Anion asymmetric

Not only cyanide but also an isocyanide behaves as a nucleophile to attack a carbonyl compound or an imine that is prepared in situ from an carbonyl compound. " In these reactions, an isocyanide is a synthetic equivalent to an aminocarbonyl anion. Asymmetric version of this reaction appeared in 2003. Using a combination of Lewis acid SiCU and a Lewis base chiral bisphosphora-mide, the corresponding a-hydroxyamide is obtained in 96% yield with >98% ee (Scheme 4.23). 

The alkylations of the azaenolates are performed at —78 °C or below, and proceed exclusively at the C-terminus of the ambident azaally] anion. Asymmetric induction at —78 "C can be as effective as at —98 °C by slower addition of the electrophile to the azaenolate. Furthermore, higher optical yields2 result from alkylations in more dilute solutions at — 78 °C. 

Nitrate anions can be substituted on bismuth nitrate by reaction with polyethylene glycols effecting deprotonation (or ionization) of one end of the ligand chain (101). Centrosymmetric dimers of the general form 29 are observed, with nitrate anions asymmetrically coordinated, staggered, and trans to one another [Bi-0 2.240(l)-2.28(2) A] in [Bi(N03)2(Heo3)]2, [Bi(N03)2(Heo4)]2(MeOH)2, and [Bi(N03)2... 

Asymmetric Heck reaction of the conjugated diene 184 and subsequent acetate anion capture of the rr-allylpalladium intermediate afforded 185 in 80% ee. which was converted into the key intermediate 186 for the capnelle-... 

The experimental spectra are interpreted by Tozer and Sosa as follows In the Na compound, the structure is of the form NaF...F2, and it exhibits an absorption due to the complex at 455 cm, with a 460 splitting (this mode is denoted (Oj). For the other two, T-shaped compounds, the two highest frequencies resemble perturbed forms of the symmetric and asymmetric F-F-F stretching modes that we saw in the F3 anion, which we denote (O2 and (O3. The Cs compound exhibits the asymmetric F3 stretching ((O3) at 550 cm", while the K structure exhibits this vibration at 549 cm" along with a weak absorption at 467 cm". The latter may represent a weakly-active symmetric stretch ((03). 

D-Methylmalonyl-CoA, the product of this reaction, is converted to the L-isomer by methylmalonyl-CoA epunerase (Figure 24.19). (This enzyme has often and incorrectly been called methylmalonyl-CoA racemase. It is not a racemase because the CoA moiety contains five other asymmetric centers.) The epimerase reaction also appears to involve a carbanion at the a-position (Figure 24.20). The reaction is readily reversible and involves a reversible dissociation of the acidic a-proton. The L-isomer is the substrate for methylmalonyl-CoA mutase. Methylmalonyl-CoA epimerase is an impressive catalyst. The for the proton that must dissociate to initiate this reaction is approximately 21 If binding of a proton to the a-anion is diffusion-limited, with = 10 M sec then the initial proton dissociation must be rate-limiting, and the rate constant must be... 

The Creutz-Taube anion, [(NH3)5Ru- N(CH=CH)2N Ru(NH3)5] + displays more obvious redox properties, yielding both 4+ and 6- - species, and much interest has focused on the extent to which the pyrazine bridge facilitates electron transfer. A variety of spectroscopic studies supports the view that low-energy electron tunnelling across the bridge delocalizes the charge, making the 5- - ion symmetrical. Other complexes, such as the anion [(CN)5Ru (/z-CN)Ru (CN)5] , are asymmetric... 

Chiral oxazolines developed by Albert I. Meyers and coworkers have been employed as activating groups and/or chiral auxiliaries in nucleophilic addition and substitution reactions that lead to the asymmetric construction of carbon-carbon bonds. For example, metalation of chiral oxazoline 1 followed by alkylation and hydrolysis affords enantioenriched carboxylic acid 2. Enantioenriched dihydronaphthalenes are produced via addition of alkyllithium reagents to 1-naphthyloxazoline 3 followed by alkylation of the resulting anion with an alkyl halide to give 4, which is subjected to reductive cleavage of the oxazoline moiety to yield aldehyde 5. Chiral oxazolines have also found numerous applications as ligands in asymmetric catalysis these applications have been recently reviewed, and are not discussed in this chapter. ... 

