Mechanistic Investigation into the Regio-Controllable Hydroallylations of Alkynes with Allylborons under Pd-Based Synergetic Catalyses

Density
functional theory calculations were employed
to investigate
the Pd-catalyzed regio-selective hydroallylations of alkynes with
allylborons: cooperation of Cu­(OAc)2 and dppe resulting
in 1,4-dienes while combination of AdCO2H and PCy3 leading to 1,5-dienes. A unified rationalization mechanism called
“Lewis-acid–base-interaction promoted deprotonation/3,3-rearrangement”
was proposed. Compared with the commonly reported metathesis pathway
to only afford the metal-allyl intermediate, in the newly established
mechanism, an additional Brønsted acid (as an initiator of the
Pd0 oxidative addition) is generated by the interaction
of the allylboron (Lewis acid) B atom with the nBuOH (Lewis base) O atom, and subsequent 3,3-rearrangement
ensures the thermodynamic feasibility of the reaction. In addition,
it was found that excess Cu­(OAc)2 plays two potential roles
in the oxidative addition/alkyne insertion: (i) the participation
of one AcO– of Cu­(OAc)2 ensures a large
orbital overlap between the migrating H and Pd atoms, facilitating
the formal AcO–H cleavage and (ii) the extra (OAc)2Cu···O­(carboxyl) σ-coordination indirectly contributes
to the (Me)­CC­(Ph) insertion into the Pd–H bond. Further
analysis showed that the origin of the regioselectivity is closely
related to the employed phosphorus ligand. These revealed results,
which have been overlooked in the previous documents, would aid the
development of new related catalytic reactions.