Pseudorotation lifetime

By definition, the retention mechanism involving axial attack and axial exit must involve a pentacoordinated intermediate of sufficient lifetime to allow pseudorotation. The evidence for pseudorotation in the retentive process is now reviewed. 

Pseudorotation about pivot ligand 1 is illustrated by Eq. (9). To be operationally detectable, it is necessary that the energy barriers for pseudorotation are accessible, and that the phosphorane has a sufficient lifetime, relative to its tetracoordinate reaction partners. 

Although the possibility of pseudorotation during enzymic phosphoryl transfer has been raised many times, there is as yet no unambiguous example of it. If groups enter and leave from apical positions, the trigonal bipyramidal intermediate may have a lifetime less than the period of a vibration and thus be too unstable to exist. In practice, therefore, the rule retention = double displacement, inversion = single displacement appears to hold for phosphoryl transfer at least as well as for glycosyl transfer. 

The mechanism of retention, involving axial attack, and axial exit by definition involves an intermediate of sufficient lifetime to allow pseudorotation. Since carbon nucleophiles do not lead to exchange with other substituents in a retentive process where R2 could also be lost, the energy difference between the activated complex for equation 14 and the... 

Pseudorotation was formulated to explain the behaviour of stable phos-phoranes and has been extrapolated to TBP reaction intermediates. However, the TBP species must be long-enough lived to allow pseudorotational processes to occur. In reactions of acyclic compounds, this is often not the case, since the TBP species is a transition state or short-lived intermediate. Only when the intermediate is stabilized (i.e. by relief of ring strain in five-membered rings) is its lifetime sufficient to allow ligand exchange via pseudorotation. 

An additional consideration is the identity of the TBP species. Since this species is not stabilized by large relief of ring strain, it may be more akin to that involved in many reactions of acyclic phosphorus compounds—that is, it may be a transition state, rather than an intermediate. If the six-membered TBP species has too short a lifetime then pseudorotation will not be allowed. Ligand reorganization about six-membered pentacoordinate phosphorus is... 

Although retention of configuration should be the result of type (2) (3.85) or type (3) processes, retention should also be the result of an apical-apical type (1) process if pseudorotation is involved. This requires that the intermediate trigonal bipyramid has sufficient lifetime for pseudorotation to take place before apical elimination occurs (3.86). 

Elimination processes of this kind can be further complicated by pseudorotation if the lifetime of the intermediate trigonal bipyramid is of sufficient duration. This probably explains the observed racemisation on alcoholysis, of phosphonium salts, which is a slower process than alkaline hydrolysis (13.137). While half the molecules may invert by apical-apical elimination, pseudorotation of 50% of the trigonal bipyramidal intermediate could lead to equatorial elimination and hence retention of configuration of the remaining molecules. 

Alkaline hydrolysis of the ester is also very rapid, but in this case no cyclic acid is formed. This suggests that under these conditions the lifetime of the intermediate state is too short for pseudorotation to take place. 

The various physical techniques that we might use to study molecular species depend on a variety of proeesses. The conclusions we could draw about structures are related to the timescales associated with these proeesses, and it is important for us to understand these if we are to avoid making erroneous deductions. In relation to any one type of experiment, there are in fact four different times for us to consider the time during which a quantum of radiation or a particle can interact with a molecule the lifetime of any excited state of the molecule the minimum lifetime that the species being studied must have to allow it to be seen as a distinct species and the total duration of an experiment in which the species is observed, which may be as much as several hours or as little as 10 s. Before we consider these further, we must look at the timescales of typical molecular processes so that we can relate them to timescales associated with structural techniques. Typical vibrational frequencies are of the order of lO to 10 Hz, while rotational frequencies are around 10 ° to 10 Hz. The inversion of ammonia has a rate of about 10 Hz at room temperature, while the corresponding rate for phosphine is 10 Hz. The inversion rate for methane is 10 Hz, so any one molecule inverts, on average, once every 100 million years But remember that there are 6 x 10 molecules in a mole of gas, so in fact the inversion is by no means a rare occurrence. Pseudorotation in PF5, which switches axial and equatorial fluorine atoms, has a rate of about 10 Hz at room temperature, while the rate for PCI5 is 10 Hz. 

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