Fluxional exchange

As is the case for most indium-metallic derivatives in solution, rapid fluxional exchanges are prevalent as can be... 

The complex (328) has been obtained from /rans-cyanohalogeno-bis-(triphenylphosphine) Pt(II) by halide displacement with azide ion followed by cycloaddition of acetonitrile.619 Above 150°C fluxional exchange of the coordinated N atom around the tetrazole ring was observed.619 New dimeric zr-allyl complexes of Pd and Pt with tetrazolate groups as bridging ligands bonded at the 2-N and 3-N atoms (329) have... 

Despite the above mentioned problems in obtaining detailed experimental information about the pathways of nuclear permutations, several general empirical models for the fluxional behavior in clusters have been proposed. Several general classes of fluxional exchange have been observed in clusters, e.g. ... 

Hilmersson and Davidsson studied by means of intensive NMR investigations a mixed 1 1 complex of n-BuLi and the Hthiated methoxyamine 6 (Scheme 1) in diethyl ether at -80°C [68]. A fluxional exchange between tetrameric and dimeric structures of the chiral Hthiated pyrrolidine 14 and -BuLi is apparent in diethyl ether solution [69]. With the Hthiated amine 15, a derivative of 6,75% ee of (S)-l-phenyl-l-pentanol was obtained in n-BuLi additions to PhCHO, using a ratio of 1.0 0.45 0.25 in diethyl ether at -116°C [37]. The N-methyl derivative gave only 2% ee, while the N-isopropyl derivative 16 yielded 82% ee under the same conditions. This demonstrates Ae crucial role of the N-isopropyl substituent in 16. Addition of dimethoxymethane increased the enantioselectivity of 16 to 91% ee of (S)-l-phenyl-l-pentanol. With Hthiated 16, enantioselectivities of up to 98.5% ee were achieved in butylations of aHphatic aldehydes [70]. Hilmersson demonstrated that the mixed Hthium amide/ -BuLi aggregate alkylates aldehydes faster than the pure -BuLi oHgomers, Eq. (2) [71]. 

The axial and equatorial substituents in these compounds are symmetry inequivalent, and are therefore expected give separate absorption peaks in NMR spectra. SF4, SeF4 and Te(CH3)4 are, however, all found to be fluxional. Exchange of axial and equatorial ligands probably occurs by Berry pseudorotation with the electron lone pair as pivotal ligand. ... 

It sometimes happens that a fluxional exchange process is suspected on chemical grounds but the low-temperature spectrum is seen at all accessible temperatures, and so the exchange is slow on the NMR timescale. An example is shown in Eq. 10.8, where we have to postulate exchange to account for the chemistry of the system, but it is too slow to affect the NMR lineshapes. In such circumstances, we can sometimes use spin saturation transfer. The principle of the method is to irradiate one of the resonances in the spectrum of one of the two species and watch for the effects on the spectrum of the other species. If we irradiate the MeA protons in 10.9a, we see a diminution in the intensity of the resonance for Meg in 10.9b. This shows that Mca in 10.9a becomes Mcb in 10.9b in the course of the exchange likewise, irradiation at the frequency of He affects the intensity of the Hd- In this way we can obtain mechanistic information about the fluxional process. 

One Ind could be in which case we should see two distinct sets of Ind resonances. If the two rings were rapidly fluxional, exchanging between T and T forms, one set of C resonances would be seen but the presence of an IPR effect (see Section 10.8) in this case should make it distinguishable. 

When fluxional exchange is too slow to detect by the methods of Section 10.5, we may still be able to use spin saturation transfer. To... 

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