Chemistry Reference
In-Depth Information
reaction mechanisms [
261
]. It may be interesting to note that the transition state
TS2 interconverting M1 and M2 has to have a lower energy than TS1, due to the
steepest descent path leading from TS1 to TS2. This is in accord with the prediction
of Theorem IV of Stanton and McIver, which is not based on gradient paths [
270
].
4 Symmetry of BAEs
4.1 Conformations and Point Groups of BAEs
4.1.1 Topological Classification
Overcrowded bistricyclic aromatic enes (1) may be classified as homomerous
(X
Y) BAEs, or, according to their substitution
pattern, as unsubstituted, monosubstituted, and disubstituted BAEs. These classifi-
cations are helpful in analyzing the symmetry of these systems. In this context any
modification making one (or more) of the four aromatic rings A, B, C, and D unique
will be considered a “substitution.” Persubstituted homomerous BAEs (e.g
.
,
hexadecachloro-9,9
0
-bi(9
H
-fluoren-9-ylidene)), and symmetrically tetra-substituted
homomerous BAEs (e.g., 2,2
0
,7,7
0
-tetramethyl substituted BAE) fall into the same
class as unsubstituted BAEs. Analogously, a 2,2
0
,7-trisubstituted BAE (R
¼
Y) and heteromerous (X
6¼
R
0
¼
¼
R
00
and R
R
0
6¼
R
00
;X
Y) would formally fall into the class of
monosubstituted BAEs. 2,7-Disubstituted homomerous BAEs have the same sym-
metry properties as unsubstituted heteromerous BAEs. For symmetry classification,
only the more narrowly defined group of 1,1
0
-, 2,2
0
-, 3,3
0
-, and 4,4
0
-disubstituted
BAEs with R
¼
¼
Y and X
6¼
¼
R
0
, and X
¼
Y are considered disubstituted homomerous BAEs.
In the class of disubstituted heteromerous BAEs (X
R
0
, and
1,2
0
-substitution patterns etc. are included, as long as the two substituents are not on
the same moiety. In highly substituted BAEs, substituents may be subdivided
in equivalent groups (i.e., identical substituents in equivalent positions). For each
group the class may be assigned separately. The appropriate class of such a highly
substituted BAE will be the “smallest common denominator,” i.e.,
6¼
¼
R
0
,R
6¼
Y), R
the class
corresponding to the common subgroup of all symmetry groups.
The above analysis leads to five classes of overcrowded bistricyclic aromatic
enes:
Unsubstituted homomerous BAEs
Unsubstituted heteromerous BAEs
Monosubstituted homomerous BAEs and monosubstituted heteromerous BAEs
Disubstituted homomerous BAEs
Disubstituted heteromerous BAEs
This classification has particular relevance for the topological considerations
used to derive the feasible permutation-inversion operators and the molecular
symmetry group.
