Cyclooctatetraene ring inversion

The numerous transformations of cyclooctatetraene 189 and its derivatives include three types of structural changes, viz. ring inversion, bond shift and valence isomerizations (for reviews, see References 83-85). One of the major transformations is the interconversion of the cyclooctatetraene and bicyclo[4.2.0]octa-2,4,7-triene. However, the rearrangement of cyclooctatetraene into the semibullvalene system is little known. For example, the thermolysis of l,2,3,4-tetra(trifluoromethyl)cyclooctatetraene 221 in pentane solution at 170-180 °C for 6 days gave three isomers which were separated by preparative GLC. They were identified as l,2,7,8-tetrakis(trifluoromethyl)bicyclo[4.2.0]octa-2,4,7-triene 222 and tetrakis(trifluoromethyl)semibullvalenes 223 and 224 (equation 71)86. It was shown that a thermal equilibrium exists between the precursor 221 and its bond-shift isomer 225 which undergoes a rapid cyclization to form the triene 222. The cyclooctatetraenes 221 and 225 are in equilibrium with diene 223, followed by irreversible rearrangement to the most stable isomer 224 (equation 72)86. 

D. A. Hrovat, J. H. Hammons, C. D. Stevenson, and W. T. Borden, Calculations of the Equilibrium Isotope Effects on the Reductions of Benzene-dg and Cyclooctatetraene-dg, J. Am. Chem. Soc. 1997,119, 9523. B3LYP/6-31+G calculations on the title compounds and on the radical anions formed from them show that the very large difference between the equilibrium isotope effects, found by Stevenson, is due to an inverse isotope effect on the planarization of the COT ring. This explanation was subsequently confirmed by KIE measurements, carried out by C. D. Stevenson, E. C. Brown, D. A. Hrovat, and W. T. Borden, Isotope Effects on the Ring Inversion of Cyclooctatetraene, J. Am. Chem. Soc. 1998, 120, 8864. 

FIGURE 45. Schematic representation of the ring inversion (ri) transition state of cyclooctatetraene. The calculated energies are given in Table 38. Reprinted with permission from Reference 164. Copyright 1998 American Chemical Society... 

TABLE 38. Calculated energies of the ring inversion transition state (A.Eri) shown in Figure 45 for substituted cyclooctatetraenes. a... 

Gyclooctatrienyne zirconocene complexes of type 704 can be prepared by /3-hydrogen elimination from zirconocene biscyclooctatetraenyl complexes (Scheme 171).528 These complexes are fluxional by a ring inversion process, with activation barriers in the range typical of cyclooctatetraenes. An X-ray crystal structure (R = Ph) reveals a boat conformation of the cyclooctatrienyne ring with no significant flattening when compared with cyclooctatetraene, thus the structure more closely resembles substituted cyclooctatetraenes than that expected of a cyclooctatrienyne. 

Cyclooctatetraene can undergo three different isomerizations ring inversion la lb due to simple rotation about the C-C single bonds, reversible bond migration 1 a 1 b involving all four double bonds and dynamic equilibration with bicyclo[4.2.0]octatriene 2. ... 

Other applications are studies of rotation about double bonds 136>, ring inversion in cyclooctatetraene 137>, the insertion of carbon into ethylene and. fraws-2-butene to give allenes 133), the barrier height to inversion of nitrogen in hydrazine and alkylamines 133>, the Cope... 

All the protons of cyclooctatetraene (4mz, with = 2) resonate at a single frequency of t 4.309 68>. This magnetic equivalence is brought about by two processes, a ring inversion between two tub-shaped forms, and bond shifts, yielding the two structures 3 a and 3b, which, in the absence of substitution are equivalent. Studies of the n.m.r. temperature... 

Cyclooctatetraene is involved in two other dynamic processes in addition to. the valence tautomerism with bicyclo[4.2.0]octatriene. They are ring inversion (a) and exchange of the positions of the single and double bonds (p) which may be depicted as follows ... 

Reaction of a digermyne with cyclooctatetraene has been reported to give a Ge(II) inverse sandwich, a result of complete Ge=Ge bond cleavage and formation of a 7T-bound cyclooctatetraene ring. " This isomerizes to the thermodynamic product, a tetracyclic diene-digermane, in which a single-bonded Ge-Ge moiety has inserted into a C=C bond of the cyclooctatetraene carbocycle (Scheme 26). 

Cyclooctatetraene, i.e., [8]annulene 3, was the first [4n] i-electron hydrocarbon to be studied [57, 64]. It was found that 3 adopts a tub conformation, that minimizes the electronic interaction between the double bonds, and makes the system less conjugated and thus nonaromatic [65]. Compound 3 undergoes a dynamic process of ring inversion, which involves D4h transition states that have been intensively studied [66]. 

The ring inversion presumably involves a planar Dih transition state having alternate single and double bonds, and the bond shift is believed to proceed via a planar Dsh transition state in which all of the C—C bonds become equal in length. The inversion barrier has been measured in the gas phase for cyclooctatetraene via a measurement of the electron affinity of COT and was found to be 12.7 0.5 kcal/mol. ° The bond shift process has a somewhat higher barrier, 14 kcal/mol. The antiaromatic de-stabilization of the Dsh form is then about 2 kcal/mol Theoretical calculations are in good accord with the experimental results. 

This inversion barrier is much lower than that observed for derivatives of cyclooctatetraene (see Section X,B), and recognition that the fluorinated ring in complexes 119 may more closely resemble a cyclooctatriene than a cyclooctatetraene provides one rationale for the difference in its dynamic behavior compared to its rj1 -heptafluorocyclooctatetraeny 1 relatives. 

The protonation of cyclooctatetraene with D SO at —15 °C resulted in the exo-endo-inversion of the ring, seemingly with the. intermediate formation of ai planar cation 462 (AG 22.3 kcal/mole) ... 

The transition state for this reaction is a superantiaromatic (anti-Hiickel-dibenzo)-normal-Huckel-cyclooctatetraene. If the phase relationships in these reactions are examined, it is clear that there will be a phase inversion at both ring junctions. By proper choice of the basis set of AOs, these phase inversions can be eliminated, so the transition state is of Hiickel type. 

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