Studies of the Reactivity of Selected Nitrogen-Rich Compounds

Abstract

Alkyl diazonium ions are extremely potent electrophiles; however, this potency causes them to be unstable and only transiently available for reaction. An esterification methodology has been developed wherein hexyl triazabutadienes decompose under acidic conditions to form hexyl diazonium ions in situ, allowing alkylation of the conjugate base. A panel of carboxylic and sulfonic acids were screened and all were found to be reactive, with the weaker conjugate bases producing greater yields of esters, contrary to expectations. The guanidine byproduct of triazabutadiene decomposition was observed to be more basic than the original triazabutadiene, and inhibited further reaction by forming a guanidinium salt with the acid substrate. An alternative electrophile, tosyl isocyanate, was employed to activate the triazabutadiene for fragmentation, allowing the full equivalent of acid to be available for alkylation, and improving ester yields dramatically. It was discovered that the absence of a nucleophile could drive the substitution reaction toward elimination, allowing the synthesis of terminal alkenes. In the first synthesis of a triazabutadiene (TBD) derived from a natural product, an N-alkylated histamine azide was subjected to TBD forming conditions to produce the histamine TBD in good yield. The compound was assessed for elimination activity under a variety of conditions, most of which produced at least some terminal alkene. A combination of silica gel and phenol resulted in complete conversion of the TBD to terminal alkenes, and an alcohol due to trace water in the reaction. Silica gel alone was found to produce 1-hexene from hexyl TBD. The elimination reactivity was probed much further and in a variety of directions, most significantly in pursuit of a promising method for selective functionalization of lysine residues in proteins. The project is based on the in situ formation of an aryl diazonium ion, that would proceed to diazotize a lysine residue, which would then eliminate to form a terminal alkene. The reaction is strategically planned to occur at near neutral pH, thereby excluding the labeling of other residues such as tyrosine. The broader impacts of the project are that the terminal alkene is quite useful in chemical biology for attachment of cargo via the thiol-ene reaction, and additionally in the synthesis of stapled peptides. This methodology is still under development; stability issues and future directions to achieve success are discussed. In a separate project, the cause of long-lasting fluorescence of an azobenzene derivative after exposure to UV light was investigated. Azobenzene compounds have been shown to isomerize from trans to cis photochemically, and this behavior has been extensively studied since it was discovered in 1937. Although isomerization was suspected to be responsible for the increase in fluorescence, the results of instrumental analysis indicate that trace decomposition to the highly fluorescent aminocoumarin starting material used in the synthesis of the compound was occurring, via homolytic cleavage during irradiation. The decomposition occurs at a slow rate and reaches a photostationary equilibrium that may be useful for long term fluorescence studies of biomolecules.