EPR studies of anion-radicals formed by reduction of perfluorinated α-triketones with some metals of groups I-III

Article abstract of DOI:10.1016/S0022-1139(99)00258-4

Reaction of α,β-bis-fluorosulfatoketones i-C₃F₇(CFOSO₂F)₂C(O)R_F, (R_F=C₂F₅, i-C₃F₇) with AcONa/AcOH afforded perfluoro-α-triketones i-C₃

Full text of DOI:10.1016/S0022-1139(99)00258-4

Journal of Fluorine Chemistry 103 (2000) 85±91
EPR studies of anion±radicals formed by reduction of per¯uorinated
a-triketones with some metals of groups I±III
E.N. Shaposhnikova*, S.R. Sterlin, S.P. Solodovnikov,
N.N. Bubnov, B.L. Tumanskii
Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov St., 117813 Moscow, Russia
Received 21 July 1999; accepted 20 October 1999
Abstract
Reaction of a,b-bis-¯uorosulfatoketones i-C3F7(CFOSO2F)2C(O)RF, (RFˆC2F5, i-C3F7) with AcONa/AcOH afforded per¯uoro-a-
triketones i-C3F7C(O)C(O)C(O)RF, (RFˆC2F5, i-C3F7). Reduction of these triketones with metals (Li, Na, K, Mg, Cd, Zn, Tl) in THF was
studied by EPR spectroscopy. Anion±radicals with unsymmetrical distribution of charge and spin densities are formed in one-electron
reduction of the triketones. Unlike hydrocarbon a-polyketones, per¯uorinated triketones can undergo three-electron reduction to form
paramagnetic allyl tisalkoxides. # 2000 Elsevier Science S.A. All rights reserved.
Keywords: EPR; Anion±radical; Reduction; Per¯uoroalkyl a-triketones; Radical pairs
1. Introduction
reduction potential was shifted to the anode region as the
value of n increased [5]), formation of other paramagnetic
species as a result of further reduction of I was not observed.
It was shown by CVA method that reduction of I is restricted
by transfer of only two electrons, evidently, to form a
diamagnetic bis-enolate.
Obviously, reduction potentials of a-polyketones are
determined not only by the number of carbonyl groups,
but also by acceptor properties of substituents R and R0. In
this connection we carried out EPR studies of reduction of
¯uoroaliphatic a-triones i-C3F7C(O)C(O)C(O)RF, (RFˆ
C2F5 (II), i-C3F7 (III)). We expected that II and III would
be readily reduced owing to the electron-withdrawing effect
of per¯uoroalkyl groups affording to the paramagnetic
products of not only one-electron transfer, but those of
three-electron reduction.
It is well known that ketones R(CO)nR (nˆ1,2) readily
undergo one-electron reduction to form corresponding
anion±radicals [1,2,3]. If we consider a-polyketones
R(CO)nR (nˆ3±5) as higher homologues of this series we
would expect that such compounds would be able to undergo
reactions of multihelectron reduction to give paramagnetic
particles of several types in the course of the reaction as a
result of transfer of various numbers of electrons.
Until now formation and recording of paramagnetic par-
ticles in the process of reduction of a-polyketones R(CO)nR
(nꢀ3) has been reported only for one example, namely, H.
Rock and co-authors showed that dimesityl-a-tetraone (I)
gave anion±radicals (AR) by the action of alkaline metals
and barium, the structure ketosemitriones was assigned to
these AR [4]1. The authors [4] emphasized that although
one-electron reduction of I was relatively easy (in the series
of ketones and polyketones R(CO)nR0 (nˆ1±5) the ®rst
2. Synthesis of bis-fluoroalkyl-a-triones
Until now only one representative of ¯uoroaliphatic a-
triones has been reported, per¯uoroethylisopropyl-a-trione
II, obtained in 36% yield by reacting 4,5-bis(¯uorosulfo-
nyloxy)per¯uoro-6-methylheptane-3-one (IV) with MeOH
[6]. Further studies showed that the relatively poor yield of
a-trione II is explained by the formation of by-products,
isomeric dimethylketals (VIa,b), that is well described by a
mechanism involving intermediate epoxides (Va,b):
* Corresponding author. Fax: ‡7-951-355-085.
