Novel copper N-tosyl nitrenes from bispyrazolylmethane ligands: Investigation of their reactivity towards catalysis

The significance of the C-N structural motive in biological and technological relevant molecules is unquestionable. Thus, the C-N bond formation by direct incorporation of a nitrogen functionality into unreactive C-H bonds remains an important objective of chemical synthesis. Copper-nitrenes derived from organic azides and iminoiodinanes have been postulated as key reactive intermediates in the C-H amination of hydrocarbons and nitrogen-group transfer reactions, such as olefin aziridinations. Current examples comprise the late-stage -amination of acylpyrazoles using copper(II) salts and iminoiodinanes and the formation of sulfonamides and isothiazoles from PhINTs and alkynes. Moreover, Prez et. al. suggested, based on DFT-calculations, that a copper-acylnitrene intermediate in conjunction with alkynes enables the formation of their obtained oxazoles.
Only very recently, such elusive and reactive intermediates have been isolated by the Warren and Ray group, respectively. Their findings include neutral and cationic (Sc³⁺-stabilized) copper-nitrenes stemming from either azides or iminoiodinanes as nitrene sources.
Novel copper-nitrenes generated from a copper(I) precursor and scorpionates with a bis(pyrazolyl)methane scaffold will be presented. These colorful compounds provide direct access to active species for C-H-insertions into hydrocarbons and nitrene transfer reactions.
The substrate reactivity follows pseudo-first-order rates and the specific copper-nitrene UV/Vis features allow time-resolved reaction monitoring. Catalytic procedures for the aziridination of olefins and C-H amination of alkanes are presented. With the aid of DFT-calculations and spectroscopic methods the open-valence electronic structures were elucidated. The impact of the ligand design on the copper-nitrene electronic attributes is revealed. Specifically, the consequences of alteration of the substitution at the pyrazole-3-position and the direct comparison of azoles in the scorpionate scaffold are rationalized in terms of the copper-nitrene stability and reactivity.