Catalytic C−O Bond Cleavage of 2-Aryloxy-1-arylethanols and Its Application to the Depolymerization of Lignin-Related Polymers

Abstract

A ruthenium-catalyzed, redox neutral C-O bond cleavage of 2-aryloxy-1-arylethanols was developed that yields cleavage products in 62-98% isolated yield. This reaction is applicable to breaking the key ethereal bond found in lignin-related polymers. The bond transformation proceeds by a tandem dehydrogenation/ reductive ether cleavage. Initial mechanistic investigations indicate that the ether cleavage is most likely an organometallic C-O activation. A catalytic depolymerization of a lignin-related polymer quantitatively yields the corresponding monomer with no added reagent.

Oxidation of Alcohols and Activated Alkanes with Lewis Acid-Activated TEMPO

“…For both reactions, compound 4 is the only organic product observable in the reaction mixture by 1 H NMR spectroscopy. 25 Similarly, oxidation of 1-phenyl-2-phenoxyethanol (5), which has been previously employed as a -O-4 lignin model compound, [21][22][23] with complex 1 results in complete consumption of the alcohol within 3 h, and formation of 2-phenoxyacetophenone (6) in 65% yield (Table 1, Entry 2).…”

“…Herein, we report the reactivity of 1 and 2 towards a variety of alcohols and activated alkanes, including xanthene. 10,[16][17][18][19][20][21][22][23][24] The latter substrate is significant, because its reactivity confirms that these oxidations can proceed via a concerted proton coupled electron transfer (CPET) step, as was previously surmised. 14 To test the role of the Lewis acid in activating the TEMPO moiety, we also explored the reactivity of TEMPO with FeBr 3 in Et 2 O.…”

Hydrogenation and cleavage of the C-O bonds in the lignin model compound phenethyl phenyl ether over a nickel-based catalyst

“…Hartwig's group [6] recently reported the use of a nickel carbene complex as an effective catalyst for the hydrogenolysis of the aryl ether bonds in several lignin model compounds in the presence of the organic base NaO t Bu, achieving 99% conversion. Bergman's group [7] reported the use of a ruthenium xantphos catalyst for the cleavage of the C-O bond of a lignin model compound using transfer hydrogenation with a 62%-98% conversion.…”

Electronic and bite angle effects in catalytic C–O bond cleavage of a lignin model compound using ruthenium Xantphos complexes

“…Toluene and TMEDA were distilled from sodium, THF and diethyl ether were distilled from sodium/benzophenone, hexanes from sodium/benzophenone/triglyme and dichloromethane from CaH 2 . 2-Phenoxy-1-phenylethanol, 1 6,7-bis(diphenyl- proceedures. [4][5][6][7][8] All reagents were purchased from commercial suppliers and used as received, unless otherwise noted.…”