Fluoro spin adducts and their modes of formation

Article abstract of DOI:10.1039/a607509f

The reactions of two fluorinating reagents, XeF₂ and N-fluorodibenzenesulfonamide [(PhSO₂)₂N-F], with several spin traps have been investigated. In dichloromethane, the strong oxidant XeF₂ cleanly gives fluoro spin adducts with N-tert-butyl-α-phenylnitrone (PBN) or 5,5-dimethyl-1-pyrroline 1-oxide (DMPO) according to a mechanism mediated by the radical cation of the spin trap. In both cases, further fluorination takes place with replacement of the a hydrogen by fluorine. The much weaker oxidant (PhSO₂)₂N-F reacts with PBN or DMPO in dichloromethane giving both the fluoro adduct and an adduct formally derived from an N-centred radical, assigned the structure of PhSO₂N(F)-PBN^? or (PhSO₂)₂N-DMPO^?, respectively. This type of reaction proceeds by a version of the Forrester-Hepburn mechanism, in which an acid HA, in this case HF, initially adds to the nitrone function to give a hydroxylamine derivative which is oxidized by (PhSO₂)N-F giving the fluoro spin adduct, a proton and the highly labile radical anion (PhSO₂)₂N-F^?-. By decomposition of the latter to PhSO₂(F)N^- and PhSO₂^?, conditions are set up for propagation of the reaction by a new molecule of HA [now PhSO₂(F)NH] and thus formation of the PhSO₂(F)N spin adduct.

Full text of DOI:10.1039/a607509f

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Fluoro spin adducts and their modes of formation
Lennart Eberson* and Ola Persson
Department of Chemistry, Lund University, PO Box 124, S-221 00 Lund, Sweden
The reactions of two fluorinating reagents, XeF 2 and N-fluorodibenzenesulfonamide [(PhSO2)2N –F ], with
several spin traps have been investigated. In dichloromethane, the strong oxidant XeF 2 cleanly gives fluoro
spin adducts with N-tert-butyl-α-phenylnitrone (PBN ) or 5,5-dimethyl-1-pyrroline 1-oxide (D M PO)
according to a mechanism mediated by the radical cation of the spin trap. In both cases, further
fluorination takes place with replacement of the α hydrogen by fluorine.
The much weaker oxidant (PhSO2)2N –F reacts with PBN or D M PO in dichloromethane giving both the
fluoro adduct and an adduct formally derived from an N-centred radical, assigned the structure of
ؒ
ؒ
PhSO2N (F )–PBN or (PhSO2)2N –D M PO , respectively. This type of reaction proceeds by a version of the
Forrester–H epburn mechanism, in which an acid H A, in this case H F, initially adds to the nitrone function
to give a hydroxylamine derivative which is oxidized by (PhSO2)N –F giving the fluoro spin adduct, a
proton and the highly labile radical anion (PhSO2)2N –F Ϫ. By decomposition of the latter to PhSO2(F )N Ϫ
᝽
ؒ
and PhSO2 , conditions are set up for propagation of the reaction by a new molecule of H A [now
PhSO2(F )N H ] and thus formation of the PhSO2(F )N spin adduct.
Me Me
NF
Introduction
The formation of fluoro adducts of spin traps, like N-tert-butyl-
(4-BrC6H4)3N
(PhSO2)2NF
α-phenylnitrone† (PBN, 1) or 5,5-dimethyl-1-pyrroline 1-oxide
(DMPO, 2) has been achieved by treatment with fluorinating
agents such as AgF2 or F3NO.1,2 In the case of PBN, treatment
with F3NO in benzene gave the fluoro adduct 3 [reaction (1)],
whereas AgF2 gave the α,α-difluorobenzyl tert-butyl aminoxyl
radical (4) by further rapid substitution of the α-hydrogen of 3
by fluorine [reaction (2)]. Similarly, DMPO gave the corre-
sponding difluoro compound 5 by treatment with AgF2 [reac-
tion (3)]. These reactions were suggested to proceed via the
formation of fluorine atoms.
