Page 617 - Advanced Organic Chemistry Part B - Reactions & Synthesis

activation energy requirements and stereochemistry of concerted elimination processes 591
can be analyzed in terms of orbital symmetry considerations. Cheletropic eliminations
are discussed in Section 6.6.1 and elimination of nitrogen from azo compounds in SECTION 6.6
Section 6.6.2. We consider an important group of unimolecular -elimination reactions Unimolecular Thermal
Elimination Reactions
in Section 6.6.3.

6.6.1. Cheletropic Elimination
Cheletropic processes are defined as reactions in which two bonds are broken at a
single atom. Concerted cheletropic reactions are subject to orbital symmetry analysis in
the same way as cycloadditions and sigmatropic processes. In the elimination processes
of interest here, the atom X is normally bound to other atoms in such a way that
elimination gives rise to a stable molecule. In particular, elimination of SO ,N ,or
2 2
CO from five-membered 3,4-unsaturated rings can be a facile process.
C C
C C
X Y X Y
C C
C C
X Y= C O, N N, SO 2
A good example of a concerted cheletropic elimination is the reaction of
3-pyrroline with N-nitrohydroxylamine, which gives rise the the diazene 21, which
then undergoes elimination of nitrogen.

Na N O +
N 2 2 3 N CH 2 CHCH CH + N
2 2
H + 21
H :N: +
Use of substituted systems has shown that the reaction is stereospecific. 300 The groups
on C(2) and C(5) of the pyrroline ring rotate in the disrotatory mode on going to
product. This stereochemistry is consistent with conservation of orbital symmetry.
CH 3 H CH 3 CH 3
+ CH 3 H + CH
N CH H N H CH 3 3
H 3 H CH H H
N – 3 N –
The most synthetically useful cheletropic elimination involves 2,5-
dihydrothiophene-1,1-dioxides (sulfolene dioxides). At moderate temperatures they
fragment to give dienes and sulfur dioxide. 301 The reaction is stereospecific. For
example, the dimethyl derivatives 22 and 23 give the E,E- and Z,E-isomers of
2,4-hexadiene, respectively, at temperatures of 100 –150 C. 302 This stereospecificity

corresponds to disrotatory elimination.
CH 3 CH 3 H CH 3
CH 3 CH 3 S H H CH 3
S CH
H H H H 3 CH H
O 2 O 2 3
22 23
300
D. M. Lemal and S. D. McGregor, J. Am. Chem. Soc., 88, 1335 (1966).
301 W. L. Mock, in Pericyclic Reactions, Vol. II, A. P. Marchand and R. E. Lehr, eds., Academic Press,
New York, 1977, Chap. 3.
302
W. L. Mock, J. Am. Chem. Soc., 88, 2857 (1966); S. D. McGregor and D. M. Lemal, J. Am. Chem.
Soc., 88, 2858 (1966).