Page 558 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

protonation can either collapse to give the vinyl halide or capture solvent to give the 539
acetate. Aryl-substituted acetylenes give mainly the syn addition product.
SECTION 5.9
Cl – Cl H Additions to Alkynes and
H C C Allenes
+
Ar C C R + HCl Ar C C Ar R
R CH CO H
CH CO H 3 2
3
2
C C
Ar R
Alkyl-substituted acetylenes can react with HCl by either the Ad 3 or the Ad 2
E
E
mechanism. The Ad 3 mechanism leads to anti addition. The preference for one or the
E
other mechanism depends on the reactant structure and the reaction conditions. Added
halide ion promotes the Ad 3 mechanism and increases the overall rate of reaction. 243
E
−
Reaction of 4-octyne with TFA in CH Cl containing 0 1–1 0M Br leads mainly to
2 2
−
Z-4-bromo-4-octene by an anti addition. The presence of Br greatly accelerates the
reaction as compared to reaction with TFA alone, indicating the involvement of the
−
Br in the rate-determining protonation step. 244
(CH ) H
CH 3 2 2
TFA, Br – C C
CH (CH ) C C(CH ) CH 3
2 2
2 2
3
Br (CH ) CH 3
2 2
1-Octyne and 2-octyne also give more than 95% anti addition under these conditions.
The reactions are formulated as concerted Ad 3 processes, involving bromide attack
E
on an alkene-acid complex.
HO CCF 3
2
R H
R C C R C C
Br R
Br –
Under these conditions, 1-arylalkynes react by a mixture of Ad 2 and Ad 3
E E
mechanisms with the proportion of Ad 3 reaction increasing with increasing
E
bromide concentration and decreasing effective acidity. 245 The Ad 3 reaction of
E
1-phenylpropyne gives the anti-Markovnikov product. This is believed to be due to a
steric effect of the phenyl group.
H +
H CH 3
CH 3
Ph Br
H
Br –
Compared to alkene additions carried out under similar conditions, there is less likely
to be involvement of a cationic intermediate because of the higher energy of the vinyl
cation (see Section 3.4.1).

243 R. C. Fahey, M. T. Payne, and D.-J. Lee, J. Org. Chem., 39, 1124 (1974).
244 H. M. Weiss and K. M. Touchette, J. Chem. Soc., Perkin Trans. 2, 1523 (1998).
245
H. M. Weiss, K. M. Touchette, F. Andersen, and D. Iskhakov, Org. Biomol. Chem., 1, 2148 (2003);
H. M. Weiss, K. M. Touchette, S. Angell, and J. Khan, Org. Biomol. Chem., 1, 2152 (2003).