Keto-enol and enol-enol tautomerism in trifluoromethyl-β-diketones

Article abstract of DOI:10.1016/j.jfluchem.2006.02.012

The keto-enol (K?E) and enol-enol (E?E) equilibria of a variety of trifluoromethyl-β-diketones were investigated using ¹H, ¹³C, ¹⁹F NMR spectroscopy, infrared spectroscopy and ultraviolet-visible spectrophotometry in nonpolar solvents. In general, NMR, IR and UV spectral evidence indicates that trifluoromethyl-β-diketones exist as mixtures of two chelated cis-enol forms in nonpolar media. Infrared spectroscopy and ultraviolet spectrophotometry show the E?E equilibrium lies in the direction of the enol form which maximizes conjugation in most cases. Exceptions are noted and discussed.

Full text of DOI:10.1016/j.jfluchem.2006.02.012

Journal of Fluorine Chemistry 127 (2006) 780–786
www.elsevier.com/locate/fluor
Keto–enol and enol–enol tautomerism in
trifluoromethyl-b-diketones
a,
Joseph C. Sloop , Carl L. Bumgardner , Gary Washington ,
b
a
*
a
W. David Loehle , Sabapathy S. Sankar , Adam B. Lewis
b
a
a
Department of Chemistry and Life Science, United States Military Academy, 646 Swift Road, West Point, NY 10996, USA
b
Department of Chemistry, North Carolina State University, P.O. Box 8204, Raleigh, NC 27695, USA
Received 6 January 2006; received in revised form 15 February 2006; accepted 16 February 2006
Available online 17 April 2006
Abstract
1
13
19
The keto–enol (KÐE) and enol–enol (EÐE) equilibria of a variety of trifluoromethyl-b-diketones were investigated using H, C, F NMR
spectroscopy, infrared spectroscopy and ultraviolet-visible spectrophotometry in nonpolar solvents. In general, NMR, IR and UV spectral evidence
indicates that trifluoromethyl-b-diketones exist as mixtures of two chelated cis-enol forms in nonpolar media. Infrared spectroscopy and ultraviolet
spectrophotometry show the EÐE equilibrium lies in the direction of the enol form which maximizes conjugation in most cases. Exceptions are
noted and discussed.
#
2006 Elsevier B.V. All rights reserved.
19
Keywords: b-Diketone; Tautomerism; Enol; F NMR; Chelated-OH; Enone
1
. Introduction
the enols which can hydrogen bond intramolecularly are the
more stable [3].
In earlier work we reported the regioselective synthesis of
fluorine and trifluoromethyl containing b-diketones and their
conversion to various heterocycles [1]. While monitoring these
Analyzing the system diagrammed in Fig. 2 is especially
challenging since isomers a and b may be interconverted by
intramolecular movement of a hydrogen atom from one oxygen
to another over a small distance. In several special cases the
equilibrium aÐb has been studied using NMR, IR and UV data
[4] (see Table 1).
1
9
cyclizations with F NMR, we noted, as expected, the
predominance of enolic forms of the b-diketones. Notable
exceptions were the 2-fluoro derivatives and b-diketones with
large alkyl groups substituted at C-2. These compounds shown
In the present paper these spectral techniques are applied to a
wider variety of trifluoromethyl-b-diketones, including alipha-
tic, aromatic and cyclic examples. The results, K
in Fig. 1 exhibit only the keto form in CDCl [2].
3
Since fluorine in the 2-position of b-diketones has such a
dramatic effect on the keto–enol equilibrium (K ), we
ketoÐtotal enol
(K ) and K
kÐe
(K ), determined by each of the
aÐb
kÐe
enol aÐenol b
decided to explore the effect of fluorine substitution in other
positions. In this paper, we describe the preparation and
examination of a number of b-diketones where a –CF group is
three spectroscopic methods are compared and trends and
differences discussed.
3
a terminus. In these cases the equilibrium potentially involves
the diketone and two chelated, hydrogen-bonding enols, a and
b, Fig. 2.
2. Results
2.1. b-Diketone synthesis
While non-chelated enols may be considered, these are
observed in rare instances. In non-hydrogen bonding solvents,
Table 2 shows trifluoromethyl-b-diketones selected for the
keto–enol and enol–enol equilibrium study. These include
commercially available diketones (1a–d, 1g, 1h and 1u) and
those prepared by literature methods [1,2,10]. For comparison
purposes, two nonfluorinated diketones, 1w and 1x, were
included in the investigation.
