Journal of the American Chemical Society
Article
the [3,5]-sigmatropic rearrangements are calculated to be less
positively charged and, hence, would be expected to attract the
carbonyl lone pair less strongly than for the [3,3]-sigmatropic
rearrangements. In addition, in all three structures (compounds 3, 9,
and 10), it is the lone pair and not the π system of the ester that is
pointed toward the carbon. It would be this lone pair that is calculated
to be involved in bond formation in the pseudopericyclic [3,5]-
sigmatropic rearrangement. While closer approach of the carbonyl
oxygens to the carbon to which they will bond in the [3,5]-sigmatropic
rearrangements does not in itself require that the eight-centered
pseudopericyclic pathway be followed, both the distances and the lone-
pair orientations of the carbonyls in the crystal structures prefigure the
calculated pseudopericyclic [3,5]-sigmatropic rearrangement transition
states. These are in accordance with the structure−reactivity
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AUTHOR INFORMATION
Corresponding Author
■
Present Address
‡David B. Cordes: EaStCHEM, School of Chemistry,
University of St. Andrews, North Haugh, St. Andrews, Fife
KY16 9ST, United Kingdom.
Author Contributions
†Shikha Sharma and Trideep Rajale contributed equally to this
work.
correlation by Burgi and Dunitz, which suggests that ground states
̈
tend to distort along allowed reaction coordinates.4e,13,14
Notes
The authors declare no competing financial interest.
CONCLUSION
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ACKNOWLEDGMENTS
The combined experimental and computational studies
reported here provide strong evidence that the [3,5]-
sigmatropic rearrangements of esters are allowed via a
pseudopericyclic pathway and are kinetically favored (at 0.0%
conversion) over the more familiar [3,3]-sigmatropic rearrange-
ments. The thermal rearrangements of two 6-acetoxy-2,4-
cyclohexadienones 3 and 10 were investigated by FVP at
temperatures ranging from 300 to 500 °C. For each reaction,
the ratio of [3,5]/[3,3] products (isolated after tautomerization
to the corresponding phenols) gives a straight line when
plotted versus percent conversion, between 300 and approx-
imately 400 °C. When extrapolated to 0.0% conversion, the
initial ratio for compound 3 is 3.17:1, and for compound 10, it
is 1.2:1, favoring the [3,5]-sigmatropic rearrangement products
in both cases. A pericyclic [3s,5s]-sigmatropic rearrangement
would be orbital-symmetry-forbidden, but the ester rearrange-
ment is allowed via a pseudopericyclic transition state, in which
bond breaking and forming occurs in the plane of the ester.
These transition states are prefigured by the ground-state
geometries from X-ray crystal structures of compounds 3, 9,
and 10. The immediate product from [3,5]-sigmatropic
rearrangement of compound 9 is compound 11, and this
compound can be isolated without tautomerization. DFT
calculations also predict pseudopericyclic transition states and
reproduce the qualitative preference for [3,5]-sigmatropic
rearrangements.
As the FVP temperature is raised and, particularly, at
temperatures above 400 °C, the selectivity for the [3,5]-
sigmatropic rearrangement products increases. This unusual
situation can be explained by invoking a non-ionic (gas-phase)
pathway for the formation of a tetrahedral intermediate (ortho-
acid ester 4′) that is formed irreversibly from compound 4 via
another pseudopericyclic transition state TS-5. Thus, at higher
temperatures, compounds 3, 4, and 7 are approaching
equilibrium, but the irreversible reaction to form 4′ shifts the
equilibrium. This intermediate (4′) can then open via a similar
transition state to give either isomeric o-acetoxyphenol (5 or 6).
This may be a general mechanism for acyl migrations.
■
We gratefully acknowledge generous support from the Robert
A. Welch Foundation (Grant D-1239). We also are grateful for
the use of a 400 MHz MNH spectrometer made available
through the NSF CRIF MU Grant CHE-1048553 and access to
the Robinson cluster, which is funded by CRIF MU
Instrumentation Grant CHE-0840493 from the National
Science Foundation. Both instruments are in the Department
of Chemistry and Biochemistry, Texas Tech University. We
thank Professor Stephane Quideau for introducing us to
Wessely’s chemistry, David Purkiss for assistance with the
NMR spectra, Dr. George Tamas for assistance with electronic
structure theory calculations, and Daniel A. Stroud for synthetic
assistance.
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ASSOCIATED CONTENT
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S
* Supporting Information
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Experimental information, including synthetic procedures, FVP
and GC conditions, H and 13C NMR spectra of all new
1
compounds, details of X-ray crystal structures, and computa-
tional information, including energies, pictures, and Cartesian
coordinates of all conformations of all calculated structures.
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dx.doi.org/10.1021/ja4077364 | J. Am. Chem. Soc. 2013, 135, 14438−14447