Silyl anion reagents

Use of main-group metal silyls to prepare transition-metal silyls appears to be a generally applicable method that is primarily limited by the availability of suitable starting materials, since these silyl anion reagents are sometimes rather difficult to obtain. Typically an alkali-metal silyl is generated in solution and then treated with the appropriate transition-metal halide. Displacement of halide by the silyl anion, with salt elimination, then leads to product (equations 17-19)46 49. The lithium silyl in equation... 

Early transition-metal silyl compounds have received much less attention, largely due to the fact that general, straightforward synthetic routes have not been available. Syntheses based on oxidative additions are less applicable, given the more electropositive character of the early metals. Particularly for d° silyl complexes, most syntheses are based on nucleophilic displacement of halide by a silyl anion reagent, usually an alkali metal derivative. 

On the other hand, the fluorine-induced addition of the diastereomeric silyl-subsliluted sulfides 36 A and 36B to benzaldehyde proceeds without loss of stereochemical information and with retention of configuration32. Since, however, the anionic reagent 35A/35B is known to be configurationally labile, the observed retention of configuration in the fluorine-induced desi-lylative hydroxy alkylation lends experimental evidence to the notion that these reactions proceed via hypervalent silicon species rather than anionic reagents. 

Germyl, Stannyl, and Plumbyl Anions The preparative methods for the synthesis of the germyl, stannyl, and plumbyl anions are essentially the same as those mentioned above for the silyl anions. The most widely used methods are (1) reduction of halides R3EX (R = alkyl, aryl E = Ge, Sn, Pb X = Cl, Br) with alkali metals and (2) reductive cleavage of the E-E bond of R3E-ER3 (R = alkyl, aryl E = Ge, Sn, Pb) with alkali metals or organolithium reagents. Due to the favorable polarization of the (E = Ge, Sn, Pb) bond, the direct metalation... 

The final product of the deprotonation depends strongly on the deprotonation reagent and/or the reaction conditions. Thus, in THF and using MeLi (or NaH) as base, an anionotropic l,3-Si,0-trimethylsilyl migration occurs in the alkoxymethylsilane 191 with formation of the silyl anion 192 instead of elimination of silanolate. Therefore, after hydrolytic work-up only trimethylsiloxy(bis(trimethylsilyl)silyl)alkanes 193 were obtained (equation 48).108,117... 

More recent examples of nucleophilic aromatic substitution reactions include the reactions of C6F6 with the superoxide ion, 02 to give F and, presumably, C6F502 278 and with the acetic acid enolate anion, as shown in Scheme 42, which also indicates how the anionic reagent was formed279. It should be noted that reaction of gas-phase F" with a suitably silylated precursor is one of the best and most specific reactions to prepare gas-phase anions280. 

As noted in Equation (46), thiasiliranide 86 was prepared through deprotonation of a silyl thiol. The reagent was prepared by sulfuration of an a-silyl anion and quenching with aqueous NH4CI <2002JOM504>. Thiastannirane 87 was prepared by a reaction analogous to Equation (52) <19930M4>. 

The alkali metal hydrides NaH and KH are also effective as metalating reagents for hydrosilane and disilane to give trialkylsilyl anions in an aprotic solvent such as THE, DME, or HMPA 8) (Scheme 4) (for the mechanism, see Section III-A-2). The reaction of disilane with 2 equivalents of metal hydride converts both silyl groups into silyl anions in two steps, in contrast to the reaction with MeOM described earlier. 

Certain silyl anions are useful silylating reagents in organic synthesis. Since some leading reviews have covered the synthetic applications Ud,e/,2), only a few typical examples are mentioned here. 

Reactions of silylcuprates provide additional examples of a silyl anion based mechanism (Scheme 3). The reagent (19), prepared from silyllithium and copper(I) cyanide, reacts with a C=C bond to give, after aqueous work-up, c/j-hydrosilylated products (20). Conjugate addition of (21) to a,P-unsaturated... 

Since the preparation of anionic species or organosilicon compounds is largely restricted by its reaction conditions, the synthetic utility of silyl anions for the synthesis of organosilicon compounds is limited. However silyllithiums possessing one or more phenyl groups on a silicon atom are readily prepared by the reductive metallation of the corresponding halosilanes with lithium. Allylsilanes are synthesized by the reaction of allyl acetates with the cuprate prepared from such a reagent (eq (47)) [43]. 

Reactions of methyllithium or methyl Grignard reagents resulted in the formation of optically active silyl anions which were characterized after hydrolysis and reaction with allyl bromide (243). The observed overall predominant retention of configuration provides evidence for the optical stability of silyl anions. Their first preparation was reported by Sommer and Mason (245) and they appeared configurationally less stable than the chiral germyl anions (31). 

Corey, Enders and Bock were among the first to describe the utility of lithium dimethylhydrazone anions for crossed aldol reactions. In the reaction shown in equation (14), an azaallyllithium reagent derived from an aldehyde dimethylhydrazone was first silylated with trimethylsilyl chloride to yield a silyl aldehyde dimethylhydrazone. Subsequent lithiation using lithium diethylamide at -20 C for 1 h generated the silylated azaallyllithium reagent (29). Subsequent addition of one equivalent of an aldehyde or ketone at -78 C and warming to -20 C then yielded the product a,p-unsaturated aldehyde dimethylhydrazone in yields of 85-95%. Hydrolysis produced the unsaturated aldehyde in 75% overall yield. 

With substituents like 9-methylfluorene and diphenylmethane, Si-C bonds can be activated for a cleavage under mild conditions. In contrast to the 9-methylfluorenyl-substituted silanes 7a and 7b, diphenylmethyl-substituted tetraorganosilanes of types 10a, 10b and roc-20 have proven to be valuable precursors for the synthesis of silyllithium reagents like 11a, 11b and rac-21 (Eq. 5). Therefore they correspond well to the silyl anion synthons B. Furthermore the bis(diphenylmethyl)-substituted silane 15 allows a sequential synthesis of unsymmetrical trisilanes and thus is a valuable silyl dianion synthon D (Eq. 6). 

The reason for this seems to be the enhanced reduction properties of the silyl anion, which are caused by the removal of a charge-stabilizing group. This called for a reactivity moderation, which can easily be accomplished via the transmetallation of potassium against magnesium [6], The reagent obtained then reacts smoothly with the metallocene dichloride (Scheme 2). 

Recently, it has been found that one doesn t need to resort to organolithium reagents to generate stoichiometric quantities of silyl anions. Potassium t-butoxide in THE, which is easier to handle than methyllithium, rapidly generates supersilylpotassium from tetrakis(trimethylsilyl)silane, as shown below ... 

The necessary silyl triflone reagents proved difficult or irr5>os-sible to make with less hindered silanes (TMS or TBDMS) by the reaction of the silyl-acetylene anion ( -BuLi/Et20/—78 °C) with Tf20. It was this triflation which failed with the proximal ethers and substituted acetylenes above. 

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