REACTIVITY OF ETHYLENIC COMPOUNDS .42. MULTI-PATHWAY MECHANISM IN STYRENE BROMINATION - COMPETITION BETWEEN BROMONIUM AND CARBONIUM-ION INTERMEDIATES

The structure of transition states and intermediates in styrene bromination is reinvestigated on the basis of the multi-pathway scheme where bromonium and carbonium ions are formed via competitive pathways. From the bromination in methanol of ring substituted styrenes 1, α-methylstyrenes 2, and trans -β-methylstyrenes 3, several criteria have been devised to estimate the relative importance of the different paths: dependence of the kα.Mc/kH ratio on the substituents; demonstration of curvature in the pσ plot by extension of the substituent scale; stereochemistry of product formation. Results lead to a coherent view of kinetic and stereochemical data. Bromination of styrenes and β-methylstyrenes goes through carbonium ion intermediates only when they bear electron-donating substituents. The intermediate of 1 and 3 substituted by the most electron-attracting group is almost exclusively the bromonium ion. For the other substituents, there is competition between the two intermediates. The reaction constants of each pathway are determined. For styrene bromination in methanol, pc of the carbonium ion pathway is -4.36 and pB of the bromonium ion one is about -2.8; in acetic acid, pc is as high as -5.6. For TRANS-β-methylstyr-enes in methanol, these pc and pB values are attenuated by a factor of 0.94. Whereas a methyl group in the β position does not change the relative stabilities of the two intermediates, a methyl group in the a position enhances the stability of the carbonium ion substantially more than that of the corresponding bromonium ion. Thus, α-methylstyrene bromination goes through carbonium ions exclusively, with a p value of -4.26, whatever the substituent. The stereochemistry of dibromide formation in the bromination of 3 confirms the scheme deduced from the kinetic data: from nonstereoselective for trans-4-methoxy-β-methyl-styrene, the reaction passes to antistereospecific for 3,5-bis(trifluoromethyl) 3. Extensive comparisons of substituent effects on bromination and on other electrophilic additions to styrene, hydration and sulfenylation in particular, lead us to discuss the different factors (charge magnitude at the transition state, distance between the ring and the cationic center) which control the p value. © 1978, American Chemical Society. All rights reserved.