Asymmetry lithium

As seen in Figure 10a, the C CP/MAS NMR spectrum of the quinuclidine complex shows the expected asymmetry (for instance, different signals for C-8a and C-9a) due to the asymmetric position of the lithium cation relative to the carbon framework in accordance with the X-ray investigation . The observed linewidths are not exceptionally large, and only small effects on the linewidths were observed when Li was substituted by Li, as shown in Figure 10b. 

A similar sequence was reported where the asymmetry was introduced by the reaction of weio-3-substituted glutanc anhydrides and (S)-methylbenzylamines to give diastereomeric hemiamides that could be separated by recrystallization The asymmetnc desymmetrization of certain 4-aryl substituted glutanmides has also been accomplished with high levels of selectivity (up to 97% ee) by enolization with a chiral bis-lithium amide base. The selectivity of the reaction was shown to be the result of asymmetric enolization, followed by a kinetic resolution." ... 

Sodium cuprate yielded less reduction product and less thiophilic addition product than lithium cuprate at both —50 and 0 °C. In the case of silyl thiones chiral at silicon, the reactions with organolithium derivatives and Grignard reagents produce a-silylsulfides with medium to good levels of asymmetric induction and, interestingly, the asymmetry induced at the a-carbon is retained in the subsequent desilylation431 (equation 118), the process being stereoselective. 

In other reports, /i-cyclodcxtrins have been used to induce asymmetry in borohydride reduction of ketones,166 a diastereoselective reduction has been controlled167 by a real lyltricarbonyl iron lactone tether , a phosphinamide has been combined with a dioxaborolidine unit as an activated, directed catalyst for ketone reduction,168 reductive amination using benzylamine-cyanoborohydride converts 3-hydroxy ketones into syn-1,3-amino alcohols,169 l-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propan-l-one has been reduced diastereoselectively,170 and production of chiral alcohols via (i) Itsuno-Corey and Brown procedures171 and (ii) lithium aluminium hydride modified by chiral nucleophiles172 has been reviewed. 

Attempts to induce asymmetry in the cyclisation of 57 using the chiral lithium amides, which had worked well with simple benzamides, were frustrated by the inherent chirality, at low temperature, of the naphthamide itself. This feature is common to all 2-substituted tertiary aromatic amides, which may become atropisomeric at low temperature due to slow rotation about the Ar-CO bond.48 We therefore sought to... 

The details of the EPR investigation of the graphite compound MCl6BF4M merit some discussion. Because the material is an electronic conductor, we suspended the polycrystalline sample in eicosane, melted the mixture under hot water, vibrated the sample to get random orientation, and froze the mixture under cold water. Because "CifcBF is an anisotropic conductor (it does not have metallic conductivity orthogonal to the planes), the interpretation of the EPR results requires care. Thus, in a thick plate of an isotropic metal as lithium, one observes a Dysonian asymmetry parameter A/B which is dependent on temperature, varying between... 

The high propensity of organolithium compounds to form mixed complexes with other organolithium species in solution has been utilized successfully in synthesis using chiral lithium amides. Either the chiral lithium amides have been added to organolithium reagents in an effort to achieve asymmetry in addition reactions, or various additives have been introduced to alter the reactivity or selectivity of the chiral lithium amides themselves, e.g. in deprotonation reactions. 

As shown below (Section IV), the lithium enolates are remarkable vectors of asymmetry. Indeed, the development of many chiral auxiliaries has been associated (in particular through their ester derivatives) with the enolate chemistry. We conclude this section with the contribution of a group of mathematical chemists who have tried to quantify the desymmetrization induced on enolate orbitals by common chiral auxiliaries219. This unusual viewpoint suggests that when the allylic stereogenic center is in the / position, the (Z) isomer has more chirality content than its (E) counterpart. This paper also concludes that in the enolates derived from Meyers oxazolines, the lithium cation distorts the structure but has little influence on its chirality. 

To authors opinion, the observed asymmetry arises from an increased number of neighboring oxygen atoms and indicates two different positions for lithium ions tetrahedral and octahedral. It was considered that if all lithium ions occupy high symmetric tetrahedral 8a sites, NMR... 

Optically active esters in which the activity is due to asymmetry of the a-carbon atom ate racemized by the Bouveault-Blanc and catalytic hydrogenation procedures. The optically active alcohols may be prepared by the addition of small pieces of sodium to a stirred mixture of an ethereal solution of the ester and aqueous sodium acetate at 0°. A slight acidity is maintained by periodic additions of acetic acid (Prin s method).An asymmetric center in the alpha position to a carboxyl group is not racemized by lithium aluminum hydride. ... 

The theoretical treatment of asymmetry is rather generally tested for lithium by the energy-dependent measurements at three angles of Baum et al. (1986) for the ground (2s) state and Baum et al. (1989) for the first-excited (2p) state. 

Fig. 9.2. Spin asymmetries for electron scattering to the 2s and 2p states of lithium plotted against incident energy for the indicated scattering angles (Bray et al, 1993). 2s experiment, Baum et al. (1986) 2p experiment, Baumet al. (1989) full curve, coupled channels optical broken curve, 13-state coupled channels.

In this style too, asymmetry has become a major concern.42 An asymmetric version 147 of this homoenolate was made from natural nor-ephedrine and could be alkylated stereoselectively using lithium tetramethylpiperidide (LiTMP) as base. The product could be hydrolysed to the bicyclic amide43 149. 

Carrying the concept of asymmetric lithiation one step further, Nozaki and co-workers incorporated the asymmetry-inducing complexing reagent with the metalated molecule (ferrocene) itself (37, 38). 1-Ferro-cenylmethyl-2-methylpiperidine was resolved and treated with n-butyl-lithium to give a mixture of diastereomeric lithio intermediates by directed metalation (Reaction 34). An optical yield of 93% was initially claimed for this reaction, but subsequent work by Ugi and co-workers (39) resulted in the suggestion that only a 67% optical yield was obtained. 

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