C O M M U N I C A T I O N S
This conclusion is consistent with the information derived from
recent quantum calculations,18 secondary isotopic effects analysis,8a
and nonsteady-state analysis,19 which point out that CTCs are not
transition state-like, even when the reaction centers in the CTC
are ideally positioned to achieve the cycloaddition.18,19
X-ray-single-crystal analyses performed on crystals of 5 after 6
months of storage show no chemical transformations. As a conse-
quence, no solid-state topochemical DA reactions can be envisioned
in our CT crystal. Obviously, substantial CT stabilization inhibits the
DA reaction in the crystalline state. Even if it is still difficult to conclude
on the role of the CT interactions in solution, and therefore on the
preferred pathway of the DA reaction, the X-ray analysis of the
intermediate CTC is of primary importance in the field of DA reactions
and further experimental efforts are being carried out.
Acknowledgment. This work was supported by grants from
the Ministry of Research and Education. We thank Prof. H. Mayr,
Prof. F. Terrier, and Dr. S. Lakhdar for helpful discussions.
Figure 2. X-ray structure of CTC 5. (a) Side view of the unit cell along
the a axis. (b) Bond lengths in Å of 1 and 2 in CTC 5. (c) Calculated DFT
structure of the corresponding DA transition state at a B3LYP level.
Supporting Information Available: Crystallographic data for 5;
HRMS and NMR for 8-10; Determination of electron affinity of 1
and kinetics procedures and methods. This material is available free
the CTC is an intermediate on the reaction coordinate leading to 9 (pathway
1) or is formed in a side, and therefore nonproductive, equilibrium
(pathway 2). In a study of the reaction of TCNE with 9,10-dimethylan-
thracene 4 at different temperatures, Kislev and Miller have found an
overall negative enthalpy of activation (∆H#exp) and suggested that this
particular situation is only consistent with the corresponding CTC being
an intermediate on the reaction coordinate.8c It appears that the above
reaction is the unique case where the intermediate CTC is stable enough
to exhibit a large negative enthalpy of formation (∆H°) value which
overcomes the activation energy (∆H#) value pertaining to the cycload-
dition step.7f,8c Apart from this exception, it remains that DA reactions
are associated with positive ∆H#exp values so that thermodynamic and
kinetic studies are mechanistically unconclusive.7-9 No negative values
of ∆H#exp were here determined for the reaction of 2, 3, or 4 with NBDF
1 (1.1, 8.8, and 12.2 kcal.mol-1 respectively) so that no conclusion
regarding the preferred pathway in Scheme 1 can be drawn.
A most informative result has come from the isolation of a stable
crystal, suitable for an X-ray analysis, upon evaporation of a
chloroform solution of CTC 5 under a flow of argon. In Figure 2,
the X-ray structural determination confirmed the formation of the
anticipated 5 as a face to face π-π complex with a 1:1
donor-acceptor ratio. The cofacially oriented anthracene and NBDF
moieties afford infinite alternate stacks along the crystallographic
b-axis with a close interplanar distance of 3.45 Å.
Because of the facile subsequent formation of the corresponding
cycloadducts, it is only in very few instances that CTCs involved in
the reaction of anthracene with electrodeficient dienophiles could be
successfully characterized by X-ray.10,16 It is therefore a significant
result that the X-ray structure of 5 is the first example of CTCs in
which the diene 2 and the dienophile 1 lie in a parallel arrangement
and are not suitably preoriented for a direct conversion to the expected
cycloadduct 8.16 The X-ray data indicate that the NBDF moiety is
not appreciably altered upon complexation, ruling out a possible
mechanism with a single electron transfer. Interestingly, there is no
structural relationship between the X-ray structure, where 1 and 2 are
probably oriented according to Mulliken’s “Overlap and Orientation”
principle,17 and the optimized DFT structure of the TS, where
secondary orbital interactions between the 1-LUMO and the 2-HOMO
are ideal for a DA reaction. In fact, the anthracene moiety has to rotate
75° to adopt the orientation required for the cycloaddition. Obviously,
this situation does not support the assumption that CTCs resemble the
TS in DA reactions. It also reveals that the CTC has to dissociate
back to the reactant, allowing the two partners to undergo cyclization
through another different prereaction complex.
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