Porous surface of an achiral trimer in the chiral conglomerate phase catalyzes a direct aldol reaction
Porous materials having a large surface area and nanosized walls are of scientific and technological interest because of their abilities to interact with molecules at their surfaces. Recently, nanostructures in optically isotropic and layered chiral conglomerate phases, which are driven by spontaneous mirror symmetry breaking, have attracted much attention from researchers. Especially, helical nanofilaments in HNF phases can act as a porous nanoconfinement medium of large internal area to produce nanocomposites for applications. However, no catalytic activity has been reported for the layered chiral conglomerates so far. We recently reported flexible achiral liquid crystal trimers exhibiting novel dark chiral conglomerate (DC) phases. They were found to form a periodic porous structure at the surface in the DC phase. We attempted to apply the porous surface as a catalyst for aldol reactions. The racemic aldol product was obtained in 40% yield for the direct aldol reaction of acetone with benzaldehyde in DMSO/acetone (4/1) with 10 mol% of trimer I-(9,9) in the DC phase. When we used the trimer in the crystalline state or a bent-core molecule possessing Shiff base units in the HNF phase, no aldol product was obtained. These results reveal that the porous surface consisting of the trimer molecules plays an important role in the catalytic direct aldol reaction. To the best of our knowledge, this is the only known example that a layered chiral conglomerate phase driven by the spontaneous mirror symmetry breaking catalyzes the carbon–carbon bond formation in organic synthesis.