Formaldehyde protonated

Because of the variety of degradation reactions, POM is stabilized using multi-component systems consisting of antioxidants and compounds that bind the secondary products (e.g., formaldehyde, protons) of autoxidation. The stabilizers allow numerous combinations that often have synergetic effectiveness. Sterically hindered phenols are mostly used as antioxidants here, polynuclear phenols are preferred because of their low migration tendency. In addition, sterically hindered amines are used as radical scavengers. 

Scheme 55) (235. 236). -The product obtained (77) is probably formed via the protonated form of the thiazole, whose reactivity is treated in Section IV, 1. The light-yellow leucobase (77) is reported to be oxidized by PbOj to the red-black carbinol (78) (236). This condensation reaction is also successful when benzaidehyde is replaced by formaldehyde, bis(2-amino-4-phenylthiazolyl-5)methane (79 i beine obtained (Scheme 56) (237). 

Learning By Model ing includes models of formaldehyde (H2C=0) and its protonated form (H2C=0H ) Compare the two with respect to their electrostatic potential maps and the degree of positive charge at carbon... 

The cyanoacryhc esters are prepared via the Knoevenagel condensation reaction (5), in which the corresponding alkyl cyanoacetate reacts with formaldehyde in the presence of a basic catalyst to form a low molecular weight polymer. The polymer slurry is acidified and the water is removed. Subsequendy, the polymer is cracked and redistilled at a high temperature onto a suitable stabilizer combination to prevent premature repolymerization. Strong protonic or Lewis acids are normally used in combination with small amounts of a free-radical stabilizer. 

Reactions with Aldehydes and Ketones. An important use for alkylphenols is ia phenol—formaldehyde resias. These resias are classified as resoles or aovolaks (see Phenolic resins). Resoles are produced whea oae or more moles of formaldehyde react with oae mole of pheaol uader basic catalysis. These resias are thermosets. Novolaks are thermoplastic resias formed whea an excess of phenol reacts with formaldehyde under acidic conditions. The acid protonates formaldehyde to generate the alkylating electrophile (17). 

When DMSO is mixed with concentrated hydrochloric acid, protonated DMSO is in equiUbtium with the chlorodimethylsiilfonium ion. Pummerer reactions and subsequent reaction of the initial products give a complex mixture of products including formaldehyde, bis(methylthio)methane, methanethiol, dimethyl disulfide, dimethyl sulfide, and others. 

The chain polymerization of formaldehyde CH2O was the first example of a chemical conversion for which the low-temperature limit of the rate constant was discovered (see reviews by Goldanskii [1976, 1979]). As found by Mansueto et al. [1989] and Mansueto and Wight [1989], the chain growth is driven by proton transfer at each step of adding a new link... 

As with resoles, we can use a three-phase model to discuss formation of a novolac. Whereas the resole is activated through the phenol, activation in novolacs occurs with protonation of the aldehyde as depicted in Scheme 12. The reader will note that the starting material for the methylolation has been depicted in hydrated form. The equilibrium level of dissolved formaldehyde gas in a 50% aqueous solution is on the order of one part in 10,000. Thus, the hydrated form is prevalent. Whereas protonation of the hydrate would be expected to promote dehydration, we do not mean to imply that the dehydrated cation is the primary reacting species, though it seems possible. 

The mechanism for the formations of 83, 85, and 93 can be explained as illustrated in Scheme 13. Initial deprotonation of the 1-methoxy group of 82 liberates formaldehyde and an indolyl anion 95, and then protonation of 95 affords 83. Following the reaction path a, 95 reacts with formaldehyde to produce indole-3-methanol 96. Unstable 96 collapses into 3-methyleneindolenine 97, which adds... 

Surely these views are not very plausible, depending as they do on the formation of such an improbable system as (23). How much more simple is it to view acid catalysis as involving enhancement of the electrophilic character of the reagent. Thus, in the formaldehyde condensation, it is protonated formaldehyde,... 

DFT molecular dynamics simulations were used to investigate the kinetics of the chemical reactions that occur during the induction phase of acid-catalyzed polymerization of 205 [97JA7218]. These calculations support the experimental finding that the induction phase is characterized by the protolysis of 205 followed by a rapid decomposition into two formaldehyde molecules plus a methylenic carbocation (Scheme 135). For the second phase of the polymerization process, a reaction of the protonated 1,3,5-trioxane 208 with formaldehyde yielding 1,3,5,7-tetroxane 209 is discussed (Scheme 136). 

