Chemistry of axially chiral binaphthol-based oligomers and dendrimers: Synthesis, characterization and catalysis.

摘要

A new and efficient method for the resolution of substituted binaphthols was developed. The resolution relies on the chromatographic separation of a diastereomeric mixture of sulfonates generated from substituted binaphthols and camphorsulfonyl chloride. The absolute configuration of the resolved binaphthols could be assigned from their chromatographic mobility.;Three series of axially chiral, binaphthol-based oligomers were prepared by an oligo-controlled synthesis strategy. In the first two series of compounds, the optically active (R)-binaphthyl ether served as the chiron units connected by either monoacetylene or ethylene spacer units. The monoacetylene-linked oligomer series was prepared by the reaction of binaphthol-based bromides and protected binaphthol-based acetylenes under the phase transfer Sonogashira coupling conditions. Because of the terminal acetylene was generated in situ, the formation of Hay coupling side products was suppressed. The ethylene-linked oligomer series was prepared by hydrogenation of monoacetylene-linked series. For the third series, the (S)-binaphthyl chiron units were linked directly to each other without any spacer moieties. This directly-linked series could be prepared by Suzuki coupling of binaphthol-based boronic acids and binaphthol-based halides. Unexpectedly, a high rotation barrier of the newly formed C-C bonds was noted. The results of the chiroptical measurements of these three series of oligomers showed that their molar rotation was directly proportional to the number of the binaphthyl units, indicating that the chirons were not arranged in a coherent fashion to exert cooperative chiroptical amplification. The ethylene- and directly-linked binaphthol, oligomers were used as ligands for kinetic study of a Diels-Alder reaction. The results of the kinetic study suggested that each binaphthol unit in these oligomers possessed nearly the same catalytic reactivity. This is probably due to the electronic segregation of the binaphthol catalytic centres in the oligomer chain.;In addition, the (S)-binaphthyl ethers were used as the peripheral groups for the construction of the zeroth and first generations of several aromatic dendrimers. From the chiroptical data, the molar rotation of these dendrimers was again proportional to the number of the chiron units. These dendrimers were also converted into the corresponding catalysts for the kinetic study of the same Diels-Alder reaction. Both the reactivity and enantioselectivity of these dendritic catalysts were slightly better than those of a model monomeric catalyst.