Xueli Zheng

Releasing 17.8 wt% H2 from lithium borohydride ammoniate

Release of ca. 17.8 wt% of hydrogen was observed from the Co-catalyzed lithium borohydride ammoniate, Li(NH3)4/3BH4 (with equivalent protic and hydridic hydrogen atoms, composed of solid Li(NH3)BH4 and liquid Li(NH3)2BH4), in the temperature range of 135 to 250 °C in a closed vessel. The low NH3 equilibrium vapor pressure of the ammoniate in the vessel results in the retention of the majority of NH3 in the vicinity of LiBH4, and thus, creates an environment favorable for the direct dehydrogenation rather than deammoniation. The dehydrogenation is a two-step process forming the intermediates Li4BN3H10 and LiBH4. The final solid residue is a mixture of BN and Li3BN2. The presence of nanosized Co catalyst effectively promote the hydrogen release.

Aqueous Phase Hydrogenation of Quinoline to Decahydroquinoline Catalyzed by Ruthenium Nanoparticles Supported on Glucose‐Derived Carbon Spheres

A new ruthenium catalyst supported on glucose‐derived carbon spheres was prepared by using the impregnation method and then characterized by X‐ray photoelectron spectroscopy, XRD, TEM, SEM, FTIR, and solid‐state 13C MAS NMR techniques. With water as the environmentally friendly solvent, the conversion of quinoline and the selectivity toward decahydroquinoline could reach 98.2 and 95.2 %, respectively, under mild conditions (reaction temperature=393 K and H2 pressure=2.0 MPa). The catalyst could be recycled thrice without decrease in activity and selectivity. The excellent catalytic performance of this catalyst may be attributed to the hydrogen bond between the hydroxyl group of the support surface and the nitrogen atom in quinoline as well as to the π–π interaction between the organic support and quinoline.

Ambient temperature hydrogen desorption from LiAlH4-LiNH2 mediated by HMPA

By adding a small amount of HMPA approximately 2.5 equivalent or ca. 8.1 wt% H2 can be released from the LiAlH4–LiNH2 mixture at ambient temperature, which is ∼370 °C lower than the corresponding solid-state dehydrogenation. NMR characterizations reveal the formation of intermediates, such as [AlH] and [AlN] species, during the dehydrogenation. LiNH2 induces the enrichment of [AlH] species in the liquid phase which further reacts with LiNH2 to produce H2, Li2NH and Al.

Pd/tetraphosphine catalytic system for Cu-free Sonogashira reaction "on water"

An efficient copper-free Sonogashira coupling reaction was performed on water at 100 °C with N,N,N′,N′-tetra(diphenylphosphinomethyl)pyridine-2,6-diamine (1) as a ligand, [Pd(η3-C3H5)Cl]2 as a catalyst precursor and K3PO4 as a base. Both aryl and heteroaryl halides were successfully alkynylated in this system, and a high turnover number (TON) up to 860 000 was obtained with a catalyst loading as low as 1 ppm.

Construction of pincer-type symmetrical ruthenium(II) complexes bearing pyridyl-2,6-pyrazolyl arms: catalytic behavior in transfer hydrogenation of ketones

Convenient synthesis of four new distorted octahedral ruthenium(II) complexes (1, 2, 3, 4) having general molecular formula [RuCl2LPAr3] (L = pyridine-based tridentate ligands not containing N–H bonds) is described. Their composition and structure were determined by elemental analysis and NMR spectra, and complexes 2 and 4 were also identified by X-ray single-crystal diffraction. All ruthenium(II) complexes exhibited good to excellent catalytic activity in the transfer hydrogenation of ketones. Among them, complex 4 achieved the highest final TOF value of 51 600 h−1 for a high molar ratio of substrate to catalyst (2000 : 1).

Combination of RuCl3·xH2O with PEG – a simple and recyclable catalytic system for direct arylation of heteroarenes via C–H bond activation

A simple and recyclable catalytic system for direct arylation of heteroarenes via C–H bond activation was developed with a relatively inexpensive RuCl3·xH2O as a catalyst and PEG-400 as a green medium without any additive or ligand. This system not only showed excellent functional group compatibility, but also the ratio of mono- to diarylated product was easily regulated by varying the reaction conditions. Moreover, this transformation could proceed under air and be easily scaled up to gram-scale in a low catalyst loading of 0.3 mol%. Particularly, a good yield of 85% was obtained after this catalyst was recycled six times.

Direct C–H Functionalization of Pyridine via a Transient Activator Strategy: Synthesis of 2,6-Diarylpyridines

A Pd-catalyzed highly selective direct diarylation of pyridines has been developed using a transient activator strategy. Both (MeO)2SO2 and Cu2O are required for this transformation. The in situ generated N-methylpyridinium salt can be arylated at both 2- and 6-positions under the cooperative Pd/Cu catalysis. A subsequent N-demethylation then gives the 2,6-diarylpyridines. This protocol provides a novel synthetic route for the symmetric 2,6-diarylpyridines.

Synthesis of phosphorus amidite ligand and investigation of its flexibility impact on rhodium-catalyzed hydroformylation of 1-octene

A new flexible 2,2′-bis((dipyrrolylphosphinooxy)methyl)-1,1′-(±)-biphenyl ligand (L1) was synthesized, characterized and employed in the hydroformylation of 1-octene. In order to investigate the influence of ligand flexibility on the catalytic performance, its analogue 2,2′-bis (dipyrrolylphosphinooxy)-1,1′-(±)-biphenyl (L3) was also applied in the hydroformylation of 1-octene. With the presence of L1, the aldehyde selectivity was approximately 10% higher than that with the relevant less flexible ligand L3. Theoretical calculation indicated that the olefin-insertion into the Rh–H bond of intermediate III and the CO-insertion into Rh–alkyl bond of intermediate V were favorable in terms of thermodynamics, which were vital to the generation of aldehyde. Meanwhile, the ligand flexibility was indeed improved by adding a methylene between benzene and oxygen atom that connected with P, as the calculation showed the variation of the bite angle ∠P–Rh–P of the intermediates I–VI was 8.7° in L1-system, as for L3-system, the variation was 14.0°. This structural feature might make the olefin-insertion and the CO-insertion occur more easily in the L1-system and resulted in higher aldehyde selectivity.

Rh(III)-Catalyzed [4 + 2] Self-Annulation of N-Vinylarylamides

An efficient rhodium(III)-catalyzed self-annulation of N-vinylarylamide has been developed. This reaction features a simple system and good reactivity with complete regioselectivity. The protocol provides easy access to an aminal incorporated dihydroisoquinolinone, which proved to be a versatile synthetic synthon. The kinetic isotope effect experiments showed that C-H activation is the rate-limiting step, and competition studies revealed the annulation exhibits a strong self-recognition mode. In addition, a seven-membered rhodacycle species was isolated and established as the key reaction intermediate.

Palladium-Catalyzed Domino Reaction for Stereoselective Synthesis of Multisubstituted Olefins: Construction of Blue Luminogens

The first Pd-catalyzed multicomponent reaction of aryl iodides, alkenyl bromides, and strained alkenes has been developed, which allowed us to synthesize a variety of multisubsituted olefins in yields of 45-96% with excellent stereoselectivity. The configuration of the product was controlled by the configuration of the alkenyl bromides. Moreover, this practical methodology employing readily available substrates was found to be tolerant to a wide range of functional groups. Fifty six examples of highly stereoselective tri- or tetrasubstituted olefins have been successfully synthesized via this methodology. Most of the synthesized tetrasubstituted olefins are good aggregation-induced emission (AIE) luminogens.