Elucidation, by Scanning Microcalorimetry, of the Detailed Mechanism of the Thermal Rearrangement of [18]Annulene into Benzene and 1,2-Benzo-1,3,7-cyclooctatriene. Determination and Interpretation of the Thermochemical and Kinetic Parameters of the Three Consecutive Reactions Implicated

Abstract

The thermograms Q 4 r (T) recorded for the thermal rearrangement of [18]annulene 1 have been reinvestigated using an iterative method (CALIT-3 program) based on the numerical integration of the kinetic equations established for the following reaction mechanism: [18]annulene 1 98 k 1 tetracyclic intermediates 2 98 k 2 transbicyclo[6.4.0]dodeca-2,4,6,9,11-pentaene 3 + benzene 5; 3 98 k 3 1,2-benzo-1,3, 7-cyclooctatriene 4. Independent 13 C-NMR experiments indicate that three chiral tetracyclic intermediates 2a, 2b, and 2c are implicated in the reaction. They are considered to play the same role in the mechanism and are therefore considered as a single compound 2 in the kinetic equations. A programmed differential microcalorimeter with linearly increasing temperature was used for producing the thermograms. The reaction enthalpies ∆ r , i H of each individual reaction (i ) 1, 2, or 3) as well as the Arrhenius parameters E a,i , log A i and the activation thermodynamic quantities ∆H q i , ∆S q i , and ∆G q i (at 298.2 K) have been established by the iterative numerical simulation of the thermograms. These results are discussed. The enthalpy of formation of [18]annulene 1, as well as the stabilization and π-bond delocalization energies of this molecule have been deduced: ∆H°f (1, g, 298.2 K) ) 123.4 ( 3.7 kcal mol -1 ; ∆H stab (1) ) -37.6 ( 4.5 kcal mol -1 ; ∆H deloc (π-bonds in 1) = -120.5 kcal mol -1 . The stabilization enthalpy ∆H stab (1) has been used for discussing the activation enthalpy of the conformational mobility in 1 previously reported. The pathway multiplicities deduced from the detailed mechanisms for each individual reaction are discussed in view of the interpretation of the large activation entropies found. Of special interest is the discussion of the mechanism of the two suprafacial 1,5 hydrogen shifts implied in the reaction 3 98 k 3 4 and of the high activation entropy observed. In independent measurements, the rate of disappearence of the [18]annulene 1 has been followed by UV spectroscopy in order to obtain the kinetic parameters for the first reaction. The results of these independent measurements are in good agreement with those obtained by the analysis of the thermograms. The reported investigation points to the fact that the thermograms recorded with our calorimetric method constitute very precise data and that their correct analysis allows the elucidation of complex reaction mechanisms.

Experimental and Theoretical Evidence for Aromatic Stabilization Energy in Large Macrocycles

“…This alternation in barrier height, from experimental NMR data, suggests that the ASE of [18]annulene is about 11 kJ mol -1 (compared with 36.9 kJ mol -1 from DFT calculations of hyperhomodesmotic reactions), which is significantly lower than previously reported estimates. 15,19,[20][21][22][23]25 To the best of our knowledge, this is the first study to provide experimental evidence for ASE in systems with Hückel electron counts greater than 18. Our results show that aromaticity has a small, but measurable, effect on the stability of large π-conjugated macrocycles.…”

“…To the best of our knowledge, the largest π-conjugated macrocycle for which experimental thermochemical data have been used to estimate the ASE is [18]­annulene (C 18 H 18 ). In this case, two wildly different determinations of the heat of formation (Δ H f (g) = 280 ± 25 and 516 ± 16 kJ mol –1 ) ,− lead to estimates of the ASE of 162 and 398 kJ mol –1 , both defined by comparison with a hypothetical “Kekulé [18]­annulene” in which every double bond is isolated. Various theoretical estimates of the ASE of [18]­annulene have been reported, ,, and the best approach appears to be to calculate the enthalpy for a homodesmotic reaction of the type shown in Figure a .…”

“…Oth and co-workers estimated the ASE of [18]­annulene from the activation energy for conformational exchange of internal and external hydrogen atoms, that is, the exchange of protons H a and H b in Figure a, measured by 1 H NMR spectroscopy, giving ASE ≈ Δ G ‡ ≈ 67 kJ mol –1 . , Later, they suggested that this method underestimates the ASE, because the transition state for exchange, in which one alkene unit is twisted out of conjugation, still has the resonance energy of a linear polyene. , In other words, the exchange barrier is nearer the Dewar resonance energy , than the ASE relative to the Kekulé [18]­annulene. Conversely, other factors may destabilize the transition state, so that Δ G ‡ overestimates the ASE, as indicated by the observation that antiaromatic [16] and [20]­annulenes also have significant barriers for this type of exchange process, even when the ASE is expected to be negative.…”

A Flexible Porphyrin–Annulene Hybrid: A Nonporphyrin Conformation formeso‐Tetraaryldivacataporphyrin

“…[24,25] Although an [18]annulene complies with the spectral and structural criteria of aromaticity, as a consequence of the conformational flexibility it is a relatively reactive molecule, which decomposes at elevated temperatures with the tendency for addition/polymerization rather than electrophilic substitution reactions. [26,27] The last decade of the 20th century heralded a renaissance of annulene chemistry. [28] A number of synthetic discoveries in organic chemistry gave the ability to create new annulenes with a variety of uses or interest from the theoretical point of view, including the very first Mçbius annulenes [29,30] or porphyrinoids.…”

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“…Oth and Gilles studied the thermal rearrangement of [18]annulene by scanning microcalorimetry. 44 The annulene rearranges to three isomeric tetracyclic intermediates through a series of three electrocyclic ring closures, then stepwise dissociation gives benzene and a benzannulated cyclooctatriene, which undergoes two [1,5]-hydrogen migrations. Numerical simulation of the thermograms provides reaction enthalpies and activation parameters.…”