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, was prepared using AgCN and found to form a comparable, isolable dimer (Figure 1.31). At the same time, variation of the amido component was probed, with (HMDS)2Ag(CN)Li2(THF) (180, HMDS = hexamethyldisilamide), (DA)2Ag(CN)Li2(THF) 181, and (Cy2N)2Ag(CN)Li2(THF) (182, Cy = cyclohexyl) all being formed but only 179 and 182 proving effective in proof‐of‐concept iodinations of N,N‐diisopropylbenzamide. Thereafter, 179 was deployed in a range of directed ortho‐deprotometalations using various DMG and ancillary functional group (FG) permutations (Scheme 1.36). Work demonstrated the ability to use tertiary carboxylic amide DMGs of variable lability (yielding 183–185). The success of nitrile (giving 186) and methyl ester (giving 187) DMGs was remarkable given their incompatibility with e.g. organolithiums. Likewise, in contrast to the difficulty of metalating nitroarenes using traditional strong bases [243], and the fact that this has hitherto been achievable using only specific substrates [244], 188 and 189 were now smoothly generated by 179. Notably, the yield of 188 (80%) contrasted with just 6% obtained when using 159. Elsewhere (forming 190–192), ancillary styrene‐type FGs could be employed without detectable polymerization, while halides and pseudohalides were tolerated, enabling the demonstration that OTf [(trifluoromethanesulfonyl)oxy] could survive a deprotonative metalation sequence [245].
Figure 1.31 The dimer of lithium argentate (TMP)2Ag(CN)Li2(THF) 179.
Scheme 1.36 Examples of directed deprotometalation using lithium argentate 179. a Argentation at −40 °C. b ortho : meta >29 : 1. c ortho : meta >16 : 1 (Tf = trifluoromethanesulfonyl).
The compatibility of traditionally