A R T I C L E S
Birney et al.
Table 9. Calculated Distances (Å) between the Bridgehead
There are also some other consistent patterns in the geom-
etries. The first is that the C2-C3 and C7-C8 bond lengths
(bonds a and b) are consistently longer (generally 0.01 to 0.02
Å) in those molecules that can undergo a retro-Diels-Alder
reactions compared to those that cannot. Tables 3 and 6 also
reveal that the magnitude of these structural effects is larger in
the cyclopentadiene cycloadducts than in the corresponding
cyclohexadiene cycloadducts. For example, the C2-C3 and
C7-C8 bond distances for the unsaturated cyclopentadiene
cycloadducts 5, 9, and 11 are calculated to be 0.022, 0.020,
and 0.021 Å, respectively, longer than the corresponding
distances in the saturated derivatives 6, 9s, and 11s. The
observed difference between these bonds in 11 and 11s was
0.022 Å, again showing an excellent agreement between theory
and experiment. The corresponding differences calculated for
the cyclohexadiene cycloadducts 13 and 13s; 10 and 10s; and
12 and 12s were 0.012, 0.010, and 0.011 Å, respectively, and
are clearly smaller. Similar conclusions are drawn by comparing
the 13C-13C scalar coupling constants for these bonds in
compounds 9-12 and 9s-12s. Thus the differences in the 13C-
13C scalar coupling constants for the cyclopentadiene cycload-
ducts 9 and 10 and the saturated analogues 9s and 10s were 3.3
and 3.6 Hz, respectively. By comparison the difference in the
13C-13C scalar coupling constants for the cyclohexadiene
cycloadducts 10 and 12 and the saturated analogues 10s and
12s was only 1.2 and 1.1 Hz, respectively.
Carbons and the Ethano Bridge Carbons in Compounds 10, 10s,
12, and 12s
C −CH
b
2
10
10s
12
1.555
1.543
1.555
1.547
12s
Figure 2.
A second possibility is that the additional ring strain of the
221 system directly manifests itself in longer endocyclic bonds
and shorter exocyclic bonds. This trend is commonly observed
in a wide range of strained systems and has been interpreted as
a change toward higher π-character in the endocyclic bonds
and higher σ-character in the exocyclic bonds.16 The greater
strain in the 221 systems is indirectly manifested in the
calculated energy of hydrogenation. These energies are consis-
tently more exothermic (4-5 kcal/mol) for the 221 systems than
for the 222 system (see Table 8).
A third alternative explanation for the smaller structural
effects observed in the [222]-alkenes arises upon examination
of the bond distances between the bridgehead carbons and the
ethano bridge carbons for 10 and 12. Comparison of these bond
distances with those in the saturated analogues 10s and 12s
(Table 9) reveals these bonds are ca. 0.012 and 0.013 Å,
respectively, longer in the unsaturated molecules; these are the
bonds which break in the alternative retro-Diels-Alder reaction
for loss of ethylene. In fact the degree of lengthening of the
bonds to the ethylene bridge is slightly greater (than that for
the bonds to the maleimide/anhydride moiety) and furthermore
the 13C-13C coupling constants between the bridgehead carbons
and the ethylene bridge increase by ca. 1.5-2.1 Hz upon
saturation (Table 5). It is therefore interesting to note that closely
related cyclohexadiene cycloadducts undergo loss of ethylene
upon heating in sealed tubes.17 Perhaps interaction of the π
orbitals with two sets of σ orbitals (Figure 2) results in a dilution
of the structural effects over the two sets of C-C σ bonds.
To further probe this particular question, we calculated the
transition structures for the retro-Diels-Alder reactions for the
loss of ethylene and for the loss of cyclohexadiene from 10.
The transition structure for loss of ethylene (10TS2) is only
7.9 kcal/mol higher in energy than that for the rDA for loss of
cyclohexadiene via 10TS. This result suggests that the Diels-
Alder reaction between maleic anhydride and cyclohexadiene
is reversible, but that the loss of ethylene is irreversible under
these conditions.
Similarly, the calculated C2-C7 bond distances are shorter
in those molecules that can undergo the rDA reaction than in
the saturated ones. For example, the C2-C7 distance is 0.005
Å shorter in 9 than in 9s, and 0.012 Å shorter in 13 than in
13s. Also, the H2-C2-C7 angle is more open in the molecules
that can fragment, as C2 becomes more sp2 hybridized, e.g. 9
(114.4°) as compared to 9s (114.0°) and 13 (109.0°) as compared
to 13s (108.7°). These trends are consistent with more double
bond character in those molecules where fragmentation is
possible.
We offer three possible explanations for the larger structural
effects observed in the cyclopentadiene cycloadducts as com-
pared to the cyclohexadiene adducts. The first two explanations
are based on the additional strain present in the norbornene
(bicyclo[2.2.1]hept-2-ene) ring system that is relieved in the
retro-Diels-Alder reaction. First, it can be argued that lengthen-
ing of the C2-C3 and C6-C7 bonds in both sets of cycload-
ducts arises from interaction between the σ and σ* orbitals of
these bonds with the π and π* orbitals of the double bond. The
greater angle strain could be postulated to raise the energy of
the σ orbitals of the breaking a and b bonds (C2-C3 and C6-
C7) in the 221 systems and similarly lower the energy of the
σ* orbitals.15 In simple frontier orbital terms this would result
in a stronger interaction between these σ orbitals with the π*
orbital and between the σ* and π orbitals in the 221 systems
compared with the 222 systems. Both of these effects would
weaken and lengthen bonds a and b.
Is there a relationship between the degree of lengthening of
the bonds that break in the retro-Diels-Alder reaction and the
reactivity of the cycloadduct toward this reaction? Our data
(14) (a) Wiberg, K. B.; Bader, R. F. W.; Lau, C. D. H. J. Am. Chem. Soc. 1897,
109, 1001. (b) Stanger, A. J. Am. Chem. Soc. 1998, 120. These effects
have been recently utilized to stabilize planar cyclooctatetraenes. (c)
Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 2001, 123, 1755-1759.
(d) Matsuura, A.; Komatsu, K. J. Am. Chem. Soc. 2001, 123, 1768.
(15) (a) Brown, R. S.; Traylor, T. G., J. Am. Chem. Soc. 1970, 92, 5228. (b)
Streitwieser, A., Jr.; S. Alexandratos, J. Am. Chem. Soc. 1978, 100, 1979.
(16) Engler, E. M.; Andose, J. D.; Schleyer, P. v. R. J. Am. Chem. Soc. 1973,
95, 8005.
(17) (a) Hugo, V. I.; Nicholson, J. L.; Snijman, P. W. Synth. Commun. 1994,
24, 23. (b) Dimitriadis, C.; Gill, M.; Harte, M. F. Tetrahedron Assym. 1997,
8, 2153.
9
5096 J. AM. CHEM. SOC. VOL. 124, NO. 18, 2002