SO2
6
8
9
ϩ
Bu4NF
6
ϩ
᝽
1
1
3
᝽
-eϪ, -Hϩ,-eϪ
3
4
ϩFϪ
Scheme 1
A third mechanism already suggested by Forrester and
Hepburn7 in 1971 was utilized to produce 3 under weakly oxidiz-
ing conditions, namely by air oxidation of PBN in the presence
of tetrabutylammonium fluoride in chloroform.8 Fluoride ion
adds to the nitrone function and the resulting hydroxylamine 7
is highly redox reactive, E1/2 for oxidation at a mercury electrode
F
O
O•
F3NO
(1)
PhCH
N
PhCH
N
But
But
1
3
O•
being Ϫ(0.2–0.3) V.9 Thus oxygen with EЊ(O2/O2 Ϫ) at ca. Ϫ0.4
AgF2
᝽
(2)
(3)
PhCF2
N
1
3
V can act as an oxidant to give 3 [reaction (4)] Under these
conditions 4 was not formed, as expected.
not seen
But
4
O2
1 ϩ FϪ ϩ Hϩ
PhCH(F)–N(OH)But
3
AgF2
(4)
Me
Me
Me
Me
F
F
7
N
O
2
N
In view of the high energy required to produce fluorine atoms
from fluoride-ion containing species, a process which according
to dogma cannot take place in homogenous medium by any
known oxidizing agent, we deem it unlikely that any fluoro spin
adduct can be formed by proper trapping of the fluorine atom.
In what follows, two fluorinating agents of widely differing
redox reactivity, XeF2 and N-fluorodibenzenesulfonamide (8),
are used to probe this problem. XeF2 is a strong oxidant capable
of forming radical cations of aromatic compounds with
Eo(ArH ϩ/ArH) ≤ 2.0 V,10 whereas Eo(8/8 Ϫ) should be much
O
5
Later,3 a mixture of 3 and 4 was generated by the oxidation
of PBN to its radical cation by the strong oxidant, tris(4-
bromophenyl)aminium ion (6 ϩ) in the presence of tetrabutyl-
᝽
ammonium fluoride, in agreement with the redox properties of
the reagents involved.4 EЊ(6 ϩ/6) is 1.06 V (vs. SCE, all poten-
᝽
tials given here will be referred to this electrode), meaning that
ϩ
6
can undergo a fast electron transfer reaction with PBN
᝽
᝽
᝽
[Eo(PBN ϩ/PBN) = 1.46 V]5 to give PBN ϩ, which then reacts
᝽
᝽
lower, in the range found for other N-fluoro reagents like the N-
with fluoride ion [EЊ(F /F ) = 2.9 V] to give 3. E (3 ϩ/3)
Ϫ
o
ؒ
᝽
fluorosultam 9 with Eo(9/9 Ϫ) of Ϫ0.14 V.11 The peak potential
᝽
should be of the order of 0.7 V, as judged from a comparison
Epc for cathodic reduction of 8 in acetonitrile was found to be
with known redox potentials of aminoxyl radicals;6 thus fur-
Ϫ0.54 V.12
ther oxidation in an electrochemical–chemical–electrochemical
(ECE) type reaction with 6 and some base present should
ϩ
᝽
Results and discussion
eventually give 4 (Scheme 1).
Formation of fluoro spin adducts by XeF2 oxidation of spin traps
in dichloromethane
Treatment of PBN with a deficit of XeF2 (ca. 50% of the
† The IUPAC name for PBN is N-(benzylidene)-tert-butylamine N-
oxide.
