Palladium‐Catalyzed C(sp ³ )−H Activation in A Monoterpene‐Based Compound Under Mild Conditions: A Combined Experimental and Theoretical Mechanistic Study

A rare example of the palladium-catalyzed sp³ C-H bond activation in a monoterpene-based compound has been observed in the reaction of PdCl₂ with a (+)-3-carene-based ligand HL (HL=N-((1aS,3S,7bR)-1,1,3-trimethyl-7-phenyl-5-(pyridin-2-yl)-1a,2,3,7b-tetrahydro-1H-cyclopropa[f]quinolin-3-yl)acetamide), which yielded the [PdLCl] complex. In contrast to the vast majority of C(sp³)-H activation reactions which require prolonged heating and mixing due to the inert character of the corresponding bond, the reaction reported herein proceeds rapidly under mild conditions. A theoretical insight into the ligand deprotonation has been performed by DFT calculations. The mechanism of the C-H activation involves (i) simultaneous coordination of the CH₃ group of HL to the Pd²⁺ ion and decoordination of the Cl^- anion with consequent formation of a Cl⋅⋅⋅H-N hydrogen bond with the amide group, (ii) approximation of the out-of-sphere Cl^- anion to one of the hydrogen atoms of the CH₃ group mediated by the crane motion of the amide group and (iii) the ejection of the HCl molecule, which increases the entropy of the system and serves as a driving force for the reaction.