The Mannich reaction is detailed, which was carried out on benzoxazine dimers under various conditions, that is, temperature, reaction time, and solvents. Against our expectation, in any condition, instead of generating a disubstitution oxazine compound, the reaction gives a product with only a single oxazine ring, a mono-oxazine benzoxazine dimer, as characterized by FT-IR, 1H NMR, 13C NMR, 2D-NMR (1H-1H COSY, 1H-13C HMQC, and 1H-13C HMBC), and EA. The asymmetrical reaction is found to be based on the original structure of the benzoxazine dimer which has two phenol rings in a different stability as clarified by X-ray structure analysis of the single crystal. All types of benzoxazine dimers indicate the specific structure with a pair of inter- and intramolecular hydrogen bonds. The bond distance indicates that the intramolecular hydrogen bonding is very strong, while the packing structure emphasizes the high stability of the dimer unit and implies the deactivation of one phenol ring in the benzoxazine dimer. In this contribution, we demonstrate one of the quite rare examples, showing how the stereostructure of the reactant molecule is an important factor to control the reaction and give an asymmetric product which we never expected when considering only the chemical formula.
in Wiley InterScience (www.interscience.wiley.com).The ring opening polymerizations of p-substituted phenol-based benzoxazines are self-terminated as soon as dimers form. The polymerization of benzoxazine monomers does not proceed according to the theoretical mechanism even though the conditions, temperature, molar ratio, solvent polarity, and reactant ratio are varied. The speculated mechanism, involving the unique structure of a dimer with interand intramolecular hydrogen bonds, is applied to explain an obstructive effect on ring opening polymerization. In this article, we clarify an important case which the stereo structure of the compound controls the reaction and prevents the polymerization expected from the theoretical mechanism.
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