J. Chem. Soc., Perkin Trans. 2, 1997
893

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Table 1 Fluoro adducts obtained from the reaction between spin traps and XeF2 in dichloromethane or HFP
Spin trap
Solventα
DCM
Ratio [spin trap]/[XeF2]
αN /mT
αH /mT
αF /mT
Assignment
b,c
ؒ
1
2
1.26
1.27
1.98
1.24
1.38
1.99
1.27
1.27
None
1.22
1.18
1.12
0.117
—
4.58
2.11 (2 F)
—
4.78
4.03
—
4.48
2.08 (2 F)
F–PBN
4d,e
ϩf,g
1
10
10
HFP
DCM
HFP
10
2
10
2.74
0.107
0.133
2.76
0.127
—
14
᝽
ؒ
ؒ
F–10
F–10
g,h
ؒ
p-NO2–14
b
ؒ
12
DCM
20
10
2
20
2
F–12
p-MeO–4i
12
11
2
HFP
DCM
DCM
ؒ
0.288
0.616
5.60
5.46
F–11
ؒ
Also 0.139 (1 H), 0.200 mT (2 H): F–DMPO
(see Fig. 2)
2
13
13
DCM
DCM
HFP
20
10
—
2.18 (2 F)
0.196 (2 H): 5j
None
None
ϩ k
13
᝽
a
DCM = dichloromethane, HFP = 1,1,1,3,3,3-hexafluoropropan-2-ol. b Ca. 3 min after mixing. c Lit.2 in benzene: 1.22, 0.118 and 4.56 mT. d Ca. 15
min after mixing. e Lit.2 in benzene: 1.26, 2.14 (2 F) mT. f Lit.5 in HFP: 2.00 and 2.80 mT. The small couplings to the 4,4-methyl groups were not
g
detectable due to the low intensity of the spectrum. Lit.5 in HFP: 1.95 and 2.73 mT. i Ca. 2 h after mixing. j Lit.1 in benzene: 1.08, 2.16 (2 F) and
h
0.175 (2 H) mT. k Identical to spectrum published in ref. 5.
required amount, see Scheme 2) in dichloromethane produced
ؒ
an intense EPR spectrum of F–PBN (3) together with a weak
signal of the difluoro derivative 4. Hyperfine coupling constants
(hfs) are given in Table 1. Upon standing, 3 was rapidly con-
verted into 4, the transformation being essentially complete
after 15 min. Thus it appears that the formation of 3 is a
relatively slow reaction leaving enough XeF2 intact to allow for
further oxidative fluorination of 3. This behaviour would be
expected for a radical cation mediated mechanism, as outlined
in Scheme 2.
ϩ
ؒ
2 PBN ϩ XeF2
F–PBN
2 PBN ϩ Xe ϩ 2FϪ
2 F–PBN
᝽
ϪeϪ
ϪHϩ, ϪeϪ, ϩFϪ
F–PBNϩ
4
ؒ
Scheme 2
The 4-nitro derivative of PBN, 4-NO2PBN (10), upon treat-
ment with a deficit of XeF2 (ca. 50% of the required amount)
in dichloromethane gave the fluoro spin adduct, the signal of
which kept growing over 2 h and then slowly decayed. This spin
ؒ
Fig. 1 (a) EPR spectrum of F–DMPO generated by the reaction
between DMPO (0.10 mol dmϪ3) and XeF2 (5 mmol dmϪ3) in dichloro-
methane. (b) The simulated spectrum used the hfs constants given for
trap is more difficult to oxidize than PBN, Epa(10 ϩ/10) = 1.7 V,5
ؒ
᝽
F–DMPO of Table 1.
and apparently its fluoro spin adduct is stable against further
oxidation by XeF2. A still more oxidation resistant spin trap,
pentafluoro-PBN (11) [Epa (11 ϩ/11) = 1.9 V], exhibited similar
upon treatment with XeF2 in dichloromethane gave no para-
magnetic species.
᝽
behaviour to 4-NO2PBN.
In order to obtain the fluoro spin adduct from a more easily
DMPO 2 upon treatment with XeF2 (ratio [DMPO]:[XeF2] =
20:1) in dichloromethane gave its fluoro spin adduct (EPR
spectrum, see Fig. 1). This aminoxyl radical has not been char-
acterized before. Under the conditions employed it decomposed
with k = 0.08 minϪ1. Use of a lower concentration ratio (2:1)
gave only the known 1 difluoro derivative of DMPO [see reaction
(3)].
