Synthesis of Phosphonium Salts—Phosphine Structure and Inorganic Salts Effects

Abstract

Solvent-free reactions of 2- and 3-halopyridines with PPh ₃ , PBu ₃ , and PCy ₃ were studied under conventional heating, as well as under microwave irradiation. No difference was observed in the reaction course between classical and microwave reactions. 2-Bromopyridine gave quantitative yields of 2-pyridyltriphenylphosphonium bromide within few minutes at 190°C. Equimolar amounts of some inorganic salts (LiPF ₆ , LiOTf, LiBr, NaPF ₆ , KPF ₆ ) were necessary for the reactions of the other 2-halopyridines. 3-Halopyridines did not react with PPh ₃ even in the presence of LiPF ₆ . Their reactions with PCy ₃ in the presence of LiPF ₆ resulted in the quantitative formation of dicyclohexylphosphine oxide.

Keywords:

• Phosphonium salts
• inorganic salt effects
• phosphine structure effects
• halopyridines
• S_NAr mechanism
• microwave irradiation

Acknowledgments

This work was carried out under the auspices of COST D32 project No. 0010/04 and Slovak grant agency VTP grant No. 1012/2003. NMR measurements were performed on the equipment supported by the Slovak State Program Project No. 2003SP200280203.

Notes

^a Only starting materials were detected in the reaction mixture.

^a Only starting materials were recovered from the reaction mixture.