Bromination of Bicyclobutanes: A Possible Case of an Electron-Transfer Mechanism

The kinetics and product distribution of the bromination of 3-R-bicyclobutanecarbonitrile (R = H, Me, Cl, and Ph) by Br₂and Br₃“ were studied in MeOH at 25 °C. For the first three substrates, the addition of Br-Br and Br-OMe across the central bond of the bicyclobutane moiety was (within experimental error) 100% syn stereospecific. For the phenylated substrate, where a relatively stable benzylic cation is formed, a mixture of syn and anti addition products was obtained. Reaction rate constants are best correlated by cr⁺with p⁺values of-6.2 and -6.4 for Br₂and Br₃', respectively. In both cases the phenylated substrate deviates positively from the Hammett-type plot. with use of ab initio calculations it was shown that the mechanism that promotes the stereospecific syn additions in the acid-catalyzed additions to the same substrates is not operative in the case of the bromination reactions. Analogous to the electrophilic additions to aromatic and olefinic substrates, an electron-transfer mechanism is suggested for these reactions. Ab initio calculations (URHF) show that the bicyclobutane radical cation formed in the first step is more stable in the puckered form by ca. 18 kcal/mol as compared to the planar one.