Phlorins Bearing Different Substituents at the sp³-Hybridized Meso-Position

Abstract

Phlorins bearing different substituents at the sp(3)-hybridized meso-position were investigated. The extent to which different substituents at this unique position can influence phlorin spectroscopic properties, structure, and stability is of interest given that such substituents are not in direct conjugation with the phlorin macrocycle. While the effect of various substituents at the sp(2)-hybridized positions has been the subject of prior investigations, the impact of different substituents at the saturated carbon atom has not been systematically examined. In this study, phlorins with different combinations of geminal methyl and phenyl substituents were prepared in yields of 24-49% via dipyrromethane + dipyrromethanedicarbinol routes, and their NMR spectra, UV-vis spectra, X-ray crystal structures, and stability toward light and air were compared. The nature of the substituents at the sp(3)-hybridized position was found to impact spectroscopic properties, structure, and stability to varying degrees. Thus, the choice of substituents at the sp(3)-hybridized meso-position provides a further option for altering phlorin properties.

Synthesis and Aromaticity of Benzene‐Fused Doubly N‐Confused Porphyrins

“…3.42-3.43 ,w hicha re typical for porphyrins. [18] ORTEP drawings of 1a,M 1o f2a,a nd M2 of 2c are illustrated in Figure 3a sr epresentative structures.…”

Mechanical Trapping of the Phlorin Intermediate

“…In principle, phlorins can be prepared by two major strategies: (1) starting from porphyrin, which can be attacked by nucleophiles-for example, alkyl lithium reagents or hydride-at its meso-position [13][14][15][16] and (2) starting from acyclic (oligo)pyrrole building blocks and aldehydes or ketones employing an acid-catalyzed condensation followed by the oxidative dehydrogenation of intermediates. 1,[17][18][19][20][21] However, native phlorins with unmodified core-structures are highly unstable and readily oxidized, 9,19,22,23 making it difficult to isolate them. To stabilize phlorins during their synthesis, chemists have proposed five methods: (1) introducing bulky substituents, for example, mesityl and phenyl, at the meso-sp 3 -carbon atom to hinder oxidation 1,13,18,19,22,[24][25][26][27] and electron-withdrawing substituents, for example, pentafluorophenyl, 1,19,24,27 to enhance the oxidation potential of the phlorin; (2) N-substitution 16,28,29 so as to distort its conformation; (3) metalation with metal ions in the core of the phlorin 30 ; (4) replacing pyrrole with thiophene 31 ; and (5) introducing electron-withdrawing groups at the β-position of these pyrroles.…”

“…1,[17][18][19][20][21] However, native phlorins with unmodified core-structures are highly unstable and readily oxidized, 9,19,22,23 making it difficult to isolate them. To stabilize phlorins during their synthesis, chemists have proposed five methods: (1) introducing bulky substituents, for example, mesityl and phenyl, at the meso-sp 3 -carbon atom to hinder oxidation 1,13,18,19,22,[24][25][26][27] and electron-withdrawing substituents, for example, pentafluorophenyl, 1,19,24,27 to enhance the oxidation potential of the phlorin; (2) N-substitution 16,28,29 so as to distort its conformation; (3) metalation with metal ions in the core of the phlorin 30 ; (4) replacing pyrrole with thiophene 31 ; and (5) introducing electron-withdrawing groups at the β-position of these pyrroles. 4 Most, if not all, of these methods only provide stable phlorins with significantly modified core structures, while no native phlorin has been trapped in situ during porphyrin synthesis or catalysis, making it difficult to establish its role as an intermediate.…”

Two-Step, One-Flask Synthesis of a Meso-Substituted Phlorin

“…11 We have also applied our stepwise synthesis of phlorin to the preparation of phlorins bearing different substituents at the sp 3 -hybridized meso-position. 5 The stepwise methodology has been recently utilized by Rosenthal and co-workers in their independent studies of phlorin spectroscopy, structure, electrochemistry, and anion binding. 12−15 Despite the utility of stepwise syntheses of phlorin and 5isocorrole, these routes have important limitations as illustrated by the stepwise synthesis of a phlorin (Scheme 1).…”