Aryl trimethoxysilane

An efficient protocol for solvent-free N-arylation of N-heterocycles, including imidazoles and triazoles, by aryl trimethoxysilanes using FeCla/Cu as co-catalyst and TBAF as the base under air has been demonstrated by Li et al. Various imidazoles and triazoles are compatible with this procedure (Scheme 5.47). 

In the context of NHC/metal catalysed cross-coupling reactions, the only example of a Hiyama reaction was reported by Nolan using an in situ protocol by mixing Pd(OAc)j and IPr HCl for the formation of the catalyst. Activated aryl bromides and chlorides, such as 2-chloropyridine, were coupled with phenyl and vinyl-trimethoxysilane in good yields [123] (Scheme 6.39). 

Scheme 6.39 Hiyama coupling between activated aryl bromides/chlorides and phenyl/vinyl-trimethoxysilane...

Shibata and co-workers have reported an effective protocol for the cyclization/hydrosilylation of functionalized eneallenes catalyzed by mononuclear rhodium carbonyl complexes.For example, reaction of tosylamide 13 (X = NTs, R = Me) with triethoxysilane catalyzed by Rh(acac)(GO)2 in toluene at 60 °G gave protected pyrrolidine 14 in 82% yield with >20 1 diastereoselectivity and with exclusive delivery of the silane to the G=G bond of the eneallene (Equation (10)). Whereas trimethoxysilane gave results comparable to those obtained with triethoxysilane, employment of dimethylphenylsilane or a trialkylsilane led to significantly diminished yields of 14. Although effective rhodium-catalyzed cyclization/hydrosilylation was restricted to eneallenes that possessed terminal disubstitution of the allene moiety, the protocol tolerated both alkyl and aryl substitution on the terminal alkyne carbon atom and was applicable to the synthesis of cyclopentanes, pyrrolidines, and tetrahydrofurans (Equation (10)). 

Asymmetric reduction of alkyl aryl ketones with trialkoxysilanes is promoted by a catalytic amount of chiral nucleophiles [39]. The reactive species is a transiently prepared hypervalent silicon hydride. 2, 4, 6 -Trimethylacetophenone was reduced with equimolecular amounts of trimethoxysilane in the presence of the monolithio salt of (R)-BINAPHTHOL (substrate Li=20 l) in a 30 1 ether-TMEDA mixed solvent at 0 °C to afford the R product in 90% ee (Scheme 21) [40]. The presence of TMEDA was crucial to achieve high yield and enantiose-lectivity. Reduction of less hindered ketonic substrates preferentially gave the... 

Reduction The asymmetric reduction of a series of aryl alkyl ketones with quaternary ammonium fluorides and silanes was reported by Drew and Lawrence [55]. In these reactions, the best catalysts (e.g., 6f) were from the qui ni ne/quinidine series in fact, a fluoride salt prepared from cinchonine gave no induction. The use of trimethoxysilane resulted in faster rates but lower enantioselectivites when compared with tris(trimethoxy)silane. It is interesting that, with the... 

Aryltrimethoxysilanes, for use in cross couplings, can be prepared by a palladium-catalysed reaction of aryl iodides with trimethoxysilane, but this silane is extremely toxic ... 

Clarke recently reported the first MW-accelerated Hiyama coupling [89, 90]. It was noted that the availability and nontoxic attributes of the organosilicon reactants make them very attractive in synthesis, but their low nucleophilicity limits their potential. Use of microwaves enabled aryl bromides and activated aryl chlorides to react under palladium catalysis with an electron-rich N-methyl piperazine/cyclohexyl phosphine ligand (Scheme 15.48). A vinylation reaction with vinyl trimethoxysilane was also reported [90]. 

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