Photoredox-HAT Catalysis for Primary Amine α-C–H Alkylation: Mechanistic Insight with Transient Absorption Spectroscopy

Mahima Sneha, Georgia Thornton, Luke Borrell, Alison Ryder, Sam Espley, Ian P. Clark, Alex Cresswell, Matthew Grayson, Andrew Orr-Ewing

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6 Citations (SciVal)

Abstract

The synergistic use of (organo)photoredox catalysts with hydrogen-atom transfer (HAT) cocatalysts has emerged as a powerful strategy for innate C(sp3)–H bond functionalization, particularly for C–H bonds α- to nitrogen. Azide ion (N3–) was recently identified as an effective HAT catalyst for the challenging α-C–H alkylation of unprotected, primary alkylamines, in combination with dicyanoarene photocatalysts such as 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN). Here, time-resolved transient absorption spectroscopy over sub-picosecond to microsecond timescales provides kinetic and mechanistic details of the photoredox catalytic cycle in acetonitrile solution. Direct observation of the electron transfer from N3– to photoexcited 4CzIPN reveals the participation of the S1 excited electronic state of the organic photocatalyst as an electron acceptor, but the N3• radical product of this reaction is not observed. Instead, both time-resolved infrared and UV–visible spectroscopic measurements implicate rapid association of N3• with N3– (a favorable process in acetonitrile) to form the N6•– radical anion. Electronic structure calculations indicate that N3• is the active participant in the HAT reaction, suggesting a role for N6•– as a reservoir that regulates the concentration of N3•.
Original languageEnglish
Pages (from-to)8004-8013
Number of pages10
JournalACS Catalysis
Volume13
Issue number12
Early online date30 May 2023
DOIs
Publication statusPublished - 16 Jun 2023

Bibliographical note

Data Availability Statement
Data are available at the University of Bristol data repository, data.bris,athttps://doi.org/10.5523/bris. 10lppiug4bgdk2nljkbto8qm73.

This research was funded by EPSRC Grant No.EP/R012695/1 and EP/L016354/1 and previous ERC Advanced Grant CAPRI 290966. G.L.T. thanks EPSRC for postgraduate studentship funding (EP/N509619/1). M.S. gratefully acknowledges award of a Marie Skłodowska - Curie Fellowship (MARCUS793799). A.J.C. thanks the Royal Society for a University Research Fellowship (UF150533) and the Centre for Sustainable Chemical Technologies (CSCT) for an EPSRCCDT PhD studentship(ASHR).S.G.E. thanks EPSRC and Astra Zeneca for an iCASE PhD studentship (EP/V519637/1). Thea uthors gratefully acknowledge the University of Bath’s Research.

Funding Information:
This research was funded by EPSRC Grant No. EP/R012695/1 and EP/L016354/1 and previous ERC Advanced Grant CAPRI 290966. G.L.T. thanks EPSRC for postgraduate studentship funding (EP/N509619/1). M.S. gratefully acknowledges award of a Marie Skłodowska-Curie Fellowship (MARCUS 793799). A.J.C. thanks the Royal Society for a University Research Fellowship (UF150533) and the Centre for Sustainable Chemical Technologies (CSCT) for an EPSRC CDT PhD studentship (ASHR). S.G.E. thanks EPSRC and AstraZeneca for an iCASE PhD studentship (EP/V519637/1). The authors gratefully acknowledge the University of Bath’s Research Computing Group (doi.org/10.15125/b6cd-s854) for their support in this work.

Keywords

  • azidyl radical
  • hydrogen-atom transfer
  • organic photocatalyst
  • photoredox catalysis
  • transient absorption spectroscopy

ASJC Scopus subject areas

  • General Chemistry
  • Catalysis

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