Chunming Cui

[(NHC)Yb{N(SiMe3)2}2]-catalyzed cross-dehydrogenative coupling of silanes with amines.

Top cat: [(NHC)Yb{N(SiMe(3))(2)}(2)] adducts (NHC = N-heterocyclic carbene) are efficient catalysts for catalytic cross-dehydrogenative coupling of silanes with a range of primary and secondary amines to yield silylamines in high yields (82-100%) under mild reaction conditions. The catalytic activity and selectivity of the rare-earth-metal silylamides are modulated by altering the steric bulk of the NHC.

N-Aryl substituted heterocyclic silylenes

Two N-aryl substituted five-membered heterocyclic silylenes and their nickel complexes have been isolated and structurally characterized.

Comparison of Anionic and Lewis Acid Stabilized N‐Heterocyclic Oxoboranes: Their Facile Synthesis from a Borinic Acid

There is great current interest in the synthesis and characterization of boron species containing multiple bonds. Oxoboranes RBO, compounds containing a boron–oxygen multiple bond, have long been known to be highly reactive and tend to form oligomers owing to the oxophilicity of the unsaturated boron atoms. Consequently, for a long time oxoboranes were only invoked as intermediates and studied in the gas phase or low-temperature matrix. In 2005, Cowley and co-workers reported the first isolable Lewis acid stabilized oxoborane supported by a b-diketiminato ligand. More recently, Braunschweig and co-workers reported the platinum oxoboryl complex trans-[(PCy3)2PtBr(BO)] (Cy = cyclohexyl), in which the low-coordinate boron center is stabilized by a late-transition-metal fragment. It has been shown that three-coordinate anionic boron species may also possess double-bonding character, as demonstrated by the anionic (Mes2BCH2) and dianionic (R2BBR2) 2 species reported by Power and others, which contain a boron– carbon and a boron–boron double bond, respectively. Inspired by the pioneering work on the stabilization of elusive oxoboranes and anionic boron p-bonding species, we anticipated that free monomeric oxoboranes (R2BO) would be accessible and may exhibit boron–oxygen double-bond character if suitable ligand frameworks are employed. A fair number of main-group and transition-metal complexes supported by diaryl and dialkyl boroxides have been synthesized and structurally characterized by several groups. However, metal-free anionic oxoboranes (R2BO) are still elusive. Herein, we report on the synthesis and structural characterization of the anionic oxoboranes 3a,b and the Lewis acid stabilized oxoboranes 4a,b prepared from the borinic acid 2 by deprotonation of the hydroxy group with N-heterocyclic carbenes (NHCs) and the hydrogen migration promoted by Lewis acids, respectively, as outlined in Scheme 1 and Scheme 2. We recently reported the synthesis and structural characterization of the first boron compounds containing B=S and B=Se double bonds. These species were prepared by insertion of the chalcogen atom into a B H bond and subsequent hydrogen migration to the exocyclic methylene group of a deprotonated b-diketiminato ligand. This finding prompted us to investigate the possibility to isolate the analogous oxoboranes that are free of Lewis acids by taking advantage of the unique electronic and steric effects of this family of ubiquitous ligands. As the direct introduction of an oxygen atom using dioxygen was not successful, an alternative route has been designed. Thus, the boron bromide 1 was prepared by the reaction of LLi (L = [HC(CMe)2(NAr)2] , Ar = 2,6iPr2C6H3) [11] with the donor–acceptor adduct Et3N·BBr3 (Scheme 1). The employment of a Br3B·base adduct is crucial for the clean synthesis of 1, as a suitable base such as NEt3 deprotonates one of the b-methyl groups on the ligand backbone and acts as an HCl acceptor. A similar approach has been employed for the generation of heavy Group 14 carbene analogues and a monomeric boron hydride. 12] In contrast, direct reaction of LLi with BBr3 led to insoluble materials, which cannot be completely identified to date. Hydrolysis of 1 in the presence of 1,3-diisopropyl-4,5-dimethylimidazol-2ylidene (IMe2iPr2) resulted in the formation of the borinic acid 2 in high yield. Unlike the analogous sulfur and selenium derivatives, which easily undergo hydrogen migration from the SH and SeH groups, 2 is rather stable, and hydrogen migration from the OH group did not occur at ambient temperature, probably owing to the lower acidity of the hydrogen atom of the OH group compared to the heavy analogues. NHCs are well-known proton acceptors. It is anticipated that the addition of an NHC to 2 might result in the weakening of the O H bond, and thus either facilitate the hydrogen migration or form an anionic oxoborane. As expected, reaction of 2 with 1,3,4,5-tetramethylimidazol-2ylidene (IMe4) [14] yielded the ionic compound 3a. The Scheme 1. Synthesis of 2 and 3a,b.