1 According to our data, in the course of reduction of diphenyl-a-
triketone with amalgams of metals of I±III groups, only single-changed
anion±radicals gave EPR spectra; this confirmed the conclusion made
in [4] that formation of paramagnetic particles, which are products of
three electron reduction of polyfluoroketones of te hydrocarbon series
is impossible.
0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 2 5 8 - 4

86
E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91
proposed scheme is con®rmed, to a certain extent, by an
increase in the relative content of dimethylketals in the
products of the reaction of bis-¯uorosulfatoketone IV with
MeOH in the presence of CsF. It is obvious that the
undesirable intramolecular substitution of the FSO3 group
could be suppressed by the action of the weaker nucleophile
in a more acidic medium to increase the yield of the target
a-trione.
Actually, trione II was obtained in 70% yield by the
reaction with AcONa/AcOH.
Bis-(hepta¯uoroisopropyl)-a-trione III was obtained
similarly from VII synthesized by electrochemical ¯uoro-
sulfation of F-2,6-dimethyl-3-heptene-5-one. However,
unlike its less branched analogue, the preparation of
trione III was accompanied by the formation of a multi-
component mixture of by-products including per¯uoroiso-
butyric acid (VIII) and 2-hydroper¯uoropropane (IX); the
appearance of these products in the reaction mixture could
be explained by a haloform type reaction of bis-¯uorosulfate
VII (but not trione III, which transformed into tris-acetox-
ysemiketal under the reaction conditions):
The ability of a-¯uorosulfonyloxyketones to undergo
This suggestion was con®rmed to a certain extent by
increasing the yield of trione III when the reaction was
carried out under conditions of incomplete conversion of
product VII (VII:AcONaˆ1:0.9).
cyclization under the action of nucleophiles to give
oxiranes as a result of intramolecular substitution of
FSO3 group has been previously reported [7]. The

E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91
87
3. EPR studies of reduction of a-triones II and III
In the process of reduction of trione III by phototransfer
of an electron from KI, the EPR spectrum of an anion±
radical (AR) was recorded characterized by the small values
of hyper®ne interaction (h®) constants of an unpaired
electron with two equivalent ¯uorine nuclei of the per¯uor-
oisopropyl group [aF (2F)ˆ0.7 G] (Fig. 1). When the trione
was reduced with sodium amalgam, interaction of the
unpaired electron with ¯uorine nuclei was not pronounced.
We assume that in both the cases, the structure in which
density of the unpaired electron is localised on the central
carbonyl group made a major contribution to the structure of
the anion±radicals formed. This type of distribution of
charge and density of an unpaired electron was con®rmed
by quantum chemical calculations conducted by the MNDO
method with UHF approximation (calculations did not take
into account the effect of a counterion on the distribution of
charge (q) and spin densities (r)) (Table 1).
Fig. 1. ESR spectrum of product of one-electron reduction of triketone III
by phototransfer of an electron from KI in THF.
The fact that the h® constants in the AR are more than
50% lower than those in the spin-adduct of trione III with
the silicon-centered radical i-C3F7C(O)Cꢁ[OSi(CH3)3]-
C(O)C3F7-i (ab-F(2F)ˆ1.7G, ag-F(12F)ˆ0.35G) [8] indi-
cates that the spin distribution is mainly determined by
mesomer structure A.
a triplet whose central component coincided with signal a
(aFˆ10.25 G). The structure in which unpaired electron
density is mainly localised on the carbonyl group, adjacent
to the per¯uoroethyl substituent was suggested for such a
particle.
