Fluxional mechanism

The metal clusters of the iron triad M3(CO)i2, 4 M = Fe, 5 M = Ru, and 6 M = Os, despite being some of the simplest carbonyl clusters known, are fascinating molecules, which still present interesting structural and dynamic problems. Because of their simplicity, they have been, and indeed remain, archetypal molecules for discussions on fluxional mechanisms. The two heavier congeners 5 and 6 have a clearer and less complicated history. In the solid state they are isostructural, and possess... 

Mo2(NMe2)e, these mechanisms are permutationally indistinguishable. However, we hope to be able to synthesize Mo2(NMe2)5(NR2) which, barring accidental anisochrony, should enable us to distinguish between the flipping and the fluxional mechanisms. 

Katakis and co-workers suggested that the fluxionality is due to the rearrangement reactions shown in Scheme 10.6. The rotation of any ring in either isomer appears to have 60 kJ mol . The relative rate constants for the interchange of the different proton types are reasonably consistent with the proposal. For the Mo analogue, bandshape analysis was found to be consistent with the same fluxionality mechanism. 

In solution, the phenyl derivative 12.29 (R = Ph) is fluxional. The mechanism of the fluxional process has been shown by an N NMR investigation of a partially N-labelled sample, Le., PhCN2 N3S3 ( N = 99% N), to involve a series of 1,3-nitrogen shifts (Section 4.8.4). Thermolysis or photolysis of 12.29 generates the corresponding 1,2,3,5-dithiadiazolyl radicals [RCNaSa]". ... 

Metathesis is a catalyzed reaction that converts two olefin molecules into two different olefins. It is an important reaction for which many mechanistic approaches have been proposed by scientists working in the fields of homogenous catalysis and polymerization. One approach is the formation of a fluxional five-membered metallocycle. The intermediate can give back the starting material or the metathetic products via a concerted mechanism ... 

The spiro dimer of a-tocopherol (9, see also Fig. 6.4) is formed as mixture of two diastereomers by dimerization of the o-QM 3 in a hetero-Diels-Alder reaction with inverse electron demand. Both isomers are linked by a fluxion process (Fig. 6.22), which was proven by NMR spectroscopy.53 The detailed mechanism of the interconversion, which is catalyzed by acids, was proposed to be either stepwise or concerted.53-55... 

The solvomercuration of alkenes has been exhaustively studied and is now well understood. The mechanism of this reaction proceeds through two steps coordination of the mercuric ion to the alkene to form a fluxional cationic intermediate and subsequent nucleophilic anti-attack. Charge stabilization of the cationic intermediate constitutes one of the factors dictating the regioselectivity of the reaction (Scheme 11). 

A completely different model of the origin of the syndiospecificity which involves the formation of a lluxional chiral site has also been proposed.147 According to this mechanism, the chirality of the growing chain determines the chirality of the fluxional site, which in turn discriminates between the two monomer enantiofaces. In particular, the assumed model site consists of a hexacoordinated metal (V) atom surrounded by four chlorine atoms assumed to be bridge bonded to other metal (i.e., Al) atoms147 149 (Figures 22a,b). 

The mechanisms of stereoselectivity which have been proposed for chain-end stereocontrolled polymerizations involving secondary monomer insertion also present a general pattern of similarity. In fact, molecular modeling studies suggest that for olefin polymerizations (both syndiospecific and isospecific, Section 4.1.2) as well as for styrene polymerization (syndiospecific, Section 4.2), the chirality of the growing chain would determine the chirality of a fluxional site, which in turn would discriminates between the two monomer enantiofaces. 

Scheme 1.10 Mechanism for the fluxional behaviour of the model Zr complex 98.

Kocak FS, Zavalij PY, Lam YF et al (2009) Substituent-dependent exchange mechanisms in highly fluxional RSng " anions. Chem Commun 4197 199... 

These complexes are effective catalysts in epoxidation reactions with H2O2 and alkyl hydroperoxides. Several detailed mechanistic studies have been carried out in particular, it has been shown that, when the alkyl chain contains a double bond, no autoepoxidation is observed both in the solid state and in solution. Nevertheless, if f-BuOOH is added, the epoxidation of the olefinic moiety immediately takes place. Therefore, it has been suggested that these complexes are not the active species in the oxygen transfer step to the substrate, but they behave as catalysts for the primary peroxidic oxidant. On the basis of kinetic, spectroscopic and theoretical studies, the authors provided a mechanism, whose key steps are sketched in Scheme 12. In this context a major role appears to be played by the fluxionality of the particular ligands used . ... 

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