O-confused oxaporphyrin--an intermediate in formation of 3-substituted 2-oxa-21-carbaporphyrins

Reaction: see text]. A condensation of 2,4-bis(phenylhydroxymethyl)furan with pyrrole and p-toluylaldehyde in the presence of ethanol formed 5,20-diphenyl-10,15-di(p-tolyl)-2-oxa-3-ethoxy-3-hydro-21-carbaporphyrin (H,EtO)OCPH]H2. The new carbaporphyrinoid has 1H NMR features of an aromatic molecule, including the upfield shift of the inner H(21) atom (-5.48 ppm). Addition of acid removes the ethoxy substituent and converts (H,EtO)OCPH]H2 into the dication of "true" O-confused oxaporphyrin {(H)OCPH]H3}2+ via an exocyclic C(3)-O bond cleavage followed by an elimination of the ethoxy group as determined by 1H NMR. Addition of ethanol, water, or pyrrole converts {(H)OCPH]H3]2+ into (H,EtO)OCPH]H2, (H,OH)OCPH]H2, or pyrrole appended O-confused porphyrin (H,pyrrole)OCPH]H2, respectively. The reaction of (H,OEt)OCPH]H2 with silver(I) acetate yields a stable Ag(III) complex (H,OEt)OCP]AgIII substituted at the C(3) position by the ethoxy group and hydrogen. Coordination through the nitrogen donors is reflected by the presence of 107/109Ag scalar splitting seen for the selected -H pyrrolic signals. Addition of TFA to (H,OEt)OCP]AgIII produces a weakly aromatic O-confused porphyrin complex {(H)OCP]AgIII}+. In the course of this reversible process the tetrahedral-trigonal rearrangements originate at the C(3) atom but extend its consequences on the whole structure. Insertion of silver into the hydroxy analogue of (H,OEt)OCPH]H2, i.e., (H,OH)OCPH]H2, was accompanied by ligand oxidation, yielding carbaporpholactone which contains the lactone functionality instead of the regular furan moiety embedded in the carbaporphyrin ligand of (O)OCP]AgIII. The structure was determined by X-ray crystallography. The macrocycle is only slightly distorted from planarity, and silver(III) is located in the NNNC plane.