Synthesis and Reactivity of N-Methyl and N-Phenyl meso-Unsubstituted N-Confused Porphyrins

Abstract

Condensations of 1-methyl and 1-phenyl-2,4-pyrroledicarbaldehydes with a tripyrrane in TFA-dichloromethane, followed by oxidation with aqueous FeCl(3), gave novel cross-conjugated meso-unsubstituted N-confused porphyrins (NCPs; 12). These porphyrin analogues showed significant diatropic ring currents that were enhanced upon protonation. Reactions with nickel(II) acetate in refluxing DMF, or palladium(II) acetate in acetonitrile, gave good yields of the corresponding nickel(II) or palladium(II) organometallic derivatives 18 and 19. These complexes were stable and the proton NMR spectra showed slightly increased downfield shifts to the external protons. Addition of TFA resulted in C-protonation at the internal carbon to give aromatic cations that showed the inner CH resonance between -2.5 and -4.0 ppm. The nickel(II) cations 20a and 20b slowly underwent demetalation but the related palladium cations 20c and 20d were quite robust and showed no loss of palladium after 1 week at room temperature. Reaction of NCPs 12 with silver(I) acetate gave silver(III) derivatives 21a and 21b where an oxidation had occurred at C-3 to afford a lactam unit. The silver complexes showed strong diatropic ring currents and porphyrin-like UV-vis spectra with a Soret band near 430 nm. N-Methyl NCP 21a also reacted with gold(III) acetate to give the gold(III) NCP 21c, albeit in low yield, and this species showed similar spectroscopic properties to silver(III) NCP 21a. Syntheses of N-phenyl NCP 12b were accompanied by the formation of the 3-oxo derivative 15b, and the related N-methyl product 16a could also be obtained when the reaction mixtures were oxidized with silver(I) acetate under acidic conditions. The proton NMR spectra for these aromatic NCPs in CDCl(3) show the internal CH shifted upfield to near -6.5 ppm, while the external meso-protons are strongly deshielded giving 4 singlets between 9 and 10 ppm. This study demonstrates that meso-unsubstituted NCPs have unusual reactivity and unique spectroscopic properties, and these results complement and extend the work on the much better known meso-tetraaryl NCPs.

Enantiomer Resolution of Intrinsically Chiral C21‐Alkylated N‐Confused Porphyrin Complexes

“…It has been shown that the inner carbon may be a target for protonation. [24,38] However, protonation does not secure the configuration stability of C21, which reversibly adopts a pyramidal hybridization of the cation [2ÀH] + , so the enantiomers cannot be separated. On the other hand, in the reaction with haloalkanes a new C À C bond is formed between the coordinated C21 and a carbocation derived from R À X.…”

6-Oxopyriphlorins

“…23,24 During investigations into the synthesis of 2-substituted NCPs 15 by this approach (Scheme 3), it was noted that silver(I) acetate could be used to oxidize the initially formed N-confused phlorin 16 to give a lactam-type derivative 17. 24 In addition, pyrazolecontaining phlorins 18 were similarly shown to be oxidized to the corresponding oxophlorins 19 in the presence of silver(I) acetate (Scheme 3). 25 Based on these observations, we speculated that this methodology could be used to obtain superior yields of oxopyriphlorins 6 (Scheme 4).…”

Metal Complexes of Carbaporphyrinoid Systems

“…[46a] N-Methyl and N-phenyl NCPs 25 have also been prepared, and these readily formed the corresponding Ni II and Pd II complexes 26 and 27 (Scheme 8). [25] Addition of TFA to solutions of these metallo-derivatives also gave aromatic C-protonated species 26H + and 27H + , respectively. A higher concentration of TFA was required to form the protonated palladium complexes 27H + compared to the nickel species 26H + , but the palladium complexes proved to be far more stable under acidic conditions.…”

“…NCP 25 a also reacted with goldA C H T U N G T R E N N U N G (III) acetate to give a low yield of the related goldA C H T U N G T R E N N U N G (III) complex 29. [25] The metalated derivatives demonstrate modified reactivity that enables the synthesis of substituted NCPs. For instance, silverA C H T U N G T R E N N U N G (III) NCP 17 reacted with dimethylamine to give the 21-dimethylamino-NCP 30, [55] while reaction with potassium diphenylphosphide afforded the 21-diphenylphosphanyl-NCP 31 (Scheme 9).…”