Methylene quinone intermediate

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). 

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). 

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-... 

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). 

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

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... 

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... 

The geometry of the zwitterions with its exocylic out-of-plane methylene group was quasi-preserved in the recently reported dibenzodioxocine derivative (18) that was formed in rather small amounts by rapidly degrading the NMMO complex at elevated temperatures.45 Strictly speaking, dibenzodioxocine dimer 18 is actually not a dimer of ortho-quinone methide 3, but of its zwitterionic precursor or rotamer 3a (Fig. 6.17). As soon as the out-of-plane methylene group in this intermediate rotates into the ring plane, the o-QM 3 is formed irreversibly and the spiro dimer 9 results... 

Reaction XIX. (6) Condensation of Anthranol Derivatives with Formaldehyde. (A., 420,134.)—The anthranols can also give rise to other intermediates by condensation with formaldehyde. Methylene anthra-quinone is thus obtained from anthranol. 

The phenol derivative 271 undergoes oxidation to the quinone 272 by constant current electrolysis. Concomitant irradiation of this quinone transforms it into the novel (2 -h 2)-cycloadduct 273. This was the key intermediate in a synthetic approach to racemic isoitalicene 274. The oxidation of a-tocopherol 275 can be brought about using Methylene Blue as the sensitizer for the production of 02( Aj). This converts the compound into the previously unknown enedione 276 as well as the usual quinone . The reaction of phenols with 02( A ) is a common process. This involves attack on the phenol, usually at the p-position. A typical example of this is the conversion of the phenols 277 and 278 into the corresponding cyclohexadienones 279 and 280 . The oxidation of phenols has been the subject of reviews . ... 

CYP-catalyzed demethylenation of the methylenedioxyphenyl (1,3-benzdioxole) group in natural products and/or medicinal agents also results in quinone formation via the intermediate catechol intermediate. The mechanism (see Scheme 1) involves an initial hydroxylation at the methylene carbon followed by partitioning between demethylenation yielding a catechol intermediate and formaldehyde/formate or dehydration to a carbene (Murray, 2000). Further oxidation of the catechol generates the ort/zo-benzoquinone species. The selective serotonin reuptake inhibitor paroxetine is a classic example of a drug that undergoes this pathway (Zhao et al., 2007). As such, the mechanistic details of quinone formation with paroxetine will be discussed later (see Scheme 25). 

As shown in Scheme 11.2, protonation of formaldehyde (6) is followed by nucleophilic attack by the 71-system of resorcinol leading to hydroxymethylation of resorcinol (7). In an acidic environment, protonation of hydroxymethyl groups leads to -OH2" group formation (8), which, as good leaving groups, cleaves further unimolecularly to form o-quinone methide-type intermediates (9). o-Quinone methides can lead to condensation via (4), or alternatively the 71-system of another resorcinol can attack the protonated hydroxymethylated resorcinol moieties to form methylene linkages (10). Those... 

Recently, the synthetic utility of dimethyl formamide (DMF) as the nucleophilic trigger in aryne MCRs was developed independently by the Miyabe and Yoshida groups. The three-component coupling reaction between aryne, DMF, and an active methylene compound such as acetyl acetone, dibenzoyl methane, cyclohexane 1,3-dione, etc. resulted in the formation of either 2H-chromenes 105 or coumarin derivatives 106 [75,76]. The insertion of fhe carbonyl group of DMF to the aryne generates the o-quinone methide intermediate 104. Subsequent reaction of 104 with cyclic or acyclic 1,3-diketones as the third component resulted in the formation of 2H-chromenes 105. Intriguingly, furnished coumarin derivatives 106 were formed when 3-ketoester or a-(hetero)aryl esters were used as the third component (Scheme 53). 

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