Meyers has demonstrated that chiral oxazolines derived from valine or rert-leucine are also effective auxiliaries for asymmetric additions to naphthalene. These chiral oxazolines (39 and 40) are more readily available than the methoxymethyl substituted compounds (3) described above but provide comparable yields and stereoselectivities in the tandem alkylation reactions. For example, addition of -butyllithium to naphthyl oxazoline 39 followed by treatment of the resulting anion with iodomethane afforded 41 in 99% yield as a 99 1 mixture of diastereomers. The identical transformation of valine derived substrate 40 led to a 97% yield of 42 with 94% de. As described above, sequential treatment of the oxazoline products 41 and 42 with MeOTf, NaBKi and aqueous oxalic acid afforded aldehydes 43 in > 98% ee and 90% ee, respectively. These experiments demonstrate that a chelating (methoxymethyl) group is not necessary for reactions to proceed with high asymmetric induction. 

The asymmetric addition of organolithium reagents to arylox azolines has been used to construct highly complex polycyclic terpene structures found in natural products. For example, the asymmetric addition of vinyllithium to chiral naphthyloxazoline 3 followed by treatment of the resulting anionic intermediate with iodoethyl dioxolane 61... 

The triazole 76, which is more accurately portrayed as the nucleophilic carbene structure 76a, acts as a formyl anion equivalent by reaction with alkyl halides and subsequent reductive cleavage to give aldehydes as shown (75TL1889). The benzoin reaction may be considered as resulting in the net addition of a benzoyl anion to a benzaldehyde, and the chiral triazolium salt 77 has been reported to be an efficient asymmetric catalyst for this, giving the products (/ )-ArCH(OH)COAr, in up to 86% e.e. (96HCA1217). In the closely related intramolecular Stetter reaction e.e.s of up to 74% were obtained (96HCA1899). 

Among the most successful classes of asymmetric acyl anion equivalents are the dioxane-containing a-amino nitriles 99 introduced by Enders and coworkers. These are deprotonated by EDA, and the resulting anions act as efficient equivalents of RCO for addition to a, (3-unsaturated esters [90AG(E)179],... 

Oxathiane 101 is readily deprotonated using s-BuLi, and the resulting anion reacts with alkyl halides, ketones, and benzonitrile (85JOC657). The majority of work in this area, however, is due to Eliel and coworkers and has involved chiral 1,3-oxathianes as asymmetric acyl anion equivalents. In the earliest work it was demonstrated that the oxathianes 102 and 103, obtained in enantiomeri-cally pure form by a sequence involving resolution, could be deprotonated with butyllithium and added to benzaldehyde. The products were formed with poor selectivity at the new stereocenter, however, and oxidation followed by addition... 

Asymmetric induction by sulfoxide is a very attractive feature. Enantiomerically pure cyclic a-sulfonimidoyl carbanions have been prepared (98S919) through base-catalyzed cyclization of the corresponding tosyloxyalkylsulfoximine 87 to 88 followed by deprotonation with BuLi. The alkylation with Mel or BuBr affords the diastereomerically pure sulfoximine 89, showing that the attack of the electrophile at the anionic C-atom occurs, preferentially, from the side of the sulfoximine O-atom independently from the substituent at Ca-carbon. The reaction of cuprates 90 with cyclic a,p-unsaturated ketones 91 was studied but very low asymmetric induction was observed in 92. 

The 1,3-dipoles consist of elements from main groups IV, V, and VI. The parent 1,3-dipoles consist of elements from the second row and the central atom of the dipole is limited to N or O [10]. Thus, a limited number of structures can be formed by permutations of N, C, and O. If higher row elements are excluded twelve allyl anion type and six propargyl/allenyl anion type 1,3-dipoles can be obtained. However, metal-catalyzed asymmetric 1,3-dipolar cycloaddition reactions have only been explored for the five types of dipole shown in Scheme 6.2. 

The size of the cation in the chloroaluminate ionic liquids also appears to have an impact on the viscosity. For ionic liquids with the same anion(s) and compositions, the trend is for greater viscosity with larger cation size (Table 3.2-2). An additional contributing factor to the effect of the cation on viscosity is the asymmetry of the alkyl substitution. Highly asymmetric substitution has been identified as important for obtaining low viscosities [17]. 

The existence of an electrical potential causes not only cation and anion movement but also migration of moisture toward the cathode. This movement of water (electroendosmosis) is due to the asymmetrical nature of the polar groups of the water molecule. In arid regions water leaving the anode area may cause the soil surrounding the anodes to become so dry that proper current densities cannot be maintained along the line. To alleviate this, some pipe-line companies have had to transport water into desert areas to re-moisten anode beds. 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

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