A transient form of 2-aza-21-carbaporphyrin prearranged for fusion: DFT studies

The formation mechanism of the fused porphyrin (CTPPH-NF) involves a rotati on of the N(2) pyrrole ring of 2-aza-21-carba-5,10,15,20-tetraarylporphrin (CTPPH2) which is subsequently followed by formation of the C(3)-N(24) bond to yield the macrocycle with the fused pyrrole tripentacyclic ring. To app roach the problem of the relative stability of the prearranged transient sp ecies theoretical investigations have been performed applying density funct ional theory (DFT). The molecular structures and electronic energy have bee n studied for idealized 2-aza-21-carbaporphyrin (CPPH2) and porphyrin (PH2) macrocycles created by a replacement of phenyl or other substituents with hydrogen or methyl groups. The following forms of 2-aza-21-carbaporphyrin a nd porphyrin were studied: CPH2-I, 2-NH-CPH-I and PH2-I in relation to CPH2 -P, 2-NH-CPH-P and PH2-P, respectively (P indicates regular geometry (N(21) or C(21) located in the inner perimeter) and I indicates inverted geometry (N(21) or C(21) located in the outer perimeter). A pi delocalization exists through the inverted macrocycles: PH2-I, CPH2-I and 2-NH-CPH-I. The B3LYP/6-31G** optimized bond distances of I macrocycles reproduce the pattern of the P counterparts. The B3LYP/6-31G**//B3LYP/6-31 G** calculated energy differences between the P and I structures are very s imilar for two considered 2-aza-21-carbaporphyrin tautomers: CPH2-P --> CPH 2-I, 18.50 kcal mol(-1), 2-NH-CPH-P --> 2-NH-CPH-I, 17.55 kcal mol(-1); but essentially different for the regular porphyrin PH2-P --> PH2-I, 45.56 kca l mol(-1). The methyl substitution at the 21-carbon or 5 and 20 meso positi ons preserved the order of stability as the calculated energy differences e qual 21-CH3-CPH2-P --> 21-CH3-CPH2-I, 13.39 kcal mol(-1); 5,20-CH3-CPH2-P - -> 5,20-CH3-CPH2-I, 17.87 kcal mol(-1). Copyright (C) 2001 John Wiley & Son s, Ltd.