Insight into the Mechanism of the CuAAC Reaction by Capturing the Crucial AuCu-π-Alkyne Intermediate.

The classic Fokin mechanism of the CuAAC reaction of terminal alkynes using a variety of Cu(I) catalysts is well-known to include alkyne deprotonation involving a bimetallic σ,π-alkynyl intermediate. In this study, we have designed a CNT-supported atomically precise nanocluster AuCu (noted AuCu/CNT) that heterogeneously catalyzes the CuAAC reaction of terminal alkynes σ,π. Therefore, three nanocluster-π-alkyne intermediates [AuCu(π-CH≡C-p-CHR)], R = H, Cl, and CH, have been captured and characterized by MALDI-MS. This AuCu/CNT system efficiently catalyzed the CuAAC reaction of terminal alkynes, and internal alkynes also undergo this reaction. DFT results further confirmed that HC≡CPh was activated by π-complexation with AuCu, unlike the classic dehydrogenation mechanism involving the bimetallic σ,π-alkynyl intermediate. On the other hand, a Cu/CNT catalyst was shown to catalyze the reaction of terminal alkynes following the classic deprotonation mechanism, and both Au/CNT and Cu/CNT catalysts were inactive for the AAC reaction of internal alkynes under the same conditions, which shows the specificity of AuCu involving synergy between Cu and Au in this precise nanocluster. This will offer important guidance for subsequent catalyst design.