P-Aryl ether cleavage

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]. 

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]. 

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...

Masai E, A Ichimura, Y Sato, K Miyauchi, Y Katayama, M Fukuda (2003) Roles of enantioselective glutathione 5-transferases in cleavage of p-aryl ether. J Bacteriol 185 1768-1775. 

The alkaline cleavage of a- and P-aryl ether linkages as well as condensation reactions have been extensively studied using lignin model compounds [22,24,27,32,277,326-343], but it is still not entirely clarified in the degradation of wood lignin. 

Etherified units. Consistent with the generally accepted mechanism (D, Fig. 14), the erythro dimer of etherified P-aryl ether was about four times more reactive than the threo isomer [350]. The ether cleavage, besides being enhanced by increasing alkalinity, was facilitated in the presence of monoetha-nolamine [351] or in a DMSO-potassium-tertiary butoxide solution [330]. 

ID Suckling. Enhanced Cleavage of P-Aryl Ether Bonds in Lignin Model Compounds during Sulfite-Anthraquinone Pulping. J Wood Chem Technol 8 43, 1988. 

Fig. 4 Ni catalyzed cleavage of lignin model p-aryl ethers...

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