A highly efficient synthesis of (Z)‐1‐aryl‐2‐silyl‐1‐ stannylethenes and their conversion to (E)‐2‐ arylethenyl‐, (Z)‐2‐(2‐pyridyl)ethenyl‐ and allenyl‐silanes

A Pd(dba)₂–P(OEt)₃ combination allowed the silastannation of arylacetylenes, 1‐hexyne or propargyl alcohols with tributyl(trimethylsilyl)stannane to take place at room temperature, producing (Z)‐2‐silyl‐1‐stannyl‐1‐substituted ethenes in high yields. Novel silyl(stannyl)ethenes were fully characterized by ¹H‐, ¹³C‐, ²⁹Si‐ and ¹¹⁹Sn‐NMR as well as infrared and mass analyses. Treatment of a series of (Z)‐1‐aryl‐2‐silyl‐1‐stannylethenes and (Z)‐1‐(3‐pyridyl)‐2‐silyl‐1‐stannylethene with hydrochloric acid or hydroiodic acid in the presence of tetraethylammonium chloride (TEACl) or tetrabutylammonium iodide (TBAI) led to the exclusive formation of (E)‐trimethyl(2‐arylethenyl)silanes with high stereoselectivity. A similar reaction of (Z)‐1‐(2‐anisyl)‐2‐silyl‐1‐stannylethene also produced E‐type trimethyl[2‐(2‐anisyl)ethenyl]silane, while (Z)‐trimethyl [2‐(2‐pyridyl)ethenyl]silane was produced exclusively from (Z)‐1‐(2‐pyridyl)‐2‐silyl‐1‐stannylethene. Protodestannylation of (Z)‐1‐[hydroxy(phenyl)methyl]‐2‐silyl‐1‐stannylethene with trifluoroacetic acid took place via the β‐elimination of hydroxystannane, providing trimethyl(3‐phenylpropa‐1,2‐dienyl)silane quite easily. The destannylation products were also fully characterized. Copyright © 2007 John Wiley & Sons, Ltd.     

An Alternative Mechanistic Paradigm for the β‐Z Hydrosilylation of Terminal Alkynes: The Role of Acetone as a Silane Shuttle

The β‐Z selectivity in the hydrosilylation of terminal alkynes has been hitherto explained by introduction of isomerisation steps in classical mechanisms. DFT calculations and experimental observations on the system [M(I)₂{κ‐C,C,O,O‐(bis‐NHC)}]BF₄ (M=Ir ( 3 a ), Rh ( 3 b ); bis‐NHC=methylenebis(N‐2‐methoxyethyl)imidazole‐2‐ylidene) support a new mechanism, alternative to classical postulations, based on an outer‐sphere model. Heterolytic splitting of the silane molecule by the metal centre and acetone (solvent) affords a metal hydride and the oxocarbenium ion [R₃Si? O(CH₃)₂]⁺, which reacts with the corresponding alkyne in solution to give the silylation product [R₃Si? CH?C? R]⁺. Thus, acetone acts as a silane shuttle by transferring the silyl moiety from the silane to the alkyne. Finally, nucleophilic attack of the hydrido ligand over [R₃Si? CH?C? R]⁺ affords selectively the β‐(Z)‐vinylsilane. The β‐Z selectivity is explained on the grounds of the steric interaction between the silyl moiety and the ligand system resulting from the geometry of the approach that leads to β‐(E)‐vinylsilanes.     

Borane‐Catalyzed Reductive α‐Silylation of Conjugated Esters and Amides Leaving Carbonyl Groups Intact

Described herein is the development of the B(C₆F₅)₃‐catalyzed hydrosilylation of α,β‐unsaturated esters and amides to afford synthetically valuable α‐silyl carbonyl products. The α‐silylation occurs chemoselectively, thus leaving the labile carbonyl groups intact. The reaction features a broad scope of both acyclic and cyclic substrates, and the synthetic utility of the obtained α‐silyl carbonyl products is also demonstrated. Mechanistic studies revealed two operative steps: fast 1,4‐hydrosilylation of conjugated carbonyls and then slow silyl group migration of a silyl ether intermediate.     

