Theoretical investigation on the mechanism of iron catalyzed cross coupling reactions via ferrate intermediates

• DFT calculations clarify the mechanism of an iron catalyzed cross coupling reaction.
• Methyl transfer proceeds from an organoferrate complex by direct substitution.
• In line with experiment, multiple methyl transfers from the complex are viable.

Iron is a versatile catalyst for cross coupling reactions. These reactions may proceed either via classical redox cycles involving low-valent iron species or via highly alkylated organoferrate complexes. Experimentally, it is difficult to trap reactive intermediates, but it has been possible to prepare iron complexes similar to the supposed active catalyst that are able to methylate activated electrophiles (J. Am. Chem. Soc. 130 (2008) 8773–8787). Motivated by these experiments we studied the methylation of 4-chlorobenzoyl chloride by the organoferrate complex [(Me)4 Fe(MeLi)][Li(OEt2)]2 employing density functional theory at the OPBE/6-311+G** level, as well as B3LYP/6-311+G** calculations with explicit inclusion of dispersion and solvent effects (describing iron with the QZVP basis and SDD pseudopotential). In the preferred mechanism, methyl transfer takes place via substitution at the organoferrate complex, with the leaving methyl group being replaced by chloride. In line with the experimental findings, up to four methyl groups can be transferred in this manner. By locating all conceivable transition states and intermediates, the calculations shed light on the relative ease of substitution at the various positions of the organoferrate complex, both in the first and subsequent methyl tranfers. Transition states for an alternative redox mechanism could not be located.

Density functional calculations show that the complex [(Me)4Fe(MeLi)][Li(OEt2)]2 can transfer methyl groups to 4-chlorobenzoyl chloride by a direct substitution mechanism. In these reactions, the ferrate complex remains intact and may thus support multiple transfers. This indicates that iron catalyzed cross coupling reactions involving methyl donors can proceed via organoferrate intermediates.Figure optionsDownload as PowerPoint slide