Oxidative homocoupling reactions

Novel heteroquaterphenoquinones were synthesized by a stepwise cross-coupling reaction or by a more convenient one-pot oxidative homocoupling reaction of the heterocycle-substituted phenols (Scheme 20, <96JOC4784 see also 95TL8055>). 

For the development of the oxidative homocoupling reaction, in 1955 Chodkiewicz and Cadiot explored a Cu(I)-catalyzed heterocoupling reaction of terminal alkynes with 1-bromoalkyne in the... 

Some of the most useful synthetic transformations of terminal alkynes involve intermolecular and intramolecular homo- and cross-coupling reactions between their. sp-carbon centers, leading to butadiyne or polyyne derivatives. The two most widely used and practical systems are (i) oxidative homocoupling reactions, i.e. Glaser and Eglington reactions and (ii) heterocoupling reactions, i.e. Chodkiewicz-Cadiot coupling of a terminal alkyne with a haloalkyne. 

JS2 OXIDATIVE HOMOCOUPLING REACTIONS OF TERMINAL ALKYNES... 

Pd-Catalyzed Aryl-Aryl Bond Formation via Oxidative Homocoupling Reaction. 170... 

Arylpyridines underwent palladium-catalyzed regioselective oxidative homocoupling reactions through chelation assistance, using [Pd(OAc)2] as catalyst and Oxone as stoichiometric oxidant in 2-propanol as solvent (Scheme 9.48) [124], Interestingly,... 

As the previous chapters have demonstrated, chiral auxiliaries have found a widespread application in the asymmetric synthesis of lignans. Among them, chiral oxazolidinones have been used extensively due to their ability to produce excellent diastereoselectivities in aldol as well as in numerous other reactions. For example, Kise et al. reported the use of (5)-4-isopropyl-3-(phenylacetyl)-2-oxazolidinone (141) in oxidative homocoupling reactions and its application in the asymmetric synthesis of dibenzylbutyrolactones and dibenzylbutandiols, Scheme (26) [86,87]. Treatment of 3-arylpropanoic acid derivative 142 with LDA in the presence of TiCU yielded a mixture of the dimeric compounds 143 in a ratio of 85 15 to 87 13. The major product having (R,R) configuration was converted into dibenzylbutyrolactones 145 in a three step sequence... 

Interestingly, the chemoselectivity of this catalytic system changed dramatically when using aryl oxazolines along with substituted allyl acetate 104. Thus, oxidative homocoupling reactions occurred under these reaction conditions (Scheme 9.37) [50]. Imidazole, pyrazole or thiazole derivatives could also be efEciently homocoupled. 

Ponomarenko and Kirchmeyer synthesized didecyloUgothiophenes 2.49b by employing Kumada cross-coupling and/or Cu-catalyzed oxidative homocoupling reactions [112]. Due to the formation of high order in its LC phase, a,a -didecylquaterthiophene 2.49b (n = 2) is reported to be the most promising material for solution processable OFETs as compared with quinque- or sexithiophene derivatives 2.49b (n = 3, 4). 

The oxidative homocoupling reaction of organometallic reagents provides a simple method for synthesizing symmetrical biaryls. However, the reaction has not been extensively studied because the dimerization of aryl halides, rather than arylmetal reagents, is synthetically a straightforward route to such biaryls. 

Three oxidative reactions of benzene with Pd(OAc)2 via reactive rr-aryl-Pd complexes are known. The insertion of alkenes and elimination afford arylalk-enes. The oxidative functionalization of alkenes with aromatics is treated in Section 2.8. Two other reactions, oxidative homocoupling[324,325] and the acetoxylation[326], are treated in this section. The palladation of aromatic compounds is possible only with Pd(OAc)2. No reaction takes place with PdCl2. 

The oxidative homocoupling of benzene with Pd(OAc)2, generated in situ from PdCl2 and. AcONa, affords biphenyl in 81% yield. In the absence of AcONa, no reaction took place. Pd(OAc)2 itself is a good reagent for the coupling[324-326]. The scope of the reaction has been studied[327,328]. 

Abstract This chapter highlights the use of iV-heterocyclic carbenes as supporting ligands in arylation reactions different than the more common cross-coupling reactions, including C-F bond activation, catalytic arylation, homocoupling, direct arylation and oxidative Heck reactions. 

Ranu and Banerjee developed a [bmim][OH] TSIL for oxidative homocoupling of terminal alkynes to 1,4-disubstituted 1,3-diynes in atmospheric conditions using Cu(ii) without using either palladium catalyst, amines, oxidants or organic solvents. Significant advantages stated by the authors include fast kinetics, high yields and mild reaction conditions. 

Oxidative homocoupling of aromatic and heteroaromatic rings proceeds with Pd(OAc)2 in AcOH. Biphenyl (165) is prepared by the oxidative coupling of benzene [104,105], The reaction is accelerated by the addition of perchloric acid. Biphenyl-tetracarboxylic acid (169), used for polyimide synthesis, is produced from dimethyl phthalate (168) commercially [106], Intramolecular coupling of the indole rings 170 is useful for the synthesis of staurosporine aglycone 171 [107]. 

Scheme 1. General description of (a) the oxidative Glaser-type homocoupling reaction and (b) nonoxidative Cadiot-Chodkiewicz heterocoupling (X= Br, I).

As already mentioned, there have been few mechanistic examinations of the copper-catalyzed Cadiot-Chodkiewicz heterocoupling reaction. Kinetic studies with the less reactive chloroalkynes [11a] have led to the assumption, shown in Scheme 7, that coupling between alkynes and haloalkynes proceeds through initial formation of copper(I) acetylides, probably formed by an acetylenic activation process similar to that described above for oxidative homocouplings. Subsequently, two reaction pathways may be reasonable ... 

The last two side reactions are fairly easy to control. The reductive dehalogenation appears to be caused by the concomitant oxidation of a primary alcohol (either from the boronic ester or from an alcohol cosolvent). Consequently, it can be minimized simply by utilizing boronic esters from tertiary alcohols (such as pinacol) and by avoiding the use of alcohol cosolvents. Since oxidative homocoupling is facilitated by ambient oxygen, the impact of this side reaction... 

The same research group has recently reported that the oxidative homocoupling of chiral aroylacetic acid derivatives proceeds stereoselectively when the sodium enolate derived from 38 is oxidized with bromine (equation 21). Good stereoselectivity was also observed in the oxidative homo- and heterocoupUng reactions of the lithium eno-lates of chiral 3-phenylpropionamides with iodine, copper(II) pentanoate and ferrocenium hexafluorophosphate. ... 

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