*
Corresponding author. Tel.: +1 845 938 3904.
E-mail address: joseph.sloop@usma.edu (J.C. Sloop).
0022-1139/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2006.02.012

J.C. Sloop et al. / Journal of Fluorine Chemistry 127 (2006) 780–786
781
Fig. 2. Keto and chelated enol forms.
Fig. 1. b-Diketones, which exhibit the keto form.
infrared data in Table 1 [6,7b,11]. Our results are reported in
Table 4.
2
2
.2. Spectroscopic studies
.2.1. NMR spectroscopy
In Table 3 are presented H, C and F NMR data.
2.2.3. Ultraviolet–vis spectrophotometry
1
13
19
As indicated in Table 1, UV–vis spectrophotometry in
some cases may also provide resolution of the enol forms
[6b,7]. Consequently, a wider variety of trifluoromethyl-b-
diketones was examined by UV–vis spectrophotometry. The
data for our series of trifluoromethyl-b-diketones are
presented in Table 5. The lmax values are consistent with
carbonyl compounds that have extended conjugation. Where
available, lmax values for both the keto and enol tautomers are
presented. In several cases, resolution of the enols was
observed.
Tautomeric assignments are based on work previously reported
in the literature [5].
2
.2.2. Infrared spectroscopy
Infrared spectroscopy was used to detect the various enol
and keto species of selected trifluoromethyl-b-diketones
present and to determine the direction of enolization. Band
assignments in the IR spectra of b-diketones that clearly
distinguish the keto and enol forms a and b are based on the
Table 1
Spectral Information for trifluoromethyl-b-diketones
Spectral Method
Units
R
a
1
1
1
1
1
NMR
dppm
Alkyl/Aryl
H: H
2
ꢀ 3.90,
H: H
3
ꢀ 15–18,
F: ꢀ ꢁ73– ꢁ78,
C: C
H: H
3
ꢀ 12–15,
F: ꢀ ꢁ73– ꢁ78,
C: C
9
19
13
1
19
13
1
F: ꢀ ꢁ80– ꢁ84,
3
C: C
b
> 200
b
ꢀ 170,
b
ꢀ 160,
J (Hz)
JCa-F ꢀ 285
JCa-F ꢀ 270
2
2
JCb-F ꢀ 34–36
JCb-F ꢀ 32–36
g
ꢁ1
b
c
c
IR
(cm
)
Alkyl
1700–1790
1687–1689
1700–1780
1700–1780
1580–1640
1595–1620
1590–1620
1550–1640
1650–1700
1650–1678
1650–1680
1655–1700
b
b
b
c
c
f
d
e
f
Aryl
Heteroaromatic
Cyclic
h,i
j
UV
(nm)
Alkyl
180–225
270–>300
>300
>300
240–270
270–300
240–>300
230–290
Aryl
225–265
230–290
Heteroaromatic
Cyclic
j
190–260
>300
a
Ref. [5].
b
Unconjugated C O.
c
Chelated C O.
d
Chelated benzoyl.
e
Chelated aroyl.
f
Ref. [6b].
g
Refs. [6,7].
h
Unless noted, bands listed are p ! p* transitions.
i
j
Refs [8,9].
n ! s* transition.

