Intramolecular Exchanges

In classical kinetics, intemiolecular exchange processes are quite different from the uniniolecular, first-order kinetics associated with intramolecular exchange. However, the NMR of chemical exchange can still be treated as pseudo-first-order kinetics, and all the previous results apply. One way of rationalizing this is as... 

Alexander S 1962 Exchange of interacting nuclear spin in nuclear magnetic resonance. I. Intramolecular exchange J. Chem. Phys. 37 967-74... 

Studies of the reaction mechanism of the catalytic oxidation suggest that a tit-hydroxyethylene—palladium 7t-complex is formed initially, followed by an intramolecular exchange of hydrogen and palladium to give a i yW-hydtoxyethylpalladium species that leads to acetaldehyde and metallic palladium (88-90). 

JesJ73 Jesson, J. P., Meakin, P. Determination of mechanistic information for nuclear magnetic resonance line shape for intramolecular exchange. Accounts of Chem. Res. 6 (1973) 269-275. 

Olefin metathesis is the transition-metal-catalyzed inter- or intramolecular exchange of alkylidene units of alkenes. The metathesis of propene is the most simple example in the presence of a suitable catalyst, an equilibrium mixture of ethene, 2-butene, and unreacted propene is obtained (Eq. 1). This example illustrates one of the most important features of olefin metathesis its reversibility. The metathesis of propene was the first technical process exploiting the olefin metathesis reaction. It is known as the Phillips triolefin process and was run from 1966 till 1972 for the production of 2-butene (feedstock propene) and from 1985 for the production of propene (feedstock ethene and 2-butene, which is nowadays obtained by dimerization of ethene). Typical catalysts are oxides of tungsten, molybdenum or rhenium supported on silica or alumina [ 1 ]. 

C. Pseudorotation.—An alternative intramolecular exchange process to Berry pseudorotation has been suggested, which also occurs with conservation of angular momentum. It has been called a turnstile-rotation process because it involves the rotation of an apical-radial pair of ligands... 

One must assume that the two examples below (ones with more electronegative R groups) that only exhibit one signal in their fluorine NMR spectra must be undergoing rapid intramolecular exchange of their axial and equatorial fluorines (Scheme 7.8). 

The phenomenological equations proposed by Felix Bloch in 19462 have had a profound effect on the development of magnetic resonance, both ESR and NMR, on the ways in which the experiments are described (particularly in NMR), and on the analysis of line widths and saturation behavior. Here we will describe the phenomenological model, derive the Bloch equations and solve them for steady-state conditions. We will also show how the Bloch equations can be extended to treat inter- and intramolecular exchange phenomena and give examples of applications. 

As indicated earlier, type 1 structures will usually lead to zero or weak intramolecular exchange coupling, while type 2 may lead to weak to strong ex-... 

If Q-symmetric ligands are employed in asymmetric hydrogenation instead of the corresponding C2-symmetric ligands, there coexist principally four stereoiso-meric substrate complexes, namely two pairs of each diastereomeric substrate complex. Furthermore, it has been shown that, for particular catalytic systems, intramolecular exchange processes between the diastereomeric substrate complexes should in principle be taken into account [57]. Finally, the possibility of non-estab-hshed pre-equilibria must be considered [58]. The consideration of four intermediates, with possible intramolecular equilibria and disturbed pre-equihbria, results in the reaction sequence shown in Scheme 10.3. This is an example of the asymmetric hydrogenation of dimethyl itaconate with a Rh-complex, which contains a Q-symmetrical aminophosphine phosphinite as the chiral ligand. 

Scheme 10.3 Reaction sequence for the sideration of intramolecular exchange pro-...

This relationship corresponds to the simplest Michaelis-Menten kinetics (Eq. (3)). In addition to the equation derived earlier by Halpern et al. for the simplest model case of a C2-symmetric ligand without intramolecular exchange [21b], every other possibility of reaction sequence corresponding to Scheme 10.3 can be reduced to Eq. (13). Only the physical content of the values of kobs and Km, which must be determined macroscopically, differs depending upon the approach (see [59] for details). Nonetheless, the constants k0bs and KM allow conclusions to be made about the catalyses ... 

