What distortions of a pyrroledione would be expected for
a planar transition state? A pseudopericyclic reaction can
often be recognized if the π-electrons can be rearranged
independently of the σ-electrons, as in resonance structures
B and C (eq 2). Thus, in 3-5, the N1-C2, C2-C3, and
carbonyls14 or a negative hyperconjugative interaction be-
tween the oxygen lone pairs and the adjacent C-C σ*
1
5
orbital. These cannot be full explanations, as this bond in
6 is significantly shorter at 1.533 Å. The benzannulation in
5 increases the strain in the five-membered ring, contributing
to longer endocyclic bonds N1-C2 and C2-C3. The
aromatic ring would interrupt resonance as in B and since
9
b
decarbonylation of 5 is more endothermic, resonance
structure C would also contribute less and A more.
In 3 and 4, the N1-C2 bonds are longer (1.395 and 1.378
Å) and the C2dO bonds are shorter (1.202 and 1.210 Å)
than in the model systems 6 and 8. This is consistent with a
reduction in the amide resonance (A), attributable to reso-
nance structures B or C. Comparing 3 with 4, N1-C5 and
C2dO are shorter in 3, while N1-C2 is longer, consistent
with less amide resonance A in 3. This likely reflects the
conjugation of two carbonyls in 3. The N1-C2 bonds in 4
and 7 are equal (1.378 Å). This is not an appropriate
comparison, however, because the nitrogen substituents are
different, Pr in 4 and Ph in 7. The phenyl conjugation in 7
would be expected to compete with amide resonance,
lengthening the N1-C2 bond. The appropriate comparisons
for the N1-C2 bonds are between 3 and 7 (1.395 and 1.378
Å) and between 4 and 6 or 8 (1.378 and 1.342 or 1.363 Å).
In each of these comparisons, the molecule that can decar-
C3dO bonds (in italics in Figure 2) as well as the C4-C5
bonds should be lengthened relative to model systems, while
the C2dO bonds (bold) as well as the C3-C4 and N1-C5
bonds should be shorter. In the calculated transition states
(Figure 1A,B), the departing CO bends toward N1. Thus,
the N1-C2dO angle should be reduced and the OdC2-
C3 angle should be opened. The C4-C3dO angle should
also be opened, toward the linear ketene. The angles
predicted to be more open are shown in green. With only
two exceptions (shown in red, both for 5 which should show
the least distortion) all of the distances and angles in 3-5
follow these trends, as discussed below. The fact that the
rings are planar and the geometries are distorted toward
fragmentation strongly suggests that the transition states are
also planar and therefore pseudopericyclic. These distortions
16
bonylate has the longer N1-C2 bond.
The trends in the bond angles of 3 and 4 also reflect
distortions toward the calculated, pseudopericyclic transition
states. Most significantly, the C3-C2dO angles in 3 and 4
can also be found in previous MP2/6-31G* calculations of
8
1
and 2. These data are reproduced in the Supporting
Information.
(128.2° [127.5°] and 126.9°) are wider than the N1-C2dO
Many factors can influence bond lengths and angles,
including hybridization, resonance, and strain. To accurately
assess the importance (or not) of the structure correlation
principle in this context requires consideration of these
effects. In the discussion below, the comparisons are between
systems in which these are as similar as possible; situations
where the comparisons are more complicated are noted.
angles (126.7° and 126.8°). This is consistent with the angle
of the departing CO in the calculated transition state (Figure
1
). In contrast, in 5-8, the N1-C2dO angles are the larger
ones. Formation of the ketene also requires the OdC3-C4
angle to become more linear and indeed, in 3 and 4 these
angles (131.9° [132.3°] and 133.0°) are larger than the
OdC3-C2 angles (122.4° [122.2°] and 122.3°). However,
this trend is the same in 5-8, so it is not unambiguous. It is
noteworthy that in 5, OdC3-C4 (131.6°) is more linear than
N1-C2dO (128.1°) while the angles are almost identical
in 6. This suggests that resonance structure C contributes
even to 5.
A search of the Cambridge Structural Database17 (CSD)
revealed five other pyrrolediones and numerous structures
analogous to 6-8. Several additional substructures were also
examined, having most, but not all of the structural features
of pyrolediones and thus being unable to decarbonylate via
a planar, pseudopericyclic transition state. These structures
are discussed in detail in the Supporting Information. The
trends in the bond distances and angles in these structures
are uniformly consistent with the trends discussed above and
The trends in the bond lengths are the most straightforward
and these predictions are uniformly followed in compounds
3
and 4. Although decarbonylation of 5 is more endothermic
and thus would be expected to show less distortion, these
predictions are generally followed in 5 as well. In the absence
of other effects, amide resonance (as in A) is stronger than
in a vinylogous amide (B, see S7 in the Supporting
Information). And this is clearly the case for 6, in which the
C3dO bond is significantly shorter than the C2dO bond.
Yet, the C2dO carbonyl is shorter than C3dO (1.202 Å vs
1.211 Å in 3 and 1.210 Å vs 1.221 Å in 4). This is consistent
with a contribution from resonance structure C. The C3dO
bonds in 3 and 6 are the same within experimental error,
arguably because the vinylogous ester resonance in 3 is
mitigated by resonance with the exocyclic carbonyl.
(
14) Palenik, G. S.; Koziol, A. E.; Katrizky, A. R.; Fan, W. Q. J. Chem.
Soc., Chem. Commun. 1990, 715.
15) Rathna, A.; Chandrasekhar, J. J. Chem. Soc., Perkin Trans. 2 1991,
661-1666.
16) This comparison is also validated by comparing 3 (N-Ph) with 4
(N-Pr); again, the N-Ph has the longer N1-C2 bond (1.395 vs 1.378 Å).
17) CSD Version 5.24 (Oct 2003): (a) Allen, F. H. Acta Crystallogr.
002, B58, 380-388. (b) Allen, F. H.; Motherwell, W. D. S. Acta
Crystallogr. 2002, B58, 407-422.
The C2-C3 bonds in 3-5 are long by any standard (1.550,
.554, and 1.569 Å respectively). They are much longer than
(
1
1
those in the comparison structures 6-8 (1.533, 1.492, 1.492
(
2
Å) even though these bonds are all between sp carbons with
(
an oxygen attached. Others have discussed the lengthening
of this bond in 5 as due to lone-pair repulsion between the
2
Org. Lett., Vol. 6, No. 23, 2004
4291