Aryl-copper complexes, substitution

Available information on the mechanism of cyclocondensation is rather contradictory. According to one hypothesis, both the condensation of aryl halides with copper acetylides and the cyclization occur in the same copper complex (63JOC2163 63JOC3313). An alternative two-stage reaction route has also been considered condensation followed by cyclization (66JOC4071 69JA6464). However, there is no clear evidence for this assumption in the literature and information on the reaction of acetylenyl-substituted acids in conditions of acetylide synthesis is absent. 

Bis(oxazoline)-copper complexes 158 have been used by Evans group as chiral catalysts for the enantioselective aziridination of olefins.116 Aryl-substituted olefins have been found to be particularly suitable substrates, which can be efficiently converted to A-tosylaziridines with ee of up to 97% (R = Ph... 

Ni complexes are used in place of copper metal and 2) for the preparation of highly substituted biaryls the use of preformed aryl copper species has been successful (Ziegler modification). °... 

Copper-mediated alkynylations of alkenyl- or aryl-substituted compounds should be considered as variations of the original Castro-Stephens reaction under catalytic reaction conditions. Copper(I) salts stabilized by aryl-substituted phosphines can mediate the coupling of aryl iodides in polar aprotic solvents using either conventional heating [87] or microwave irradiation [88]. More recently, weU-defined 1,10-phenanthroline-derived copper complexes were shown to be effective catalysts for the coupling of activated and deactivated aryl iodides at 10mol% catalyst loading in an apolar solvent (Scheme 6.29) [89]. 

As 1,4-disubstituted 1,2,3-triazoles are usually prepared through copper-catalyzed 1,3-dipolar cycloadditions of terminal alkynes with organic azides, the use of a single copper complex for a direct arylation-based sequential catalysis was probed. Thereby, a modular chemo- and regioselective synthesis of fully-substituted 1,2,3-triazoles was achieved (Scheme 9.43). Notably, the overall reaction involved the selective coupUng of four components through the formation of one C—C- and three C—N-bonds [58]. 

There has been a wide-ranging review of transmetalation reactions of arenes concentrating on the use of boron-containing compounds. The reaction of arylboronate esters and related compounds with alkyl halides is catalysed by copper(I) formation of an aryl—copper intermediate followed by an 5N2-type reaction with the alkyl electrophile is likely. Palladium complexed with a diimine is an excellent catalyst in the phenylation reaction of Michael acceptors with phenylboronic acid so as to yield products such as (16). The nickel-catalysed reaction of phenylboronic acid with styryl epoxides has been shown to yield a-substituted alcohols such as (17). 

Copper complexes have been used as reagents and as catalysts for the formation of carbon-carbon bonds. The most utilized reactions mediated by copper have been couplings of alkyl halides and sulfonates because copper complexes were unique for many years as reagents that would mediate the nucleophilic substitution of alkyl and aryl nucleophiles with alkyl halides. In recent years, work has been conducted to develop copper-catalyzed versions of cross couplings with aryl halides to address the issues of the cost of palladium catalysts. Although few examples of these processes currently rival those catalyzed by palladium complexes, they do illustrate the potential of copper complexes to catalyst these types of cross-coupling processes. 

Several types of nitrogen substituents occur in known dye stmetures. The most useful are the acid-substituted alkyl N-substituents such as sulfopropyl, which provide desirable solubiUty and adsorption characteristics for practical cyanine and merocyanine sensitizers. Patents in this area are numerous. Other types of substituents include N-aryl groups, heterocycHc substituents, and complexes of dye bases with metal ions (iridium, platinum, zinc, copper, nickel). Heteroatom substituents directly bonded to nitrogen (N—O, N—NR2, N—OR) provide photochemically reactive dyes. 

Especially in the early steps of the synthesis of a complex molecule, there are plenty of examples in which epoxides are allowed to react with organometallic reagents. In particular, treatment of enantiomerically pure terminal epoxides with alkyl-, alkenyl-, or aryl-Grignard reagents in the presence of catalytic amounts of a copper salt, corresponding cuprates, or metal acetylides via alanate chemistry, provides a general route to optically active substituted alcohols useful as valuable building blocks in complex syntheses. 

According to these conclusions, it is possible to propose a catalytic cycle (Fig. 20) involving no radical species, but a copper(I) complex with the classical oxidative addition, nucleophilic substitution and reductive elimination resulting lastly in the arylated nucleophile. 

The coupling of terminal alkynes with aryl or alkenyl halides catalysed by palladium and a copper co-catalyst in a basic medium is known as the Sonogashira reaction. A Cu(I)-acetylide complex is formed in situ and transmetallates to the Pd(II) complex obtained after oxidative addition of the halide. Through a reductive elimination pathway the reaction delivers substituted alkynes as products. 

The utility of a palladium catalyst in the synthesis of substituted aryl acetylenes is well established.(7,8,9,10) The end-capping agent I was produced by using a standard catalyst system, dichlorobls(triphenylphosphlne)palladlum (II)/copper (I) iodide/triphenylphosphlne mixture, which has been employed in previously developed ethynylation procedures.(10) The copper (I) iodide is believed to act as a cocatalyst, reducing the palladium (II) complex to the active palladium (0) catalyst. The scheme is shown in Figure 3 (diethylamine is the solvent).(11)... 

---