Lewis acid catalysts boron trifluoride

Lithiated ynamides react stereoselectively with chiral A-sulfonyl imides without Lewis acid catalysts. Boron trifluoride etherate completely reverses the selectivity a switch (g) from a chelated to an open TS is proposed. 

The polymerization o oxetanes with cationic catalysts has been studied by many investigators. (1.H2) RoseC. ), in particular, first reported the homopolymerization of the parent compound, tri-methylene oxide (TMO), with a Lewis acid catalyst, boron trifluoride. The use of coordination catalysts to polymerize oxetanes has been reported in the patent literature by Vandenberg.W In this work, Vandenberg polymerized oxetanes with the aluminum trialkyl -water-acetylacetone coordination catalyst (referred to as chelate catalyst) that he discovered for epoxide polymerization . This paper describes the homo- and co-polymerization of TMO with these coordination catalysts. Specific TMO copolymers, particularly with unsaturated epoxides such as allyl glycidyl ether (AGE), are shown to provide the basis for a new family o polyether elastomers. These new elastomers are compared with the related propylene oxide-allyl glycidyl ether (PO-AGE) copolymer elastomers. The historical development and general characteristics of polyether elastomers and, in particular, the propylene oxide elastomers, are reviewed below. 

Cationic mechanisms are much more characteristic of the polymerization of oxygen heterocycles, both ethers and acetals. A wide variety of catalysts has been used, including protonic acids, such Lewis acids as boron trifluoride, phosphorus pentafluoride, stannic chloride, antimony pentachloride, titanium tetrachloride, zinc chloride, and ferric chloride, and salts of carbocations or tri-alkyloxonium ions having anions derived from Lewis acids. Some complex, coordination catalysts that appear to operate by a mechanism... 

Korshak and coworkers20 polymerized levoglucosan in p-dioxane at 80-90°, using, as the catalyst, benzenesulfonic acid or such Lewis acids as boron trifluoride, ferric chloride, or aluminum chloride. Highly branched, amorphous products having Ma 38,000-68,000 (by light-scattering) were obtained. 

Monomers with electron-donating groups like isobutylene form stable positive charges and are readily converted to polymers by cationic catalysts. Any strong Lewis acid like boron trifluoride (BF3) or Friedel-Crafts catalysts such as AICI3 can readily initiate cationic polymerization in the presence of a cocatalyst like water, which serves as a Lewis base or source of protons. During initiation, a proton adds to the monomer to form a carbonium ion, which forms an association with the counterion. This is illustrated for isobutylene and boron trifluoride in Equation 2.19 ... 

These are additions to double bonds, like the Prins reaction. They can be carried out on natural and synthetic rubbers. They take place rapidly in the presence of acid catalysts. Aqueous formaldehyde, or paraform in CQ4, can be used. The catalysts are inorganic acids or anhydrous Lewis acids, like boron trifluoride in acetic acid solution ... 

Generally, alcohols, readily generating carbenium cations in the presence of acidic catalysts, were found to react with A H-unsubstituted 5-trifluoromethyltetrazole 96 yielding N -alkylated products 112 [28, 103], The reaction can be carried out in neutral organic solvents (chloroform, dichloromethane, acetonitrile, nitromethane) in the presence of catalytic amounts of sulfuric or p-toluenesulfonic acids as well as Lewis acids like boron trifluoride etherate or zinc triflate. 

Cationic polymerization of coal-tar fractions has been commercially achieved through the use of strong protic acids, as well as various Lewis acids. Sulfuric acid was the first polymerization catalyst (11). More recent technology has focused on the Friedel-Crafts polymerization of coal fractions to yield resins with higher softening points and better color. Typical Lewis acid catalysts used in these processes are aluminum chloride, boron trifluoride, and various boron trifluoride complexes (12). Cmde feedstocks typically contain 25—75% reactive components and may be refined prior to polymerization (eg, acid or alkali treatment) to remove sulfur and other undesired components. Table 1 illustrates the typical components found in coal-tar fractions and their corresponding properties. 

Catalytic curing agents initiate resin homopolymerization, either cationic or anionic, as a consequence of using a Lewis acid or base in the curing process. The Lewis acid catalysts frequently employed are complexes of boron trifluoride with amines or ethers. 

