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

by Cai, Yi; Wang, Jiang; Zhang, Yuexiang; Li, Zhi; Hu, David; Zheng, Nan; Chen, Hao

Visible-light-mediated photoredox reactions have recently emerged as a powerful means for org. synthesis and thus have generated significant interest from the org. chem. community. Although the mechanisms of these reactions have been probed by a no. of techniques such as NMR, fluorescence quenching, and laser flash photolysis and various degrees of success has been achieved, mechanistic ambiguity still exists (for instance, the involvement of the chain mechanism is still under debate) because of the lack of structural information about the proposed and short-lived intermediates. Herein, we present the detection of transient amine radical cations involved in the intermol. [3 + 2] annulation reaction of N-cyclopropylaniline (CPA, 1) and styrene 2 by electrospray ionization mass spectrometry (ESI-MS) in combination with online laser irradn. of the reaction mixt. In particular, the reactive CPA radical cation 1+.bul., the reduced photocatalyst Ru(I)(bpz)3+, and the [3 + 2] annulation product radical cation 3+.bul. are all successfully detected and confirmed by high-resoln. MS. More importantly, the post-irradn. reaction with an addnl. substrate, isotope-labeled CPA, following photolysis of 1, 2, and Ru catalyst provides strong evidence to support the chain mechanism in the [3 + 2] annulation reaction. Furthermore, the key step of the proposed chain reaction, the oxidn. of CPA 1 to amine radical cation 1+.bul. by product radical cation 3+.bul. (generated using online electrochem. oxidn. of 3), is successfully established. Addnl., the coupling of ESI-MS with online laser irradn. has been successfully applied to probe the photostability of photocatalysts. [on SciFinder(R)]

Journal of the American Chemical Society
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1520-5126; 0002-7863