Light- and thermal-driven gold-catalyzed reaction of o-alkynylphenols with aryldiazonium salts: Computational insights into mechanistic similarities and differences

It was experimentally found that the gold-catalyzed reaction between o-alkynylphenols and aryldiazonium salts can lead to different products under thermo- and photocatalytic conditions; however, the mechanism is elusive. Herein we have employed both MS-CASPT2 and DFT approaches to study the catalytic mechanisms of the corresponding light- and thermal-driven reactions. The results show that both the thermo- and photo-catalytic reactions share some same elementary steps from the Au(I) catalyst and o-alkynylphenol, both of which generate a vinyl Au(I) intermediate with the aid of HCO3−. In these steps, the formation of a structurally distorted complex of the Au(I) catalyst and o-alkynylphenol has a free energy barrier of 14.8 kcal/mol in that the C≡C triple bond of o-alkynylphenol is seriously activated. Importantly, the thermo- and photo- catalytic reactions start to diverge from the complex formed between the generated vinyl Au(I) intermediate and the aryldiazonium salt. Under the dark condition, the reaction proceeds to generate the final thermal product after overcoming a free energy barrier of 15.7 kcal/mol, in which the terminal N atom of the aryldiazonium salt is bonded directly to the C atom of the Au(I) species. In the photoirradiation condition, the complex is first excited to its electronically excited singlet state, which then decays to the T1 state with an efficient intersystem crossing process of 9.8x109 s−1. In the T1 state, the denitrogen process completes easily after overcoming a free energy barrier of 7.8 kcal/mol resulting into an aryl radical interacting with the Au atom of the vinyl Au(I) species, which is followed by another intersystem crossing process from T1 to S0. In the S0 state, the final photocatalytic product is formed. The present work provides important mechanistic details on understanding both thermo- and photo-catalytic reactions of Au(I) catalysts and aryldiazonium salts, and most importantly, it is found that nonradiative transitions play an essential role in regulating photocatalytic reactions.