Page 560 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

For alkyl-substituted alkynes, there is a difference in stereochemistry between 541
mono- and disubstituted derivatives. The former give syn addition, whereas the latter
react by anti addition. The disubstituted (internal) compounds are considerably (∼ 100 SECTION 5.9
times) more reactive than the monosubstituted (terminal) ones. This result suggests that Additions to Alkynes and
Allenes
the TS of the rate-determining step is stabilized by both alkyl substituents and points
to a bridged structure. This interpretation is consistent with the anti stereochemistry
of the reaction for internal alkynes.

Cl –
Cl 2 Cl + R Cl
R C C R
C C
R R
Cl R
The monosubstituted intermediates do not seem to be effectively bridged, since syn
addition predominates. A very short-lived vinyl cation appears to be the best description
of the intermediate in this case. 251
The stereochemistry of bromination is usually anti for alkyl-substituted alkynes. 252
A series of substituted arylalkynes has been examined in dichloroethane. 253 As with
alkenes, a -complex intermediate was observable. The H for formation of the
complex with 1-phenylpropyne is about −3 0kcal/mol. The overall kinetics are third
order, as for an Ad 3 mechanism. The rate-determining step is the reaction of Br
E 2
with the complex to form a vinyl cation, and both syn and anti addition products
are formed.
–
Br 2 Br 3
+ Br Br Br Ar Br
Br 2 Br 2
ArC CR ArC CR ArC C C C + C C
R Ar R Br R

For the aryl-substituted alkynes, the reaction stereochemistry is sensitive to the
aryl substitution. With EWG substituents (NO , CN) the reaction becomes stereospecif-
2
ically anti and the same is true for 2-hexyne, reflecting the diminished stability of the
vinyl cation in these cases. Aryl-substituted alkynes can be shifted toward anti addition
by including a bromide salt in the reaction medium. Under these conditions, a species
preceding the vinyl cation must be intercepted by a bromide ion. This intermediate is
presumably the complex of molecular bromine with the alkyne. An overall mechanistic
summary is shown in the following equations.

Br 2 Br + Br – R Br
Br 2 Ad E 2
RC CH RC CH C C C C bridged
R H Br H intermediate
–
Br
Br –
R Br Br Br Br R Br Ad E 2
Ad E 3 +
C C RC C C C + C C vinyl cation
anti
Br H H R H Br H intermediate
251 K. Yates and T. A. Go, J. Org. Chem., 45, 2385 (1980).
252 J. A. Pincock and K. Yates, Can. J. Chem., 48, 3332 (1970).
253
R. Bianchini, C. Chiappe, G. Lo Moro, D. Lenoir, P. Lemmen, and N. Goldberg, Chem. Eur. J., 1570
(1999).