oxidized PBN derivative, 4-MeOPBN (12) [Eo(12 ϩ/12) = 1.16
᝽
V],5 it was found necessary to decrease the likelihood of further
fluorination by using a large excess of spin trap over XeF2
(ratio > 20:1). The signal from the fluoro adduct increased over
30 min and then decayed; after 2 h practically only the difluoro
derivative of 4-MeOPBN was present. The still more redox
reactive nitrone, 4-Me2NPBN (13) [Eo(13 ϩ/13) = 0.64 V],5
The results described above are best interpreted in terms of
the radical cation mechanism of Scheme 2. A way of further
testing this proposal is to conduct the reactions in 1,1,1,3,3,3-
hexafluoropropan-2-ol (HFP), a solvent in which nucleophilic
reactivity is enormously attenuated due to strong hydrogen
᝽
F
O2N
F
F
F
H3CO
O
O
O
bonding to negatively charged species.13 Thus the chloride ion is
CH
N
CH
N
CH N
ϩ
᝽
about 108 times less reactive toward 6
in HFP than in
But
But
But
F
acetonitrile and it is expected that the fluoride ion should be
subject to a similar decrease in reactivity.
10
11
12
Me2N
XeF2 oxidation of spin traps in HFP
O
Reactions run as above, but in HFP instead of dichlorometh-
ane, proceeded according to the radical cation mechanism. We
have previously5 shown that the radical cations of PBN and its
CH
N
But
13
894
J. Chem. Soc., Perkin Trans. 2, 1997

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4-substituted derivatives display different chemical behaviour
when generated in HFP :PBN, PBNs which are more difficult to
oxidize (4-NO2, 4-F) and 4-MePBN are transformed into isoxa-
zolidines by a de-tert-butylation–cycloaddition sequence [reac-
tions (5) and (6)]. Under the oxidizing conditions the particular
isoxazolidine formed (14) is transformed into its radical cation
O•
PhCH N+
But
O•
Me3C+
+
+
PhCH
N
(5)
1•+
Me2C CH2
H+
Ph
(or 14•+
)
(6)
Me
Me
1 (or 1•+
)
+
Me2C CH2
N
But
14
ؒ
Fig. 2 Time dependence of F–PBN (3) (᭞) and PBNOx (18) (᭢),
when formed from a solution of PBN (0.10 mol dmϪ3) and 8 (0.09 mol
dmϪ3) in dichloromethane under thermal conditions. The curves repre-
sent the best fits to a sigmoidal curve and a double exponential curve,
respectively (Tablecurve curve fitting program).
ϩ
14 which has a characteristic EPR spectrum with αN = 2.0
᝽
and αH = 2.8 mT and small couplings of ca. 0.5 mT to the two
ring methyl groups. It may also be that 14 ϩ is formed directly
᝽
from 1 ϩ and isobutene.14
᝽
On the other hand, the radical cations of PBNs with
electron-donating groups in the ring [4-MeO, 3,4-(OCH2O), 4-
Me2N] were stable enough to allow recording of their EPR
spectra in HFP.5
shallow maximum after ca. 1.5 h (Fig. 2). Benzoyl-N-tert-
butylnitrone (18, often denoted PBNOx) was formed as the final
paramagnetic product, its concentration following a sigmoidal
curve. This situation, a spontaneous thermal reaction between a
spin trap and a weak electron acceptor to give a spin adduct is
common but has previously been little understood. We recently
investigated a similar reaction between PBN and trichloro-
acetonitrile in hexane19 and concluded that the thermal reac-
tion isinitiated byaddition of an acid HA to thenitronefunction,
followed by oxidation of the hydroxylamine by trichloro-
acetonitrile, giving dichlorocyanomethyl radical and HCl. The
former adds to PBN and the latter can initiate a new cycle of
hydroxylamine formation, oxidation, etc. [eqns. (7)–(9)].
Accordingly, upon oxidation of PBN with XeF2 in HFP, a
weak EPR signal of the isoxazolidine radical cation (14 ϩ) was
᝽
observed (Table 1); no signs of fluoro adducts 3 or 4 were
detected. Thus the nucleophilicity of the fluoride ion is strongly
decreased in HFP, allowing for the sequence of reactions (5) and
(6) to compete with capture of PBN ϩ by fluoride ion.
᝽
4NO2PBN (10), corresponding to a more reactive cation,
gave both the fluoro spin adduct and the isoxazolidine radical
cation upon oxidation by XeF2 in HFP, whereas 4-MeOPBN
(12) gave no paramagnetic species. 4-Me2NPBN (13) gave its
persistent radical cation.
