Competitive Intramolecular Amination as a Clock for Iron-Catalyzed Nitrene Transfer

Abstract

Reaction of the complex [(TpPh,Me)­FeII­(NCMe)3]­BF4, where TpPh,Me = hydrotris­(3-phenyl,5-methyl-1-pyrazolyl)­borate, with the iodonium heteroylide PhINTs (1.5 equiv) is proposed to result in the insertion of N-tosylnitrene into one C–H bond at the ortho ring position of a 3-pyrazole phenyl substituent; subsequent deprotonation of the nascent aniline and one-electron oxidation of iron forms TsNH2 (0.5 equiv) as a coproduct. The covalent ligand modification and oxidation results in an intense purple-brown anilinato–iron­(III) LMCT chromophore. This intramolecular reaction is utilized as a consistent clock to determine relative rates of competitive intermolecular nitrene transfer to added substrates, specifically to para-substituted styrenes and thioanisoles. Prior addition of substrate to the reaction of PhINTs with the iron­(II) complex attenuates the CT absorbance of the equilibrium solution. Fitting of the concentration-dependent absorption data gives the ratio of intra- versus intermolecular nitrene transfer. Because the former is independent of substrate, ratios for various substrates are directly comparable, and this approach enables acquisition of data for a single substrate under nearly stoichiometric, as opposed to competitive catalytic, conditions. Hammett analyses of such data are consistent with an electrophilic intermediate consistent with known or suspected imidoiron­(IV) complexes. Because this intermediate was not observed directly, plausible geometric and electronic structures were modeled and assessed using density functional theory.

Fe-Catalyzed Aziridination Is Governed by the Electron Affinity of the Active Imido-Iron Species

“…Interestingly, this view is supported by the recent work of Jensen and coll. 44 who reported Hammett correlations for thioanisoles sulfimidation and styrenes aziridination by their Fe scorpionate catalysts. Whereas the sulfimidation data gave a good correlation with the p purely inductive parameter with a slope p = -1.5, the aziridination data gave a poor correlation with this parameter (R 2 =0.80).…”

“…Dominant inductive effects were also revealed by Hammett correlations reported for Fe catalysts by Che and coll. 39 and Jensen and coll., 40,44 Cu catalysts by Müller and coll. 71 and Chang and coll.…”

“…By contrast to most Fe systems, 39,40,[42][43][44]46 Hammett correlations have shown that inductive effects are negligible for Betley's Fe catalyst 41 whose reactivity is ruled by radical delocalization, suggesting a two-step mechanism. Surprisingly, aziridination of cis--deuterostyrene produced exclusively cis-1-adamantyl-2-deutero-3-phenylaziridine, a feature usually associated to a concerted mechanism.…”

“…with iron scorpionates. 40,44 In both systems Hammett correlations reveal dominant inductive electronic effects ( + = -0.72 and -0.58, respectively). A similar study by Stavropoulos and coll.…”

“…38 A few limited mechanistic studies of iron-based catalytic aziridination systems have been reported in the past decade but a rationalization of their activities is still lacking, owing to the widely different if not opposite behaviors reported. [39][40][41][42][43][44][45][46] Using a similar dipyrromethene ligand ( DCP L Ad )Fe (DCP = 2,6-dichlorophenyl, Ar = p-tBu-Ph),…”