Migration of electronegative substituents. I. Relative migratory aptitude and migration tendency of the carbethoxy group in the dienone-phenol rearrangement

Abstract

The rearrangement, in trifluoroacetic acid at 38.5°, of two series of 4,4-disubstituted cyclohexadienones to give 3,4-disubstituted phenols has been studied. In the first series, a carbethoxy group is placed in intramolecular migratory competition with other common substituents (Me, Et, Ph, z'-Pr, Bz). The carbethoxy group is found to migrate in preference to Me, Et, or Ph with relative rates of 1:0.45:135. In the case of the z'-Pr and Bz substituted compounds, fragmentation occurs instead of rearrangement to give 4-carbethoxyphenol and the z'-Pr and Bz carbonium ions, respectively. In the second series, intermolecular kinetic comparisons of a series of compounds in which the substituents migrate in a methyl-substituted framework allows one to assess the migration tendency, as defined by Stiles and Mayer, of the substituents in the dienone system. The order found is Me < COOEt < Et < Ph

Fragmentation vs rearrangement of the amide and thioester groups in 4,4-disubstituted cyclohexadienones

“…The syntheses of 4-methyl-4-thiocarbethoxy-2,5-cyclohexadienone, 6b, and 4-methyl-4-diethylamido-2,5-cyclohexadienone, 6c, were first attempted via transformations from the readily available 4 4-methyl-4-carbethoxy-2-cyclohexenone, 7a. Direct treatment of ester 7a with diethyl amine under heat and pressure gave recovered starting material.…”

“…Rearrangement reactions of 6b and 6c were carried out in aqueous H 2 SO 4 , as in our previous studies [4][5][6][7][8] . The rates of the reactions were very fast, as recorded in Table 1.…”

“…The second major factor that controls this type of rearrangement process is the inherent ability of the electron-withdrawing substituent to migrate. Our data [4][5][6][7][8] clearly show that the π bond in the carbonyl group, although it is polarized adversely to stabilize positive charge, nevertheless provides resonance stabilization for transition states like 5. Evidence to support this includes a) a trichloromethyl group, although it has about the same polarity as an ester group, absolutely does not migrate 6 ; b) the vinyl group migrates about 1000 times faster than the ester in the dienone system 7 , since both groups have a π bond but only the ester is affected by adverse polarity; c) the vinyl group migrates ca.…”

“…In this method, in properly chosen systems, the MT values reflect properties of the migrating group R, since all other variables are held constant. We have had considerable success in predicting not only which group will migrate but also in rationalizing relative rates of such migrations, using the MT concept [4][5][6][7][8] . Of special interest in our work is the situation in which an electron-withdrawing substituent, such as an ester, is placed in competition with a substituent that is normally a very good migrating group, as represented schematically in Figure 1.…”

“…However, there are two major factors that determine which group migrates. The first 4,9,10 is the fact that if the electron-withdrawing group does not migrate, it is called upon to stabilize the positive charge placed α to an electron-withdrawing group in the transition state 4 and thus in the product ion 3. The second major factor that controls this type of rearrangement process is the inherent ability of the electron-withdrawing substituent to migrate.…”