Inter-action between maleic acid and N- R-furfuryl-amines: crystal structure of 2-methyl- N-[(5-phenyl-furan-2-yl)meth-yl]propan-2-aminium (2 Z)-3-carb-oxy-acrylate and N-[(5-iodo-furan-2-yl)meth-yl]-2-methyl-propan-2-aminium (2 Z)-3-carb-oxy-prop-2-enoate

Abstract

The title mol-ecular salts, C₁₅H₂₀NO⁺·C₄H₃O₄^-, (I), and C₉H₁₅INO⁺·C₄H₃O₄^-, (II), have very similar mol-ecular geometries for both cation and anion. The anions of both (I) and (II) are practically planar (r.m.s. deviations = 0.062 and 0.072 Å, respectively) and adopt a rare symmetrical geometry with the hy-droxy H atom approximately equidistant from the two O atoms. In their crystals, the cations and anions in both (I) and (II) form tight ionic pairs via strong N-H⋯O hydrogen bonds, with a roughly perpendicular disposition of the anion to the furan ring of the cation. This ion-pair conformation appears to correlate with the lack of reactivity of these salts in [4 + 2] cyclo-addition reactions. In the extended structures of (I) and (II), the ion pairs form hydrogen-bonded chains propagating along [010] and [001], respectively, via N-H⋯O hydrogen bonds.

Keywords: Diels–Alder reaction; crystal structure; furans; maleates; synchrotron radiation.

Figure 1

Synthesis of maleic salts (I) and (II) from N-[(5-R-furan-2-yl)meth­yl]-2-methyl­propan-2-amines.

Figure 2

The attempted thermal cyclization of salts (I) (R = Ph) and (II) (R = I).

Figure 3

The mol­ecular structure of salt (I). Displacement ellipsoids are shown at the 50% probability level. H atoms are presented as small spheres of arbitrary radius. Dashed lines indicate the intra­molecular O—H⋯O and inter­molecular N—H⋯O hydrogen bonds.

Figure 4

The mol­ecular structure of salt (II). Displacement ellipsoids are shown at the 50% probability level. H atoms are presented as small spheres of arbitrary radius. Dashed lines indicate the intra­molecular O—H⋯O and inter­molecular N—H⋯O hydrogen bonds.

Figure 5

The crystal structure of (I), illustrating the hydrogen-bonded chains propagating along [010]. Dashed lines indicate the intra­molecular O—H⋯O and inter­molecular N—H⋯O hydrogen bonds.

Figure 6

The crystal structure of (II), illustrating the hydrogen-bonded chains propagating along [001]. Dashed lines indicate the intra­molecular O—H⋯O and inter­molecular N—H⋯O hydrogen bonds.