Chiral Zinc(II)‐Catalyzed Enantioselective Tandem α‐Alkenyl Addition/Proton Shift Reaction of Silyl Enol Ethers with Ketimines

A new catalytic asymmetric tandem α‐alkenyl addition/proton shift reaction of silyl enol ethers with ketimines was serendipitously discovered in the presence of chiral N,N′‐dioxide/Zn^II complexes. The proton shift preferentially proceeded instead of a silyl shift after α‐alkenyl addition of silyl enol ether to the ketimine. A wide range of β‐amino silyl enol ethers were synthesized in high yields with good to excellent ee values. Control experiments suggest that the Mukaiyama–Mannich reaction and tandem α‐alkenyl addition/proton shift reaction are competitive reactions in the current catalytic system. The obtained β‐amino silyl enol ethers were easily transformed into β‐fluoroamines containing two vicinal tetrasubstituted carbon centers.     

Electrophilic Alkylations of Vinylsilanes: A Comparison of α‐ and β‐Silyl Effects

Kinetics of the reactions of benzhydrylium ions (Aryl₂CH⁺) with the vinylsilanes H₂C?C(CH₃)(SiR₃), H₂C?C(Ph)(SiR₃), and (E)‐PhCH?CHSiMe₃ have been measured photometrically in dichloromethane solution at 20 °C. All reactions follow second‐order kinetics, and the second‐order rate constants correlate linearly with the electrophilicity parameters E of the benzhydrylium ions, thus allowing us to include vinylsilanes in the benzhydrylium‐based nucleophilicity scale. The vinylsilane H₂C?C(CH₃)(SiMe₃), which is attacked by electrophiles at the CH₂ group, reacts one order of magnitude faster than propene, indicating that α‐silyl‐stabilization of the intermediate carbenium ion is significantly weaker than α‐methyl stabilization because H₂C?C(CH₃)₂ is 10³ times more reactive than propene. trans‐β‐(Trimethylsilyl)styrene, which is attacked by electrophiles at the silylated position, is even somewhat less reactive than styrene, showing that the hyperconjugative stabilization of the developing carbocation by the β‐silyl effect is not yet effective in the transition state. As a result, replacement of vinylic hydrogen atoms by SiMe₃ groups affect the nucleophilic reactivities of the corresponding C?C bonds only slightly, and vinylsilanes are significantly less nucleophilic than structurally related allylsilanes.     

Highly Stereoselective β‐Mannopyranosylation via the 1‐α‐Glycosyloxy‐isochromenylium‐4‐gold(I) Intermediates

While the gold(I)‐catalyzed glycosylation reaction with 4,6‐O‐benzylidene tethered mannosyl ortho‐alkynylbenzoates as donors falls squarely into the category of the Crich‐type β‐selective mannosylation when Ph₃PAuOTf is used as the catalyst, in that the mannosyl α‐triflates are invoked, replacement of the ^?OTf in the gold(I) complex with less nucleophilic counter anions (i.e., ^?NTf₂, ^?SbF₆, ^?BF₄, and ^?BAr₄^F) leads to complete loss of β‐selectivity with the mannosyl ortho‐alkynylbenzoate β‐donors. Nevertheless, with the α‐donors, the mannosylation reactions under the catalysis of Ph₃PAuBAr₄^F (BAr₄^F=tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate) are especially highly β‐selective and accommodate a broad scope of substrates; these include glycosylation with mannosyl donors installed with a bulky TBS group at O3, donors bearing 4,6‐di‐O‐benzoyl groups, and acceptors known as sterically unmatched or hindered. For the ortho‐alkynylbenzoate β‐donors, an anomerization and glycosylation sequence can also ensure the highly β‐selective mannosylation. The 1‐α‐mannosyloxy‐isochromenylium‐4‐gold(I) complex ( Cα ), readily generated upon activation of the α‐mannosyl ortho‐alkynylbenzoate ( 1 α ) with Ph₃PAuBAr₄^F at ?35 °C, was well characterized by NMR spectroscopy; the occurrence of this species accounts for the high β‐selectivity in the present mannosylation.     

