Coupling of organosilanes

The pioneering work of Hiyama has demonstrated that organosilanes (suitably functionalized) in the presence of a nucleophilic activator can undergo Pd-catalyzed cross-coupling reactions. The chlorosilanes, fluorosilanes and alkoxysilanes are used to couple with a variety of electrophiles. 

It is a palladium-catalyzed cross-coupling reaction between organosilanes (vinyl, ethynyl and allylsilanes) and organic halides (aryl, vinyl and allyl halides). Allylpal-ladium chloride dimmer [( ri -C3H5PdCl)2] and either tris(diethylamino)sulfonium difluorotrimethylsilicate (TASF) or tetra-n-butylammonium fluoride (TBAF) are used as catalysts. Fluoride ion acts as an activator for the coupling, forming an intermediate hypervalent anionic silicon species, which can then transmetallate with palladium as a preliminary reaction to coupling. 

Stereospecificty and regioselectivity of the reaction are noteworthy. The reaction proceeds with the retention of the double bond geometry of the vinyl halide. 

The mechanism for this transformation is not clear, but one could speculate that the reaction proceeds via a transmetallation pathway similar to fluoride-induced silicon to palladium transmetallation mechanism. 

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Sodium and magnesium do not react with tetrachlorosilane at room temperature, but do so at elevated temperatures and ia the presence of polar aprotic solvents at moderately elevated temperatures. The Wurtz-Fittig coupling of organosilanes to form disilanes (168) and polysdanes (169) is usually accomphshed usiag molten sodium ia toluene or xylene. 

Coupling of alkylsilanes with aryl triflates.3 Coupling of organosilanes with triflates is possible with fluoride ion (2 equiv.) and a Pd(0) catalyst. The striking... 

The Hiyama cross-coupling of organosilanes is attractive as the intermediates are often easy to prepare and the silicon by-products are environmentally benign. A one-pot synthesis of 2-aryl-3-methylpyridines from 2-bromo-3-methylpyridine was developed (Scheme 25) <2003JOM58>. Both 2- and 3-bromopyridine cross-couple with phenyltrimethoxysilane to afford the corresponding phenylpyridines in good yield <19990L2137>. 

Coupling of organosilane derivatives to RX species is known as the Hiyama reaction. This process is seen as more difficult to catalyze than the Stille reaction since a C-Si bond is less reactive than a C-Sn bond. However, addition of additives, such as B114NF or CsF, that form hypervalent silicon intermediates, or the use of more reactive silane precursors, such as Me3SiSiMc3, ArSiMc20H, or Cl2MeSi(vmyl), can overcome this limitation (equation 29). 

Hiyama coupling is an efficient protocol for the synthesis of biaryl derivatives. A very fast (5 min) Hiyama coupling of organosilanes with aryl bromides or iodides in aqueous medium at 90 °C, using Pd nanoparticles, has been reported recently by Ranu and his co-workers (Scheme 5.27). 

Coupling of organosilanes with aryl halides. Disilanes are split and coupled to aryl halides under rather vigorous conditions. When alkyltrifluorosilanes are used, the coupling also requires BU4NF as a promoter. 

The couplings of organosilanes discussed so far are carried out in the presence of more than equimolar amounts of promoters, typically TBAF, which is expensive. 

Palladium(O) mediates the coupling of organosilanes and organoiodides in the Fliyama reaction. 

Further studies quickly revealed that the rapid dehydrogenative coupling of primary organosilanes to oligomers and the slower coupling of secondary silanes to dimers can be effected under ambient conditions with compounds of the type CP2MR2 (M = Ti, R = alkyl M = Zr, R = alkyl or H)(11,12,13). None of the other metallocenes, metallocene alkyls, or metallocene hydrides of groups 4, 5 or 6 have shown any measurable activity for polymerization... 

Oxidative nickel-catalyzed coupling of aldehydes and alkynes to generate allylic alcohols. Intermolecular and intramolecular examples are both effective, and the transmetalating agent (MR" ) may be an organosilane, organoborane, organozinc, or alkenylzirconium. ... 

Fig. 5.3. Functions of a coupling agent (a) hydrolysis of organosilane to corresponding silanol (b) hydrogen bonding between hydroxyl groups of silanol and glass surface (c) polysiloxane bonded to glass surface (d) organofunctional R-group reacted with polymer. After Hull (1981).

In 1996, Hiyama and co-workers reported on the cross-coupling of activated aryl chlorides with aryl- and alkenyl-chlorosilanes 71 (Figure 16). The high temperatures required to activate the aryl chlorides did not affect the organosilanes an added advantage that can be attributed to their relative inertness. The system could be catalyzed by a variety of phosphine-bearing palladium complexes in the presence of either KF or TBAF as promoters. 

H. Cross-coupling Reactions of Organosilanes in the Presence of Palladium Catalysts... 

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