Acceptorless Catalytic Alcohol Dehydrogenation and Depolymerization Applied to Lignin
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A new catalytic method for the selective depolymerization of lignin via acceptorless dehydrogenation has been developed. Initially, using a Cp*-iridium bipyridonate catalyst under basic conditions, primary alcohol dehydrogenation and retro-aldol cleavage of the Cα-Cβ bond in the β-O-4 linkage was successfully performed in lignin model compounds. The reaction conditions were optimized in additional model studies, achieving high yields and selectivities. It was then demonstrated that, under the optimized conditions, softwood enzyme lignin could be depolymerized successfully, as was confirmed by GPC, GC-FID and NMR analyses. The product mixture included a large amount of nonphenolic, monoaromatic compounds. By refluxing the product mixture after reaction the formation of carboxylic acids was promoted significantly. Hardwood dioxasolv poplar lignin was depolymerized, however it was found to be more inert to the catalyst system as compared to the softwood.