Silyl anions carbonyls

While silyl metal carbonyl derivatives are unreactive towards neutral metal carbonyls (7), metal carbonyl anions give rise to various displacement processes (entries 1-3). The order of displacing ability seems generally similar to that noted earlier for nucleophilicities (Section II, A,1) ... 

Despite the paucity of data for 9 itself, there now exists a wide range of derivatives in the cyclopropabenzene and -[Z ]naphthalene series that have been expanded upon since a 1987 accountThe preparation of these derivatives can be effected by one of three distinct routes depending upon the particular nature of the compound sought. Each has its limitations and none has provided a parent methylenecycloproparene. The first method depends upon the availability of the cycloproparenyl anion (Section IV.B) which can be intercepted by trimethylsilyl chloride to give silane 93 (R=H). In turn, deprotonation of 93 at the benzylic position affords the stabilized a-silyl anion that gives alkylidene derivatives 94 (R=H) from interaction with an appropriate carbonyl compound in a Peterson olefination (Scheme 12). The reaction sequence can be effected as a one-pot operation... 

In contrast to triorganosilyl anions, functionalized silyl anions have been studied less extensively. Functional groups reported so far are hydrogen, chlorine, amino, alkoxy, and carbonyl groups. In addition to these species, silacyclopentadienide anions are also described in this section. Theoretical studies on the effect of functional groups are described in Section... 

Silyl anions in which a carbonyl group is attached to silicon have been reported (60). An acyltris(trimethylsilyl)silane is treated with tris(trimeth-... 

Because silyl anions are much more reactive in substitution reactions than the isosteric phosphanes [10], we examined the reaction of 1 with transition metal carbonyls. 

As mentioned in the previous section, the Peterson reaction proceeds by an irreversible addition of the silyl-substituted carbanion to a carbonyl. It has generally been assumed that an intermediate p-oxidosi-lane is formed and then eliminated. In support of this mechanistic hypothesis, if an anion-stabilizing group is not present in the silyl anion, the p-hydroxysilanes can be isolated fixrm the reaction, and elimination to the alkene carried out in a separate step. Recent studies by Hudrlik indicate that, in analogy to the Wittig reaction, an oxasiletane (304) may be formed directly by simultaneous C—C and Si—O bond formation (Scheme 43). The p-hyd xysilanes were synthesized by addition to the silyl epoxide. When the base-induced elimination was carried out, dramatically different ratios of cis- to rranr-alkenes were obtained than from the direct Peterson alkenation. While conclusions of the mechanism in general await further study, the Peterson alkenation may prove to be more closely allied with the Wittig reaction than with -elimination reactions. 

In the original study by Peterson, the alkenation procedure was found to be compatible with sulfur and phosphorus substitution. The alkenation reaction has been tqrplied successfully to a variety of substituted alkenes. Because of the aiuon-stabilizing nature of the thiophenyl, the p-hydroxysiliuie is not isolated and the elimination to the alkene takes place directly to form a 1 1 mixture of ( )- and (Z)-isomers. Ager studied the reaction of the lithio anions of phenyl (trimethylsilyl)methyl sulfides (318) with a variety of carbonyl compounds (equation 72). Yields of this process were good, and addition occurred even with enolizable substrates. This reaction was extended to vinyl sulfones. In contrast to the sulfide case, the substituted sulfone silyl anion behaves as a base, leading to undesired enolization. The best yields were observed for the case where R is a hydrogen or phenyl. 

We urge you to try to write a mechanism for the above process. You may find it useful to think of silyl groups as fat protons. Start off with silylating the carbonyl oxygen and proceed from there. Silyl enol ethers function as less reactive analogs of enolate anions. Thus, with titanium tetrachloride as a catalyst, silyl enol ethers undergo aldol condensations with a variety of carbonyl compounds. This is the Mukaiyama aldol condensation ... 

Treatment of a disilane with CuOTf generates a silyl anion which can add in conjugate fashion to a, -unsaturated carbonyl compounds (eq 111). Optimal conditions consist of heating the reaction mixture in DMF in the presence of tributylphosphine. Absence of the latter gives very low conjugate addition product (20%). 

Silyl Anion Equivalent. Silylboronic ester 1 reacts as a silyl anion equivalent in the presence of transition metal catalysts. Cyclic and acyclic a,/3-unsaturated carbonyl compounds serve as good acceptors of the silyl groups in conjugate addition of 1 catalyzed by rhodium and copper complexes, giving /3-silylcarbonyl compounds (eq 30). The silylation takes place with high enan-tioselectivity when Rh/(5)-BINAP or Cu/chiral NHC catalysts are used. Three-component coupling of 1, a,/3-unsaturated carbonyl compounds, and aldehydes affords 8-hydroxyketone stereoselec-tively in the presence of a copper catalyst (eq 31). The copper enolate 32 is presumed as an intermediate of the reaction. 

