Silyl anions reaction with imines

Lithium Enolates. The control of mixed aldol additions between aldehydes and ketones that present several possible sites for enolization is a challenging problem. Such reactions are normally carried out by complete conversion of the carbonyl compound that is to serve as the nucleophile to an enolate, silyl enol ether, or imine anion. The reactive nucleophile is then allowed to react with the second reaction component. As long as the addition step is faster than proton transfer, or other mechanisms of interconversion of the nucleophilic and electrophilic components, the adduct will have the desired... 

Lithiation of compound 560 with s-BuLi-TMEDA in THF at —78 °C following an inverse addition protocol provided the anion 561. It reacts with primary alkyl iodides and triflates, silyl chlorides, diphenyl disulfide, epoxides, aldehydes, ketones, imines, acyl chlorides, isocyanates and sulfonyl fluorides to yield the expected compounds 562 (Scheme 152). The transmetallation of compound 561 with ZnBr2 allowed the palladium-catalyzed cross-coupling reaction with aryl and vinyl bromides837. When the reaction was quenched with 1,2-dibromotetrafluoroethane, the corresponding bromide 562 (X = Br) is obtained838. 

A method that makes available aromatic and aliphatic aldehyde derived sulfin-imines 47, for the first time, was recently introduced by Davis and co-workers.23,36 This one-pot procedure entails treatment of the Andersen reagent 40 with LiHMDS to generate 44 which subsequently reacts with the lithium methoxide by-product to produce silyl sulfinamide anion 46. Reaction of 46 with the aldehyde in a Peterson-type olefination reaction affords the sulfinimine 47 in >96% ee. This method was highly effective for the preparation of arylidene sulfmamides 47 (R = aryl) which were usually obtained in 60-76% yield although the alkyl counterparts... 

For acceptor-substituted alkynes, it is possible to use trimethylsilyl azide as transfer reagent (cyanogen azide does not react). The reaction (2-90) is not regiospecific, but the silylated triazoles 2.225 can be hydrolyzed and deprotonated to the anion 2.226. The latter reacts regiospecifically with cyanogen bromide to form the triazole-carbonitrile 2.227, which is in equilibrium with the a-diazo-A -cy-ano-imine 2.228 (Regitz et al., 1981 b). 

An interesting extension of this method involves the reaction of Af-silyl oxyketene imines derived from cyanohydrins (Scheme 19) [81]. By judicious selection of the protecting group on the oxygen, highly functionalized (3-hydroxy cyanohydrins can be accessed with high levels of enantio- and diastereoselectivity. These products can then be transformed into a diversity of structural motifs (amines, aldehydes, imines, ketones) important for the synthesis of polyketide and other classes of natural products. In addition, the ethers can be easily converted to enantiomerically enriched unsymmetrical benzoins, thus revealing the synthetic equivalency of A-silyl oxyketene imines as acyl anions (Scheme 19). 

The modification of the Peterson reaction using an N-trimethylsilylamide anion instead of an a-silyl carbanion offers a promising route to the corresponding imines. Treatment of N-(p-tolyl)-N-trimethylsilylamide anion with carbonyl compounds yields the corresponding ketimines [400]. In particular, LiHMDS has been utilized for the preparation of N-trimethylsilylimines, which are useful as masked imine derivatives in the synthesis of yS-lactam antibiotics [401-407]. Reactions of LiHMDS with non-enolizable aldehydes, enolizable aldehydes, ketones, a diketone, and a-keto esters give the respective imines (Scheme 2.153) [408-413]. Chloro-trimethylsilane is added to convert the generated lithium trimethylsilanolate into hexamethyldisiloxane. 

The reaction conditions for the synthesis of [Si2(pz )g] and [Si(pz )4] have been investigated in detail in order to obtain suitable precursor compoxmds for the synthesis of the hitherto unknown Janus-head ligand tris(3,5-dimethylpyrazolyl) silanide ([Si(pz )3] [241]. The X-ray stmctures of several pentacoordinated disilanes of type 95 with X = Cl or 3,5-dimethylpyrazolyl have been presented in this work. The chemistry of 3,5-dimethylpyrazolyl silicon complexes has been further explored. Although pyrazolide is an anionic substituent, its second N atom (the imine N atom of N-silylated pyrazole) also coordinates to silicon, thus exhibiting features of a neutral nitrogen donor as well. 

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