Extending motifs in lithiocuprate chemistry: unexpected structural diversity in thiocyanate complexes

Abstract

The new area of lithio(thiocyanato)cuprates has been developed. Using inexpensive, stable and safe CuSCN for their preparation, these complexes revealed Lipshutz-type dimeric motifs with solvent-dependent point group identities; planar, boat-shaped and chair shaped conformers are seen in the solid state. In solution, both Lipshutz-type and Gilman structures are clearly seen. Since the advent in 2007 of directed ortho cupration, effort has gone into understanding the structure-reactivity effects of amide ligand variation in and alkali metal salt abstraction from Lipshutz-type cuprates such as (TMP)₂Cu(CN)Li₂(THF) 1 (TMP = 2,2,6,6-tetramethylpiperidide). The replacement of CN⁻ with SCN⁻ is investigated presently as a means of improving the safety of lithium cuprates. The synthesis and solid state structural characterization of reference cuprate (TMP)₂Cu(CN)Li₂(THP) 8 (THP = tetrahydropyran) precedes that of the thiocyanate series (TMP)₂Cu(SCN)Li₂(L) (L = OEt₂9, THF 10, THP 11). For each of 9–11, preformed TMPLi was combined with CuSCN (2 : 1) in the presence of sub-stoichiometric Lewis base (0.5 eq. wrt Li). The avoidance of Lewis basic solvents incurs formation of the unsolvated Gilman cuprate (TMP)₂CuLi 12, whilst multidimensional NMR spectroscopy has evidenced the abstraction of LiSCN from 9–11 in hydrocarbon solution and the in situ formation of Gilman reagents. The synthetic utility of 10 is established in the selective deprotometalation of chloropyridine substrates, including effecting transition metal-free homocoupling in 51–69% yield.