Page 580 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

N,N,N-trimethylnorbornylammonium ion is exclusively syn. 289 This is another case 561
where the rigid ring prohibits attainment of an anti-elimination process. There is also a
steric effect operating against removal of an endo proton, which is required for anti elimi- SECTION 5.10
nation. Syn elimination is especially prevalent in the medium-sized ring compounds. 290 Elimination Reactions

D
anti
+
D N (CH ) syn H
3 3
H
H H
The energy difference between the anti and syn transition structures has been
examined computationally using fluoride as the base and alkyl chlorides as the
reactants. Simple primary and secondary chlorides show no barriers for anti elimi-
nation at the MP4SDQ/6-31+G** level. The syn TSs show positive barriers and the
total difference between the syn and anti TSs is on the order of 13 kcal/mol. 291

−
C H Cl+F E anti =−5 7kcal/mole E syn−anti = 12 7kcal/mol
2 5
n-C H Cl+F E anti =−9 9kcal/mole E syn−anti = 12 8kcal/mol
−
3
7
−
CH CHCl+F E anti =−10 2kcal/mole E syn−anti = 12 8kcal/mol
3 2
The preferred TS for syn elimination is not periplanar, but rather has a torsion angle

of about 30 . The syn TS has more E1cb character than the anti.
MP2/6-31+G** computations have been used to compare cyclopentyl and cyclo-
hexyl systems. 292 As noted above, cyclohexyl systems have a much stronger preference
for the anti stereochemistry. 293 The optimum TSs are shown in Figure 5.15. Both
the anti and syn TSs have negative barriers in the cyclopentyl system ( −10 2
and −0 9kcal/mol), whereas the syn system shows a positive barrier in the cyclohexyl
system (−10 9 and +5 4kcal/mol).
The factors that determine whether syn or anti elimination predominates are
complex. 294 One factor that is believed to be important is whether the base is free
or present as an ion pair. 295 The evidence suggests that an ion pair promotes syn
elimination of anionic leaving groups. This effect can be explained by a TS in which
the anion functions as a base and the cation assists in the departure of the leaving group.

C C C C
H
X
- +
ROH X M
RO –+ M
This interpretation is in agreement with the solvent effect that is evident for the
5-decyl system data in Table 5.15. The extent of syn elimination is much higher in
the nondissociating solvent benzene than in DMSO. The ion pair interpretation is also
supported by the fact that addition of specific metal ion–complexing agents (crown

289 J. P. Coke and M. P. Cooke, J. Am. Chem. Soc., 89, 6701 (1967).
290
J. Sicher, Angew. Chem. Int. Ed. Engl., 11, 200 (1972).
291 S. Gronert, J. Am. Chem. Soc., 113, 6041 (1991); J. Am. Chem. Soc., 115, 652 (1993).
292
S. Gronert, J. Org. Chem., 59, 7046 (1994).
293 C. H. DePuy, G. F. Morris, J. S. Smith, and R. J. Smat, J. Am. Chem. Soc., 87, 2421 (1965).
294 R. A. Bartsch and J. Zavada, Chem. Rev., 80, 453 (1980).
295
R. A. Bartsch, G. M. Pruss, R. L. Buswell, and B. A. Bushaw, Tetrahedron Lett., 2621 (1972); J. K.
Borchardt, J. C. Swanson, and W. H. Saunders, Jr., J. Am. Chem. Soc., 96, 3918 (1974).