Group 6 shield

Group 3 Shield. The Group 5 Suppressive Shield is designed for use with pyrotechnic material ( ). 

Figure 11.11 Ion pairs in an S l reaction. The leaving group shields one side of the carbocation intermediate from reaction with the nucleophile, thereby leading to some inversion ol configuration rather than complete racemization.

The asymmetric induction that has been observed in this reaction can be explained in terms of the model shown in Scheme 9. In the most stable conformation the appropriately positioned phenyl group shields selectively the Re,Re face of the chromadiene by 7r,7r-orbital overlap forcing the nucleophile to attack preferentially on the opposite side. 

H and 13C NMR Data. A ketone or aldehyde carbonyl group bound to a CF2H group shields its proton slightly (0.1 ppm), and even more surprisingly, it also has a shielding effect upon its carbon chemical... 

A chlorine substituent at the 2-position of a 1,1-difluoroalkene will deshield the fluorines modestly, but as seen in Scheme 4.46, a vicinal alkoxy group shields both fluorines, the trans fluorine more greatly. With the additional electronegative substituent, three-bond F—H coupling constants for such compounds become even smaller. 

Anisotropic shielding in unsaturated groups. (Shielding zones + deshielding zones -). (a) Ethyne. (b) Ethene. (c) Ethanal. (d) Benzene. 

Qualitatively, the bilayer structures that result from DPD simulations are reasonable [65], In the simulation box, it is possible to find a stable bilayer in which the head groups shield the apolar core from the water phase. This means that the model effectively features a start-and-stop mechanism for... 

It has been noted that electron-withdrawing substituents (e.g., halogens) on the carbon bearing the proton deshield the proton, while electron releasing substituents (e.g. alkyl group) shield the protons. 

As shown in Tables 1 and 2, iV-protectcd amino acids have been widely used as CDAs mainly because of their ready availability. However, it was noticed that chromatographic resolution of the diastereomeric amides increased when the space-filling protecting group (e.g., Boc) was cleaved, indicating that the bulky Boc group shields the diastereodifferentiation and the specific adsorption sites of the analyte with the stationary phase94. 

The observations clearly indicated that bromine groups shield the carbene center better than methyl groups, as expected. One may wonder whether through-space interaction between the triplet carbene center and the o-bromine groups may play a role in stabilizing the triplet. The EPR data clearly indicate, however, that there is no such interaction at least in a matrix at low temperature. 

There should only be one protecting group. The incorporation of two protecting groups would result in amino groups shielded from methylation. This would result in contamination of the product with non-methylated peptide, which in most cases is difficult to separate from the main product. 

His(+), Lys, Arg Destabilizing. Unfavorable interaction with helix dipole. Cannot form hydrogen bonds with terminal NH groups. Shield terminal NH groups from solvent. 

Electron withdrawing substituents such as aldehyde or imonium functions at the other end of the alkene or the 1,3-diene transform enamines and 1,3-dienamines into push-pull systems characteristic of dyes. Electron release (push) of the donor group shields the / , S, e,... carbons and electron withdrawal (pull) of the acceptor group (carbonyl oxygen or imonium nitrogen) deshields in the a, y,... position [343],... 

Starling with the N-acyl derivative, sodium hexamethyldisilazide selectively forms the Z-enulate 38 (amides usually form Z-eno-lates). Chelate formation between sodium and the two oxygen atoms requires a conformation for 38 in which the isopropyl group shields the bottom of the molecule, so attack by the nucleophile oc curs from above. This method generally provides outstanding selectivity. The Fmns auxiliary has also been used successfully to achieve stereocontrol in aldol15 and Diels-Atder reactions.16... 

Acetylide anion adds to the carbonyl from the less sterically hindered side. The methyl group shields the top face of the carbonyl, and so acetylide adds from the bottom. 

Methyl group shields top face of double bond. 

Examples of azide bound to a CF2 group also are unusual, but some examples are also provided in Scheme 4.30, as are examples of CF2 bound to a nitro group. Interestingly, the highly electron-withdrawing N02 group shields the CF2 fluorines considerably more than the azide group. 

This reaction is a formal asymmetric aldol addition following a modified Evans protocol. The enolate 26 is formed at 0 °C in the presence of one equivalent of titanium tetrachloride as Lewis acid and two equivalents diisopropylethylamine (Hunig s base) as proton acceptor. Selectively the Z-enolate is formed. The carbon-carbon bond formation takes place under substrate control of the Tvan.v-auxiliary, whose benzyl group shields the, v/-face of the enolate. 

The angular CH3 group shields the 3-face of A. Hence, the a-face is more accessible for epoxidation, resulting in epoxide ratio of 95 5 a to 3. 

The research below focusses on the NMR parameters for poly(dA-dT) in 1 M tetramethylammonium chloride (TMA+) relative to their value in the same concentration of sodium chloride. The methyl groups shield the charged nitrogen in the TMA+ ion and it was of interest to determine whether conformational changes occur in the synthetic DNA when the counterion was changed from Na+ to TMA+. 

Covalent addition at the 10a- and 9a-positions are not likely since the 10-(2, 6 -dimethylphenyl) (I) and 5-ethyl-10-(2, 6 -dimethylphenyl) (II) isoalloxazines behave as ordinary flavins in all the reactions described in this chapter. With these analogues, the 2 - and 6 -methyl groups shield the 9a- and lOa-positions from nucleophilic attack. For instance, the 10a- and 4-positions are opened on alkaline hydrolysis of N(3)-substituted isoalloxazines (23). In the case of I, hydrolysis only occurs at the 4-position because the lOa-position is shielded. 

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