In a first reaction step the formaldehyde 2 is protonated, which increases its reactivity for the subsequent electrophilic aromatic substitution at the benzene ring. The cationic species 4 thus formed loses a proton to give the aromatic hydroxymethyl derivative 5, which further reacts with hydrogen chloride to yield the chloromethylated product 3 ... 

There have been extensive investigations on the reaction mechanism. In most cases the reaction proceeds via initial nucleophilic addition of ammonia 2 to formaldehyde 1 to give adduct 5, which is converted into an iminium ion species 6 (note that a resonance structure—an aminocarbenium ion can be formulated) through protonation and subsequent loss of water. The iminium ion species 6 then reacts with the enol 7 of the CH-acidic substrate by overall loss of a proton ... 

The initial step is the protonation of the aldehyde—e.g. formaldehyde—at the carbonyl oxygen. The hydroxycarbenium ion 6 is thus formed as reactive species, which reacts as electrophile with the carbon-carbon double bond of the olefinic substrate by formation of a carbenium ion species 7. A subsequent loss of a proton from 7 leads to formation of an allylic alcohol 4, while reaction with water, followed by loss of a proton, leads to formation of a 1,3-diol 3 " ... 

A rather more complex amino alcohol side chain is accessible by a variation of the Mannich reaction. Taking advantage of the acidic proton in acetylenes, propargyl acetate (62) is condensed with formaldehyde and dimethylamine to give the acetylated amino... 

Novolacs are prepared with an excess of phenol over formaldehyde under acidic conditions (Fig. 7.6). A methylene glycol is protonated by an acid from the reaction medium, which then releases water to form a hydroxymethylene cation (step 1 in Fig. 7.6). This ion hydroxyalkylates a phenol via electrophilic aromatic substitution. The rate-determining step of the sequence occurs in step 2 where a pair of electrons from the phenol ring attacks the electrophile forming a car-bocation intermediate. The methylol group of the hydroxymethylated phenol is unstable in the presence of acid and loses water readily to form a benzylic carbo-nium ion (step 3). This ion then reacts with another phenol to form a methylene bridge in another electrophilic aromatic substitution. This major process repeats until the formaldehyde is exhausted. 

Simple imines are poor dienophiles and must be activated by protonation or by attaching an electron-withdrawing group to the nitrogen atom. Scheme 6.10 illustrates the Diels-Alder reactions of benzyliminium ion 25, generated in situ from an aqueous solution of benzylamine hydrochloride and commercial aqueous formaldehyde, with methylsubstituted 1,3-butadienes [22]. This aqueous Diels-Alder reaction combines three components (an aldehyde, an amine... 

The addition of an alkene to formaldehyde in the presence of an acid catalyst is called the Prins reaction.Three main products are possible which one predominates depends on the alkene and the conditions. When the product is the 1,3-diol or the dioxane, the reaction involves addition to the C=C as well as to the C=0. The mechanism is one of electrophilic attack on both double bonds. The acid first protonates the C=0, and the resulting carbocation attacks the C=C ... 

Pd, or Ni (Scheme 5-3). First, P-H oxidative addition of PH3 or hydroxymethyl-substituted derivatives gives a phosphido hydride complex. P-C bond formation was then suggested to occur in two possible pathways. In one, formaldehyde insertion into the M-H bond gives a hydroxymethyl complex, which undergoes P-C reductive elimination to give the product. Alternatively, nucleophilic attack of the phosphido group on formaldehyde gives a zwitterionic species, followed by proton transfer to form the O-H bond [7]. 

An extension of this method can be used to prepare allylic alcohols. Instead of being protonated, the (3-oxido ylide is allowed to react with formaldehyde. The (J-oxido ylide and formaldehyde react to give, on warming, an allylic alcohol. Entry 12 is an example of this reaction. The reaction is valuable for the stereoselective synthesis of Z-allylic alcohols from aldehydes.245... 

Another elegant example of the thermal generation and subsequent intramolecular cycloaddition of an o-QM can be found in Snider s biomimetic synthesis of the tetracyclic core of bisabosquals.2 Treatment of the starting material with acid causes the MOM ethers to cleave from the phenol core (Fig. 4.3). Under thermal conditions, a proton transfer ensues from one of the phenols to its neighboring benzylic alcohol residue. Upon expulsion of water, an o-QM forms. The E or Z geometry of the o-QM intermediate and its propensity toward interception by formaldehyde, water, or itself, again prove inconsequential as the outcome is decided by the relative thermodynamic stabilities among accessible products. 

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