Page 381 - Mechanism and Theory in Organic Chemistry

proportion of Saytzeff olefin in the ammonium bromide-catalyzed elimination is
even higher than when the same substrate undergoes E, eliminati~n.~~

H3C H CH3
I I H3C\ ,H I
CH,-C-CCH, base C-C + CH3-C-CH=CH2
I I \ I
H OTs '333 H (7.38)
H. C. Brown has suggested that steric factors are of primary and almost sole
importance in determining the position of the double bond. According to Brown,
Hofmann product predominates when a large leaving group makes it even more
difficult for the base to abstract the more hindered protons.96 He has asserted that
data similar to those of Table 7.12, which seem at first glance to be contrary to his
theory, support it further: He says that fluorine takes up more space in the transi-
tion state than iodine because fluorine is more sol~ated.~~ However, the entropies
of activation for Reaction 7.37 with X = F, C1, Br, or I are all very similar;
therefore increased solvation of fluorine seems not to be the proper explanation
for the preponderance of Hofmann product when X = F.98
More recently, Bartsch and co-workers have shown that in E,H eliminations
of HI from 2-iodobutane, the positional orientation of the double bond is con-
trolled almost entirely by the strength of the base (if the attacking atom is kept
constant) unless really outsized bases are used.99 In Table 7.14 are listed some of

Table 7.14 RELATIVE OLEFIN~C PROPORTIONS FROM REACTIONS 2-IODOBUTANE
OF
WITH OXYANION BASES IN DIMETHYL SULFOXIDE AT 50.0°C
Percent 1- trans-2-
pK, of Conjugate Butene in Butene : cis-
System Base Acid in DMSO Total Butenes 2-Butene
1 Potassium p-nitrobenzoate 8.9 5.8 + 0.1 3.51
2 Potassium benzoate 11.0 7.2 + 0.2 3.24
3 Potassium p-nitrophenoxide 11.0 7.5 + 0.1 3.58
4 Potassium o-nitrophenoxide 11.0 7.5 + 0.2 3.82
5 Potassium acetate 11.6 7.4 + 0.1 3.47
6 Potassium p-aminobenzoate 12.7 8.0 + 0.2 3.42
7 Potassium 2,6-di-tert-butyl- 15.0 19.2 + 0.4 3.70
phenoxide
8 Potassium phenoxide 16.4 11.4 + 0.2 3.34
9 Sodium 2,2,2-trifluoroethoxide 21.6 14.3 + 0.2 3.32
10 Sodium methoxide 27.0 17.0 + 0.5 3.14
11 Sodium ethoxide 27.4 17.1 + 0.4 3.31
12 Sodium n-propoxide 28.0 18.5 + 0.3 3.34
13 Potassium tert-butoxide 29.2 20.7 + 0.4 3.00
SOURCE: A. Bartsch, G. M. Pruss, B. A. Bushaw, and K. E. Wiegers, J. Amer. Chem. Soc., 95, 3405
R.
(1973). Reprinted by permission of the American Chemical Society.
9S G. Biale, D. Cook, D. J. Lloyd, A. J. Parker, I. D. R. Stevens, J. Takahashi, and S. Winstein,
J. Amer. Chem. SOC., 93, 4735 (1971).
9"ee note 82, p. 362.
H. C. Brown and R. L. Klimisch, J. Amer. Chem. Soc., 88, 1425 (1966).
O8 See note 94, p. 367.
" (a) R. A. Bartsch, G. M. Pruss, B. A. Bushaw, and K. E. Wiegers, J. Amer. Chem. Soc., 95, 3405
(1973); (b) R. A. Bartsch, K. E. Wiegers, and D. R. Guritz, J. Amer. Chem. Soc., 96, 430 (1974).