Electron-deficient aryl β-diketones: Synthesis and novel tautomeric preferences

Article abstract of DOI:10.1002/ejoc.201001451

Fluorinated aryl β-diketones were prepared using Claisen and electrophilic fluorination methods. The keto-enol and enol-enol tautomerism of these compounds were examined in the solid state, as neat liquids and in polar, aprotic solution by crystallography and spectroscopy. Neat-liquid spectroscopic measurements as well as single crystal X-ray crystallographic results for selected electron-deficient aryl β-diketones suggest a single, chelated cis-enol isomer that is conjugated with the aryl ring. In polar aprotic solvents, nonfluorinated aryl β-diketones equilibrate rapidly from the chelated cis-enol form to a tautomeric mixture of cis-chelated enol and a substantial proportion of the diketone form, trifluoromethylated aryl β-diketones show only limited equilibration from the chelated cis-enol to the diketone form, with 2-fluoro-1-aryl β-diketones again displaying only the diketonic form.

Full text of DOI:10.1002/ejoc.201001451

FULL PAPER
DOI: 10.1002/ejoc.201001451
Electron-Deficient Aryl β-Diketones: Synthesis and Novel Tautomeric
Preferences
Joseph C. Sloop,*[a] Paul D. Boyle,[b] Augustus W. Fountain,[c] William F. Pearman,[d] and
Jacob A. Swann[d]
Keywords: Ketones / Enols / Tautomerism / Keto–enol equilibrium / 1,3-Diketones
Fluorinated aryl β-diketones were prepared using Claisen
and electrophilic fluorination methods. The keto–enol and
enol–enol tautomerism of these compounds were examined
in the solid state, as neat liquids and in polar, aprotic solution
by crystallography and spectroscopy. Neat-liquid spectro-
scopic measurements as well as single crystal X-ray crystallo-
graphic results for selected electron-deficient aryl β-di-
ketones suggest a single, chelated cis-enol isomer that is con-
jugated with the aryl ring. In polar aprotic solvents, non-
fluorinated aryl β-diketones equilibrate rapidly from the che-
lated cis-enol form to a tautomeric mixture of cis-chelated
enol and a substantial proportion of the diketone form, tri-
fluoromethylated aryl β-diketones show only limited equili-
bration from the chelated cis-enol to the diketone form, with
2-fluoro-1-aryl β-diketones again displaying only the di-
ketonic form.
Introduction
Keto–enol tautomerism in trifluoromethyl β-diketones
has been studied extensively in solution.[1–8] Scheme 1 de-
picts the principal tautomeric structures for nonfluorinated
and trifluoromethylated β-diketones (frame a), 2-fluoro β-
diketones (frame b) and atoms of spectral interest in this
study (frame c). Table 1 lists spectroscopic indicators for the
various tautomers.[1–5,7–9]
Prior researchers have probed the keto–enol and enol–
enol equilibria of various trifluoromethylated and nonfluor-
inated aryl β-diketones using computational methods and
experimentally in non-polar and polar media.[1,2,5–9] Ab ini-
tio calculations confirm a general thermodynamic stability
trend: chelated cis-enol A Ͼ chelated cis-enol B Ͼ diketo
form in the gas phase as well as in non-polar and polar,
protic solvents.[1,2] In general, while solution state NMR
spectroscopic studies can easily distinguish between the di-
keto and enol tautomers of β-diketones, resolution of the
enol tautomeric forms has remained contentious. Enol–enol
equilibria in aryl trifluoromethyl β-diketones, for example,
have been estimated using a combination of NMR chemical
Scheme 1. β-Diketone tautomeric species of interest.
Table 1. Spectral information for trifluoromethyl β-diketones.
[a] School of Science and Technology, Georgia Gwinnett College,
1000 University Center Lane, Lawrenceville, GA 30043, USA
Fax: +1-678-407-5938
Method
Units
R
Diketo form
Enol A
Enol B
1H NMR
δ / ppm aryl
X = H; H2: 3.9–5.0 H1 5–7
X = F; H2: 5.5–6.5 H3: 13–18 H3: 13–18
1685–1700
1700–1780
H1 5–7
E-mail: jsloop@ggc.edu
Raman/IR
ν / cm–1 aryl
1595–1620 1650–1680
[b] Department of Chemistry, North Carolina State University,
P. O. Box 8204, Raleigh, NC 27695, USA
[c] Edgewood Chemical Biological Center,
5183 Blackhawk Road, Aberdeen Proving Ground, MD 21010,
USA
[d] Department of Chemistry and Life Science, United States
Military Academy,
˜
shift arguments, model compounds and 13C-19F coupling
constants; the results, however, are often ambiguous. These
studies have generally concluded that most aryl β-diketones
exist as near-unity mixtures of cis-chelated enol forms a and
646 Swift Road, West Point, NY 10996, USA
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201001451.