782
J.C. Sloop et al. / Journal of Fluorine Chemistry 127 (2006) 780–786
Table 2
Table 3
b-diketones
Diketone NMR spectral data
1
a,g
13
a
19 d
F
Entry Compd
H
C
b
c
(Hz) CDCl
Enol-OH C
1
C
b
(Hz)
C
a
3
Enol
Keto
0
R
1
2
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
1q
1r
14.0
14.5
14.0
14.7
12.4
13.8
12.7
15.0
14.4
194 176 (36) 117 (280) ꢁ77.4
199 173 (35) 117 (281) ꢁ77.4
201 174.5 (35) 116 (281) ꢁ77.1
202 175 (34) 117 (271) ꢁ77.1
201 162 (36) 118 (276) ꢁ77.1
198 175 (35) 117 (281) ꢁ77.5
Compound
R
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
s
CH
3
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
C
2
H
5
4
i-C
3
H
7
5
t-Bu
i-Bu
6
h
7
176 (36) 116 (281) ꢁ77.1
n-C
6
H
13
8
186 177 (34) 117 (280) ꢁ76.9
204 172 (35) 117 (278) ꢁ76.4
199 159 (35) 121 (280) ꢁ76.6
206 163 (34) 117 (280) ꢁ76.4
200 159 (39) 115 (286) ꢁ76.3
182 171 (35) 117 (277) ꢁ76.1
196 175 (35) 118 (271) ꢁ76.2
208 179 (32) 121 (279) ꢁ76.9
201 173 (34) 120 (276) ꢁ76.8
186 177 (35) 118 (279) ꢁ76.7
201 158 (33) 115 (279) ꢁ76.5
(97)
CF
Ph
3
9
10
11
12
13
14
15
16
17
18
9.00
2-FPh
10.9
14.3
2-CH
2-CH
3
Ph
3
OPh
9.50
4-FPh
11.0
14.7
14.6
4-CH
4-CH
3
Ph
3
OPh
4-NCPh
4-O NPh
8.50
e
2
11.9
ꢁ81.0 (3)
2-Naphthyl
2-Pyridyl
2-Pyrrolyl
2-Furyl
19
20
21
22
23
1s
1t
1u
1v
1w
1x
10.1
14.3
14.5
15.5
15.5
16.1
205 159 (34) 115 (279) ꢁ76.5
177 172 (34) 117 (278) ꢁ76.8
183 171 (34) 117 (276) ꢁ76.1
181 174 (36) 118 (280) ꢁ73.6
t
u
v
2-Thienyl
i
i
i
i
f
f
192
185
2.1 (81) 2.3 (19)
f
2.2 (90) 2.3 (10)
f
2
4
a
b
c
d
e
f
Referenced to CDCl
2
3
, see Fig. 2 for definitions.
CF
3
J(Cb-F)
.
1
1
1
w
x
CH
Ph
3
CH
CH
3
J(Ca-F)
.
3
Referenced to CFCl3.
19
Based on integration of F data.
Based on CH
3
group integration.
g
1
13
H and C NMR spectra obtained at 0.02M and 0.002M showed no changes
in chemical shifts or integration values.
h
Not observed.
i
Not applicable.
Table 4
Trifluoromethyl-b-diketone IR bands
a
Entry
Compound
Bands
ꢁ1
ꢁ1
ꢁ1
Keto (cm ) C
1790,w
O
Enol Chelated -OH
Enol a (cm ) Chelated C
O
Enol b (cm ) Chelated C
O
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1g
1h
1i
2570–2800
2400–2750
2400–2700
2400–2750
2400–2700
2570–2800
2700–2900
2700–2900
2700–2900
2550–2790
2550–2790
2350–2850
2650–2880
2600–2870
2400–2700
2400–2800
1610,vs
1603,vs
1596,s
1650,s
1650,s
1647,m
1650,s
1599,vs
1630,s; 1690,m
1680,w
1602,s
1601,s
1685,m
1670,s
1664,s
1649,w
1658,w
1j
1586,m
1586,m
1618,m
1592,m
1610,vs
1618,vs
1600,m
1k
1o
1p
1q
1t
1u
1w
1x
1
1
1
1
1
1
1
0
1
2
3
4
5
6
1650,s
1640,m
1710,m
1620,vs
1610,m
a
4
Spectra obtained as 0.02M solutions in CCl .

