Benzoxazines phenol reaction

Lim et al. also investigated HMTA-phenolic reactions with somewhat larger model compounds (e.g., two- and four-ring compounds) and established that similar reaction pathways to those described previously occurred.50 For these model compounds (as opposed to one-ring model compounds), which are more representative of typical oligomeric systems, increased molecular weight favored die formation of hydroxybenzylamines but not benzoxazines. This was suggested to be a steric effect. 

In the reaction of phenol and bisphenol F with hexa, nmr spectra show the transient appearance of benzoxazine intermediates after 2 h at 103°C, all the benzoxazine decomposed to the diphenyknethylene and benzylamine intermediates (15). 

Reaction of 9,10-difluoro-3-methyl-2,3-dihydro-7-oxo-7//-pyrido[l,2,3- /e]-l,4-benzoxazine-6-carboxylic acid (252 X = F, R = H, R = Me) with 8 M aquous solution of KOH under reflux for 6 h, and in the presence of an alcohol or phenol afforded 10-hydroxy, 10-alkoxy and 10-aryloxy derivatives, respectively (96JAP(K)96/291144, 99MI5). 

Benzoxazines are heterocyclic compounds obtained from reaction of phenols, primary amines, and formaldehyde.98,99 As described previously, they are key reaction intermediates in the HMTA-novolac cure reaction.40,43 Crosslinking benzoxazine monomers at high temperatures gives rise to void-free networks with high Tgs, excellent heat resistance, good flame retardance, and low smoke toxicity.100 As in HMTA-cured novolac networks, further structural rearrangement may occur at higher temperatures. 

Figure 7.40 Reaction of benzoxazines with free ortho positions on phenolic compounds.

Dihydro-1,3-benzoxazines (196) are formed by the reaction of phenols with a mixture of formaldehyde and primary aromatic amines in the molar ratio 2 1. Presumably the phenol first reacts with the appropriate iminium species to form an intermediate amine (195), which is then cyclized in a Pictet-Spengler type reaction (Scheme 81) (44JA1875). If 2-hydroxybenzylamines are employed then methylene derivatives are obtained, and if the formaldehyde is replaced by a-dicarbonyl compounds dehydro dimers (197) are produced (Scheme 82) <70BCJ226>. 

Nair 482 reacted benzoxazole with a molar quantity of DM AD at room temperature for 10 days, then he heated the product for 6 hours at 100°, and obtained 3-methoxycarbonylmethylene-2-oxo-3,4-dihydro-2H-1,4-benzoxazine (18), identical with that obtained from o-amino-phenol and the acetylenic ester his benzoxazole may have hydrolyzed before or during reaction. 

Dihydroxybenzoquinones BenzothiazoUnone hydrazones 980, 987 Benzoxazine, reactions with model phenols 1473, 1474 Benzoxazoles,... 

The study found that the reaction pathways for the ortho and para sites are different and that distinctly different mechanisms apply. In the case of the free ortho site, the reaction is dependent on the breakdown of the hydrogen bonding complex to form benzoxazine, in contrast to the free para position where the reaction is governed by the breakdown of the amine intermediates. These results strongly suggested that interaction between phenolic entities is primarily controlled by hydrogen bonding, especially when... 

The reactivity of the maleimide monomer was dependent on the substitution pattern of the phenyl ring, with the substituents in the ortho position tending to lower the molecular weight of the polymer formed. The THP substituent is readily removed either chemically or thermally to yield poly(iV-(hydroxyphenyl)maleimides). All polymers exhibited excellent thermal stability and showed no evidence of degradation below 360 °C. Reaction occurs between the phenolic ring of the polyimide and HMTA, to form benzoxazine-type derivatives. These reactions have been studied comprehensively using the monomeric model systems, Ai-(hydroxyphenyl)succinimides (Figure 8) . 

The mechanistic pathway taken during the reaction of the Af-(hydroxyphenyl)succini-mides and HMTA is dependent on the substitution of the phenolic ring. With compounds containing a free ortho position, the initial intermediates are benzoxazine-type species (Scheme 31). 

The main substrates that have been investigated as partners in cyclization reactions are phenols. As with reactions involving secondary amines, phenols react with aldehydes and primary amines principally ortho to the hydroxy group. A reaction of the first-formed secondary amine with a second molecule of aldehyde can therefore lead to the formation of a benzoxazine derivative. A wide range of phenols, includ-... 

Reaction of phenols and hexahydro-l,3,5-triazines at 150°C also provides access to dihydro-1,3-benzoxazines (272) (Equation (24)). Similar constructions use hexamethylenetetramine, rather than hexahydro-l,3,5-triazines, but now the products are 3-benzyl derivatives (e.g. 273) (Equation (25)) <85ZOR2243>. It seems likely that, in these reactions, the nitrogen-containing reagents act as sources of methylenimine, which convert the phenols into the corresponding benzylamines. Further reactions with more methylenimine then results in cyclization if so these preparations are closely related to those which employ hydroxybenzylamines and carbonyl compounds. 

This reaction is an autocatalytic one. It is catalyzed by the dimers and higher oligomers with free ortho positions at the phenolic ring that are contained in the 3,4-dihydro-3-substituted- 1,3-benzoxazine precursors. Acids catalyze the reaction in equation 44 with adipie aeid showing the best performanee [133]. 

Schreiber, for the preparation of phenolic resins with improved thermal, mechanical, and dielectric characteristics. However, no systematic study was reported in Schreiber s patents. Thus, it was of interest to examine in more detail several points such as 1) the synthesis of an homologeous series of benzoxazines, 2) their reaction with phenols, which may be considered as the initiation step of the polymerization by a ring opening mechanism, 3) the pol3nnerization kinetics of benzoxazines, initiated either thermally or with phenols, and 4) the application possibilities of these monomers and polymers. 

A qualitative study carried out by these authors, on different N substituted benzoxazines and various phenols, revealed two Important features, namely the orthohydroxy selectivity of the reaction and the important steric effect of the N substituent of benzoxazines. 

Since no quantitative information was disclosed by Burke and coworkers, we have carried out a systematic study of the reaction between benzoxazine and phenol. The latter reaction may be considered as the initiation step of the ring opening poljrmerization, described later,... 

As a model approach to better understand the bulk pol3nnerization of benzoxazines, we initially examined the neat reaction of a "non-pol3nneriz-able benzoxazine, such as 3,6,8-trlmethyl-3,4-dihydro-2H-l,3-benzoxazlne (la), with phenols having either a free ortho position (2, 2 or a free para position (2c, 2d), The reactions run and products obtained (3a-3c) are shown below ... 

With the exception of 2,6-ditert-butylphenol, which remained unreac-tive under the used experimental conditions, the reaction of benzoxazine with phenols and were shown to follow second order kinetics. 

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