Mechanism and stereochemistry of the reaction of nitric oxide with secondary amines

The reaction of nitric oxide with secondary amines to form diazeniumdiolate anions with the structure R2NN2O2 - has been studied in order to investigate the stereochemistry and mechanism of this reaction. The stereochemical preference of the cis (Z) isomer for these compounds was proved using vibrational spectroscopy, namely infrared spectroscopy (IR) and Raman. Theoretical (B3LYP/aug-cc-pVDZ) rotation barriers for the simple diazeniumdiolate anion, Me2NN(O)=NO- are calculated in the gas phase to be 26.2 kcal/mol with the cis isomer predicted to be stabilized by only 0.5 kcal/mol compared to the trans isomer, therefore stereochemical preference for the cis is due to kinetic rather than thermodynamic control. Using vibrational spectroscopy to characterize the anion, it was determined that if the trans (E) isomer is present it represents <0.002% of the total composition.;The rate of these NO-amine condensation reactions was found to be dependent upon the solvent in which the reaction was conducted, with modest rate enhancements obtained in heterocyclic organic bases. In addition to differences in rate, the overall yield of diazeniumdiolate recovered was found to be highly dependent on solvent choice as well as amine structure.;Kinetic measurements for the formation of these compounds indicate that the rate law is multi-term and the order of reaction with respect to nitric oxide depends greatly on the concentration of nitric oxide. At low nitric oxide concentrations the dependency on nitric oxide is first order, however as the NO concentration increases the order of the reaction becomes second order in [NO].;The decomposition profile of amine derived diazeniumdiolates in aqueous media is significantly different than in organic solutions. In alkaline aqueous solutions the rate of decomposition is slow, with no significant decomposition occurring after several hours. This decomposition can be accelerated by lowering the pH. In non-aqueous solutions, the rate decomposition of the anion was determined to be comparable to the rate in neutral aqueous solutions.;The mechanism of nitric oxide's reaction with secondary amines was elucidated using both synthetic and kinetic methods. Attempts at preparing diazeniumdiolates (NONOates) by reacting nitric oxide directly to various radical anions proved to be unsuccessful. The result of these reactions is the formation of many reaction products, many of which could not be conclusively identified.;Finally the equilibrium constant for the rate determining step for the reaction of nitric oxide with the secondary amine, pyrrolidine in pyridine was elucidated. The equilibrium constant for the rate limiting step was found to be 1.1 M-1.