J.C. Sloop et al. / Journal of Fluorine Chemistry 127 (2006) 780–786
783
Table 5
Diketone lmax values
Entry
Compound
l
max (nm)
K
kÐe
K
aÐb
e
Keto
Enol a
Enol b
b
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
1q
1r
1s
284 ; e = 77,800
c
c
205 (7) ; e = 808
282 (93) ; e = 10,135
282 (93) ; e = 4,926
13.3
13.3
9.00
c
c
203 (7) ; e = 380
c
c
199 (10) ; e = 2,293
282 (90) ; e = 20,887
c
310 ; e = 1,950
c
c
217 (13) ; e = 5,293
200 (11) ; e = 1,460
282 (87) ; e = 35,429
6.69
8.09
c
c
274 (89) ; e = 12,202
d
336 ; e = 10,870
c
c
336 (56) ; e = 3,536
305 (40) ; e = 2,644
299 (44) ; e = 2,794
0.79
c
c
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
288 (60) ; e = 3,561
1.50
3.54
c
c
344 (22) ; e = 7,958
300 (78) ; e = 28,167
c
318 ; e = 11,497
c
313 ; e = 4,420
c
352 ; e = 2,630
c
c
323 (53) ; e = 9,285
310 (47) ; e = 8,350
0.90
0.99
c
c
328(50.3) ; e = 3,785
317(49.7) ; e = 3,745
c
338 ; e = 26,691
c
c
330 (44) ; e = 1,340
327 (25) ; e = 497
320 (72) ; e = 2,014
328 (66) ; e = 11,977
314 (56) ; e = 1,680
1.27
3.00
0.39
0.52
1.03
b
b
278 (75) ; e = 1,577
b
b
1t
272 (28) ; e = 769
d
d
1u
1v
1w
1x
278 (34) ; e = 814
c,f
265 (19) ; e = 287
c
c
a
296 (40) ; e = 605
286 (41) ; e = 620
4.26
6.69
11.5
b
b
200 (13) ; e = 202
272 (87) ; e = 1,351
c,f
c
242 (8) ; e = 1,132
306 (92) ; e = 13,018
a
b
c
d
e
f
K
aÐb normalized for the keto form.
Cyclohexane.
Hexane.
Benzene.
Keto form, n ! s*.
Keto form, p ! p*.
2
2
.3. Estimation of K
and K
kÐe
aÐb
1
3
Earlier work on compounds 1a, 1d, 1h and 1v used C data
and model compounds to calculate the EÐE equilibrium
.3.1. K
kÐe
1
9
NMR: The absence of a F signal in the ꢁ80 to ꢁ84 ppm
range (see Table 1) and the absence of methylene H’s with
the exception of Table 3, entry 18 indicates that in CDCl3
constant, K
their results in Table 6.
= [b]/[a] [5,13]. Our findings are compared with
aÐb
The similar trends in C , C , and C for the aliphatic cases
1
b
a
only enol forms are detectable and that K
ꢂ 1 in all
are also consistent with a preference for b [5,12–16]. For
comparison, these equilibrium constants and those described
below are collected in Table 7.
kÐe
cases.
IR: Using assignments from Table 1 and data from Table 4,
the absence of keto forms may be inferred which indicates
that in CCl K
ꢂ 1 in the cases studied.
IR: As shown in Table 5, the presence of both enols a and b
can be detected in several cases. Based on the relative
intensity of bands associated with the enol tautomers, a
4
kÐe
UV: Comparison of the correlations in Table 1 with data in
Table 5 indicates that in non-polar solvents K
ꢂ 1. The
kÐe
absence or low intensity of absorptions in the 195–265 nm
range shows that little or no diketone form is present.
Table 6
13
19
Comparison of C and F NMR data for selected trifluoromethyl-b-diketones
1
9
2
.3.2. K
NMR: The H NMR enolic proton chemical shifts observed
Entry Compound C1
Cb
Ca
F
K
Reference
aÐb
aÐb
1
1
2
3
4
1a
1d
1h
1v
194.0 176.0 117 ꢁ77.4 1.35
This work
[6,13]
by previous workers suggest a correlation between the
dominant enol form in solution and the enolic OH proton
chemical shift [6,10b-c,12]. The aliphatic enolic OH protons
in our study range in chemical shift from 12.4 to 14.7 ppm,
indicating that form b is the major enol isomer in solution
1
94.0 175.7 118 ꢁ77.5 1.38
203.4 175.4 117 ꢁ77.1 1.33
This work
[13]
2
03.3 175.7 117 – 1.32
185.6 177.4 117 ꢁ76.9 1.12
85.6 179.3 118 ꢁ77.0 1.02
This work
[13]
1
[
6]. The data for the aromatic compounds are more scattered,
181.4 174.3 118 ꢁ73.6 1.00
This work
[5]
ranging from 8.5 to 15.5 ppm, rendering any correlation
difficult to assess.
1
82.0 175.0 118 ꢁ72.1 1.00

784
J.C. Sloop et al. / Journal of Fluorine Chemistry 127 (2006) 780–786
Table 7
a
K
aÐb
b
c
d
UV
Compound
NMR
IR
1
1
1
1
1
1
1
1
1
1
1
a
d
h
i
1.35
1.33
1.12
<1
<1
<1
<1
>1
>1
<1
<1
<1
<1
0.79
1.50
3.54
0.90
0.99
0.39
0.52
1.03
j
k
o
p
t
u
v
Fig. 3. Dihedral angles in enol form a of 1j and 1k.
e
f
preference for enol a shown by 1h-i, 1o-q and 1t-u may be due
in part to conjugation of the aromatic ring with the C C of enol
a. Although form a is preferred by these diketones, form b is
favored by 1j, 1k, 1r and 1s.
