Coupling of arylsilanes

Phenyltrimethoxysilane 75 is activated by fluoride anion to 76, which undergoes palladium-catalyzed couphng with 4-methyliodobenzene 77 to give a nearly quantitative yield of 4-methyldiphenyl 78 [90]. Such Heck-, Stille-, or Suzuki-type C-C coupling of arylsilanes such as 75 were recently reviewed [91] (Scheme 2.12). 

Scheme 36. Pd-catalyzed cross-coupling of arylsilanes with aryl chlorides [129]...

Scheme 4.7 The Au-catalyzed oxidative coupling of arylsilanes with arenes as described by Lloyd-Jones and coworkers [32].

The coupling reaction of arylsilanes with aryl iodides is also mediated by a palladium catalyst and a fluoride ion [23,24]. Optimized reaction conditions are as... 

Allyl carbonates (Scheme 5) and diene monoxides (Eq. 32) were also employed in the palladium-catalyzed coupling reaction of arylsilanes 136, 37]. The reaction does not require activation by a fluoride ion or an additional base like a hydroxide ion. 

Lee and Fu reported that the TBAF-promoted, Pd-catalyzed coupling of alkyl halides with arylsilanes gave fairly good yields (65-81%) of the alkylated arenes at room temperature. The catalyst was formed in situ from the electron-rich phosphine P(f-Bu)2Me and PdBr2 (eq 30). TBAF was used in excess. 

The coupling reaction of arylsilanes with aryl iodides is also mediated by a palladium catalyst and a fluoride ion.f " Optimized reaction conditions are as follows (i) two fluorine atoms on silicon are required (ii) an ethyl or propyl group as a dummy alkyl ligand is preferred, because a methyl group competitively participates in the crosscoupling reaction and (iii) TBAF, a highly effective fluoride ion source, can be replaced by inexpensive KF. Various unsymmetrical biaryls are synthesized under the conditions (Scheme 10). 

Cross-coupling reactions of arylsilanes with aryl halides... 

The only side-reaction of considerable interest is the homo-coupling reaction of the parent arylsilane. For that reason, Masuda s method is usually conducted with a slight excess, e.g. 20 mol%, of arylsilane. Once again, methods involving significantly higher excesses of arylsilanes cause several difficulties during the purification procedure of the final biaryl product. 

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. 

Protiodesilylation of the starting material is occasionally encountered in the coupling reactions of arylsilanes with a nucleophihc promoter. This side-reaction can be suppressed by the use of a stoichiometric amount of a copper(I) salt to promote the cross-coupling of arylsilyl ethers with iodoarenes (Scheme 7.36) [63]. The preferred salt is CuOCgFj, generated in situ by mixing Cul and NaOC Fj, to effectively promote this reaction. Unhke other arylsilyl ether cross-couphng reactions wherein a trialkoxysilane is used, the mono-methoxysilane reacted much more efficiently than either the di- or trimethoxysilane. Despite the harsh conditions used, this is an impressive fluoride- and paUadium-ffee reaction. 

Aroyl chlorides undergo silylative decarbonylation to give arylsilanes by the reaction of disilanes. Oxidative addition of aroyl chlorides and decarbonylation are followed by transmetallation with disilane to generate (aryl)silylpalladium, and its reductive elimination gives arylsilanes. For example, neat trimellitic anhydride add chloride reacted with dichlorotetramethyldisilane at 145 °C to afford 4-(chlorodimethylsilyl)phthalic anhydride by decarbonylation and reductive elimination. Also, trimellitic anhydride add chloride was converted to biphenyltetracarboxylic anhydride at 165 °C in refluxing mesitylene by reaction of the disilane. Thus, decarbonylation-coupling of aroyl chlorides offers a good synthetic method of biaryls (Scheme 7). ... 

Silylation of Aryl and Alkenyl Halides. Aryl halides can be successfully silylated in the presence of palladium and rhodium catalysts. The palladium(0)-catalyzed coupling reaction of triethoxysilane with aryl iodides and bromides was developed by Masuda and Murata (eq 21), and later improved on by DeShong. This is an efficient process in most cases, resulting in facile formation of arylsilanes. One limitation of this palladium(0)-catalyzed process is the decreased effectiveness in the presence of ort/zo-substituted and electron-deficient aryl halides. The successively developed rhodium(I)-catalyzed process by Murata and Masuda is broadly applicable to a wide range... 

Scheme 3-181. Cross-coupling of 2-(2-hydroxyprop-2-yl)cyclohexyl arylsilanes...

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