Aryl ethers, hydrolysis

Table 9 shows the relative hydrolysis rate of lignin models reported by Johansson and Miksche [302]. Both a- and P-aryl ether hydrolysis were enhanced by the presence of a phenolic hydroxyl groups. It is evident that a-aryl ether is much more reactive than the P-ether by a factor of 25 and 65... 

Amides can also be alkylated with diazo compounds, as in 10-49. Salts of sulfonamides (ArS02NH ) can be used to attack alkyl halides to prepare N-alkyl sulfonamides (ArS02NHR) that can be further alkylated to ArS02NRR. Hydrolysis of the latter is a good method for the preparation of secondary amines. Secondary amines can also be made by crown ether assisted alkylation of F3CCONHR (R = alkyl or aryl) and hydrolysis of the resulting F3CCONRR. ... 

The quinone methide can also be generated in situ, at least in aqueous NaOH, directly from the peracetate, as hydrolysis of the phenolic acetate is faster than the benzylic acetate (see an example in Section 12.5.3). This method was used to demonstrate the addition of anthrahydroquinone (AHQ) and anthranol to (actual polymeric) lignin quinone methides in studies elucidating the anthraquinone (AQ)-catalyzed 8-0-4-aryl ether cleavage mechanisms in alkaline pulping.64-66... 

Prior to imide formation, the imide-aryl ether ketimine copolymers were converted to the imide-aryl ether ketone analogue by hydrolysis of the ketimine moiety with para-toluene sulfonic acid hydrate (PTS) according to a literature procedure [51,52,57-59]. The copolymers were dissolved in NMP and heated to 50 °C and subjected to excess PTS for 8 h. The reaction mixtures were isolated in excess water and then rinsed with methanol and dried in a vacuum oven to afford the amic ester-aryl ether ether ketone copolymer, 2e (Scheme 8.)... 

This paper has provided the reader with an introduction to a class of polymers that show great potential as reverse osmosis membrane materials — poly(aryl ethers). Resistance to degradation and hydrolysis as well as resistance to stress Induced creep make membranes of these polymers particularly attractive. It has been demonstrated that through sulfonation the hydrophilic/hydrophobic, flux/separation, and structural stability characteristics of these membranes can be altered to suit the specific application. It has been Illustrated that the nature of the counter-ion of the sulfonation plays a role in determining performance characteristics. In the preliminary studies reported here, one particular poly(aryl ether) has been studied — the sulfonated derivative of Blsphenol A - polysulfone. This polymer was selected to serve as a model for the development of experimental techniques as well as to permit the investigation of variables... 

Duloxetine hydrochloride is an example of an aryl ether that is particularly unstable to hydrolysis under acidic conditions (84). The acid instability led to the development of an enteric-coated formulation to protect the compound from the acidic environment of the stomach. The reason for the susceptibility to hydrolysis is the stability of the cationic intermediate (Fig. 70), which is stabilized by delocalization into the aromatic thiophene ring. See Chapter 2 for additional discussion of the chemistry of this compound. 

Ruveda and co-workers have shown that the /5-hydroxyleucine, of which the aryl ether function in frangulanine is constructed, is present in the erythro-L-form (21). Dihydrofrangulanine was reduced with lithium in methylamine to an enol ether of /3-hydroxyleucine which on hydrolysis generated the free amino acid. It was shown that the hydroxyleucine in the hydrolysate is degraded by snake venom l-aminooxidase but not by pig kidney D-aminooxidase. Inasmuch as reo-hydroxyamino acids are attacked by neither enzyme it follows that the /3-hydroxyleucine, of hydrolytic origin, is the L-erythro form. 

Hymenocardine (58), (32, 40) has a p-hydroxy-w-aminoacetophenone unit (81), instead of the usual styrylamine, in its cyclic system which can be recognized in addition to N,N-dimethylisoleucylvaline and tryptophan in its acid hydrolysate. Mild alkaline hydrolysis results in ring opening via /3 elimination on the hydroxyamino acid and severance of the phenolate to a tetrapeptide 82 whose structure was determined by mass spectrometry and further hydrolysis. It is the only peptide alkaloid in which /3-hydroxy valine is involved in the aryl ether bridge. 

Biphenyl structures and a-carbonyl-p-aryl ether structures, which are both assumed to be present in native lignin with a higher abundance than coniferyl alcohol structures (22,23) and both considered to be important leucochromophores, were not observed among die products, presumably because they are not present in spruce lignin as end groups. Both these types of structures are very stable and unlikely to be structurally changed during mild acid hydrolysis (24). 

Another factor which may influence the yield of acids obtained from lignin in wood is the presence of easily hydrolyzable aryl ether linkages such as those represented by phenylpropane-a-aryl ether structures In phenolic units, such structures are easily cleaved even under very mild alkaline conditions (Adler et al 1968) Therefore, it can be assumed that, in the alkylation step of the oxidative degradation procedure, a partial hydrolysis of such structures may occur, thereby creating new phenolic end groups that may affect the analysis... 

Balousek PJ, McDonough TJ, McKelvey RD, Johnson DC (1981) The effects of ozone upon a lignin model containing the /1-aryl ether linkage Sven Papperstidn 84 R49-R54 Bes RS, Gas G, Molimer J, Vidal P, Mathieu J, Mora JC (1989) Enhancement of poplar cellulose susceptibility to cellulase enzyme hydrolysis by ozonation Ozone Sci Eng 11 217-226... 

P-Aryl ether. Solvents were shown to significantly affect the hydrolysis rate of p-aryl ether dimers (29), being higher in aqueous dioxane than in water or aqueous ethanol media [306]. Both dioxane and ethanol favor the C6C2 enol ether (55) formation, especially at high temperatures [307]. 

Mild hydrolysis. Nimz 318,319] subjected wood to water percolation at 100 C for several weeks. Approximately 20% and 40% of the lignin from spruce and beech wood, respectively, became soluble. Sakakibara [320] obtained similar results using a 50% aqueous dioxane at 180°C. These soluble products were assumed to come mainly from the cleavages of a-aryl ether units. 

Treatments of spruce MWL in methanol-dioxane solution containing p-toluenesulfonic acid (at 30°C) resulted in hydrolysis of the a-aryl ether units and methylation of the benzylic hydroxyl groups [360,361]. A complete... 

Acyl cations are involved as propagating species in the synthesis of poly-(ether ketone)s. Poly (ether ketone)s are a class of thermoplastic crystalline polymers that have many desirable properties that make them useful as high-performance engineering materials [153,154]. The poly(ether ke-tone)s with the most useful properties are actually para-linked poly(aryl-ether ketone)s (PAEKs). They have excellent chemical resistance to oxidation and hydrolysis, high thermal stability, and many useful mechanical properties. Unlike some other materials with similar properties they are readily melt processable using conventional equipment. In addition, their mechanical properties are not affected deleteriously by most solvents. These polymers are usually crystalline. PAEKs contain arene groups joined by ether and carbonyl linkages. For example, two commercial poly-(ether ketone)s are PEK and PEEK (Fig. 36). 

There are several methods for the direct introduction of an aldehyde group into an aromatic compound. In the Vilsmeier-Haack reaction, activated aromatic systems such as aryl ethers and dialkylanilines are formylated by a mixture of dimethylformamide, HCONMe2, and phosphorus oxychloride, POCL, (Scheme 6.4). The process involves elec-trophilic attack by a chloroiminium ion, Me2N=CHCl, formed by interaction of dimethylformamide and phosphorus oxychloride. Hydrolysis of the dimethyl imine completes the synthesis. 

---