Mechanistic Insights into the Nitrogen Transfer Reactivity of Synthetic Metalloporphyrins

Abstract

Nitrogen insertion is a significant synthetic tool as multiple drug targets feature an aziridine or amine moiety. Adaptation of effective oxygen functionalization metalloporphyrin catalysts can be achieved with nitrogen-based oxidants. However, limited mechanistic understanding of the active species for the nitrogen transfer has hindered their development. The present work improves the field of nitrogen transfer by elucidating oxidation mechanisms of manganese porphyrins for nitrogen transfer as well as the interaction of nitrogen-based oxidants with μ-nitridoiron porphyrin dimers.

The mechanism of nitridomanganese(V) porphyrin formation was shown to involve chloramine as the oxidant formed in situ from an oxygen-based oxidant and ammonium hydroxide. Oxidation of manganese(III) porphyrins with authentic chloramine showed a great dependence on the identity of the metal's axial ligands, which can help oxidation by deprotonating chloramine and reducing electronic rearrangement requirements. Chloramine has also been shown to oxidize manganese(II) porphyrins to a mixture of nitridomanganese(V) and manganese(III) porphyrins likely via disproportionation of an initially formed imidomanganese(IV) porphyrin.

Olefin aziridination by nitridomanganese(V) porphyrins has been proposed to involve imidomanganese(V) intermediates. Reexamination of this mechanism has demonstrated that all spectral changes correspond to a protonation event at the nitrido nitrogen. EPR investigation of the activated nitridomanganese porphyrin showed signals for both a porphyrin radical-cation and

a MnIV complex. It is proposed that previous characterization ascribed to the acylimidomanganese(V) porphyrin can be explained by disproportionation between the nitridomanganese(V) porphyrin and its protonated form to an unreactive nitridomanganese(V) porphyrin radical-cation and an imidomanganese(IV) porphyrin. Furthermore, protonation of the nitridomanganese(V) porphyrin has been shown as a viable method to achieve unsubstituted aziridine formation via direct [NH] transfer.

Finally, this work expands the utility of mu-nitridoiron porphyrin dimers in oxidative reactions by studying their interaction with nitrogen-based oxidants. Generally, addition of nitrogen-based oxidants to mu-nitridoiron porphyrin dimers showed only formation of their one-electron oxidized cations. The cation of (FeT-pOMe-PP)2N was further oxidized by PhINTs to a proposed imido-FeIVT-pOMe-PP+.-N-FeIIIT-pOMe-PP. Unexpected C-H chlorination was observed in the reaction of silica-loaded (FeTPP)2N with adamantane and chloramine-T. The catalytic chlorination is believed to proceed with (FeTPP)2N+ acting as an oxidatively-stable Lewis acid, indicating possible future applications of this catalyst.