A series of a-aryl and diarylthioforrnarnidyl cations were observed by low-temperature nuclear magnetic resonance spectroscopy. These ions undergo efficient cyclization and deprotonation to give benzothiophenes.
, 1343 (1988). The photoisomerization of a series of protonated, methyl substituted phenols to protonated bicyclo[3.1 .O]hexenones has been examined. These reactions, which were carried out in CF3S03H as a strong acid solvent at ambient temperatures, provide a convenient route to a variety of bicyclo[3.1 .O]hexenones. The quantum yields for these photoisomerizations vary from 0.01 8 for protonated 3,5-dimethylphenol to 0.65 for protonated 2,6-dimethylphenol. This variation in efficiency can be understood in terms of a competition between ring opening, to regenerate the starting phenol, or cyclopropyl migration, to give product, of an initially formed intermediate. On peut interpreter cette variation en fonction d'une compktition entre une ouverture de cycle qui regknere le phCnol de dCpart et une rCaction qui conduit au produit et qui implique la migration du cyclopropyle d'un intermkdiaire qui se formerait initialement.[Traduit par la revue]Most often on dissolution of a phenol in a strong acid medium, proton attachment occurs on the carbon para to the oxygen atom of the phenol to give what can be considered to be an 0-protonated cyclohexa-2,5-dienone, eq. [ l ] (1). These protonated phenols are stable in solution provided the strong acid solvent is a non-oxidizing and non-sulphonating medium. Triflic acid, CF3S03H, meets these criteria and is an excellent medium for these cations, with solutions being stable for long periods even at elevated temperatures (2). The only limitation on the use of triflic acid would seem to be its lower acidity compared to the other commonly used super-acid solvents (3) and the requirement for a more acidic medium to ensure ring protonation of some of the least basic phenols.Ring protonated phenols have been shown to photoisomerize to give protonated bicyclo[3.1 .O]hexenones on irradiation, eq.[2] (4, 5). This transformation parallels the analogous photoisomerization of these protonated phenols, can also be converted back to a protonated phenol thermally. In many instances the activation energies for these isomerizations are large and the acid solutions have to be heated to relatively high temperatures for the thermal isomerizations to proceed. Indeed, the thermal stability and highly exothermic nature of these rearrangements (8) have led us to suggest that the photochemical and reverse thermal isomerizations of protonated phenols are potential candidates for a solar energy, photochemical/latent heat storage system (9). To evaluate the potential use of protonated phenols for solar energy storage it is important to know considerably more about the photoreactions themselves. For this purpose we have undertaken a quantitative examination of the photoisomerization of a series of methyl substituted protonated phenols. The results of this study are reported here. Results and discussion Characterization of the protonated phenolsThe methyl substituted phenols 1-10 were protonated by dissolving them in CF3S03H at 25°C. The resulting cations, 1H-lOH, were characterized by comparison of their...
, 1350 (1988). The thermal isomerization of a wide range of protonated methyl substituted bicyclo[3.1 .O]hex-3-en-2-ones to protonated phenols has been examined using triflic acid as a strong acid solvent. The rate constants and activation energies of these isomerizations have been determined. The barriers to the isomerizations were shown to be dependent on the number and position of the methyl substituents. The results show that three different mechanisms are needed to account for these isomerizations, two of which involve a preliminary circumambulatory rearrangement prior to ring opening and the other process involving a direct ring opening of the initial protonated bicyclohexenone to give an intermediate meta-protonated phenol. RONALD F. CHILDS et BAHA E. GEORGE. Can. J. Chem. 66, 1350 (1988). OpCrant dans I'acide triflique qui agit comme solvant fortement acide, on a CtudiC I'isomCrisation thermique d'un grand nombre de bicyclo[3.1 .O]hexkne-3 ones-2 substituies par des groupements mCthyles en phenols protonCs. On a dCterminC les constantes de vitesse ainsi que les Cnergies d'activation de ces isomCrisations. On a dCmontrC que les barrikres B ces isomCrisations dCpendent du nombre et de la position des substituants mCthyles. Les rksultats dCmontrent que trois mCcanismes diffkrents sont nCcessaires pour expliquer ces isomCrisations : deux impliquent une transposition circumambulatoire prkliminaire prkckdant l'ouverture du cycle alors que I'autre processus implique une ouverture directe du cycle de la bicyclohexknone protonte initiale conduisant a un phCnol intermediaire proton6 en mito.[Traduit par la revue]The thermal isomerization of bicyclo[3.1 .O]hexenyl cations to the isomeric benzenium ions is a well-known reaction (1-4). This 4e-disrotatory electrocyclic reaction is formally forbidden in the ground state and as a consequence these reactions, despite their strongly exothermic nature, involve relatively high activation energies ( 5 ) .When these bicyclic cations bear a hydroxy substituent, the corresponding thermal isomerization to hydroxybenzenium cations appears to be more complicated than that of the parent benzenium ions. The isomerization of the hexamethyl substituted cation, 1 , has been studied in detail ( l , 6 ) . Careful studies with deuterium labeled derivatives showed that this reaction proceeds by a circumambulation of the cyclopropyl ring around the five-membered ring followed by a higher energy ringopening reaction of the unobserved intermediate 3. There have been no further reports of studies of the ring-opening reactions of other protonated bicyclo[3.l .O]hexenones and it is not clear whether the isomerization pathway observed with 1 is a general feature of these systems.In the isomerization of 1 the direct ring-opening reaction to SCHEME 1 '~uthor to whom correspondence may be addressed.give 4 and thence 5 is of higher energy than the observed pathway involving 3. The indirect route involving the intermediacy of 3 does not eliminate the need for the disrotatory electrocyclic ring openin...
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