60th Annual Report on Research 2015

In the past year we have largely put aside our initial focus of trying to render the conversion from azides to diazonium ions catalytic in N-heterocyclic carbene (NHC). We did this upon recognizing that a more thorough understanding of the fundamental reactivity of triazabutadiene adducts, formed from the reaction of an organic azide with an NHC, was necessary. As mentioned in last year’s report, we observed intriguing reactivity upon rendering these systems water-soluble. Indeed these compounds degrade to liberate aryl diazonium ions via a protic mechanism under exceptionally mild conditions (up to pH 8). We published these findings in Angewandte Chemie and Flora Kimani, the student responsible for the work, presented a poster at the American Chemical Society conference in Boston (Fall 2015).

In the course of determining the reactivity of the aryl triazabutadienes we observed that they readily photo-isomerize. The isomer is significantly more susceptible to degradation and we concluded that it too was reacting via a protic mechanism. Remarkably this reaction can occur in water at pH 14. This reactivity indicates that the isomer brought about by light is significantly more basic than the isomer that is predominant in the absence of light. Upon bringing these compounds back into organic solvents we used this photobasic compound to catalyze a Henry reaction in a light-dependent manner. This work was recently reported in the Journal of the American Chemical Society.

All of the work described above was accomplished with aryl triazabutadienes, but once alkyl triazabutadienes are used in their place a new subset of reactivity is uncovered. We have observed that these compounds can be activated to provide alkyl diazonium species that can be trapped by acids to provide esters or simply eliminate to provide alkenes. We are actively pursuing this area in order to better control addition versus elimination (an age-old problem) and to establish a substrate scope. Some of the triazabutadienes have proven challenging to synthesize. In order to overcome our synthetic challenges we have developed silyl-protected NHC precursors that can provide carbenes using fluorides instead of strong bases. We are also looking to synthesize thermally generated carbenes, but the chemistry of these unstable compounds has yet to provide a practical route to these synthetically important species.

The student that has benefited most directly from this grant mechanism is Flora Kimani. Flora has recently trained an undergraduate student, Jean-Laurent Blanche, and he is established substrate scope for the alkyl triazabutadienes. Flora’s work and the projects tangentially related have become the core of our lab and this grant was truly enabling. As with many scientific pursuits the questions that became interesting and as such project driving evolved over time. We are segueing back out of the aqueous milieu, but the insights observed therein have proved essential to our understanding of the reactivity of this interesting and still understudied functional group.