Optimization of the synthesis of phenanthrene-2- and -3-sulfonyl chlorides

Sulfonation of phenanthrene with sulfuric acid, followed by neutralization with sodium hydroxide, gave a mixture of isomeric sodium phenanthrenesulfonates in an overall yield of 83%. Sodium phenanthrene-2-, -3-, and -9-sulfonates were isolated in 17, 43, and 4% yield, respectively. Sulfonation of phenanthrene with ClSO₃H or SO₃ did not improve the yields of phenanthrene-2- and -3-sulfonic acids. Treatment of sodium phenanthrene-2- and -3-sulfonates with PCl₅-POCl₃ or SOCl₂-DMF afforded the corresponding sulfonyl chlorides in 89 and 87 or 69 and 70% yield, respectively. Pure phenanthrene-2- and -3-sulfonyl chlorides were also synthesized in 27 and 51% yield by reaction of a mixture of 2- and 3-sulfonates with PCl₅-POCl₃.