Methylene quinone

Similarly reduction of o- or p-hydroxy-substituted aryl ketones with NaBHa in refluxing aqueous NaOH solution gives rise to an intermediate methylene quinone (26), which is further reduced to the desired methylene product (equation 14). When the hydroxy group is situated only at the meta position, there is no possibility to make such a methylene quinone intermediate, and the corresponding benzyl alcohol (27) is formed (equation 15). 

The serie of Methylene quinones abietanes (286-303) comprising taxodone (288), possessing an antitumour activity [142] have been reported from a large number of Salvidi species. 

Particularly interesting has been the case of Orthosphenia mexicana which yielded five new triterpene methylene quinones with a new carbon skeleton, a greater degree of conjugation than hitherto reported, an extra 14-15 double bond and a rearranged methyl at C-15. Its structure was elucidated by a succession of chemical transformations, spectroscopic methods and X ray diffraction which determined the absolute configuration of this compound [49,50],... 

Two mechanisms have been proposed for resin curing of butyl rubber. One proposal involves an ortho-methylene-quinone intermediate that abstracts an allyl hydrogen from the isoprenoid group of the butyl polymer chain and would occur via the formation of a six-membered ring ene intermediate. This would be followed by formation of a second ortho-methylene-quinone intermediate with another isoprenoid group of an adjacent butyl polymer chain to form a monophenol crosslink (Scheme 4.3). 

Reactions o/In yielding nonradical products that influence markedly on the reaction inhibiting or promoting it. Thus, disproportionation of 2,4,6-trialkyl-substituted phenoxyl radicals results in end products, the appropriate methylene quinone and the initial phenolic inhibitor, e.g. ... 

The first systematic study on the vulcanization of rubber with phenolic resins was undertaken at the Dutch research institute Rub-ber-Stichting during World War II (Van der Meer, 1944 1945). Whilst the mechanism of cure remains to be fully established it is generally considered that the first stage involves the formation of a methylene quinone structure by elimination of water from an o-methylol phenol ... 

An alternative theory suggests that reaction occurs at the a-methylene group with, once again, methylene quinone or a derivative as a reactive intermediate ... 

On the other hand, it is well known that phenol-formaldehyde resins are able to vulcanize diene rubber. Two basic types of mechanisms have been postulated for this interaction. The first, associated with van der Meer, involves the formation of a methylene quinone intermediate, formed from the methylolated re-... 

When camphor (I) is heated with selenium dioxide in acetic acid, the methylene group next to the carbonyl group is oxidised also to a carbonyl group, to form camphorquinone (II). Note that the compound (II) is not a true quinone but a 1,2-diketone ... 

Rate studies show that base-cataly2ed reactions are second order and depend on the phenolate and methylene glycol concentrations. The most likely path involves a nucleophilic displacement by the phenoxide on the methylene glycol (1), with the hydroxyl as the leaving group. In alkaline media, the methylolated quinone intermediate is readily converted to the phenoxide by hydrogen-ion abstraction (21). 

Quinone monoacetals such as 2-methoxyben2oquinonemonoacetal [64701-03-7] (66) show regiospeciftc addition of active methylene compounds (66), yielding 83% (67) and 63% (68) on reactions with ethyl malonate. 

This derivative is prepared from an A-protected amino acid and the anthrylmethyl alcohol in the presence of DCC/hydroxybenzotriazole. It can also be prepared from 2-(bromomethyl)-9,10-anthraquinone (Cs2C03). It is stable to moderately acidic conditions (e.g., CF3COOH, 20°, 1 h HBr/HOAc, / 2 = 65 h HCl/ CH2CI2, 20°, 1 h). Cleavage is effected by reduction of the quinone to the hy-droquinone i in the latter, electron release from the —OH group of the hydroqui-none results in facile cleavage of the methylene-carboxylate bond. The related 2-phenyl-2-(9,10-dioxo)anthrylmethyl ester has also been prepared, but is cleaved by electrolysis (—0.9 V, DMF, 0.1 M LiC104, 80% yield). ... 

Some other inhibitors from the patent literature include hydroquinone [129], ionoP [130], and quinone [131]. Other inhibitors used to stabilize MMA include butylated hydroxy toluene (BHT), phenothiazine, methylene blue, hydroxy-diphenylamine and di-/jc/<3-napthol [132]. Several good reviews of inhibition and inhibitors have been written [133-136]. The mechanisms of inhibition are subtle and complicated. For example, it has been reported that highly purified benzo-quinone acts as a retarder rather than an inhibitor [137]. It has been proposed... 

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]. 

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. 

The prototype o-quinone methide (o-QM) and / -quinone methide (p-QM) are reactive intermediates. In fact, they have only been detected spectroscopically at low temperatures (10 K) in an argon matrix,1 or as a transient species by laser flash photolysis.2 Such a reactivity is mainly due to their electrophilic nature, which is remarkable in comparison to that of other neutral electrophiles. In fact, QMs are excellent Michael acceptors, and nucleophiles add very fast under mild conditions at the QM exocyclic methylene group to form benzylic adducts, according to Scheme 2.1.2a 3... 

Da Silva, G. Chen, C.-C. Bozzelli, J. W. Quantum chemical study of the thermal decomposition of o-quinone methide (6-methylene-2,4-cyclohexadien-l-one). J. Phys. Chem. A 2007, 111, 7987-7994. 

Quinone methides (QMs), especially the simple ones (those not having substituents at the exocyclic methylene group), are very unstable compounds. Their isolation is very difficult and normally requires very dilute solutions and low temperatures.2 Due to the aromatic z witterionic form (Scheme 3.1), quinone methides react very rapidly with both electrophiles and nucleophiles, with the medium, or in self-condensation reactions. 

As observed with the pincer-type quinone methide complexes (Section 3.2), the one-site coordinated QMs can also undergo chemical transformation to other quinonoid compounds. For instance, reaction of complex 20 with MeOTf resulted in the methylene arenium complex 21 (Scheme 3.13).22... 

Amouri and coworkers also demonstrated that the nucleophilic reactivity of the exocyclic carbon of Cp Ir(T 4-QM) complex 24 could be utilized to form carbon -carbon bonds with electron-poor alkenes and alkynes serving as electrophiles or cycloaddition partners (Scheme 3.17).29 For example, when complex 24 was treated with the electron-poor methyl propynoate, a new o-quinone methide complex 28 was formed. The authors suggest that the reaction could be initiated by nucleophilic attack of the terminal carbon of the exocyclic methylene group on the terminal carbon of the alkyne, generating a zwitterionic oxo-dienyl intermediate, followed by proton transfer... 

Vigalok, A. Shimon, L. J. W. Milstein, D. Methylene arenium cations via quinone methides and xylylenes stabilized by metal complexation. J. Am. Chem. Soc. 1998, 120, 477 183. 

Gauvin, R. M. Rozenberg, H. Shimon, L. J. W. Ben-David, Y. Milstein, D. Osmium-mediated C—H and C—C bond cleavage of a phenolic substrate p-quinone methide and methylene arenium pincer complexes. Chem. Eur. J. 2007, 13, 1382-1393. 

Rabin, O. Vigalok, A. Milstein, D. A novel approach towards intermolecular stabilization of para-quinone methides. First complexation of the elusive, simplest quinone methide, 4-methylene-2,5-cyclohexadien-l-one. Chem. Eur. J. 2000, 6, 454-462. 

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