Template-Controlled Supramolecular Photodimerization of 2-Substituted Anthracenes Mediated by Metal–NHC Complexes

Abstract

The photochemical cycloaddition reaction has been extensively investigated for many years due to its efficient accessibility for intriguing photochemical products. Nevertheless, such a photoreaction is still an ongoing challenge to control regio- and stereoselectivity during the reaction process. Herein, a series of mononuclear M(I)–NHC (M = metal, NHC = N-heterocyclic carbene) complexes [M(1)2](PF6) (M = Ag, Au; 1 = 1a–c) were designed and synthesized from 2-substituted anthracene derivatives H-1(PF6) (1 = 1a–c) and characterized by nuclear magnetic resonance (NMR) spectroscopy, electrospray ionization-mass spectrometry (ESI-MS), and single-crystal X-ray diffraction analyses. Upon irradiation at 365 nm, the [4 + 4] photodimerization of complexes [M(1)2](PF6) (M = Ag, Au; 1 = 1a–c) successfully proceeded with 100% conversion and gave the syn-head-to-head (HH) photodimers syn-HH-[M(2)](PF6) (M = Ag, Au) as major products. Based on M(I)–NHC templates, syn-HH-[Au(2)](PF6) (2 = 2a–c) was generated in 87–92% relative yields and syn-HH-[Ag(2)](PF6) (2 = 2a–c) in 65–70% relative yields as determined by 1H NMR spectra. This phenomenon revealed the efficient supramolecular control of the M(I)–NHC templates on the regio- and stereoselectivity of the [4 + 4] photodimerization in solution, which can be applied to a series of photoreactions in a controllable manner.