936
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Keto–Enol Equilibria in the Solid State
b in nonpolar and slightly polar solvents.[5] The diketo tau- liquids and in polar, aprotic media. A combination of crys-
tomer and other nonchelated enol forms, when observed, tallographic, Raman, infrared, and NMR methods allow
are present in small proportions.
determination of whether discreet tautomeric forms may be
However, this equilibrium shifts toward the diketo form identified.
in solvents like ethanol rapidly, appears to be concentration
dependent and leads to acetal formation over extended
periods.[1] The limited studies conducted in polar aprotic
Results and Discussion
solvents show a shift in equilibrium toward the diketo form
for non-fluorinated aryl β-diketones whereas trifluorometh-
yl aryl β-diketones and aryl 2-fluoro β-diketones give hy-
drates over extended periods.[5,6]
Infrared spectroscopy has also been used to identify the
presence of specific tautomeric forms of nonfluorinated aryl
β-diketones in solution. Studies of benzoylacetone deriva-
tives conducted by Lowe and Ferguson confirm a marked
preference for enol form A, pointing to characteristic infra-
red bands (Table 1) that differentiate the tautomers.[7] Com-
plementary Raman investigations have not been reported.
β-Diketone Synthesis
Figure 1 shows the β-diketones investigated in this study.
Compounds 1a and 1d are commercially available; 1b, 1c,
2a, 2b, 2d, 2e were prepared by literature methods,[1,4,12–14]
while 2c and 3b were prepared by via Scheme 2.
Solid-State/Neat-Liquid Studies
Solid-state experiments were performed on all molecules,
Solid-state investigations of this phenomenon are sparse. and in instances where the solids had melting points
While examining benzoylacetone’s enol–enol equilibrium by below 70 °C, neat liquid studies were also conducted. Com-
neutron diffraction, Madsen and co-workers found that the pelling evidence for preferential tautomerization toward
enol proton occupied a “centered” location between the two chelated cis-enol A was found in the case of 4,4,4-trifluoro-
oxygens,[10] implying the possibility of a cis-enolic equilib- 1-(4-nitrophenyl)-1,3-butanedione (1b) where
a crystal
rium in the solid state. Vila’s group also posited a dynamic structure was obtained from X-ray quality crystals. The
enol–enol equilibrium for benzoylacetone based on 13C ORTEP diagram is shown in Figure 2 and measurements
CPMAS and solution state NMR spectroscopic studies.[11] are compared to those reported by Madsen for 1d in
Neat liquid investigations have likewise received little atten- Table 2.
tion in the literature.
Key differences are noted in our X-ray results when com-
Our efforts address these experimental gaps with a sys- pared to the neutron diffraction data of Madsen and co-
tematic, comparative study of fluorinated aryl β-diketone workers. The C7–C8 and C9–O4 bond lengths for 1b show a
enol–enol and keto–enol equilibria in the solid-state, as neat higher degree of double bond character than those of 1d
Figure 1. β-Diketones investigated.
Scheme 2. Diketone synthesis.
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937

J. C. Sloop et al.
FULL PAPER
δH3 were determined from the neat liquid, νketo C=O was ob-
tained in the (solid state) and νenolA C=O was acquired in the
neat liquid and/or (solid state).
Table 3. Diketone equilibria in neat liquid/solid state.
Method
KKǞEA δH2 / pm
KEǞE
δH1, δH3 / ppm
Figure 2. ORTEP drawing of 1b depicting ellipsoids at the 50%
probability level and hydrogen atoms drawn with arbitrary radii for
clarity.
ν˜keto C=O / cm–1 (bǞa)
ν˜enolA C=O / cm–1
1a NMR
Raman
IR
1b Raman
IR
1c Raman
IR
1d Raman
IR
ϾϾ 1 not detected
ϾϾ 1
ϾϾ 1
ϾϾ 1
ϾϾ 1
ϾϾ 1
ϾϾ 1
ϾϾ 1
ϾϾ 1
ϾϾ 1
6.5, 15.2
ϾϾ 1 not detected
ϾϾ 1 not detected
ϾϾ 1 not detected
ϾϾ 1 not detected
ϾϾ 1 not detected
ϾϾ 1 not detected
ϾϾ 1 not detected
ϾϾ 1 not detected
ϽϽ 1 5.6
ϽϽ 1 (1685, 1710)
ϽϽ 1 (1688, 1708)
ϽϽ 1 (1696, 1715)
ϽϽ 1 (1687, 1700)
ϽϽ 1 (1690, 1702)
1602 (1598)
1603 (1599)
1607 (1603)
1610
1610 (1612)
(1585)
Table 2. Selected interatomic distances and bond lengths of 1b and
1d.
