Methidate

NONDESTRUCTIVE MAGNETIC METHID OF INSPECTION OF THE MECHANICAL PROPERTIES OF CAST STEELS. 1. CONSTRUCTION OF CORRELATION MODELS and II. PRACTICAL APPLICATION OF CORRELATION... 

If pure triphenylchloromethane and freshly prepared sodium amalgam are used, the yield of sodium triphenyl-methide should be almost quantitative but is usually 0 15 mol per htre (1). The reagent should be used as soon as possible after its preparation. 

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). 

The alkylate contains a mixture of isoparaffins, ranging from pentanes to decanes and higher, regardless of the olefins used. The dominant paraffin in the product is 2,2,4-trimethylpentane, also called isooctane. The reaction involves methide-ion transfer and carbenium-ion chain reaction, which is cataly2ed by strong acid. 

Proton loss from alkyl groups a or 7 to a cationic center in an azolium ring is often easy. The resulting neutral anhydro bases or methides (cf. 381) can sometimes be isolated they react readily with electrophilic reagents to give products which can often lose another proton to give new resonance-stabilized anhydro bases. Thus the trithione methides are anhydro bases derived from 3-alkyl-l,2-dithiolylium salts (382 383) (66AHC(7)39). These... 

Naphthoquinone 1-methide thermal dissociation, 3, 785 N aphtho[2,3 -d ][2,1,3]selenadiazole... 

Pyridin-4-one, 1 -hydroxy-2,6-dimethyl-hydrogen-deuterium exchange reactions, 2, 196 Pyridin-4-one, 1-methyl-hydrogen-deuterium exchange, 2, 287 pK 2, 150 Pyridin-2-one imine tautomerism, 2, 158 Pyridin-2-one imine, 1-methyl-quaternization, 4, 503 Pyridin-4-one imine tautomerism, 2, 158 Pyridinone methides, 2, 331 tautomerism, 2, 158 Pyridinones acylation, 2, 352 alkylation, 2, 349 aromaticity, 2, 148 association... 

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). 

CH3CH2CH2(CH2)3CH3 CH3CH2CH(CH2)3CH3 1,2-Methide-Hydride Shift ... 

Abstraction of a hydride ion from a tertiary carbon is easier than from a secondary, which is easier than from a primary position. The formed car-bocation can rearrange through a methide-hydride shift similar to what has been explained in catalytic reforming. This isomerization reaction is responsible for a high ratio of branched isomers in the products. 

The new carbocation can rearrange again through a methide/hydride shift as shown in the following equation ... 

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