A. Evans Olefin Bromination -1 Chem 206 a Bromonium ion origin of the anti(trans )selectivity first suggested by Roberts, JACS 1937. 59. 947 Introduction Reaction is first order in alkene At low concentrations of Br. rxn is also first order in At higher concentrations of Br2 in nonpolar solvents rxn is second order in Br2 I First X-ray Structure of a bromonium ion: Brown, JACS 1985, 107, 4504 Substituent Effects on Bromination Rates Alkene eq -2 eq Br2 n-PrCH=CH2 t-BuCH=O (CH3)2C=CH CiS-CH3 CHECHCH 2620 trans-CH3 CH=CHCH Br-2 1,800,000 Br-4 X-ray structure a Stereochemical outcome versus structure( Br2 in HOAC 25) Alkene anti addition Alkene anti addition 2.116A A 100% 100% 73%C C Br R R Br R R Br Br3 – C C R R R R Br Br– Br-1 Br-4 Br-2 Br-3 Br2 C C R R R R C C Br R R Br R Br2 R Ph H Me H Ph Me H Me Ph Me Me H R R R R Ph H H Me Me H H Me Me H Me H D. A. Evans Olefin Bromination-1 Chem 206 Introduction ■ Reaction is first order in alkene At low concentrations of Br2, rxn is also first order in Br2 At higher concentrations of Br2 in nonpolar solvents rxn is second order in Br2 . ■ Stereochemical outcome versus structure (Br2 in HOAc @ 25°) Alkene % anti addition 100% 100% 73% 83% 63% 68% Alkene % anti addition ■ Bromonium ion origin of the anti (trans) selectivity first suggested by Roberts, JACS 1937, 59, 947 ■ First X-ray Structure of a bromonuium ion: Brown, JACS 1985, 107, 4504 + Alkene krel 1 61 70 57 27 5470 2620 1700 130,000 1,800,000 CH2=CH2 CH3CH=CH2 n-PrCH=CH2 i-PrCH=CH2 t-BuCH=CH2 (CH3 )2C=CH2 cis-CH3CH=CHCH3 trans-CH3CH=CHCH3 (CH3)2C=CHCH3 (CH3 )2C=C(CH3 )2 ■ Substituent Effects on Bromination Rates 2 eq Br2 -2 eq Br2 X-ray structure 2.116 Å 2.194 Å 1.497 Å