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

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 ¹H, ¹³C and ¹¹³Cd 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.