Copper-alkynyl species

Controlled oxidation of organometallic species may lead to the creation of a C-C bond. In acetylene chemistry, the Glaser reaction and its variations [29] are among the most ancient reactions of this type, and rely on the oxidation by air (or cupric ions) of alkynyl copper(I) species generated in situ. These very mild conditions allow the use of a large range of terminal alkynes, such as ethynylated heterocycles (Figure 7a) [26b]. 

Alternatively, carbene-complexed copper hydride species is involved in the transformation of alkynyl epoxides to allenyl carbinols using Cul, t-BuONa, PMHS and a imidazo-lium salt. ... 

This reaction process takes advantage of the ease with which a copper acetylide will oxidatively insert into an alkynyl halide bond. The postulated mechanism begins with and in situ base- and Cu(I)-induced formation of a copper acetylide (1) from a terminal alkyne (33). This intermediate undergoes oxidative addition into the activated C-X bond of an alkynyl halide (34) to afford the copper(III) species 35. Reductive elimination of the bis-alkyne 32 from complex 35 delivers the reaction product and regenerates the copper(l) halide 36 which may re-enter the catalytic cycle. 

The Cadiot-Chodkiewicz coupling typically proceeds under conditions which are considerably milder than Castro-Stephens reactions. Triethylsilylacetylene 74 rapidly undergoes Cadiot-Chodkiewicz coupling with alkynyl bromide 75 to generate the unsymmetrical bisalkyne 76 in nearly quantitative yield when those two reactants are treated with catalytic cuprous chloride and catalytic ammonium hydroxide in -butylamine solution. This coupling process affords one of the best entries into compounds such as 76 and is permissive of TES and larger silylated copper acetylide species because of the lower reaction temperature. ... 

The DPT calculations showed that the possible copper-copper cooperation effect is crucial for determining the catalytic efficiency of the present cycloaddition reaction. The azide interacts with the di-copper(I)-alkynyl species containing one a,Jt-bridging acetylide unit to form the monoalkynyl monoazido di-copper(I) intermediate. Then, the nucleophilic attack of the terminal nitrogen atom of an azide on the positively charged carbon atom of the acetylide species takes place to form the six-membered di-copper metallacycle intermediate (Fig. 6). This is the rate-limiting step for the present 1,3-dipolar cycloaddition, and the activation energy was calculated to be 59 kj moP, This is the first example for the remarkable enhancement of the catalytic activity by the construction of di-copper active sites. 

The reaction of 38 with nucleophiles gave a dinuclear species 39 in which each copper center is bonded to three C=C units, in contrast with the usual two-alkynyl-based bridge observed in many dinuclear alkynylcoppers.18 This... 

A different approach towards titanium-mediated allene synthesis was used by Hayashi et al. [55], who recently reported rhodium-catalyzed enantioselective 1,6-addition reactions of aryltitanate reagents to 3-alkynyl-2-cycloalkenones 180 (Scheme 2.57). In the presence of chlorotrimethylsilane and (R)-segphos as chiral ligand, alle-nic silyl enol ethers 181 were obtained with good to excellent enantioselectivities and these can be converted further into allenic enol esters or triflates. In contrast to the corresponding copper-mediated 1,6-addition reactions (Section 2.2.2), these transformations probably proceed via alkenylrhodium species (formed by insertion of the C-C triple bond into a rhodium-aryl bond) and subsequent isomerization towards the thermodynamically more stable oxa-jt-allylrhodium intermediates [55],... 

Better results can be obtained by generating the boronate species with the aid of sodium methoxide. In this case, satisfactory transmetalation occurs on treatment with Cul. Thus, the functionalized copper reagent 55 can be alkynylated with 1-bromo-l-hexyne at —40 °C, furnishing the enyne 56 in 75% yield (Scheme 2.15) [32]. 

Fig. 16.30. Pd(0)-catalyzed arytation of a copper acetytide at the beginning of a three-step synthesis of an ethynyt aromatic compound. Mechanistic details of the C,C coupling Step 1 formation of a complex between the catalytically active Pd(0) complex and the arylating agent. Step 2 oxidative addition of the arylating agent and formation of a Pd(II) complex with a cr-bonded aryl moiety. Step 3 formation of a Cu-acetylide. Step 4 trans-metalation the alkynyl-Pd compound is formed from the alkynyl-Cu compound via ligand exchange. Step 5 reductive elimination to form the -complex of the arylated alkyne. Step 6 decomposition of the complex into the coupling product and the unsaturated Pd(0) species, which reenters the catalytic cycle anew with step 1.

The copper(I) alkynyls displayed rich photochemistry and particularly strong photoreducing properties. The transient absorption difference spectrum of [Cu3(dppm)3(/X3-) -C=CPh)2]+ and the electron acceptor 4-(methoxycarbonyl)-A-methylpyridinium ion showed an intense characteristic pyridinyl radical absorption band at ca. 400 nm. An additional broad near-infrared absorption band was also observed and it was assigned as an intervalence-transfer transition of the mixed-valence transient species [Cu Cu Cu (dppm)3(/x3- -C=CPh)2] +. The interesting photophysical and photochemical properties of other copper(I) alkynyl complexes such as [Cu(BTA)(hfac)], 2 [Cui6(hfac)8(C=C Bu)8], and [Cn2o(hfac)8(CsCCH2Ph)i2] have also been studied. 

The mechanism consists of an oxidative addition of the organic halide 48 to the palladium(O) catalyst to give a palladium(II) intermediate 49 that undergoes transmetallation with the alkynyl copper species 50,... 

In the scheme the oxidative addition of aryl halide to a Pd(0) species gives an arylpalladium halide and the halide ligand is then replaced by an alkynyl group in the alkynyl-copper intermediate generated by interaction of the alkyne with cop-... 

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