Synthesis and Crystal Structures of t‐Butyl‐Pyridine Adducts of ZnR2 (R = Me,i‐Pr,t‐Bu,Cp*)

Lewis acid‐base adducts of the general type R₂Zn(4‐tBuPy)_x (R = Me 1 , iPr 2 , tBu 3 , Cp* 4 ; x = 1, 2) were obtained in high yields from reactions of ZnR₂ with the Lewis base 4‐tBu‐Pyridine. Compounds 1 – 4 were characterized by multinuclear NMR (¹H, ¹³C) and IR spectroscopy and elemental analyses, 1 and 4 also by X‐ray diffraction at single crystals.     

Beryllium Phosphine Complexes: Synthesis,Properties, and Reactivity of (PMe3)2BeCl2 and (Ph2PC3H6PPh2)BeCl2

The directed synthesis, spectroscopic properties, and reactivity of bis(trimethylphosphine) beryllium dichloride ( 1 ) and bis(diphenylphosphino)propane beryllium dichloride ( 2 ) are reported, including the crystal structure of (PMe₃)₂BeCl₂ ( 1 ). These four‐coordinate beryllium compounds can be alkylated with n‐butyllithium (ⁿBuLi) to give three‐coordinate (Ph₂PC₃H₆PPh₂)BeⁿBu₂ ( 3 ) and (PMe₃)BeⁿBu₂ ( 4 ). PMe₃ can be removed from (PMe₃)BeⁿBu₂ ( 4 ) in vacuo to yield [ⁿBu₂Be]₂ ( 5 ). For the first time, the presence of [ⁿBu₂Be]₂ as a dimer in solution, which has been postulated for decades, could be observed spectroscopically. This novel, ether‐free pathway provides access to beryllium dialkyl compounds that have never been in contact with oxygen‐atom‐containing reagents or solvents. This “freeness from oxygen” is crucial for semiconductor applications where oxygen is often unwanted and must be avoided at all costs.     

"Designer acids": combined acid catalysis for asymmetric synthesis

Lewis and Brønsted acids can be utilized as more‐effective tools for chemical reactions by sophisticated engineering (“designer acids”). The ultimate goal of such “designer acids” is to form a combination of acids with higher reactivity, selectivity, and versatility than the individual acid catalysts. One possible way to take advantage of such abilities may be to apply a “combined acids system” to the catalyst design. The concept of combined acids, which can be classified into Brønsted acid assisted Lewis acid (BLA), Lewis acid assisted Lewis acid (LLA), Lewis acid assisted Brønsted acid (LBA), and Brønsted acid assisted Brønsted acid (BBA), can be a particularly useful tool for the design of asymmetric catalysis, because combining such acids will bring out their inherent reactivity by associative interaction, and also provide more‐organized structures that allow an effective asymmetric environment.     

Methylene Bridging Effect on the Structures,Lewis Acidities and Optical Properties of Semi-planar Triarylboranes

Three synthetic methods towards semi-planar triarylboranes with two aryl rings connected by a methylene bridge have been developed. The fine-tuning of their stereoelectronic properties and Lewis acidities was achieved by introducing fluorine, methyl, methoxy, n-butyl and phenyl groups either at their exocyclic or bridged aryl rings. X-ray diffraction analysis and quantum-chemical calculations provided quantitative information on the structural distortion experienced by the near planar hydro-boraanthracene skeleton during the association with Lewis bases such as NH₃ and F⁻. Though the methylene bridge between the ortho-positions of two aryl rings of triarylboranes decreased the Gibbs free energies of complexation with small Lewis bases by less than 5 kJ mol⁻¹ relative to the classical Lewis acid BAr₃, the steric shielding of the CH₂ bridge is sufficient to avoid the formation of Lewis adducts with larger Lewis bases such as triarylphosphines. A newly synthesized spirocyclic amino-borane with a long intramolecular B−N bond that could be dissociated under thermal process, UV-irradiation, or acidic conditions might be a potential candidate in Lewis pairs catalysis.     

