Rational synthesis of trans-substituted porphyrin building blocks containing one sulfur or oxygen atom in place of nitrogen at a designated site

초록

The use of heteroatom-substituted porphyrins in bioorganic and materials chemistry requires the ability to position a variety of substituents in a controlled manner about the porphyrin periphery. We describe a rational route to trans-AB<inf>2</inf>C-type porphyrins bearing one oxygen atom (N<inf>3</inf>O) or one sulfur atom (N<inf>3</inf>S) in a designated location in the porphyrin core. The synthesis involved four stages: (1) Acid-catalyzed condensation of a furyl- or thienylcarbinol in excess pyrrole afforded the aryl-substituted furyl- or thienylpyrromethane in high yield. (2) Treatment of the furyl- or thienylpyrromethane with an acid chloride catalyzed by SnCl<inf>4</inf> or AlCl<inf>3</inf> afforded the corresponding diketo product. (3) Reduction with NaBH<inf>4</inf> in alcoholic solvents gave the furyl- or thienylpyrromethanediols. (4) Reaction of a furylpyrromethanediol, thienylpyrromethanediol, or dipyrromethanediol with a dipyrromethane in a one-flask process of condensation followed by oxidation gave the corresponding porphyrin. Reaction conditions previously identified to minimize scrambling in a dipyrromethane-aldehyde condensation were found to be effective in this application. Thus, reaction with 10 mM reactants in acetonitrile at 0 °C containing BF3·Et<inf>2</inf>O and NH<inf>4</inf>Cl followed by oxidation with DDQ resulted in the desired porphyrin (10-20% yields) without acidolysis. In this manner, N<inf>3</inf>O-, N<inf>3</inf>S-, or N<inf>4</inf>-porphyrins bearing 5-(p-iodophenyl), 15-[4-(2-(trimethylsilyl)ethynyl)-phenyl], and 10,20-di-p- tolyl groups have been made. This set of trans-substituted porphyrin building blocks is expected to be useful in the synthesis of biomimetic energy transduction systems.