Quantum Chemical Investigation of Dimerization in the Schlenk Equilibrium of Thiophene Grignard Reagents

The
Schlenk equilibrium of Grignard reagents describes the intricate
relationships between monomers, aggregates, and exchange products.
The core step of the Schlenk equilibrium, formally 2RMgX ⇌
R2Mg + MgX2, has been subject to computational studies of simple methyl
Grignards and NMR determination of thermodynamics. These studies neglect
the effect the R group may have on the accessibility of intermediates
in the Schlenk equilibrium. In this study, computational reaction
discovery tools were employed to thoroughly search the chemical space
for feasible dimerizations and pathways to ligand exchange for thiophene
Grignards. Three bridged dimers, μ-(Cl, C), μ-(Cl, Cl),
and μ-Cl, were found to be vital intermediates, which are stabilized
by π-interactions involving the thiophene group. These dimers
are approximately as thermodynamically stable as the Grignard monomers
and its ligand exchange products, and therefore, their reactivity
should be considered when examining mechanisms for aryl Grignard or
cross-coupling reactions.