1.00
a
Entries include only cases where data from at least two methods are
available.
b
3
CDCl .
c
d
e
f
This reversal noted for 1j and 1k may be due to the bulky
ortho-methyl and methoxy groups, respectively, which prevent
coplanarity of the aromatic ring with the double bond of enol a.
Optimized enol geometries obtained by quantum mechanical
computations predict substantial dihedral angles between the
aromatic ring and the enol double bond in 1j and 1k [19]. These
are depicted in Fig. 3.
4
CCl .
Hexane.
Cyclohexane.
Benzene.
qualitative measure of the b/a ratio may be obtained [17].
The results are displayed in Table 7. In addition, the absence
ꢁ
1
of free O-H stretches in the 3200–3500 cm range indicate
that non-chelated enols are not present in solution.
The preference of enol b for diketone 1r, while initially
puzzling, may be part of a larger trend. Two other cases, 1o and
1p, which also contain aromatic rings with electron with-
drawing groups, were found to shift the EÐE equilibrium
UV: In several cases absorptions due to enols a and b could
be discerned. Analysis of these resolved bands gave a
measure of the b/a ratio and the resulting K
summarized in Table 7.
values are
toward enol b, with K
= 0.90 and 0.99, respectively. It
aÐb
aÐb
appears that the electron withdrawing properties of the pyridine
ring may have a similar and stronger effect. Comparison of the
contributing resonance structures 2 and 3 shown in Fig. 4 may
provide a clue to the apparent anomaly where b is favored over
a in an aromatic compound.
3
. Discussion
3
.1. K
kÐe
In 2 a positive charge is developed adjacent to a trifluoroacyl
moiety, whereas in 3 the positive charge is next to a carbonyl
conjugated with a b-hydroxy group. The latter (structure 3)
would be expected to be more stable and contribute to a larger
extent to the enol.
In all cases involving nonpolar solvents, the three analytical
methods gave K
ꢂ 1 as expected. Molecular orbital
kÐe
calculations support the spectroscopic results in that the enol
forms were found to be generally 2–3 kcal/mol lower in energy
than the diketone form [3]. A further finding appears to be that
the trifluoromethyl group enhances the enolic preference over
non-fluorinated b-diketones (cf. 1a with 1w and 1h with 1x).
A rationale for the preference of 1s for enol b is shown in
Fig. 5. With enol b, the availability of dual intramolecular
hydrogen bonding between the pyrrole hydrogen and the
carbonyl as well as the enolic hydrogen adjacent to the
trifluoromethyl group would provide for additional stabilization
not expected in enol a. This is supported by ab initio calculations,
which predict an interatomic bond distance between the pyrrole
3
.2. K
aÐb
In the case of 1a, 1d and 1h, IR favors enol a whereas NMR
results favor enol b. In one case, 1v, the NMR and UV K
aÐb
values are in good agreement. All of the methods rely
ultimately on model compounds [7,8,11] but the NMR method
involves time averaging of signals and therefore the necessity of
using weighting factors [14,18]. Even though the NMR
indicates a preference for b, the preference decreases when
an aromatic group is introduced (cf. 1a with 1h). This trend is
consistent with the importance of conjugation.
The direct correlations possible with IR and UV suggest that
these methods are more reliable for determination of K
.
aÐb
They give results, which agree in all cases, as shown by 1i-k,
o-p and 1t-u. In general, aromatic compounds prefer to
enolize toward the aromatic ring giving rise to enol a. The
1
Fig. 4. Resonance contributors to compound 1r.

J.C. Sloop et al. / Journal of Fluorine Chemistry 127 (2006) 780–786
785
5. Experimental
5.1. Chemicals
All chemicals were obtained from the Aldrich Chemical
Company, Eastman Kodak, or Fisher Chemical Company. All
solvents (spectrophotometric grade) and starting materials were
checked for purity by mass spectrometry prior to use.
