Hypervalent silicates

It was found that treatment of a mixture of 120 and 121 with tris(diethylamino-sulfonium) trimethyldifluorosilicate [TASF(Et)] resulted in smooth addition-elimination to the naphthoquinone to form the y-alkylation product 125 (85 %). TASF(Et) is a convenient source of soluble, anhydrous fluoride ion [47]. It is believed that exposure of 121 to TASF(Et) results in fluoride transfer to generate a hypervalent silicate anion, as depicted in structure 124. The transfer of fluoride between TASF(Et) and 121 may be driven by stabilization of the anionic species 124 by delocalization of the carbon-silicon bond into the LUMO of the unsaturated ketone. 1,4-Addition-elimination of this species to the naphthoquinone 120 would then form the observed product. 

Two patterns are possible in the activation mechanism by simple chiral Lewis base catalysts. One is through the activation of nucleophiles such as aUyltrichlorosilanes or ketene trichlorosilyl acetals via hypervalent silicate formation using organic Lewis bases such as chiral phosphoramides or A-oxides. " In this case, catalysts are pure organic compounds (see Chapter 11). The other is through the activation of nucleophiles by anionic Lewis base conjugated to metals. In this case, transmetal-lation is the key for the nucleophile activation. This type of asymmetric catalysis is the main focus of this section. 

Mowery and DeShong used the commercially available hypervalent silicate complex TBAT as a phenylating agent for the cross-coupling reaction with allylic esters. They later reported on the use of the same organosilane for the coupling with aryl iodides and triflates and electron-deficient aryl bromides. The reactions were catalyzed by either Pd(dba)2 or [Pd(allyl)Cl]2 without the need of added phosphine ligands. 

The Strecker reaction of silyl cyanide (H3SiCN) with benzaldehyde A-methylimine (PhCH=NMe), catalysed by an axially chiral 2,2 -bipyridine A,A -dioxide has been explored computationally as a model for the corresponding reaction using TMSCN, PhCH=NCH2Ph, and a biquinoline dioxide.70 The non-catalysed reaction is found to be concerted (via a five-membered ring TS), whereas the catalysis is stepwise, via a hexacoordinate hypervalent silicate. 

Alkynyl nucleophiles, (R10)3Si-C=C-R2, have been added to aldehydes, ketones, and imines, using a strong Lewis base, KOEt.212 Evidence for ethoxide attack at silicon, to give a hypervalent silicate intermediate, which then coordinates with the carbonyl (or imine), is presented. 29Si NMR is particularly informative when R1 = OEt, the alkyne silicon shows up at -72 ppm [similar to (EtO Si at -80 ppm], but a new peak is seen very far upheld at -126 ppm, beyond a similar known silicate, (EtO)4SiPhK at -117 ppm, and indeed close to (EtO Si-, at —130 ppm. 

The Fewis base-catalysed additions of alkynyl nucleophiles to aldehydes, ketones, and imines have been described.161 Mechanistic studies strongly indicated that the use of new triethoxysilylalkynes facilitate access of a reactive hypervalent silicate intermediate (Scheme 24). 

When Me3SiSnBu3 is treated with CsF, the fluoride anion should coordinate to the silyl group and not the stannyl group to produce a hypervalent silicate, and as a result, a stannyl anion would be generated.282 The stannyl anion reacts with vinyl iodide to produce a vinyl anion via a halogen-metal exchange and it reacts with the carbonyl group intramolecularly (Equation (113)). Aryl halides or allyl halides are also used in similar cyclizations.283,284... 

The intermediate and key species proposed for the reaction in Scheme 3.2c are hypervalent silicates based on the silicon NMR spectra of (Z)-crotyltrichlorosilane in DMF. This hypervalent silicate has sufficient Lewis acidity based on the electron-withdrawing chlorine groups as well as nucleophilicity due to electron donation from the hypervalent silicon atom to the allyl systems, which enables the reaction to proceed smoothly. Thus, the high levels of diastereoselectivity can be explained by a six-membered cyclic transition state (Scheme 3.2d). 

The secondary amine of the AEP group is responsible for the supported enamine formation with acetone (aldol reaction), the deprotonation of nitromethane (Henry reaction) and the generation of a potential nucleophile from trimethylsilyl cyanide through hypervalent silicate formation (cyanosilylation reaction). Therefore, the presence of both AEP and UDP groups in close proximity can cooperatively activate the electrophile (through hydrogen bond) and the nucleophile by enamine formation, thus enhancing the reaction rate. 

Nitriles by substitution. Glycosyl cyanides are formed from glycal esters (Pd-catalyzed reaction), whereas 8 2 reaction of alkyl halides requires fluoride ion to activate Me SiCN (hypervalent silicate). ... 

B. Smith, Azide and cyanide displacements via hypervalent silicate intermediates, J. Org. Chem., 64 (1999) 3171-3177. 

Feng developed a highly enantioselective cyanosilylation of ketones catalysed by L-phenylglycine sodium salt 54 to give the corresponding cyanohydrins (Scheme 2.34). H, and Si NMR analyses suggested the possible formation of hypervalent silicate species from the carboxylate ion of 54 and trimethylsilylcyanide. Introduction of i-PrOH greatly enhanced the reactivity without a loss of enantioselectivity. 

Recently, Levacher and co-workers reported on the total synthesis of two natural homoisoflavones using their asymmetric protonation protocol as a key step. Their concept focused on the use of chiral ammonium fluoride as a chiral ion pair to generate a naked enolate or a hypervalent silicate species, which might be trapped in an asymmetric concerted fashion by the proton coming from the chiral counter anion (Figure 31.1). 

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