Remarkable effect of water on functionalization of the phenyl ring in methyl-substituted benzene derivatives with F-TEDA-BF4.
Various N-F reagents reacted with hexamethylbenzene (1) forming side chain substituted alkoxides or esters in protic solvents, Ritter type side chain functionalization was observed in acetonitrile in the presence of trifluoroacetic acid, while in aqueous acetonitrile solution phenyl ring transformation took place, starting with ipso attack of water and further rearrangement of the methyl group as the main process. Rearranged 2,3,4,5,6,6-hexamethylcyclohexa-2,4-dienone (7) was transformed to 5-fluoro-2,3,5,6,6-pentamethyl-4-methylenecyclohex-2-en-1-one (8) or 5-hydroxy-2,3,5,6,6-pentamethyl-4-methylenecyclohex-2-en-1-one (9). 1,2,3,4,5,6-Hexamethyl-bicyclo[2.2.0]hexa-2,5-diene reacted with F-TEDA-BF4 in the presence of water and 7 was formed in high yield. Durene (12) followed similar ipso attack of water as 1, but on the other hand 1,2,3,4-tetramethylbenzene displayed different regioselectivity and 2,3,4,5-tetramethylphenol was formed, further transforming to 4-fluoro-2,3,4,5-tetramethylcyclohexa-2,5-dienone. The functionalizations of methylbenzenes obeyed a second-order rate equation v = d[N-F]/dt = k2[N-F][substrate], and DeltaG# values between 77 and 94 kJ/mol were determined. The presence of water did not significantly influence DeltaG# but considerably affected DeltaS# and positive values were found where methyl group migration was the dominant process (9.1 J/(mol K) for 1 and 0.5 J/(mol K) for 12). A higher reactivity of durene than mesitylene (k2(MES)/k2(DUR) = 0.23) was found, supporting the assumption that single electron transfer (SET) is the dominant process in the functionalizations of methyl-substituted benzene derivatives with F-TEDA-BF4.
Kralj
Petra
P
Laboratory of Organic and Bioorganic Chemistry, "Jozef Stefan" Institute, Jamova 39, 1000 Ljubljana, Slovenia.
Zupan
Marko
M
Stavber
Stojan
S