Solvation stereoselective

S-N bond cleavage 159 S-O bond lengths 543 Solvated electrons 897, 905 Solvent effects 672 in elimination reactions 772 S-O stretching frequencies 543, 545, 546, 552-555, 560-562 Spiroconjugation 390 Stereoselectivity 779, 789 of cylcoaddition reactions 799 of sulphones 761 Steroids... 

The introduction of an alkyl substituent at the a-carbon in the enolate enhances stereoselectivity somewhat. This is attributed to a steric effect in the enolate minimize steric interaction with the solvated oxygen, the alkyl group is d t °... 

The stereoselectivity is enhanced if there is an alkyl substituent at C(l). The factors operating in this case are similar to those described for 4-r-butylcyclohexanone. The tnms-decalone framework is conformationally rigid. Axial attack from the lower face leads directly to the chair conformation of the product. The 1-alkyl group enhances this stereoselectivity because a steric interaction with the solvated enolate oxygen distorts the enolate to favor the axial attack.57 The placement of an axial methyl group at C(10) in a 2(l)-decalone enolate introduces a 1,3-diaxial interaction with the approaching electrophile. The preferred alkylation product results from approach on the opposite side of the enolate. 

In polar solvents the reaction is more stereoselective (Table 7.2). This is explained by preferential solvation of the diastereoisomeric zwitterionic intermediates which lead to the cyclopropyl ketones (36) and (37). 

The possible formation of a delocalised benzyl type carbocation (16) results in much lower (70%) ANTI stereoselectivity than with trans 2-butene (5 =100% ANTI stereoselectivity, p. 180), where no such delocalisation is possible. It is also found that increasing the polarity, and ion-solvating ability, of the solvent also stabilises the carbocation, relative to the bromium ion, intermediate with consequent decrease in ANTI stereoselectivity. Thus addition of bromine to 1,2-diphenylethene (stilbene) was found to proceed 90-100% ANTI in solvents of low dielectric constant, but =50% ANTI only in a solvent with e = 35. 

The degree of stereoselectivity may be influenced to some extent by the polarity and ion-solvating ability of the solvent. 

In summary, water appears as an extremely unsuitable solvent for coordination of hard Lewis acids to hard Lewis bases, as it strongly solvates both species and hinders their interaction. Hence, catalysis of Diels-Alder reactions in water is expected to be difficult due to the relative inefficiency of the interactions between the Diels-Alder reactants and the Lewis acid catalyst. On the other hand, the high stereoselectivities and yields observed in biosyntheses, with water as the solvent, indicate that these rationalizations cannot entirely be true. As a matter of fact, we will demonstrate in the following that Lewis acid catalysis in water is not only possible, but also allows for effective as well as environmentally friendly reaction conditions. 

Moreover, it is seen from Table 1.7 that the equilibrium constants at both the nonstereoselective site (fens) and the stereoselective site (fcs) were decreased when the temperature was raised, while concomitantly more nonstereoselective as well as stereoselective adsorption sites became available or accessible at elevated temperatures. The larger number of accessible binding sites at elevated temperatures has been explained by solvation effects The sorbent surface and thus the interaction sites are solvated to a lesser degree so that they are better available for solute interactions [51]. 

Because of their usefulness in aldol additions and other synthetic methods (see especially Section 6.5.2), there has been a good deal of interest in the factors that control the stereoselectivity of enolate formation from esters. For simple esters such as ethyl propanoate, the /r-enolate is preferred under kinetic conditions using a strong base such as LDA in THF solution. Inclusion of a strong cation solvating co-solvent, such as HMPA or tetrahydro-1,3 -dimethyl-2(1 Z/)p y r i m i d o nc (DMPU) favors the Z-enolate.13... 

Figure 13. Schematic view of brush-type CSPs showing the chiral selector substituents oriented towards the liquid phase. Solvent molecules and the respective solvation are not shown. Stereoselective [SO-SA] interactions, attractive or repulsive, are located invariably within the heterogeneously structured chiral stationary phase.

The lower stereoselectivity in the transmetalation of the 21/ -stanny epimer has been attributed to jr-propynyllithium and-copper intermediates, the formation of which would be unfavorable starting from the (A7)-21/j-epimer due to steric interference of the solvated metal groups with the angular methyl group of the steroid21. 

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