De Novo Synthesis of Bacteriochlorins Bearing Four Trideuteriomethyl Groups

Haoyu Jing,Qun Tang,David F Bocian,Jonathan S Lindsey

doi:10.3390/org3010002

Haoyu Jing, Qun Tang + Show 2 more

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https://doi.org/10.3390/org3010002

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Abstract

Site-specific introduction of isotopes in tetrapyrrole macrocycles provides the foundation for probing physicochemical features germane to photosynthetic energy-transduction processes, but has chiefly been done with porphyrins rather than the more biologically relevant hydroporphyrin analogues of native photosynthetic pigments. A prior study incorporated pairwise 13C or 15N atoms in the skeleton of a bacteriochlorin containing a gem-dimethyl group in each pyrroline ring. Here, a complementary effort is reported that installs deuterium atoms in substituents at the perimeter of a bacteriochlorin. Thus, perdeuteriated 3-methyl-2,4-pentanedione was converted in an 8-step synthesis via the intermediacy of tert-butyl 5-formyl-3,4-bis(trideuteriomethyl)pyrrole-2-carboxylate to the 2,3,12,13-tetrakis(trideuteriomethyl)-8,8,18,18-tetramethylbacteriochlorin (BC-2). The fidelity of isotope substitution was maintained throughout the synthesis. Resonance Raman spectroscopy of the copper chelate (CuBC-2) revealed that addition of the four β-pyrrolic substituents alone is not sufficient to account for the vibronic complexity observed for the copper chelate of bacteriochlorophyll a (CuBChl a). The increased vibronic activity exhibited by the natural pigments and CuBChl a must arise from the increased structural complexity of the macrocycle.

Highlights

Academic Editors: Wim Dehaen, Michal Szostak and Huaping XuReceived: 22 December 2021Accepted: 30 January 2022Published: 7 February 2022Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.Licensee MDPI, Basel, Switzerland.Bacteriochlorophylls and their free base analogues are the chief chromophores for solar light harvesting, energy transduction and charge separation in anoxygenic photosynthetic bacteria (Chart 1) [1]
Bacteriochlorophylls are distinguished from the other members of the tetrapyrrole family by the saturation of the chromophore and the resulting absorption in the near infrared region
Acetylacetone (1) underwent alkylation with CD3I to give the the reported 3-trideuteriomethyl-2,4-pentadione 2 [45], which was exhaustively deuteriated reported 3-trideuteriomethyl-2,4-pentadione 2 [45], which was exhaustively deuteriated via underlying keto-enol tautomerism to give 1,3-diketone 3 [46] (Scheme 2)

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. There areareseveral methodsforfor preparation of isotopically substituted tetrapyrrole macrocycles. The general synthetic route and the resulting isotopically substituted bacteriochlorins chlorins are shown in Chart 2. The isotopic substitutions in the macrocycles shown effects of peripheral substituents. Rich inAccordingly, vibronic modes relative to thatanof the unsubstituted bacteriochlorin counterpart we sought to prepare analogue wherein the four methyl groups are(BC-0). Such compounds exhibit structural complexity analogue wherein the four methyl groups are per-deuteriated (BC-2). Forcarbon the carbon and nitrogen the tetrapyrrole macrocycle precursors thereto In this regard, and coworkers reported the partial exchange of the protons on the β-pyrrolic methyl.

Results and Discussion

Synthesis

Copper

Outlook

Experimental Section