Nitrogen-bound diazeniumdiolated amidines

Aminolysis of an N-diazeniumdiolated amidine as an approach to diazeniumdiolated ammonia

Recent theoretical studies have suggested that the parent diazeniumdiolate ion, H₂N–N(O)=NO^– (“diazeniumdiolated ammonia”), might be stable enough to be isolated and that it could potentially serve as a uniquely advantageous prodrug form of bioactive nitroxyl (HNO). Here, we report on an attempt to isolate its O²-benzylated derivative by aminolysis of the C=N bond in PhC(NH₂)=N–N(O)=NOBn. The reaction proved remarkably sluggish in comparison to aminolysis of unsubstituted benzamidine, and the desired product could not be isolated, apparently because of base sensitivity of the NH₂ group. Consistent with this interpretation, O-benzylhydroxylamine and N₂O were recovered from the reaction mixture in high yields, along with N,N′-dibutylbenzamidine. Theoretical calculations rationalize the observed slow aminolysis by demonstrating that the diazeniumdiolate group greatly suppresses the electrophilicity of the adjacent C=N carbon center, rendering attack at that position endothermic. The data provide significant insights into the challenges inherent to the pursuit of diazeniumdiolated ammonia.

O2-functionalized methylamine diazeniumdiolates: evidence for E ⇄ Z equilibration in an acyclic system

Diazeniumdiolates that have the structure RHN-N(O)═NOR' are of interest as prodrug (caged) forms of the bioeffectors nitric oxide (NO) and nitroxyl (HNO). Previous work has focused on examples possessing α-branched R groups, with isopropylamine (IPA)/NO (R = isopropyl) being the smallest examined to date. To probe the effect of minimizing the alkyl-group size on the chemistry of IPA/NO, we prepared the corresponding methylamine derivative as a sodium salt that was highly unstable but could be trapped in very low overall yield as the stable O(2)-benzyl derivative. To prepare enough for efficient characterization, we devised an alternate synthesis involving a novel N-dealkylation route. CH(3)HN-N(O)═NOBn, synthesized in high yield and crystallized as the Z isomer as determined by X-ray crystallography, was observed to exist as a 11:1 mixture of two isomeric forms in dynamic equilibrium in solution. Similar results were seen for the O(2)-ethyl derivative, whose two equilibrium constituents were partially separated by HPLC to reveal essentially identical UV and mass spectra, indicating them to be Z and E isomers of CH(3)HN-N(O)═NOEt. The results could lead the way to a fuller understanding of the chemistry of the acyclic (E)-diazeniumdiolates.