1) The document studies the 1,5-hydrogen shift in decalin-1,8-dione, a rigid β-diketone that constrains the carbon-oxygen bonds to be parallel. 2) NMR spectroscopy and X-ray crystallography show the presence of the enol tautomer (5) and suggest a low barrier, in-plane hydrogen transfer consistent with a pseudopericyclic reaction mechanism. 3) Isotopic substitution studies using Saunders' method and examination of potential proton tunneling effects aim to further characterize the reaction by distinguishing between symmetrical and asymmetrical transition states.

1. Characterization
Nuclear Magnetic Resonance (NMR) has been a power tool in investigating the structure of enols,
from those present in small amount at equilibrium to highly stabilized enol species. In solution, the
1H-NMR spectra of 4 shows exclusively the presence of 5. The spectrum of 5 shows proton transfer
is fast on the NMR timescale independent of temperature. This complicates the NMR by the
equilibration of both cis and trans isomers of 4 in solution as well as 5. Due to this complication, it
is theorized in order to properly study the compound, two factors need to examined; the isotope
perturbation effect and the symmetry for the hydrogen bond.
Isotopic Perturbation of Equilibria
According to Saunders Isotopic Perturbation of Equilibrium5, the experimental difference between a
symmetrical structure versus two degenerate equilibrating structures of lower symmetry has been a
common problem in chemistry. This issue is perplexing when the equilibration is quick and available
spectroscopic techniques have a low time resolution. To address this problem, Saunders developed
one of the most elegant experiments of classical physical organic chemistry in the observation of
isotopic perturbation effects on the NMR spectra of molecules of interest.
The effect of isotopic substitution on geometry is often assumed to be small, and this assumption is
important in many experiments. The idea behind the Saunders experiment is that desymmetrizing
isotopic substitution affects the equilibrium between two equilibrating structures but cannot have an
effect on a single symmetrical structure where no equilibration is present.
As seen in the structures above, 5a and 5b, the use of Saunders method would help in the mono-
deuterated structure. This desymmetrization can help distinguish not only the peaks for the a-
deuteration but the perturbation shifts can help distinguish whether the compound has a symmetrical
or asymmetrical hydrogen bond.
Symmetry of Hydrogen Bond
In using Saunders method, we can use the carbon isotope effects to examine the possibility of proton
tunneling. The consequences of tunneling on reaction kinetics indicate that temperature independence
leads to nonlinear Arrhenius plots. A minimum energy for tunneling represents the slope at low
temperature. This also indicates when the hydrogen atom without its electron, is reduced to a proton,
it could tunnel either in a single well or in a double well. When proton tunneling occurs, the
hydrogen bond and the covalent bonds are switched.
Figure 3 illustrates how in-plane proton transfer from one oxygen to another leads to the
interchange of bonding and nonbonding orbitals, along a pseudopericyclic pathway. A
planar transition state is recognized as one of the hallmarks of a pseudopericyclic
reaction.4
Enols are important intermediates in a wide variety of reactions and have been studied
extensively.1 In general, the enol tautomers of simple ketones are present only at very
low concentrations. In contrast, the enol tautomers of β-diketones are are often present
in much higher concentrations. Their rich enol chemistry often dominates their
reactivity.
The acidic proton in the enol tautomers of flexible β-diketones is hydrogen bonded to
the other carbonyl and undergoes a rapid exchange from one oxygen to the other. This
reaction can be formally defined as a [1,5] sigmatropic rearrangement. However, if the
hydrogen is transferred in the plane of the conjugated system (as suggested by the
hydrogen bonding), it is not a pericyclic reaction as defined by Woodward and
Hoffmann.2 Rather, it would be better classified as a pseudopericyclic reaction, initially
described by Lemal and coworkers3 to explain the rapid degenerate rearrangement of
Dewarthiophene 1 (Figure 1). He defined a pseudopericyclic reaction as “a concerted
transformation whose primary changes in bonding compassed a cyclic array of atoms,
at one (or more) of which nonbonding and bonding atomic orbitals interchange roles.”
