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Opening the black box of mixed-metal TMP metallating reagents : direct cadmation or lithium-cadmium transmetallation?

Armstrong, David R. and Kennedy, Alan and Mulvey, Robert E. and Parkinson, John A. and Robertson, Stuart D. (2012) Opening the black box of mixed-metal TMP metallating reagents : direct cadmation or lithium-cadmium transmetallation? Chemical Science, 3 (9). pp. 2700-2707. ISSN 2041-6520

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Abstract

Designed to remove some of the mystery surrounding mixed-metal TMP (2,2,6,6-tetramethylpiperidide) metallating reagents, this study examines in detail "LiCd(TMP)(3)'' in its own right. Previously established as an excellent "cadmating'' (Cd-H exchange) reagent towards a wide variety of aromatic substrates, "LiCd(TMP)(3)'' has been investigated by H-1, C-13 and Cd-113 NMR studies as well as by DOSY NMR spectroscopy. This evidence puts a question mark against its ate formulation implying it exists in THF solution as two independent homometallic amides. Exploring the reactivity of "LiCd(TMP)(3)'' with anisole as a test substrate, both experimentally by NMR studies and theoretically by DFT studies suggests a two-step lithiation/transmetallation process in which the initially formed ortho-lithiated species undergoes a reaction with Cd(TMP)(2) to form new Cd-C and Li-N bonds. For completeness, the homometallic cadmium component Cd(TMP)(2) has been comprehensively characterised for the first time including a crystal structure determination revealing a near-linear N-Cd-N arrangement.