Synthesis and Studies of Stable Nonaromatic Dithia Pyribenzihexaphyrins

Abstract

We report here one of the rare examples of expanded hexaphyrins named as dithia pyribenzihexaphyrin macrocycles containing six-membered rings such as pyridine and p-phenylene along with five-membered heterocycles such as pyrrole and thiophene as a part of a macrocyclic frame. Trifluoroacetic acid catalyzed [3 + 3] condensation of equimolar mixture of [10,10′bis(p-tert-butyl phenyl)hydroxymethyl]-1,3-bis(2-thienyl)pyridine diol (2,6-pyri diol) and 1,4-bis(phenyl(1H-pyrrol-2-yl)methyl)benzene (p-benzidipyrrane) in CH 2 Cl 2 followed by oxidation with DDQ afforded stable nonaromatic dithia 2,6-pyri-para-benzihexapyrins 1 and 2 in 6−8% yields. The macrocycles were characterized by high-resolution mass spectroscopy and 1D and 2D NMR spectroscopy. NMR studies revealed the nonaromatic nature of dithia 2,6-pyri-p-benzihexaphyrins and indicated that the para-phenylene ring prefers to be in quininoid form rather than in benzenoid form. The macrocycles displayed sharp absorption bands in the region of ∼380−500 nm and a broad band at ∼700 nm, reflecting their nonaromatic nature. Upon protonation, these macrocycles showed NIR absorption properties. The redox studies of macrocycles indicated their electron-deficient nature. The DFT/TD-DFT studies are in line with the experimental observations.

Synthesis of Pyridine‐Containing Crowned Fused Expanded Porphyrins

“…Interestingly, the lower energy band maxima of compounds 4 – 6 and their protonated forms 4.2H + – 6.2H + was blue shifted compared to our previously reported macrocycles 1 and 2 . This was attributed to the disruption in the π‐conjugation in the macrocycles 4 – 6 and enhanced flexibility due to incorporation of polyether linkage into the macrocyclic framework [24,25] …”

“…This was attributed to the disruption in the π-conjugation in the macrocycles 4-6 and enhanced flexibility due to incorporation of polyether linkage into the macrocyclic framework. [24,25] The redox properties of pyridine containing crowned expanded porphyrins 4-6 were investigated using cyclic voltammetry and differential pulse voltammetry in CH 2 Cl 2 and the relevant data are included in Table 1. The cyclic voltammograms along with differential pulse voltammograms of compounds 4-6 presented in Figure 4c showed that the compounds 4-6 in general showed three reversible/quasi-reversible oxidations and one irreversible reduction.…”

“…[20][21][22][23] Our group is involved in synthesis and studies of different types of porphyrinoids and recently our attention is focussed on the synthesis of pyridine containing porphyrinoid macrocycles 1 & 2 in which the pyridine is a part of the macrocycle (Figure 1). [24][25][26] Such pyridine containing macrocycles are difficult to synthesize because of their inherent instability, lack of proper precursors and limited synthetic strategies. A perusal of literature revealed that the reports on pyridine containing porphyrinoids are still few in number.…”

Recent Advances in the Design and Syntheses of Porphyrinoids by Embedding Higher Analogues of Arene and Pyridine Units

“…The first attempt to synthesize pyriporphyrin was done by Berlin and Breitmaier through [3 + 1] MacDonald type condensation between pyridine-2,6-dicarbaldehyde with tripyrran to obtain pyriporphyrinone instead of pyriporphyrin . Thereafter, a plethora of pyridyl derivatives are reported which encompassed, true pyriporphyrin, confused pyriporphyrin, expanded pyriporphyrin, and its coordination complexes. ,− …”