By using EXSY measurements, it was shown that at 323 K there is only intramolecular exchange, whereas at 348 K intermolecular exchange also occurs via complete dissociation of the complex, producing a free substrate and a solvate complex (Fig. 11.9). 

Fig. 31.2 Typically (DIPAMP, CHIRAPHOS), the intramolecular exchange of Re and Si complexes is several times faster than the dissociation to reform the solvate complex.

The added primary amine may facilitate the cleavage of the Ti-N bond of the key intermediate 107 through the coordination to the titanium center followed by u-bond metathesis. Such an intramolecular exchange process is expected to be facile. The amine exchange product is 106, which can then be rapidly converted to 105 and the corresponding silylated amine to complete the catalytic cycle. 

Fig. 5. Reaction scheme proposed for intramolecular exchange process in [V3Oi0]5. Local anionic charges are ignored. Subscripts t and c refer to terminal and central vanadium atoms. (Reprinted with permission from Andersson, I., Petterson, L., Hastings, J. J., Howarth, O. W., J. Chem. Soc. Dalton Trans. 1996, 3357.)...

In solution, the 170 NMR spectra of tributyltin acetate and dibutyltin diacetate show a single signal in a variety of solvents over the range 200-300 K. This is interpreted to imply the existence, in solution, of a cyclic monomer in which the carboxylate groups ate anisobidentate, and are undergoing rapid intramolecular exchange 34.350... 

There is ample evidence that the reductive elimination of alkanes (and the reverse) is a not single-step process, but involves a o-alkane complex as the intermediate. Thus, looking at the kinetics, reductive elimination and oxidative addition do not correspond to the elementary steps. These terms were introduced at a point in time when o-alkane complexes were unknown, and therefore new terms have been introduced by Jones to describe the mechanism and the kinetics of the reaction [5], The reaction of the o-alkane complex to the hydride-alkyl metal complex is called reductive cleavage and its reverse is called oxidative coupling. The second part of the scheme involves the association of alkane and metal and the dissociation of the o-alkane complex to unsaturated metal and free alkane. The intermediacy of o-alkane complexes can be seen for instance from the intramolecular exchange of isotopes in D-M-CH3 to the more stable H-M-CH2D prior to loss of CH3D. 

This type of temperature dependence, and in particular the N.M.R. spectrum in Fig. 4b, is a convincing proof of the intramolecular exchange reaction (b) in this system. Similar situations apply in the case of durene (1,2,4,5-) and prehnitene (1,2,3,4-tetramethylbenzene) (MacLean and Mackor, 1962 Brouwer et al., 1965a). 

The splitting patterns due to ligand protons and nuclei differ in the free ligand and the complexes. There is rapid intramolecular exchange of between two possible sites (shown in 13) and this leads to line broadening which can be used to measure the exchange rate constants (5 x 10 s to 2 x 10 s ). See Table 7.4. 

G. Binsch, Top. Stereochem. 3, 97 (1968) Y. Ikeda, H. Tomiyasu and H. Fukutomi, Inorg. Chem. 23, 1356 (1984) study intramolecular exchange in U02(acac)2Me2S0 in 0-CJH4CI2. 

A number of cyano-bridged complexes are included here even though they strictly do not fall in the general family-type defined for the section. The syntheses and photophysical properties of [(NC)(bpy)2Ru(/r-NC)Cr(CN)5] and [(NC)5Cr(Ai-CI Ru(bpy)2(M-NC)Cr(CN)5] have been described. Absorption of visible light by the Ru(bpy)2 unit results in phosphorescence from the Cr(CN)g luminophore, and the results evidence fast intramolecular exchange energy transfer from the MLCT state of the Ru(bpy)2 chromophore to the doublet state of the Cr -based unit. Time-resolved resonance Raman and transient UV-vis absorption spectroscopies have been employed to investigate the MLCT excited states of [(NC)(bpy)2Ru(//-CN)Ru (bpy)2(CN)], [(NC)(bpy)2Ru(//-CN)Ru(phen)2(CN)]+, [(NC)(phen)2Ru(//-CN)Ru (bpy)2(CN)]+, [(NC)(bpy)2... 

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