Interaction between maleic acid and N-R-furfurylamines: crystal structure of 2-methyl-N-[(5-phenylfuran-2-yl)methyl]propan-2-aminium (2Z)-3-carboxyacrylate and N-[(5-iodofuran-2-yl)methyl]-2-methylpropan-2-aminium (2Z)-3-carboxyprop-2-enoate

The molecular and crystal structures of two N-(furylmethyl)propan-2-aminium salts – the products of interaction between maleic acid and N-R-furfurylamines – were studied by X-ray diffraction and correlated with their lack of reactivity in [4 + 2] cycloaddition reactions.


Chemical context
Owing to the fact that the furan ring contains a system of conjugated double bonds, it usually acts as an effective diene in intra-and intermolecular Diels-Alder reactions with electron-deficient dienophiles. The [4 + 2] cycloaddition of furans with maleic acid leading to structurally diverse 7-oxabicyclo-[2.2.1]heptenes has been investigated for a long time (Diels & Alder, 1931;Berson & Swidler, 1953, 1954Eggelte et al., 1973;Sprague et al., 1985). However, there are only fragmentary data concerning the reactions of halogen-or aryl-substituted furans with maleic acid (Sheinkman et al., 1972;Shih et al., 1975). It is known that the interaction between maleic acid and furfurylamines leads usually to the formation of the salts, but is not accompanied by the [4 + 2] cycloaddition (Clitherow, 1983;Price et al., 1985;Brown, 1986;Pelosi et al., 2002;Craig et al., 2008;Metsger et al., 2010).
The main goal of this work was to study the cycloaddition reaction between 5-R-furfuryl-tert-butylamines and maleic acid. The interaction between the corresponding amines and maleic acid at room temperature leads to the salts (I) and (II) only (Fig. 1). Unexpectedly, attempts to achieve thermal cyclization of salts (I) and (II) did not result in isolation of the ISSN 2056-9890 targeted 7-oxabicyclo[2.2.1]heptenes: the initial maleates remained unchanged at temperatures up to 413 K (Fig. 2). In order to explain this fact by an understanding of their stereochemical features, an X-ray diffraction study of compounds (I) and (II) was undertaken. , represent secondary amine salts of maleic acid and have very similar molecular geometries (Figs. 3 and 4) for both cation and anion. The saturated C2-C1-N1-C(t-Bu) backbone of the ammonium cation is twisted by 72.66 (7) and 63.2 (2) relative to the furan ring in (I) and (II), respectively. The phenyl substituent of the cation in (I) is almost coplanar to the furan ring (r.m.s. deviation is 0.006 Å ). The anions of (I) and (II) are practically planar (r.m.s. deviations are 0.062 and 0.072 Å , respectively). It interesting to note that the hydrogen atom of the hydroxy group of the Synthesis of maleic salts (I) and (II) from N-[(5-R-furan-2-yl)methyl]-2methylpropan-2-amines.

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-HÁ Á ÁO and intermolecular N-HÁ Á ÁO 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-HÁ Á ÁO and intermolecular N-HÁ Á ÁO hydrogen bonds.
anion is arranged at almost equal distances from the two oxygen atoms in both (I) and (II) (Tables 1 and 2, Figs. 3 and 4). Thus, the anions of (I) and (II) adopt a rare symmetrical geometry.
Importantly, the cations and anions in both (I) and (II) form tight ion pairs via strong N1-H1AÁ Á ÁO2 hydrogen bonds (Tables 1 and 2 ,Figs. 3 and 4). Within the tight ion pairs, the anion is roughly perpendicular to the furan ring of the cation, the interplanar angles being 72.01 (4) and 67.94 (12) in (I) and (II), respectively. Apparently, the formation of the robust tight ion pairs with a definite cation-anion conformation inhibits the desired cyclization reaction, preventing the closure of the cations and anions.

Supramolecular features
Despite the sterically different substituents at the furyl ring of the aminium cations, compounds (I) and (II) organize similar supramolecular structures in the solid state. So, in the crystal of (I), the tight ion pairs form hydrogen-bonded chains propagating along [010] via strong N1-H1BÁ Á ÁO4 links (Table 1, Fig. 5). In the crystal of (II), the analogous hydrogenbonded chains propagate along [001] ( Table 2

Figure 6
The crystal structure of (II), illustrating the hydrogen-bonded chains propagating along

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. X-ray diffraction studies for (II) were carried out on the 'Belok' beamline of the National Research Center "Kurchatov Institute" (Moscow, Russian Federation).
The hydrogen atoms of the amino and hydroxy groups were localized in a difference-Fourier map and refined isotropically with fixed displacement parameters [U iso (H) = 1.2U eq (N) and 1.5U eq (O)]. All other hydrogen atoms were placed in calculated positions with C-H = 0.95-0.99 Å and refined using the riding model with fixed isotropic displacement parameters [U iso (H) = 1.5U eq (C) for the CH 3 groups and 1.2U eq (C) for all other atoms].    (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.31 e Å −3 Δρ min = −0.26 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.