Fluxional structure

A number of azabullvalenes (106 containing heterocyclic nitrogen) have been synthesized. They also have fluxional structures when heated, though with fewer tautomeric forms than bullvalene itself Paquette, L.A. Malpass, J.R. Krow, G.R. Barton, T.J. J. Am. Chem. Soc., 1969, 91, 5296. 

The spiro polymerization is a novel reaction type that uses the spiro dimerization of o-QMs to build up linear oligomers and polymers. The basic properties of the spiro dimer of a-tocopherol, that is, its fluxional structure and its ready reduction to the ethano-dimer, remain also active when such structural units are bound in the polymer. The products of the reaction, both in its poly(spiro dimeric) form (41) and in the form of the reduced polytocopherols (42), are interesting materials for application as high-capacity antioxidants, polyradical precursors, or organic metals, to name but a few. 

The CH cation 1, protonated methane, is the parent of hypercoordinated carbocations containing a five coordinated carbon atom. It is elusive in solution and has not been observed by NMR spectroscopy but gas-phase infrared investigations have shown its fluxional structure which has been proven by ab initio molecular dynamic simulation.18... 

Although a-bond metathesis of five-membered zirconacydes with EtMgBr [51] (Scheme 1.4) and H2ZrCp2 (Scheme 1.68) has been implicated, there are as yet very few well-established examples. The reaction of zirconacyclopentanes with alkyllithiums is interesting since it involves (i) the displacement of one of the two Cp groups, and (ii) the generation of a bimetallic species, the NMR spectroscopic data of which are consistent only with a fluxional structure as shown in Scheme 1.71 [66],... 

The bridging carbonate has an unusual T-T 1 T 2-rnode of coordination to U atoms with a fluxional structure, whereas the squarate species has a p-q2 ]2 structure (Figure 4.14). This is the first example of the formation of a carbocycle derived purely from a C02 carbon source. 

At least, it ought to have given the benzene oxide The compound turned out to have a fluxional structure—it was a mixture of compounds that equilibrate by a reversible disrotatory electrocyclic reaction. 

The pentacoordinate organotin compounds 416a-c also contain a tridentate dianionic N,N,0-chelate ligand . The structure of 416c was determined by X-ray diffraction which conformed the rigid polycyclic structure proposed on the basis of H, B, C, and Sn NMR data in solution (S Sn —150.4 ppm) . In contrast, the dimethyltin compound 416a displays a fluxional structure in solution. 

These results cannot be explained by a fluxional structure with a tetrahedral structure. A moving butterfly species is proposed for these clusters in solution. 

There are other types of proton shift tautomers, such as phenol/dienone, nitroso/oxime and imine/enamine, but these are less often encountered. There are also valence tautomers that exhibit fluxional structures, which undergo rapid sigmatropic rearrangements. Molecules that exhibit this type of isomerism are very interesting, but are not often encountered in undergraduate courses. An example of a valence tautomer is illustrated by the Cope system. 

The structure of the dihydride tautomer could not be determined in the solid state because it is an equilibrium species that could not be isolated from solution (where it has a fluxional structure). However calculations indicated that a pentagonal bipyramid with distal hydrides is the lowest energy structure [24]. Thermodynamic and kinetic investigations of the equilibrium reactions of the tungsten complexes showed the following parameters [25]. 

The retro-ene reaction was used to generate 6,6-dimethyl-6-silafulvene from allylcyclo-pentadienyldimethylsilane (equation 61)215. From this reaction the dimer of the presumably first formed 6-silafulvene was isolated. Unlike other silene dimers, the isolable product does not contain a four-membered ring, although the initially formed dimer may be a 1,3-disilacyclobutane. In solution, the isolable dimer has a fluxional structure due to rapid 1,5-silyl migrations. 

---