21 -Monoheteroatom-substituted

The first synthetic approach to 21-monoheteroatom-substituted porphyrins involved a "3+1" condensation approach using 2,5-diformyl furan 219 (X=0) or thiophene 219 (X=S) and tripyrrane 220 (Y=NH), to obtain meso-unsubstituted 21-oxa 221 (X=0, Y=NH) or 21-thia 221 (X=S, Y=NH) porphyrins (Scheme 89) (1970JCS(D)807,1971JCS(C)3681,1969JCS(CC)1480). 

The isolation of XIIc was of importance since it demonstrated for the first time that monoheteroatom-substituted carbenes could be stable, indefinitely, at room temperature. The next step was to investigate whether a single electronically active substituent is sufficient to isolate a carbene. 

A controversial issue of heteroatom-stabilized cations is the relative stabilization of carbocationic centers adjacent to oxygen and sulfur.541 In solution studies, a-O-substituted carbocations were found to be stabilized more than a-iS -substituted carbocations.677 Gas-phase studies reached an opposite conclusion,678 679 whereas subsequent theoretical studies (high-level ab initio methods) supported the findings of solution chemistry. Recent results, namely, basicities of various vinylic compounds (365-370) measured in the gas phase also support this conclusion.680 Although monoheteroatom-substituted compounds 365 and 366 were found to have similar proton affinities, an additional a-methyl group increased the stability of the carbenium ion derived from 367 more than that of the sulfur counterpart 368. Even larger differences were found between proton affinities of the bis-heteroatom-substituted compounds 369 and 370. 

At that stage, we found necessary to increase the stability of such monoheteroatom substituted earbene and we investigated the possibility of replacing... 

Like porphyrins, heteroporphyrins are aromatic and confirm the [4n+2] it-elecfron Huckel rule (00MI3). Replacing N with O, S, Se, and Te perturbs jt-delocalization and electronic properties. The alteration in jt-delocalization is evidenced from downfield shifts in H NMR spectra. The extent of shift in monoheteroatom is more than diheteroatom-substituted porphyrins (1979JA7055, 19840MR561). The large heteroatoms... 

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