure, the corresponding phenylpropiolic acid will, in all likelihood, have the same structure.This is exemplified by the pair of nearly isomorphous compounds 3,4-(methylenedioxy)cinnamic acid and <3,4-(methylenedioxy)phenyl>propiolic acid.The structure of the latter is triclinic, P<*>, Z = 2, a = 3.807(2) Angstroem, b = 10.297(2) Angstroem, c = 10.995(3) Angstroem, α = 84.07(2) deg, β = 96.46(3) deg, γ = 98.13(3) deg.Even if a cinnamic acid does not have a 4-Angstroem structure, the removal of two hydrogen atoms to give the propiolic acid could result in the stack structure for the latter.Accordingly, (3,4-dimethoxyphenyl)propiolic acid is triclinic, P<*>, Z = 2, a = 3.891(1) Angstroem, b = 11.361(3) Angstroem, c = 12.089(4) Angstroem, α = 112.50(2) deg, β = 92.53(3) deg, γ = 96.12(3) deg.A large number of oxygen atoms acting as C-H...O bond acceptors can lead to a stack structure for significantly nonplanar molecules.An example is (3,4,5-trimethoxyphenyl)propiolic acid which is monoclinic, P2₁/n, Z = 4, a = 20.806(6) Angstroem, b = 14.159(3) Angstroem, c = 3.942(5) Angstroem, β = 94.79(7) deg.In contrast, (4-methoxyphenyl)propiolic acid, which does not have the critical number of oxygen atoms, does not have a 4-Angstroem stack structure being triclinic, P<*>, Z = 2, with a = 10.767(3) Angstroem, b = 8.494(3) Angstroem, c = 7.499(2) Angstroem, α = 99.01(2) deg, β = 125.62(2) deg, γ = 112.42(2) deg.The crystal structures of these and other phenylpropiolic acids are closely paralleled by their solid-state thermal reactivities.Acids with a 4-Angstroem stack structure participate in an intermolecular Diels-Alder reaction to give derivatives of 1-phenylnaphthalene-2,3-dicarboxylic acid anhydride, while those with short axes greater than 4.2 Angstroem are unreactive.The products of these reactions may be rationalized by assuming that adjacent double and triple bonds, within a threshold distance of ca. 4.5 Angstroem in the crystal, are potentially reactive.In general, any crystalline phenylpropiolic acid may be expected to form Diels-Alder products upon heating if the triple bonds are sufficiently close for topochemical reaction.The formation of these lignan derivatives from acetylenic precursors under mild conditions could be of biosynthetic significance.