Synthesis of bis-phosphonium salts from 6-bromo-1,2-naphthoquinone and α,ω-bis(diphenylphosphino)alkanes

1,4-Bis(diphenylphosphino)butane and 1,5-bis(diphenylphosphino)pentane add two molecules of 6-bromo-1,2-naphthoquinone to afford bis-ylides which on treatment with protic acids are converted into the corresponding P,P'-bis(7-bromo-3,4-dihydroxy-1-naphthyl)-substituted bis-phosphonium salts.