Mechanistic Study on Thermal Isomerization of 1-Methylbenzocyclobutenol to 2-Methylacetophenone

Abstract

Heating 1-trideuteriomethylbenzocyclobutenol 7 in benzene-d 6 at 160 °C gave 2-monodeuteriomethyl- and 2-methylacetophenone 9 and 10 in a ratio of 96:4. Thermolysis of 7 in nonpolar solvents (hexane, toluene, mesitylene) gave similar results. On the contrary, heating 7 in polar solvents (ethanol, acetonitrile, chloroform) or in benzene-d 6 in the presence of proton source (PhCO2H) gave 10 as the major product. However, heating a mixture of 7 and N-phenylmaleimide at 160 °C in benzene-d 6 or acetonitrile-d 3 gave adduct 12 of N-phenylmaleimide and the dienol generated by ring opening of 7 almost quantitatively. These results indicate that 1-methylbenzocyclobutenol undergoes selective thermal opening to the E-dienol. The resulting E-dienol isomerizes to 2-methylacetophenone by 1,5-sigmatropic shift of hydrogen from the methyl group in nonpolar solvent. In polar solvent, the E-dienol isomerizes to 2-methylacetophenone by both intra- and intermolecular processes. The k H/k D value for isomerization was 1.13. Since this relatively low value is a secondary kinetic effect, overall reaction is governed by the ring opening step. The selective opening to the E-dienol was supported by calculation.

Thermal reaction of diastereomeric benzocyclobutenols. Evidence for reversible opening of 1,2-dihydrobenzocyclobutenols to hydroxy-o-xylylenes

“…9 The resulting (E)-dienol undergoes recyclization to give 1,2-dihydrobenzocyclobutenols 2 and 3 along with intermolecular hydrogen transfers to give hydroxy and acetoxy ketones 1. 10 The lower yields of hydroxy ketones 1a,b compared with 1c in the thermolysis of 3a-c without solvent at 120 ЊC may be due to a bulkier substituent in 3a,b than in 3c. The bulky substituent may prevent the approach of the initially formed (E)-dienols.…”

Stereospecific Thermal Isomerization of 2,2-Dimethylbenzocyclobutenols to 2-Isopropenylphenyl Alcohols

“…We have recently reported that 1-methylbenzocyclobutenol undergoes selective opening to the ( E )-dienol and that the resulting ( E )-dienol undergoes isomerization to 2-methylacetophenone by two processes, viz. intramolecular 1,5-sigmatropic hydrogen shift from the methyl group and intermolecular proton transfers 7b. The 2-isopropylphenyl ketones 9a and 9b would arise through the ( E )-dienol by a hydrogen shift from the alkyl group on the carbon bearing OH group and/or by intermolecular process.…”

“…o -Xylylenes are also generated by thermal ring opening of benzocyclobutenes . Benzocyclobutenols are known to undergo selective outward rotation of the OH group to give ( E )- o -xylylenols, also named ( E )-dienols . The ( E )-dienols generated from 2,2-dimethylbenzocyclobutenols having a bulky C 1 -alkyl group are expected to have a twisted nonplanar structure because of the steric repulsion between the bulky C 7 -alkyl group and a C 8 -methyl group oriented inside.…”

Silyl Migration in the Photochemical Reactions of 2-Trimethylsilylmethylphenyl Ketones

“…This would lead to formation of the isomer wherein the phenyl and the trimethylsilylmethyl groups are cis. Benzocyclobutenol 2a is thermally unstable and converted to the starting ketone over 1 day at room temperature via E -xylylenol 3a . , Prolonged irradiation of 1a , however, gives siloxybenzocyclobutene 4a (10%) and less hindered benzocyclobutenol 5a (12%) together with 2a (8%) (Scheme ) . In 1 H NMR, the trimethylsilyl protons of 2a appear at higher field (−0.29 ppm) compared to those for 5a (0.14 ppm) because of the strong shielding effect of the phenyl group located in a syn fashion.…”

“…Siloxybenzocyclobutene 4a probably arises from siloxyxylylene 6a formed by the photoinduced 1,5-migration of a trimethylsilyl group to the carbonyl oxygen. Although the benzocyclobutenol 2a reverts to the starting ketone 1a through the xylylenol 3a by intermolecular 1,5-hydrogen shift, the siloxyxylylene 6a formed by 1,5-silyl migration cyclizes to siloxybenzocyclobutene 4a because it lacks hydrogen to undergo 1,5-shift. The hindered benzocyclobutenols are unstable and revert to the parent ketones .…”