Organocopper II Compounds

Although there are numerous examples of dimerizations of organo-metallic reagents by copper(II) salts, one particularly interesting example concerns 2,2 -dilithiobiphenyl 304). Wittig and Klar have proposed several transient copper(II) ate complexes and a stable copper(I) complex to account for the high yields of biphenylene and o-tetraphenyl-ene. Diarylcopper(II) compounds were supposedly isolated, and characterized by elemental analysis, from diarylmercury compounds and copper at 120°C 11, 197) an unlikely reaction. 

and Burba, G., in Methoden der organischen Chemie (Houben-Weyl, ed.), Vol. XIIl/1, Thieme, Stuttgart, 1970. 

Cairncross, A., and Sheppard, W. A., 157th Meeting, Amer. Chem. Soc., 1969, Abstr. ORGN 118. 

Chodowska-Palicka, J., and Nilsson, M., Acta Chem. Scand. 24, 3353 (1970). 

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The intrinsic instability of organocopper(II) compounds is most probably associated with the redox properties of copper. Decomposition of organocoppeifll) compounds can occur by two different routes (i) formation of an organocopper(I) compound and an organic radical R that can undergo further reactions which formally represents a one-electron reduction process and (it) direct formation of R R and Cu(0) which is formally a two-electron reduction process (reductive elimination cf Eqns. 1 and 2 in Scheme 1.3). 

Since isolable organocopper(II) compounds do not apparently exist, it is rather surprising that oxidation of the cuprate CdI [(CF3)2Cu ] (prepared in situ) with thiuram disulfide affords (CFj)2Cu S2CNEt2 (see Eqn. 1 in Scheme 1.6), the first and so far only example of an organocopper compound with the copper atom in the trivalent oxidation state. The structure of this compound was unambiguously proven by an X-ray crystal structure determination (see Rg. 1.2) [37]. 

While a large number of studies have been reported for conjugate addition and Sn2 alkylation reactions, the mechanisms of many important organocopper-promoted reactions have not been discussed. These include substitution on sp carbons, acylation with acyl halides [168], additions to carbonyl compounds, oxidative couplings [169], nucleophilic opening of electrophilic cyclopropanes [170], and the Kocienski reaction [171]. The chemistry of organocopper(II) species has rarely been studied experimentally [172-174], nor theoretically, save for some trapping experiments on the reaction of alkyl radicals with Cu(I) species in aqueous solution [175]. 

Copper(II) chloride has often been employed in reactions with lithium and Grignard reagents to obtain organocopper(I) compounds. Two equivalents of the organometallic reagent are required, one of which is used to reduce Cu(II) to Cu(I). Unstable alkylcopper(II) compounds may be intermediates. The reaction is usually represented by the equation... 

In inorganic and coordination chemistry, the Cu(II) state is the most abundant one, and is regarded as more stable than the Cu(II) state under normal conditions [32]. Although numerous examples of Cu(I) coordination complexes are known, their chemistry is rather limited and they are readily oxidized to Cu(II) species [32]. Of the common oxidation states, compounds derived from copper(III) are rare, with only 30-40 reported examples [32]. Despite the small number of isolated Cu(III) compounds, however, organocopper] 111) species have been proposed as important intermediates in copper-mediated organic reactions (Chapts. 4 and 10). 

The above analysis for copper chemistry also applies to the same-class element gold, which, however, forms much more stable C-Au bonds [176] and so is un-reactive. On the other hand, the d-orbitals of zinc(II), a main group neighbor, are too low-lying to make organozinc compounds as nucleophilic as organocopper compounds [92],... 

Arylations, Alkenylations and Alkynylations of Neutral Organocopper Compounds II... 

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