Polar Organometallic Reagents. Группа авторов

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Scheme 1.24 Synthesis of lithium dimethylcuprate 83.

Figure 1.11 Structures of phenylcuprate species (a) [Ph₆Cu₃Li₂]⁻ in 98 and (b) [Ph₂CuLi(Et₂O)]₂99₂.

      Sources: Adapted from Hope et al. [131]; Lorenzen and Weiss [133].

Structures of complexes such as 96 [129] and 97 [130] exhibit unusual ‘higher‐order’ copper centres that act as bridges towards lower‐order [Ph₂Cu]⁻ units. On the other hand, compounds like Ph₉Cu₄Li₅(SMe₂)₄100 [134] and Ph₅Cu₂Li₃(SMe₂)₄101 [135] incorporate [Ph₃Cu]²⁻ units as one of the primary cuprate moieties, making a straightforward higher‐order description more appropriate (Figure 1.12a). Indeed, recent work has shown that a higher Cu coordination number is attainable in spirocuprate (biph)₂CuLi₃(THF)₆102 (biph = 2,2′‐biphenyl, Figure 1.12b), with Cu now displaying a remarkable distorted tetrahedral geometry [dihedral angle between cuprocycles = 84.1(1)°] [136].

Moving from simple phenylcuprates, the use of aminoaryl ligands capable of providing internal coordination has enabled the isolation of neutral (DMBA)₂CuLi 103, whose dimeric structure revealed a near‐planar arrangement of alternating Cu and Li atoms, bridged by aryl ligands and with only the Li centres interacting with the pendant amine functions (Figure 1.13a) [137]. In this case, the bridging mode of the aryl ligand differed from earlier reports on the tetramer of similar organocopper species [2‐(Me₂NCH₂)C₆H₄‐Me‐5]Cu 104 [138], its asymmetric nature suggesting primary σ‐type interaction of the C‐based sp² lone pair with Cu (C–Cu 1.942(3) and C–Li 2.385(6) Å, respectively). A much more pronounced contrast in σ/π‐bonding has been reported in (Mes)₂CuLi 105, where dimerization results in each Li centre adopting both η¹ and η⁶‐coordination towards mesityl groups, leaving Cu free to adopt a preferred linear geometry (C–Cu–C = 178.34(7)°, Figure 1.13b) [139]. The dominance of Li…π interaction here can be attributed to the absence of donor solvent in the structure.

Figure 1.12 Selected higher‐order cuprates (a) Ph₅Cu₂Li₃ (SMe₂)₄101 and (b) (biph)₂CuLi₃(THF)₆102 (biph = 2,2′‐biphenyl).

      Sources: Adapted from Olmstead et al. [135]; Liu et al. [136].

Figure 1.13 Molecular structures of the dimers of (a) (DMBA)₂CuLi 103 and (b) (Mes)₂CuLi 105.

      Sources: Adapted from Van Koten et al. [137]; Davies et al. [139].

Simple alkylcuprates of the type routinely used in synthesis have proved difficult to study due to their relatively low thermal stability. In 1984, the crystal structure of SIP [{(Me₃Si)₃C}₂Cu][Li(THF)₄] 106 was described, providing the first solid‐state evidence for the structure of a dialkylcuprate (Figure 1.14a) [140]. Crystallography revealed linear, two‐coordinate copper, though the possibility that these features were imposed by the steric bulk of the anion could not be excluded. Other breakthroughs in the alkylcuprate field have included characterization of the polymer of (Me₃SiCH₂)₂CuLi(SMe₂) 107 [141], a structure consisting of dimeric units (similar to those seen in the structure of 99 joined by SMe₂ ligands. Meanwhile, only two lithium dimethylcuprate structures have been reported for reagents; SIPs [Me₂Cu][Li(12‐crown‐4)₂] 108 [142] and [Me₂Cu][Li(DME)₃] 109 [143] (DME = 1,2‐dimethoxyethane). Recently, these have been added to by a possible pre‐reaction π‐complex (fluorenone)CuMe₂Li(THF)₃110 (Figure 1.14b) [144]. In contrast to the linear cuprate ion geometry observed in the first two cases, the C=O π‐complex in Figure 1.14b reveals a C–Cu–C angle of 104°. Ion separation would appear to be induced by strongly coordinating Lewis base additive. On the other hand, in the more weakly coordinating ethereal solvents in which lithium dimethylcuprate is typically used, it is believed that CIPs dominate and that these forms of reagent are responsible for observed reactivity [143].

A number of solution‐state studies on the lithium methylcuprate species (Me _m+n Cu _m Li _n ) have been undertaken. Detailed solution work is covered elsewhere in this volume. However, briefly, ¹H and ⁷Li NMR spectroscopies have revealed that the addition of MeLi to MeCu in THF/Et₂O results in an equilibrium between Me₂CuLi 83 and Me₃Cu₂Li 111 plus MeLi, though the existence of this equilibrium has proved to be strongly dependent upon both the solvent (it does not occur in Et₂O) and the presence of LiI (which promotes the formation of a different discrete entity). This work highlighted the fact that reagents presumed to be either ‘lower‐order’ or ‘higher‐order’ according to the stoichiometry of their preparation were in fact composed of varying quantities of the same species [145]. More recent DOSY studies using PFG (pulsed field gradient) NMR spectroscopy have indicated the existence of dimethylcuprate aggregates (based on homodimeric cores) larger than dimers in Et₂O, though these exhibited depleted reactivity [146]. Likewise, multi‐dimensional NMR spectroscopy showed that the addition of small amounts of THF to Et₂O‐based cuprate preparations had surprisingly different effects on aggregation depending on the identity of inorganic Li salts present [147].

Figure 1.14 Structures of (a) SIP [{(Me₃Si)₃C}₂Cu][Li(THF)₄] 106, and (b) bent cuprate anion in CIP (fluorenone)CuMe₂Li(THF)₃110.

      Sources: Adapted from Eaborn et al. [140]; Bertz et al. [144].

The dramatic effects on reactivity of incorporating a Li salt with a polar organometallic reagent have already been discussed in the context of magnesium amides and will be returned to in the context of