The radical cation 2 is also able to attack the C-N double bond in imines in an intermolecular fashion, a reaction not possible for photochemical 1,3-dipolar cycloaddition.16] The reaction of azirines with imines leads to the formation of the N-substituted imidazoles.The course of the reaction is as follows: Addition of the radical cation 2 to the imine is followed by ring closure to form a dihydroimidazole. Under these reaction conditions the dihydroimidazole is not stable and undergoes subsequent aromatization, so that, even in the crude product, no dihydroimidazole is detected. Huisgen[71 did not find a dihydroimidazole under comparable conditions either. The reaction of azirines with imines offers a ready approach for the synthesis of N-substituted imidazoles with a wide variety of substituents.Since the C-N double bonds of the imine and the azirine compete for reaction, the yields of imidazoles vary rather widely ( Table 1). The remaining products are the azirine
Formation constants for the hydrogen-bonded complexes between phenol (donor) and selected carbonyl or phosphoryl compounds (acceptors) were determined from IR Δν shifts. The P=O group is about two orders of magnitude stronger as an acceptor than the C=O group, but no evidence for the enolization of a β-diphosphoryl system was found. The IR absorption band of the phosphoryl group (νP=O) showed weak response to the involvement in the hydrogen bonding, and the shifts of the donor molecule (νOH) provide a better measure of the formation constants. Key words: carbonyl and phosphoryl groups as hydrogen bond acceptors, enolization of a β-difunctional system, determination of Kassoc from the IR Δνshifts.
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