Page 204 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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Scheme 2.7. Examples of Stereospecific Reactions
CHAPTER 2
A. Bromination of Alkenes (see Section 2.4.2.1 for additional discussion)
Stereochemistry,
Conformation,
and Stereoselectivity Bromination of simple alkenes normally proceeds via a bromonium ion and is stereospecifically anti.
Exceptions occur when the bromonium ion is in equilibrium with a corresponding carbocation.
H CH
a CH H Br CH CH Br 3
1 3 Br 2 CH 3 3 3 Br 2 H
CH 3 CH 3
H CH 3 H Br H H H Br
anti addition anti addition
2 b Ph H Br Br H Ph H H Br 2 Br H Ph Br H Ph
2
Ph CH NO Ph + H
H Ph H Br Ph Ph 3 2 H Br Ph Br
10%
anti addition 90%
loss of stereospecificity
B. Dihydroxylation and Epoxidation-Hydrolysis of Alkenes (see Section 2.4.2.2 for additional discussion)

Dihydroxylation of epoxides can be carried out with syn stereospecificity using OsO as the active oxidant.
4
The reaction occurs by a cycloaddition mechanism. Epoxidation is also a stereospecific syn addition. Ring
opening of epoxides by hydrolysis also leads to diols. This is usually an anti addition with inversion of
configuration at the site of nucleophilic attack, leading to overall anti dihydroxylation.
OH
3 c H ) CH OsO 4
3 t-BuOOH (CH 2 2
(CH 2 2
) CH
CH (CH ) 3
2 2
3
+ –
CH (CH ) H Et 4 N OAc
3 2 2 OH racemic
4 d H H OH
CH 3 H CH
1) RCO 3 3
2) H2O, H + CH
CH 3
H 3 H OH racemic
racemic
C. Hydroboration-Oxidation (see Section 2.4.2.3 for additional discussion)
Hydroboration-oxidation occurs by syn addition. The reagents are borane or an alkyl or dialkyl derivative,
–
O and OH. The oxidation occurs with retention of configuration of the
followed by oxidation, usually with H 2 2
alkyl group. The regioselectivity favors addition of the boron at the less-substituted carbon of the double bond.
As a result, the reaction sequence provides a stereospecific syn, anti-Markovnikov hydration of alkenes.
5 e CH 3 CH 3 H CH 3
BH 3 ( ) BH H 2 O 2 OH
2
–
OH
> 99%
6 d CH 3 CH 3 CH 3
H H O H
1) BH 3 2 2 OH
( ) 2 BH
–
OH
85%
a. J. H. Rolston and K. Yates, J. Am. Chem. Soc., 91, 1469, 1477 (1969).
b. R. E. Buckles, J. M. Bader, and R. L. Thurmaier, J. Org. Chem., 27, 4523 (1962).
c. K. Akashi, R. E. Palermo, and K. B. Sharpless, J. Org. Chem., 43, 2063 (1978).
d. B. Rickborn and D. K. Murphy, J. Org. Chem., 34, 3209 (1969).
e. H. C. Brown and G. Zweifel, J. Am. Chem. Soc., 83, 2544 (1961).
f. G. Zweifel and H. C. Brown, Org. Synth., 52, 59 (1972).