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

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.