Chelation shift

Fig. 1-13. NMR coordination shifts [<5(complex) — <5(free ligand)] (------) and chelation shifts ...

Table II shows the H NMR data on 0.1 M iso-HMTT and 0.1M iso-HMTT LiBr in CH2C12 and in benzene. Of primary interest is the last entry in the table, AChei, the chelation shift for iso-HMTT LiBr relative to free iso-HMTT in each of the two solvents. The values of Achei in CH2C12 are analogous to what is observed for lithium alkyl systems such as TMED LiBu in paraffinic solvents. The small 0.1 ppm) down-field shift of the N-CH3 protons accompanied by essentially no shift of the N-CH2- protons is typical for lithium chelates in non-aromatic solvents and is what we have referred to as a normal chelation shift. Observation of this normal AChei is evidence that iso-HMTT LiBr does exist as the chelate in CH2C12.

Table II shows that the chelation shift Achei is quite different in benzene solution. Here again the behavior of iso-HMTT LiBr is analogous to TMED LiBu, with a large upfield shift of the N-CH2- protons. This large upfield methylene shift (large positive Achei) in benzene is general for chelated lithium compounds and is seen as a manifestation of a stereospecific collision complex between benzene and the positive end of the molecular dipole moment of Chel LiX. To study this unusual interaction we have carried out extensive experiments in mixed CH2C12— benzene solvents, some results of which are discussed below. In addition to studying the origin of this upfield methylene chelation shift in benzene, we have taken advantage of its existence in a number of ways described below.

With iso-HMTP Nal, the bulk of the N—CH2 signal is shifted upfield to the point where N-methylene and N-methyl resonances show considerable overlap. Peak assignment can tentatively be made with n-HMTP Nal, however, and the magnitude of the chelation shift can be estmiated. In n-HMTP Nal, the upfield shifts of the N—CH2 protons average +0.13 ppm the downfield shifts of the N—CH3 resonances are... 

An explanation offered for the upheld chelation shift envisions a loose association of benzene molecules about the positive end of the chelate complex dipole (21). A similar solvation model has been proposed for TMED -ZnCl2 in mixtures of dioxane and benzene (22). According to the model, the N—CH2 protons of the ligand would be near the benzene 7r-molecular orbitals, the associated diamagnetic anisotropy of which would induce an upheld shift in the N—CH2 proton resonances. 

On the basis of the studies described in the preceding chapters, we anticipated that chelation is a requirement for efficient Lewis-acid catalysis. This notion was confirmed by an investigation of the coordination behaviour of dienophiles 4.11 and 4.12 (Scheme 4.4). In contrast to 4.10, these compounds failed to reveal a significant shift in the UV absorption band maxima in the presence of concentrations up to one molar of copper(ir)nitrate in water. Also the rate of the reaction of these dienophiles with cyclopentadiene was not significantly increased upon addition of copper(II)nitrate or y tterbium(III)triflate. 

Because the stmcture of 1,3-diketones comprise a methylene group between two activating carbonyls, equiUbrium is shifted toward the enol form. The equihbrium distribution varies with stmcture and solvent (303,306) (Table 13). The enol forms are cycHc and acidic and form covalent, colored, soHd chelates with metals ... 

Hydrogen bonding plays a major role in pyrazolone tautomerism, and the formation of a chelate structure can shift the equilibrium towards the chelated form. Structures (135) and (136) are two representative examples of such stabilized tautomers. Structure (137) is a hypothetical example of stabilization of the NH tautomer. 

C and H chemical shifts and the corresponding coupling constants have been determined for the chelate (225 M = Zn(II)) (81M105). 

Figure 2.24, Determination of the enantiomeric excess of 1-phenylethanol [30, 0.1 mmol in 0.3 ml CDCI3, 25 °C] by addition of the chiral praseodymium chelate 29b (0.1 mmol), (a, b) H NMR spectra (400 MHz), (a) without and (b) with the shift reagent 29b. (c, d) C NMR spectra (100 MHz), (c) without and (d) with the shift reagent 29b. In the C NMR spectrum (d) only the C-a atoms of enantiomers 30R and 30S are resolved. The H and C signals of the phenyl residues are not shifted these are not shown for reasons of space. The evaluation of the integrals gives 73 % R and 27 % S, i.e. an enantiomeric excess (ee) of 46 %...

In low ionic strength solutions (I < 0.005 panel A), the curves obtained with the pH buffers MOPS and glycylglycine and also with a weak Ca2+-chelator citric acid are nearly superimposable on each other, whereas those obtained with the buffers that contain strong chelators EDTA and NTA are shifted almost 2 pCa units to the right. The curve for NTA is somewhat atypical probably the Ca2+-NTA ratio of the Ca2+-NTA complex used was not exactly 1.0. In the... 

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