SUBSTITUENTS EFFECT ON THE CLAISEN REARRANGEMENT

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°C in C6D 6. 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-attracting character 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 rationalized by assuming for the Claisen rearrangement a transition state having significant dipolar character, with a partial negative charge developed on the oxygenated fragment. © 1990.