Epoxies Lewis acid catalysts

The epoxidalion is generally conducted in iwo steps (11 the polyol is added to cpichlorohydrin in the presence of a Lewis acid catalyst (stannic chloride, boron irifluoride) to pruduce the chlorollydrin intermediate, and (2] the intermediate is dchydrohalogenaled with sodium hydroxide iu yield the aliphatic glycidyl ether. Solid epoxy resins are prepared by the Taffy or Advancement processes. 

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,... 

The IPNs prepared were composed of a rubbery polyurethane and a glassy epoxy component. For the polyurethane portion, a carbodiimide-modified diphenyl-methane diisocyanate (Isonate 143L) was used with a polycaprolactone glycol (TONE polyol 0230) and a dibutyltin dilaurate catalyst (T-12). For the epoxy, a bisphenol-A epichlorohydrin (DER 330) was used with a Lewis acid catalyst system (BF -etherate). The catalysts crosslink via a ring-opening mechanism and were intentionally selected to provide minimum grafting with any of the polyurethane components. The urethane/epoxy ratio was maintained constant at 50/50. A number of fillers were included in the IPN formulations. The materials used are shown in Table I. 

The partially epoxidized compound was shown to have an average of two epoxy groups per cube. Both polyepoxides readily polymerized in the presence of Lewis acid catalysts or in reaction with amines. This suggested their potential as coupling agents in the synthesis of novel inorganic-organic hybrids. 

Sterically hindered ketones were more difficult to reduce with tin hydrides but reduction at high pressure (1 GPa) without radical or Lewis acid catalyst in methanol at 55 °C was effective.Reduction of the very hindered ketone (92) did not occur at atmospheric pressure but proceeded usefully at high pressure (equation 25). The absence of radical intermediates allowed even a,P-epoxy and cyclopropyl ketones such as (93) to be reduced in high yield, largely without cleavage of the strained ring (equation 26). Under conventional AIBN-initiated conditions radical-mediated processes predominated. 

Epoxy resins must be cured with cross-linking agents (hardeners) or catalysts to develop desired properties. Cross-linking takes place at the epoxy and hydroxyl groups that are the reaction sites. Useful agents are amines, anhydrides, aldehyde condensation products, and Lewis acid catalysts. To achieve a balance of application properties and initial handling... 

As discussed earlier, the use of unmodified Lewis acid catalysts as epoxy curatives has been restricted due to the extremely rapid rate of cure of such systems, and the poor physical properties of the cured resins. Lewis acids are frequently modified by the formation of heat-activatable complexes with amines or glycols. Such complexes display moderate room temperature... 

Acrylic or methacrylic monomer(s) Lewis acid catalyst (for epoxy curing)... 

Intramolecular reaction of epoxides with a hydroxy group provides a practical and effective method for the stereoselective synthesis of substituted tetrahydrofurans and tetrahydropyrans. Bronsted and Lewis acid catalysts are employed for this purpose. In general, the reaction of epoxy alcohols such as (94) favors 5-exo-cyclization over 6-endo-cyclization according to the Baldwin s rule. A recent study has revealed... 

The method is a modification of one used by Barton and McCombie.8 Reduction of ketones.9 Ketones can be reduced to alcohols by Bu3SnH in the presence of either AIBN or a Lewis acid, but this reaction is limited to unhindered ketones. However, even sterically hindered ketones, such as f-butyl methyl ketone, can be reduced under high pressure (10 kbar) in the absence of a catalyst. This method is particularly useful in the case of cyclopropyl and a,p-epoxy ketones, which are reduced to the corresponding alcohols. Reduction of these ketones with Bu3SnH under radical conditions results in ring-opened products. 

Similarly, Lewis acid-catalyzed rearrangement of epoxy alcohols 84 (Scheme 37) gives in most cases the exo-cyclized tetrahydropyrans 85 as the main products (80-97%) however, with La(OTf)3 as the catalyst, the main products (80-90%) are endo-cyclized oxepanes 86 <1998TL393>. 

In practice, epoxy-amine reactions in carbon fiber prepregs, and epoxyphenol reactions in molding compounds, are often accelerated by the addition of a Lewis acid (typically a BF3 - amine complex) or a Lewis base (often a tertiary amine), as catalysts. ... 

The catalytic curing agents commonly used include tertiary amines, Lewis acids and bases, and dicyandiamide. Since their function is truly catalytic, the catalyst is added at relatively low concentrations (0 to 5% by weight) to the epoxy formulation. Homopolymerization generally requires both the presence of catalysts and elevated temperatures for the reaction to proceed. Like the polyaddition reaction, the homopolymerization reaction is accelerated by hydroxyl groups or tertiary amines. 

Some catalysts, such as certain Lewis acids, are so reactive that they can provide extremely short gel times with epoxy resins at room temperature.15 For example, BC13, can polymerize epoxy resins, resulting in gel times of less than 60 s. However, these reactive systems result in a very rigid adhesive with low peel strength properties and poor impact strength. As a result, less reactive catalysts are commonly employed in adhesive formulations. 

A mild and efficient method for the aminolysis of oxiranes in aprotic solvents using metal ion salts of Li+, Na+, Mg2+, Ca2+, and Zn2+ as Lewis acid has been reported <90TL4661). The reaction rates depend, in addition to the nature of the amine and epoxide, also on the type of the metal ion of the catalyst salt. The stereoselectivity observed in these reactions is complete inversion of configuration. Epoxy carboxylic acids are cleanly ring-opened by primary amines at C-2 to provide a-amino-/ -hydroxy acids <92TL2497>. 

Allyltri- -butyltin usually requires activation to perform effective allylation of carbonyl compounds. Stereoselective reactions occasionally occur in the presence of the strong Lewis acids TiCU, SnCU, and BF3-OEt2. These conventional Lewis acids cannot, however, be used for chemoselective allylation of carbonyl compounds bearing other reactive functional groups, because of further transformation and decomposition of the products. It has been found that the lead diiodide-HMPA complex is a good catalyst for chemo- and diastereoselective allylation of a,73-epoxy ketones under mild neutral conditions (Scheme 13.62) [81]. 

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