Reasons Two Nonstrained C-C σ-Bonds Can Be Easily Cleaved in Decyanative [4+2] Cycloaddition Catalyzed by Nickel(0)/Lewis Acid Systems. Theoretical Insight

Two nonstrained C-C sigma-bonds are cleaved in a novel nickel(0)/LA-catalyzed decyanative [4 + 2] cycloaddition of o-arylcarboxybenzonitrile with alkyne, where LA represents a Lewis acid such as methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide). The catalytic cycle of this reaction is systematically investigated here by DFT method to clarify the reasons two nonstrained C-C sigma-bonds are successfully cleaved in this reaction. DFT calculations indicate that this reaction occurs via the oxidative addition of the C-CN sigma-bond of o-arylcarboxybenzonitrile to the Ni(0) center, alkyne insertion into the Ni(II) aryl bond, C-C coupling between the vinyl carbon and the carboxyl carbon atoms, and beta-aryl elimination followed by reductive elimination. One LA interacts with the cyano nitrogen atom of o-arylcarboxybenzonitrile to accelerate the oxidative addition by stabilizing the unoccupied sigma* + pi* C-CN antibonding orbital. One more LA interacts with the carbonyl oxygen of o-arylcarboxybenzonitrile. This LA enhances the electrophilic nature of the carbonyl carbon to accelerate the C C coupling, because this step occurs through the nudeophilic attack of the vinyl carbon at the carbonyl carbon atom. The second C-C sigma-bond activation occurs via beta-aryl elimination, the transition state of which is stabilized by the interaction between LA and the carbonyl oxygen atom. These results lead to the dear conclusion that the presence of two LA molecules is crucial to achieve the dual C-C sigma-bond cleavages. The reasons LA accelerates the oxidative addition of the C-CN sigma-bond to the nickel(0) center and the C-C coupling followed by the beta-aryl elimination are discussed in detail.