Palladium‐Catalyzed Aerobic Intermolecular Cyclization of Acrylic Acid with 1‐Octene to Afford α‐Methylene‐γ‐butyrolactones: The Remarkable Effect of Continuous Water Removal from the Reaction Mixture and Analysis of the Reaction by Kinetic,ESI‐MS,and XAFS Measurements

Further study of our aerobic intermolecular cyclization of acrylic acid with 1‐octene to afford α‐methylene‐γ‐butyrolactones, catalyzed by the Pd(OCOCF₃)₂/Cu(OAc)₂ ? H₂O system, has clarified that the accumulation of water generated from oxygen during the reaction causes deactivation of the Cu cocatalyst. This prevents regeneration of the active Pd catalyst and, thus, has a harmful influence on the progress of the cyclization. As a result, both the substrate conversion and product yield are efficiently improved by continuous removal of water from the reaction mixture. Detailed analysis of the kinetic and spectroscopic measurements performed under the condition of continuous water removal demonstrates that the cyclization proceeds in four steps: 1) equilibrium coordination of 1‐octene to the Pd acrylate species, 2) Markovnikov‐type acryloxy palladation of 1‐octene (1,2‐addition), 3) intramolecular carbopalladation, and 4) β‐hydride elimination. Byproduct 2‐acryloxy‐1‐octene is formed by β‐hydride elimination after step 2). These cyclization steps fit the Michaelis–Menten equation well and β‐hydride elimination is considered to be a rate‐limiting step in the formation of the products. Spectroscopic data agree sufficiently with the existence of the intermediates bearing acrylate (Pd? O bond), η³‐C₈H₁₅ (Pd? C bond), or C₁₁H₁₉O₂ (Pd? C bond) moieties on the Pd center as the resting‐state compounds. Furthermore, not only Cu^II, but also Cu^I, species are observed during the reaction time of 2–8 h when the reaction proceeds efficiently. This result suggests that the Cu^II species is partially reduced to the Cu^I species when the active Pd catalytic species are regenerated.     

2‐Chloro‐3‐(dimethylamino)prop‐2‐enoyl Fluoride,a Stable Acyl Fluoride

The title compound is formed as a side‐product in the reaction of CF₃CCl₃ with Zn/DMF and dimethyl(thexyl)silyl chloride (=dimethyl(1,1,2‐trimethylpropyl)silyl chloride). The structure and the double‐bond configuration are deduced from its ¹³C‐NMR data. Its formation is discussed in terms of a Vilsmeier‐type formylation and a reductive elimination.     

Low‐Concentration 1,2‐trans β‐Selective Glycosylation Strategy and Its Applications in Oligosaccharide Synthesis

This study develops an operationally easy, efficient, and general 1,2‐trans β‐selective glycosylation reaction that proceeds in the absence of a C2 acyl function. This process employs chemically stable thioglycosyl donors and low substrate concentrations to achieve excellent β‐selectivities in glycosylation reactions. This method is widely applicable to a range of glycosyl substrates irrespective of their structures and hydroxyl‐protecting functions. This low‐concentration 1,2‐trans β‐selective glycosylation in carbohydrate chemistry removes the restriction of using highly reactive thioglycosides to construct 1,2‐trans β‐glycosidic bonds. This is beneficial to the design of new strategies for oligosaccharide synthesis, as illustrated in the preparation of the biologically relevant β‐(1→6)‐glucan trisaccharide, β‐linked Gb₃ and isoGb₃ derivatives.     

Synthesis of β‐Keto Sulfones by a Catalyst‐Free Reaction of Aryldiazonium Tetrafluoroborates,Sulfur Dioxide,and Silyl Enol Ethers

A green approach for the generation of β‐keto sulfones through a reaction of aryldiazonium tetrafluoroborates and sulfur dioxide with silyl enol ether under catalyst‐ and additive‐free conditions has been realized. This reaction proceeds efficiently at room temperature and goes to completion in half an hour. During the reaction process, aryldiazonium tetrafluoroborate is treated with DABCO ⋅ (SO₂)₂ (DABCO=1,4‐diazabicyclo[2.2.2]octane) to provide a sulfonyl radical as the key intermediate, which then initiates the transformation. Oxidants or metal catalysts are avoided, and the presence of DABCO also plays an important role in the reaction.     

