Selective 1,4-Addition of Arenes to 3-Chloro-3-cyclobutene-1,2-dione under Friedel−Crafts Conditions. Synthesis and Reactivity of 4-Aryl-3-chloro-2-hydroxy-2-cyclobuten-1-ones1

The reaction of semisquaric chloride (7) with arenes 2 has been investigated. In the presence of 1.1 equiv of AlCl3 and in the temperature range of −15° C to rt the arenes 2a-q afford the 4-aryl-3-chloro-2-hydroxy-2-cyclobuten-1-ones (chloroenols) 8a-q in good yield. By contrast, 7 reacts with 1,4-dimethoxybenzene (2l) in boiling CH2Cl2 to give a mixture of (2,5-dimethoxyphenyl)cyclobutenedione (9a) (27% yield) and bis(2,5-dimethoxyphenyl)cyclobutenedione (10a) (8% yield). With 1,2,4-trimethoxybenzene (2r) in the presence of trifluoroacetic acid is generated (2,4,5-trimethoxyphenyl)cyclobutenedione (9b) in 21% yield. The chloroenols 8 allow a series of valuable transformation reactions: with diazomethane the chloroenol methyl ethers 11 are generated, with chlorine the 3-aryl-4-chlorocyclobutenediones 12, and with bromine in MeOH the 3-aryl-4-methoxycyclobutenediones 13. In DMSO or in acetone/H2O the chloroenols 8 eliminate HCl, furnishing the arylcyclobutenediones 14. In a mixture of acetone-d 6/D2O/DCl are obtained 4-aryl-cyclobutenediones-3-d 15. For the latter two processes the corresponding 3-aryl-4-chlorocyclobutane-1,2-diones 16 are postulated as intermediates. Thermolysis of the chloroenols 8 and the chloroenol methyl ethers 11 in refluxing m-xylene afforded the 3-chloro-1,2-dihydroxynaphthalenes 17 and the 3-chloro-1-hydroxy-2-methoxynaphthalenes 18, respectively.