Page 562 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

543
Br –0.041 Br –0.068 Br –0.061
SECTION 5.9
2.317 2.327 2.328
(2.355) (2.373) (2.371) Additions to Alkynes and
Allenes
Br –0.005 Br –0.018 Br –0.019

3.053(2.952) 3.031(3.003)
3.193(3.087) 90.0 (90.0) 85.16(81.31)
90.0 (90.0) C C C C C
1.343 1.313 1.319
C C (1.337) (1.300) (1.311)
1.218
(1.202)
–1.38 (–1.54) kcal mol –1 –1.59 (–2.14) kcal mol –1 –1.75 (–1.93) kcal mol –1
Fig. 5.10. Optimized structures and E for formation of (a) ethyne-Br 2 , (b) ethene-Br 2 , and
(c)allene-Br 2 complexes. Values of E from MP2/6-311+G ∗∗ and B3LYP/6-311G ∗∗ (the latter in
parentheses). Reproduced from Chem. Eur. J., 967 (2002), by permission of Wiley-VCH.

structures and E for formation of the complex are shown in Figure 5.10. 259 The
structures and energies are quite similar. This indicates that it is the formation of the
cationic intermediate that is more difficult for alkynes.
Calculations comparing the open -halovinyl and bridged cations have been
reported using MP2/6-311G++(3df,3pd)- and B3LYP/6-31+G(d)-level computa-
tions. 260 The bridged ions are found to be favored for chlorine and bromine but the
open ion is favored for fluorine. The -chloro and -bromovinyl cations are found
not to be minima. They rearrange to the much more stable -halovinyl cations by a
hydride shift. The bridged ions tend to be more strongly stabilized by solvation than
the open ions. As was noted for the halonium ions derived from alkenes (p. 495), the
charge on halogen is positive for Cl and Br, but negative for F.

+ F F+ + Cl Cl + + Br Br +

–15.2 0.0 +5.1 0.0 0.0
F Cl Br
+ + +
– 39.3 – 21.8 – 24.1

When a methyl group is added, the vinyl cation is favored. The open cation was also
favored for ions derived from 2-butyne.
Computational studies have also explored the issue of how the complex is
converted to the intermediate and several potential mechanisms have been described. 261

259 C. Chiappe, A. de Rubertis, H. Detert, D. Lenoir, C. S. Wannere, and P. v. R. Schleyer, Chem. Eur. J.,
967 (2002).
260 T. Okazaki and K. K. Laali, J. Org. Chem., 70, 9139 (2005).
261
R. Herges, A. Papafilippopoulos, K. Hess, C. Chiappe, D. Lenoir, and H. Detert, Angew. Chem. Int.
Ed., 44, 1412 (2005); M. Zabalov, S. S. Karlov, D. A. Le menovskii, and G. S. Zaitseva, J. Org. Chem.,
70, 9175 (2005).