Abnormal upfield shifts for the external azulene protons are seen in the 1H NMR spectra on oxidation of azuliporphyrins with copper(II) acetate to give highly distorted copper(II) complexes (see structure, brown Cu, blue N, red O, green H). These undergo demetalation with 10 % trifluoroacetic acid/chloroform to give unique cross-conjugated ketones.
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Colby et al. (2004) studied this question.
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