Interatomic distances/bond length / Å
O3–O4
C7–O3
O3–H
C9–O4
C7–C8
C8–C9
1594 (1601)
(1599)
1b 2.553
1d 2.498
1.317
1.293
0.96
1.245
1.243
1.286
1.378
1.405
1.413
1.414
2a NMR
IR
not detected not detected
not detected not detected
not detected not detected
not detected not detected
not detected not detected
not detected not detected
not detected not detected
2b IR
2c IR
2d IR
2e IR
3b IR
which exhibit bond lengths intermediate between typical
C=C, C–C, C=O, and C–O bonds. Conversely, the C7–O3
bond of 1b is closer in length to a typical C–O single bond
than that of 1d. Perhaps the most revealing feature, how-
ever, is the large difference between the O3–H bond
NA
(1732, 1761)
The results of this solid state-neat liquid study are in-
lengths – 0.285 Å. Unlike Madsen’s assertion that the near formative. Given the chelated cis-enol A crystal structure
equality of interatomic distances between O3–H and O4–H for 1b, solid state and neat liquid IR and Raman spectro-
(1.26 Å) for 1d shows a likely enol A/enol B equilibrium scopic experiments confirm that the resonances at
approaching unity, the much smaller O3–H interatomic dis- 1610 cm–1, 1607 cm–1, and 1603 cm–1 arise from the che-
tance of 0.96 Å as compared to the O4–H distance (1.68 Å) lated cis-enol form A. Likewise, the neat liquid 1H NMR of
for 1b supports the presence of a single enol isomer, enol 1a is indicative of the chelated cis-enolic structure. Vide in-
A. Finally, it is important to note that several crystals of fra, this supports the presence of a single enol form for
compound 1b were examined; no crystal structure data con- diketones 1a–d, that of enol A. Another trend apparent
sistent with either a dimeric species or enol B were ob- from the IR data is the observation of two C=O bands for
tained.
the 2-fluoro-1-aryl β-diketones 2a–e, evidence that these
Based on these findings, spectroscopic studies were con- compounds exist solely as diketo tautomers. This is again
ducted on selected species in the solid state and/or neat li- corroborated by the neat liquid 1H NMR of 2a, which
quid. The data are presented in Table 3 where δH1, δH2 and shows a resonance arising from the H2 proton centered at
Figure 3. Raman spectra of 1a.
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Keto–Enol Equilibria in the Solid State
1
δ = 5.6 ppm, (d, 1 H, JH,F = 50.1 Hz). Likewise, the solid- series corroborate the presence of a single chemical species.
state IR of compound 3b, which can only exist as the di- Taken together, these facts are consistent with a chelated
ketone, displays two C=O bands at 1732 and 1761 cm–1.
A more detailed Raman spectroscopic study was under-
cis-enol structure (enol A) for β-diketone 1a, see Table 1.
taken on 4,4,4-trifluoro-1-phenyl-1,3-butanedione (1a) to Solution State Studies: NMR/Raman
ascertain whether multiple chemical species were present in
the solid-state and neat liquid. The results are shown in
Figure 3, where frame a depicts Raman spectra collected
from 1a as the sample underwent a solidǞliquid transition
from 20–84 °C and frame b shows a Hilbert–Noda synchro-
nous two-dimensional (2-D) correlation spectrum.
Spectroscopy
To assess whether the keto–enol and enol–enol equilibria
for β-diketones 1a–d and 2a–e were modulated in the solu-
tion state, 0.20 m solutions of compounds 1a–d were pre-
pared in sealed NMR tubes and H NMR (CDCl3 or [D6]-
DMSO) and Raman spectra (CDCl3 or CH3CN or [D6]-
1
The solidǞliquid phase transition study of 1a involved
a series of spectra collected at each temperature. The sample
temperature was held constant for a minimum of ten min-
utes between the incremental temperature increases. These
spectra showed no change over time or the temperature
range of the study. In Figure 3, frame a, the observation of
a lone, well-defined enolic C=O band at 1598 cm–1 in the
solid state (1602 cm–1 as a neat liquid) coupled with the
absence of bands above 1650 cm–1 suggests the presence of
enol form A which apparently does not undergo any ap-
preciable tautomerization during phase transition. If 1a
were a mixture of enol tautomers in the solid state or neat
liquid, one would anticipate that the C=O band would be
shifted to higher wavenumber and be somewhat broader in
appearance than the sharp C=O band observed.
