Here,
aiming to adopt the phenyl–perfluorophenyl interaction
to regulate molecular alignment and arrangement for crystal engineering,
we examined and compared in detail the crystal structures of N,N′-diphenylurea compounds 1–6. We found that phenyl–perfluorophenyl
interaction greatly influenced the intermolecular arrangement in the
crystal, and we were able to prepare a cocrystal of 1 and 2, in which the molecules were alternately arranged
under the control of the phenyl–perfluorophenyl interaction.
This arrangement was driven by the asymmetric geometry of the hydrogen
bonds in the cocrystal (1·2), in which 2, bearing two perfluorophenyl groups, worked as a better hydrogen
bond donor. In contrast, NH connected to the phenyl group in 3 proved to be a better hydrogen bond donor due to the intramolecular
resonance effect. N,N′-Dimethylated
derivatives, 4–6, existed in cis-cis form in the crystal. Antiparallel
carbonyl–carbonyl arrangements were observed in 4 and 6, while an unexpected carbonyl–perfluorophenyl
interaction was observed in the crystal of 5. These findings
will be helpful in the design of diphenylurea-based functional molecules,
especially for solid-state application.
Activated amide bonds have been attracting intense attention; however, most of the studied moieties have twisted amide character. To add a new strategy to activate amide bonds while maintaining its planarity, we envisioned the introduction of an alkynyl group on the amide nitrogen to disrupt amide resonance by nN→Csp conjugation. In this context, the conformations and properties of N‐ethynyl‐substituted aromatic amides were investigated by DFT calculations, crystallography, and NMR spectroscopic analysis. In contrast to the cis conformational preference of N‐ethyl‐ and vinyl‐substituted acetanilides, N‐ethynyl‐substituted acetanilide favors the trans conformation in the crystal and in solution. It also has a decreased double bond character of the C(O)−N bond, without twisting of the amide. N‐Ethynyl‐substituted acetanilides undergo selective C(O)−N bond or N−C(sp) bond cleavage reactions and have potential applications as activated amides for coupling reactions or easily cleavable tethers.
Amide-based molecular switches had its limitation on structural diversities. In this work, we designed and synthesized a series of pentafluorobenzoyl-based benzanilide compounds. The conformational ratio of these compounds in solution was correlated linearly with Hammett's σ p value of the substituent on the anilide ring, reflecting the repulsive interaction between the carbonyl group and the electron-rich aryl group. The addition of acid into the solution of 6, bearing pentafluorobenzoyl group, switched the stable amide conformation. In addition, the sizeable rotational barrier of 6 induced by the pentafluorobenzoyl moiety enabled us to monitor the conformational transition by means of 1 H NMR spectroscopy.
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