Homocoupling reactions scope

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]. 

Neither the synthetic scope nor mechanism was significantly investigated until 1935, when Baddeley and co-workers provided new insights into the Scholl reaction. Their work provided the first mechanistic interpretation, suggesting that the homocoupling reaction of naphthalene proceeds via an arenium cation intermediate. Based on experiments in... 

Earlier this year, the Grubbs group reported the preparation of the Ru-based catalyst with a chelating iV-heterocyclic carbene (NHC) ligand that catalyzes highly Z-selective olefin metathesis (Fig. 31) [69, 70]. This catalytic system provided similar levels of efficiency and selectivity to the W-alkylidene complexes for homocoupling reactions. The reason for the Z-selectivity is not clear at this point. Extension of the substrate scope of this catalytic system is expected. 

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]. 

Suzuki [201] reported in 2007 that pyridines, which are electron-deficient heteroarenes, could undergo a homocoupling by dinuclear ruthenium complexes. For example, Cp Ru( a-H)4RuCp and (Cp Ru)2(]i.-PMe2)(]i.-H)( a- ) 7 -C5H 5) [202] promote the dimerization of 4-methoxypyridine to give 4,4 -dimethoxy-2,2 -bipyridine. Although the reaction suffers some limitations such as stoichiometric Ru complexes, high temperature, and narrow substrate scope, this is so far the only report in which simple pyridines can be homocoupled at their C-H bonds. 

Naphthol is another electron-rich aromatic compound which can also lead to sp -sp CDC-type products. Thus, a new type of Betti base was formed via the CDC reaction of N-phenyl-THIQ with 2-naphthol derivatives under our CuBr/TBHP system with a small amount of homocoupled 2,2-binaphthol (BINOL) (Scheme 1.10). Subsequently, the scope of cross-dehydrogenative Friedel-Crafts type aiylations was significantly improved by the development of highly efficient catalyst systems, and an intramolecular Cu-catalyzed aerobic synthesis of functionalized cinnolines via a Friedel-Crafts-type CDC aiylation was reported by Zhang et al. ... 

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