Methides quinone

Lower reaction temperatures favor the reversible formation of the Diels-Alder products, and higher reaction temperatures promote the irreversible ene reactions. Related observations of the competing or predominating intramolecular Diels-Alder reactions of a-cyano or a-acetyl a,jS-unsaturated esters have been reported in studies of the thermal and Lewis acid-promoted ene reaction (Table 7-II, entry 16). The complementary use of o-quinone methide intermediates in intra- as well as inter-molecular [4 + 2] cycloaddition reactions is discussed in the following section. 

The various methods of generating o-quinone methides, including the thermal or (Lewis) acid-catalyzed elimination of a phenol Mannich base, the thermal or (Lewis) acid-catalyzed dehydration of an o-hydroxybenzyl alcohol (ether), 

The general approach employed for the in situ generation of o-quinone methides relies on the elimination of water [Eq. (41)], alcohol, or second- 

The reaction of chlorotrimethylsilane with hydroxymethylspiroepoxycy-clohexadienone (20) affords the spiroannelated chroman 21. The reaction presumably proceeds through an o-quinone methide intermediate which dimerizes to the chroman [Eq. (42)].  

Two recent reports have detailed the [4 + 2] cycloadditions of o-quinone methides with heterodienophiles, thiones [Eq. (43)]  

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Quinone dyes Quinoneimine Quinoneimine dye Quinone methides Quinonemonoimine Quinones... 

Quinone Methides. The reaction between aldehydes and alkylphenols can also be base-cataly2ed. Under mild conditions, 2,6-DTBP reacts with formaldehyde in the presence of a base to produce the methylol derivative (22) which reacts further with base to eliminate a molecule of water and form a reactive intermediate, the quinone methide (23). Quinone methides undergo a broad array of transformations by way of addition reactions. These molecules ate conjugated homologues of vinyl ketones, but are more reactive because of the driving force associated with rearomatization after addition. An example of this type of addition is between the quinone methide and methanol to produce the substituted ben2yl methyl ether (24). 

This addition is general, extending to nitrogen, oxygen, carbon, and sulfur nucleophiles. This reactivity of the quinone methide (23) is appHed in the synthesis of a variety of stabili2ers for plastics. The presence of two tert-huty groups ortho to the hydroxyl group, is the stmctural feature responsible for the antioxidant activity that these molecules exhibit (see Antioxidants). 

Methylenebis(2,6-di-/ /f-butylphenol) (25) (R = H) [118-82-17, the reaction product of two molecules of 2,6-DTBP with formaldehyde under basic conditions, is a bisphenoHc antioxidant. The quinone methide in this case is generated in situ. The product results from the addition of 2,6-di-/ /f-butylphenolate to (23) (12). 

A series of carbamates have been prepared that are cleaved by liberation of a phenol, which, when treated with base, cleaves the carbamate by quinone methide formation through a 1,6-elimination. ... 

Above 160°C it is believed that additional cross-linking reactions take place involving the formation and reaction of quinone methides by condensation of the ether linkages with the phenolic hydroxyl groups (Figure 23.14). 

These quinone methide structures are capable of polymerisation and of other chemical reactions. 

It is likely that the quinone methide and related structures formed at these temperatures account for the dark colour of phenolic compression mouldings. It is to be noted that cast phenol-formaldehyde resins, which are hardened at much... 

In addition to the above possible mechanisms the possibility of reaction at w-positions should not be excluded. For example, it has been shown by Koebner that o- and p-cresols, ostensibly difunctional, can, under certain conditions, react with formaldehyde to give insoluble and infusible resins. Furthermore, Megson has shown that 2,4,6-trimethylphenol, in which the two ortho- and the one para-positions are blocked, can condense with formaldehyde under strongly acidic conditions. It is of interest to note that Redfam produced an infusible resin from 3,4,5,-trimethylphenol under alkaline conditions. Here the two m- and the p-positions were blocked and this experimental observation provides supplementary evidence that additional functionalities are developed during reaction, for example in the formation of quinone methides. 

In order that the rate of cure of phenolic moulding compositions is sufficiently rapid to be economically attractive, curing is carried out at a temperature which leads to the formation of quinone methides and their derivatives which impart a dark colour to the resin. Thus the range of pigments available is limited to blacks, browns and relatively dark blues, greens, reds and oranges. 

Scheme 10. Mechanislic possibililies for PF condensalion. Mechanism a involves an SN2-like attack of a phenolic ring on a methylol. This attack would be face-on. Such a mechanism is necessarily second-order. Mechanism b involves formation of a quinone methide intermediate and should be Hrst-order. The quinone methide should react with any nucleophile and should show ethers through both the phenolic and hydroxymethyl oxygens. Reaction c would not be likely in an alkaline solution and is probably illustrative of the mechanism for novolac condensation. The slow step should be formation of the benzyl carbocation. Therefore, this should be a first-order reaction also. Though carbocation formation responds to proton concentration, the effects of acidity will not usually be seen in the reaction kinetics in a given experiment because proton concentration will not vary.