DMSO) collected. Compounds 2a–e and 3b were examined
by H NMR in CDCl3. The results are recorded in Table 4.
1
The data in Table 4 reflect three distinct trends. For β-
diketones which are not fluorinated in the 2-position (1a–
d), a preference for cis-enol form A is indicated by the pre-
1
dominance of a H NMR resonance at δ(enol-H1) = 6.4–
6.9 ppm ([D6]DMSO), and Raman bands for enol A at
ν(enol) = 1590–1620 cm–1 ([D6]DMSO, CH3CN). The ob-
˜
servation that δH3 (enol-H3) moves to a lower chemical shift
in [D6]DMSO relative to that of CDCl3 suggests that polar,
aprotic solvents may disrupt the intramolecular H-bonding
of the chelated cis-enol form, see Figure 4.
Two-dimensional (2-D) correlation spectroscopy was
used to study which vibrational features were more suscep-
tible to change during the phase transition from solid to
liquid state.[15] The 2-D spectrum of 1a shows a strong, di-
rect correlation between the C=O band and the C–C–O
bend at 1230 cm–1, while being somewhat weakly correlated
to the C–F stretches at 1169, 1181 and 1353 cm–1 and the
C–H out-of-plane bend at 990 cm–1. This indicates possible
torsional adjustment of the molecule during the phase tran-
Figure 4. Disuption of cis-enol chelation.
Furthermore, and in accord with previous results,[1] the
sition, but the lack of complexity in the 2-D spectrum and 1-aryl-2-fluoro β-diketones 2a–e demonstrate only the di-
1
the absence of any new vibrational features in the Raman keto form, as evidenced by H NMR resonances: δ(H2) =
Table 4. Solution state diketone equilibria constants.
Method
NMR
KKǞE(A)
KEǞE
% Hydrate
δ / ppm
δ / ppm
[ ]: δH3 in [D6]DMSO
[ ]: δH1 in [D6]DMSO
( ): δH2 in [D6]DMSO
{ }: δH2 in CDCl3
(bǞa)
[D6]DMSO
(600 h)
{ }: δH3 in CDCl3
ν / cm–1
˜
Raman
ν / cm–1
[ ]: in [D6]DMSO
( ): in CH3CN
˜
[ ]: in [D6]DMSO
1a
1b
1c
1d
NMR
Raman
NMR
Raman
NMR
Raman
NMR
Raman
NMR
NMR
NMR
NMR
NMR
NMR
39.0
[6.8], (4.8)
not detected
[6.9]
not detected
[6.4]
not detected
[6.6], (4.2)
[1703]
(5.6), {5.7}
(5.5), {5.6}
(5.5), {5.6}
(5.4), {5.5}
(5.6), {5.7}
not detected
ϾϾ 1
ϾϾ 1
ϾϾ 1
ϾϾ 1
ϾϾ 1
ϾϾ 1
ϾϾ 1
ϾϾ 1
not detected
not detected
not detected
not detected
not detected
not detected
[14.2], {15.2}
[1594], (1599)
[10.8], {11.0}
(1608)
[14.1], {14.7}
(1613)
[16.3], (16.1)
[1603], (1620)
not detected
not detected
not detected
not detected
not detected
not detected
41
50
46
29
ϾϾ 1
ϾϾ 1
ϾϾ 1
ϾϾ 1
ϾϾ 1
2.2
Ͼ 1
2a
2b
2c
2d
2e
3b
ϽϽ 1
ϽϽ 1
ϽϽ 1
ϽϽ 1
ϽϽ 1
ϽϽ 1
45
60
52
35
41
70
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J. C. Sloop et al.
FULL PAPER
Figure 5. Enol-F FMO interaction.
Table 5. 1,3-Diketone solution component mixtures for 1a and 1d.
