Migratory dynamics

A catalyst used for the u-regioselective hydroformylation of internal olefins has to combine a set of properties, which include high olefin isomerization activity, see reaction b in Scheme 1 outlined for 4-octene. Thus the olefin migratory insertion step into the rhodium hydride bond must be highly reversible, a feature which is undesired in the hydroformylation of 1-alkenes. Additionally, p-hydride elimination should be favoured over migratory insertion of carbon monoxide of the secondary alkyl rhodium, otherwise Ao-aldehydes are formed (reactions a, c). Then, the fast regioselective terminal hydroformylation of the 1-olefin present in a low equilibrium concentration only, will lead to enhanced formation of n-aldehyde (reaction d) as result of a dynamic kinetic control. 

The migratory insertion of CO into the Ti-Me bond in Cp2TiMe2 has been investigated by both static and dynamic density functional theory calculations. CO coordination prior to insertion has been analyzed considering both lateral and central approaches, and the two pathways were found to be kinetically equivalent. The 0- outside r -bound acyl complex is more stable than the 0- inside isomer by 4.0kcal mol-1, with an isomerization energy barrier of 9.6kcal mol-1.1334... 

A dynamic density functional study of methane photo-carbonylation by the Rh(PH3)2Cl catalyst has been reported [73]. It involves C-H bond activation to produce Rh(PH3)3Cl(H)(CH3)(CO) followed by migratory insertion of CO into the Rh-CH3 bond to generate Rh(PH3)2Cl(H)(CH3CO) and, finally, the elimination of acetaldehyde to form Rh(PH3)2Cl(CO) H)(CO)(CH3CO)... 

The suggestion is then made that the stereoconlrolling step in asymmetric Mizoroki-Heck reactions is oxidative addition (via dynamic kinetic resolution) rather than alkene association or migratory insertion. The implication is that only substrates capable of a dynamic kinetic resolution may cyclize with high enantioselectivity. This would limit the substrate scope of the asymmetric intramolecular Mizoroki-Heck reaction. While the dynamic kinetic resolution during the oxidative addition may be a component of the overall stereoselectivity, it does not rule out contributions from later events in the mechanistic pathway and does not explain the effect of additives on selectivity. What has been shown is that the axial chirality of the o-iodoanilides (as with any enantioenriched isomer of a chiral precursor) influences the stereochemical outcome of their reactions. 

This fundamental experiment has strong implications on related catalyst-controlled Mizoroki-Heck cyclizations of precursors of this type. As axial chirality in 113 sets the stereochemistry in 114, enantioinduction was rationalized to arise from a dynamic kinetic resolution of (at elevated temperature) rapidly interconverting enanhomers of 113 in the oxidative addition step, rather than in the alkene coordination-migratory insertion event. Such a dynamic kinetic resolution process has been previously proposed by Stephenson et al. within their mechanistic study regarding the conformations of helically chiral 2-iodoanilides in intramolecular asymmetric Mizoroki-Heck reactions [72],... 

Pathways to form trans addition products by Qialk-Harrod and modified Qialk-Harrod mechanisms are also shown in Scheme 16.10. The formation of trans addition products is rationalized by the dynamics of ri -vinyl complexes or by a zwitterionic intermediate." AVinyl complexes are known to exchange stereochemistry and are thought to do so by one of the mechanisms shown at the bottom left of tire two cycles in Scheme 16.10. One pathway involves rotation of the C-C bond upon reopening of the -r)--vinyl complex and a second involves formation of a zwitterionic intermediate. The steric interactions in the initial cis T -vinyl intermediate make it less stable than the trans T -vinyl complex. Such trans insertions of alkynes were discussed in detail in Chapter 9 (migratory insertions). 

In addition to migratory insertion/BHE, the aforementioned complexes are subject to further dynamical processes. Four nuclear (C/H) scrambling processes have been examined theoretically their computed activation energies agree very well with data from NMR studies [44]. 

In this context, it is also noteworthy to mention another ligand/transition metal related, dynamic behavior, namely, the lateral displacement of the whole ligand system around the imaginary axis connecting the transition metal bonds to the centroids, which was first described by Petersen [164]. This movement can be described as a kind of windshield wiper-type oscillation of the MCI2 moiety within the fixed ligand framework and could facilitate or influence the steps involved in the counter-ion-assisted site epimerization and chain migratory insertion processes (Fig. 19, top). 

Fig. 20 Cation/anion (A ) dynamics in the normal propagation process and in site epimerization. (a) The concerted process of monomer coordination and anion dissociation, (b) Diffusion-controlled process of anion rearrangement, (c) Normal chain migratory insertion/propagation. (d) Site epimerization...

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