Double bond, shielding

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OC//j, SC//j, N(C//j)2] attached to a C=C double bond shields the (l-C atom and the -proton (+M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

The resultam structure has e very high and unusual symmetry, with the remaining 18 double bonds shielded from addition by the bulky bromine atoms. Suggest a structure. H.3 Turn 10 Chapter 12 and do Problem 12.28... 

Conjugation does not significantly change olefinic carbon shifts in 1,3-dienes relative to those of an isolated double bond. Shieldings observed for the central carbons are usually caused by steric repulsions. 

The shielding at a given nucleus arises from the virtually instantaneous response of the nearby electrons to the magnetic field. It therefore fluctuates rapidly as the molecule rotates, vibrates and interacts with solvent molecules. The changes of shift widi rotation can be large, particularly when double bonds are present. For... 

The induced field of a carbonyl group (C=0) deshields protons in much Ihe same way lhal a carbon-carbon double bond does and Ihe presence of oxygen makes il even more eleclron wilhdrawmg Thus protons attached to C=0 m aldehydes are Ihe leasl shielded of any protons bonded to carbon They have chemical shifts m Ihe range 8 9-10... 

The reversed polarity of the double bond is induced by a n electron-accepting substituent A (A = C=0, C=N, NO2) the carbon and proton in the p-position are deshielded (-A/effect, larger shifts). These substituents have analogous effects on the C atoms of aromatic and heteroaromatic rings. An electron donor D (see above) attached to the benzene ring deshields the (substituted) a-C atom (-/ effect). In contrast, in the ortho and para positions (or comparable positions in heteroaromatic rings) it causes a shielding +M effect, smaller H and C shifts), whereas the meta positions remain almost unaffected. 

Multiple bonds are revealed clearly by anisotropic effects. Textbook examples include alkynes, shielded along the C=C triple bond, and alkenes and carbonyl compounds, where the nuclei are deshielded in the plane of the C=C and C=0 double bonds, respectively One criterion for distinguishing methyl groups attached to the double bond of pulegone (31), for example, is the carbonyl anisotropic effect. 

The isomerization of isopentenyl diphosphate to dimethylally diphos phate is catalyzed by JPP isomerase and occurs through a carbocation pathway Protonation of the IPP double bond by a hydrogen-bonded cysteine residue ir the enzyme gives a tertiary carbocation intermediate, which is deprotonated b a glutamate residue as base to yield DMAPP. X-ray structural studies on the enzyme show that it holds the substrate in an unusually deep, well-protectec pocket to shield the highly reactive carbocation from reaction with solvent 01 other external substances. 

These results may be rationalized by assuming a chelation model 4. The nucleophile preferentially attacks the Re-face of the A -acylimine double bond, because the S/ -face is shielded by the phenyl group of the auxiliary9. 

In 1988 Wiberg et al. presented reactions of afree, stable iminosilane.14,27 Since that time the number of products has rapidly increased.28 Although the double bond is sterically shielded, the reactivity of iminosilanes is immense. 

Placing a carbonyl function next to an allylic CF2 group leads to the usual shielding, but its impact appears to be dampened by the influence of the double bond (Scheme 4.52). 

The pair of electrons in the n orbital are more diffuse and less firmly held by the carbon nuclei, and so more readily polarisable, than those of the a bond, leading to the characteristic reactivity of such unsaturated compounds. As the it electrons are the most readily accessible feature of the carbon-carbon double bond, we should expect them to shield the molecule from attack by nucleophilic reagents and this is indeed found to be the case (cf. p. 198, however). The most characteristic reactions of the system are, hardly surprisingly, found to be initiated by electron-deficient species such as X and X (radicals can be considered electron-deficient species as they are seeking a further electron with which to form a bond), cations inducing heterolytic, and... 

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