Sulfonamide‐Promoted Palladium(II)‐Catalyzed Alkylation of Unactivated Methylene C(sp3)H Bonds with Alkyl Iodides

The alkylation of unactivated β‐methylene C(sp³)? H bonds of α‐amino acid substrates with a broad range of alkyl iodides using Pd(OAc)₂ as the catalyst is described. The addition of NaOCN and 4‐Cl‐C₆H₄SO₂NH₂ was found to be crucial for the success of this transformation. The reaction is compatible with a diverse array of functional groups and proceeds with high diastereoselectivity. Furthermore, various β,β‐hetero‐dialkyl‐ and β‐alkyl‐β‐aryl‐α‐amino acids were prepared by sequential C(sp³)? H functionalization of an alanine‐derived substrate, thus providing a versatile strategy for the stereoselective synthesis of unnatural β‐disubstituted α‐amino acids.     

Palladium‐Catalyzed Pyrazole‐Directed sp3 C−H Bond Arylation for the Synthesis of β‐Phenethylamines

We have developed a method for palladium‐catalyzed, pyrazole‐directed sp³ C−H bond arylation by aryl iodides. The reaction employs a Pd(OAc)₂ catalyst at 5–10 mol % loading and silver(I) oxide as a halide‐removal agent, and it proceeds in acetic acid or acetic acid/hexafluoroisopropanol solvent. Ozonolysis of the pyrazole moiety affords pharmaceutically important β‐phenethylamines.     

Divergent and Stereoselective Synthesis of β‐Silyl‐α‐Amino Acids through Palladium‐Catalyzed Intermolecular Silylation of Unactivated Primary and Secondary C−H Bonds

A general and practical Pd^II‐catalyzed intermolecular silylation of primary and secondary C?H bonds of α‐amino acids and simple aliphatic acids is reported. This method provides divergent and stereoselective access to a variety of optical pure β‐silyl‐α‐amino acids, which are useful for genetic technologies and proteomics. It can also be readily performed on a gram scale and the auxiliary can be easily removed with retention of configuration. The synthetic importance of this method is further demonstrated by the late‐stage functionalization of biological small molecules, such as (?)‐santonin and β‐cholic acid. Moreover, several key palladacycles were successfully isolated and characterized to elucidate the mechanism of this β?C(sp³)‐H silylation process.     

β‐Cyclodextrin‐catalyzed decomposition of Caro's acid

The kinetics and mechanism of decomposition of peroxomonosulphate (PMS) in aqueous sodium hydroxide medium in the presence β‐cyclodextrin has been investigated. The rate of decomposition of PMS is considerably enhanced by the added β‐cyclodextrin. The reaction follows first order kinetics with respect to [PMS]. The experimental results suggest the formation of β‐cyclodextrin peroxy anion by the interaction between SO₅^2?, and β‐cyclodextrin anion (BCDO^?). β‐Cyclodextrin peroxy anion subsequently reacts with PMS to give O₂, SO₄^2? and β‐cyclodextrin anion. The rate constant for the β‐cyclodextrin‐catalyzed decomposition of SO₅^2? (BCD + SO₅^2?) is of the same order of magnitude as that of the reaction HSO₅^2? + SO₅^2? products. © 2002 Wiley Periodic mals, Inc. Int J Chem Kinet 34: 508–513, 2002     

Manganese(I)‐Catalyzed β‐Methylation of Alcohols Using Methanol as C1 Source

Highly selective β‐methylation of alcohols was achieved using an earth‐abundant first row transition metal in the air stable molecular manganese complex [Mn(CO)₂Br[HN(C₂H₄PⁱPr₂)₂]] 1 ([HN(C₂H₄PⁱPr₂)₂]=MACHO‐ⁱPr). The reaction requires only low loadings of 1 (0.5 mol %), methanolate as base and MeOH as methylation reagent as well as solvent. Various alcohols were β‐methylated with very good selectivity (>99 %) and excellent yield (up to 94 %). Biomass derived aliphatic alcohols and diols were also selectively methylated on the β‐position, opening a pathway to “biohybrid” molecules constructed entirely from non‐fossil carbon. Mechanistic studies indicate that the reaction proceeds through a borrowing hydrogen pathway involving metal–ligand cooperation at the Mn‐pincer complex. This transformation provides a convenient, economical, and environmentally benign pathway for the selective C?C bond formation with potential applications for the preparation of advanced biofuels, fine chemicals, and biologically active molecules     

Stereoselective Construction of 1,3‐Disilylcyclopentane Derivatives by Scandium‐Catalyzed [3+2] Cycloaddition of Allylsilanes to β‐Silylenones

The Sc(OTf)₃‐catalyzed [3+2] cycloaddition of allylsilanes to β‐silyl‐α,β‐unsaturated ketones (β‐silylenones) has been developed to form five‐membered syn‐1,3‐disilylketones diastereoselectively through the rearrangement of the silicon substituents on the allylsilane. Stabilization of the carbocation intermediates by a double silicon effect plays a key role in directing the course of the reaction to favor the [3+2] cycloaddition pathway over simple allylation.     