Fig. 5. Dual hydrogen bonding capability of enol b in 1s.
hydrogenandcarbonylofenolbof2.436,sufficientlyclosefor an
intramolecular hydrogen bonding interaction. In addition, the
standard equilibrium constant, K , was also determined by ab
5.2. Instrumentation
Melting points were obtained on a Mel-Temp melting
point apparatus and are uncorrected. NMR data were
collected using a Varian VXR-200 spectrometer with a
aÐb
initio methods and found to be equal to 2.0[20].
Notwithstanding the exceptions noted above, the importance
of conjugation in the aromatic series, 1h–1t, may have a
counterpart in the aliphatic series where alkyl hyperconjugation
with the C C and distal carbonyl of enol a provides some
stabilization not available to form b. Calculations are being
carried out on several cases and will be published separately.
1
broad band probe operating at 200.0 MHz for H, 188.2 MHz
1
9
for F and 50.3 MHz for C, and/or a Br u¨ ker Avance 300
13
1
spectrometer operating at 300.0 MHz for H and 75.4 MHz
1
3
for C. IR data were collected on a Nicolet 5 DXB FT-IR
spectrometer and/or an HP FT-IR spectrometer with resolu-
ꢁ
1
tions of 1 cm . UV–vis spectrophotometric data were
collected by a Hewlett-Packard 8452 A Diode Array spectro-
photometer and/or an Agilent 8453 spectrophotometer with
resolutions of 1 nm.
4
. Conclusions
For the trifluoromethyl-b-diketone cases studied, K
ꢂ 1
kÐe
in nonpolar solvents [16]. This is in contrast to trifluoromethyl-
b-diketones with a fluorine atom on C-2, Fig. 1, X=F. The
results of our work suggest that IR and UV are more reliable
5.3. General procedure for the preparation of
trifluoromethyl-b-diketones [1,10]
than NMR in determining the EÐE equilibrium constant K
.
aÐb
In both IR and UV some resolution of enols a and b is possible
To 50 mL diethyl ether in a round bottom flask equipped
with a magnetic stirrer is added 60 mmol of sodium
methoxide slowly. Then, 1eq (60 mmol) of trifluoromethyl
ethyl acetate is added dropwise slowly while stirring. After
5 min, 1eq (60 mmol) of the ketone is added dropwise and
stirred overnight at room temperature under a calcium
chloride drying tube. The resulting solution is evaporated to
dryness under reduced pressure and the solid residue
dissolved in 30 mL 1 M sulfuric acid. This solution is
19
in certain instances. No such resolution was noted in F NMR
1
measurements down to 223 K; nor was it observed in H and
1
3
C NMR experiments. In general, we found K
< 1 in most
aÐb
cases, indicating a preference for enol a. See Fig. 6.
This preference may be rationalized by considering the
importance of extended conjugation with aromatic systems
when available. In the same manner, hyperconjugative
stabilization may be partially responsible for the observed
preference of enol form a in alkyl-substituted trifluoromethyl-
b-diketones.
extracted with ether, and the organic layer dried over MgSO .
4
The solvent is evaporated under reduced pressure, the crude
diketone purified by distillation or recrystallization,
We can now identify three circumstances, however, where
the preference for enol a can diminish and form b may even
become dominant. The first of these is when R represents an
aromatic ring possessing electron withdrawing properties,
1
13
19
and purity confirmed by H, C, and F NMR spectroscopy,
mass spectrometry and literature melting or boiling
points.
with K
approaching or, as in the case of 1r, surpassing unity.
aÐb
In addition, enol b may predominate if an ortho substituent on
the aromatic ring prevents coplanarity of the ring and enol, thus
limiting conjugative stabilization. Finally, the EÐE equili-
brium may shift in favor of enol b when intramolecular
hydrogen bonding opportunities of the carbonyl with adjacent
heteroatoms become significant.
5.4. Trifluoromethyl-b-diketone NMR studies
Solutions of each 1,3-diketone (0.20 M) were volumetrically
prepared using anhydrous CDCl . All NMR studies were
3
1
conducted at ꢀ298 K. For the typical H NMR experiment, 4
1
3
19
scans were gathered, for C NMR, 1024 scans, and F NMR, 4
scans.
5.5. Trifluoromethyl-b-diketone infrared studies
Solutions of each 1,3-diketone (0.020 M) were prepared
using anhydrous CCl . All IR studies were conducted at
4
ꢀ
298 K. For the typical experiment, 16 scans from 400 to
ꢁ
1
4000 cm were conducted.
