4-Sulfanyl-1,2,3-triazole as a Powerful Ligand in CuAAC to Synthesize 1,4-Substituted 1,2,3-Triazoles Under Solvent-Free and Low Catalyst Loading

4-Sulfanyl-1,2,3-triazole (L1) accelerated the solvent-free CuAAC efficiently with low catalyst loading (0.1 mol% for common azides and 1 mol% for sulfonyl azides). L1 exhibited higher catalytic activity compared to 1,4-substituted 1,2,3-triazole without sulfanyl group (5a) and sulfide, demonstrating that coordination of both sulfanyl and 1,2,3-triazole moieties with copper was critical to enhance the activity of L1. The Cu(OAc)2/L1 catalytic system displayed high selectivity in synthesis of alkynyl- or azido-involved 1,2,3-triazoles. The di-copper system Cu(OAc)2/CuBr/L1 promoted the reaction of electron-deficient and less reactive sulfonyl azides well, generating N-sulfonyl-1,2,3-triazoles in good yields, and L1 showed better performance than 1,3-di-o-tolylthiourea (L′). Other features of this protocol included recyclable ligand, 1:1 substance ratio, high yields, wide substance scope, and easily scaled up and facile purification of most products.