Homocoupling reactions systems, arylation

Synthetic reactions involving activation of alkenyl- and arylsilanes by copper salts have also been reported. A copper-mediated system is effective in the cross-coupling reaction of aryl- and heteroarylsilanes with aryl halides under Pd-free conditions (Equation (13)).66 Alkenyl- and arylsilanes as well as alkynylsilanes undergo homocoupling in the presence of Cul and air.65,65a Functionalized alkenyl- and arylsilanes bearing a coordination site in the vicinity of silicon (e.g., 7) are easily activated by copper salts even at room temperature (Scheme 5).67,67l 67e 68 q-jle organocoppers such as 8 are useful for carbon-carbon bond-forming reaction with carbon electrophiles. 

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. 

The use of imidazolium salts for in situ catalyst formation was shown to be optimal for the coupling of TMS-protected alkynes even with sterically demanding aryl bromides and avoids the formation of homocoupling-derived products. For this reaction, Nolan reported that the activation of chlorobenzene by this catalytic system was possible in moderate yield [125] (Scheme 6.41). 

A second example from the same group is the synthesis of an elaborate diethynyltriphenylene derivative (Scheme 7 Table 8,entries 12,13) [58].Zn/Pd-promoted homocoupling of a 4-iodo-l,2-dialkoxybenzene furnishes the desired tetraalkoxybiphenyl, an electron-rich aromatic system. Iron trichloride-catalyzed Friedel-Crafts arylation of the biphenyl derivative with dimethoxy-benzene furnishes an unsymmetrical triphenylene derivative. Deprotection, oxidation, and subsequent Diels-Alder reaction with cyclohexadiene is followed by catalytic hydrogenation and reoxidation. TMS-CC-Li attack on the quinone delivers the alkyne modules, treatment with SnCl2 aromatizes the six-mem-bered ring, while KOH in MeOH removes the TMS groups cleanly to give the elaborate monomer. 

The formation of the undesired homocoupling product could be decreased using a larger excess of silane and reducing the reaction temperature to 60 °C. This catalytic system could also mediate the coupling of aryl halides with vinyltrimethoxysilane to form substituted styrenes in good yields. 

Additionally, the reactions with alkynes and iron catalyst were also applied to the carbonylative homocoupling of aryl iodides to give benzophenones [49]. As catalyst system, Fe(CO)5-Co2(CO)g was used under phase-transfer conditions to give carbonylate aryl iodides in moderate yields. 

Though the hydrogenation of the double bond is one of the basic catalytic reactions in Pd chemistry, httle has been published on aqueous variants of this process. Hydrogenation of o ,j8-unsaturated aldehydes in recyclable biphasic systems in the presence of TPPTS complex of palladium has been studied. - Also, a biphasic hydrogenation of nitrocompounds in the presence of PdCla and TPPTS under mild conditions has been reported. Room temperature homocoupling of aryl iodides was achieved in neat water or water-acetone mixture in the presence of Pd/C or Pd(OAc)2, and Zn dust (Scheme 87). The addition of 18-crown-6 was required for the reaction in neat water. The reactions are run under aerobic conditions. The effect of phosphine is negative. - ... 

To date, most research on NHCP transition-metal catalysis has been devoted to cross-couplings [13] or related reactions such as hydroarylation of alkenes [18], direct arylation of alkynes [17], or oxidative homocoupling of terminal alkynes [19]. All NHCP systems used in these studies feature one or two... 

The homocoupling of aryl halides was also studied in the presence of various palladium systems, and dimer (1) was found to be the best precursor. 7 For example, 4,4 diacetylbiphenyl was obtained in the presence of 2.5 mol % of 1 in 75% yield (eq 82). The use of tetrabutylammonium fluoride was crucial for the good outcome of the reaction. 

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