Hydrogen-Borrowing and Interrupted-Hydrogen-Borrowing Reactions of Ketones and Methanol Catalyzed by Iridium
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The use of catalytic [{Ir(cod)Cl}2] was shown to facilitate hydrogen-borrowing reactions of ketone enolates with methanol at 65°C. An oxygen atmosphere accelerated he process, and when combined to a bulky monodentatephosphine ligand, the catalytic cycle could be interrupted by preventing the enone reduction.
Subsequent addition of pronucleophiles to the reaction mixture allowed a one-pot methylenation/conjugate addition protocol to be developed (see scheme): the enone and methoxy adduct were not isolated but reacted in situ with an external nucleophile and extra base. In most cases we found to be beneficial to treat the crude reaction mixture with the Ir scavenger SiliaMetS DMT while stirring the solution, open to the atmosphere, for 1 hour, before the addition of base and nucleophile. The team suggested that the removal of most of the metal catalyst from solution and prevented complications caused by methanol oxidation during the second phase of the reaction.
Related publication to mention: Donohoe, J. et al. Angew. Chem. Int. Ed., 2014, 126, 1 - 5
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SiliaMetS DMT (Dimercaptotriazine) (R79030B)
SiliaMetS DMT is the silica - bound equivalent of 2,4,6 - trimercaptotriazine (trithiocyanuric acid, TMT). It is a versatile metal scavenger for a variety of metals and the preferred metal scavenger for ruthenium catalysts and hindered Pd complexes. Best scavenger for: As, Ir, Ni, Os, Pd, Pt, Rh, Ru & Se. Good scavenger for: Cd, Co, Cu, Fe, Sc & Zn.
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