Alkynylcopper compounds

It is interesting to note that a copper(II)-mediated coupling reaction of alkenyldialkyl- or trialkylboranes with alkynylcopper compounds, generated in situ, in the presence of various solvents and a small amount of water, gives (E)-l,3-enynes (or disubstituted alkynes) with various functional groups in reasonable yields (Eq. 4.3).12... 

Neutral alkynylcopper compounds are not prepared by transmetalation of alkynyllithium compounds. Rather, they are obtained by partially deprotonating terminal alkynes with amines and capturing the ammonium acetylide formed at equilibrium with Cul (—> R-C=C-Cu + R3NH I example Figure 16.7). Copper(I) cyanide couples with aryl iodides and -bromides in a similar fashion as alkynylcopper compounds (which may well be conceived as their carba analogs). 

As Figure 16.7 reveals, an aryl iodide reacts more rapidly with an alkynylcopper compound than an aryl bromide. The palladium-catalyzed C,C coupling reactions, which will be discussed later in the present chapter, also proceed more rapidly with an aryl iodide than with an aryl bromide (example Suzuki coupling in Figure 16.22) or an aryl chloride (example Stille reaction in Figure 16.27). There are still some details that are not fully understood one is inclined to assume that in accordance with the Hammond postulate the weaker C-I bond (dissociation energy DE = 51 keal/mol) breaks more rapidly with the initial oxidative addition... 

We have found that alkynylsilanes are smoothly converted into alkynylcopper compounds by treatment with CuCl in l,3-dimethyl-2-imidazohdinone (DMI) the copper reagents can be isolated in good yields [563]. This study was the first example of preparation and isolation of organocopper compounds by use of organosilicon reagents. The Si-Cu transmetalation is applicable to the synthesis of alkynyl ketones by Cu-catalyzed alkynylation of acid chlorides (Scheme 10.217). We have also shown that a Cu-mediated system is effective in the cross-coupling reaction between arylsilanes or heteroarylsilanes and aryl hahdes [564]. 

The advantage of the process is that prior preparation of alkynylcopper compounds is not required but use of copper halide in the presence of an amine suffices for driving the catalytic process. The catalytic cycle is shown in Scheme 1.7. 

Alkynylcoppers constitute a class of compounds relevant to several synthetic organic reactions,47 where they have been proposed as key intermediates. The interest in this area has supposed that the number of structurally characterized alkynylcopper complexes has considerably expanded in the last few years. The most common route toward alkynylcoppers is based on the reaction of a terminal alkyne with a copper source, either a salt or an organocopper compound (Equations (8) and (9)). 

Oxidative addition of aryl (or vinyl) halide to Pd(0) precursor forms the monoarylpalladium complex that is the common intermediate in the catalytic cross-coupling reactions of haloarene with organometallic compounds of main group elements such as Mg, Si, and Sn. Alkynylcopper, formed from alkyne, Cu(I) salt and base in the reaction mixture, transfers the ligand to the above Pd complex, giving an intermediate complex with aryl (or vinyl) and alkynyl ligands bonded to Pd. Reductive elimination of arylacetylene (or enyne) occurs... 

The other organometallic compounds, such as alkynylcoppers, alkenylzincs, and alkenylzirconocenes, have also been utilized for the tran -selective couphng reaction of 1,1-dihaloalkenes (Scheme 4). The monoalkynylation of 1,1-dichloroethylene in the presence of palladium and copper catalysts is troublesome. Since dialkynylation is not easily suppressed, an excess amount of the substrate, 1,1-dichloroethylene, is required to produce the desired product. 

Organocopper compounds, RCu, though less reactive toward acid halides, couple to give the corresponding ketones as well. Reaction of alkynylcoppers with acyl halides produces acetylenic ketones in good yields (Normant and Bourgain, 1970). 

The most frequently encountered and generally robust methods for the synthesis of alkynylpalladium(ii) complexes are copper(i)-catalyzed dehydrohalogenation and transmetallation involving isolated organotin(iv) compounds or well-defined alkynylcopper(i) species (Scheme 2). 

Oxidative addition of alkenyl halides, triflates, and other esters to zerovalent palladium compounds has been long known as a viable route to palladium(ii)-alkenyl complexes. Stereospecific coupling reactions involving mono- and (E)- or (Z)-dihalo-alkenes with palladium-copper catalysis under modified Sonogashira conditions are quite versatile and useful. These proceed through oxidative addition of the alkenyl halide via palladium(ii) alkenyl complexes, followed by coupling with a nucleophile (usually an alkynylcopper reagent obtained in situ with co-catalytic copper(i) from terminal alkynes in the presence of, in this case, a base like piperidine instead of diethylamine). ... 

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