Expanded scope of synthetic bacteriochlorins via improved acid catalysis conditions and diverse dihydrodipyrrin-acetals

J Org Chem. 2010 Feb 19;75(4):1016-39. doi: 10.1021/jo9025572.

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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Acetals / chemistry*
  • Acids / chemistry*
  • Catalysis
  • Crystallography, X-Ray
  • Metalloporphyrins / chemical synthesis*
  • Metalloporphyrins / chemistry
  • Models, Molecular
  • Molecular Structure
  • Photochemistry
  • Porphyrins / chemical synthesis*
  • Porphyrins / chemistry*
  • Pyrroles / chemistry*
  • Spectroscopy, Near-Infrared

Substances

  • Acetals
  • Acids
  • Metalloporphyrins
  • Porphyrins
  • Pyrroles
  • bacteriochlorin