Theoretical Study of Ruthenium(0)-Catalyzed Transfer Hydrogenative Cycloaddition of Cyclohexadiene and Norbornadiene with 1,2-Diols to Form Bridged Carbocycles

The
recent success of Krische et al. (Angew. Chem., Int. Ed. 2017, 56, 14667–14671) in achieving a ruthenium(0)-catalyzed transfer hydrogenative
cycloaddition of 1,2-diols with cyclohexadiene and norbornadiene in
excellent yield with exo- and diastereoselectivity represents an exciting
development in the synthesis of bridged carbocycles. In the present
work, the possible catalytic mechanisms and origin of the exo- and
diastereoselectivity for cyclohexadiene and norbornadiene were studied
in detail by density functional theory calculations. The theoretical
results indicate that the exoselective pathway for the cyclohexadiene
substrate proceeds by a novel two-step successive C–C coupling,
while the endoselective pathway undergoes the C–C coupling
reaction in a conventional concerted manner. The origin of the preferential
chemoselectivity of dione–cyclohexadiene C–C coupling
over aromatization to benzene was investigated. Aromatization to benzene
is unfavorable because of the large distortion energy of the three-membered
ring in the transition state of hydrogen migration. From distortion/interaction
analysis, for norbornadiene, the distortion energy plays the main
role in determining the exoselectivity.