Modern Organocopper Chemistry

81 Structures and Reactivities of Organocopper Compounds

1.2

Homoleptic Organocopper Compounds CunRn

In the early days of organocopper chemistry, synthesis and structural characterization of pure organocopper compounds were hampered by several factors. One of the problems was the tendency of organocopper compounds to associate with metal halides (even copper halides itself ) to form highly aggregated species. Therefore, products containing both the organocopper compound and metal halides were regarded as impure, although they were in fact pure compounds in which metal halides constituted an integral part of the complex. Definite proof for this view was obtained as early as 1975 [44].

On the basis of this information, techniques for the synthesis of pure copper compounds were developed. The following parameters played an important role:

(1)purity and nature of the starting materials,

(2)the reaction temperature,

(3)the nature of the solvent, and

(4)the presence of co-solvents.

It is consequently not possible to give one general synthetic procedure (for a detailed discussion see refs. 29 and 45). It also became evident that the order of addition of the reagents can play a crucial role, which is nicely illustrated by the following examples.

When a suspension of [Li(C6H4CH2NMe2-2)] is added in stoichiometric amounts to a suspension of CuX (X ¼ Cl or Br) in Et2O (see Scheme 1.7), an insoluble red compound with the composition [Cu(C6H4CH2NMe2-2) CuX] is formed. This compound does not undergo any further reaction, even with an excess of the corresponding organolithium reagent [35].

Scheme 1.7.

When the order of addition is reversed (i.e., when a suspension of CuBr is gradually added to a suspension of [Li(C6H4CH2NMe2-2)]), a pure, yellow-colored organocopper compound [Cu4(C6H4CH2NMe2-2)4] is isolated from the reaction mixture in about 40% yield [19]. This was the first organocopper compound to be fully structurally characterized by X-ray crystal structure determination (see Fig. 1.1 in the previous section). The latter reaction sequence proceeds through a ‘‘cuprate stage’’, a stable, soluble intermediate with [Cu2Li2(C6H4CH2NMe2-2)4] stoichio-

1.2 Homoleptic Organocopper Compounds CunRn 9

metry, which was also fully characterized and which is discussed later in this chapter. When the copper arenethiolate [Cu3(SC6H4CH2NMe2-2)3] was used in place of CuBr, [Cu4(C6H4CH2NMe2-2)4] was formed and subsequently isolated in almost quantitative yield [46].

An entirely di erent methodology is applied for the synthesis of the closely related organocopper compound [Cu(C6H4NMe2-2)]. This proceeds by way of a stable organocopper-CuBr aggregate [47] (see Scheme 1.8). Although structural characterization of [Cu(C6H4NMe2-2)] was hampered by its insolubility in all common solvents, it most probably has a polymeric structure.

Scheme 1.8.

A major problem that often complicates the structural characterization of simple organocopper compounds is their insolubility in common organic solvents, which precludes the application of NMR techniques or the growing of suitable single crystals for X-ray structure determination. Attempts have been made to solubilize these compounds by addition of additional donor molecules such as amines [48] and phosphines [48–50]. It was expected that binding of additional donor molecules would increase the intrinsic thermal stability of simple organocopper compounds. However, the opposite result was often observed, whilst complexation of additional ligands also caused the breaking down of the original aggregated structures into mononuclear copper compounds.

The stabilizing e ect of additional donor molecules is nicely illustrated by the increase in decomposition temperatures seen on going from MeCu (< 15 C) to CuMe(PPh3)3, which decomposes at about 75 C [49]. The structure of the latter compound in the solid state (see Fig. 1.5) comprises a mononuclear complex with

Fig. 1.5. Structure of CuMe(PPh3)3 in the solid state.

10 1 Structures and Reactivities of Organocopper Compounds

Fig. 1.6. Structure of CuCp(PPh3) in the solid state.

the methyl group and the three PPh3 groups in a tetrahedral arrangement around the copper atom [51].

Another illustration of the stabilizing e ect of phosphines is supplied by cyclopentadienylcopper triphenylphosphine, one of the very few examples in which a cyclopentadienyl group is h5-bonded to copper (see Fig. 1.6) [52].

An enhancement in the solubility of otherwise almost insoluble, and most probably polymeric, phenylcopper species has been achieved by treatment with a tridentate phosphine. The resulting soluble phenylcopper compound was fully structurally characterized (see Scheme 1.9) [53].

Scheme 1.9. Structure of CuPh(P3) in the solid state (Ph groups at P are omitted for clarity).

A remarkably stable organocopper compound was obtained on treatment of Me3PbCH2 with CuCl (see Scheme 1.10) [54, 55]. Formally, this product should be regarded as an ‘‘ate’’ complex, with positive charges on the phosphorus atoms and negative charges on the copper atoms.

