From models to lignin: Transition metal catalysis for selective bond cleavage reactions

In the coming decades major changes are expected in the chemical industry regarding the utilized raw material inputs. Depleting fossil resources will gradually be replaced by renewable feedstocks wherever possible. Because of this transition, new and efficient methodologies are required that enable depolymerization and defunctionalization of these complex, highly oxygenated biopolymers. Additionally, utilization of all components of lignocellulose is of great importance. In particular, depolymerization of lignin into its aromatic subunits or defined aromatic platform chemicals has proven challenging. Various approaches to overcome these difficulties have been attempted and resulted in new and exciting developments in many fields. In this review we will give an overview of bond cleavage strategies relevant for lignin depolymerization using homogeneous catalysts, focusing especially on reductive and hydrogen transfer methods.