As we can see in Fig. 2, three types of particles were
recorded in the process of reduction of triketone II with
potassium iodide. The anion±radical in which unpaired
electron density is mainly localised on the central carbonyl
group, corresponded to the predominant signal a. In addition
to a, signal b was recorded, characterised by interaction of
an unpaired electron with one ¯uorine atom (aFˆ5.5 G); this
may be explained by reduction of the carbonyl group,
adjacent to the isopropyl substituent. The third signal was
4. Reduction of a-triketones with metals of groups
II-III
Reduction of triketones II and III with magnesium, zinc,
cadmium or thallium occurs in stages. Thus, at the ®rst stage
of reduction of III (which lasted for 1 min), the EPR
spectrum of the product of one-electron reduction was
recorded (for the case of Zn, see Fig. 3), which contained
two magnetically non-equivalent per¯uoroisopropyl groups
Table 1
MNDO/UHF calculation data for the anion±radical-product of one-electron reduction of trione III
C(1)
C(2)
C(3)
O(6)
O(5)
O(4)
Linkage order
q
0.26
0.055
0.2
0.23
0.34
0.2
0.4
0.32
0.15
r
0.002
0.011
0.44
C(1)-O(4)
1.77
C(2)-O(5)
1.46
C(3)-O(6)
1.83

88
E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91
Table 2
Hfi constants (G) of anion±radical products of one-electron reduction of
triketone III
Reducing ab-F(F)a
ab-F(F)b
ag-F(2CF3)a ag-F(2CF3)b aM
agent
Mg/Hg
Zn/Hg
Cd/Hg
Tl/Hg
5.0
5.1
5.0
5.2
0.25
1.35
1.35
1.65
1.5
0.25
9.6
40.25
Table 3
Hfi constants (G) of anion±radicals derived from (CF3)2CFCOCOCF3
Fig. 2. ESR spectrum of product of one-electron reduction of triketone II
by phototransfer of an electron from KI in THF.
Reducing
agent
ab-F (CF3)
ab-F (1F)
Ag-F(2CF3)
aM
Mg/Hg
Zn/Hg
Cd/Hg
TlCp
11.5
10.5
11.25
12
1.75
1.25
1.5
1.75
3
3.1
3.5
and interaction of the unpaired electron with magnetic
isotopes of cations appeared (Table 2).
1.75
52.25
It follows from these data that species were formed in
which the density of the unpaired electron and its charge are
distributed in the main between two adjacent carbonyl
groups (their h® parameters are close to those for anion±
radicals derived from F-methyl-isopropyl-a-dione (Table 3)
[9]).
Thus, ketosemidione structure (X) can be assigned to the
particles formed at the ®rst step of reduction of III by metals
of II and III groups.
Fig. 4. ESR spectra of products of one-electron reduction of triketone II
with magnesium amalgam: a signals are assigned to AR of XIa type; b
signals are assigned to AR of XIb type.
Where nˆ2, MˆZn, Mg, Cd and nˆ1, MˆIn, Tl.
When reduction of II was carried out with metals of the
second and the third groups, paramagnetic particles of two
types were recorded (Figs. 4 and 5). The particles of the ®rst
type were characterized by a high value of the h® constant
with b-¯uorine nuclei of the ethyl group (Table 4). On the
contrary, interaction of the unpaired electron with the
¯uorines of the isopropyl group was recorded in the particles
Fig. 5. ESR spectrum of product of one-electron reduction of triketone II
by phototransfer of an electron from CdI2 in THF: a signals are assigned
to AR of XIa type; b signals are assigned to AR of XIb type.
Fig. 3. ESR spectrum of product of one-electron reduction of triketone III
with zinc amalgam in THF.

E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91
89
Table 4
Hfi constants for anion±radicals of type XIa
Reducing agent
ab-F(CF2)
ag-F(CF3)
aM
Mg/Hg
Zn/Hg
CdI2
11.25
10.25
10.25
11.0
1.0
1.0
0.85
7.2
55
Tl/Hg
of the second type (Table 5). Therefore, ketosemidione
structures (XIa,b) can be assigned to these particles.
