Complex amine-boron

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. 

Complexes of boron trifluoride and amines such as monoethylamine are of interest because of the very long pot lives possible. The disadvantages of these complexes are their hygroscopic nature and the corrosive effects of BF3 liberated during cure. 

In practice the amine curing reactions are often accelerated by the addition of Lewis acids, especially amine complexes of boron trifluoride. Such materials can also initiate epoxide homopolymerisation in which chain propagation occurs through a carbocation ... 

A similar trend has been noted for the association constants for ground state amine-boron trifluoride complexes (126). 

The molybdenum catalyst 2 has been used extensively for ADMET polymerization. This complex is easier to handle than the tungsten analog and is more tolerant of functionality. This complex has allowed the synthesis of polymers containing esters, carbonates, ethers, sulfides, aromatic amines, boronates, dichlorosilanes, siloxanes, acetals, and conjugated carbon-carbon double bonds [38-45]. Aldehydes, ketones, and protic functionahty are not tolerated. The molybdenum alkylidene will react with aldehydes and ketones, but not esters, in a Wittig fashion [64]. 

Hoffmann et al have reported the addition of allyl(dimethoxy)borane to linear and branched a-aryl-aldimines (entries 16-19, Table 2). The absence of a-deprotonation may be explained by a delicate balance between the basicity and the reactivity of the allylboronate. Allyl(dimethoxy)borane should thus be considered the reagent of choice in reactions with enolizable aldimines. Reactions are conveniently carried out at 25 °C in CH2CI2 and work-up is performed using triethanolamine to break up amine-boronate complexes. Allyl(dimethoxy)borane also adds to the cyclic imine, A -piperideine, in 90% yield. The reported yields for the addition of allyllithium and allylmagnesium chloride to A -piperideine are low. 

Epoxy resins can also be cured by polycondensation with amino resins or phenolic resins. Epoxy resins can be polymerized with catalysts such as tertiary amines, boron trifluoride complexes, ferrocenes, and triarylsulfonium salts. 

Lewis Acids. Lewis acids, eg, boron trihalides, contain an empty outer orbital and therefore seek reaction with areas of high electron density. Boron trifluoride, BF3, a corrosive gas, reacts easily with epoxy resins, causing gelation within a few minutes. Complexation of boron trihalides with amines enhances the curing action. Reasonable pot lives using these complexes can be achieved because elevated temperatures are required for cure. Reactivity is controlled by the choices of the halide and the amine. The amine choice also affects other properties such as solubility in resin and moisture-sensitivity. Boron trifluoride monoethylamine (BF3 NH2C2H5), a crystalline material which is a commonly used catalyst, cures epoxy resins at 80-100°C. A chloride version is also commercially available. Other Lewis acids used in epoxy curing include stannic chloride and tin octanate. 

Bimolecular Lewis Acid-Base Complexation under Non-aqueous Conditions As evidenced by the high pH required in the formation of boronate anions, boronic acids and most dialkyl esters are weak Lewis acids. This behavior contrasts sharply with trialkylboranes, which form strong adducts with phosphines, amines, and other Lewis bases [66]. Aside from the formation of boronate anions, discussed in the previous section, very few stable intermolecular acid-base adducts of boronic acids (esters) exist. Long ago, aliphatic amines and pyridine were found to form complexes in a 1 3 amine boronic acid stoichiometry [67]. Cbmbustion analyses of these air-stable solids suggested that two molecules of water are lost in the process, which led the authors to propose structure 18 (Equation 7, Figure 1.8). Subsequently, Snyder... 

The catalytic process uses Lewis acids, such as boron trifluoride as the etherate, or a tertiary amine, or the more reactive imidazoles. Amine complexes of boron trifluoride are stable in epoxy resins until heated, when rapid homopolymerization of the resin takes place. The reaction mechanism for tertiary amines has been represented by Garnish [3] (see Figure 3). 

The mechanism of epoxy-prepolymer curing with amine-boron trifluoride complexes is controversially discussed in the literature. It k often postulated that the BFj-amine complex reacts at its dissociation temperature [83,86], but this dissociation temperature has not been determined experimentally. Arnold [87] postulated that a proton k released in the dissociation of the amine complex,... 

The second reason is that the enthalpy-entropy compensation is generally limited to (i) acid complexes of specific base types (i.e. correlations are family dependent) and (ii) unhindered bases. An example is given for a set of complexes of boron acids with nifiogen and phosphorus bases [72, 91]. Table 1.10 shows the continuous increase in the quality of the AH-AG correlation when the sample of 31 nitrogen and phosphorus bases is gradually restricted to 21 unhindered primary and secondary amines. The limited correlation is displayed in Figure 1.3. 

It has been shown that some latent hardeners of the epoxy resins may corrode the aluminum wires or bonding pads. This corrosivity has been demonstrated for the complex of boron trifluoride and monoethyl-amine. In the specification NSA 77-25A, small dots of adhesive are applied to the aluminized side of a Mylar polyester film (www.dupont.com) and are allowed to stand in flie room ambient without cure. After 48 h, the dots are removed from the film by washing with acetone and the requirement is that there are no changes in the light transmission of the film. 

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

Air-Stable boron-containing polymers can be prepared by the reaction of dicyano compounds with the / fZ-butylborane—trimethyl amine complex (449). 

Primary nitroparaffins react with two moles of formaldehyde and two moles of amines to yield 2-nitro-l,3-propanediamines. With excess formaldehyde, Mannich bases from primary nitroparaffins and primary amines can react further to give nitro-substituted cycHc derivatives, such as tetrahydro-l,3-oxa2iaes or hexahydropyrimidines (38,39). Pyrolysis of salts of Mannich bases, particularly of the boron trifluoride complex (40), yields nitro olefins by loss of the amine moiety. Closely related to the Mannich reaction is the formation of sodium 2-nitrobutane-1-sulfonate [76794-27-9] by warming 1-nitropropane with formaldehyde and sodium sulfite (41). 

Aromatic amines form addition compounds and complexes with many inorganic substances, such as ziac chloride, copper chloride, uranium tetrachloride, or boron trifluoride. Various metals react with the amino group to form metal anilides and hydrochloric, sulfuric, or phosphoric acid salts of aniline are important intermediates in the dye industry. 

Borane complexes are the most widely used commercial boron compounds, after sodium borobydride. Examples used in organic synthesis are amine borane complexes and borane complexes of tetrahydrofuran and dimethyl sulfide. 

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