Scheme 1.10. Reaction between copper(I) chloride and Me3PbCH2.

Complete degradation of organocopper aggregates may occur when they react with tertiary phosphines. This is illustrated by treatment of arylcopper compounds with bis-(diphenylphosphino)methane (DPPM) and with 1,2-bis-(diphenylphosphino)- ethane (DPPE) (see Scheme 1.11).

1.2 Homoleptic Organocopper Compounds CunRn 11

Scheme 1.11. Reactions between arylcopper compounds and diphosphines.

Treatment of 2-, 3- or 4-MePhCu with DPPM a ords the trimeric aggregate Cu3[CH(PPh2)2]3 in quantitative yield [56]. This is in fact a simple acid-base reaction (Cu/H exchange). It should be noted that the latter compound is also accessible by means of a transmetalation reaction between Li[CH(PPh2)2] and CuCl. In contrast, treatment of [Cu4(C6H4CH2NMe2-2)4] with DPPE results in selective CaP bond cleavage and formation of Cu2(PPh2)2(DPPE)2 (see Scheme 1.11) [57].

As mentioned above, one of the thermal decomposition pathways of alkylcopper compounds involves a b-hydrogen elimination process, and so it is not surprising that the first well characterized alkylcopper compounds lacked such b-hydrogens. Treatment of LiCH2SiMe3 with CuI a orded a tetrameric aggregate, the structure of which was unambiguously proven by an X-ray crystal structure determination (see Fig. 1.1B in the previous section). This represented the first example of a well characterized alkylcopper compound [17].

On the basis of molecular weight determinations, simple arylcopper compounds such as 4-Me and 2-MePhCu are tetrameric [58] or (in the case of CuPhCF3-3 [59]) octameric. Polymeric structures are implied for insoluble compounds such as CuPh [60]. It has been proposed that the tetrameric aggregates are isostructural with [Cu4(C6H4CH2NMe2-2)4], containing four copper atoms in a butterfly arrangement and with each of the aryl groups bridging between adjacent copper atoms (see Fig. 1.1 in the previous section).

More recently, several arylcopper compound syntheses that make use of a soluble form of a copper halide precursor, CuBr DMS (DMS ¼ dimethylsulfide) in DMS as the solvent have been reported. Some of these compounds, such as [Cu4(C6H5)4(DMS)2] [61] and [Cu4(C6H4Me-2)4(DMS)2] [62], appeared to be DMS adducts and were fully characterized by X-ray crystal structure determination (see Fig. 1.7). It is interesting to note that these structures contain twoand threecoordinate copper atoms in trans positions. These structures may be envisaged as ion-pairs comprising Cu(Aryl)2 anions bound to Cu(DMS) cations through the Cipso atoms.

The overall structural motif of pure organocopper compounds can be changed dramatically by the addition or the presence of coordinating solvents such as DMS or THT (THT ¼ tetrahydrothiophene). This is illustrated by comparison of the structures of [Cu4Mesityl4(THT)2], and [Cu5Mesityl5] (see Fig. 1.8) [63, 64].

12 1 Structures and Reactivities of Organocopper Compounds

Fig. 1.7. Structures of [Cu4(C6H5)4(DMS)2] and [Cu4(C6H4Me-2)4(DMS)2] in the solid state.

There is also evidence for the influence of steric crowding exerted by large groups present near the CuaC bond on the aggregation state of the organocopper compound. When methyl substituents, as present in [Cu5Mesityl5], are replaced by i-Pr groups, the corresponding organocopper compound [CuC6H2(i-Pr)3-2,4,6] becomes tetrameric [65]. If the even more sterically demanding t-Bu groups are subsequently introduced in the presence of DMS, a mononuclear copper compound [CuC6H2(t-Bu)3-2,4,6] DMS is isolated [66]. Both compounds have been characterized by X-ray crystal structure determinations (see Fig. 1.9). The latter compound is one of the very few examples of a monomeric organocopper compound.

The triphenyl analogue of mesitylcopper, prepared from the corresponding lithium compound and CuBr DMS, has a rather unexpected structure (see Fig. 1.10) [66]. Two 2,4,6-triphenyl groups are bound to one copper atom in an almost linear arrangement, while one of the aryl groups is bound to a Cu(DMS)2 unit. Formally, this compound should be regarded as consisting of Ar2Cu anions coordinated to a Cu(DMS)2 cation (cf. the interpretation of the structure of [Cu4(C6H4Me-2)4(DMS)2]).

Fig. 1.8. Structures of [Cu4Mesityl4(THT)2] and [Cu5Mesityl5] in the solid state.