Synthesis and Characterization of Linear and Square-Planar Nickel Complexes with Primary Amido Ligands

The synthesis and characterization of the two homoleptic mononuclear nickel complexes (2,6-Dipp2C6H3NH)2Ni ( 1) and [2-C(H)NDippC6H4NH] 2Ni (2) (Dipp = 2,6-Pr(i)2C6H3) are described. 1 is formally two-coordinate and adopts a strictly linear geometry, while 2 features a slightly distorted square-planar geometry. Electrochemistry of 1 and 2 shows that they can be reduced to the corresponding nickel(I) species and oxidized to the corresponding nickel(III) species reversibly or quasi-reversibly. A solid-state magnetic measurement (mu(eff )= 2.79 mu(B)) for paramagnetic 1 indicates the presence of two unpaired electrons on the nickel center.

Access to B=S and B=Se Double Bonds via Sulfur and Selenium Insertion into a B―H Bond and Hydrogen Migration

Stable compounds with a boron-chalcogen (S or Se) valence double bond have been prepared via sequences involving insertion of the chalcogen into a B-H bond and subsequent hydrogen migration. X-ray diffraction studies and density functional theory calculations on the resulting compounds provide convincing evidence for the boron-chalcogen multiple bonding.

Synthesis of a Base-Stabilized 1-Hydrosilanimine via NHC-Mediated Dehydrohalogenation of Hydrochlorosilane

In recent years, unsaturated silicon compounds (silylenes and silicon multiple-bonding species) have attracted much attention owing to their significant roles in the development of silicon chemistry. Silanimines (R2Si =NR), as silicon analogues of imines, are, in general, short-lived and highly reactive species in the absences of efficient protecting groups on both the silicon and nitrogen atoms. By taking advantage of donor–acceptor stabilization concepts, a fair number of donor-stabilized silanimines have been isolated and structurally characterized since the first isolable silanimine was reported in 1986. It is well-known that organosilicon hydrides have been widely used as hydrosilylation reagents in organic synthesis. However, unsaturated silicon hydrides, in which the hydride ligands are bonded to an unsaturated silicon atom, are extremely rare. The lack of these species may be related to their intrinsic high reactivity arising from a reactive hydride ligand being attached to the unsaturated silicon atom. Among these, hydrosilanimines (RHSi=NR) are interesting and attractive synthetic targets because of their fundamental interest considering the relative stability of hydrosilanimines and their corresponding silylene isomers (RSiNHR). Furthermore, it has been recently shown that transition-metal–hydrosilanimine complexes are useful for some stoichiometric and catalytic transformations. However, stable hydrosilanimines are still elusive, presumably owing to their high reactivity and lack of synthetic methods for their synthesis. Herein, we report the first donor-stabilized hydrosilanimine prepared by the reaction of RSiHCl2 (1, R= ArACHTUNGTRENNUNG(SiMe3)N , Ar= 2,6-iPr2C6H3) with N-heterocyclic carbenes (NHCs; Scheme 1). In addition, we demonstrate that the reactions of 1 with NHCs are significantly influenced by the steric effects of the substituents on the NHC nitrogen atoms. We and others have previously reported NHC-mediated dehydrochlorination reactions for the generation of stable and transient silylenes. To extend that methodology, we examined the reactions of the silylamino-substituted dichlorosilane RSiHCl2 (1) with NHCs. As previously observed in the reactions of various N-heterocyclic chlorosilanes with NHCs, the products varied largely depending on the reaction conditions and the substrates employed. Therefore, it is essential to probe the steric effects of NHCs on the reaction. Thus, the less-hindered 1,3,4,5-tetramethylimidazol-2-ylidene (IMe4) and the bulky 1,3-bis(tert-butyl)imidazol-2-ylidene (ItBu2) were chosen for this study. [9] The reaction of 1 with one equivalent of IMe4 in C6D6 at room temperature gave a complicated mixture of products as indicated by NMR analysis. However, unexpectedly, the reaction of 1 with two equivalents of IMe4 in tetrahydrofuran at low temperature gave 1-hydrosilanimine 2 (Scheme 1) as yellow crystals in 26 % yield along with some insoluble materials. Compound 2 is air and moisture sensitive, and it is soluble in common hydrocarbon solvents such as n-hexane and toluene. The proton NMR spectrum of 2 indicated the presence of one Si H (d= 6.21 ppm, JSi-H = 218 Hz) bond, two Ar rings, one SiMe3 group, and one IMe4 group in the structure. The detailed structure of 2 was finally determined by single-crystal X-ray analysis. X-ray structural analysis of 2 revealed that it was an NHC-stabilized 1-hydrosilanimine (Figure 1). The central silicon atom was four-coordinate and adopted a distorted [a] H. Cui, Prof. Dr. C. Cui State Key Laboratory of Elemento-organic Chemistry Nankai University Tianjin, 300071 (China) Fax: (+86)-22-23503461 E-mail : cmcui@nankai.edu.cn Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201100022. Scheme 1. Synthesis of 2 and 3. IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene.

