Phenols, self-condensation

Diarylamines are manufactured by the self-condensation of a primary aromatic amine in the presence of an acid, or the reaction of an arylamine with a phenol, at high temperatures. 

Benzyl chloride reacts with benzene in the presence of a Lewis acid catalyst to give dipbenylmetbane [101 -81-5]. It undergoes self-condensation to form polymeric oils and soHds (21). With phenol, benzyl chloride produces a mixture of o- andp-her zylpbeno1. 

Liquid crystal polyesters are made by a different route. Because they are phenolic esters, they cannot be made by direct ester exchange between a diphenol and a lower dialkyl ester due to unfavorable reactivities. The usual method is the so-called reverse ester exchange or acidolysis reaction (96) where the phenolic hydroxyl groups are acylated with a lower aliphatic acid anhydride, eg, acetic or propionic anhydride, and the acetate or propionate ester is heated with an aromatic dicarboxylic acid, sometimes in the presence of a catalyst. The phenolic polyester forms readily as the volatile lower acid distills from the reaction mixture. Many liquid crystal polymers are derived formally from hydroxyacids (97,98) and their acetates readily undeigo self-condensation in the melt, stoichiometric balance being automatically obtained. 

Only one other synthesis from this review, the aldol self-condensation of cyclohexanone, followed by dehydrogenation of the initial products, is of preparative importance. It has even been used in the large scale industrial syntheses of 2,6-diphenyl-phenol (see 2.4). All the other routes mentioned in this account, are primarily of scientific interest and are not useful for larger scale preparative work. They ate, however, of interest insofar as they emphasize differences between the chemical properties of alkyl and aryl substituted phenols. 

The most important industrial process for the production of 2-phenyl- and 2,6-diphenyl-phenol, is based on the long established self-condensations of cyclohexanone under either acidic, or preferentially basic conditions to give mono- or di-substituted aldol-like condensation products79). These easily loose water giving semicyclic or endocyclic ring double bonds. Plesec et al. have studied these reactions extensively 80). 

When 2,4,6-triphenylphenoxyl is heated for several hours in non-reactive solvents, such as benzene, self condensation to a higher molecular phenol is observed. Pyridine, but also visible light catalyse this reaction preferently at the 4 -positions of the 2,4,6-phenyl groups to give TPPO-ethers 3 ... 

Phenols add intramolecularly to Michael acceptors. " Under acidic conditions, a one-pot sequence starts with initial electrophilic acetylation of the activated aromatic ring and is followed by cyclization." With an appropriate leaving group in the /f-position (OMe. or other amines such as in the unsaturated carbonyl compound (e.g., 4) is formed. Other approaches to pyroncs include the self-condensation of protected //-hydroxy acrylates,intramolecular aldol reactions followed by condensation,thermal cycli-zations of unsaturated ()-chloro esters,and an iodo-cyclization-elimination sequence w th Michael acceptors.Oxymercuration of an unsaturated alcohol is an alternative cyclization approach to tetrahydropyrans. 

Condensation of carbonyl compounds. This magnesium phenolate catalyzes the self-condensation of linear aliphatie aldehydes. When the reaction is conducted in HMPT, 1,3-diol monoesters (3 and 4) are formed (equation I). When the reaction is conducted in benzene, 2,3-dialkylacrylaldehydes (5) are formed, also in high yield (equation II). Magnesium ions (MgBr and Mg ) are much more effective in these reactions than Li or Na . ... 

In industry, several dendrimers are already being synthesized on a hundred kilogram scale. The hopes for effective solubilizing systems suitable for medical applications may, however, not be justified. Phenolic ether or polyamine compounds are both normally not biodegradable so that major problems are foreseeable. An alternative is the employment of hydrolysable polyesters. One such compound already reported, is the imperfectly hyperbranched dendrimer which is easily accessible via thermal self-condensation of 3,5-bis(trimethyl-siloxyjbenzoyl chloride. This one-step procedure leads to a dendritic polyester... 