Platinum Oxoboryl Complexes as Substrates for the Formation of 1:1, 1:2, and 2:1 Lewis Acid–Base Adducts and 1,2‐Dipolar Additions

The oxoboryl complex trans‐[(Cy₃P)₂BrPt(B?O)] ( 2 ) reacts with the Group 13 Lewis acids EBr₃ (E=Al, Ga, In) to form the 1:1 Lewis acid–base adducts trans‐[(Cy₃P)₂BrPt(B?OEBr₃)] ( 6 – 8 ). This reactivity can be extended by using two equivalents of the respective Lewis acid EBr₃ (E=Al, Ga) to form the 2:1 Lewis acid–base adducts trans‐[(Cy₃P)₂(Br₃Al‐Br)Pt(B?OAlBr₃)] ( 18 ) and trans‐[(Cy₃P)₂(Br₃Ga‐Br)Pt(B?OGaBr₃)] ( 15 ). Another reactivity pattern was demonstrated by coordinating two oxoboryl complexes 2 to InBr₃, forming the 1:2 Lewis acid–base adduct trans‐[{(Cy₃P)₂BrPt(B?O)}₂InBr₃] ( 20 ). It was also possible to functionalize the B?O triple bond itself. Trimethylsilylisothiocyanate reacts with 2 in a 1,2‐dipolar addition to form the boryl complex trans‐[(Cy₃P)₂BrPt{B(NCS)(OSiMe₃)}] ( 27 ).     

Electron-Deficient Borinic Acid Polymers: Synthesis,Supramolecular Assembly,and Examination as Catalysts in Amide Bond Formation

The Lewis acidic character of borinic-acid-functionalized polymers suggests broad potential applications in supramolecular materials, chemo- and biosensors, as well as supported catalysts. Two highly electron-deficient borinic acid copolymers ( 3 a and 3 b ) with variable steric hindrance at the boron center were prepared by reaction of aryldibromoboranes ArBBr₂ ( 2 , Ar=2,4-Cl₂Ph, 3,5-Cl₂Ph) with a 10 % stannylated polystyrene random copolymer, followed by conversion to the desired PS-B(Ar)OH functionalities. The supramolecular assembly of these polymers through Lewis acid–Lewis base interactions and reversible covalent B−O−B bond formation was investigated. Exposure of a polymer solution of 3 a to pyridine triggered spontaneous gelation, whereas 3 b only gelled upon addition of molecular sieves to favor formation of boroxane crosslinks. The crosslinking process was readily reversed by addition of small amounts of water or wet solvent. The dynamic processes were studied in detail by variable-temperature (VT) NMR by using molecular model compounds. The polymers and their corresponding model compounds were also examined as catalysts in the amide bond formation reaction between phenylacetic acid and benzylamine. The 3,5-dichlorophenyl borinic acid derivatives proved to be the more effective catalysts. Mechanistic studies suggested that the borane Lewis acid-catalyzed coupling involves initial acid-induced protodeboronation to release the dichlorophenyl boronic acid as the active catalyst.     

Dihydrogen Activation with a Neutral,Intermolecular Silicon(IV)-Amine Frustrated Lewis Pair

The heterolytic cleavage of dihydrogen constitutes the hallmark reaction of frustrated Lewis pairs (FLP). While being well-established for planar Lewis acids, such as boranes or silylium ions, the observation of the primary H₂ splitting products with non-planar Lewis acid FLPs remained elusive. In the present work, we report bis(perfluoro-N-phenyl-ortho-amidophenolato)silane and its application in dihydrogen activation to a fully characterized hydridosilicate. The strict design of the Lewis acid, the limited selection of the Lewis base, and the distinct reaction conditions emphasize the narrow tolerance to achieve this fascinating process with a tetrahedral Lewis acid.     

Synthesis and Catalytic Activity of Atrane-type Hard and Soft Lewis Superacids with a Silyl,Germyl, or Stannyl Cationic Center

The synthesis and isolation of atrane-type molecules 1 E⁺ (E=Si, Ge, or Sn) having a cationic group 14 elemental center are reported. The cations 1 E⁺ act as hard and soft Lewis superacids, which readily interact with various hard and soft Lewis basic substrates. The rigid atrane framework stabilizes the localized positive charge on the elemental center and assists the formation of the well-defined highly coordinated states of 1 E⁺. The cations were applied to the hydrodefluorination, Friedel-Crafts reaction, alkyne cyclization, and carbonyl reduction as Lewis acid catalysts. Most notably, [ 1 Si][ClO₄] exhibits unique chemoselectivity that depends on a solvent in the competitive reaction of silyl enol ether with a mixture of benzaldehyde dimethyl acetal and benzaldehyde. Our findings indicate the potential of hard and soft Lewis superacids in organic synthesis.     

