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. 

Among the arylsilane reagents, ArSi(OR)3, where R = Me [130], Et [131], CH2CF3 [130], are the most convenient and easily available. The aryldialkylhydroxy- and aiylalkyldihydroxy-silanes, ArSiR20H and ArSiR(OH)2, where R = Me, Et, have also been used in the synthesis of biaryls [132]. Moreover, polysiloxanes bearing a phenyl... 

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. "" ... 

Li and co-workers also reported a highly efficient conjugate addition reaction with arylsilanes as nucleophilic reagents. The reaction of 2-cyclohexenone with 4 equiv. of either diphenyldichlorosilane or phenylmethyldichlorosilane in water generated the conjugate addition product in 97% and 95% yields, respectively (Scheme 55).143 An excess of sodium fluoride additive was important in this reaction. 

Platinum complexes, 39-43 Pnicogen atoms coordinated to E-E bonds, homoleptic dielement compounds with, 61-65 Polysilanes, 1-52 characterization, 34-37 mechanistic proposals, 13-18 Primary alkylsilanes, condensation, 23 Primary arylsilanes, 18-22 Primary silanes, 18, 22-24 condensation by metallocene catalysts, 19 Protic reagents, 170 2-Pyridylboronic esters, 223... 

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... 

The silyl halides can now" be prepared in high purity and high yield by the facile hydrogen halide cleavage of the carbon-silicon bond in arylsilanes. " No catalyst is required, and the use of the hazardous intermediate reagent, silane, is avoided. Bromosilane is prepared by the reaction of hydrogen bromide and phenylsilane. The latter is obtained by lithium hydro-aluminate reduction of the commercially available phenyltri-chlorosilane. lodosilane can be prepared in a similar fashion however, mixtures of iodosilane and benzene are difficult to separate. The preferred alternative procedure described below utilizes an isomeric mixture of 2-, 3-, and 4-chlorophenylsilanes as the intermediate. This intermediate is obtained by the chlorination of phenyltrichlorosilane, and is then reduced to the hydride. 

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]. 

Silanes are widely recognized as efficient reagents for reduction of carbonyl and heterocarbonyl functionality. In the case of alkyl and arylsilanes, the reaction requires catalysis by Lewis acids or transition metal complexes 1, 3] however, with more Lewis acidic trichloro or trialkoxysilanes, an altemative metal free activation can be accomplished. Thus, it has been demonstrated that extracoordinate silicon hydrides, formed by the coordination of silanes to Lewis bases, such as tertiary amines 7a], DMF [7b] or MeCN, and so on [7], can serve as mild reagents for the reduction of imines to amines [8]. In the case of trichlorosilane, an inexpensive and relatively easy to handle reducing reagent, and DMF as a Lewis basic promoter, the intermediacy of hexacoordinate species has been confirmed by Si NMR spectro scopy [7b]. 

Addition reactions. The same reagent system is active in hydroarylation and hydro-alkenylation of aUcynes. The arylsilanes submit the addends and thereby are converted into benzoxasiloles. ... 

A new route to arylsilanes which can be adapted to industrial scale utilizes chloromethyl-disilanes, which are byproducts in the reaction of methyl chloride with silicon. This new method is the most general of those currently available and is compatible with functional groups that cannot be used with Grignard reagents. The reagent required for this route is an acid chloride and substituents that are electron withdrawing increase the rate of the reaction. The general process is shown in equation 15160. 

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. 

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