Effect of solvent type on the formation rate of benzyl cation intermediate in acidolysis of lignin

The purpose of this study was to examine how the type of solvent among aqueous 1,4-dioxane, tetrahydrofuran, ethanol, iso-propyl alcohol, or ethylene glycol and its content (mol%) affect the formation rate of benzyl cation intermediate (BC) in the acidolysis of lignin, using a simple model compound, 1,2-dimethoxy-4-methoxymethylbenzene. Because the BC forms from the model compound via two steps, i.e., protonation of the benzyl methoxymethyl group as the pre-equilibrium step and liberation of the methanol as the rate-determining step, the observed variation of the formation rate with type of solvent and solvent content originates from the effects on both steps undistinguishably. The formation rate of BC decreased with increasing mol% of any of the organic solvents for a range of relatively low mol%, but increased with it for relatively high mol%. The formation rate varied more in the ether than in the alcohol systems. These results seem to be regulated by the effect of changing the mol% on the pre-equilibrium step, i.e., on the proton activity, rather than on the rate-determining step. Two reaction products, 4-alkoxymethyl-1,2-dimethoxybenzene and 4-hydroxymethyl-1,2-dimethoxybenzene, exclusively formed in the aqueous alcohol systems. The former compound was confirmed to be thermodynamically more stable and kinetically the more favorable product.

Revisiting the mechanism of β-O-4 bond cleavage during acidolysis of lignin VII: acidolyses of non-phenolic C6-C2-type model compounds using HBr, HCl and H2SO4, and a proposal on the characteristic action of Br− and Cl−

A non-phenolic C6-C2-type lignin model compound with the β-O-4 bond, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)ethanol (I), was acidolyzed in aqueous 82% 1,4-dioxane containing HBr, HCl, or H2SO4 with a concentration of 0.2 mol/L at 85 ℃ to examine the differences between these acidolyses. Compound I primarily converted to an enol ether compound, 1-(2-methoxyphenoxy)-2-(3,4-dimethoxyphenyl)ethene (II), via the benzyl cation followed by acidolytic β-O-4 bond cleavage regardless of the acid-type, although the disappearance rates of compound I were remarkably different (HBr > HCl >> H2SO4). Acidolyses of compound II using these acids under the same conditions showed a similar tendency, but the rate differences were much smaller than in the acidolyses of compound I. Acidolyses of the α-methyl-etherified derivative of compound I (I-α-OMe) using these acids under the same conditions suggested that the formation rates of the benzyl cation from compound I-α-OMe (also from compound I) are not largely different between the acidolyses using these acids, but those of compound II from the benzyl cation are remarkably different. Acidolysis of the α-bromo-substituting derivative of compound I (I-α-Br) using HBr under the same conditions showed a characteristic action of Br¯ in the acidolysis. Br¯ adds to the benzyl cation generated from compound I or I-α-OMe to afford unstable compound I-α-Br, resulting in acceleration of the formation of compound II and of the whole acidolysis reaction.

Mechanistic insights into the Ag(I)-catalyzed aryl migration

Theoretical investigation has been conducted into the mechanism for the Ag(I)/PhI(OTFA)2-catalyzed 1,4-radical aryl migration from carbon center to nitrogen center in sulfonamides by using density functional theory (DFT). The calculations show that 11 elementary reactions compose the whole catalytic reaction, which includes the deprotonation, the oxidation of Ag(I) intermediate to Ag(II) intermediate, the formation of nitrogen radical, the 1,4-radical aryl migration, the oxidation of benzyl radical to benzyl cation, etc. The difference between sulfonamides and other amides in the deprotonation process accounts for the experimental phenomenon that only the sulfonamides perform the deprotonation and subsequent 1,4-radical aryl migrations. Moreover, migrations of this kind proceed via a five-membered spirocyclic radical intermediate and favor the electron-rich phenyl group.