Thioamidate Ion as Effective Cocatalyst for Photoinduced C–H Alkylation via Multisite Proton-coupled Electron Transfer

Abstract

Thioamidate ions are introduced as competent cocatalysts for multisite proton-coupled electron transfer (MS-PCET) catalysis. The effectiveness of these anions is ascribed to the high pKa values of their conjugate acids. The combined use of bench-stable tetrabutylammonium thioamidate with an appropriate Ir-based photosensitizer allows the C–H alkylation of a range of C–H donors, including simple cyclic alkanes, under visible-light irradiation.

Zwitterionic Diphenylphosphinyl Amidate as a Powerful Photoinduced Hydrogen-Atom-Transfer Catalyst for C–H Alkylation of Simple Alkanes

“…The Stern–Volmer luminescence quenching experiments confirmed the fast and predominant SET between the excited states of 3b and 2a (quenching rate constant; k q = 2.23 × 10 9 [M –1 s –1 ]). Since NMR analysis of a mixture of 2a and 3b showed no meaningful interaction between these two catalysts, the multisite proton-coupled electron transfer (MS-PCET) process would not be involved in this catalysis. , Therefore, catalytic C–H alkylation would proceed through SET to generate an amidyl radical from 2a followed by HAT with C–H donors. To gain insight into the HAT process, we examined deuterium kinetic isotope effects (KIEs) by performing the reactions of cyclohexane and cyclohexane- d 12 with benzalmalononitrile in two different vessels and in a single vessel, which resulted in k H / k D values of 1.34 and 1.90, respectively.…”

“…The cationic triazolium component renders N -benzoyl and N -acyl amidates, such as 1 , stable enough to be isolated under ambient conditions (Figure ) yet sufficiently competent to engage in single-electron transfer (SET) with a photoexcited Ir­(III) complex to form a reactive amidyl radical. During our continuous research to enhance the performance of this class of amidates as photoinduced HAT catalysts, we became interested in the alteration of the functional groups on amide nitrogen and their influence on the properties relevant to HAT activity under photoredox conditions. Here, we report the preparation and photophysical properties of N -phosphinyl amidates, which act as superior HAT catalysts compared to N -acyl analogs, thereby enabling the efficient C–H alkylation of simple alkanes in combination with Ir-based photocatalysts under visible-light irradiation.…”

“…With these data of the physical properties in hand, we evaluated the catalytic performance of triazolium amidates in the photoredox C–H alkylation. For this purpose, cyclohexane and benzalmalononitrile were selected as model substrate combination (Table ), and preliminary studies were performed to assess the feasibility of the bond formation with N -benzoyl amidate 1 as the HAT catalyst. Thus, a mixture of the substrates, 1 (5 mol %), and [Ir­(dF­(CF 3 )­ppy) 2 (4,4 ′ -dCF 3 bpy)]­PF 6 ( 3a , * E 1/2 red = 1.65 V vs SCE) (2 mol %) in 1,2-dichloroethane (DCE) was irradiated with blue LED light (Techno Sigma-PER-AMP, 800 W m –2 ) for 24 h, which gave a trace amount of alkylated product 4a (entry 1).…”