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DDQ-Supported Alkoxylation of 2‑Aza-21-carbaporphyrin and Noncatalyzed Transetherification of Its 3,21-Dialkoxy Derivatives

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posted on 21.09.2012, 00:00 by Xiaofang Li, Bin Liu, Xin Xu, Piotr J. Chmielewski
An oxidative addition of primary alkoxyls into two sites of N-confused porphyrin (NCP) has been accomplished by means of alcohols in the presence of a stoichiometric amount of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The resulting aromatic monocationic species 1-R3 (R = Me, Et) were characterized by the NMR and, in the case of triethoxy-NCP, by monocrystal X-ray diffraction analysis. One alkoxy group is located in position 3, on the macrocycle’s perimeter, while two −OR moieties are attached to the internal carbon (position 21) of the confused pyrrole. The 3-EtO-21-Cl-NCP 2, which is formed as a byproduct, was also structurally characterized by means of X-ray diffraction. Reduction of 3-RO-21-(RO)2-NCP with hydrazine hydrate gave selectively a neutral and intrinsically chiral 3,21-bis­(alkoxy)­NCP 3-R2. Dealkylation of the externally bonded 3-OR fragment under basic conditions leading to a zwitterionic aromatic 3-oxo-species 4-R2, which still possesses the internal ketal functionality, was established by the NMR and X-ray diffraction methods. An unprecedented transetherification for the internal alkoxyl of 3-R2 can be achieved under very mild conditions and without catalyst. One of the alkoxyl-exchange products, i.e., 3-ethoxy-21-methoxy-NCP, was characterized by the X-ray diffraction method. The substitution proceeds via an associative (SN2) mechanism resulting in an inversion of the chirality of 3-RR′, which was shown by means of the NMR and chirooptical methods.