(Figure 2).
Introduction
X-Ray Crystallography
Pseudopericyclic Study of [1, 5] Hydrogen Shift
of a Decalin-1, 8-dione
Josmalen M. Ramos-Lewis, Brett M. Casserly and David M. Birney*
Texas Tech University Department of Chemistry and Biochemistry, Lubbock, Texas 79409-1061
Abstract
Enols of β-diketones undergo facile 1,5-hydrogen shifts. Decalin-1,8-dione has been
synthesized, to provide a rigid and conformationally constrained platform for the study
of such a 1,5-hydrogen shift. Qualitative analysis of the orbitals and DFT calculations
(B3LYP/6-31G*d,p) predict a planar transition state; implying a pseudopericyclic
reaction. Spectroscopic studies (NMR and X-ray crystallography) are consistent with a
low barrier, in-plane hydrogen transfer.
Acknowledgements
Thanks are gratefully given to my research advisor and group. Financial support was given
from Robert A. Welch Foundation, Texas Tech Provost Doctoral Fellowship, National Science
Foundation GK-12 (NSF) Fellowship. NMR spectrometers were obtained through NSF grant,
CHE-1048553 and the NSF CRIF Program. Thanks to Dr. Daniel Unruh for X-ray structures.
Computational Studies
Based on B3LYP/6-31G(d,p) calculations (Figure 8), the β-diketone system 5 was calculated
to have a planar transition state (TS side view) for the [1,5] sigmatropic rearrangement. The
migrating hydrogen is calculated to be in the plane of the π-system, indicating a planar,
pseudopericyclic pathway.
References
1. Rappaport, Z.; The Chemistry of Enols. John Wiley & Sons. England, 1990.
2. Woodward, R.B.; Hoffmann, R.; The Conservation of Orbital Symmetry. 3rd ed; Academic
Press, Germany, 1971. (b) Woodward, R. B.; Hoffmann, R. Angew. Chem. Int. Ed. Engl.
1969, 8, 781
3. Ross, J. A.; Seiders, R. P.; Lemal, D. M. J. Am. Chem. Soc. 1976, 98, 4325.
4. Birney, D. M.; J. Org. Chem. 1996, 61, 243–251.
5. Saunders, M., et. al.; J. Am. Chem. Soc. 1989, 111, 8989
Figure 8. Energy Profile at the B3LYP/6-31G(d,p) level
Figure 1. Lemal’s NMR Rearrangement Study on Dewarthiophene.
Figure 2. Orbital representation of Lemal’s work.
4 5
Figure 5. Equilibrium species of 4 in solution with 5.
5 (1.0) 5 (1.0)
4 (0.0)
TS Side View
Bonding
Non-Bonding
Bonding
Non-Bonding
Bonding
Figure 3. Orbital representation of a pseudopericyclic reaction.
Non-Bonding
X-ray crystallographic studies has shown that
the enolic form 5 is present in the crystal. The
π-system of the enol is planar, but the precise
position of the hydrogen is uncertain.
Nevertheless, the geometry is consistent with
the proposed pseudopericyclic pathway.
Since proton-tunneling may be occurring in
this compound, the true picture of the b-
diketone is unknown. With a-deuteration
with a heavy atom, this answer can be solved.
As the distance between the heavy atom
decreases, the height of the barrier separating
the wells is either decreasing or increasing.
The essence of isotopic perturbation is that it
makes it possible to distinguish these
structures which would have an effect on the
bonding.
In this work, 1,8-decalindione (4) and its enol tautomer 5 were studied. This system has
a fused-ring framework present in various classes of natural products and constrains the
C-O bonds to be parallel.
Figure 4. β-diketone (4) and its enol tautomer (5)
TS (12.0)
5 5
4
4
4
Figure 6. Isotopic substitution of 5 into its isomeric forms 5a and 5b.
Figure 7. Schematic representations of some of the various proton potential wells: (a)
single well (b) low-barrier double well (c) high-barrier double well.
5 Theorized structure
of 5
In-plane side view
of enol crystal 5