Page 619 - Advanced Organic Chemistry Part B - Reactions & Synthesis

The elimination of carbon monoxide can occur by a concerted process in some 593
cyclic ketones. The elimination of carbon monoxide from bicyclo[2.2.1]heptadien-7-
ones is very facile. In fact, generation of bicyclo[2.2.1]heptadien-7-ones is usually SECTION 6.6
accompanied by spontaneous decarbonylation. Unimolecular Thermal
Elimination Reactions
O R
R R
R R
R + CO
R R
R R
R R
The ring system can be generated by D-A addition of a substituted cyclopentadienone
and an alkyne. A reaction sequence involving addition followed by CO elimination
can be used for the synthesis of highly substituted benzene rings. 308

Ph Ph
Ph Ph Ph
O + PhC CPh
Ph Ph Ph
Ph Ph
Ref. 309

The synthetic utility of cyclopentadienones is limited, however, because they are quite
unstable. Exceptionally facile elimination of CO also takes place from 22, in which
homoaromaticity can facilitate elimination.

O
O
C
+ CO
22
Ref. 310

6.6.2. Decomposition of Cyclic Azo Compounds

Another significant group of elimination reactions involves processes in which a
small molecule is eliminated from a ring system and the two reactive sites that remain
re-form a ring.
X Y
+ X Y

The most common example is decomposition of azo compounds, where −X−Y−
is −N=N−. 311 The elimination of nitrogen from cyclic azo compounds can be carried

308 M. A. Ogliaruso, M. G. Romanelli, and E. I. Becker, Chem. Rev., 65, 261 (1965).
309
L. F. Fieser, Org. Synth., V, 604 (1973).
310 B. A. Halton, M. A. Battiste, R. Rehberg, C. L. Deyrup, and M. E. Brennan, J. Am. Chem. Soc., 89,
5964 (1967).
311
P. S. Engel, Chem. Rev., 80, 99 (1980).