1.2 Homoleptic Organocopper Compounds CunRn 13

Fig. 1.9. Structures of [Cu4(C6H2(i-Pr)3-2,4,6)4] and [CuC6H2(t-Bu)3-2,4,6 DMS] in the solid state.

The introduction, initiated by Van Koten et al. in the early 1970s [18, 19], of the concept of stabilization of organocopper compounds through the use of organic groups, thus permitting additional intramolecular coordination, provided more detailed insight into the factors determining the formation of specific aggregates. The first example of this approach was the synthesis and structural characterization of [Cu4(C6H4CH2NMe2-2)4] (vide supra). As the overall structural features found in this compound – bridging three-center, two-electron-bonded aryl groups – are comparable to those in simple aryl copper compounds, the influence of the coordinating ortho-(dimethylamino)methyl substituent on the stability of the compound is twofold in nature. Firstly, the ortho substituent stabilizes the rotamer with the aryl ring perpendicular to the CuaCu vector, thus increasing the electron density between Cipso and Cu. Secondly, the Lewis base stabilizes the tetrameric aggregate relative to other possible aggregation states. This last point is illustrated by comparison of the features of closely related ligand systems. Whereas CuC6H4NMe2-2

Fig. 1.10. Structure of [Cu2(C6H2Ph3-2,4,6)2(DMS)2] in the solid state.

14 1 Structures and Reactivities of Organocopper Compounds

Fig. 1.11. Structures of [Cu(Me3SiCH(Py-2))]4 and [Cu((Me3Si)2C(Py-2))]2 in the solid state.

is insoluble, most probably pointing to a polymeric structure [47], CuC6H4OMe-2 is soluble and exists as an octameric aggregate in the solid state [67].

That subtle variations in the ligand system can have a large influence on the overall structure of the copper compound is also attested to by the di erent structures of [Cu(Me3SiCH(Py-2))]4 (Py-2 ¼ 2-pyridyl) and [Cu((Me3Si)2C(Py-2))]2. In both compounds, the 2-methylpyridyl group is h1-bonded to a copper atom, while a linear coordination geometry at the copper center is achieved through intermolecular coordination of the nitrogen atom of an adjacent pyridyl unit. However, [Cu(Me3SiCH(Py-2))]4 exists as a tetramer in solution and in the solid state [68], whereas [Cu((Me3Si)2C(Py-2))]2 has a dimeric structure (see Fig. 1.11) [68, 69]. This di erence is probably a consequence of the presence of a second bulky Me3Si substituent at the carbon atom bound to copper in [Cu((Me3Si)2C(Py-2))]2.

Finally, an important role is also played by the flexibility of the chelate ring formed upon coordination of a heteroatom-containing substituent. This is obvious when the structures of [Cu4(C6H4CH2NMe2-2)4] and the corresponding arylcopper compound containing a 2-oxazoline ligand are compared. Although both ligand systems contain a potentially coordinating nitrogen atom in the g-position (with respect to the copper atom), [Cu4(C6H4CH2NMe2-2)4] is a tetramer while the latter compound (see Fig. 1.12, left) is a dimer [70].

The use of the more rigid 8-(dimethylamino)naphthyl group a ords an organocopper compound with some remarkable features (see Fig. 1.12, right). It comprises a tetranuclear aggregate in which each of the naphthyl groups bridge between two adjacent copper atoms. However, the heteroatom-containing substituents are pairwise coordinated to two, mutually trans-positioned copper atoms [71], or in other words, the structure contains two CuAr2 anions with two-coordinate copper atoms and two four-coordinate CuN2 cations. Ion-pair formation involving coordination of Cipso to the CuN2 cations a ords the neutral tetranuclear aggregate seen in the solid state structure. This organocopper compound shows an unusual reactivity – usually observed only for cuprates – towards organic substrates. These observations provide a direct link between the structural features of this compound and its reactivity in organic synthesis.

1.2 Homoleptic Organocopper Compounds CunRn 15

Fig. 1.12. Structures of [Cu(C6H4(oxazolyl)-2(Me)-4]2 and [1-CuC10H6NMe2-8]4 in the solid state.

When cuprates are used as reagents in organic chemistry, the compounds are usually prepared in situ from copper salts, starting either from a Grignard reagent or from an organolithium compound. Because of the presence of magnesium or lithium halides, these systems are not always suitable for mechanistic studies or structural characterization of the cuprate involved (see Sect. 1.3). An excellent synthetic pathway to pure organocuprates, free from additional metal halides or other impurities, involves treatment of the pure organocopper compound with the pure organolithium compound in the required stoichiometry [72] (see Eqn. 1 in Scheme 1.12).

