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A ring whizzer is a fluxional molecule frequently encountered in organometallic chemistry in which rapid rearrangements occur by migrations about unsaturated organic rings.[1] Ring whizzing, also called ring walking, is an intramolecular isomerization that features fluxional process. It results in the change of the relative positions of the substituent groups on a ring. In a typical ring whizzing process, the hapticity of the migrational substituent group doesn’t change.[2]


Shown in figure 1 is a ring whizzer molecule. W(Tp)(PMe)3NO could migrate around the bezene ring which results in the change of the relative positions of W(Tp)PMe3NO substituent group and CF3 substituent ligand.[3]

Fluxionality

A chemical species is said to be fluxional if it undergoes rapid degenerate rearrangements (generally detectable by methods which allow the observation of the behaviour of individual nuclei in a rearranged chemical species, e.g. NMR, X-ray).[4] Example (shown in figure 2): bullvalene: interconvertible arrangements of the ten CH groups.[5]


NMR

Ring whizzers are studied by H NMR spectroscopy. The number of peaks doesn't correlate with the number of peaks implied by the structure. The number of peaks is usually smaller because the protons become magnetic equivalent due to the fluxional behavior of the molecule. One example is the hydrogen shifting observed in cyclopentadiene.The H NMR spectra of it is shown in figure 3. From the structure of this compound, it could be predicted that the H NMR spectrum should show a triplet at about δ4 ppm corresponding to the sp3 protons and more peaks at about δ 5-6 corresponding to the sp2 protons. The ratio should be 1:2.[6]


In fact, the room temperature spectrum shows only one peak at about 4.8 ppm. The reason for this is that a series of successive [1,5] hydrogen shifts is occurring so quickly that only the 'averaged' proton position is manifested in the NMR spectrum. Slowing down the exchange by cooling to about -50°C produces the expected spectrum described above. Many NMR spectra are affected by these “ring whizzing” processes, or so called 'degenerate' pericyclic processes.

The study of the molecule TpW(NO)(PMe3)-(η2-arene) showed similar behavior in NMR spectroscopy. The monohapto nature of TpW(NO)(PMe3)-(η2-arene) has been confirmed by single crystal x-ray diffraction. The molecular structure is shown in figure 4. Curiously, the NMR data do not suggest any unusual coordination mode in the molecule, showing only one signal for the Cp ring in both the 1H and 13C NMR spectra. As an 5 coordination mode would give a 22-electron complex, the only plausible explanation is that this complex is fluxional, rapidly hopping from one carbon to another. The barrier to this interconversion is so low that the fluxionality persists even at -87 degrees °C.[7]

File:Re real-1.png
Figure 4.TpW(NO)(PMe3)-(η2-arene)

Possible mechanism of ring whizzing

Bennet et al. studied the compound Fe(η5-C5H5) (η1- C5H5)(CO)2 to explore the ring whizzing mechanism. The compound has the structure shown in figure 5.

Figure 5. The molecular structure of Fe(η5-C5H5) (η1-C5H5)(CO)2


It could be seen from figure 6 that at 30°C, the H NMR for this compound shows only two peaks, one typical of the η5-C5H5 hydrogens and the other one for all of the η1-C5H5 hydrogens. The peak with the chemical shift of 5.6 is assigned to the protons on the π- C5H5 ring. The two peaks gradually collapse as the temperature goes down. If the latter, η1-C5H5, is not fluxional, then it should show two peaks(due to a and b protons in the structure) plus the unique H on the same C as the Fe. As is shown on figure 6, the single peak for the η1- C5H5 protons collapses into two peaks as temperature decrease. The two peaks may appear because the fluxional motion of η1- C5H5 ring slows down. These observations are in correspondence with the Fe moving around the η1- C5H5 ring. That the peaks don't collapse at the same rate indicates that the shift might not be a random process. The researcher excluded other possibilities and came to the conclusion that the process could be a series of either 1,2 or 1,3 shifts of the Fe around the η1-C5H5 ring.[8] The two shift mechanisms are shown in figure 7. For the 1, 2 shift, the B' acts as a magnetical equivalent of B. For the 1, 3 shift, the A acts as magnetical equivalent of A'. The schematic presentation of the corresponding orbitals are shown in figure 8. If the A resonance is collapsing more rapidly the mechanism is a 1,2 shift. If the B resonance is the one collapsing more rapidly, the mechanism is a 1, 3 shift.[9]

