Benzyl aryl ethers

Quideau, S. Ralph, J. A biomimetic route to lignin model compounds via silver(I) oxide oxidation. 1. Synthesis of dilignols and non-cyclic benzyl aryl ethers. Holzforschung 1994, 48, 12-22. 

Naturally lignins that have been modified by chemical reactions, e.g. condensed or sulfonated by pulping, are far less susceptible to biological decay than natural lignins, because the weak, readily hydrolyzable benzyl aryl ether bonds or the readily oxidisable benzyl alcohols of guaiacyl-glycerol units have been sulfonated or transformed by condensation reactions into strong carbon-carbon bonds. 

The reduction of benzyl aryl ethers has been thoroughly investigated by voltammetric reduction, homogeneous redox catalysis,and currently, by convolution analysis. A family of ethers activated by proper substitution on the phenoxy side were chosen to provide a wide variation in the ET and bond cleavage properties of the molecule. ... 

Table 2 Electrochemical, kinetic, and thermodynamic parameters for ET to benzyl aryl ethers in DMF/0.2M TBAP at 25C. ...

Selective cleavage of aryI methyl ethers.1 The sodium salt (NaH) of N-melhylaniline in the presence of HMPT cleaves methyl (and benzyl) aryl ethers in... 

Both XLII and XLI 11 are p-hydroxybenzyl aryl ethers so is the polymerization product (XLIV) 21) of the quinonemethide (XXI) mentioned above. Such benzyl aryl ether bonds are not very rare in lignin and are easily attacked by acids or nucleophilic reagents containing sulfur. 

Phenylcoumarone (VIII) has a characteristic ultraviolet and ioniza-tion-Ae spectrum, which enabled us to detect dimeric structures of this type in reaction mixtures obtained when Bjorkman spruce lignin was subjected to acidolysis for 20 hours. From the spectrophotometric estimation of the amount of the phenylcoumarone systems formed, we concluded that from a total of 100 phenylpropane units of Bjorkman lignin, about 20 are involved in phenylcoumaran systems (I) in other words, about every 10th phenylpropane unit is linked to one of its neighbors by the cyclic benzyl aryl ether linkage characteristic of I. 

Recently, Freudenberg and co-workers (//) briefly reported the isolation of small amounts of dehydrodiconiferyl alcohol (XXVII) from the methanolysis of wood at room temperature. By this procedure, again, only benzyl aryl ether-linked terminal dimers can be obtained whereas acidolysis liberates (a- and) /3-ether-linked phenylcoumaran systems from the core of the lignin molecule as well. 

Dissolution of almost 40% of powdered beechwood in this way 20) suggests that the benzyl aryl ether content of beech lignin is much higher than that of spruce lignin. 

Castellan, A., Vanucci, C., Bous-Laurent, H., "Photochemical Degradation of Lignin Through a C-0 Bond Cleavage of Non-phenolic Benzyl Aryl Ether Units. A Study of the Photochemistry of a(2 ,4 -Trimethyl-Phenoxy)-3,4 Dimethoxy Toluene.", Hohforschmg, 1987, 41(4), 231. 

Benzyl aryl ethers are also cleaved when irradiated in the presence of e.g. tetracyanoethylene (Timpe and Weschke, 1980). 

Inflection possibly due to H in pinoresinol units and in noncyclic benzyl aryl ethers (threo forms). 

Acidolysis causes the selective cleavage of arylglycerol-)6-aryl ethers (4, Fig. 6.1.2) and some other types of labile ether linkages, and this has been utilized on several occasions, for example by Kirk and Adler (1970). Treatment with the acidolysis reagent at 50°C (Adler et al. 1968) has been used in lignin chemistry for the selective hydrolysis of noncyclic benzyl aryl ethers. 

Eisch, J. J., Kovacs, C. A., Rhee, S.-G. Rearrangements of organometallic compounds. X. Mechanism of 1,2-aryl migration in the Wittig rearrangement of a-metallated benzyl aryl ethers. J. Organomet. Chem. 1974, 65, 289-301. 

---