Enantioselective cyanosilylation of aldehydes catalyzed by a novel N,N′-dioxide-Ti(OPr)4 bifunctional catalyst

A novel bifunctional asymmetric catalyst containing N-oxide and titanium(IV) was developed and applied to the asymmetric cyanosilylation of aldehydes. Optically active trimethylsilyl cyanohydrin ethers were obtained up to 99% yield and 80% ee in the presence of 5 mol % catalyst loading at −78 °C. Based on the experimental results, the catalytic cycle was proposed as a pathway in which Lewis acid and Lewis base activated aldehyde and trimethylsilylcyanide (TMSCN), respectively.     

苯系物Lewis酸碱性的定量化尝试及估算

Lewis酸碱理论研究目前尚处于定性阶段,在环境化学上应用甚少,Gutmann等曾采用热力学与~(31)P NMR方法定量了少量有机溶剂的酸碱性,Kamlet等也利用~(19)F NMR测定了一些有机物的碱性.本文提出了一种利用多种溶剂/水分配系数来定量苯系物酸碱性的方法,该法简便、快速,有广泛的适用性。     

Synthesis of 1-indanones by intramolecular Friedel-Crafts reaction of 3-arylpropionic acids catalyzed by Tb(OTf)3

Intramolecular Friedel-Crafts acylation reaction of 3-arylpropionic acids was efficiently catalyzed by Tb(OTf)₃ at 250 °C to give 1-indanones. Even deactivated 3-arylpropionic acids with halogen atoms on the aromatic ring can be cyclized in moderation to good yields.     

Synthesis of Functional Bis(pentafluoroethyl)silanes (C2F5)2SiX2, with X=H,F, Cl,Br, OPh,and O2CCF3

As recently shown, the introduction of pentafluoroethyl functionalities into silicon compounds is of general interest due to an enhanced Lewis acidity of the resulting species. By this means, the synthesis of previously inaccessible hypervalent silicon derivatives is enabled. 1 While an easy access to tris(pentafluoroethyl)silanes has already been published, synthetic strategies for the selective preparation of bis derivatives are yet unknown. In this contribution, a convenient protocol for the synthesis of functional bis(pentafluoroethyl)silicon compounds is presented. These compounds represent precursors for the synthesis of pentafluoroethylated polysiloxanes. 2 Furthermore, they prove to be resistant to oxonium cations, which is a key feature for the preparation of stable pentafluoroethylsilic acids. 3 Treatment of dichlorodiphenoxysilane with in situ generated pentafluoroethyl lithium leads to the corresponding bis(pentafluoroethyl)silane in high yields. (C₂F₅)₂Si(OPh)₂ serves as a starting material for further functionalized bis(pentafluoroethyl)silanes. These silanes have been isolated and their reactivity towards N bases studied. The pronounced Lewis acidity of the obtained compounds has been documented by the formation of octahedral adducts with nitrogen donors such as 1,10‐phenanthroline and acetonitrile.     

Convenient synthesis of selectively substituted tribenzo[a,d,g]cyclononatrienes

Convenient laboratory procedures for obtaining selectively substituted dihydro-5H-tribenzo[a,d,g]cyclononatrienes have been achieved. X-ray structure determination indicates that 2d is present in the solid state in the crown conformation to yield H-bonded columns and pillars with a hydrophilic interior and hydrophobic exterior that can be used for the design of specific sensor materials.     

Synthesis,Structure, and Reactivity of Lewis Base Stabilized Plumbacyclopentadienylidenes

Plumbacyclopentadienylidenes, in which the lead atoms have divalent states and are coordinated by THF, pyridine and N‐heterocyclic carbene, were synthesized and characterized. The THF‐ and pyridine‐stabilized compounds can be regarded as rare examples of hypervalent 10‐X‐4 species. The equilibrium between the THF adduct and the free plumbacyclopentadienylidene was evidenced by spectroscopic analysis and theoretical calculations. The THF adduct in benzene converted into a plumbylene dimer, where one of the lead centers is coordinated by THF and the other lead atom is coordinated by a divalent lead atom, the dimer gradually decomposing into spiroplumbole. The THF adduct unexpectedly reacted with trifluoroborane and trichlorogallane to afford fluoroborole and chlorogallole, which are the first examples of non‐annulated fluoroborole and gallole, respectively.