Highly Efficient Polyacrylonitrile Fiber Catalysts Functionalized by Aminopyridines for the Synthesis of 3‑Substituted 2‑Aminothiophenes in Water

Four aminopyridine (AP) functionalized polyacrylonitrile fibers (PANAPFs) were developed. UV–vis, FTIR, EA, and SEM were used to demonstrate the successful preparation of the fiber catalysts. Among the prepared catalysts, the PANp‑AP‑3F (with a C3 chain linker) and PANp‑AP‑6F (with a C6 chain linker) exhibited the best catalytic activity to efficiently catalyze the Gewald reaction of ethyl cyanoacetate and 2,5-dihydroxy-1,4-dithiane in water with yields up to 92% under a low catalyst dosage of 1 mol %. The influences of the position and length of the linker, i.e., the carbon chain between the fiber backbone and the aminopyridine moiety, on the catalytic activities were investigated in detail. In addition, the selected catalyst PANp‑AP‑3F was applied to the Gewald reaction of a series of active nitriles and 2,5-dihydroxy-1,4-dithiane and reused without further treatments. A possible mechanism and a microenvironment promoting process were conceived to explain the excellent catalytic properties of this catalytic system. Furthermore, the PANp‑AP‑3F performed well in a scaled-up experiment indicating its potential application in industry.