Organocopper Compounds: From Elusive to Isolable Species, from Early Supramolecular Chemistry with RCu^I Building Blocks to Mononuclear R_2–nCu^II and R_3–mCu^III Compounds. A Personal View

Abstract

The first reports on copper-mediated organic reactions and speculations about the role of presumed organocopper compounds as intermediates or transient species date back to the early 20th century. Since that time, copper salt mediated and much later, copper-catalyzed C−X bond forming reactions (X = C, O, N) have been developed. Phenyl-and methylcopper(I) (highly explosive as a dry solid) were the first organocopper materials synthesized by Reich (1923) and Gilman (1952), respectively. However, it was not until the late 1960s and early 1970s that the first pure organocopper(I) compounds, which are also stable at room temperature, were isolated and structurally characterized. Recently, by the application of clever design and new synthetic approaches even organocopper(II) and -(III) compounds have been isolated, species that during the greater part of the last century were considered elusive. Significantly, these species had for some time been surmised, via kinetic and computational studies, to play a key role in copper-mediated and -catalyzed C−X bond forming reactions. In this personal account the various stages along which organocopper chemistry developed, with a steady pace, will be highlighted.

Highlights of 45 years of research: A personal account

“…Initially, the impurity of the synthesized organocopper(I) material created a major problem which hampered their testing as biocides. Careful study of this phenomenon showed, however, that not only RCu aggregates undergo interaggregate exchange but that also, and sometime with high selectivity, R'Cu and CuX (and MX in general) species can become involved to the effect that thermodynamically stable hetero aggregate species can be formed and isolated in almost quantitative yield; an example of this is Cu 6 Br 2 (C 6 H 4 NMe 2 -2) 4 2 [3]. In this case, the heteroaggregate is more stable than the homoaggregate Cu n (C 6 H 4 NMe 2 -2) n to the effect that isolation of the pure organocopper (from the reaction of Li(C 6 H 4 NMe 2 -2) with CuBr in Et 2 O) is impossible.…”

“…2. Structures of 1 Cu 4 (C 6 H 3 CH 2 NMe 2 -2-Me-4) 4 [6] and 3 Cu 2 Li 2 (C 6 H 4 CH 2 NMe 2 -2) 4 [7]. (Reproduced with permission from ref 3; copyright Wiley-VCH Verlag GmbH & Co.…”

“…Organocopper chemistry: self-assemblage, stereochemistry and catalysis [3,4] Previous work taught us that organocopper compounds R n Cu n exist as aggregated species both in solution and in the solid state, and that this situation is quite comparable with that encountered in organolithium chemistry. This notion has helped us enormously in the synthesis of novel aggregated organocopper species that contain next to alkyl, aryl, alkenyl or alkynyl anions also alkoxy, thiolate, halide or cyanide anions.…”

“…X ¼ halide can be substituted for X ¼ acetylide with retention of the initial [Cu 6 R 4 ] 2þ -core structure. Without going into detail, the N-donor substituent and 2e-3c bonded C ipso are playing a crucial role during the assemblage process and provides the spatial arrangement by each bridging one Cu 3 -face of the Cu 6 -bipyramide that leads to the observed stability of Cu 6 R 4 X 2 aggregates [4].…”

Quantum chemistry of organocuprates as intermediates of catalytic and photochemical reactions

“…[3] Some experimental evidences of formation of adducts between copper complexes and organic radicals were obtained, [4][5][6][7][8][9][10][11][12][13][14] but the information on their structure was very scarce. Over the last years organometallic chemistry of Cu(I) and Cu(III) was vastly in progress [15,16] in contrast to Cu(II); only few stable organocuprates (II) are known to date. [17][18][19][20][21][22][23][24] Meanwhile, due to spectral data not a few labile transients, probably compounds with Cu(II)AC bond, may form in conditions of catalytic and photochemical reactions or directly by reaction of Cu(I) complexes with organic radicals.…”