A manganese catalyst for highly reactive yet chemoselective intramolecular C(sp3)–H amination

Abstract

C—H bond oxidation reactions underscore the existing paradigm wherein high reactivity and high selectivity are inversely correlated. The development of catalysts capable of oxidizing strong aliphatic C(sp3)—H bonds while displaying chemoselectivity (i.e. tolerance of more oxidizable functionality) remains an unsolved problem. Herein, we describe a catalyst, manganese tert-butylphthalocyanine [Mn(tBuPc)], that is an outlier to the reactivity-selectivity paradigm. It is unique in its capacity to functionalize all types of C(sp3)—H bonds intramolecularly, while displaying excellent chemoselectivity in the presence of π-functionality. Mechanistic studies indicate that [Mn(tBuPc)] transfers bound nitrenes to C(sp3)—H bonds via a pathway that lies between concerted C—H insertion, observed with reactive noble metals (e.g. rhodium), and stepwise radical C—H abstraction/rebound, observed with chemoselective base metals (e.g. iron). Rather than achieving a blending of effects, [Mn(tBuPc)] aminates even 1° aliphatic and propargylic C—H bonds, reactivity and selectivity unusual for previously known catalysts.

Inverting Steric Effects: Using “Attractive” Noncovalent Interactions To Direct Silver-Catalyzed Nitrene Transfer

“…On the other hand, Ru and Fe catalysts furnished 2 with varying degrees of selectivity. 8c,20 These results fit the prevailing NT mechanistic paradigm, where catalysts proceeding via stepwise H-transfer/radical recombination (S-HT:RR) tend to show higher kinetic isotope effect (KIE) values (Scheme 1B) and favor amination of weaker C−H bonds (BDE: 2° benzylic C− H ~ 85 kcal/mol; 3° alkyl C(sp 3 )−H ∼ 95 kcal/mol).…”

“…22 As with all metal-catalyzed NT reactions reported to date, the negative ρ value indicates a buildup of positive charge in the transition state (TS). 13,17a,20 These results imply an earlier TS for reactions catalyzed by Ag(tpa)OTf, as compared to other catalysts proceeding via stepwise NT; however, the exact reasons for these mechanistic differences are currently unclear. 17e …”

“…8c,20,23 While this might be attributed to stepwise NT promoted by Fe, Ru, Co, and Ag, we considered the possibility that additional factors might be responsible for the behavior observed with Ag(tpa)OTf. For example, previous studies found that Ag(I) complexes supported by N-donor ligands show highly fluxional behavior in solution, depending on the counteranion and ligand identity.…”

Chemoselective single-site Earth-abundant metal catalysts at metal–organic framework nodes

“…Developing iron catalysts for sp 3 C–H amination is of particular interest due to the high abundance and low toxicity of iron484950. Although most Fe-catalysed C–H amination reactions use sulfamides or azides as a nitrogen source4546485051, we report here the first example of using aniline as a nitrogen source. Heating aniline with neat substrates such as indane, tetraline and cyclohexene gave a C–H aminated product in 41–53% yield with UiO-FeBr (2–10 mol% Fe, Table 4 and Supplementary Fig.…”

Catalyst-Controlled and Tunable, Chemoselective Silver-Catalyzed Intermolecular Nitrene Transfer: Experimental and Computational Studies

“…9 More recently, it has been shown that the chemoselectivity of nitrene transfer can be altered to favor C‒H insertion by changing the metal identity to Ru, Co, Mn, or Fe, which results in a change from a concerted to a stepwise nitrene transfer event. 3,4b,5,8 …”