Deuterium isotope effects competition studied with

We have mentioned above some attempts to explain these facts theoretically. Preliminary calculations show that Cu insertion of a C-H bond in methane is competitive with the abstraction reaction in H2, so that the activation of hydrogen (specially when the deuterium isotope is used) is less effective in the presence of methane. Also CIPSI calculations on the Cu -f N2 interactions shows the formation of the Cu(N2)n complexes which were first proposed by Gracie (Gracie, C., M.Sc. Thesis, University of Toronto, 1985). Such complexes make the interaction of Cu with D2 much less effective, raising the activation barriers and explaining the scarcity of the CuD and D subproducts (Sanchez-Zamora, M. Novaro, 0. and Ruiz, M.E., submitted for publication). These modest attempts however only make it more imperative to carry out more theoretical studies both to obtain better potential energy surfaces and to do trajectory calculations for Cu -fD2 reactions. 

The existence of a free carbonium ion such as VII in a strongly solvating medium is highly improbable. Only if VII could exist in association with the palladium could decomposition to vinyl acetate be expected to occur with a reasonable degree of frequency, in competition with the reaction with acetate to form ethylidene diacetate. Similar results have been reported in the Wacker acetaldehyde synthesis when D2O is used as the solvent (25). Stern (54) has reported results in which 2-deuteropropylene was used as substrate in the reaction. Based on assumed /J-acetoxyalkylpalladium intermediates, on the absence of an appreciable isotope effect in the proton-loss step, and on the product distribution observed, excellent agreement between calculated (71%) and observed (75%) deuterium retention was obtained. Several problems inherent in this study (54) have been discussed in a recent review (I). Hence, considerable additional effort must be expended before a clear-cut decision can be made between a simple / -hydrogen elimination and a palladium-assisted hydride shift in this reaction. 

Finally, the involvement of deoxy-Breslow species as intermediates and related to the reactivity profile of acrylates under NHC catalysis has been reported twice. The first report dealt with detailed mechanistic studies of the well-known tail-to-tail dimerization of methyl acrylate. By means of complementary and robust experiments (including kinetic isotope effects, deuterium-labelling studies and competitive reactions), the formation of the dimer (148) has been unambiguously rationalized. The second report has described/or the first time the NHC-catalysed cyclotetramerization of acrylates. Using imidazolium chloride (135) as NHC source, various trisubstituted cyclopen-tenones (149), thus resulting from the cyclotetramerization of acrylates, have been obtained in moderate yields. 

A detailed study using X-ray crystallography, H, C, and P NMR (nuclear magnetic resonance) spectroscopies, product studies, a secondary a-deuterium KIE (kinetic isotope effect) of 1.13, competition experiments, and a stereochemical study has shown that the nickel-catalysed Suzuki reaction of N,0 acetals with (PhBO)3, and a DPEPhos ligand occurs by the 6 1-like mechanism shown in Scheme 1. 

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