Mechanism of Counterion-Controlled Regioselective Hydrothiolation of 1,3-Dienes: Insights from a Density Functional Theory Study

Recently, it has been reported that
Rh-catalyzed counterion-controlled
hydrothiolation of unsymmetrical 1,3-dienes results in allylic or
homoallylic sulfides under highly regiocontrol. However, it remains
unclear about the reasons for this unusual selectivity profile. Therefore,
the present density functional theory (DFT) study was aimed at determining
the mechanism of hydrothiolation of unsymmetrical 1,3-dienes and the
origin of regioselectivity. The study results show that when SbF6– is used as the counterion, the hydrothiolation
product 3a of 1,2-Markovnikov is the primary product,
and oxidative addition is a rate-limiting step. However, 3,4-anti-Markovnikov hydrothiolation product 3b became the major product when a Cl– counterion
was employed in the system. The relevant data show that the one step
via an initially proposed oxidation addition pathway produces the
opposite regioisomer in comparison to the experimental results. Instead,
the one step via a distinctive kind of the Cl– counterion-assisted
deprotonation pathway accounts for the “3,4-anti-Markovnikov” regioselectivity observed in the experiments,
which has not yet been reported. The origin of the experimentally
observed regioselectivity can be attributed to counterion-controlled
electronic and steric effects. These results can serve as the basis
for designing better catalytic systems with transition-metal complexes.