Mechanisms of Reactions of a Lithium Boryl with Organohalides

Reactions of the lithium boryl LiB(R'NCH=CHNR') (R' = 2,6-(Pr2C6H3)-Pr-i), 1, with organohalides (RX) giving RB(R'NCH=CHNR') and/or XB(R'NCH=CHNR') were studied computationally using density functional theory calculations at the B3LYP level. Our calculations indicate that the boryl anion in the lithium boryl can undergo nucleophilic attack at an organohalide on the halide-bonded carbon atom and/or the halogen atom to form RB(MeNCH=CHNMe) (an expected S(N)2 substitution product) and/or XB(MeNCH=CHNMe) (a halogen-abstraction product), respectively. Our energetic calculation results show that an organohalide having a halogen with lower electronegativity and higher ability to engage in hypervalent bonding promotes the halogen-abstraction pathway. Benzyl halides were also found to promote the halogen-abstraction pathway due to conjugation effects that stabilize a benzyl anion in the transition state during the halogen-abstraction process.