Following the Molecular Mechanism of Decarbonylation of Unsaturated Cyclic Ketones Using Bonding Evolution Theory Coupled with NCI Analysis.

The synergetic use of bonding evolution theory (BET) and noncovalent interaction (NCI) analysis allows to obtain new insight into the bond breaking/forming processes and electron redistribution along the reaction path to understand the molecular mechanism of a reaction and recognize regions of strong and weak electron pairing. This viewpoint has been considered for cheletropic extrusion of CO from unsaturated cyclic ketones cyclohepta-3,5-dien-1-one CHD, cyclopent-3-en-1-one CPE, and bicyclo[2.2.1]hept-2-en-7-one BCH by using hybrid functional MPWB1K in conjugation with aug-cc-pVTZ basis set. Decarbonylation of CHD, CPE, and BCH are nonpolar cyclo-elimination reactions that are characterized by the sequence of turning points (TPs) as CHD, 1-11-C[CC]C†C†FFFTSC†C†C†-0:HT + CO; CPE, 1-8-CC[C†C†F†][FF][FF]FTS[C†C†]-0:BD + CO; and BCH, 1-8-CC[C†C†]F[FF]FTS[C†C†]-0:CD + CO. Breaking of C-C bond between the terminal carbon atoms of diene/triene framework and carbon atom of CO fragment starts at a distance of ca. 1.9-2.0 Å in the vicinity of the transition structure where the transition states are not reached yet. NCI analysis explains that the noncovalent interactions between two fragments appeared after the breaking of C-C bonds.