The mechanism of this reaction has been studied by several groups [133,174-177]. The consensus is that interaction of ester with the phenolic resole leads to a quinone methide at relatively low temperature. The quinone methide then reacts rapidly leading to cure. Scheme 11 shows the mechanism that we believe is operative. This mechanism is also supported by the work of Lemon, Murray, and Conner. It is challenged by Pizzi et al. Murray has made the most complete study available in the literature [133]. Ester accelerators include cyclic esters (such as y-butyrolactone and propylene carbonate), aliphatic esters (especially methyl formate and triacetin), aromatic esters (phthalates) and phenolic-resin esters [178]. Carbamates give analogous results but may raise toxicity concerns not usually seen with esters. 

Scheme 11. Proposed quinone methide condensation mechanism. Work by Murray (and Lemon unpublished) showed clearly that the quinone methides formed from o-hydroxymethyl and not /7-hydroxymethyl groups in the presenee of ester.

The absence of methylol (-CH2OH) groups in all six lower molecular weight resorcinol-formaldehyde condensates which have been isolated [119] reflects the high reactivity of resorcinol under acid or alkaline conditions. It also shows the instability of its para-hydroxybenzyl alcohol groups and their rapid conversion to jpara-hydroxybenzyl carbonium ions or quinone methides. This explains how identical condensation products are obtained under acid or alkaline reaction conditions [119]. In acid reaction conditions methylene ether-linked condensates are also formed, but they are highly unstable and decompose to form stable methylene links in 0.25 to 1 h at ambient temperature [121,122]. 

Estrone is rapidly oxidized by DDQ at room temperature to the A E om-pound (76) which can be obtained from the A" -3-ketone under more drastic conditions. The quinone methide (77) is suggested as an inter-... 

A tautomeric equilibrium between quinone and quinone methide tautomers has been proposed to exist for the compounds which are obtained by oxidation of 5,6-dihydroxy indole (Scheme 18) (92TL3045). 

Quinone methides are the key intermediates in both resole resin syntheses and crosslinking reactions. They form by the dehydration of hydroxymethylphenols or dimethylether linkages (Fig. 7.24). Resonance forms for quinone methides include both quinoid and benzoid structures (Fig. 7.25). The oligomerization or crosslinking reaction proceeds by nucleophilic attack on the quinone methide carbon. 

The ortho-quinone methides are difficult to isolate due to their high reactivity, which leads to rapid Diels-Alder dimerization or trimerization (Fig. 7.26). At 150°C, a partial retro-Diels-Alder reaction of the trimer can occur to form ortho-quinone methide and bis(2-hydroxy-3,5-dimethylphenyl) ethane (dimer).51... 

Figure 7.24 Dehydration of methylols or benzylic ethers to form quinone methides.

Figure 7.26 Dimer and trimer structures of ortho-quinone methides.

The mechanisms for model condensation reactions of para-hydroxymethyl-substituted phenol (and therefore para-quinone methide) with reactive ortho positions are described in Fig. 7.29. The phenolate derivatives react with para-quinone... 

Figure 7.28 Preferential formation of para-quinone methides.

Figure 7.29 Reactions of a quinone methide with ahydroxymethyl-substitutedphenolate.

Resole resins are generally crosslinked under neutral conditions between 130 and 200° C or in the presence of an acid catalyst such as hydrochloric acid, phosphoric acid, p-toluenesulfonic acid, and phenolsulfonic acid under ambient conditions.3 The mechanisms for crosslinking under acidic conditions are similar to acid-catalyzed novolac formation. Quinone methides are the key reaction intermediates. Further condensation reactions in resole resin syntheses under basic conditions at elevated temperatures also lead to crosslinking. 

Figure 7.31 Ethane and ethene linkages derived from quinone methide structures.

In addition to methylene and dimethylether linkages, cured networks contain ethane and ethene linkages (Fig. 7.31). These side products are proposed to form through quinone methide intermediates. 

Orthopedic fixations, 27 Ortho-quinone methides, 400, 401 Oxalic acid, 379... 

Para-quinone methide intermediates, 403 Para-quinone methides, 404 para-Trishydroxybenzylamine, reaction with 2,4-xylenol, 397 Partial aromatic polyamides, 136, 143, 180-184... 

Quasi-prepolymers, 236, 237 Quinoid resonance forms, 402 Quinone methide reactions, with... 

Keywords Lewis acids, asymmetric reactions, tandem, tethered. Intramolecular reactions, o-quinodimethanes, o-quinone methides, befera-Dlels-Alder reactions... 

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