Electrophotocatalysis with a Trisaminocyclopropenium Radical Dication

Abstract

Visible-light photocatalysis and electrocatalysis are two powerful strategies for the promotion of chemical reactions. Here, these two modalities are combined in an electrophotocatalytic oxidation platform. This chemistry employs a trisaminocyclopropenium (TAC) ion catalyst, which is electrochemically oxidized to form a cyclopropenium radical dication intermediate. The radical dication undergoes photoexcitation with visible light to produce an excited-state species with oxidizing power (3.33 V vs. SCE) sufficient to oxidize benzene and halogenated benzenes via single-electron transfer (SET), resulting in C-H/N-H coupling with azoles. A rationale for the strongly oxidizing behavior of the photoexcited species is provided, while the stability of the catalyst is rationalized by a particular conformation of the cis-2,6-dimethylpiperidine moieties.

Keywords: C−H functionalization; Electrophotocatalysis; oxidation; radical dication; trisaminocyclopropenium ion.

Figure 1.

Generic paradigms for (A) photocatalytic and (B) electrocatalytic oxidations. (C) Electrophotocatalytic oxidation with TAC ion 1.

Figure 2.

(A) Electrophotocatalytic C–H/N–H coupling of benzene and pyrazole 4. (B) ¹H NMR spectra of (1) the reaction shown in Figure 2A; (2) the same conditions without catalyst 1 showing no reaction occurred; (3) the same reaction under direct electrolysis at 3.0 V; and (4) direct electrolysis of benzene at 3.0 V. (C) Visual comparison of attempted direct electrolysis and electrophotocatalytic reactions.

Figure 3.

Mechanistic rationale.

Figure 4.

(A) Level inversion of a photoexcited TAC radical dication. (B) TD-DFT calculated orbitals of TAC radical dication 2. (C) X-ray structure of TAC radical dication 2. (D) Synthesis of TAC catalyst 1.