Cation chelating substituents

Hydroxyalkylation of these allyltitanium derivatives by carbonyl compounds proceeds at the more highly substituted terminus. Exceptions are observed for those hetero-substitutcd derivatives in which the chelating substituent Y ties the cation to the 1-position. 

The mechanism of the asymmetric alkylation of chiral oxazolines is believed to occur through initial metalation of the oxazoline to afford a rapidly interconverting mixture of 12 and 13 with the methoxy group forming a chelate with the lithium cation." Alkylation of the lithiooxazoline occurs on the less hindered face of the oxazoline 13 (opposite the bulky phenyl substituent) to provide 14 the alkylation may proceed via complexation of the halide to the lithium cation. The fact that decreased enantioselectivity is observed with chiral oxazoline derivatives bearing substituents smaller than the phenyl group of 3 is consistent with this hypothesis. Intermediate 13 is believed to react faster than 12 because the approach of the electrophile is impeded by the alkyl group in 12. 

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. 

In these reactions (Table 3, entries 1 - 6) the diastereoselectivity increases with increasing size of the alkoxy substituent. The results can be explained by postulating a six-membered chelate 6 involving a N-atom (or perhaps both N-atoms), the lithium cation and the acetal oxygen which is the farthest away from the alkoxy substituent. Attack of the nucleophile takes place from the less hindered side4. 

The new arsino(phosphino)methanes with bulky substituents at the two donor centers can behave both as chelating and bridging ligands toward palladium(II). Besides neutral and mono- as well as di-nuclear cationic compounds, in which these ligands are bonded in a chelating fashion, a di-nuclear complex of the A-frame type could also be generated (see Scheme 5).396... 

The interpretation and prediction of the relationship between the configuration of the newly formed chiral center and the configuration of the amine are usually based on steric differentiation of the two faces of the imine anion. Most imine anions that show high stereoselectivity incorporate a substituent which can hold the metal cation in a compact transition state by chelation. In the case of entry 2 in Table 1.3, for example, the observed enantioselectivity is rationalized on the basis of transition state L. 

Secondly, a calix[4]arene with four polyether substituents prepared by the Cragg group demonstrated no K+ transport but significant Na+ transport [55], Here, as with the Gokel group s more complex molecule described above, the calixarene was in the 1,3-alt conformation so that at best the cation could only bind to two -OCH2CH2O- chelation sites at any time. This would allow two water molecules to bind the cation giving a sixfold coordination centre ideal for Na+. Consequently 1,3-alt calix[4]arenes with simple polyether substituents appear to be selective for Na+ over K+. 

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