Aryl ethers, cleavage

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... 

Figure 3.15 Mechanism of aryl ether cleavage during alkaline pulping.

Figure 10. Enhancement of all l aryl ether cleavage resulting from introduction of carbonyl group into para position of aromatic ring. Unimolecular rate coefficients for 4.8 X 10 M reactants in aqueous 30% ethyleneglycol monomethyl ether containing 0.53 - 0.54 M hydroxide and 0.08 - 0.09 M bisulfide at 170 C (63).

Standard peptide coupling chemistry is performed (Box 25). The acid component is transformed to an activated ester derivative and then treated with the amine [17-19]. This procedure is repeated twice. In the final step the ligands la-le-H4 are deprotected by aryl ether cleavage with BBr3. By use of this simple reaction sequence derivatives with glycine, alanine, phenylalanine, valine, leucine, and other amino acids as spacers were obtained [16]. 

From an inspection of the values in Table 7.2.2, it is clear that the phenolic content of lignin decreases in the order kraft lignin kraft pulp > wood, a trend which underscores the significance of aryl ether cleavage in delignification and of the effect of the phenolic hydroxyl groups on lignin solubility. 

On the other hand, alkaline cleavage of the etherified P-aryl ether unit (47) proceeds by an intramolecular displacement mechanism (D). Under kraft cooking conditions, the p-aryl ether cleavage of phenolic type could be 12-50 times faster than that of the etherified type depending on the hydroxide and sulfide ion concentrations [331]. 

F Lu, J Ralph. DFRC method for lignin analysis. 1 New method for (3-aryl ether cleavage Lignin model studies. J AgrFood Chem 45 4655 660, 1997. 

How do reduction reactions that lead to a-CH2-groups affect pulping efficiencies Several typical (P-aryl ether) cleavage pathways would be blocked, which would have a negative effect on efficient delignification. However, because quinone methide formation would be prohibited, undesirable vinyl ether formation and condensation reactions would also be blocked. Bulk-phase fragmentation reactions, involving a... 

One of the difficulties in establishing the detailed chemistry of initial phase pulping is the dominance of the delignification rate by the first step in the process— formation of a quinone methide intermediate. The rates of subsequent steps are difficult to determine however, the rates of these steps are critical to the partitioning of the QMs toward fragmentation processes (p-aryl ether cleavage) as opposed to undesirable competing reactions, such as vinyl ether formation and condensation reactions [57]. 

S Ljunggren. The Significance of Aryl Ether Cleavage in Kraft Delignification of Softwood. Svensk Papperstidn 83(13) 363-369, 1980. 

R Kondo, Y Tsutsumi, H Imamura. Kinetics of (3-aryl Ether Cleavage of Phenolic Syringyl Type Lignin Model Compounds in Soda and Kraft Systems. Holzforschung 41 83-88, 1987. 

Multiple reactions. The reagent is prepared from chromium(III) chloride and 4 equiv of BuLi in THF. It can be isolated as a dark red-brown LiCrH2-THF complex, free from LiCl. As shown in the following diagram, it can perform hydrodehalogenation, aryl ether cleavage, coupling reactions, and polymerization. 

A unique heterogeneous Pd/C/Zn catalyst also induces p-aryl ether cleavage in a number of lignin models. Treatment of 9 with 5 wt% catalyst and 2 MPa H2 in MeOH at 150°C produces propylguaiacol 10 and guaiacol 11 in 85% yield. A small amount of alcohol 12 is also formed as a byproduct. Hydrogenolysis of a polymeric lignin model under these conditions afforded 10 and 11 as major products in 56% and 44% yield, respectively (Fig. 6) [71]. 

Fig. 7 Oxidative p-aryl ether cleavage with V(dipic) catalysts...

Fig. 11 Reactivity control as a function of catalyst in vanadium catalyzed p-aryl ether cleavage...

Chmely SC, Kim S, Ciesielski PN, Jimenez-Oses G, Baton RS, Beckham GT (2013) Mechanistic study of a Ru-xantphos catalyst for tandem alcohol dehydrogenation and reductive aryl ether cleavage. ACS Catal 3 963... 

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