Selectivity, steric factors influence

With a-alkyl-substituted chiral carbonyl compounds bearing an alkoxy group in the -position, the diastereoselectivity of nucleophilic addition reactions is influenced not only by steric factors, which can be described by the models of Cram and Felkin (see Section 1.3.1.1.), but also by a possible coordination of the nucleophile counterion with the /J-oxygen atom. Thus, coordination of the metal cation with the carbonyl oxygen and the /J-alkoxy substituent leads to a chelated transition state 1 which implies attack of the nucleophile from the least hindered side, opposite to the pseudoequatorial substituent R1. Therefore, the anb-diastereomer 2 should be formed in excess. With respect to the stereogenic center in the a-position, the predominant formation of the anft-diastereomer means that anti-Cram selectivity has occurred. 

Methyl a-D-glucopyranoside may be converted,271 in a remarkably selective reaction, into its 2,4,6-tribenzyl ether (in 62% yield) on treatment with three molar equivalents of sodium hydride in benzyl chloride at 110°, a substitution pattern that might have been predicted in view of the low, relative reactivity of HO-3 towards alkylation in the mechanistically related, Haworth procedure.268 Similar, selective benzylations have also been achieved on partially substituted derivatives of methyl a- and /3-D-galactopyranoside272 and on methyl 6-deoxy-a-L-galactopyranoside62 in all of these, an unexpectedly high relative-reactivity of HO-4 (ax) compared to that of HO-3 (eq) was noted, indicating that steric factors are not the sole influence on reactivity in these cases. Nevertheless, the primary hydroxyl... 

Size, shape, and density The shielding effects of dendritic shells can likewise be caused by steric factors. Thus, the access of foreign molecules to the central functional unit can be hindered or prevented according to size and density of the dendritic shell. Sometimes, even a certain size selectivity is observed. These effects are especially interesting for electrochemically, catalytically active, redox-and photo-active functional units, since interactions with foreign molecules, such as oxygen quenching of the luminescence (photo-active units) or the access of substrates (catalytically active units) can be influenced.14 11 17,221... 

In contrast, nitrene addition under the same conditions (80JCS(P2)385) is non-selective, affording the diastereomeric sulfimides in 1 1 mixture (equation 13). Again the dominant factors may be steric as reaction of the same nitrenes with mixed acyclic sulfides shows little selectivity. Sulfimines are also formed by the action of N- chloroarenesulfonamide sodium salts on the sulfide, and an extensive kinetic investigation (76T2763) has sliown the minor influence of steric factors in this reaction as well. Equation (14) sets out the proposed course of the reaction. 

Steric factors can also play a part in the reactivity of acid derivatives. For example, a bulky group attached to the carbonyl group can hinder the approach nucleophiles and hence lower reactivity. The steric bulk of the nucleophile can also have an influence in slowing down the reaction. For example, acid chloride react faster with primary alcohols than they do with secondary or tertiary alcohols. This allows selective esterification if a molecule has more than one alcohol group present ... 

The common properties attributed to helper ligands in improving lanthanide-based detection techniques include enhanced stability, sensitivity, and selectivity. We will start with the effects of ancillary ligands on lanthanide photophysics and then explore the impacts of these ligands on sensor properties (Section II). Other factors influencing stability, such as sterics and oxophilicity, will be summarized in Section III. A brief discussion of future applications will follow (Section IV). 

It was shown that with a Pd/C catalyst in the liquid phase terminal triple bonds were saturated faster than internal ones, and both hydrogenated faster than terminal or internal double bonds in competitive processes (Eqn. 16.5). Further, alkene isomerization generally does not take place over palladium catalysts when alkynes are present. This selective hydrogenation depends on the stronger adsorption of an alkyne compared to an alkene. It is also possible that steric factors can influence the selectivity in the competitive semihydrogenation of an acetylene and an olefmic group in the same molecule. When the double bond and the triple bond are c/s to each other as in 7, selective adsorption of the acetylene... 

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