BF3-amine complexes

The cure reactions, the viscosity-time-temperature profile, the processing conditions, the resultant epoxy chemical and physical structure, and the mechanical response of a C-fiber/TGDDM-DDS cured epoxy composite are modified by the presence of a BF3-amine complex catalyst within the prepreg. These factors also will be modified... 

In this Sect, we discuss 1H, 19F and nB NMR studies of BF3 NH2C2H5 and BF3 NHC5H10 complexes, with principal emphasis on the former. We present the chemical composition of commercial BF3 amine complexes, their thermal stability in the solid state and in solution, the effect of moisture and heat upon their composition, the nature of their interaction with the epoxide and amine components utilized in TGDDM-DDS commercial prepregs, the composition of BF3 amine complexes in commercial prepregs, their thermal stability in the prepregs, and the chemical structure of the predominant catalytic species of the cure reactions of the prepreg. 

Solid BF3 NH2C2H5 samples that were annealed at 85, 115, or 139 °C for 1 h and then subsequently dissolved in dimethyl sulfoxide (DMSO) exhibited no significant dissociation as detected by 1H. These data are consistent with observations by Harris and Temin 18) that BF3 amine complexes do not dissociate irreversibly to gaseous BF3 and amine products. (The BF3 NH2C2HS was observed to melt near 85 °C during these studies.)... 

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. ... 

Property Method BF3 amine complex DTA MPD Aliphatic diamine... 

In spite of the great importance of polyaddition (polyetherification ) of epoxy groups in many formulations, a rigorous branching theory is still to be developed. Polyetherification is often released by a primary reaction of epoxide with amines, carboxyl groups, dicyandiamide, or is initiated by BF3-amine complexes, imidazoles, etc. The polyetherification reaction is to be regarded as an initiated reaction and the initiator is either added at the beginning of the reaction of formed in the first reaction... 

The general conclusions were confirmed by the practical results related to the CO-curing of CEC/ACEC compositions with amines in the presence of BF3-amine complexes [87]. The idea presented here was also applied to the synthesis of novel epoxy compounds which contained epoxy groups differing in structure and reactivity [88-90] to be used in adhesive formulations. 

In the meanwhile. Smith et al. [88] proposed another path of BF3—amine complex dissociation ... 

Bouillon et al. [94] compare these results with the experimental data obtained in the case of homopolymerization of epoxy-prepolymers and monomers initiated by a BF3-amine complex in the presence of low molecular weight poly (ethylene oxide). Based on these data, they proposed the following mechanism of polymerization of the system epoxy-prepolymer-monomer-BF3 amine complex. 

Reaction of 9,10-difluoro-7-oxo-2,3-dihydro-7//-pyrido[l, 2,3- e]-1,4-ben-zothiazine-6-carboxylic acid and its ethyl ester with B(OH)3 in AC2O in the presence of ZnCl2 afforded 6-[(diacetoxyboryl)oxycarbonyl] derivative 323 (R = OAc)], which was reacted with primary and cyclic amines to give 10-amino-9-fluoro-7-carboxylic acid derivatives 324 (97MI41, 98MI30). 6-[(Difluoroboryl)oxycarbonyl derivative 323 (R = F) was obtained from ethyl 9,10-difluoro-7-oxo-2,3-dihydro-7//-pyrido[l,2,3- fe]-l,4-benzothiazine-6-carboxylate with BF3-THF complex. Reaction of 323 (R = F) and 1-methylpiperazine in DMF at 50-60 °C and subsequent acidic hydrolysis afforded 7 (97MI1). 

Under non-aqueous conditions the epoxy function in the epoxylactones can be opened only by Lewis acid assistance. Thus, 2-fluoro-2-deoxy-lactones have been prepared from 2,3-epoxylactones by treatment with HF-amine complexes [38,49,50], while 5,6-epoxylactones yield 6-deoxy-6-fluoro-lactones by this treatment [49, 50]. Likewise, BF3-assisted opening of a 2,3- epoxy function with TMSN3 [51] gave a 2-azido-2-deoxy-lactone [52]. In all cases the opening of the epoxide is a trans-opening, and it is noteworthy that under acidic conditions the nucleophile attacks at C-2 or at the primary position, similar to the opening of acetoxonium ions by bromide ions in the... 

