Carbene complexes deuterium exchange

Kreiter (67) first demonstrated by XH NMR spectroscopic studies the acidity of hydrogen atoms that are bonded at the a-C atom of alkoxy-(alkyl) carbene complexes. In CH3OD solution, in the presence of catalytic amounts of sodium methanolate, pen tacarbonyl[methoxy (methyl) car-bene cliromiumfO) exchanges for deuterium all hydrogen atoms of the methyl group that is situated next to the carbene carbon ... 

Protons attached to the a-carbon atom in metal-carbene complexes undergo rapid base-catalyzed hydrogen-deuterium exchange with hydroxylic solvents (Kreiter, 1968). For example, the half-life for exchange of the a protons in VII in acetone-D20 with no added base is 23 min at 40° (Casey, 1970). This rapid exchange implies the existence of an intermediate carbanion. These... 

C. P. Casey and W. R. Brunsvold, J. Organometallic Chem, 1976,118, 309. Base-catalysed deuterium exchange of complexes [Cr(CO)5(carbene)]. 

It is interesting to note that C-H activation on ruthenium NHC complexes is not limited to intramolecular protons located in the N-sidechain of the carbene, but occurs inter-molecularly as well. Leimer et al. reacted [MesIRuH PCyj] with toluene-dg at ambient temperature and observed a rapid H/D exchange reaction involving the four hydride hydrogen atoms on ruthenium, the methyl protons of the mesityl substituents of the carbene ligand and the deuterium atoms on the meta positions of toluene-dg. The ortho-, para- and methyl-deuterium atoms of the solvent did not participate [145]. 

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