Induction solvent complex

Coupling attempts conducted with (R,S)-214 led to lower enantioselection upon C-C bond formation, an observation that points to the significant role played by the relative configuration (R,R) of the binaphthyl and ethylenediamine units in promoting asymmetric induction. This complex was found to be the most efficient among several different structural variations. Solvent effects on this transformation were also studied (Table 35), with toluene and chlorobenzene giving the best results. Low solubility of the catalyst (diethyl ether and diiso-... 

For the deprotonation of less acidic precursors, which do not lead to mesomerically stabilized anions, butyllithium/TMEDA in THF or diethyl ether, or the more reactive, but more expensive,. seobutyllithium under these conditions usually are the most promising bases. Het-eroatomic substitution on the allylic substrate, which docs not contribute to the mesomeric or inductive stabilization often facilitates lithiation dramatically 58. In lithiations, in contrast to most other metalations, the kinetic acidity, caused by complexing heteroatom substituents, may override the thermodynamic acidity, which is estimated from the stabilization of the competing anions. These directed lithiations59 should be performed in the least polar solvent possible, e.g.. diethyl ether, toluene, or even hexane. 

Herrmann et al. reported for the first time in 1996 the use of chiral NHC complexes in asymmetric hydrosilylation [12]. An achiral version of this reaction with diaminocarbene rhodium complexes was previously reported by Lappert et al. in 1984 [40]. The Rh(I) complexes 53a-b were obtained in 71-79% yield by reaction of the free chiral carbene with 0.5 equiv of [Rh(cod)Cl]2 in THF (Scheme 30). The carbene was not isolated but generated in solution by deprotonation of the corresponding imidazolium salt by sodium hydride in liquid ammonia and THF at - 33 °C. The rhodium complexes 53 are stable in air both as a solid and in solution, and their thermal stability is also remarkable. The hydrosilylation of acetophenone in the presence of 1% mol of catalyst 53b gave almost quantitative conversions and optical inductions up to 32%. These complexes are active in hydrosilylation without an induction period even at low temperatures (- 34 °C). The optical induction is clearly temperature-dependent it decreases at higher temperatures. No significant solvent dependence could be observed. In spite of moderate ee values, this first report on asymmetric hydrosilylation demonstrated the advantage of such rhodium carbene complexes in terms of stability. No dissociation of the ligand was observed in the course of the reaction. 

The solvent isotope effect suggests that no O-H cleavage is involved in the slow step and the effect of O-methylation indicates that a cyclic complex is involved. The induction factor is probably obscured by the reaction of Mn(III) and Mn02 with pinacol itself. The typical glycol-cleavage mechanism advocated for oxidations by Pb(IV) and I(VII) (p. 349) may well operate, viz. 

Solvent selectivity is a measure of the relative capacity of a solvent to enter into specific solute-solvent interactions, characterized as dispersion, induction, orientation and coaplexation interactions, unfortunately, fundamental aiq>roaches have not advanced to the point where an exact model can be put forward to describe the principal intermolecular forces between complex molecules. Chromatograidters, therefore, have come to rely on empirical models to estimate the solvent selectivity of stationary phases. The Rohrschneider/McReynolds system of phase constants [6,15,318,327,328,380,397,401-403], solubility... 

Hydrolyses of p-nitrophenyl and 2,4-dinitrophenyl sulfate are accelerated fourfold and eightfold, respectively, by cycloheptaamylose at pH 9.98 and 50.3° (Congdon and Bender, 1972). These accelerations have been attributed to stabilization of the transition state by delocalization of charge in the activated complex and have been interpreted as evidence for the induction of strain into the substrates upon inclusion within the cycloheptaamylose cavity. Alternatively, accelerated rates of hydrolysis of aryl sulfates may be derived from a microsolvent effect. A comparison of the effect of cycloheptaamylose with the effect of mixed 2-propanol-water solvents may be of considerable value in distinguishing between these possibilities. 

At 20°C, an induction period was observed in hexane. At 5°C an induction period of about 2 hours was seen in hexane, toluene, and styrene. This suggests that the Pt(II) complex [C6H5CH=CH2PtCl2]2 became catalytic after conversion to C6H5CH=CH2PtH(—Si=). This conversion was not instantaneous thus an induction period was noted, influenced by temperature and by solvents. The kinetic rate constant for formation of the phenylethylsilane was temperature dependent but nearly free of solvent effects. 

By chance, the existence of the borane complex 330 of 329 was discovered. The liberation of 330 occurred with the best efficiency with sodium bis(trimethylsilyl)-amide from the borane complex 327 of 326. When styrene or furan was used as the solvent, three diastereomeric [2 + 2]-cycloadducts 328 and [4 + 2]-cycloadducts 331, respectively, were obtained in 30and 20% yield (Scheme 6.70) [156]. With no lone pair on the nitrogen atom, 330 cannot be polarized towards a zwitterionic structure, which is why its allene subunit, apart from the inductive effect of the nitrogen atom, resembles that of 1,2-cydohexadiene (6) and hence undergoes cycloaddition with activated alkenes. It is noted that the carbacephalosporin derivative 323 (Scheme 6.69) also does not have a lone pair on the nitrogen atom next to the allene system because of the amide resonance. 

In many studies of asymmetric reductions no attempts were made to rationalize either the extent or the sense of the observed asymmetric induction, that is, the absolute configuration of the predominant enantiomer. It is believed that it is premature in certain cases to attempt to construct a model of the transition state of the key reaction step, given the present state of knowledge about the mechanism of these reduction processes. The complexity of many of the reducing systems developed is shown by the fact that the enantiomeric excess or even the sense of asymmetric induction may depend not only on the nature of the reducing agent and substrate, but also on temperature, solvent, concentration, stoichiometry of the reaction, and in some cases the age of the reagent. 

A marked solvent effect on the sense of asymmetric induction was observed. For example, reduction of acetophenone with 65 in refluxing ether gave the (R)-alcohol in 48% optical yield, and reduction in boiling THF gave die (S)-alcohol in 9.5% optical yield. A number of other similar reversals were observed. In ether solvent, an empirical relationship can be drawn between the configuration of the alcohol used for preparation of the reducing complex and the configuration of the enantiomeric product alcohol formed in excess. The relationship depends on the type of substrate used and is summarized in Table 8. 

The selection of the solvent is based on the retention mechanism. The retention of analytes on stationary phase material is based on the physicochemical interactions. The molecular interactions in thin-layer chromatography have been extensively discussed, and are related to the solubility of solutes in the solvent. The solubility is explained as the sum of the London dispersion (van der Waals force for non-polar molecules), repulsion, Coulombic forces (compounds form a complex by ion-ion interaction, e.g. ionic crystals dissolve in solvents with a strong conductivity), dipole-dipole interactions, inductive effects, charge-transfer interactions, covalent bonding, hydrogen bonding, and ion-dipole interactions. The steric effect should be included in the above interactions in liquid chromatographic separation. 

Asymmetric induction of between 30 and 66% has been reported for the addition of perfluoroalkyl iodides to chiral arene/chromiumtricarbonyl complexes using ultraso-nically dispersed zinc at room temperature (Scheme 3.19) [101], Photochemical decomposition of the organometallic intermediate affords a chiral alcohol product. The reaction is carried out in DMF as solvent and high overall yields are reported e.g.80 % (R = Et). Only a small excess of the perfluoroalkyl iodide is required and the conversion is complete in under 1 h. 

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