For instance, condensation of a variety of 1,3-dicarbonyl componnds 102 with diethyl /3-ketoglutarate (103) afforded the phenols 104 by treatment with sodinm in ethanol (equation 101) . Naphthalene 106 has been obtained by self condensation of 2,4,6-heptatrione (105) (equation 102) °. 

SCHEME 21. Base-catalysed self-condensation of 2-(hydroxymethyl)phenol... 

In the above condensation resist designs, the phenolic resin offers a reaction site as well as base solubility. Self-condensation of polymeric furan derivatives has been utilized as an alternative crosslinking mechanism for aqueous base development (Fig. 126) [375]. The copolymer resist is based on poly[4-hydroxy-styrene-co-4-(3-furyl-3-hydroxypropyl)styrene], which was prepared by radical copolymerization of the acetyl-protected furan monomer with BOCST followed by base hydrolysis. The furan methanol residue, highly reactive toward electrophiles due to a mesomeric electron release from oxygen that facilitates the attack on the ring carbons, readily yields a stable carbocation upon acid treatment. Thus, the pendant furfuryl groups serve as both the latent electrophile and the nucleophile. Model reactions indicated that the furfuryl carbocation reacts more preferentially with the furan nucleus than the phenolic functionality. 

Acetic acid is known to undergo a vapor-phase ketonization reaction with formation of acetone on Brpnsted acids in general, and on proton-zeolites in particular. On large-pore zeolites in their proton form, the ketonization reaction is followed by acid-catalysed self-condensation amounting to mesitylene, mesityl oxide and phorone as main products [1], the chemistry being essentially identical to that in mineral acids. In H-pentasil zeolites with suitable acid site density, phorone isomerises to isophorone, which is cracked to yield 2,4-xylenol [1]. With propionic acid a similar chemistry occurs, but the formation of phenolics is severely suppressed by transition-state shape-selectivity effects... 

The reaction of phenol and formaldehyde in alkaline conditions results in the formation of o- and p-methylol phenols. These are more reactive towards formaldehyde than the original phenol and undergo rapid substitution with the formation of di- and trimethylol derivatives. The methylol phenols obtained are relatively stable in an alkaline medium but can undergo self-condensation to form dinuclear and polynuclear phenols (of low molecular weight) in which the phenolic nuclei are bridged by methylene groups. Thus in the base-catalyzed condensation of phenol and formaldehyde, there is a tendency for polynuclear phenols, as well as mono-, di-, and trimethylol phenols to be formed. 

Thermal properties can also be influenced by a furan ring in the para position of a phenolic ring. A phenolic novolac was prepared by reaction of phenol with methanal using oxalic acid as catalyst, and the furfuryl alcohol was added in a proportion of 0.5 mole furfuryl alcohol per mole phenol. The reaction steps in Scheme 11 show that the condensation of furfuryl alcohol at the para position of the phenolic rings is accompanied by the self-condensation of furfuryl alcohol. 

Amino and phenolic resin adhesives for plywood, fiberglass insulation, laminates, friction products, and abrasive products are reactive oligomers that cure by self-condensation... 

Various synthetic approaches have been demonstrated for the synthesis of PAEs since early days [35 0], PAEs were synthesized by Ullmann condensation between bisphenols and aryl fcis-halide monomers using Cu(I) salt/pyridine as catalyst [36], General Electric developed the first commercially successful PAE poly(2,6-dimethyl phenylene oxide) (PPO) [38], It was prepared by oxidative coupling of 2,6-dimethyI phenol. However, this process has its own restrictions, because it does not allow much structural variation or inclusion of any electron-withdrawing group into the polymer main chain. First attempts to synthesize polysulfones (PSF) were successfully done by Friedel-Crafts sulfonylation reaction of arylenedisulfonyl chlorides, for example, diphenyl ether-4,4 -disulfonyl chloride with diaryl ethers, for example, diphenyl ether, or by self-condensation of 4-phenoxy benzene sulfonyl chloride in the presence of FeCls [41], Whereas the former reaction involves side reactions (sulfonylation not only in the para- but also in the ort/io-position), the latter produces only the desired linear all-para products. 

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