The radical mechanism of cobalt(ii) porphyrin-catalyzed olefin aziridination and the importance of cooperative H-bonding

Abstract

The mechanism of cobalt(II) porphyrin-mediated aziridination of styrene with PhSO(2)N(3) was studied by means of DFT calculations. The computations clearly indicate the involvement of a cobalt 'nitrene radical' intermediate in the Co(II)(por)-catalyzed alkene aziridination. The addition of styrene to this species proceeds in a stepwise fashion via radical addition of the 'nitrene radical'C to the C=C double bond of styrene to form a γ-alkyl radical intermediate D. The thus formed tri-radical species D easily collapses in an almost barrierless ring closure reaction (TS3) to form the aziridine, thereby regenerating the cobalt(II) porphyrin catalyst. The radical addition of the 'nitrene radical'C to the olefin (TS2) proceeds with a comparable barrier as its formation (TS1), thus providing a good explanation for the first order kinetics in both substrates and the catalyst observed experimentally. Formation of C is clearly accelerated by stabilization of C and TS1 via hydrogen bonding between the S=O and N-H units. The computed radical-type mechanism agrees well with all available mechanistic and kinetic information. The computed free energy profile readily explains the superior performance of the Co(II)(porAmide) system with H-bond donor functionalities over the non-functionalized Co(TPP).

Characterization of Porphyrin-Co(III)-‘Nitrene Radical’ Species Relevant in Catalytic Nitrene Transfer Reactions

“…4 Given the ubiquity of nitrogen atoms in biologically active compounds, 8 nitrene transfer reactions also have very important applications in making molecules of interest. However, the applicability of metal-nitrenoids has so far been limited to alkene aziridination, 9,10 C–H amination 11,12 and amidation. 13 …”

“…9,43 The mechanism of C–H bond amination of ethylbenzene, toluene, and 1,2,3,4-tetrahydronaphthalene (tetralin) using a series of different organic azides (N 3 C(O)OMe, N 3 SO 2 Ph, N 3 C(O)Ph and N 3 P(O)(OMe) 2 ) as nitrene sources was studied using density functional theory (DFT) and electron paramagnetic resonance (EPR) spectroscopy (Scheme 1a). 43 The mechanism of cobalt(II) porphyrin-mediated aziridination of styrene with PhSO 2 N 3 was also studied (Scheme 1b).…”

“…43 The mechanism of cobalt(II) porphyrin-mediated aziridination of styrene with PhSO 2 N 3 was also studied (Scheme 1b). 9 For both amination and aziridination reactions, the DFT calculations revealed a stepwise radical process involving coordination of the azide to the cobalt(II) center, followed by release of dinitrogen to produce an unusual ‘nitrene radical’ intermediate C (Scheme 1). In addition, experimental EPR spectroscopic studies, combined with DFT calculated EPR properties being in good agreement with the experimental data, revealed the formation of a (por)Co III –N • Y nitrene radical adduct C from the catalyst in the presence of an excess of the azide in benzene.…”

“…These nitrene radicals have thus far only been detected by means of X-band EPR spectroscopy. 9,43 Here we disclose their detection and characterization with a variety of spectroscopic and mass spectrometric techniques.…”

“…Thus, well-studied complexes with redox active aminyl ligand radicals are limited to only a few well-characterized examples, 65 and related complexes bearing redox non-innocent nitrene ligands are even scarcer. 9,43,65–67 Given the importance of these radicaloid nitrene species in metal-catalyzed nitrene transfer and C–H functionalization reactions, 10,12,13,39–42,68 we gathered more experimental evidence for the formation of the previously reported mono-nitrene radical complexes upon reaction of cobalt(II) porphyrins with nitrene transfer reagents. 9,43 Furthermore, we here reveal the first example of a bis-nitrene species of 1 P1 upon reacting the stronger oxidizing nitrene transfer agent N -nosyl iminoiodane 4 Ns with 1 P1 .…”