Tandem Catalytic Depolymerization of Lignin by Water-Tolerant Lewis Acids and Rhodium Complexes

P.C.A.B. thanks the Netherlands Organization for Scientific Research (NWO) for a Vernieuwingsimpuls Veni Grant. The CatchBio program is acknowledged for financial support. N.J.W. thanks EPSRC for funding by the EP/J018139/1, EP/K00445X/1 grants and an EPSRC Doctoral Prize Fellowship to C.S.L. Saumya Dabral and Prof. Carsten Bolm (RWTH Aachen) are gratefully acknowledged for providing an authentic sample of 5b. Dr. Wouter Huijgen (ECN) is acknowledged for Klason lignin determination. SuBiCat Initial Training Network, Call FP7-PEOPLE-2013-ITN, #607044) is gratefully acknowledged. Lignin is an attractive renewable feedstock for aromatic bulk and fine chemicals production, provided that suitable depolymerization procedures are developed. Here, we describe a tandem catalysis strategy for ether linkage cleavage within lignin, involving ether hydrolysis by water-tolerant Lewis acids followed by aldehyde decarbonylation by a rhodium complex. In situ decarbonylation of the reactive aldehydes limits loss of monomers by recondensation, a major issue in acid-catalyzed lignin depolymerization. Rate of hydrolysis and decarbonylation were matched using lignin model compounds, allowing the method to be successfully applied to softwood, hardwood and herbaceous dioxasolv lignins as well as poplar sawdust to give the anticipated decarbonylation products, and, rather surprisingly, 4-(1-propenyl)phenols. Promisingly, product selectivity can be tuned by variation of Lewis acid strength and lignin source. Publisher PDF