File:Two shift.png
Figure 7. The two shift mechanisms of Fe about the η1-C5H5 ring


Both of the paths are compatible with the unsymmetrical collapse of the low-field resonance. However, from the observations of Bennett, the resonance which collapses most rapidly has been shown on the basis of the limiting low temperature spectrum to be most likely due to the A protons. In this way, it is concluded that the 1,2 shift pathway should be the major path of rearrangement.[10]

Symmetry rules in explaining the mechanism of ring whizzing

Researchers have been trying to apply the symmetry rules like Woodward-Hoffmann rules to inorganic transition-metal systems.[11] For organic reaction like sigmatropic shift, the configuration of the products could be predicted based on Woodward-hoffmann rules. According to the rule, 1,3 sigmatropic shift would result in the inversion of configuration while the 1,5 shift would result in retention of the configuration.[12]

The migration or rearrangement is symmetry allowed if the migrating group can maintain proper overlap. In the meantime,the migrating group has to stay on the same side of the molecule.[13] It could be seen from figure 9 and 10 that the migrating groups maintain proper overlap in a favorable 1,5 shift.


File:1,5 orbital.png
Figure 10. The orbital of the migrating group in a 1,5 shift could maintain proper overlap as it moves.

A 1,5 sigmatropic shift is symmetry allowed and occurs suprafacially with the migrating group staying on the same side of the molecule, as is shown in Figrue 8. The orbital of the migrating group could maintain proper overlap as it moves.

Researchers then extended the symmetry rules to pi-bonded cyclic organometallic systems.[14] The strategy is to draw the valence bond structure and move electron pairs in the standard way of organic chemistry to obtain the product. If the rearrangement is symmetry allowed and suparafacial, then the fluxional process is expected to proceed with a low energy barrier. For example, with the Fe(η1-C5H5) system discussed below, the process is shown by the shown in figure 11.It corresponds to a 1,5 shift and would be predicted to be of low energy. It is the same as 1,2 shift mentioned previously in a C5 ring.

File:1,5 shift cyclic.png
Figure 11.1,5 shift in a Fe(η1-C5H5) system

Same approach could be applied to systems with higher hapticity. For example, η2-C6H6 systems undergo a favorable 1,5 shift, but η4-C6H6 systems have an unfavorable 1,3 shift, as is shown in the reactions in figure 12 and figure 13:

File:1,5 shift hap2.png
Figure 12. Favorable 1,5 shift in η2-C6H6 system
File:1,3 shift hap4.png
Figure 13. Unfavorable 1,3 shift on η4-C6H6

Application of ring whizzers

Ring whizzers are ideal choice for molecular nanotechnology.[15] These are very simple molecules, which are not rigid and undergo intra-molecular rearrangements very easily. The rearrangement leaves the structure of the molecules unchanged, but does change the arrangement of the atoms in the molecule, and is usually studied using dynamic NMR spectroscopy.[16] In particular, ring whizzers are ring containing organometallic compounds for which the M-C bond, keep on changing at ordinary temperatures. The experimentally known ring whizzers Cyclopentadienyltrimethylsilane [(η1-C5H5)Si(CH3)3] and Cyclopentadienyl-trimethylgermane [(η1-C5H5)Ge(CH3)3] and Cyclopentadienyl-trimethyl-stannane [(η1-C5H5)Sn(CH3)3] are possible candidates for this use. The barriers to their rotational motion are studied. (Then we ask: what would happen if the cyclopentadienyl ring is bonded to an M atom, which is a part of an M surface. Calculations show that cyclopentadienyl adsorbed to Si, Ge or Sn surfaces (co-adsorbed with H, so that all neighboring sites are blocked), can exhibit interesting spinning motion due to their fluxional behavior. The motion involves movement of the point of attachment around the ring, resulting in a wheel like motion of the molecule, though there is no net motion of the molecule in space (see figure 9). Our studies predict the activation energy for this to be rather low [~13.5 kcal/mol for Si(111), ~11.0 kcal/mol for Ge(111) surface and ~6.0 kcal/mol for Sn (111) surface], and hence it should be possible to observe this experimentally. Shown in figure 14 is a schematic diagram of cyclopentadienyl adsorbed on M surface. The ring changes the position of C atom being bonded to the same semiconductor site The calculations were done for a cluster containing 13 M atoms, using GAUSSIAN 94 using the B3LYP hybrid-correlation-functional. 