PBN ϩ HA
PBN(H)A
(7)
Formation of fluoro and N-centred spin adducts by treatment of
spin traps with N-fluorodibenzenesulfonamide (8) in
dichloromethane
PBN(H)A ϩ Cl3CCN
ϩ
Ϫ
ؒ
ؒ
A–PBN ϩ H ϩ Cl ϩ Cl C᎐C᎐N
(8)
(9)
᎐ ᎐
2
Compound 8 is a weak oxidant used for the fluorination of
nucleophilic species like carbanions via a mechanism involving
nucleophilic attack upon the fluorine atom.15 We tested its one-
electron oxidizing ability by allowing it to react with tris(4-
ؒ
ؒ
Cl C᎐C᎐N ϩ PBN
᎐ ᎐
Cl C᎐C᎐N–PBN
᎐ ᎐
2
2
With the promoting acid HA present in a low but finite con-
anisyl)amine (15) [E o(15 ϩ/15) = 0.52 V] in HFP which pro-
ؒ
᝽
centration equimolar amounts of the spin adducts A–PBN
ؒ
and Cl C᎐C᎐N–PBN were formed. This mechanism is a
᎐ ᎐
2
OMe
version of the Forrester–Hepburn mechanism; a truly auto-
catalytic version of it was recently found in the reaction
between N-chlorobenzotriazole and PBN, with benzotriazole as
the autocatalyst.20
O•
OMe
PhCO
N
(4-MeOC6H4)3N
(4-MeC6H4)3N
But
OMe
A similar situation can be envisaged for the reaction between
PBN and 8 in dichloromethane. It is likely that the formation of
a trace of HF will always accompany the dissolution of 8 in a
OMe
15
16
18
17
solvent, leading to the initiation of a mechanism of the same
ϩ
Ϫ
duced the radical cation 15 in a rapid reaction (within sec-
type as above [eqns. (10) and (11)]. The radical anion 8
,
᝽
᝽
onds). Tris(4-tolyl)amine (16) [Eo(16 ϩ/16) = 0.75 V] reacted
᝽
slower (within minutes) and 2,3-dimethyl-1,4-dimethoxy-
HF ϩ PBN
PBN(H)F
(10)
benzene (17) [Eo(17 ϩ/17) = 1.18 V]16 gave a weak EPR spec-
᝽
trum of 17 ϩ only after standing over night. To provide a cali-
PBN(H)F ϩ (PhSO2)2NF
F–PBN ϩ Hϩ ϩ (PhSO2)2NF
᝽
Ϫ
ؒ
bration mark, it should be mentioned that N-bromo-
(11)
᝽
succinimide (Eo = 0.2 V)17 oxidized 17 instantaneously to 17
ϩ
᝽
in HFP,18 thus indicating that Eo(8/8 Ϫ) must be considerably
presumably a highly unstable species, has two pathways open
for cleavage [eqns. (12) and (13)] and it is not immediately
᝽
lower than 0.2 V. Therefore it is not possible for 8 to oxidize spin
traps like PBN or DMPO via an electron transfer mechanism.
Yet the spontaneous reaction between 8 and PBN in
dichloromethane gave a weak EPR spectrum of the fluoro spin
adduct 3, the intensity of which increased and reached a
Ϫ
Ϫ
ؒ
(PhSO2)2NF
(PhSO2)2N ϩ F
(12)
(13)
᝽
Ϫ
Ϫ
ؒ
(PhSO2)2NF
PhSO2 ϩ (PhSO2)(F)N
᝽
J. Chem. Soc., Perkin Trans. 2, 1997
895

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ؒ
Fig. 4 EPR spectrum of PhSO2(F)N–PBN 21, generated from a
solution of PBN (0.1 mol dmϪ3), 8 (0.09 mol dmϪ3) and HF (7 mmol
dmϪ3) in dichloromethane under thermal conditions. The spectrum was
recorded ca. 15 min after mixing of the reagents. The simulated spec-
trum used the hfs coupling constants from Table 2. The remaining lines
ؒ
belong to the EPR spectrum of F–PBN .
