THP = tetrahydropyran) when introduced to LTMP in a 1 : 2 ratio in the presence of donor solvent [228]. A strong dependence of the geometry of the solid‐state dimers of these thiocyanatocuprates on the Lewis base additives was uncovered, apparently resulting from the ability of the metallacyclic (LiSCN)2 core to adopt boat‐like, chair‐like or planar conformations (Figure 1.27). The influence of the donor solvent was not limited to the solid‐state either: Lipshutz‐type thiocyanato(amido)cuprates were found to convert to Gilman cuprate in benzene solution, with the degree of conversion being strongly influenced by the identity of the donor solvent incorporated in the cuprate (and being most pronounced for Et2O). In a synthetic setting, thiocyanatocuprates performed competitively with CuCl‐derived bases in the directed ortho‐cupration of halopyridines.
Cyanato(amido)cuprate analogues of the thiocyanate systems described above have also been investigated. Attempts to prepare these cuprates from the direct reaction of CuOCN with LTMP were not successful, with spectroscopy suggesting multiple products and crystallography indicating Cu/Li substitution in the solid‐state in some of these [229]. Nonetheless, Lipshutz‐type (TMP)2Cu(OCN)Li2(THF) 172 proved accessible by inserting LiOCN into 156 in THF. The resulting solid‐state dimer offered a geometry that differed substantially from those of the known THF‐solvated thiocyanatocuprates, but otherwise retained all the features now established to be typical for Lipshutz‐type cuprates (Figure 1.28). Curiously, when reacted with CuOCN, less sterically encumbered lithium diisopropylamide (LDA) furnished a novel amidocuprate‐amidolithium adduct (DA)4Cu(OCN)Li4(TMEDA)2173, which could be viewed as arising from the attachment of units of LDA(TMEDA) to a Lipshutz‐type monomer. Meanwhile, spectroscopic investigations on CuOCN/LTMP reaction mixtures suggested the production of a new amidocopper‐amidolithium aggregate (CuTMP)2(LTMP)2174 (Figure 1.29) – a structural isomer of previously reported Gilman dimer [(TMP)2CuLi]2156 2 (Figure 1.24a). The reactivity of the Gilman dimer was previously established to be low. However, the knowledge that monomeric Gilman amidocuprates are reactive led to an interest in the solution behaviour and reactivity of this new adduct.
Figure 1.27 Molecular structures of the dimers of thiocyananto(amido)cuprates (a) (TMP)2Cu(SCN)Li2(THF) 169, (b) (TMP)2Cu(SCN)Li2(Et2O) 170, and (c) (TMP)2Cu(SCN)Li2(THP) 171.
Source: Adapted from Peel et al. [228].
Figure 1.28 Examples of cyanatocuprates (a) [(TMP)2Cu(OCN)Li2(THF)]21722 and (b) (DA)4Cu(OCN)Li4(TMEDA)2173.
Source: Adapted from Peel et al. [229].
The recent realization that 174 could be accessed in a pure form by reaction of CuCl with LTMP made possible investigations into its synthetic utility. This proved both highly unexpected and potentially useful. Notably, the adduct demonstrated an uncanny ability to smoothly deprotonate benzene. As such it suggests the use of currently underutilized chemical feedstocks in aromatic elaboration. Physical mixtures of CuTMP and LTMP replicated this reactivity towards benzene, in so doing leading to the production of a series of organoamidocuprates Ph(TMP)3Cu n Li4−n (n = 1–4 175–178; Figure 1.30) [230]. This contrasted with conventional Gilman cuprate 156 2, which was unreactive under the same conditions. Crystallography revealed metallacyclic structures for these product organoamidocuprates in the solid‐state; the amido ligand adopted the by now usual metal‐bridging mode, whilst coordination of the Ph‐group varied substantially depending on metal content. In all cases, where Ph bridged Cu and Li, Cu–C σ‐bonding took precedence, the C···Li interactions being π‐type and suggesting an increase in hapticity with increasing Li content (a conclusion which was also supported by spectroscopy). Detailed spectroscopic discussions are reserved for Chapter 5. However, briefly, 1H,1H–NOESY/EXSY revealed that in solution, Ph(TMP)3Cu2Li2176 was conformationally fluxional but otherwise retains its integrity. More dramatically, 7Li,7Li–EXSY established a dissociative‐associative equilibrium for the Li‐rich species Ph(TMP)3CuLi3175.
Figure 1.29 Isomers of (TMP)4Cu2Li2; (a) dimer of conventional Gilman cuprate 156 (see Figure 1.24a) and, (b) isomeric adduct (CuTMP)2(LTMP)2174.
Figure 1.30 Structurally characterized organoamidocuprate aggregates (a) Ph(TMP)3Cu2Li2176 and (b) Ph(TMP)3Cu3Li 177, illustrating changes to the hapticity of the Ph moiety in the solid‐state as a function of metal composition.
Source: Adapted from Peel et al. [230].
1.4.6 Argentates
To bring the current narrative right up to date, the similarities between copper and silver, vertically related in the periodic table, have led to a recent interest in lithium argentates. However, whereas applications of organocopper compounds have proved extensive [231–233], those of their silver congeners have been much more limited due to the difficulties posed by their preparation. Hence, the Ag(0 → I) redox potential [234] of 0.8 V means that, unlike organocopper(I) compounds, organosilver(I) species cannot be made by oxidatively metalating organic halides [235]. Meanwhile, neither hydro (or carbo)‐argentation nor halogen–silver exchange have become well established, though isolated examples of borylargentation [236], fluoroargentation [237], and carboargentation [238] have been described. Meanwhile, transmetalation has been limited by the high reactivity of e.g. s‐block organometallics towards silver salts [239–241]. This has led to efforts to expand the field of DoM [242] to encompass silver chemistry. To engender development, the argentate analogue of Lipshutz cuprate 159 [213], (TMP)2Ag(CN)Li2(THF) 179,