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References

  1. ^ Heinekey, D.M.; Graham, W.A.G. J.Am.Chem.Soc. 1979,101,6115
  2. ^ John W. Faller Stereochemical Nonrigidity of Organometallic Complexes Encyclopedia of Inorganic and Bioinorganic Chemistry 2011 DOI: 10.1002/9781119951438.eibc0211
  3. ^ 1. Kevin D. Welch, Daniel P. Harrison, Edward C. Lis, Jr., Weijun Liu, Rebecca J. Salomon, and W. Dean Harman (2007) “Large-Scale Syntheses of Several Synthons to the Dearomatization of Agent {TpW(NO)(PMe3)} and Convenient Spectroscopic Tools for Product Analysis”, Organometallics, 26 (10), pp 2791–2794
  4. ^ Braga, D.; Grepion, F.;Orpen, A.G. Organometallics 1993, 12, 1481
  5. ^ Haijun Jiao, Paul von Rague Schleyer (1995) “Electrostatic Acceleration of Electrolytic Reactions by Metal Cation Complexation: The Cyclization of 1,3-cis-5-Hexatriene into 1,3-Cyclohexadiene and the 1,5-Hydrogen Shift in Cyclopentadiene. The Aromaticity of the Transition Structures.” J. Am. Chem. Soc., 117 (46), pp 11529–11535
  6. ^ Piper, T.S.; Wilkinson, G.J.Inorg.Nucl.Chem. 1956,3,104
  7. ^ Yu.A. Ustynyuk, A.V. Kisin, A.A. Zenkin (1972) “NMR spectroscopy of metal cyclopentadienyls IX. Non-degenerate metallotropic rearrangement in bis(trimethylgermyl)- and bis(trimethylstannyl)-cyclopentadienes” Journal of Organometallic Chemistry, 37, (1), pp101–112
  8. ^ Bennett, M.J.; Cotton, F.A.;Davison,A.; Faller, J.W.;Lippard, S.J.; Morehouse, S.M. J.Am.Chem.Soc. 1966, 88,4371
  9. ^ Robert B. Jordan, Reaction Mechanisms of Inorganic and Organometallic Systems (Topics in Inorganic Chemistry),June 18, 2007 ISBN 978-0195301007
  10. ^ M.J. Bennett, F.A. Cotton, A. Davison, J.W. Faller, S.J. Lippard, S.M. Morehouse, J. Am. Chem.Soc. 88 (1966) 4371
  11. ^ Jerome A. Berson 1968, The stereochemistry of sigmatropic rearrangements. tests of the predictive power of orbital symmetry rules Acc. Chem. Res., 1 (5), pp 152–160
  12. ^ F. Albert Cotton (2002) “A Half-Century of Nonclassical Organometallic Chemistry:  A Personal Perspective” Inorg. Chem.,41 (4), pp 643–658
  13. ^ Robert B. Jordan, Reaction Mechanisms of Inorganic and Organometallic Systems (Topics in Inorganic Chemistry),June 18, 2007 ISBN 978-0195301007
  14. ^ 3.Mingos, D.M.P.(1977) J.Chem.Soc., Dalton Trans.602
  15. ^ 10th Foresight Conference on Molecular Nanotechnology
  16. ^ Gregg S. Kottas, (2005), Chem. Rev. Artificial Molecular Rotors 105, 1281-1376 1281
source: https://en.wikipedia.org/w/index.php?title=Ring_whizzer&oldid=492879448

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