Poly[[diaquabis[μ2‐2‐(4‐pyridinio)propane‐1,3‐dionato‐κ2O:O′]cobalt(II)] dinitrate]

The title compound, {[Co(C₈H₇NO₂)₂(H₂O)₂](NO₃)₂}_n, is the first d‐metal ion complex involving bidentate bridging of a β‐dialdehyde group. The Co²⁺ ion is situated on an inversion centre and adopts an octahedral coordination with four equatorial aldehyde O atoms [Co—O = 2.0910 (14) and 2.1083 (14) Å] and two axial aqua ligands [Co—O = 2.0631 (13) Å]. The title compound has a two‐dimensional square‐grid framework structure supported by propane‐1,3‐dionate O:O′‐bridges between the metal ions. The organic ligand itself possesses a zwitterionic structure, involving conjugated anionic propane‐1,3‐dionate and cationic pyridinium fragments. Hydrogen bonding between coordinated water molecules, the pyridinium NH group and the nitrate anions [O...O = 2.749 (2) and 2.766 (3) Å, and N...O = 2.864 (3) Å] is essential for the crystal packing.     

B(C6F5)3‐Catalyzed Chemoselective Defunctionalization of Ether‐Containing Primary Alkyl Tosylates with Hydrosilanes

Catalytic C(sp³)−O bond cleavage promoted by B(C₆F₅)₃ /Et₃SiH proceeds preferentially with primary tosylates in the presence of primary and secondary silyl ethers and aryl ethers. This reactivity difference enables the chemoselective defunctionalization of several 1,n ‐diols, and the efficiency of the new procedure is highlighted by the selective deoxygenation of the hydroxymethyl group of an orthogonally protected carbohydrate. Tosylates with an adjacent phenyl group are cleaved with anchimeric assistance.     

Complexation of dodecyl‐substituted poly(acrylate) by linked β‐cyclodextrin dimers and trimers in aqueous solution

Two‐dimensional NOESY ¹H NMR, isothermal titration calorimetric (ITC), and rheological studies of host–guest complexation by β‐cyclodextrin, β‐CD, and the β‐CD groups of the linked β‐CD dimers, β‐CD₂ur and β‐CD₂su and trimers, β‐CD₃bz and β‐CDen₃bz, of the dodecyl, C12, substituents of the 3.0% substituted poly(acrylate), PAAC12, in aqueous solution are reported. Complexations by β‐CD, β‐CD₂ur, β‐CD₂su, β‐CD₃bz, and β‐CDen₃bz of the C12 substituents of PAAC12 in 0.2 wt % solution exhibit complexation constants 10^?4K₁₁ (298.2 K) = 0.83, 5.80, 4.40, 15.0, and 1.50 dm³ mol^?1, respectively. (The corresponding ΔH₁₁ and TΔS₁₁ show a linear relationship.) The rheologically determined zero‐shear viscosities of 3.3 wt % aqueous solutions of PAAC12 alone and in the presence of β‐CD, β‐CD₂ur, β‐CD₂su, β‐CD₃bz, and β‐CDen₃bz (where the β‐CD groups and C12 substituents are equimolar) are 0.016, 0.03, 0.12, 0.25, 0.12, and 0.08 Pa s (298.2 K), respectively, and show PAAC12 to form interstrand cross‐links through complexation. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1278–1286     

Nickel‐Catalyzed CO2 Rearrangement of Enol Metal Carbonates for the Efficient Synthesis of β‐Ketocarboxylic Acids

4‐Methylene‐1,3‐dioxolan‐2‐ones underwent oxidative addition of a Ni⁰ catalyst in the presence of Me₂Al(OMe), followed by a coupling reaction with alkynes, to form δ,ϵ‐unsaturated β‐ketocarboxylic acids with high regio‐ and stereoselectivity. The reaction proceeds by [1,3] rearrangement of an enol metal carbonate intermediate and the formal reinsertion of CO₂.