Fig. 6. Enol–enol tautomers.

786
J.C. Sloop et al. / Journal of Fluorine Chemistry 127 (2006) 780–786
[
[
[
7] J.U. Lowe Jr., L.N. Ferguson, J. Org. Chem. 30 (1965) 3000.
5
.6. Trifluoromethyl-b-diketone UV–vis studies
8] E.M. Larsen, et al. J. Am. Chem. Soc. 75 (1953) 5107.
9] (a) R.M. Silverstein, G.C. Bassler, T.C. Morrill, Spectrometric Identifica-
tion of Organic Compounds, fifth edition, John Wiley & Sons, New York,
Solutions of each 1,3-diketone (ꢀ0.0020 M) were volume-
trically prepared using anhydrous benzene, cyclohexane, and
n-hexane. All UV–vis studies were conducted at ꢀ298 K. For
the typical experiment, a single scan from 180 to 800 nm was
conducted.
1
991, p. 302;
(b) USMA, Center for Molecular Science, 2005. Energies obtained from
TDDFT/aug-cc-pvdz calculations indicate that keto n ! s* and enol
p ! p* transitions for aliphatic trifluoromethyl-b-diketones occur at
ꢀ200 and ꢀ300 nm respectively.
[
10] (a) J.C. Reid, M. Calvin, J. Am. Chem. Soc. 72 (1950) 2948;
Acknowledgements
(
(
(
(
(
(
(
b) S. Forsen, et al. Acta. Chem. Scand. 18 (1964) 1208;
c) S. Hunig, H. Hoch, Justus Liebigs Ann. Chem. 716 (1968) 68;
d) J.C. Sloop, M.S. thesis, North Carolina State University, 1990.
e) M.A. Oligaruso, Chem. Rev. (1965) 261;
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. The authors would like to thank
the USMA Faculty Research Fund for providing financial
support for this work, the USMA Center for Molecular Science
for computational support, and the NCSU NMR Facility for
low temperature NMR studies.
f) R. Ho, S. Livingstone, Aus. J. Chem. 21 (1968) 1781;
g) R. Charles, E.J. Reidel, Inorg. Nuc. Chem. 29 (1967) 715;
h) S. Dilli, K.J. Robards, Chromatogr. 312 (1984) 109;
(i) I. Katsuyama, et al. Synthesis 11 (1997) 1321;
(
(
j) T.J. Larsen, J. Am. Chem. Soc. 73 (1951) 500;
k) J. Park, et al. J. Am. Chem. Soc. 75 (1953) 4753.
[
[
11] R.S. Rasmussen, D.D. Tunnicliff, R.R. Brattain, J. Am. Chem. Soc. 71
1949) 1069.
12] K.I. Pashkevich, et al. Dokl. Akad. Nauk SSSR 255 (1980) 1140.
(
References
[13] For equations to determine KaÐb, see C.F.G.C. Geraldes, M.T. Barros,
C.D. Maycock, M.I. Silva. J. Mol. Struct. 238 (1990) 335.
[
[
[
14] K.I. Lazaar, S.H. Bauer, J. Phys. Chem. 87 (1983) 2411.
[
1] J.C. Sloop, C.L. Bumgardner, W.D. Loehle, J. Fluorine Chem. 118 (2002)
15] G.A. Olah, C.U. Pittman, J. Am. Chem. Soc. 88 (1966) 3310.
16] Christian Reichardt, Solvents and Solvent Effects in Organic Chemistry,
third ed., Wiley-VCH Verlag GmbH & Co., KGaA, Weinheim, vol. 107,
135.
[
[
2] J.C. Sloop, C.L. Bumgardner, J. Fluorine Chem. 56 (1992) 141.
3] (a) USMA, Center for Molecular Science, 2005. Data based on MP2/cc-
pvdz energies and B3LYP/6-31G(d) optimization with frequency calcula-
tions followed by solvent calculations. Ab initio calculations show that the
s-cis chelated enol forms are usually thermodynamically 2–3 kcal/mol
lower in energy than the diketo form. Additionally, depending on the
molecule, a substantial difference in the energy of the enol forms is also
indicated. Non-chelated enols forms have energies higher than the che-
lated enol forms. A detailed computational study of this and other b-
diketone equilibria will be published separately.
