High‐field NMR study of the tautomers resulting from the condensation of 2‐methylindole and phenalenone in the presence of <i>p</i>‐toluenesulphonic acid

Tort, F.; Waegell, B.; Zahra, Jean-Pierre; Bernasconi, Christian; Germanaud, L.; Bessibes, C.; Bounoure, J.

doi:10.1002/mrc.1260300802

Cited by 3 publications

(8 citation statements)

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“…42 Various features have been suggested to determine this ratio, such as the relative geometries of the radical pairs required for the disproportionation and the recombination reactions and the reorientation of the radicals in the pair. 42,46 There exists a distinct difference between k RD /k RC for the self-reaction of alkyl radicals 51 and for hydroaromatic (benzylic type) radicals. For example, the k RD /k RC (on a per hydrogen basis and around 330 K) changes from 0.3 (tert-butyl) to 0.0045 (cumyl) which cannot be explained on the basis of geometry or thermodyamic consideration.…”

Section: Scheme 3 Dehydrogenation Of Some Linear Dihydroaromaticsmentioning

confidence: 99%

“…The various tautomers of 1-methylphenalene have been observed in a NMR study at 298 K. 51 It has been hypothesized that the tautomerization proceeds through a RRD mechanism. 52 The migration of the double bond in (the model) phenalene sets in with a RRD reaction between two phenalene molecules (Scheme 9).…”

Section: Scheme 3 Dehydrogenation Of Some Linear Dihydroaromaticsmentioning

confidence: 99%

“…Rather, the interconversion must be suspected to be an (acid) catalyzed process. 51 The benzylic C−H bonds in the methylenedihydroacenes 1t and 2t are rather weak with BDE(C−H)s of of 61.5 31 and 67.7 32 kcal mol −1 , respectively (see Table 2). Therefore, these compounds are expected to be quite susceptible toward oxygen even at ambient temperature.…”

Section: Scheme 3 Dehydrogenation Of Some Linear Dihydroaromaticsmentioning

confidence: 99%

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Tautomerization of Some Methylacenes and the Role of Reverse Radical Disproportionation

Korth

2015

J. Org. Chem.

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The thermokinetics for the tautomerization of a series of methylenedihydroacenes to the corresponding methylacenes (toluene to 6-methylpentacene) have been investigated by means of CBS-QB3 calculations. Only for 6-methylpentacene does the methylenedihydro form predominate at room temperature. The obtained equilibrium ratios are consistent with various theoretical methods, but the agreement with the scarce experimental data is only qualitative. The noncatalyzed thermal tautomerization of the methylenedihydroacene in an inert solvent may proceed by means of a reverse radical disproportionation reaction (RRD) as the rate-determining step. The benzylic BDE(C-H)s and the hydrogen atom affinities (HA) of the tautomers have been used to calculate the reaction enthalpy, ΔRRDH. It appears that the Ea,RRD is substantially higher than ΔRRDH. This implies that the opposite reaction (and the tautomer forming step), a radical-radical disproportionation (RD), is an activated process. This is an often ignored or overlooked kinetic feature. The consequence is that although the RRD reaction may be kinetically feasible at elevated temperatures, the products are not the tautomers but rather dimers stemming from radical-radical recombination reactions, with p-isotoluene as a clear exception. It is further shown that the RRD self-reaction of phenalene is too slow at 298 K, despite claims to the contrary.

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Section: Scheme 3 Dehydrogenation Of Some Linear Dihydroaromaticsmentioning

confidence: 99%

Section: Scheme 3 Dehydrogenation Of Some Linear Dihydroaromaticsmentioning

confidence: 99%

Section: Scheme 3 Dehydrogenation Of Some Linear Dihydroaromaticsmentioning

confidence: 99%

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Tautomerization of Some Methylacenes and the Role of Reverse Radical Disproportionation