The two most common BF3 amine catalysts used commercially to cure epoxies are boron trifluoride monoethylamine, BF3 NH2C2H5, and boron trifluoride piperidine, BF3 NHCsHi0, complexes. Such complexes are latent catalysts at room temperature but enhance epoxide group reactivity at higher temperatures. 

Other latent curing agents that are used in solid adhesives are dihydrazides and BF3-MEA complexes. These compositions are also stable at room temperature but cure when heated. Solid anhydrides can be used in one-component powder blends (e.g., 10 pph of trimellitic anhydride accelerated with 0.5 pph of 2-methylimidazole). Solid systems with aromatic diamines are prepared by comelting the solid epoxy with the amine. Typically 30 pph of curing agent is used.1... 

Many phosphine-borane complexes Y3P BZ3 have been characterized. They include compouuds where Y = alkoxy, aUcyl, amino, halide, and hydride groups, and Z = alkyl, halide, and hydride. The stabilities of these complexes vary widely depending on the Lewis acidity and basicity of the boron and phosphorus moieties, respectively. The relative stabilities of Lewis acid-base complexes with BH3 are R3P > R3N > R3AS > R3Sb, but with BF3 the order is R3N > R3P > R3AS > RsSb. The stabihties of the borou halide complexes of phosphines follow the same order as the amine complexes BI3 > BBrs > BCI3 > BF3. 

Salts of tetra-amine phthalogyanine such as Cu, Co, and Ni have also been used as curatives with success to give cured resins with considerable improvements in heat resistance compared with resins prepared with conventional curatives. Evidence for this is the considerable reduction in cure temperature by use of the catalytic BF3(MeNH) complex. 

However, the primary amine complexes of BF3 have not yet been reported in the literature to dissociate reversibly. Therefore, it seems clear that dissociation cannot be the primary reaction proceeding the cure of epoxy oligomers by BF3 complexes. 

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. 

The reaction of diazo compounds with amines is similar to 10-15. The acidity of amines is not great enough for the reaction to proceed without a catalyst, but BF3, which converts the amine to the F3B-NHR2 complex, enables the reaction to take place. Cuprous cyanide can also be used as a catalyst. The most common substrate is diazomethane, in which case this is a method for the methylation of amines. Ammonia has been used as the amine but, as in the case of 10-44, mixtures of primary, secondary, and tertiary amines are obtained. Primary aliphatic amines give mixtures of secondary and tertiary amines. Secondary amines give successful alkylation. Primary aromatic amines also give the reaction, but diaryl or arylalkyl-amines react very poorly. 

In 1991, Kessar and coworkers demonstrated that the kinetic barrier could be lowered by complexing the tertiary amine with BF3, snch that i-BuLi is able to deprotonate the ammoninm compound, which can be added to aldehydes and ketones as shown by the example in Scheme 4a. Note the selectivity of deprotonation over vinyl and allyl sites. A limitation of this methodology is that the ylide intermediate does not react well with alkyl hahde electrophiles. To get aronnd this, a seqnence that begins with the stannylation and decomplexation shown in Scheme 4b was developed. The stannane can be isolated in 94% yield (Scheme 4b) and snbseqnently snbjected to tin-lithium exchange to afford an unstabilized lithiomethylpiperidine that is a very good nucleophile. However, isolation of the stannane is not necessary and a procedure was devised in which the amine is activated with BF3, deprotonated, stannylated, decomplexed from BF3 with CsF, transmetalated back to lithium and alkylated, all in one pot (Scheme 4c). ... 

