Umbrella effect

An effect observed with a number of compounds which have apparent chiral centers on elements other than carbon. Eor example, secondary and tertiary amines have a pyramidal structure in which the unshared pair of electrons is at the top of the pyramid. If the three substituents hnked to the nitrogen are all different, one might suspect that the tertiary amine would give rise to optical activity and be resolvable. However, rapid oscillation of the unshared pair of electrons on one side of the nitrogen to the other (hence, pyramidal inversion) in effect causes interconversion of the two enantiomers and prevents resolution. If the nitrogen is at a bridgehead, this umbrella effect is inhibited and optical isomers can be isolated. 

Other elements besides nitrogen can exhibit an umbrella effect e.g., phosphorus, sulfur, and arsenic. Fortunately, the effect is slow in most of these cases. Hence, one can prepare optically pure phosphoesters (such as,... 

A simple tertiary amine cannot be resolved into its enantiomers at normal temperatures, because there is rapid interconversion between the two forms. This rapid interconversion is called the umbrella effect. A similar effect occurs in simple carbanions, and so they cannot be resolved either. This means that any stereochemical information at such a centre is lost. 

The rate of SE1 reactions is affected by similar considerations as those that affect SN1 reactions. Further, generally any stereochemical information at the carbanion centre is lost, either due to the umbrella effect or due to the formation of a planar intermediate as a result of delocalisation to a -M substituent such as a carbonyl group. However, vinyl carbanions can maintain their configuration, and so retain their stereochemistry. 

Pyramidal inversion The inversion of the configuration of a molecule that has a trigonal pyramidal geometry, such as simple nitrogen compounds. Also called the umbrella effect. 

Any trisubstituted, pyramidal, nitrogen atom bonded to three different groups is chiral. In such an example, the lone pair constitutes the fourth group. However, at room temperature there is a rapid inversion of the molecule, which means that effectively there is an equal concentration of both enantiomers, and so such a molecule does not exhibit any optical activity in normal circumstances. This inversion is called the umbrella effect. 

The umbrella effect is also seen in other types of compounds, e.g. sulphoxides. However, the inversion process is slowed down sufficiently in substituted sulphoxides, R R SO, for the optical activity to be measured, e.g. [a]280=+0.71° for (+) Ph12CH2S013CH2Ph. 

EDX analysis (number 1 and 2) of the relation of Si and O in the sample SHF is realized in TEM micrograph of Fig. lb. The same analysis is made on SAHMI (number 3) and SAHMII (number 4 and 5) samples and the results are shown in Table 2. EDX analysis corroborates the presence inside of silica framework of the two modifiers, which besides, are identified in particular form as it can be observed on micrographs and by EDX analysis. The presence of HS induces hydrophobic properties on the synthesized silicas so they seem to float on water when the HS quantities showed on Table 1 are used. It is important to emphasize that when ethanol is used to wet the tested silicas, they interact with their free surface OH groups and get wet. This behaviour could be due to the umbrella effect [7] which is produced by the AP that covered the surface OH. 

Figure 16.2 Illustration of the umbrella effect in which W-10 protects adjacent VAc groups from hydrolysis...

The presence of a building may reduce the beneficial effect of an adjacent chimney at short distances, reducing the already mentioned umbrella effect. The infiuence of the building can be considered completely absent only if the height of the chimney is equal to at least 2-2.5 times that of the building otherwise the effect of the chimney will be reduced or possibly eliminated by the turbulence generated by the building. 

Umbrella" Effect in the Purple Isomer of [Ru(S2CH)(PMe2Ph)4] ... 

For the synthesis of highly defined and complex polymer architectures, the Ugi-4CR serves as conjugation method for two kinds of polymers benzaldehyde-terminated PMMA and aniline-terminated PEG 5000 (Scheme 11) [73], For this purpose, the remaining components, isocyanide and carboxyhc acid, were added in high excess (ten and five equivalents, respectively) to achieve full conversion of the parent polymers. Removal of the excess was accomplished via dialysis. Attractively, this approach not only combines two different polymers, but also facilitates the synthesis of mid-functionalized block copolymers by simple introduction of functional isocyanides or carboxylic acids. In this way, Tao and coworkers introduced a fluorescent molecule via the carboxylic acid (using dansylglycine) or a terminal double bond (using 2-acrylamido acetic acid) to obtain a mid-vinyl PMMA-Z -PEG polymer. Mid-reactive polymers are known to behave differently to their end-functionalized counterparts due to the umbrella-effect [74]. Therefore, it was... 

Every attempt to detect the cis/trans isomerism of the model compound NTDA-M went to failure, presumably due to the umbrella effect, in which the phenyl group bonded to tlie nitrogen atom with the unshared electron pair rapidly oscillates from one side to the other. In NA-PIs the phenylene ring attched to the nitrogen atom is enormously heavy, because it is linked with a long chain at 4-position, so that it can not rapidly oscillate any more. Therefore, the cis/trans isomerism should be observed in NA-PIs. 

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