Page 330 - Advanced Organic Chemistry Part B - Reactions & Synthesis

302 of the bromide ion as low as possible. One method for accomplishing this is to use
N-bromosuccinimide (NBS) as the brominating reagent. 38 39 High yields of bromohy-
CHAPTER 4
drins are obtained by using NBS in aqueous DMSO. The reaction is a stereospecific
Electrophilic Additions anti addition. As in bromination, a bromonium ion intermediate can explain the anti
to Carbon-Carbon
Multiple Bonds stereospecificity. It has been shown that the reactions in DMSO involve nucleophilic
attack by the sulfoxide oxygen. The resulting alkoxysulfonium ion intermediate reacts
with water to give the bromohydrin.

Br + R R
Br + + H O
RCH CH 2 R CH CH 2 (CH ) S O CHCH Br 2 HOCHCH Br
2
2
3 2
(CH ) S O H O H +
3 2
2
In accord with the Markovnikov rule, the hydroxy group is introduced at the carbon
best able to support positive charge.
CH CH 3
3
NBS
(CH ) CC CH (CH ) CC CH Br
3 3
2
3 3
2
DMSO 60%
H 2 O OH
Ref. 40
NBS
PhCH CH CH 2 PhCH CHCH Br
2
2
2
DMSO 89%
H O OH
2
Ref. 41
The participation of sulfoxy groups can be used to control the stereochemistry in acyclic
systems. In the reaction shown below, the internal sulfoxide captures the bromonium
ion and then undergoes inversion at sulfur in the hydrolytic step.
O – H O –
OTBDMS 2 O OTBDMS
NBS OTBDMS OH
S + Ar S O Ph S +
Ar Ph H 2 O/toluene Br Ar Ph
Br
Ar = 4-methylphenyl
Ref. 42

A procedure that is useful for the preparation of both bromohydrins and iodohy-
drins involves in situ generation of the hypohalous acid from NaBrO or NaIO by
3
4
reduction with bisulfite. 43
38
A. J. Sisti and M. Meyers, J. Org. Chem., 38, 4431 (1973).
39 C. O. Guss and R. Rosenthal, J. Am. Chem. Soc., 77, 2549 (1965).
40 D. R. Dalton, V. P. Dutta, and D. C. Jones, J. Am. Chem. Soc., 90, 5498 (1968).
41
A. W. Langman and D. R. Dalton, Org. Synth., 59, 16 (1979).
42 S. Raghavan and M. A. Rasheed, Tetrahedron, 59, 10307 (2003).
43
H. Masuda, K. Takase, M. Nishio, A. Hasegawa, Y. Nishiyama, and Y. Ishii, J. Org. Chem., 59, 5550
(1994).