Method
Component % of solution
diketo
enol A
enol hydrate
NMR (δ / ppm)
H1: 4.8
H2: 6.8,
H3: 14.2
1594
H4: 6.8, H5: 7.1,
H6: 3.4 (var)
Ͼ 3000
1a
Raman (ν / cm–1)
not observed
˜
Time / h
Conc. / mol/L
0.20
0.0016
0.20
0.0016
0.20
0.0016
0
600
1750
2.4
0.0
0.0
2.3
0.0
0.0
94.0
59.0
0.0
94.2
58.0
0.0
3.6
41.0
100
3.5
42.0
100
NMR (δ / ppm)
H1: 4.2
H2: 6.6,
H3: 16.3
1603
H4, H5: not observed
1d
H6: 3.3
Raman (ν / cm–1)
1703
0.20
Ͼ 3000
˜
Time / h
Conc. / mol/L
0.0016
0.20
0.0016
0.20
0.0016
0
600
1750
21.0
23.2
23.0
21.2
23.3
23.0
75.9
47.8
47.9
75.8
48.1
48.3
3.1
29.0
29.1
3.0
28.6
28.7
5.4–5.7 ppm in both the weakly polar CDCl3 and strongly
polar [D6]DMSO. We rationalize this on the basis of fron- over three orders of magnitude shows no appreciable effect
tier molecular orbital theory as depicted in Figure 5. on the ketoǞenol equilibrium for either β-diketone. For 1a,
The net destabilizing effect displayed in Figure 5 arises the measured ketoǞenol equilibrium constant (Kketo
Based on the results in Table 5, varying the concentration
)
enol
h
from the two-orbital, four-electron interaction of the enol ≈ 39 is transitory in nature due to the continued growth of
HOMO (pentadienyl equivalent) and the filled fluorine sp3 the enol hydrate species. For 1d, however, the determination
hybrid orbital. In addition, the enol LUMO has a node at of Kketo enol ≈ 2.2 is likely a true constant given the absence
h
the carbon center adjacent to the fluorine, which provides of change in the component ratios over the final 1150 hours
no stabilization for the enol form.
of the investigation.
Finally, formation of hydration products caused by the
slow encroachment of water into the solution was noted.
As the data from Table 4 show, the extent of hydration for
trifluoromethyl 1,3-diketones 1a–c and 2a–c was generally
found to be higher than that of the non-trifluoromethylated
Conclusions
Our efforts reveal new facts concerning the keto–enol
1,3-diketones. A more detailed study of the trifluoromethyl and enol–enol behavior exhibited by electron deficient aro-
1,3-diketone rate of hydration is ongoing and will be ad- matic β-diketones in the solid state, as neat liquids and in
dressed separately.
polar, aprotic solvents. In contrast to previous work,[10,11]
To investigate the influence of diketone concentration on our X-ray crystallographic results as well as solid state and
the solution state equilibria, solutions of compounds 1a and neat liquid spectroscopic measurements support the pres-
1d (0.0016–0.20 m in [D6]DMSO) were analyzed by 1H ence of a single chelated cis-enol isomer that is conjugated
NMR and Raman spectroscopy at specified time intervals with the aryl ring for the β-diketones in this study which
over a 1750 h period. Table 5 shows representative results are not fluorinated in the 2-position. Whereas solid state,
of this study at two different concentrations, 0.20 m and neat liquid and solvent studies reveal that 2-fluoro-1-aryl β-
0.0016 m. Raman spectra were collected to qualitatively diketones exist in the diketonic form. In the polar, aprotic
confirm the presence of tautomeric forms and the forma- solvent [D6]DMSO, trifluoromethylated aryl β-diketones
tion of hydrated species. Solution mixture component ratios equilibrate from the cis-chelated enol A form and a small
were determined by noting 1H NMR chemical shifts for the proportion of the diketone to a hydrated species, while 2-
species of interest, δ(H1–H6), and integrating the signals to fluoro-1-aryl β-diketones exhibit the diketonic form but
obtain relative amounts of each component for each con- likewise undergo complete hydration if the solutions are al-
centration.
lowed to stand. Non-fluorinated β-diketones equilibrate
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Eur. J. Org. Chem. 2011, 936–941

Keto–Enol Equilibria in the Solid State
rapidly from the chelated cis-enol A form to a larger pro-
portion of the diketo form and, in general, form enol hy-
drates in much lower proportion than fluorinated β-di-
ketones.
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Experimental Section
CCDC-797427 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
[9] R. M. Silverstein, G. C. Bassler, T. C. Morrill, Spectrometric
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Supporting Information (see footnote on the first page of this arti-
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Acknowledgments
[11] A. J. Vila, C. M. Lagier, A. C. Olivieri, J. Phys. Chem. 2009,
95, 5069.
The authors would like to thank the Georgia Gwinnett College
(GGC) and USMA Faculty Research Funds for providing financial
support for this work, USMA Photonics Research Center for Ra-
man support and the NCSU X-ray facility for crystallographic sup-
port. The views expressed in this academic research paper are those
of the authors and do not necessarily reflect the official policy or
position of the U. S. Government, the Department of Defense, or
any of its agencies.
[12] J. C. Sloop, J. L. Jackson, R. D. Schmidt, Heteroat. Chem.
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Received: October 25, 2010
Published Online: December 27, 2010
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