Korth

2015

J. Org. Chem.

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“…4 Phenalene 6, the hydrogenated phenalenyl, undergoes six-fold tautomerization (Scheme 2) owing to the hydrogen shift of the benzylic-allylic hydrogen, 5 according to NMR evidence. 6 Inspired by these phenomena, we proposed that the phenalenyl s-dimers undergo six-fold sigmatropic rearrangements by breaking and forming the inter-phenalenyl CC s-bond that is the counterpart of the C(sp 3 )-H bond in phenalene. However, such s-bond shift has not been observed because most of the experimentally isolated phenalenyl derivatives exist in the form of p-dimeric structures.…”

Section: Introductionmentioning

confidence: 99%

Fluxional σ-bonds of 2,5,8-tri-tert-butyl-1,3-diazaphenalenyl dimers: stepwise [3,3], [5,5] and [7,7] sigmatropic rearrangements viaπ-dimer intermediates

Tian

Huang

Kertész

2010

Phys. Chem. Chem. Phys.

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Inspired by experimental evidence of the thermally accessible pi-dimer of the title compound, DAzPh (7), we propose that the sigma-dimer (8) can undergo a variety of sigma-bond shifts representing very unusual multi-faceted fluxional bonding between two neutral pi-radicals. In this paper, we present a theoretical study of the sigmatropic rearrangement of the DAzPh sigma-dimers. Out of the six sigma-bonded tautomers three are competitive: a degenerate pair resulting from a [5,5] sigmatropic rearrangement and a non-degenerate product of a [3,3] sigmatropic rearrangement with barriers of 10.21 kcal mol(-1) and 10.00 kcal mol(-1), respectively. Both of these rearrangements occur stepwise through a pi-dimer intermediate (9), which is 1.33 kcal mol(-1) higher in energy than the sigma-dimer (8). These data are consistent with optical and paramagnetic susceptibility experiments and offer a natural interpretation for the unusual C-C contact distance of 2.153 A obtained by X-ray diffraction by Morita et al. Another new sigma-dimer (15) with a different dipole-dipole stacking pattern is predicted, the energy of which is very close to that of 8, and is likely to be isolable under suitable conditions. The new sigma-dimer (15) is expected to undergo stepwise [7,7] sigmatropic rearrangement. Thus we observed a complete spectrum of sigmatropic rearrangement reactions in these DAzPh dimers. The pi-dimers 4, 9 and 17 show decreasing order of SOMO-SOMO splittings consistent with the UV-vis absorbance. The calculated paramagnetism is in good agreement with experiments providing further evidence for the presented interpretation of fluxional bonding in the DAzPh sigma-dimers.

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“…[2][3][4] Phenalene, the precursor of the phenalenyl radical, has been proposed to undergo a six-fold tautomerism as described in Scheme 1, in which the benzylic-allylic hydrogen transfer reactions are hypothesized to occur through an unimolecular process. 6 NMR spectroscopy studies 7 showed that the reactions designed to obtain 1-alkyl-phenalenes, as a result, yielded a mixture of five kinds of isomers indicating that hydrogen transfer occurs 8 in solution. However, the mechanism of this reaction has not been discussed in the literature.…”

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confidence: 99%

Bimolecular hydrogen transfer in phenalene by a stepwise ene-like reaction mechanism

Tian

Kertész

2010

Chem. Commun.

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For the hydrogen transfer of phenalene, a bimolecular ene-like mechanism is proposed, which is preferable over the hypothesized unimolecular rearrangement in the literature. Unique SOMO-SOMO pi-bonding of phenalenyl reduces the barriers of pericyclic reactions significantly. Pi-bonding between radicals is being recognized as a novel type of bonding interaction. This paper adds to the use of this interaction by pointing out its effect on reaction barriers.

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High‐field NMR study of the tautomers resulting from the condensation of 2‐methylindole and phenalenone in the presence of p‐toluenesulphonic acid

Cited by 3 publications

References 12 publications

Tautomerization of Some Methylacenes and the Role of Reverse Radical Disproportionation

Tautomerization of Some Methylacenes and the Role of Reverse Radical Disproportionation

Fluxional σ-bonds of 2,5,8-tri-tert-butyl-1,3-diazaphenalenyl dimers: stepwise [3,3], [5,5] and [7,7] sigmatropic rearrangements viaπ-dimer intermediates

Bimolecular hydrogen transfer in phenalene by a stepwise ene-like reaction mechanism

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