A new route for the efficient metalation of unfunctionalized aromatics

Abstract

The synthesis and isolation of a novel bimetallic species formed by reacting two equivalents of TMPLi with CuCl in the presence of Et₂O are reported. X-ray crystallography reveals the Et₂O-free tetranuclear aggregate (TMPCu)₂(TMPLi)₂1, which formally results from the catenation of dimers of TMPLi and TMPCu. NMR spectroscopy confirms that, upon dissolution in hydrocarbon media, the crystals fail to form a conventional Gilman cuprate dimer. Instead they exhibit a spectrum which is consistent with that recently proposed for an isomer of dimeric Gilman cuprate. Moreover, while pre-isolated Gilman cuprate is inert to benzene solvent, this new isomer smoothly affects aromatic deprotonation to give mainly Ph(TMP)₃Cu₂Li₂3, which is formally a heterodimer of Gilman cuprate TMPCu(μ-TMP)Li 2 and PhCu(μ-TMP)Li 4. Attempts to synthesise 3 through explicit combination of pre-isolated 2 and 4 were successful; additionally, this permitted the preparation of Ph(TMP)₃Cu₃Li 5 and Ph(TMP)₃CuLi₃7 when 4 was combined in 1 : 2 ratios with TMPCu or TMPLi, respectively. 5 was characterised as metallacyclic in the solid-state, its structural features resembling those in 3 but with reduced Li–π interactions. It also proved possible to perform Cu/Li exchange on 5 (using ^tBuOCu) to give a novel mixed organo(amido)copper species Ph(TMP)₃Cu₄6. Remarkably, the unprecedented reactivity of 1 towards benzene is reproduced by heating a 1 : 1 mixture of TMPLi and TMPCu in the same solvent; this gives predominantly 3. On the other hand, mixtures which are rich in either Cu or Li can lead to the selective in situ formation of 5 or 7. Though crystallographic data on 7 could not be obtained, DFT calculations accurately corroborated the observed structures of 3 and 5 and could be used to support 7 having the same structure type, albeit with enhanced Li–π interactions. This was consistent with NMR spectroscopic data. However, in contrast to 3 and 5, for which 2D NMR spectroscopy indicated only conformational changes, 7 was additionally found to exhibit fluxionality in a manner consistent with a dissociative process.