SUBSTITUENTS EFFECT ON THE CLAISEN REARRANGEMENT

Article abstract of DOI:10.1016/S0040-4020(01)89781-7

The Claisen rearrangement of six homogeneous series of (R,S) or (R,R) 1,4-diaryl-6-isopropenyl-6-methyl-5,6-dihydro-4H-pyran<2.3-c>pyrazoles (1-6) to give 1,2-dimethyl-5-arylcyclohexene-4-spiro-4'(1'-aryl-5'-pyrazolones) (7-12) was kinetically investigated at 125 deg C in C6D6.When the substituents are on the 1-aryl group (1,2) the rate increased with increases in the electron-attracting character of the substituent.When the substituents are on the 4-aryl group and the configuration is (R,S) (3,5), the rate decreased with increases in the electron-attractingcharacter of the substituent.If the configuration is (R,R) (4,6) a lower, opposite, effect was observed.All kinetic data correlate linearly with the ρ of the substituents.These results can be rationalyzed by assuming for the Claisen rearrangement a transition state having significant dipolar character, with a partial negative charge developed on the oxygenated fragment.