Direct Lignin C – OAr , ArO – Ar or C – Ar Bonds Cleavage without First Activation of the Adjacent Chemical Bonds

">For lignin depolymerization, the direct cleavage of C β –OAr, C α –OAr or ArO–Ar ether bonds, and C–Ar bonds provide the simplest mode to tailor the β-O-4 linkage and other lignin linkages. In this connection, some stoichiometric reaction and their derived catalytic systems have been founded for the direct cleavage of lignin linkages, but these have focused primarily on the C–O bonds rather than the cleavage of the C–C bonds. In this chapter, we first review the stoichiometric reactions and catalytic systems for the direct cleavage of ether bonds with a strong nucleophile and electrophilic reagent. Then, as the modification and update of the above methods, Brönsted/Lewis acid + metal systems, base/organometallic systems, and other heterogeneous catalysts used in the direct hydrogenative cleavage of C–OAr ether bonds are systemically introduced. In addition, for the direct reductive cleavage of ether bond, this chapter also gives a brief review of the lignin conversion methods with hydride reagents, and e – species from chemical conversion or surface of the cathode or semiconductors.