De Novo Synthesis of Stable Tetrahydroporphyrinic Macrocycles:  Bacteriochlorins and a Tetradehydrocorrin

• Published in: Journal of organic chemistry Vol. 70; no. 14; pp. 5475 - 5486
• Main Authors: Kim, Han-Je, Lindsey, Jonathan S
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 08.07.2005; Amer Chemical Soc
• Subjects: Bacteriochlorophyll A - metabolism; Catalysis; Chemistry; Chemistry, Organic; Exact sciences and technology; Heterocyclic compounds; Heterocyclic compounds with several n hetero atoms in the same ring, in separated rings or in fused rings; Hydrogenation; Macrocyclic Compounds - chemical synthesis; Molecular Structure; Organic chemistry; Photochemistry; Physical Sciences; Pigments, Biological - chemical synthesis; Porphyrins - chemical synthesis; Preparations and properties; Science & Technology; Spectrum Analysis; RATIONAL SYNTHESIS; ACID CATALYSTS; ONE-FLASK SYNTHESIS; BACTERIOPHEOPHYTIN; CORROLES; ASYMMETRIC NUCLEOPHILIC-SUBSTITUTION; CHLORIN BUILDING-BLOCKS; PHOTOPHYSICAL PROPERTIES; DERIVATIVES; ALGA TOLYPOTHRIX-NODOSA; Chemical synthesis; Porphyrin; Corrin; Macrocycle
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo050467y

Bacteriochlorins (tetrahydroporphyrins) are attractive for diverse photochemical applications owing to their strong absorption in the near-infrared spectral region, as exemplified by the bacterial photosynthetic pigment bacteriochlorophyll a, yet often are labile toward dehydrogenation to give the chlorin. Tetradehydrocorrins (ring-contracted tetrahydroporphyrins) are attractive for studies of catalysis analogous to that of vitamin B12. An eight-step synthesis toward such tetrahydroporphyrinic macrocycles begins with p-tolualdehyde and proceeds to a dihydrodipyrrin-acetal (1) bearing a geminal dimethyl group and a p-tolyl substituent. Self-condensation of 1 in CH3CN containing BF3·OEt2 at room temperature afforded a readily separable mixture of two free base bacteriochlorins and a free base B,D-tetradehydrocorrin. Each bacteriochlorin contains two geminal dimethyl groups to lock-in the bacteriochlorin hydrogenation level, p-tolyl substituents at opposing (2,12) β-positions, and the absence (H − BC) or presence (MeO − BC) of a methoxy group at the 5- (meso) position. The B,D-tetradehydrocorrin (TDC) lies equidistant between the hydrogenation levels of corrin and corrole, is enantiomeric, and contains two geminal dimethyl groups, 2,12-di-p-tolyl substituents, and an acetal group at the pyrroline−pyrrole junction. Examination of the effect of the concentrations of 1 (2.5−50 mM) and BF3·OEt2 (10−500 mM) revealed a different response surface for each of H − BC, MeO − BC, and TDC, enabling relatively selective preparation of a given macrocycle. The highest isolated yield of each was 49, 30, and 66%, respectively. The macrocycles are stable to routine handling in light and air. The bacteriochlorins display characteristic spectral features; for example, H − BC exhibits near-IR absorption (λQ y = 737 nm, εQ y = 130 000 M-1 cm-1) and emission (λem = 744 nm, Φf = 0.14). In summary, this simple entry to stable bacteriochlorins and tetradehydrocorrins should facilitate a wide variety of applications.