This paper describes a highly enantioselective metal-free hydrogenation of imines using chiral dienes as “ligands” for the generation of catalysts with HB(C6F5)2 by hydroboration in situ to furnish a ...variety of chiral amines with up to 89% ee, which provides a practical strategy for the development of novel chiral frustrated Lewis pairs for asymmetric hydrogenation.
Conspectus The use of frustrated Lewis pairs is an extremely important approach to metal-free hydrogenations. In contrast to the rapid growth of catalytic reactions, asymmetric hydrogenations are far ...less developed due to a severe shortage of readily available chiral frustrated Lewis pair catalysts with high catalytic activities and selectivities. Unlike the stable Lewis base component of frustrated Lewis pairs, the moisture-sensitive boron Lewis acid component is difficult to prepare. The development of convenient methods for the quick construction of chiral boron Lewis acids is therefore of great interest. In this Account, we summarize our recent studies on frustrated Lewis pair-catalyzed, asymmetric metal-free hydrogenations and hydrosilylations. To address the shortage of highly active and selective catalysts, we developed a novel strategy for the in situ preparation of chiral boron Lewis acids by the hydroboration of chiral dienes or diynes with Piers’ borane without further purification, which allows chiral dienes or diynes to act like ligands. This strategy ensures the construction of a useful toolbox of catalysts for asymmetric metal-free hydrogenations and hydrosilylations is rapid and operationally simple. Another strategy is using combinations of readily available Lewis acids and bases containing hydridic and acidic hydrogen atoms, respectively, as a novel type of frustrated Lewis pairs. Such systems provide a great opportunity for using simple chiral Lewis bases as the origins of asymmetric induction. With chiral diene-derived boron Lewis acids as catalysts, a broad range of unsaturated compounds, such as imines, silyl enol ethers, 2,3-disubstituted quinoxalines, and polysubstituted quinolines, are all viable substrates for asymmetric metal-free hydrogenations and give the corresponding products in good yields with high enantioselectivities and/or stereoselectivities. These chiral catalysts are very effective for bulky substrates, and the substrate scope for these metal-free asymmetric hydrogenations has been dramatically expanded. Chiral alkenylboranes were designed to enhance the rigidity of the framework and modify the Lewis acidity through the resulting double bonds. Frustrated Lewis pairs of chiral alkenylboranes and phosphines are a class of highly effective catalysts for asymmetric Piers-type hydrosilylations of 1,2-dicarbonyl compounds, and they give the desired products in high yields and enantioselectivities. Moreover, asymmetric transfer hydrogenations of imines and quinoxalines with ammonia borane as the hydrogen source have been achieved with frustrated Lewis pair of Piers’ borane and (R)-tert-butylsulfinamide as the catalyst. Mechanistic studies have suggested that the hydrogen transfer occurs via an 8-membered ring transition state, and regeneration of the reactive frustrated Lewis pair with ammonia borane occurs through a concerted 6-membered ring transition state.
A highly enantioselective hydrosilylation of 1,2-dicarbonyl compounds was successfully realized for the first time utilizing the combination of tricyclohexylphosphine and chiral alkenylborane derived ...in situ from diyne as a frustrated Lewis pair catalyst. A variety of optically active α-hydroxy ketones and esters were obtained in 52–98% yields with 86–99% ee’s.
A wide range of 2,3‐disubstituted quinoxalines have been successfully hydrogenated with H2 using borane catalysts to produce the desired tetrahydroquinoxalines in 80–99 % yields with excellent cis ...selectivity. Significantly, the asymmetric reaction employing chiral borane catalysts generated by the in situ hydroboration of chiral dienes with HB(C6F5)2 under mild reaction conditions has also been achieved with up to 96 % ee, and represents the first catalytic asymmetric system to furnish optically active cis‐2,3‐disubstituted 1,2,3,4‐tetrahydroquinoxalines.
At the core: A highly cis‐selective and enantioselective metal‐free hydrogenation of the title compounds was achieved using in situ generated chiral borane catalysts. The catalyst shown was generated from the chiral diene 1. The resulting tetrahydroquinoxalines products can be found as core structures for biologically active compounds.
Using a simple combination of tri-tert-butylphosphine and chiral borane generated in situ by the hydroboration of chiral diene with HB(C6F5)2 as a frustrated Lewis pair catalyst, a highly ...enantioselective metal-free hydrogenation of silyl enol ethers was successfully realized to furnish a variety of optically active secondary alcohols in 93–99% yields with 88->99% ee’s.
A metal-free direct hydrogenation of pyridines was successfully realized by using homogeneous borane catalysts generated from alkenes and HB(C6F5)2 via in situ hydroboration. The reaction affords a ...broad range of piperidines in high yields with excellent cis stereoselectivities.
Inspired by the zwitterion species generated from the splitting of H2 by frustrated Lewis pairs, we put forward a novel frustrated Lewis pair by the combination of Hδ‑ and Hδ+ incorporated Lewis acid ...and base together. Piers’ borane and chiral tert-butylsulfinamide were chosen as the FLP, and a metal-free asymmetric transfer hydrogenation of imines was realized with high enantioselectivities. Significantly, with ammonia borane as hydrogen source, a catalytic asymmetric reaction using 10 mol % of Piers’ borane, chiral tert-butylsulfinamide, and pyridine additive, has been successfully achieved to furnish optically active amines in 78–99% yields with 84–95% ee’s. Experimental and theoretical mechanistic studies reveal an interesting 8-membered ring hydrogen transfer transition state and an expected regeneration of reactive species with ammonia borane. Accordingly, a plausible catalytic pathway for this reaction is depicted.
Writing, erasing and computing are three fundamental operations required by any working electronic device. Magnetic skyrmions could be essential bits in promising in emerging topological spintronic ...devices. In particular, skyrmions in chiral magnets have outstanding properties like compact texture, uniform size, and high mobility. However, creating, deleting, and driving isolated skyrmions, as prototypes of aforementioned basic operations, have been a grand challenge in chiral magnets ever since the discovery of skyrmions, and achieving all these three operations in a single device is even more challenging. Here, by engineering chiral magnet Co
Zn
Mn
into the customized micro-devices for in-situ Lorentz transmission electron microscopy observations, we implement these three operations of skyrmions using nanosecond current pulses with a low current density of about 10
A·m
at room temperature. A notched structure can create or delete magnetic skyrmions depending on the direction and magnitude of current pulses. We further show that the magnetic skyrmions can be deterministically shifted step-by-step by current pulses, allowing the establishment of the universal current-velocity relationship. These experimental results have immediate significance towards the skyrmion-based memory or logic devices.
Topological charge Q classifies non-trivial spin textures and determines many of their characteristics. Most abundant are topological textures with |Q| ≤ 1, such as (anti)skyrmions, (anti)merons or ...(anti)vortices. In this study we created and imaged in real space magnetic skyrmion bundles, that is, multi-Q three-dimensional skyrmionic textures. These textures consist of a circular spin spiral that ties together a discrete number of skyrmion tubes. We observed skyrmion bundles with integer Q values up to 55. We show here that electric currents drive the collective motion of these particle-like textures similar to skyrmions. Bundles with Q ≠ 0 exhibit a skyrmion Hall effect with a Hall angle of ~62°, whereas Q = 0 bundles, the so-called skyrmioniums, propagate collinearly with respect to the current flow, that is, with a skyrmion Hall angle of ~0°. The experimental observation of multi-Q spin textures adds another member to the family of magnetic topological textures, which may serve in future spintronic devices.
PDF
