Chemistry of the diazeniumdiolates: Z ⇌ E isomerism

Here, we explore the chemistry of the previously undocumented E form of diazeniumdiolates having the structure R ¹R ²NN(O)=NOR ³. Reported crystallographic studies have uniformly revealed the Z configuration, and our attempts to observe a Z → E conversion through thermal equilibration or photochemical means have, until now, consistently failed to reveal a significant amount of a second conformer. As a typical example, the NMR spectrum of trimethyl derivative Me ₂NN(O)=NOMe revealed no evidence for a second configuration. Electronic structure calculations attribute this finding to a prohibitively high interconversion barrier of ∼40 kcal/mol. A similar result was obtained when we considered the case of R ¹ = Me = R ³ and R ² = H at the same levels of theory. However, when MeHNN(O)=NOMe was ionized by dissociating the N-H bond, the barrier was calculated to be lower by approximately 20 kcal/mol, with the E form of the anion being favored over Z. This circumstance suggested that an E isomer might be isolable if a Z anion were formed and given sufficient time to assume the E configuration, then quenched by reaction with an electrophile to trap and neutralize the E form and restore the putatively high interconversion barrier. Consistent with this prediction, basifying iPrHNN(O)=NOCH ₂CH ₂Br rapidly led to a six-membered heterocycle that was crystallographically characterized as containing the -N(O)=NO- functional group in the E configuration. The results suggest an approach for generating pairs of Z and E diazeniumdiolates for systematic comparison of the rates at which the individual isomers release bioactive NO and of other physicochemical determinants of their biomedical utility.