Scheme 1.12.

Another application of pure organocopper compounds is as starting materials for the synthesis of other organocopper compounds. Treatment of [CuC6H3CF3-3] with (CF3)3CBr, for example, a ords CuC(CF3)3 through a halogen/metal exchange reaction [73] (Eqn. 2 in Scheme 1.12). A further demonstration of the applicability of pure organocopper compounds is the insertion reaction of an isocyanide into a copper-carbon bond [74], (Eqn. 3 in Scheme 1.12).

16 1 Structures and Reactivities of Organocopper Compounds

Finally, some organocopper compounds undergo charge disproportionation under the influence of ligands that bind strongly to copper. Treatment of mesitylcopper with 1,2-bis-(diphenylphosphino)ethane (DPPE), for example, results in the formation of bis(mesityl)copper anions and a copper cation to which four phosphorus atoms of two DPPE molecules are coordinated [75].

The selective formation of symmetric biaryls in high yield through thermal or oxidative decomposition is a feature that can be directly associated with the structure of the compound involved. It has been shown that arylcopper compounds with a structure comprising three-center, two-electron-bonded bridging aryl groups undergo this selective reaction (see Scheme 1.13), while arylcopper compounds in which the Cu atom is h1-bonded to the aryl group give a mixture of unidentified decomposition products, most probably by a radical pathway. In structures incorporating bridging aryl groups, the carbon atoms are already in close proximity [76], as shown schematically in Scheme 1.13. Therefore, only a slight further distortion of this geometry is needed to bring the ipso-carbon atoms even closer together, thus promoting the CaC bond formation.

Scheme 1.13.

Furthermore, it has been demonstrated that an increase in the electrophilicity of the copper centers in aggregate structures, by incorporation of Cuþ into such structures, for example, favors CaC bond formation to give biaryls. Treatment of various organocopper compounds with Cuþ (in the form of CuOTf, OTf ¼ trifluoromethanesulfonate) has been studied [77]. For some compounds containing potential coordinating substituents, it was possible to isolate and study species such as [(Cu6R4)2þ][2 OTf ] [76], but addition of only catalytic amounts of CuOTf to simple arylcopper compounds such as Cu4(C6H4Me-2)4 and Cu4(C6H4Me-4)4 a ords the corresponding biaryls in quantitative yield. This was explained in terms of a mechanism involving a valence disproportionation reaction of two Cu(I) into Cu(II) and Cu(0) [77].

Finally, pure organocopper compounds have found applications in one-step syntheses of triand diorganotin halides. Its has now become well established that treatment of Grignard and organolithium reagents with tin(IV) halides always gives a mixture of products (Eqn. 1 in Scheme 1.14) rather than the desired trior diorganotin halides.

In contrast, treatment of SnCl4 with excess CuPh a ords SnPh3Cl as the only product [78] (Eqn. 2 in Scheme 1.14). Furthermore, it has been shown that reaction of functionally substituted arylcopper compounds with organotin halides proceeds very selectively to a ord a novel type of pentacoordinate organotin compounds possessing interesting structural features [79]. Treatment of Cu4(C6H4CH2NMe-2)4 with four equivalents of SnMeCl3, for example, gives

1.3 Heteroleptic Organocopper Compounds CunBmRnXm 17

Scheme 1.14.

SnMeCl2(C6H4CH2NMe-2) as the only product, in quantitative yield [80] (Eqn. 3 in Scheme 1.14).

1.3

Heteroleptic Organocopper Compounds CunBmRnXm

As outlined previously, aggregation of organocopper compounds is a consequence of the fact that the carbon moieties in these compounds are capable of bridging between two copper atoms. It is therefore to be expected that other anionic ligands capable of bridging between metal centers – halides, for example – might easily become incorporated into such aggregates.

By the early 1970s it was already recognized that the excess CuBr in the red product obtained on treatment of LiC6H4NMe2-2 with CuBr (for which the elemental analysis pointed to a Cu3(C6H4NMe2-2)2Br stoichiometry) is not a contaminant but an integral part of an aggregated species [47]. An X-ray crystal structure determination of this compound showed a structure (see Fig. 1.13) of Cu6(C6H4NMe2-2)4Br2 stoichiometry, with the copper atoms in an octahedral arrangement [44].

Each of the four organic moieties bridges between an equatorial and an axial copper atom through its C(1) atom, while the nitrogen atom in the substituent is coordinated to an adjacent equatorial copper atom. The two bromine atoms bridge, at opposite sites, between two equatorial copper atoms. This structural arrangement has the consequence that the aggregate incorporates two distinct types of

Fig. 1.13. Structure of Cu6(C6H4NMe2-2)4Br2 in the solid state.