Perspective

, Volume: 17( 2)

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Cobalt and Dual Photo Redox Catalysis Enable Site Specific Allylic Amination

*Correspondence:

John Newton
Department of Chemistry,
The University of Lorraine,
Lorraine,
France;
E-mail: Newton.J@yahoo.com

Abstract

A challenge in green synthetic chemistry is the creation of an extremely effective, selective and atom economical process for the synthesis of allylic amines. Lei, Qi and coworkers recently reported that the use of a photoredox catalyst in conjunction with a cobaloxime catalyst enables site selective allylic amination of different olefins with secondary alkyl amines, producing the valuable tertiary aliphatic allylamines without the need for external oxidants.

Keywords

Aliphatic allylamines; Hydrogen evolution; Photocatalysis; Cobaloxime; Hydrogen atom transfealkenes

Introduction

A diverse class of building blocks for synthetic chemistry, aliphatic allylamines is present in many physiologically and pharmacologically active molecules. One of the best ways to make allylic amines is probably to prepare them directly from aliphatic amines and olefins using hydrogen evolution. However, this approach is still mainly unexplored because of several long-standing difficulties. The catalytic activity of a transition metal catalyst can be significantly reduced by aliphatic allyl amines due to their great tendency to coordinate with metals. Second, because of the significant redox activity of tertiary alkyl allyl amines, the products are vulnerable to oxidative degradation. Additionally, the thermodynamics are not in favour of the conventional oxidative hydrogen evolution coupling strategy. It is never easy to manage the regioselectivity in the double bond formation stage of the radical addition-triggered alliylic amination. Lei, Qi and coworkers recently revealed that the use of a photo catalyst and a cobaloxime enables mild and persistent extremely site selective allylic amination of alkenes with secondary amines by H₂ evolution. Notably, this technique avoids the substrate pre functionalization phase and makes use of commonly available starting materials.

Description

Additionally, this redox neutral process does not require stoichiometric exogenous oxidants, emphasizing its atom economical potential. The allylic amination procedure developed by Lei and Qi can be used with a variety of ditrisubstituted olefins and even tetrasubstituted ones. Additionally, it showed good site selectivity, preferentially cleaving the allylic C-H bond with the least amount of steric resistance. Even the principal allylic C-H bond, which has higher bond energy, is effectively broken, allowing for the synthesis of terminal alkene products that are thermodynamically unfavourable. The approach also works well with amine sources and may handle a variety of secondary alkylamines with different structural compositions. It is possible to successfully transform amine derivatives such piperidine, piperazine, morpholine, isoquinoline and phenethylamine into the equivalent allylamine compounds. The equivalent allylamine compounds can be successfully converted. Additionally, both cyclic and acyclic simple amines, such dimethylamine, can be processed using the catalytic system. Surprisingly, the application of a variety of structurally more complex secondary amines to late stage amination provided more proof of this strategy’s good functional group. The authors next looked into the reaction’s limiting elements, such as why the olefins needed to be present in such significant excess. The discrepancy between the hydrogen yield and the amination yield suggests the presence of conflicting processes. The scientists used X-ray Absorption Fine Structure (XAFS) spectroscopy to first investigate the interaction of (Co(dmgH)2Py2)PF6 with morpholine and tetramethylethylene in order to shed further light on the mechanism. The cobaloxime’s metal centre does not coordinate with alkenes or amines, according to the XAFS spectra. Additionally, they conducted spin trapping and in situ Electron Paramagnetic Resonance (EPR) investigations. The results show that the excited state amine is oxidised. The early and late transition metals reactivity characteristics. The authors draw the conclusion from these findings that the steric barrier between the intermediate II and the oxime ligand in the HAT process should be the cause of the reaction’s site-selectivity. With less steric resistance, the reaction energy barrier on the terminal methyl group is smaller. With less steric resistance, the reaction energy barrier regarding the HAT of the terminal methyl group is smaller. Additionally, the energy barriers for various reaction pathways differ according to the degree of steric hindrance, which results in site selectivity that is the reverse of the thermodynamic trend.

Conclusion

Dual photoredox and cobalt catalysis opens a new and exciting avenue for transition metal catalyzed cross coupling reaction. The ability of this approach to access low valent cobalt species gives enormous opportunity to develop reactions that relies on stoichiometric methods. Cobalt catalyzed is highly branched and enantioselective allylic amination of racemic branched allylic carbonates bearing alkyl groups with both aromatic and aliphatic amines has been developed.