Exploration of early transition diazametallacyclohexadiene complexes as a general platform for N-heterocycle synthesis

Abstract

The Ti-mediated multicomponent pyrazole synthesis from alkynes, nitriles, and Ti imidos has been proposed to go through an electrocyclic reductive elimination from a key diazatitanacyclohexadiene intermediate. This thesis aims to examine the structure and reactivity of early transition metal diazametalacyclohexadiene complexes to build a greater understanding of the N-N coupling mechanism and expand its reactivity to prepare a variety of organic products. Previous work has shown that oxidation induced reductive elimination affords pyrazoles whereas addition of nitrosobenzene to the diazatitanacyclohexadiene generates asymmetric α-diimines through a [4+2] cycloaddition/cycloreversion mechanism. Here, 1,2-dihydropyrimidines have been prepared through addition of aldehydes to the diazatitanacycle, going through a [2+2] cycloaddition in contrast to the α-diimine mechanism. Furthermore, Ta and Nb isostructural complexes have been synthesized and found to be more challenging to oxidize, indicating some influence from the metal center on the reactivity of these complexes. Zirconocene and hafnocene diazametallacycles were also prepared, though further investigation will be required to elucidate reactivity trends down the group IV metals. Finally, a series of diazatitanacyclohexadienes were prepared to identify structure activity relationships towards developing a catalytic methodology for pyrazole synthesis using electrochemistry or photoredox chemistry. Substitution on the ligand backbone was found to be key for stabilizing the metallacycle through the electrocyclic N-N coupling mechanism. Unfortunately, mechanistic investigations identified the inner-sphere oxidation through Ti-O bond formation to be vital towards pyrazole formation, precluding successful catalysis with electrochemical or photochemical means.