<i>O</i>-Confused OxaporphyrinAn Intermediate in Formation of 3-Substituted 2-Oxa-21-Carbaporphyrins
作者:Miłosz Pawlicki、Lechosław Latos-Grażyński
DOI:10.1021/jo051141s
日期:2005.11.1
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]H-2. The new carbaporphyrinoid has H-1 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]H-2 into the dication of "true" O-confused oxaporphyrin [(H)OCPH]H-3}(2+) via an exocyclic C(3)-O bond cleavage followed by an elimination of the ethoxy group as determined by H-1 NMR. Addition of ethanol, water, or pyrrole converts [(H)OCPH] H-3](2+) into [(H,EtO)OCPH]H-2, [(H,OH)OCPH]H-2, or pyrrole appended O-confused porphyrin [(H,pyrrole)-OCPH]H-2, respectively. The reaction of [(H,OEt)OCPH]H-2 with silver(I) acetate yields a stable Ag(III) complex [(H,OEt)OCP]Ag-III substituted at the C(3) position by the ethoxy group and hydrogen. Coordination through the nitrogen donors is reflected by the presence of Ag-107/109 scalar splitting seen for the selected beta-H pyrrolic signals. Addition of TFA to [(H,OEt)OCP]Ag-III produces a weakly aromatic O-confused porphyrin complex [(H)OCP]Ag-III}(+). 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]H-2, i.e., [(H,OH)OCPH]H-2, 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]Ag-III. 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.