Page 948 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

932 There is a very powerful substituent effect for dienes having anionic oxygen
substituents at C(3), a reaction known as the anionic oxy-Cope reaction. 272 When the
CHAPTER 10
C(3) hydroxyl group is converted to its alkoxide, the reaction is accelerated by factors
17 273
10
Concerted Pericyclic of 10 –10 .
Reactions
O – O – H + O
H

The rates of anionic oxy-Cope rearrangements depend on the degree of cation coordi-
+
+
nation at the oxy anion. The reactivity trend is K > Na > Li . Crown ethers
+
catalyze reaction by promoting ion pair dissociation. 274 Catalytic amounts of tetra-
n-butylammonium salts lead to accelerated rates in some cases. This presumably
results from the dissociation of less reactive ion pair species promoted by the tetra-
n-butylammonium ion. 275 As with other [3,3]-sigmatropic rearrangements, the stereo-
chemistry of the anionic oxy-Cope rearrangement depends on TS conformation. There
is no strong stereochemical preference associated with the C−O bond, and in the
−
absence of other controlling steric factors, products arising from both equatorial and
axial orientations are formed. 276
The origin of the rate acceleration has been explored by computation. The
‡
∗
B3LYP/6-31+G computational results give a G of 6.3 kcal/mol, some 30 kcal/mol
less than the unsubstituted system. 277 Another study found the barrier to be only
8.3 kcal/mol in the gas phase. This is raised substantially (to 31.8 kcal/mol) by coordi-
+
nation of an Li cation at the oxygen. 278 As shown in Figure 10.39, the TS for the
anionic oxy-Cope reaction is much more asynchronous than for the parent system. The
TS is much looser and closer to two dissociated fragments. Note that the C(3)−C(4)
bond has lengthened substantially in the TS, whereas the C(1)−C(6) bond distance is
still quite long. Several factors probably contribute to the large rate acceleration. The
anionic oxy substituent substantially weakens the C(3)−C(4) bond. 279 The delocal-
ization of the negative charge in the enolate is also likely a factor, in view of the
dissociative nature of the TS.
3-Amino groups also accelerate the Cope rearrangement. 280 The products are
enamines and subsequent reactions of the enamine are feasible, such as -alkylation.

272
L. A. Paquette, Angew. Chem. Int. Ed. Engl., 29, 609 (1990).
273
D. A. Evans and A. M. Golob, J. Am. Chem. Soc., 97, 4765 (1975); D. A. Evans, D. J. Baillargeon,
and J. V. Nelson, J. Am. Chem. Soc., 100, 2242 (1978).
274 J. J. Gajewski and K. R. Gee, J. Am. Chem. Soc., 113, 967 (1991).
275
M. George, T.-F. Tam, and B. Fraser-Reid, J. Org. Chem., 50, 5747 (1985).
276
L. A. Paquette and G. D. Maynard, J. Am. Chem. Soc., 114, 5018 (1992); E. Lee, Y. R. Lee, B. Moon,
O. Kwon, M. S. Shim, and J. S. Yun, J. Org. Chem., 59, 1444 (1994).
277 H. Bauman and P. Chen, Helv. Chim. Acta, 84, 124 (2001).
278
F. Haeffner, K. N. Houk, S. M. Schulze, and J. K. Lee, J. Org. Chem., 68, 2310 (2003).
279 (a) M. L. Steigerwald, W. A. Goddard, III, and D. A. Evans, J. Am. Chem. Soc., 101, 1994 (1979);
(b) H. Y. Yoo, K. N. Houk, J. K. Lee, M.A. Scialdone, and A. I. Meyers, J. Am. Chem. Soc., 120, 205
(1998).
280
R. W. Jemison, W. D. Ollis, I. O. S. Sutherland, and J. Tannock, J. Chem. Soc., Perkin Trans. 1, 1462
(1980); J. P. Hagen, K. D. Lewis, S. W. Lovell, P. Rossi, and A. Z. Tescan, J. Org. Chem., 60, 7471
(1995).