Nucleus Exchange Reaction

Springer Senes in Wood Science Methods in Lignin Chemistry (Edited by SY Lin and C W Dence) 

In modified softwood lignins, NE quantitatively converts not only noncondensed guaiacyl nuclei (type Ga) having side chain structures a, b, c, d, and e (Fig. 6.6.4) into guaiacol and catechol, but also guaiacyl nuclei Ga and Gb in DPM moieties having these side chain structures into the same products. To 

In the NE reaction, lignin is degraded by boron trifluoride in the presence of excess phenol. The key step in this process is the dealkylation of DPM-type structural units formed by phenolation the Ca side chain position of the phenylpropane units. To bring this reaction sequence into clear focus, it is appropriate to discuss the alkylation and dealkylation of simple aromatic compounds. 

Under acidic conditions, the alkylation and dealkylation of aromatic compounds are reversible reactions involving several steps in which n- and CT-complexes are formed. However, dealkylation proceeds only under more drastic conditions compared with alkylation. Nevertheless, this is not always the case. For example, if the aromatic compound is of the DPM type, the dealkylation may proceed under mild conditions since the cations formed (Fig. 6.6.5) are resonance-stabilized. This statement is supported by the fact that DPM derivatives may be degraded even at room temperature by aluminum chloride to yield benzene, alkylbenzene, and alkyldiphenylmethane, together with some resinous substances (Tsuge and Tashiro 1962, 1965). 

Based on the mechanism shown in Fig. 6.6.5, it is also clear that the cations formed after dealkylation may recombine with released phenyl nuclei as well as 

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Table 6.6.1. Rate constants and activation energies for the demethylation of the methoxyl group in the nucleus exchange reaction. (Funaoka and Abe 1982e)...

M Funaoka, I Abe, VL Chiang. Nucleus exchange reaction. In SY Lin, CW Dence, eds. Methods in Lignin Chemistry. Berlin Heidelberg Springer-Verlag, 1992, pp. 385-391. 

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