Detection of Fleeting Amine Radical Cations and Elucidation of Chain Processes in Visible-Light-Mediated [3 + 2] Annulation by Online Mass Spectrometric Techniques

Abstract

Visible-light mediated photoredox reactions have recently emerged as a powerful means for organic synthesis and thus have generated significant interest from the organic chemistry community. Although the reaction mechanisms have been probed by a number of techniques such as NMR, fluorescence quenching, and laser flash photolysis and various degree of success has been achieved, mechanistic ambiguity still exists (for instance, the involvement of the chain mechanism is still under debate) due to lack of structural information about the proposed and short-lived intermediates. Herein, we present the detection of transient amine radical cations involved in intermolecular [3+2] annulation reaction of N-cyclopropylaniline (CPA, 1) and styrene 2 by electrospray ionization mass spectrometry (ESI-MS) in combination with online laser irradiation of the reaction mixture. In particular, the reactive CPA radical cation 1+·, the reduced photocatalyst Ru(I)(bpz)3 +, and the [3+2] annulation product radical cation 3+· are all successfully detected and confirmed by high resolution MS. More importantly, the post-irradiation reaction with an additional substrate, the isotope-labeled CPA following photolysis of 1, 2 and Ru catalyst provide strong evidences to support the chain mechanism in the [3+2] annulation reaction. Furthermore, the key step of the proposed chain reaction, the oxidation of CPA 1 to amine radical cation 1+· by product radical cation 3+· (generated using online electrochemical oxidation of 3) is successfully established. Additionally, the coupling of ESI-MS with online laser irradiation has been successfully applied to probe the photostability of photocatalysts.

Assigning the ESI mass spectra of organometallic and coordination compounds

“…ESI‐MS spectrum of Ru (II)(bpz) 3 (PF 6 ) 2 and N‐cyclopropylaniline acquired with light irradiation of the sample. Adapted with permission from study of Cai et al [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…This can be especially useful in identifying short‐lived key reaction intermediates in metal‐catalyzed reactions. A recent example involves the detection of amine radical cations in the ruthenium‐catalyzed light‐mediated annulation shown below (Scheme ) …”

Selectivity control in thiol–yne click reactions via visible light induced associative electron upconversion

“…31,32 Monitoring of photoredox-catalyzed reactions by coupling of spray-based ionization mass spectrometers with online laser irradiation has been established as a reliable method. [33][34][35] For the thiol-yne click reaction, we coupled a green laser source with an electrospray ionization mass spectrometer (ESI-MS). The device used consisted of a green laser connected to the ESI chamber as shown in Fig.…”

Photochemical Synthesis of 1-Aminonorbornanes via Strain-Driven, Formal [3+2] Cycloadditions

“…Additionally, Chen, Zheng, and co-workers have recently provided evidence for propagative pathways for anilinocyclopropane-based formal [3+2] cycloadditions. 31 For the system presented herein, quantum yield measurements 32 do not directly indicate a highly propagative system ( = 0.34 for aminoCP 1a to 1-aminoNB 2a), and the high quenching efficiency and presence of facile post-oxidation fragmentation (thus minimizing backelectron transfer) eliminate two common explanations for artificially low quantum yields. 33 Instead, it is suspected that the 5-exotrig cyclization is sufficiently slow relative to the electron transfer processes that it renders the measured quantum yield <1 due to photons lost during the lifetime of distonic radical cation 7.…”