The stoichiometric composition of the halogenoborane complexes is 1 1, as a rule. However, in some cases, 2 1 and 4 1 complexes are also obtained (Table 4). In the latter instances additional ligands are bonded to the 1 1 complex through hydrogen bonds.2,11 55 With ammonia only the 1 1 complex is stable, whereas (H20)2-BF3 is more stable than H20-BF3. BF3 produces 2 1 complexes of similar stability with alcohols and carboxylic acids.2,55,57 At low temperatures some tertiary amines give complexes with halogenoboranes and complexes of... 

Both unknowns,/B and /L, can be found provided two different resonance lines are observed and a separate equation written for each. Liang and Gay measured SL in an amine/BF3 complex and <5B in an amine/HCl system for 4-ethylpyridine as the probe. The low precision with which the various l3C chemical shifts were determined resulted in poor accuracy in the final calculation of /B and /L, but the method does have potential provided chemical shifts can be measured accurately. 

The strength or coordinating power of different Lewis acids can vary widely against different Lewis bases. Thus, for example, in the case of boron trihalides, boron trifluoride coordinates best with fluorides, but not with chlorides, bromides, or iodides. In coordination with Lewis bases such as amines and phosphines, BF3 shows preference to the former (as determined by equilibrium constant measurements).66 The same set of bases behaves differently with the Ag+ ion. The Ag+ ion complexes phosphines much more strongly than amines. In the case of halides (F, CP, Br, and P), fluoride is the most effective base in protic acid solution. However, the order... 

The chemoselectivity of the dioxirane oxyfunctionalization usually follows the reactivity sequence heteroatom (lone-pair electrons) oxidation > JT-bond epoxida-tion > C-H insertion, as expected of an electrophilic oxidant. Because of this chemoselectivity order, heteroatoms in a substrate will be selectively oxidized in the presence of C-H bonds and even C-C double bonds. In allylic alcohols, however, C-H oxidation of the allylic C-H bond to a,/ -unsaturated ketones may compete efficaciously with epoxidation, especially when steric factors hinder the dioxirane attack on the Jt bond. To circumvent the preferred heteroatom oxidation and thereby alter the chemoselectivity order in favor of the C-H insertion, tedious protection methodology must be used. For example, amines may be protected in the form of amides [46], ammonium salts [50], or BF3 complexes [51] however, much work must still be expended on the development of effective procedures which avoid the oxidation of heteroatoms and C-C multiple bonds. 

When compared to aliphatic amines, aromatic amines generally have reduced exotherm and reactivity. Elevated temperatures are required to achieve optimum properties. In certain cases aromatic amines can be cured at room temperature with catalysts such as phenols, BF3 complexes, and anhydrides. 

When the base is changed, 1H complexation shifts bear little relationship to the heat of dissociation of the donor-acceptor bond, as illustrated in Table II. The same is true even when the donor atom stays the same, as in a series of amine-BF3 complexes (179). A number of factors affect NMR parameters in adducts, and trends in chemical shifts in closely related series of adducts (70). 

The observation by Fischer et al.18 that the 4,1-addition of dimethylamine to compound la is thermodynamically controlled at 20°C, whereas 2,1-addition/elimination is kinetically controlled at -115°C, turned out to be limited to few cases.20 It has been shown9a 9b 42 112 113 that for most cases, three competing reaction paths must be considered (i) 2,1-addition/elimina-tion with formation of (l-amino)alkynylcarbene complexes (= 2-amino-l-metalla-l-en-3-ynes) 98 (ii) 4,1-addition to give [(2-amino)alkenyl]carbene complexes (= 4-amino-l-metalla-l,3-butadienes) 96 and (iii) 4,1-addition/ elimination to (3-amino)allenylidene complexes (= 4-amino-l-metalla-1,2,3-butatrienes) 99 (Scheme 33, M = Cr, W). The product ratio 96 98 99 depends on the bulk of substituents R and R1, as well as on the reaction conditions. Addition of lithium amides instead of amines leads to predominant formation of allenylidene complexes 99.112 Furthermore, compounds 99 also can be generated by elimination of ethanol from complexes 96 with BF3 or AlEt3114 and A1C13,113 respectively. 

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