Comparative Nitrene-Transfer Chemistry to Olefinic Substrates Mediated by a Library of Anionic Mn(II) Triphenylamido-Amine Reagents and M(II) Congeners (M = Fe, Co, Ni) Favoring Aromatic over Aliphatic Alkenes

Abstract

Selective amination of σ and π entities such as C–H and CC bonds of substrates remains a challenging endeavor for current catalytic methodologies devoted to the synthesis of abundant nitrogen-containing chemicals. The present work addresses an approach toward discriminating aromatic over aliphatic alkenes in aziridination reactions, relying on the use of anionic metal reagents (M = Mn, Fe, Co, Ni) to attenuate reactivity in a metal-dependent manner. A family of MnII reagents bearing a triphenylamido-amine scaffold and various pendant arms has been synthesized and characterized by various techniques, including cyclic voltammetry. Aziridination of styrene by PhINTs in the presence of each MnII catalyst establishes a trend of increasing yield with increasing MnII/III anodic potential. The FeII, CoII, and NiII congeners of the highest-yielding MnII catalyst have been synthesized and explored in the aziridination of aromatic and aliphatic alkenes, exhibiting good to high yields with para-substituted styrenes, low to modest yields with sterically congested styrenes, and invariably low yields with aliphatic olefins. CoII mediates faster styrene aziridination in comparison to MnII but is less selective than MnII in competitive aziridinations of conjugated versus nonconjugated olefins. Indeed, MnII proved to be highly selective even versus well-established copper and rhodium aziridination reagents. Mechanistic investigations and computational studies indicate that all metals follow a two-step styrene aziridination pathway (successive formation of two N–C bonds), featuring a turnover-limiting metal–nitrene addition to an olefinic carbon, followed by product-determining ring closure. Both steps exhibit activation barriers in the order Fe > Mn > Co, most likely stemming from relevant metal–nitrene electrophilicities and MII/III redox potentials. The aziridination of aliphatic olefins follows the same stepwise path, albeit with a considerably higher activation barrier and a weaker driving force for the formation of the initial N–C bond, succeeded by ring closure with a miniscule barrier.

Electronically Asynchronous Transition States for C–N Bond Formation by Electrophilic [CoIII(TAML)]-Nitrene Radical Complexes Involving Substrate-to-Ligand Single-Electron Transfer and a Cobalt-Centered Spin Shuttle

“…For example, the |ρ/ρ • | ratio found for [Cu I (TMG 3 trphen)(NR)] + is dependent on the N-group substituent (0.50 for R = Ts, 0.98 for R = Ns), 31 34 Radical-type pathways were proposed for anionic Mn II , Fe II , and Co II complexes of a triphenylamido-amine ligand, with |ρ/ ρ • | ratios of 0.75 (Mn), 1.17 (Fe), and 1.00 (Co). 35 Low |ρ/ρ • | ratios were reported for styrene aziridination with [Fe II (dipyrrinato)(Cl)(NAd)] (0.04) 36 and toluene C(sp 3 )− H amination with [Co II (por)(NAr)] (0.008), 37 which are both dominated by spin-delocalization effects and are meanwhile known to react via radical-type mechanisms involving nitrene radicals. 10 T he n i t r e n e t r a n s f e r r e a c t i o n s m e d i intermediates were shown to have clear nitrene radical character, 15 the Hammett data ( Figure 6) provide strong evidence that the reactions do not proceed via direct (nitrene) radical addition to styrene (as observed for [Co II (por)(NR)] species), as |ρ/ρ • | values much smaller than 1 are then expected.…”

Nitrene Insertion into Aromatic and Benzylic C−H Bonds Catalyzed by Copper Complexes of Fluorinated Bis‐ and Tris(pyrazolyl)borates

“…The optimized transition state suggests a step-wise nitrene addition followed by proton transfer rather than a concerted nitrene insertion, as was proposed by Hou et al [49] This result agrees with previous experiment-theory research on nitrene two-step aziridination using substituted styrenes as substrate studies. [52,53] The nitrene addition pathway has a lower barrier than arene sp 2 HAA (17.0 vs. 27.0 kcal mol À 1 ), suggesting it is the preferred route for functionalization of the mesitylene sp 2 CÀ H bonds. NBO analysis shows that nitrene addition leads to an amido-copper intermediate as illustrated in Figure 3, with a Cu(I)-N À (Ns)(Mes + ) configuration.…”

“…[51] Significant spin density is found on both the nitrene nitrogen and the copper, Information). [52,53] The nitrene addition pathway has a lower barrier than arene sp 2 HAA (17.0 vs. 27.0 kcal mol À 1 ), suggesting it is the preferred route for functionalization of the mesitylene sp 2 CÀ H bonds. Additionally, for mesitylene, one expects an additional steric advantage for activation at a benzylic versus an arene CÀ H bond.…”

“…[59] Geometry optimization and frequency calculations were performed with B3LYP functional and 6-31 + G(d) basis set, which have been proven accurate in previous study of nitrene aziridination catalysts. [52,53] The calculated Gibbs free energies are computed in the gas-phase, at 298.15 K and 1.0 atm.…”

Imido Cobalt Complexes with Imidyl Character

“…This is especially of interest in the atom economic direct amination of unfunctionalised C-H bonds, either via a stepwise H atom abstraction/radical recombination or a more concerted nitrene insertion mechanism. [8][9][10][11][12][13][14][15][16][17][18] Despite the resulting high demand for understanding late 3d-transition imido metal species, comprehensive knowledge about the electronic and structural factors that contribute to their bond activation reactivity is still lacking. Most structurally characterised late-3d-transition metal imido complexes are found in a low-spin state which mitigates their reactivity and thus allowed for their isolation in the first place.…”