The synthesis and characterization of four dendron-containing tetraphenylethylenes (TPEs), 1(1)-1(4), were synthesized, along with a TPE compound that contained four OCH(2) Ph groups (referred to as 1(0)) for comparison. Photophysical studies revealed that the TPE core became emissive after linking dendrons onto its periphery. Moreover, the fluorescence intensity was significantly enhanced when high-generation dendrons were linked onto the TPE core; the fluorescence intensity increased in the following order: 1(1)<1(2)<1(3)<1(4). This phenomenon was tentatively attributed to an enhancement in the energy barrier for internal rotation and torsion of the TPE core to which four dendrons were connected. In addition, the photocyclization of the TPE core into the respective 9,10-diphenylphenanthrene was facilitated when high-generation dendrons were linked to the TPE core. Again, the photocycliztion reactivity increased in the following order: 1(1)<1(2)<1(3)<1(4). We found that the fluorescence and photocyclization reactivity of TPE could be modulated by covalent interactions with dendrons, and such modulation was strongly dependent on the dendron-generation.

Dedicated to Professor Albert S. C. Chan on the occasion of his 60th birthdayThe development of new effective ligands is a continual challenge in transition-metal-catalyzed asymmetric reactions. During the last several decades, a number of excellent chiral bidentate ligands have been reported. [1] Among them, there are no universal ligands reported since catalytic asymmetric transformations are often substrate-dependent. Subtle changes in conformation, sterics, and electronic properties of chiral ligands can lead to dramatic variations of reactivity and enantioselectivity. Therefore, the development of structurally and/or conformationally tunable chiral ligands will offer a great advantage for optimizing the enantioselectivity of a reaction. [2] Herein, we report a new kind of easily available and conformationally tunable phosphorus metallacrown ethers for asymmetric hydrogenation.Metallacrown ethers are a class of metallomacrocycles that are often formed by the chelation of a,w-bis(phosphorus-donor)polyether ligands to transition metals. [3,4] Like crown ethers, they have proven to be capable of binding alkali-metal cations selectively. In addition to their receptor properties, they are expected to be of interest as catalysts for organic reactions owing to the potential bifunctionality of such hard-soft bimetallic complexes and the tunability of the catalyst conformation. Surprisingly, the application of metallacrown ethers as catalysts is less studied. [4c, e] Most re-cently, Gray and co-workers reported a Rh I complex containing an a,w-bis(phosphite) ligand with two amido groups for the hydroformylation of styrene. [4c] It was found that the addition of alkali-metal salts could result in obvious increases in the regioselectivity. However, the use of chiral metallacrown for asymmetric catalysis has never been reported to the best our knowledge. Inspired by these facts and as a continuation of our ongoing endeavor to develop effective chiral supramolecular catalysts based on crown ethers, [5] for this study we choose easily available and length-tunable oligo(ethylene glycols) (OEG) as the backbone of the ditopic ligand. First complexation of the ligand with alkali-metal ions is expected to bring the two terminal ligating groups closer together, [6] furnishing a new type of template-induced bidentate ligand. [7,8] The subsequent coordination of the preformed ditopic ligand to the catalytically active transitionmetal ions thus gives the metallacrown ether catalyst (Scheme 1). With this template-induced strategy, the catalyst would be prepared more efficiently, avoiding the formation of supramolecular coordination polymers and/or oligomers. [4a, e] To exemplify our approach, we report here for the first time a class of supramolecular chiral binol-derived diphosphite ligands and the effect of alkali-metal ions on the enantioselectivity by taking Rh-catalyzed asymmetric hydrogenation of dehydroamino acid derivatives as the reaction prototype. Up to 17 % enhancement of ee value was observed in a favorable case.Firs...