Carbonyl displacement silanes

Lastly, Antilla has disclosed a novel asymmetric desymmetrization of a wide range of aliphatic, aromatic, and heterocyclic meso-aziridines with TMS-N3 promoted by 11 and related 12 (Scheme 5.31) [56]. Uniquely, this is one of only several reports of electrophilic activation of nonimine substrates by a chiral phosphoric acid. Mechanistic studies suggest that silylation of 11 or 12 by displacement of azide generates the active catalytic species A. Consequently, the aziridine is activated through coordination of it carbonyl with chiral silane A to produce intermediate B. Nucleophilic ring opening by azide furnishes the desymmetrized product and regenerates 11 or 12. 

Mechanistic studies have been carried out for neutral and cationic Cu systems [12,13b]. The proposed mechanism for [Cu(Cl)(NHC)j complexes involves the formation of [Cu(0 Bu)(NHC)] by reaction of the chloride complex with the base (Scheme 8.3). [Cu(H)(NHC)j would be formed in situ by o-bond metathesis between the terf-butoxide copper complex and the hydrosilane. The hydride copper complex is highly unstable (observable by NMR) however, it is the active species. Hence, by addition of the hydride species to the carbonyl, a second o-bond metathesis with the silane affords the expected silyl ether and regenerates the active catalyst. In the case of cationic derivatives, dissociation of one NHC occurs as the first step, which is displaced by the fert-butoxide moiety, and is the direct precursor of the active species. The hydrosilane is activated by the nucleophilic NHC, leading to the formation of the silyl ether. The activation of the silane appears to be the decisive step for this transformation. 

The oxygen atoms of acyl silanes resonate at higher chemical shifts than do those of the analogous carbon compounds (Table 4). The displacement to higher chemical shifts (i.e. downheld) of and carbonyl group signals in acyl silanes has been attributed to lowering of the HOMO-LUMO gap Partial reversal of this effect is possible by... 

Acyl silanes are extremely sensitive towards nucleophiles and nucleophilic bases for example, alcoholic solutions of benzoyl triphenylsilane containing a trace of aqueous hydroxide ion rapidly produce triphenylsilanol and benzaldehyde . Three reasonable mechanisms may be conceived for this reaction (Scheme 60) Sat2 displacement at the silicon atom (path A) nucleophilic attack at the carbonyl carbon atom followed by Brook rearrangement, initially to give a hemiacetal (path B) and nucleophilic attack at the silicon atom to form a pentacoordinate silicon anionic intermediate, followed by migration of the nucleophile to the carbonyl group and subsequent Brook rearrangement (path C). 

The easiest way to make oxidizable silanes is by condensation of an organolithium with the inexpensive dimethyldimethoxysilane. If enolates have to be silylated, it may be preferable to use the more reactive chlorodimethylalkoxysilanes. Introduced by Dieter Enders, a-lithiated RAMP- and SAMP-hydrazones (e.g., 78) are enolate-like species having an impressive track record for stereocontrolled synthesis. The chiral auxiliary enables the stereoselective introduction of the silicon substituent. Having oxidatively cleaved the hydrazone to restore the original carbonyl function, the latter may be diastereoselective reduced to either an (/ )- or (5)-alcohoI. The ultimate silicon/oxygen displacement thus produces either a meso- or a dl-Aio (Scheme 1-55). ... 

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