Lewis acids are defined as molecules that act as electron-pair acceptors. The proton is an important special case, but many other species can play an important role in the catalysis of organic reactions. The most important in organic reactions are metal cations and covalent compounds of metals. Metal cations that play prominent roles as catalysts include the alkali-metal monocations Li+, Na+, K+, Cs+, and Rb+, divalent ions such as Mg +, Ca +, and Zn, marry of the transition-metal cations, and certain lanthanides. The most commonly employed of the covalent compounds include boron trifluoride, aluminum chloride, titanium tetrachloride, and tin tetrachloride. Various other derivatives of boron, aluminum, and titanium also are employed as Lewis acid catalysts. 

As mentioned several times Lewis acids are highly valuable catalysts but the most commonly used ones such as aluminium chloride and boron trifluoride are highly water sensitive and are not usually recovered at the end of a reaction, leading to a significant source of waste. In recent years there has been much research interest in lanthanide triflates (trifluoro-methanesulfonates) as water stable, recyclable Lewis acid catalysts. This unusual water stability opens up the possibility for either carrying out reactions in water or using water to extract and recover the catalyst from the reaction medium. 

Homogeneous catalysts are also often used in cationic and anionic polymerization processes. Lewis acid catalysts, such as boron trifluoride and stannic chloride, accept protons from co-... 

Lewis acids such as zinc chloride, boron trifluoride, aluminum chloride, and diethylaluminum chloride catalyze Diels-Alder reactions.8 The catalytic effect is the result of coordination of the Lewis acid with the dienophile. The complexed dienophile is more electrophilic and more reactive toward electron-rich dienes. The mechanism of the cycloaddition is still believed to be concerted, and high stereoselectivity is observed.9 10 Lewis acid catalysts also usually increase the regioselectivity of the reaction. 

The addition of phosphine to olefins is accelerated by acidic and basic catalysts. Under the influence of non-oxidising acids or Lewis acid such as, for example, methanesulphonic acid, benzenesulphonic acid, trifluoro-acetic acid or boron trifluoride phosphine is quickly added to olefins at pressures of 20-40 at, and temperatures of 30-60 °C. It is assumed that the reaction proceeds via a carbonium ion which is first formed thus ... 

The mono- and dibrominated chelates were also obtained by reaction of the acetylacetonates with cyanogen bromide in the presence of boron trifluoride ether-ate or aluminum chloride. In the absence of these Lewis acid catalysts cyanogen bromide does not react with these acetylacetonates. No trace of cyanogenated products was detected from these reactions. 

The epoxidation is generally conducted in two steps (/) the polyol is added to epichlorohydtin in the presence of a Lewis acid catalyst (stannic chloride, boron trifluoride) to produce the chlorohydrin intermediate, and (2) the intermediate is dehydrohalogenated with sodium hydroxide to yield the aliphatic glycidyl ether. A prominent side-reaction is the conversion of aliphatic hydroxyl groups (formed by the initial reaction) into chloromethyl groups by epichlorohydtin. The aliphatic glycidyl ether resins are used as flexibilizers for aromatic resins and as reactive diluents to reduce viscosities in resin systems. 

Boron trifluoride monoethylamine (BF3-MEA) is a Lewis acid catalyst. Lewis acids are electron pair acceptors that function as curing agents by coordinating with the epoxy oxygen,... 

Boron trifluoride is used as a Lewis acid catalyst in a wide variety of reactions. Like diborane, BF3 is a toxic gas, but BF forms a stable complex with ethers, allowing it to be conveniently stored and measured. The complex of BF3 with diethyl ether is called boron trifluoride etherate. ... 

The formation of thioacetals from aldehydes and ketones involves the reaction with a thiol such as ethane-1,2-dithiol in the presence of a Lewis acid catalyst such as boron trifluoride eiherate. These derivatives have been described earlier. 

The reaction of aldehydes or ketones with thiols, usually with a Lewis acid catalyst, leads to dithioacetals or dithioketals. The most common catalyst used is probably boron trifluoride etherate (BF3 OEt2). Similarly reactions that use 1,2-ethanedithiol or 1,3-propanedithiolleadto 1,3-dithiolanes, such as 18 or l,3-dithianes. " Dithioa-cetals can also be prepared from aldehydes or ketones by treatment with thiols in the presence of TiCU, SiCU, LiBp4, AKOTfls, with a disulfide RSSR (R = alkyl or aryl), or with methylthiotrimethylsilane (MeSSiMe3). " ... 

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