2,5-Dihydroxy-1,4-benzoquinones Appended with -P(O)(OR)₂ (R = Me or Et) Groups and Their Ammonium and Lithium Salts:...

Synthetic methods yielding 2,5-dihydroxy-1,4-benzoquinones appended with dialkylphosphonato groups were established. Reactions of 1,4-dichloro-2,5-dihydroxybenzene with dialkylchlorophosphates ClP-(O)-(OR)₂ (R = Me or Et) produced 1,4-dichloro-2,5-bis-(dimethylphosphato)-benzene (1a) and 1,4-dichloro-2,5-bis-(diethylphosphato)-benzene (1b). Low-temperature reactions of 1a-b with lithium diisopropylamide (2 equiv) proceeded with double anionic phospho-Fries rearrangement, yielding 2,5-dichloro-3,6-bis-(dialkylphosphonato)-1,4-hydroquinones 2a-b. Oxidations of 2a-b with K₂S₂O₈ under basic conditions proceeded with the nucleophilic displacement of both chlorides by the hydroxy groups. Acidification of the reaction mixtures led to the isolation of 2,5-dihydroxy-1,4-benzoquinones appended with two dimethylphosphonato (H₂ 3a) or diethylphosphonato (H₂ 3b) groups. Reactions of H₂ 3a and H₂ 3b with excess NH₃(aq) or with LiOH yielded salts (NH₄)₂ 3a, (NH₄)₂ 3b, Li₂ 3a, and Li₂ 3b. All compounds were characterized by spectroscopic (FT-IR, FT-NMR (¹H, ¹³C, and ³¹P)) methods and by high-resolution mass spectrometry or elemental analyses. Both 2,5-dihydroxy-1,4-benzoquinones H₂ 3a-b and their ammonium and lithium salts were investigated by cyclic voltammetry. Compounds 2a, H₂ 3a, H₂ 3b, and salts (NH₄)₂ 3a·H₂O, (NH₄)₂ 3b·2H₂O, Li₂ 3a·2H₂O, and Li₂ 3b were characterized by single-crystal X-ray diffraction methods. Compounds H₂ 3a/H₂ 3b and their salts represent the first examples of anillic acids appended with -P-(O)-OR₂ groups. Phosphonato functionalities were shown to be important factors in increasing the solubility of free acids as well as providing coordination/hydrogen bonding sites in their salts.