Carbonyl shift

The (S)-tropic acid, the acid moiety found in hyoscyamine (27) and scopolamine (89), is formed from phenylalanine by an intramolecular 2,3-carbonyl shift (5,124). Feeding of the four possible stereoisomers of [l-14C,3-3H]phenylalanine to Datura innoxia and D. stramonium was used to prove that during the 2,3-carbonyl shift a 3,2-hydrogen shift takes place as well (725)(Scheme 23). With D. innoxia, the 2,3-carbonyl shift was shown to... 

Streck and coworkers showed that in a range of solvents, the 13C carbonyl shifts in dialkyl ketones were affected similarly by branching at the a-position.127 In chloroform, the carbonyls of di-tert-butylketone and diisopropylketone were 11-12 ppm downfield of that of acetone, which they attributed to a mixture of inductive and steric effects. With tertiary systems, particularly in dipolar solvents, hindrance to solvent stabilisation of the polar, basic form of the carbonyl offsets the inductive stabilisation of the branched alkyl. 13C NMR data presented here support this. 

Limited carbonyl NMR data are available for these anomeric amides. However, carbonyl shifts for hydrazines 217 and 218 were on average 3 ppm higher than their hydroxamic ester precursors. This reflects a higher degree of residual amide resonance in the hydrazines relative to A-acyloxy-iV-alkoxyamides where the difference was closer to 8.0 ppm. As reported for A-acyloxy-A-alkoxyamides (see Section IV.B.2), analysis of variance in the hydrazine and hydroxamic ester shifts indicates that substituents affect the hydroxamic ester carbonyl shifts ( 2.6) more than those of the hydrazines ( 1.3 ppm). 

Particularly large carbonyl shifts (215-218 ppm) are measured for 2,4-dimethyl-,... 

The chemical shift dependence of the carbonyl resonances on ring size in cycloalkanones is particularly remarkable In the series of cycloalkanones, cyclopentanone is found to have the largest carbonyl shift (219.6 ppm). The CO signals of cyclobutanone and cyclohexanone are both observed at higher field (x 209 ppm). The carbonyl carbons of cy-clooctanone and cyclononanone are much more deshielded than those of cyclohexanone, cycloheptanone, cyclodecanone and cycloundecanone. The carbonyl resonances of the twelve to seventeen membered ring ketones occur at S values similar to those of acyclic ones [282, 288]. 

Representative benzaldehyde derivatives [73k] and phenones [294] (Tables 4.30 and 4.31) display carbonyl shifts which are essentially influenced by steric repulsions and intramolecular hydrogen bonding. Steric repulsions by bulky alkyl groups in o, o position of the carbonyl group prevent coplanarity of carbonyl double bond and phenyl ring,... 

Carbon-13 NMR spectra of the H2C On series, such as deltic, squaric, croconic and rhodizonic acids, obtained in anhydrous solvents [304] display carbonyl shifts similar to those reported for quinones (Table 4.33). Considerable shielding of the carbonyl carbon of deltic acid diethyl ester is not only attributed to the three-membered ring but also to an electron releasing effect of the ethoxy groups. 

Carboxy carbons of methyl benzoates are shielded by electron-withdrawing substituents in the oposition of the benzene ring [320] (Table 4.38). Carbonyl shifts of phthalic acid diesters and phthalimide are larger than those of phthalic anhydride [321]. fi effects of the O-alkyl group in the esters and hydrogen bonding of the imide are the obvious reasons. 

Steric repulsions between substituents with n bonds and other groups attached to the benzene ring may twist the planes of both n systems as outlined in Sections 3.1.3.8 and 4.7.2 for o, o -dialkylated acetophenones. This sterically induced hindrance of conjugation is not only monitored by an increase in the carbonyl shifts but also by a shielding of the para benzenoid carbon due to an attenuated mesomeric effect of the carbonyl group. This... 

D is correct Tautomerization involves a proton shift where the double bond of the carbonyl shifts to the carbonyl/a-carbon bond when the carbonyl oxygen is protonated. You should memorize tautomer formatio and structure. 

The 13C NMR parameters for vinylene carbonate (30) show significant shielding of the carbonyl carbon atom due most probably to the proximity of the lone pair electrons on the adjacent heteroatoms (77JOC2237). The carbonyl shift is very similar in ethylene carbonate, the saturated analog (B-72MI43000). The alkenic carbons in (30) appear at 5132.2, a typical resonance for sp2 carbons of unsaturated alkenes and aromatics. On protonation the chemical shifts for the 1,3-dioxolium ion (31) are deshielded by 6.1 p.p.m. for the alkenic carbon atoms and 9.5 p.p.m. for the carbonyl carbon atom. 

Now look at the first two groups, the aldehydes and ketones. The two aldehydes have smaller carbonyl shifts than the two ketones, but they are too similar for this distinction to be reliable. What distinguishes the aldehydes very clearly is the characteristic proton signal for CHO at 9-10 p.p.m. So you should identify aldehydes and ketones by C=0 shifts in carbon NMR and then separate the two by proton NMR. 

It is obvious that such equilibria would exist for all the other catalytic intermediates. The result of all this is coupled catalytic cycles and many simultaneous catalytic reactions. This is shown schematically in Fig. 5.5. The complicated rate expressions of hydroformylation reactions are due to the occurrence of many reactions at the same time. As indicated in Fig. 5.5, selectivity towards anti-Markovnikov product increases with more phosphinated intermediates, whereas more carbonylation shifts the selectivity towards Mar-kovnikov product. This is to be expected in view of the fact that a sterically crowded environment around the metal center favors anti-Markovnikov addition (see Section 5.2.2). 

NMR spectrum A 3H singlet at 5 2.7 is methyl next to a carbonyl, shifted slightly downfield by an aromatic ring. The other signals are seven aromatic protons. The IH at d 8.7 is a deshielded proton next to a carbonyl. Since there is only one, the carbonyl can have only one neighboring hydrogen. 

Figure 6.30. Carbonyl regions of the HMBC spectra of 6.10 recorded with (a) the conventional HMBC and (b) the semi-selective HMBC sequences. The small carbonyl shift dispersion causes considerable crosspeak overlap with the low fi resolution of 80 Hz/pt in (a), whereas the higher resolution in (b) of 6 Hz/pt removes this limitation.

Maciel et have observed an isotope effect of —0-28 p.p.m. on the chemical shift of the carbonyl group in acetone- /g with respect to that of the carbonyl shift of acetone. The authors discuss the results in terms of vibrational zero-point energies, and in addition the paper describes a useful method of observing resonances using a direct field—frequency lock system. 

Many workers have been prompted, because of the abundance of shifts of the carbonyl carbon atom, to study the factors that affect this parameter in the polar > C=0 grouping. Maciel" has considered the origin of carbonyl shifts in compounds of the general type... 

Carbonyl shifts of acetophenones (11), alkyl phenyl ketones (12) and methyl benzoates (13), p.p.m. from CS2... 

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