Coupling oxidative homocoupling

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]. 

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>). 

Although difluoroenoxysilanes are typical nucleophiles, they can also react with other nucleophiles via their oxidation into radical cations. The oxidative homocoupling and the cross-coupling with heteroaromatics and alcohols proceeds very well in the presence of Cu triflate as the oxidizing reagent (Figure 2.24). " ... 

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]. 

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 ... 

A wide series of oxidants, spanning from TiCLj to iodine, has been used in the oxidative homocoupling of chiral 3-arylpropionic acid derivatives aimed at the preparation of lignans. The /f,/f-selectivity in the reactivity of 34 has been explained by a radical coupling mechanism (equation 20). The initially formed lithium (Z)-enolate may transform into the titanium enolate 35, which undergoes oxidation to the radical intermediate 36 via a single electron transfer process. The iyw-Z-type radicals 36 couple each other at the less hindered S-side si face) to give the R,/f-isomers 37 stereoselectively. 

Cu-catalyzed Glaser coupling Preparation of symmetrical conjugated diynes and polyynes by the oxidative homocoupling of terminal alkynes in the presence of copper salts. 186... 

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. 

Intermolecular dehydrogenative oxidative homocouplings of (hetero)arenes turned out to be among the most important methods for the synthesis of symmetrically substituted biaryls [122]. A recent illustrative example is oxidative coupling reactions of 2-naphthols, which were accomplished in an asymmetric fashion employing an inexpensive iron catalyst (Scheme 9.47) [123]. 

Based on studies directed towards chelation-assisted oxidative homocouplings of 2-arylpyridines, a protocol was developed for intermolecular coupling reactions [132]. Thus, benzo h quinoline (186) was efficiently arylated with a variety of arenes using silver salts and benzoquinone as additives (Scheme 9.53). 

Oxidative biomimetic aryl coupling reactions of electron-rich (phenolic) aromatic hydrocarbons have occasionally been reported for selected examples see the reviews by Bringmann et al. [93 a] and the review on the ellagitannin chemistry by Quideau and Feldman [93b]. Oxidative homocoupling has also been achieved with palladium(II) acetate via the 5-arylpalladium complexes [78 d], but stoichiometric amounts of the palladium reagents are often required and acetylation can occur as a side reaction [93a]. 

The first method involves oxidative homocoupling of bis(terminal alkynyl) complexes in the presence of a catalytic amount of cuprous halides and O2 as the oxidizing agent (Scheme 16, Equation (58)). The use of this catalyst system in organic synthesis is extensive and is better known as Hay s coupling reaction. Extension of this methodology to organometallic synthesis was demonstrated by the conversion of... 

Three oxidative reactions of benzene with Pd(OAc)2 via reactive phenylpalla-dium acetate (397) are known. The insertion of alkenes and /3-H elimination afford arylalkenes 398. This topic is treated in Section 2.2.8. Two other reactions, oxidative homocoupling [157,158] and acetoxyladon [159] 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 affords biphenyl (399). The scope of the homocoupling reaction has been studied [160,161]. The reaction was applied to commercial production of biphenyltetracarboxylate (400) by coupling of dimethyl phthalate using Pd(OAc)2 and Cu(OAc)2. Addition of phenanthroline is important for the regioselective formation of the most important 3,4,3 4 -isomer [162]. 

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