Pseudorotation mechanism

In summary the argument for or against in-line and adjacent (pseudorotation) mechanisms are inconclusive and may both be correct in the transphosphorylation and hydrolysis steps, respectively. 

NMR data, one can only say that an intermediate is probably protonated part of the time and thus either an in-line or a pseudorotation mechanism is allowed. The total interaction can include other specific hydrogen bonds and the interaction with solvent is completely conjecture. There must be a balance between too much stabilization of any intermediate and too little stabilization of the activated complex following it in the pathway. 

It is observed that on heating, the NMe2 methyl signals become equivalent with the same AG values as the methyl groups of the pinacoxy moiety. This implies a symmetrical geometry in the transition state. The only pseudorotation mechanism which involves the degree of symmetry required for the equivalence of the methyl groups is shown in... 

Myers and coworkers40 41 have shown evidence for a pseudorotational mechanism in the reaction of the O-silyl ketene A,O-acetal 17 with aromatic aldehydes (equation 8). 

Fluxional behavior was demonstrated for pentacoordinate alkylhydridosilicates in the gas phase, by the reaction of deuterium-labeled alkylsilicate with carbon dioxide65. The monodeuteriated silicate was first obtained in the flow tube by D- addition to a neutral alkylsilane, followed by H-/D- transfer to CO2 (equation 28). A statistical distribution of HC02 /DCC>2 (3 1 ratio) indicated complete scrambling of the hydrogens and deuterium in the silicate, presumably by a Berry pseudorotation mechanism. 

Berry turnstile pseudorotation mechanism. Interestingly, with an excess of less sterically demanding nucleophiles such as PMe3, PPh3, or P(OMe)3, only one of the phosphine ligands is substituted, yielding 67 when the nucleophile is carbon monoxide, probably by a dissociative mechanism [293],... 

S T 20 K. The temperature dependence of is well represented by either exponential or power law behavior. Possible pseudorotation mechanisms, including a temperature dependent tunneling, are outlined. 

FIGURE 11 Steps in the conversion of an asymmetric phosphorane (top left) into its enantiomer (top right) by a Berry pseudorotation mechanism. In each step, labeled Kx), one of the three equatorial ligands (the pivot ligand x) remains equatorial and is at the apex of the square pyramid in the transition structure. 

FIGURE 16.17. The three types of five-coordinate d complexes and their relationship to the Berry pseudorotation mechanism (see Figure 16.9). It appears that five-coordinate complexes of Rh, Ir, and Ni can undergo pseudorotation between a TBP and e-SQP structure. Other five-coordinate complexes of Ni, plus those of Pt and Pd, form f-SQP structures and do not undergo pseudorotation. 

All MO calculations, both semiempirical and ab initio, have demonstrated that the barrier to aZ>3h-C4v-D3h interconversion, the Berry pseudorotation process, is small for acyclic phosphoranes (see also Section 2.1.). Estimates of 1.4 (extended Hiickel MO34 ), 4.8 (ab initio35 ) and 3.5 kcal/mol (CNDO/231 ) have been obtained for PFS. The barrier computed from the ab initio calculation increases to 8.5 kcal/mol if the 3c -orbitals are not included. The turnstile rotation mechanism traverses a much higher energy barrier 10.0 (extended Hiickel MO), 18.1 (ab initio) and 9.1 kcal/mol (CNDO/2). On the basis of these calculations, the Berry pseudorotation mechanism must be the preferred explanation of pentatopal isomerisation in acyclic phosphoranes (see Section 2.1.). 

The great flexibility of the trigonal bipyramidal skeleton displayed in the structures of many cyclic oxyphosphoranes suggests that the Berry pseudorotation mechanism may be an oversimplified representation for the permutational isomerisation of such species. Theoretical calculations and statistical considerations combined with these studies lead to the conclusion that this isomerisation may take place by a continuum of different routes (including turnstile rotation pathways) over a broad relatively flat potential surface, which have similar energy barriers. The Berry pseudorotation will, however, represent the energetically most favourable pathway for acyclic derivatives. 

DFT calculations of the Berry pseudorotation mechanism in SF4 was used to give 19F chemical shifts for various points along the pseudorotation trajectory.1014... 

In the following chapters of this book, when a pseudorotation mechanism is involved in the overall sequence of reactions, it will be denoted in the reaction schemes using the formalism shown below, according to the convention introduced by Oae. This formalism does not bear any significance on the nature of the pathway followed by the topological transformation. 

Recently, unsymmetrical bismuthonium salts bearing four different aryl ligands have been prepared, and the chirality at bismuth has been investigated by H-NMR [990M5668]. The activation energy of isomerization at the bismuth center is strongly dependent on the nucleophilicity of the counter anions as well as the polarity of the solvents employed. This is rationalized by a pseudorotation mechanism via pentacoordinate species at the transition state. 

Since the kinetics of isomerization of 195 and 196 are different, severalmech-anisms can be operating. Study of compound 196 when compared with 195 does not adduce any supplementary evidence for a pseudorotation mechanism. However, it confirms the multiplicity of exchange processes possible at silicon. 

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