Turnstile mechanism

Scheme 6. Turnstile mechanism for the ligand substitution reactions of oxo-rhenium(V) compounds.

Figure 5.21 Transition state optimized by ab initio DFT/B3LYP calculations for hydride-hydride exchange via the turnstile mechanism. (Reproduced with permission from ref. 32.)...

While pseudorotation is usually invoked to explain the observed exchange phenomena, alternative intramolecular processes have been proposed, such as the Turnstile Mechanism. This involves rotation of a pair of ligands on one side of the tbp and the remaining three on the other side. Both Berry and turnstile mechanisms result in a similar exchange, albeit via different transition states, ft is difficult to differentiate these mechanisms experimentally however, the structural data described in the next Section appear to support the Berry process. 

This map applies equally well to either Berry pseudorotations or Ugi turnstile mechanisms for isomerizations. It does not apply to Gielen s P3 mechanism 21> (Muetterties Process 2)22) for isomerization of trigonal-bipyramids. These isomerizations resemble a pseudorotation about an axial pivotal ligand rather than the usual equatorial of the Berry pseudorotation. Chart XX illustrates the process. The P3 reactions number 60, and each of the 20 trigonal-bipyramids can in principle enter into any of 6 different reactions. Maps of the P3 isomerizations are accordingly more complex. Three Berry pseudorotations accomplish the equivalent of a single P3 isomerization. Since the minimum number of Berry pseudorotations needed to generate an enantiomer is five, the minimum number of P3 isomerizations required is three. This relationship holds because it takes an odd number of Berry pseudorotations to get to enantiomer, and two P3 isomerizations is the equivalent of six Berry pseudorotations. 

An ab initio study of the pathway of ligand scrambling of PH4 indicated a turnstile mechanism Involving a TBP-ax transition state with a lower barrier than a Berry pseudorotation process with a C4V transition state [11] see [7]. 

The structures of the hydridorhodium complexes that are present in the catalytic system have been deduced by NMR spectroscopy. Brown showed that HRh(CO)2(PPh3)j exists as an 85 15 mixture of diequatoriakapical-equatorial isomers of HRh(CO)2(PPh3)2 (Scheme 17.11) that undergoes rapid equilibration at room temperature. This rapid equilibration of trigonal bipyramidal complexes could occur by either a Berry pseudorotation mechanism or a turnstile mechanism. In situ IR transmission spectroscopy on the catalytic system demonstrated that these two isomers are the resting state of the catalyst and were by far the predominant species present during hydroformylation of 1-octene (60-100 C, 5-20 atm, [Rh] = 1 mM, PPhj/Rh = 5). °... 

In solutions we need to distinguish intra- from inter-molecular and also intra- from inter-nuclear CO exchange in clusters. These various reaction types may easily be distinguished if /(M- C) can be observed. The axial-equatorial CO site exchange known for [OsafCO) ] could result from exchange localized at each Os (perhaps by a turnstile mechanism) or from... 

The subject of pentacoordinated phosphorus has been considered in depth in two volumes entitled Structure and Spectroscopy and Reaction Mechanisms. The latter volume covers two major topics, namely, a comparison of the Berry pseudorotation process and the turnstile mechanism for explaining the interconversion of rotamers, and the consideration of reactions (from simple to enzymatic) which involve five-coordinate phosphorus intermediates. 

The fluxional behavior of cyclic and acyclic [(diene)Fe(CO)2L] complexes (L = phosphine, phosphite, or isonitrile) is represented by Scheme 4. This can occur either by sequential Berry pseudorotation or simple rotation of the diene relative to the Fe(CO)2L fragments (turnstile mechanism). These processes are indistinguishable by NMR, but the turnstile mechanism is favored on strain energy grounds. 

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