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E versus Z Diazeniumdiolation of Acetoacetate-Derived Carbanions

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posted on 07.09.2012 by Navamoney Arulsamy, D. Scott Bohle, Carla L. Holman, Inna Perepichka
Nitric oxide adds to methyl acetoacetate in the presence of KOH in methanol at room temperature to form potassium acetylsydnonate N-oxide (K1) with an (E)-diazeniumdiolation and potassium acetate diazenium diolate (K22) from a (Z)-diazeniumdiolation. A study of the reaction with LiOH, NaOH, and NMe4OH and with ethyl acetate substrate reveals that the temperature of the reaction greatly influences the nitric oxide reactivity. At 23 °C, nitric oxide adds to give both E and Z products, whereas at −5 °C the gas reacts almost exclusively to give Z addition. The (Z)-diazeniumdiolation products, namely, the alkali metal and NMe4+ salts of methyl and ethylbutenoate-2-diazeniumdiolate-3-hydroxylate (32– and 42), are isolated in good yields. The alkali metal salts are not amenable for recrystallization because of their ready decomposition in aqueous solutions. However, [NMe4]2[MeC­(O)­C­(N2O2)­CO2Me] is readily recrystallized from a methanol/acetonitrile solvent mixture. The crystals are unambiguously characterized by X-ray crystallography. NMR spectra for all of the 32– and 42– salts reveal the presence of two isomers in aq solutions. But the structure of the NMe4+ salt contains only one of the isomers. Our attempts to cyclize the isolated and purified butenoatediazeniumdiolates from the (Z)-diazeniumdiolation to the E-containing sydnonate products were unsuccessful. TGA/DSC data for all of the products demonstrate the thermal instability of the salts at high temperatures. The salts decompose exothermally possibly with the release of N2O among other gases.