Aldol reactions (self- and cross-aldol condensations) for conjugated enone synthesis were efficiently performed on large-sized Cs+ single sites (1 wt %) confined in β-zeolite channels in toluene, which showed the highest level of catalytic aldol condensation activity among reported zeolite catalysts. In general, aldol condensation reactions for C-C bond synthesis can proceed by acids (e.g., H+), bases (e.g., OH-), enolate species, and acidic or basic solid catalysts. However, the Cs+ single site/β sample without significant acid-base property showed unprecedented, efficient, and reusable catalysis for self-aldol and cross-aldol condensations. Intrinsically inactive Cs+ single sites due to the noble-gas electronic structure were transformed to active Cs+ single sites in β-zeolite channels. Cs+/β has many advantages such as broad substrate scope, eco-friendliness, high product selectivity and yield, and simple work-up procedure. Thus, the Cs+ single site/β provides an attractive and useful methodology for practical C-C bond synthesis. On the basis of the Cs+/β characterization by X-ray photoelectron spectroscopy (XPS), in situ X-ray absorption fine structure (XAFS) (X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS)), and temperature-programmed desorption (TPD), density functional theory (DFT) calculations of the self- and cross-aldol condensation reaction pathways involving the transition states on the Cs+ single site in β-zeolite channel revealed nontraditional concerted interligand bond rearrangement mechanisms.
Keywords: catalytic C−C bond formation; concerted interligand rearrangement mechanism; confined Cs+ single site in β zeolite pore; self- and cross-aldol condensation reactions.