Hongfan Hu

[(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.

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.

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.

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.

N-Heterocyclic Carbene-Ytterbium Amide as a Recyclable Homogeneous Precatalyst for Hydrophosphination of Alkenes and Alkynes

The N-heterocyclic carbene-ytterbium(II) amides (NHC)2Yb[N(SiMe3)2]2 (1: NHC: 1,3,4,5-tetramethylimidazo-2-ylidene (IMe4); 2: NHC: 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IiPr)) and the NHC-stabilized rare-earth phosphide (IMe4)3Yb(PPh2)2 (3) have been synthesized and fully characterized. Complexes 1-3 are active precatalysts for the hydrophosphination of alkenes, alkynes, and dienes and exhibited much superior catalytic activity to that of the NHC-free amide (THF)2Yb[N(SiMe)2]2. Complex 1 is the most active precursor among the three complexes. In particular, complex 1 can be recycled and recovered from the reaction media after the catalytic reactions. Furthermore, it was found that complex 3 could catalyze the polymerization of styrene to yield atactic polystyrenes with low molecular weights. To the best of our knowledge, complex 1 represents the first rare-earth complex that can be recovered after catalytic reactions.

One-step access to luminescent pentaaryldiazaboroles via C-C double bond formation from imidoylstannanes.

A series of pentaaryl-substituted diazaboroles have been prepared for the first time by a novel strategy based on the C-C double bond formation from imidoylstannane reagents in the presence of dibromophenylboranes. The aryl substituents on the 4,5-position of the planar C(2)N(2)B core have substantial effects on their electronic structures. All the new diazaboroles are luminescent both in solution and in the solid state. DFT calculations indicate the 4,5-C-aryl substituents have significant contributions to the LUMOs.

Reaction of a bulky amine borane with lanthanide trialkyls. Formation of alkyl lanthanide imide complexes

Borane-stabilized lanthanide imides have been synthesized by the reaction of lanthanide trialkyls with a bulky amine borane. X-ray crystal structures indicate the presence of 3-center-2-electron B–H–Ln bonding and Ln–arene interactions in the solid state.

Isolable 1,1-Disubstituted Silole Dianion: a Homogeneous Two-Electron-Transfer Reducing Reagent

The 1,1-disubstituted silole dianion 2 has been isolated and characterized by single-crystal X-ray analysis for the first time. 2 can be used as a two-electron-transfer reducing reagent for the reduction of organic compounds and inorganic salts with regeneration of the corresponding neutral silole in nearly quantitative yields, indicating that it is an excellent reducing reagent. Reduction of (Mes)2SiCl2 with 2 selectively yielded the cyclotrisilane (Mes2Si)3 in high yield, which has not been isolated in pure form with the existing methods.

Synthesis and Catalytic Isoprene Polymerization of Rare-Earth Alkyl Complexes Supported by An Arylimine-amidinate Ligand

Reaction of the arylimine-amidinate ligand (NNN)H (1) ((NNN)=(2-C(H)NDippC6H3NHC(Ph)NDipp), Dipp= 2,6-i-Pr2C6H3) with Ln(CH2SiMe3)3(THF)2 (Ln=Y, Sm) in n-hexane at -78 ℃ followed by stirring the mixture for 3 h at room temperature afforded (NNN)Ln(CH2SiMe3)2 (Ln=Y, 2; Ln=Sm, 3) in 75% and 50% yields after work up. 2 and 3 have been characterized by 1 H NMR, 13 C NMR, IR spectroscopy, elemental analysis. The molecular structure of 2 has been confirmed by X-ray analysis. Complexes 2 and 3 catalyzed the polymerization of isoprene in the presence of (Ph3C)(B(C6F5)4) (1 equiv.) and AlEt3 (10 equiv.) at 25 ℃. 2 converted 250 equivalents of isoprene into polyisoprene with a narrow molecular weight distribution (Mw/Mn=1.20) and a 3,4-rich microstructure (3,4-selectivity: 78%, rr=50%) in 12 h. A significant increase of 3,4-selectivity was observed in the presence of one equivalent of AlEt3 (3,4-selectivity: 87%). When i-Bu2AlH (10 equiv.) was used instead of AlEt3, the catalytic activity and selectivity did not change noticeably while the re- sulting polyisoprenes feature a broad molecular weight distribution (Mw/Mn=2.45). When the amounts of i-Bu2AlH de- creased from 5 to 1 equivalent, a significant increase of 3,4-selectivity and number-average molecular weight have been ob- served while the molecular distributions became small. The catalyst is active within a wide range of temperatures from low temperature to 40 ℃. At 40 ℃, a similar selectivity (3,4-selectivity: 78%, Mw/Mn=1.43) to that found at 25 ℃ but the increased reaction rate has been observed. High stereo- and regioselectivity and narrow molecular weight distributions were obtained at -20 ℃ (3,4-selectivity: 88%, rr=52%, Mw/Mn=1.15). The samarium alkyl 3 exhibited a high activity but low selectivity (25 ℃, 3 h, 3,4-selectivity: 67%, Mw/Mn=1.71) compared to the yttrium alkyl 2. Keywords rare-earth metal; alkyl complex; arylimine-amidine; isoprene polymerization; 3,4-selectivity

Synthesis and study of an unprecedented 1-hydro-1-lithio-1-silafluorene anion

The 1-hydro-1-lithio-silafluorene anion 2 has been isolated and characterized by single-crystal X-ray analysis for the first time. Three tetrahydrofuran molecules coordinate with lithium atoms in the crystal structure of 2. 2 was reacted with a number of electrophilic reagents and it can be used as a useful reagent for generating various functional hydrogen-substituted silafluorenes. Treatment of 2 with 2 equiv. of chlorobis(isopropyl)phosphine afforded chloro-substituted silafluorene 4. The reaction of 8 which has a SiH-SiBr fragment with IiPr2Me2 (IiPr2: 2,5-diisopropyl-3,4-dimethylimidazol-1-ylidene) led to the formation of an Si-Si bond cleavage product, NHC-stabilized silacyclopentadienylidene 9.