Cesium Carbonate Catalyzed Chemoselective Hydrosilylation of Aldehydes and Ketones under Solvent‐Free Conditions

Cs2CO3 has been found to be an efficient and chemoselective catalyst for reduction of aldehydes and ketones to alcohols with one equivalent of Ph2SiH2 as the reductant under solvent-free conditions. Most of the aldehydes employed can be effectively hydrosilated quantitatively to give the corresponding silyl ethers in 2 h at room temperature, whereas the hydrosilylation of ketones proceeded smoothly at 80 °C. The catalyst system tolerates a number of functional groups including halogen, alkoxyl, olefin, ester, nitro, cyano, and heteroaromatic groups; the selective hydrosilylation of aldehydes in the presence of ketone can be effectively controlled by temperature; and hydrosilylation of α,β-unsaturated carbonyls resulted in the 1,2-addition products. The catalytic hydrosilylation of suitable dicarbonyls can be applied to the synthesis of poly(silyl ether)s with a high molecular weight and narrow molecular distribution.

Synthesis of 1,2-borazaronaphthalenes from imines by base-promoted borylation of C-H bond.

A new route from benzylic imines permits the synthesis of 1,2-borazaronaphthalenes in good yields. The reaction involves formation of the enamidyl dibromoborane, which undergoes base-promoted borylation of the nearby aromatic C-H bond. Electrophilic attack of the boron species onto the benzylic arene is supported by the slow borylation of arenes substituted with electron-withdrawing groups. The resultant boron bromides can be easily substituted with lithium reagents to provide a range of products. The electronic properties of these 1,2-borazaronaphthalene derivatives have been investigated by UV-vis and fluorescence spectroscopy.

N‐Heterocyclic Carbene Organocatalysts for Dehydrogenative Coupling of Silanes and Hydroxyl Compounds

Go organic! N-Heterocyclic carbene (NHC) 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IiPr) has been found to be an efficient and selective catalyst for the dehydrogenative coupling of a wide range of silanes and hydroxyl groups to form SiO bonds under mild and solvent-free conditions. Mechanistic studies indicated that the activation of hydroxyl groups by the NHC is the most plausible initial step for the process.

The Reactivity of a Silacyclopentadienylidene towards Aldehydes: Silole Ring Expansion and the Formation of Base‐Stabilized Silacyclohexadienones

Tossing aldehydes into the ring: The reaction of a silacyclopentadienylidene with aldehydes leads to C=O bond cleavage with the formation of base-stabilized silanones and cyclopropanation of the adjacent C=C bond, followed by silole ring expansion to give silicon analogues of cyclohexadienones.