Fig. 6. ESR spectrum of product of three-electron reduction of triketone
III with zinc amalgam in THF.
We studied the spectra of frozen solutions of anion±
radicals derived from a-triketones at various stages of
reduction. The EPR spectra of adducts of one- and three-
charged AR derived from III (Fig. 8) contained lines typical
of dipole±dipole interactions of unpaired electrons. This
allowed identi®cation of the formed radical pairs, to mea-
sure parameters of dipole±dipole interaction of unpaired
electrons D, and to determine an average distance between
the radical centres (Table 8). Because the value of parameter
E, which characterises deviation of spin density distribution
from axial symmetry for all biradical complexes, is equal to
zero, it can be concluded that the chelate centre of these
complexes is a tetrahedron with oxygen atoms in the tetra-
hedron apexes, and there is a cation in the centre of the
tetrahedron (cf. [12]).
Where nˆ2, MˆZn, Mg, Cd and nˆ1, MˆTl.
On a more prolonged reduction of II and III (>5 min), the
EPR spectra of products of one-electron reduction disap-
peared. Instead of them, other spectra appeared (Figs. 6 and
7) characterized by larger h® constants with b-¯uorine
atoms (Tables 6 and 7) compared to the products of one-
electron interaction. These data indicate that the density of
an unpaired electron is higher on carbons C(1) and C(3) and,
thus, the unpaired electron is delocalized in an allyl system,
therefore, the structure of triply-charged anion±radicals
(XIIa,b) can be proposed for the formed AR:
We failed to record the spectra of radical pairs for
derivatives II at 77 K, probably, because of too low a
concentration of AR.
Thus, per¯uorinated a-triketones ± contrary to their
non-¯uorinated analogs-form paramagnetic species under
the action of metals of groups I±III as products of not
only one-electron reduction but also of three-electron reduc-
tions. The formation of AR of the latter type undoubtedly
re¯ects the electron-withdrawing effect of ¯uoroalkyl sub-
stituents.
XIIa: RFˆCF(CF3)2, XIIb: RFˆCF2CF3
The assumption that these AR (XIIa,b) have allyl struc-
tures is also con®rmed by comparison of h® constants with
the b-¯uorines in XIIa with those in alkyl radicals con-
taining similar substituents (cf. (CF3)2CFCꢁFCF(CF3)2
ab-Fˆ14.8 G [10], (CF3)2CF-CꢁF-CF2CF3 ab-Fˆ29.9 G
[11]). The observed values of h® constants are smaller,
because in the allyl system, the density of an unpaired
electron on the terminal carbon atoms is 2/3.
Table 5
Hfi constants for anion±radicals of type XIb
Reducing agent
ab-F(F)
ag-F(2CF3)
Mg/Hg
Zn/Hg
Cd/Hg
4.0
4.0
±
1.0
1.0
1.0
Fig. 7. ESR spectrum of product of three-electron reduction of triketone
II with cadmium amalgam in THF.

90
E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91
Table 6
5. Experimental
Hfi constants for anion±radicals, products of three-electron reduction of
III
19F NMR spectra were recorded on a Bruker-WP200 SV
instrument (188.3 MHz, CF3COOH as the external stan-
dard). The mass spectra were obtained on a VGMS-70E
spectrometer, the energy of ionization was 70 eV. EPR
spectra were recorded on a `Varian E-12A' radiospectrom-
eter in degassed quartz tubes. Irradiation was carried out
using focussed light of a Dash lamp (DRSh-1000). An
electron Unipan regulator was used for thermostatic control
of samples. Reduction of di- and triketones was carried out
either with Li, Na, K, Zn, Mg, Cd, In, and Tl mercury
amalgams or by phototransfer of an electron with MIn,
MˆNa, K, Cd (nˆ1,2) using a procedure reported in [3].
