Kumada coupling template

Catalytic cycle of the Kumada–Tamao–Corriu coupling with a Ni(dppp) catalyst: oxidative addition of R¹–X to Ni(0), transmetalation from R²MgX with loss of MgX₂, and reductive elimination of R¹–R²; key step: four-centre transmetalation from the Grignard reagent.

What the template shows

• Ni- (or Pd-) catalysed cross-coupling of an organic halide with a Grignard reagent
• Overall: R¹–X + R²–MgX → R¹–R² + MgX₂ (cat. NiCl₂(dppp) or Pd(PPh₃)₄, Et₂O or THF, rt)
• L₂ = dppp (cis-chelating)
• The first catalytic cross-coupling (Tamao, Sumitani & Kumada; Corriu & Masse, both 1972): cheap Ni catalysts and Grignard reagents used directly.
• The precatalyst NiCl₂(dppp) is reduced to Ni(0) by two equivalents of R²MgX (releasing a little R²–R²).
• Grignard reagents are strong bases and nucleophiles, so esters, ketones, nitriles and OH/NH groups are not tolerated (Negishi and Suzuki couplings are milder).
• Ni couples aryl chlorides and vinyl halides well; the cis-chelating dppp ligand speeds reductive elimination.
• Key step: transmetalation from the Grignard reagent
• Drawn as a four-centre exchange (simplified): the C–Mg bond electrons form the new Ni–C bond, bromide leaves nickel and its lone pair binds magnesium, so MgBr₂ is released.
• L₂ = dppp (Ph₂P(CH₂)₃PPh₂), named in the captions, not drawn · every part of this figure is editable

Drawn with 19 atoms, 7 arrows, 67 shapes – every one editable in MolDraw.

Sources

• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005, Kumada cross-coupling Textbook
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 40 Textbook