Synthetic Chlorins, Possible Surrogates for Chlorophylls, Prepared by Derivatization of Porphyrins

Abstract

Chlorophylls make Earth green, are the central constituents in the engine of photosynthesis, and not surprisingly have garnered immense attention. A chlorin, the core chromophore of a chlorophyll, is a dihydroporphyrin macrocycle that contains one pyrroline ring and three pyrrole rings. The dominant method for the synthesis of chlorins has entailed the derivatization of porphyrins. The present review covers the ostensibly simple conversion of porphyrins, regardless of synthetic or biological origin, to chlorins. The period covered encompasses the entire history since the beginnings of chlorin synthetic chemistry in the early 20th century through 2015. Representative transformations include hydrogenation, cycloaddition, annulation, and diverse "breaking and mending" approaches. Altogether, the synthesis of >1000 chlorins or chlorin-like compounds (containing >50 distinct pyrroline motifs) is described. Such diversity animates the question "what structural features are essential for a chlorin to resemble chlorophyll?" To begin to address the structure-spectrum relationship, > 250 absorption spectra are provided for representative structures. The synthesis and spectral properties of the vast collection of compounds described herein are expected to illuminate the scope to which synthetic chlorins can serve as surrogates for chlorophylls and be exploited in diverse ways.

Manganese(III) Porphyrin‐Catalyzed Dehydrogenation of Alcohols to form Imines, Tertiary Amines and Quinolines

“…The main structural feature of the porphyrin ring is the aromatic 18 π‐electron conjugated network. Two pyrrole double bonds in the β‐positions are not part of this system and these two bonds are susceptible to hydrogenation to afford the chlorin and the bacteriochlorin structure . On the contrary, these hydroporphyrins are rather easily oxidized back to the parent porphyrin macrocycle.…”

“…Twop yrrole double bonds in the b-positions are not part of this system and these two bonds are susceptible to hydrogenation to afford the chlorin and the bacteriochlorin structure. [17] On the contrary, these hydroporphyrinsa re rather easily oxidized back to the parentp orphyrin macrocycle.I ts eems inconceivable that PhCD 2 OH coordinates to manganese and transfers deuterium to the b-positions in as imilarw ay as the reduction of the Schiff bases in the salen complexes. [6] It also appearshighly unlikely that the 18 p-electron pathway is disrupted during the process sincet his usually requires very strong nucleophiles such as organolithium reagents.…”

Synthesis and Oxidation of Internally Chlorinated Carbachlorins

“…been reported, but in every case low to moderate yields were obtained. As is the case for tetrapyrrolic chlorins, [23,24] carbachlorins show moderately strong absorptions at 650 nm or higher, a feature that could find applications in photodynamic therapy. [25] Carbachlorins are also more stable than their tetrapyrrolic counterparts as they require more stringent conditions in order for oxidation to carbaporphyrin to occur.…”

Modifications of Porphyrins and Hydroporphyrins for their Solubilization in Aqueous Media

“…For instance, the group of Lindsey reported the synthesis of a series of amphiphilic hydroporphyrins prepared by Suzuki coupling of a pyridyl boronic ester with meso-bromo-chlorins or bacteriochlorins [34]. Key chlorin building block 45 was prepared by regioselective meso-bromination of chlorin 44 using NBS (Scheme 12) [89].…”

“…Many of these applications will greatly benefit from an inherent solubility of the porphyrins in aqueous media, i.e., not requiring any surfactants or liposome vesicles to mediate solubility. However, even though the methodologies toward synthetic porphyrins [29,30], hydroporphyrins [2,[31][32][33][34], and their analogues [34][35][36][37][38][39], have advanced enormously in the past decades, the vast majority of the synthetic porphyrinoids presented are not natively water-soluble, preventingor at least complicating-their utilization in, for example, biological contexts.…”