Aryl with arylsilane reagents

The cross-coupling reaction of aryl halides with arylsilane reagents is an important alternative aryl-aryl bond forming strategy. Thus the reaction of various aryltrialk(ox)ylsilanes readily proceeds with aryl iodides [130] and bromides [131] in the presence of palladium(0)-catalyst to give the biaryls in good to excellent yields. Scheme 32. 

Coordination-assisted aromatic arylation reactions using ruthenium [122-125] and rhodium [125, 126] catalysts have been reported. In the reactions, substrates having a neutral heteroatom such as pyridines and aromatic imines are usually employed and are reacted with aryl halides or arylmetal reagents. The palladium(II)-promoted arylation reaction of pyridines and imines having a tert-hutyl group with arylsilanes is also known (Eq. 51) [127]. The reaction proceeds via initial transmetalation to form a PhPd(II) species, which is coordinated by the substrate, and then one of the aliphatic C-H bonds is activated. 

The hydrosilylation of unsaturated carbon-rhodium-catalyzed silylcarbocyclizations. In the presence of Rli4(CO)i2 and triethoxysilane, a rigid triyne backbone can undergo a silylcarbotricyclization cascade reaction to yield [5,6,5]-tricycles (eq 16). Similar to the results observed by Sieburth for the hydrosilylation of enamines, the alkoxysilane functionality provides significant rate enhancement in comparison to silylcarbocyclizations using alkyl- and arylsilane reagents. The incorporation of carbonyl functionality as terminal electrophiles into these cyclizations has also been successful. Rhodium-catalyzed carbonylative silylcarbocyclizations proceed in the presence of carbon monoxide (10 atm) to incorporate a carbonyl unit, usually as the aldehyde. Both of these tandem ad-dition/cyclization strategies produce functionalized carbocycles with simultaneous incorporation of sUyl functionality as aryl- and vinylsilanes. These alkenylsilanes can then be exploited for further synthetic manipulations as discussed above. "" ... 

In a limited number of cases, arylsilanes react with aldehydes as if they were aryl Grignard or aryllithium reagents. Both trimethyl(perchlorophenyl)silane and trimethyl(perfluoro-phenyl)silane react with benzaldehyde to give the corresponding 7.-(pcrhalophenyl)bcnzyl tri-methylsilyl ethers.163 Benzaldehyde reacts completely with trimethyl(perfluorophenyl)silane in diethyl ether in the presence of either a catalytic amount of the potassium cyanide/18-crown-6 complex in less than 5 hours at room temperature or potassium fluoride in dimethylform-umide.164 In the case of aryltrimcthylsilanes containing electron-withdrawing substituents in the ortho position, the reaction is observed only under the conditions of nucleophilic catalysis by potassium fluoride or cesium fluoride. 

Alkenylsilanes and -stannanes, and arylsilanes and -stannanes are useful reagents for transfer of an sp -carbon unit to electrophiles under titanium catalysis. Epoxides are opened by TiCE to generate cationic carbon, which is successfully trapped with bis(trimethylsilyl)propene as an aUcenylsilane (Eq. 122) [305]. Other Lewis acids, for example ZnCla, SnCU, and BF3 OEt2, proved less satisfactory. Cyclic epoxides such as cyclopentene and cyclohexene oxides gave poorer yields. An intramolecular version of this reaction proceeded differently (Eq. 123) [305]. Eqs (124) and (125) illustrate diastereoselective alkenylation and arylation of (A,0)-acetals that take advantage of the intramolecular delivery of alkenyl and aryl groups [306], Cyclic ethers... 

We have found that alkynylsilanes are smoothly converted into alkynylcopper compounds by treatment with CuCl in l,3-dimethyl-2-imidazohdinone (DMI) the copper reagents can be isolated in good yields [563]. This study was the first example of preparation and isolation of organocopper compounds by use of organosilicon reagents. The Si-Cu transmetalation is applicable to the synthesis of alkynyl ketones by Cu-catalyzed alkynylation of acid chlorides (Scheme 10.217). We have also shown that a Cu-mediated system is effective in the cross-coupling reaction between arylsilanes or heteroarylsilanes and aryl hahdes [564]. 

Similar palladium-catalyzed arylation reactions using arylsilane and arylzinc reagents are also possible by the judicious choice of bases and additives. As shown in Equation 5.9, the addition of a fluoride donor BU4NF promoted efficient cross-couplings of primary aUcyl bromides and iodides with aryltrimethoxysilanes [12]. While the reaction showed good functional group compatibility, electron-deficient arylsilanes, such as 4-fluorophenyltrimethoxysilane, showed a rather poor reactivity. 

Ligand exchange of monoaryl-A -iodanes with an arylsilane or arylstannane leads to analogous diaryl reagents, which have unique uses. For example, diaryl-A -iodanes bring about a-arylation of enolates, as shown below ... 

Direct Arylation of Heteroarenes, Aryl silicon reagents were employed to directly arylate heteroarenes via a Pd-catalyzed, ligand-free process. Silver(I) fluoride was the most effective source of fluoride (93%) compared with FeFs, KF, wBu4NF-H20, and CsF, although CuF2 gave a comparable performance (82%). In addition to activating the arylsilanes, silver(I) fluoride acts as a co-oxidant with Cu(0Ac)2-H20 to oxidize Pd° to Pd. The electron-rich and electron-deficient aryl (trimethoxy)silanes could be coupled to access arylated benzothiazoles in excellent yields (83-93%) (eq 19). 

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