Page 328 - Advanced Organic Chemistry Part B - Reactions & Synthesis

300 A substantial amount of syn addition is observed for Z-1-phenylpropene (27–80%
syn addition), E-1-phenylpropene (17–29% syn addition), and cis-stilbene (up to 90%
CHAPTER 4
syn addition in polar solvents).
Electrophilic Additions
to Carbon-Carbon
Multiple Bonds CH H
H H Br 3 Br
AcOH H CH 3
+ Br 2 H +
Ph CH 3 Br H Br
Ph 28% Ph 72%
Br CH 3 Br H
H CH 3 H CH
AcOH 3
H +
+ Br 2 H
Ph H Ph Br Ph Br
83% 17%
Ref. 30
A common feature of the compounds that give extensive syn addition is the presence
of a phenyl substituent on the double bond. The presence of a phenyl substituent
diminishes the strength of bromine bridging by stabilizing the cationic center. A weakly
bridged structure in equilibrium with an open benzylic cation can account for the loss
in stereospecificity.

δ+ δ+
Br Br Br
δ+ H + δ+
H H H Ph H
Ph CH 3 Ph CH 3 H CH 3

The diminished stereospecificity is similar to that noted for hydrogen halide addition
to phenyl-substituted alkenes.
Although chlorination of aliphatic alkenes usually gives anti addition, syn addition
is often dominant for phenyl-substituted alkenes. 33

Ph CH 3 AcOH Cl H Cl Cl
+ Cl 2 CH 3 + CH 3
H H Ph Cl Ph
H H H
(major) (minor)

These results, too, reflect a difference in the extent of bridging in the intermediates.
With unconjugated alkenes, there is strong bridging and high anti stereospecificity.
Phenyl substitution leads to cationic character at the benzylic site, and there is more
syn addition. Because of its smaller size and lesser polarizability, chlorine is not
as effective as bromine in bridging for any particular alkene. Bromination therefore
generally gives a higher degree of anti addition than chlorination, all other factors
being the same. 34

33 M. L. Poutsma, J. Am. Chem. Soc., 87, 2161, 2172 (1965); R. C. Fahey, J. Am. Chem. Soc., 88, 4681
(1966); R. C. Fahey and C. Shubert, J. Am. Chem. Soc., 87, 5172 (1965).
34
R. J. Abraham and J. R. Monasterios, J. Chem. Soc., Perkin Trans. 1, 1446 (1973).