THF was puri®ed by standard procedures [13].
Reducing
agent
ab-F(2F)a
ab-F(F)b
ag-F(2CF3)a
ag-F(2CF3)b
Mg/Hg
Zn/Hg
Cd/Hg
6.45
6.8
6.45
6.8
1.2
1.2
1.2
1.2
6.9
6.9
1.25
1.25
Table 7
Hfi constants for anion±radicals, products of three-electron reduction of II
Reducing agent
ab-F(CF2)
ab-F(F)
ag-F(2CF3)
Mg/Hg
Zn/Hg
Cd/Hg
19.8
20.0
19.75
3.75
3.75
4.5
1.25
1.0
6. Synthesis of perfluoroethylisopropyl-a-triketone (II)
1.0
(a) Ketone IV (15.6 g, 27 mmol) was gradually added to a
mixture of dry CsF (5 g, 33 mmol) and MeOH (25 ml); after
the exothermic reaction was complete, the resulting mixture
was stirred for 15 min. The reaction mixture was poured into
cold 30% H2SO4, the organic layer was separated, distilled
over H2SO4 to collect a fraction with b.p. 105±1658C.
Subsequent distillation over H2SO4 gave triketone II
(1.1 g, 11%, b.p. 99±1018C) (GC-identi®cation by compar-
ison with the authentic sample [6]) and a mixture of
compounds with b.p.130±1408C which were identi®ed as
dimethylketals VIa and VIb (2:1) (GC-MS).
‡
MS, m/z (Irel, %): VIa: 387 [M-CH3O] (1), 359 [M-
‡
‡
‡
‡
CH3OCO] (6), 299 [M-C2F5] (2), 259 [C6H3F8O2] (2),
‡
‡
193 [C5H6F5O2] (100), 181 [C4F7] (8), 169 [C3F7] (7),
‡
‡
‡
147 [C2F5CO] (8), 131 [C3F5] (30), 119 [C2F5] (30), 97
‡
‡
‡
[C2F3O] (10), 81 [C2F4] (13), 69 [CF3] (35), 59
‡
‡
‡
[CH3OCO] (19), 45 [CO2H] (8), 43 [C2H3O] (5), 29
‡
[CHO] , (4).
VIb: 399 [M-F] (2.0), 387 [M-MeO] (18), 354
‡
‡
‡
‡
[M-CH3O-CO] (3), 271 [M-C2F5CO] (88), 252 [M-F-
‡
‡
C2F5CO] (4), 243 [M-2F-C2F5CO] (17), 221 [M-
‡
‡
‡
C3F7CO] (100), 209 [C5F7CO] , 197 [C3F7CO] (30),
‡
‡
‡
193 [C5F7] 18, 181 [C4F7] 13, 169 [C3F7] (12),
‡
‡
‡
147 [C2F5CO] (12), 131 [C3F5] (20), 119 [C2F5]
‡
Fig. 8. ESR spectrum of solutions of anion-radicals derived from
‡
‡
(67), 100 [C2F4] (11), 81 [C2F4] (15), 69 [CF3] (65),
‡
triketone III at 77 K.
‡
‡
59 [CH3OCO] (71), 43 [C2H3O] (20), 29 [CHO] ,
(10).
(b) Sodium acetate (AcONaÁ3H2O) (7.7 g, 72 mmol) was
gradually added to a solution of compound IV (15 g,
26 mmol) in AcOH (25 ml). After the exothermic reaction
was complete, the resulting mixture was stirred for 15 min
and poured into 30% H2SO4. The lower layer was separated
and twice distilled over conc. H2SO4 to give triketone II
(6.6 g, 70%), which was identi®ed by comparison with an
authentic sample (GLC) [6]. Triketone II is an orange
liquid, b.p. 98±1008C. UV (l, nm): 289 (eˆ487), 447
(eˆ60).