2
003, 329–362, 375–388.
[
17] Quantitative IR measurements, while possible, are not practical in these
cases since vibrational frequencies observed in the IR regions ca. 1500–
ꢁ
1
1
6
700 cm
for the chelated enol tautomers frequently are within
ꢁ1
0 cm . Therefore, it is difficult to rule out possible vibrational con-
tributions made by one enol isomer to absorbances ascribed to the other
enol isomer.
19
[
[
[
18] F NMR experiments conducted at 223 K gave no indication that enol
forms a and b could be resolved.
(
(
b) M. Moriyasu, A. Kato, Y. Hashimoto, J. Chem. Soc. Perkin Trans. 2
1986) 515.
19] Optimized geometries obtained from MOPAC quantum mechanical cal-
culations.
[4] The literature concerning the study of trifluoromethyl-b-diketone
tautomerization reveals wide variations in sample preparation with
20] Optimized geometries for the enol forms of 1s obtained from NWChem
quantum mechanical calculations using the 6-31G* basis set and DFT/
B3LPY method. See Apr a` , E.; Windus, T.L.; Straatsma, T.P.; Bylaska,
E.J.; de Jong, W.; Hirata, S.; Valiev, M.; Hackler, M.; Pollack, L.;
Kowalski, K.; Harrison, R.; Dupuis, M.; Smith, D.M.A; Nieplocha, J.;
Tipparaju V.; Krishnan, M.; Auer, A.A.; Brown, E.; Cisneros, G.; Fann, G.;
Fr u¨ chtl, H.; Garza, J.; Hirao, K.; Kendall, R.; Nichols, J.; Tsemekhman,
K.; Wolinski, K.; Anchell, J.; Bernholdt, D.; Borowski, P.; Clark, T.; Clerc,
D.; Dachsel, H.; Deegan, M.; Dyall, K.; Elwood, D.; Glendening, E.;
Gutowski, M.; Hess, A.; Jaffe, J.; Johnson, B.; Ju, J.; Kobayashi, R.;
Kutteh, R.; Lin, Z.; Littlefield, R.; Long, X.; Meng, B.; Nakajima, T.; Niu,
S.; Rosing, M.; Sandrone, G.; Stave, M.; Taylor, H.; Thomas, G.; van
Lenthe, J.; Wong, A.; Zhang, Z.; NWChem, A Computational Chemistry
Package for Parallel Computers, Version 4.6 (2004). Pacific Northwest
National Laboratory, Richland, Washington 99352-0999, USA. High
Performance Computational Chemistry: an Overview of NWChem a
Distributed Parallel Application, Kendall, R.A.; Apr a` , E.; Bernholdt,
D.E.; Bylaska, E.J.; Dupuis, M.; Fann, G.I.; Harrison, R.J.; Ju, J.; Nichols,
J.A.; Nieplocha, J.; Straatsma, T.P.; Windus, T.L.; Wong, A.T. Computer
Phys. Comm., 2000. 128, 260–283.
ꢁ5
concentrations ranging from neat liquids to 10 M. Refs. [5–8,16]
Any comparative study of KÐE or EÐE equilibria must account for
these differences since equilibria may be influenced by solution ionic
strength, solute–solute and solute–solvent interactions. To mitigate the
difficulties that could arise from sample concentration variation, we
have limited the range of solution concentrations in this study from
0
.2 M for NMR, 0.02 M for IR and 0.0020 M for UV studies as these
concentrations strike a balance between instrumentation sensitivity and
species solubility. This concentration range was determined acceptable
for our study by NMR experiments conducted 0.20, 0.020 and
1
13
19
0
3
.0020 M CDCl . Under these conditions, H, C, and F NMR
for compounds 1a and 1h showed no change in chemical shifts or
integrated peak values.
[
[
5] S.R. Salman, R.D. Farrant, J.C. Lindon, Magn. Reson. Chem. 28(1991)645.
6] (a) K.A. Ebraheem, Monatshefte fur Chemie 122 (1991) 157;
(
(
b) K.A. Ebraheem, S.T. Hamdi, M.N. Khalaf, Can. J. Spectrosc. 26
1981) 225. Herein is described IR and UV data for b-diketones such as 2-
trifluoroacetylcyclopentanone, where tautomer a is defined as the exo-
cyclic enol and form b is termed the endocyclic enol.