A. Evans Olefin Bromination -2 Chem 206 Calculated Geometries of substituted bromonium ions Overall Reaction Mechanisn Ruasse. Chem commun. 1990. 898 Second order Kinetics Products Br Br -HOR 2.01 188 mH Her C… Me mer Me 147 1.51 Note;the C-Br bond lengths in previous X-ray structure are 2.116A (π complex) a Bromonium lons undergo fast exchange with olefins Products Brown. Accts. Chem. Res. 1997. 30. 131 ird Order Kinetics Unprecedented until 1991(Bennet, JACS 1991, 113, 8532 Ad-C=C-Ad Ad-C--C-Ad Bromination of Cyclohexene Derivatives Pasto, JACS 1970, 92, 7480 Ad-C-C-Ad Ad-C=C-Ad Me3C e3C r exclusive ex x=Br: exchange rate 2 x 106M-1s'1 H R=H Me Ad-C-C-Ad X=I: exchange rate: 8 X 10MS Diaxial opening of bromonium ions may be viewed as an extension of the Furst-Plattner Rule for epoxide ring opening( Lecture-3) OMe There is an intermediate in the halogen transfer(ab initio calcs) Me3C MeoH Br47% 仓Br 53% complex omplex OMe It appears that bromine attack from both olefin faces occurs wilth near equal probabilityC C H H H H Br C C Me Me Me Me Br R R R R R R R Br R C C H H Me Me Br Ad–C C–Ad Br Br3 – Ad–C C–Ad Ad–C C–Ad Br Ad–C C–Ad Br3 – Ad–C C–Ad X R R R R R R R Br R R R R R R R R Br R complex TS complex Br2 Br2 Br2 A B H Me3C R H Me3C H MeOH H Me3C Br H H H Me3C Br H H MeOH Br Br Br3 – Br–.HOR H Me3C Br Br R H H Me3C Br OMe H H H Me3C OMe Br H H D. A. Evans Olefin Bromination-2 Chem 206 ■ Calculated Geometries of Substituted Bromonium Ions Ruasse, Chem Commun. 1990, 898 1.47 2.01 1.51 2.05 1.51 2.70 1.88 Note; the C–Br bond lengths in previous X-ray structure are 2.116 Å. ■ Bromonium Ions undergo fast exchange with olefins Brown, Accts. Chem. Res. 1997, 30, 131 Unprecedented until 1991 (Bennet, JACS 1991, 113, 8532) X = Br: exchange rate: 2 x 106 M–1 s-1 X = I : exchange rate: 8 x 106 M–1 s-1 There is an intermediate in the halogen transfer (ab initio calcs): + + + Products (p-complex) Overall Reaction Mechanism s-complex s-complex Second Order Kinetics Third Order Kinetics Products Bromination of Cyclohexene Derivatives Pasto, JACS 1970, 92, 7480 Pyr–Br+ Br3 – R = H, Me exclusive product Pyr–Br+ Br3 – 47% 53% Diaxial opening of bromonium ions may be viewed as an extension of the Furst-Plattner Rule for epoxide ring opening (Lecture-3). It appears that bromine attack from both olefin faces occurs wilth near equal probability