Mechanistic Insights into Ni-Catalyzed Difunctionalization of Alkenes Using Organoboronic Acids and Organic Halides: Understanding Remarkable Substrate-Dependent Regioselectivity

Transition
metal catalyzed difunctionalization reactions of alkenes
using simple chemical feedstocks are powerful strategies for the synthesis
of valuable compounds and materials. Density functional theory (DFT)
calculations reported in the present paper reveal detailed mechanistic
insight and the origin of the substrate-dependent regioselectivity
in the titled reactions. Computational results demonstrate that these
reactions are generally composed of several steps including oxidative
addition, carbonickelation, H-shift/transmetalation, and reductive
elimination. Among these steps, the key one responsible for the regioselectivity
depends upon the organic halides utilized. Natural bond orbital (NBO)
analysis, energy decomposition analysis (EDA), and buried volume calculations
indicate that steric effect is a common contributor of the regioselectivity,
while other energy terms, such as electrostatic interaction, have
significant and even dominant effects on the specific regioselectivity.
Furthermore, the key reason for successfully suppressing Heck and/or
Suzuki products lies in that the formation of Heck and/or Suzuki products
is thermodynamically less favorable. Comparison of the total reaction
barriers of the rate-limiting step (combined transmetalation and reductive
elimination processes) demonstrates that the electron-induced effect
of various organic halides significantly causes the different bonding
ability between Ni atom and allyl moiety. As a result, transmetalation
and reductive elimination processes made distinct contributions to
reducing the total activation free energy of the rate-limiting step
of various difunctionalized reactions.