Multipathway Mechanism in Aryl Olefin Bromination: Competition between Tertiary and Secondary Carbocation Pathways

Article abstract of DOI:10.1021/jo00150a013

Rate constants, product stereochemistry, and regiochemistry of the bromination of 26 α-methylstilbenes XC6H4C(Me)=CHC6H4Y have been measured in methanol at 25 deg C.Kinetic substituent effects on the rate show the competitive formation of secondary and tertiary carbocations in the rate-determining step.The secondary pathway competes with the tertiary one only if Y is strongly electron donating and X electron accepting.The ρ values are all consistent with carbocationic transition states: in the tertiary pathway ρ_α for X is -4.59 and ρ_β for Y is -1.66, whereas in the secondary pathway ρ_β for X, which varies with Y from -1.95 (Y=4-OMe) to -0.89 (Y=4-NMe2), is in the expected range for a β effect.Product analysis is in complete agreement with the kinetic results.The formation of dibromides and methoxy bromides is nonstereoselective as expected from the absence of bromine bridging in the transition states and the intermediates.The experimental regiochemistry is correctly predicted by the relative rates of formation of the secondary and tertiary carbocations.From this rate-product correlation, the first established for an addition reaction, the unambiguous conclusion is that the positive charge distribution does not change on going from the rate-determining to the product-determining transition state.