Carbonyl group, shielding

C Chemical shifts for a number of methylcyclopentanones have been examined the cyclopentanone ring was considered to exist as a half chair and data were interpreted on the basis that an axial a-methyl is less stable than equatorial p-methyl. The chemical shift for P-carbons in 3-methyl-, cis-2,4-dimethyl-, and 2,2,4-trimethyl-cyclopentanones is 20.4 + 0.1 p.p.m., whereas in 2-methyl-, trans-2,5-dimethyl-, and ds-2,4-dimethyl-cyclopentanones the a-methyl chemical shift was centred around 14.3 p.p.m. Taking these values as representative of equatorial and pseudoequatorial groups, it was concluded that in cyclopentanones the carbonyl group shields C-2 methyl carbons less than in cyclohexanones. This was attributed to the larger 0==C—... 

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... 

Electronegativity and hybridization effects combine to make the carbon of a carbonyl group especially deshielded Normally the carbon of C=0 is the least shielded one m... 

The methoxy group is a +M substituent, and so shields ortho protons and C atoms in ortho positions the protons at 5 = 6.67 and 6.79 reflect this shielding. The carbonyl group as a -M substituent deshields ortho protons, and is ortho to the proton at <5// = 7.97. With the additional doublebond equivalent for a ring, 6-methoxytetralin-l-one (E) results. 

In this case, /6 -attack at the carbonyl group is induced by the stereogenic center at the metal. This fact is best explained by an approach of the aldehyde trans to the nitroso group, forming a chair-like six-membered transition state in the lower half-sphere which is not shielded by the cyclopentadiene residue. 

Although hydrolysis as well as other nucleophilic reactions of A-acylazoles (alcoholysis, aminolysis etc.) most likely follow the addition-elimination (AE) mechanism, there are indications that more complex mechanisms must be taken into account for hydrolysis under specific structural conditions. For example, for neutral hydrolysis of imidazolides with increasing steric shielding of the carbonyl group by one, two, and three... 

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... 

Electronegative substituents, as well as carbonyl groups or other functional groups on the carbon (3 to a CF3 group will be seen to shield its fluorines. 

Several formylfuranboronic acids and similar compounds have been prepared by regular methods and studied with respect to their 13C and H NMR spectra. As in the benzene series, the protons ortho to the boron are deshielded and the meta protons are shielded. Good conjugation exists between the furan ring (donor) and both the boron atom and the carbonyl group (acceptors). A Hammett-Jaffe analysis yielded ap+ + 0.256 for the B(OH)2 group.234... 

The thermal stability in air of the deeply colored radialenes 37 increases with the efficiency of steric shielding of the carbonyl groups. Thus, 37a was only detected in solution by its UV/Vis spectrum, whereas 37b and 37c are reduced to their precursors, 36b and 36c, when heated in air at 133 and 280 °C, respectively. In solution, this reduction is readily accomplished with hydroquinone13. 

The excellent chemoselectivity achieved with catalyst 153r may be attributed to its steric properties the bulky 3,3 -silyl substituents (R = SiPhj) ensure an effective shielding of the carbonyl group and thus prevent 1,2-addition. In the presence of catalyst 153r (5 mol%), the reaction of A-methylindoles 151 and p,y-unsaturated a-ketoesters 152 furnished the 1,4-addition products 155 in moderate to good yields and enantioselectivities (43-88%, 80-92% ee) (Scheme 64). 

A related effect is observed in the H NMR spectra of annulated species 16. The protons a- to nitrogen are deshielded by the 5-carbonyl group and appear at S 4.73-5.55 <2003RCB441> on the other hand, the terminal methyl (R = Et, Pr) is shielded by the nearby benzene ring and protons are observed at S 0.50-0.59. 

Studies of the stereochemical dependence of 1H chemical shifts in cyclobutanes, benzocyclobute-nes and /J-lactams have been collected463. In a number of cases the protons or proton groups are shielded if they are cis to vicinal halogen, hydroxy or phenyl substituents, due to a ring-current effect. Carbonyl groups, however, can give rise to deshielding effects. 

Figure 4.6 Shielding and deshielding for a carbonyl group (+ represents deshielding,...

---