The fluoride anion has a pronounced catalytic effect on the aldol reaction between enol silyl ethers and carbonyl compounds [13] This reacbon proceeds at low temperature under the influence of catalytic amounts (5-10 mol %) of tetra-butylammonium fluoride, giving the aldol silyl ethers in high yields (equation 11). 

Merck s thienamycin synthesis commences with mono (V-silylation of dibenzyl aspartate (13, Scheme 2), the bis(benzyl) ester of aspartic acid (12). Thus, treatment of a cooled (0°C) solution of 13 in ether with trimethylsilyl chloride and triethylamine, followed by filtration to remove the triethylamine hydrochloride by-product, provides 11. When 11 is exposed to the action of one equivalent of tm-butylmagnesium chloride, the active hydrogen attached to nitrogen is removed, and the resultant anion spontaneously condenses with the electrophilic ester carbonyl four atoms away. After hydrolysis of the reaction mixture with 2 n HC1 saturated with ammonium chloride, enantiomerically pure azetidinone ester 10 is formed in 65-70% yield from 13. Although it is conceivable that... 

It is known that Na2Fe(CO)4 can be silylated twice to form cri-[(H3C)3Si]2Fe(CO)4 [109]. Also the reaction of Na2Fe(CO)4 with 1.1-dichlorosilanes has been described and leads exclusively to the dimeric compounds [110, 111], In polar solvents the formation of dimers can be suppressed and monomeric base-stabilized compounds are obtained. A very elegant procedure is the in-situ generation of the carbonylate anions in solution by deprotonation of H2Fe(CO)4. 

Deprotection of 2,2-disubstituted-l,3-dithiolanes to give carbonyl compounds can be achieved using Oxone with KBr in aq. MeCN <06TL8559> and a review of silylated heterocycles as formyl anion equivalents includes reference to 64 <06CC4881>. A method for transformation of propargylic dithiolanes 43 into tetrasubstituted furans has been reported <06SL1209> and Michael addition of enolates to the chiral dithiolane dioxide 65 takes place... 

The Peterson olefination reaction involves the addition of an a-silyl substituted anion to an aldehyde or a ketone followed by the elimination of silylcarbinol either under acidic (awP -elimination) or basic (syn-elimination) conditions to furnish olefins178. Thus, Peterson olefination, just like Wittig and related reactions, is a method for regioselective conversion of a carbonyl compound to an olefin. Dienes and polyenes can be generated when the Peterson reaction is conducted using either an ,/l-unsaturated carbonyl compound or unsaturated silyl derivatives as reaction partners (Table 20)179. 

We have discussed the direct formation of anions by reaction of silyl- or germyl-lithiums with metal carbonyls (cf. Sect. 2.12) ... 

Scheldt and co-workers have reported the apphcation of silyl-protected thiazoUum carbinols as stoichiometric carbonyl anions for the intermolecnlar acylation of nilroalkenes [89]. While predominantly a discussion of racemic chemistry, a singular example illustrates that the newly formed stereocenter may be controlled by the addition of an equivalent of a chiral thionrea 136 with the desired product 135 formed in 74% ee Eq. 13. 

Overall, it is possible to divide the silyl Lewis acids into two groups, depending on how strong the counter anion interacts with the silicon atom as shown in Scheme 2. In the case where a very weakly coordinating anion is part of the compound, one could consider that a free silyl cation is present. However, the silyl cation is very strong and will be coordinated by solvent molecules like acetonitrile or toluene [25, 26]. This complex could activate, for example, a carbonyl group. Whether the carbonyl group replaces the solvent molecule is not known. In the case... 

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... 

Schiff base complex, 32 13 -selenium complex FejScj complexes, 32 348 Fc4Se3 complexes, 32 349-350 Fc4Se4 complexes, 32 348-349 -selenium-nitrosyl complexes, 32 348-350 selenocyanates, 17 295, 296 sequestration in apoferritin, 36 463-464 silicates, Mbssbauer effect of, 6 474-479 -silicon compounds, 3 250 silyl complexes anionic tetracarbonyl, 25 37 binuclear carbonyls, 25 3, 5, 16, 33, 44-45, 116... 

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