TY - JOUR
T1 - Bromination of Bicyclobutanes
T2 - A Possible Case of an Electron-Transfer Mechanism
AU - Hoz, Shmaryahu
AU - Livneh, Mordechai
AU - Cohen, Drora
PY - 1987/8/1
Y1 - 1987/8/1
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=0000794734&partnerID=8YFLogxK
U2 - 10.1021/ja00251a018
DO - 10.1021/ja00251a018
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AN - SCOPUS:0000794734
SN - 0002-7863
VL - 109
SP - 5149
EP - 5156
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 17
ER -