Table 8
Radical pairs parameters (D/E), the average distance between radical
centres (L) in symmetrical biradical complexes III with metals of II group,
ionic radii of elements (r)
Metal (reducing agent)
D/E
L, A
r, A
Mg
Zn
Cd
375/0
340/0
255/0
4.20
4.36
4.77
0.65
0.74
0.97

E.N. Shaposhnikova et al. / Journal of Fluorine Chemistry 103 (2000) 85±91
91
7. Synthesis of perfluoro-4,5-bisfluorosulfonyloxy-2,6-
dimethylheptane-3-one VII (using a procedure reproted
in [14])
fraction with b.p. 108±1118C (7.0 g), that contained III
(contaminated with AcOH). Further distillation afforded
starting bisketo¯uorosulfate VII (0.9 g) (b.p. 608C/3 mm
Hg). Subsequent distillation of the ®rst fraction over P2O5
gave 4.5 g of triketone III (52% on the basis of the reacted
VII). Triketone III is an orange-red liquid, b.p. 108±1098C.
UV (l, nm): 281 (eˆ500), 453 (eˆ186). Found: C 25.15, F
62.97%, C9F14O3. Calculated C 25.54, F 63.03%. MS, m/z
Fluorosulfonic acid (100 ml) containing NaSO3F (8 g)
was placed into a 150 ml one-compartment cell (anode-
glassy carbon, cathode Ni). Per¯uoroethyl-3-methylbutenyl
ketone [15] (70 g, 163 mmol) was added gradually into the
cell for 9 h electrolysis under galvanostatic conditions
(Iˆ1.8 A) at 28±308C. After the reaction was complete
(16.2 A±h of electricity were passed), the mixture was
poured into crushed ice, washed with water and aq.Na2CO3.
The organic layer was dried over MgSO4 and distilled to
give product VII (67.7 g, 66%) as a mixture of isomers, b.p.
54±648C/3 mm Hg. Found C 17.4; F 55.05; S 10.11%.
C9F18O7S2. Calculated: C 17.25; F 54.6; S 10.11%.
19F NMR : 4 (6F1), 103(1F2), 52 (1F3‡1F4), 113(1F5),
6(6F6), 127(1F7), 129 (1F8).
‡
‡
(Irel, %): 422 [M] (2.13), 394 [M-CO] (4.26), 225 [i-C3F7
‡
‡
‡
(CO)2] (2.13), 197 [C3F7CO] (100.0), 169 [C3F7]
‡
‡
(72.34), 150 [C3F6] (8.51), 119 [C2F5] (10.64), 100
‡
‡
‡
[C2F4] (21.28), 69 [CF3] (100.0), 31 [CF] (10.64).
19F NMR: 19F: 3,5 (12F), 117 (2F).
Acknowledgements
The work has been supported by the Russian Basic
Research Foundation (grant N98-03-32933a).
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‡
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8. Synthesis of perfluorodiisopropyl a-triketone III
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reaction it was stirred until gas liberation stopped (ca.
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sulfuric acid; the organic layer was separated and twice
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(12.6 g). It contained triketone III (50%) and a mixture of
by-products (GC), among which per¯uoroisobutyric acid
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identi®ed by GC by comparison with authentic samples
[16,17].
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Subsequent recti®cation gave a fraction (6.5 g, 35%),
b.p. 107±1108C that contained compound III and
(CF3)2CFCOOH (30%) (19F NMR and GC-MS).
(b) The reaction of compound VII ( 16 g, 25 mmol) with
AcONaÁ3H2O (6.2 g, 45 mmol) in AcOH (25 ml) was car-
ried out similarly. Treatment with 30% sulfuric acid resulted
in a two-phase organic layer. The starting bisketo¯uorosul-
fate VII (2.3 g) was separated as the lower layer. The upper
layer was twice distilled over conc. sulfuric acid to give a
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