Transition metal catalyzed carbon-carbon bond activation : from four-membered rings to less-strained systems

With the help of trasition metal catalysis, a number of previously considered inert moieties can be readily functionalized. We show special interest in carbon-carbon bond activation, where a relatively inert carbon-carbon bond is replaced by two reactive carbon-metal bonds, allowing for subsequent functionalization and rapid increase of the complexity of organic molecules. Obstacles for carbon-carbon bond activation arise from their inefficient orbital overlap with transition metals, their steric hindered environment and their statistically low abundancy. Small ring systems, such as cyclobutenones and benzocyclobutenones, were applied to circumvent these challenges and enable efficient carbon-carbon bond activation due to their inherent high ring strain. Rhodium catalytic systems were employed to activate the four-membered rings; synthetically valuable structure moieties such as β-naphthols, indenes and cyclopentenones were afforded under finely tuned conditions. The carbon-carbon bond activation strategy was also attempted in the lignin valorization field. As the depletion of fossile fuels become more rapid, lignin was considered as an alternative feedstock for fuels and chemicals. While the activation of the most abundant β-O-4 unit has been extensively explored, the less abundant but more robust 5-5’ unit, essentially an unstrained aryl-aryl carbon-carbon bond, was rarely visited. We were interested in using transiton metal catalysis to break the 5-5’ unit in an efficient and selective approach. A ruthenium catalytic system was employed to effectively cleave the unstrain carbon-carbon bond under a variety of reductive conditions, such as hydrogenation, secondary alcohol and silane