A mechanistic insights into manganese-catalyzed oxidative homocoupling reactions of Grignard reagents: A computational DFT investigation

• The oxidative addition gives an oxo-complex MnR2O2 where the O2 is η2-bonded to the metal.
• The kinetics of the reductive elimination is the result of electronic and steric effects.
• The “right” electron density on the coupling carbons is required for aryl Grignard.

We have carried out a DFT computational investigation on the mechanism of the manganese-catalyzed homocoupling reaction of aryl Grignard reagents RMgX using atmospheric oxygen as an oxidant. The oxidative addition gives an oxo-complex MnR2O2 where the oxygen molecule is η2-bonded to the metal. The free energy barrier for the subsequent reductive elimination (key-step of the cycle) is 9.4 kcal mol−1 when R = p-anisyl, which indicates a rather fast reduction step. A comparative analysis of various systems (R = p-anisyl, o- and p-nitrophenyl, pentafluorophenyl, mesityl, naphtyl and phenylethynyl) suggests that, in general, the kinetics of the reductive elimination step is the result of a complex interplay between electronic and steric effects and, in the case of aryl groups, strongly depends on the nature of the substituent and its position on the phenyl ring. The reduction is favored when the electron density on the coupling carbons is large enough to interact with the metal and form the intermediate oxo-complex. However it cannot be so large to prevent the coupling because of a too strong electron repulsion or too strong Mn-R bonds.

The “right” electron density on the coupling carbons is required to favor the Mn-catalyzed homocoupling reaction of aryl Grignard reagents RMgX. The electron density induced by an appropriate substituent, must be large enough to form the active catalytic intermediate, but not so large to give a too strong electron repulsion and prevent the coupling.Figure optionsDownload as PowerPoint slide