title: Expanded scope of synthetic bacteriochlorins via improved acid catalysis conditions and diverse dihydrodipyrrin-acetals.
abstract: Bacteriochlorins are attractive candidates for a wide variety of photochemical studies owing to their strong absorption in the near-infrared spectral region. The prior acid-catalysis conditions [BF(3) x O(Et)(2) in CH(3)CN at room temperature] for self-condensation of a dihydrodipyrrin-acetal (bearing a geminal dimethyl group in the pyrroline ring) typically afforded a mixture of three macrocycles: the expected 5-methoxybacteriochlorin (MeOBC-type), a 5-unsubstituted bacteriochlorin (HBC-type), and a free base B,D-tetradehydrocorrin (TDC-type). Here, a broad survey of >20 acids identified four promising acid catalysis conditions of which TMSOTf/2,6-di-tert-butylpyridine in CH(2)Cl(2) at room temperature was most attractive owing to formation of the 5-methoxybacteriochlorin as the sole macrocycle regardless of the pyrrolic substituents in the dihydrodipyrrin-acetal (electron-withdrawing, electron-donating, or no substituent). Eleven new dihydrodipyrrin-acetals were prepared following standard routes. Application of the new acid catalysis conditions has afforded diverse bacteriochlorins (e.g., bearing alkyl/ester, aryl/ester, diester, and no substituents) in a few days from commercially available starting materials. Consideration of the synthetic steps and yields for formation of the dihydrodipyrrin-acetal and bacteriochlorin underpins evaluation of synthetic plans for early installation of bacteriochlorin substituents via the dihydrodipyrrin-acetal versus late installation via derivatization of beta-bromobacteriochlorins. Treatment of the 5-methoxybacteriochlorins with NBS gave regioselective 15-bromination when no pyrrolic substituents were present or when each pyrrole contained two substituents; on the other hand, the presence of a beta-ethoxycarbonyl group caused loss of regioselectivity. The 15 new bacteriochlorins prepared herein exhibit a long-wavelength absorption band in the range 707-759 nm, providing tunable access to the near-infrared region. Taken together, this study expands the scope of available bacteriochlorins for fundamental studies and diverse applications.
authors.affiliation: Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Krayer
authors.forename: Marcin
authors.initials: M
authors.lastname: Ptaszek
authors.forename: Han-Je
authors.initials: HJ
authors.lastname: Kim
authors.forename: Kelly R
authors.initials: KR
authors.lastname: Meneely
authors.forename: Dazhong
authors.initials: D
authors.lastname: Fan
authors.forename: Kristen
authors.initials: K
authors.lastname: Secor
authors.forename: Jonathan S
authors.initials: JS
authors.lastname: Lindsey
mesh_terms: D000080:Acetals / Q000737:chemistry*; D000143:Acids / Q000737:chemistry*; D002384:Catalysis; D018360:Crystallography, X-Ray; D008665:Metalloporphyrins / Q000138:chemical synthesis* / Q000737:chemistry; D008958:Models, Molecular; D015394:Molecular Structure; D010777:Photochemistry; D011166:Porphyrins / Q000138:chemical synthesis* / Q000737:chemistry*; D011758:Pyrroles / Q000737:chemistry*; D019265:Spectroscopy, Near-Infrared
chemical_list: D000080:Acetals; D000143:Acids; D008665:Metalloporphyrins; D011166:Porphyrins; D011758:Pyrroles; C452433:bacteriochlorin