ؒ
ؒ
Fig. 3 Time dependence of F–PBN 3 (᭞), PhSO2(F)N–PBN 21 (᭺)
and PBNOx 18 (ᮀ), when formed from a solution of PBN (0.1 mol
dmϪ3), 8 (0.09 mol dmϪ3) and HF (7 mmol dmϪ3) in dichloromethane
under thermal conditions. The EPR spectrum was recorded after ca.
100 min.
obvious which will be the predominant one. However, the
important feature is that a new molecule HA is formed in either
step with the proton from eqn. (11), HF in eqn. (12) or
(PhSO2)(F)NH in eqn. (13) and either of these acids can in
principle propagate the reaction by adding to a new PBN mole-
ؒ
ؒ
cule. Both the neutral radicals (PhSO2)2N and PhSO2 can
ؒ
add to PBN and give spin adducts (PhSO2)2N–PBN (19) and
ؒ
PhSO2–PBN (20), of which the former might be persistent
enough for detection and the latter known 3 to be rapidly trans-
formed to PBNOx (18). Furthermore, propagation via
(PhSO2)(F)NH will lead to the spin adduct (PhSO2)(F)N–
ؒ
PBN (21).
Thus the HA propagated reaction between PBN and 8 is
ؒ
predicted to give F–PBN (3), one or two N-centred spin
adducts (19, 21) and PBNOx (18). In the spontaneous reaction
(Fig. 2) only 3 and 18 appeared, indicating that only the auto-
catalytic process of eqns. (7)–(9) was operating and that the N-
centred spin adduct 17 might be too reactive to survive under
the conditions employed, thus ending up as PBNOx.
Addition of a low concentration of the autocatalyst HF (ca.
7 mmol dmϪ3) led to the formation of F–PBN 3, at least one
ؒ
N-centred spin adduct, giving rise to an EPR spectrum consist-
ing of a triplet of septets, and PBNOx 18, developing with time
as shown in Fig. 3. By aid of experiments with [α-2H]PBN,
which gave a spectrum consisting of a triplet of sextets by loss
of one 0.19 mT coupling, it was eventually established that the
Fig. 5 EPR spectra obtained by irradiation (light of λ > 400 nm) of
(a) a solution of (PhSO2)2NH (20 mmol dmϪ3), PBN (100 mmol dmϪ3
)
t
and TAPT (ca. 1 mmol dmϪ3) in dichloromethane. The Bu 2NO
ؒ
ؒ
ؒ
lines are marked with ‘o’, those of PhSO2–PBN with ‘ϩ’. Spectrum (b)
N-centred adduct must be predominantly (PhSO2)(F)N–PBN
was obtained similarly from 4-NO2PBN.
(21). Its EPR spectrum was best described by four coupling
constants, αN = 1.39, αN Ј = 0.36, αF = 0.19 and αH = 0.20 mT
(Fig. 4). The same spectrum could also be generated by pho-
tolysis (UV) of solutions of PBN and 8 in dichloromethane and
several other solvents. Simulation of the spectrum of 21, using
the coupling constants given above, does not give perfect
agreement; one reason for this might be the formation of dia-
stereoisomeric forms of 21 due to a non-planar arrangement
of the N(F)SO2Ph group.
dichloromethane by irradiation by light of λ > 400 nm in the
presence of 2,4,6-tris(4-anisyl)pyrylium tetrafluoroborate
(TAPT), an often used photo-electron transfer reagent.21 Its use
for photo-initiated inverted spin trapping has been described
earlier 8 and is presently being extended to a large number of
nucleophilic species.22 The EPR spectrum obtained is shown in
Fig. 5(a). It was analysed as being composed of three spectra,
t
ؒ
ؒ
Attempts to record the EPR spectrum of (PhSO2)2N–PBN
namely (i) a weak signal with αN = 1.56 mT (Bu 2NO ), (ii) a
3 × 2 line spectrum with αN = 1.48 and αH = 0.45 mT (assigned
(19) were made by oxidation of PBN and (PhSO2)2NH in
896
J. Chem. Soc., Perkin Trans. 2, 1997

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Table 2 Spin adducts from reactions between spin traps and (PhSO2)2NF in dichloromethane
F adduct N adduct
αN /mT αH /mT αF /mT αN /mT
Spin trap
Reaction conditions
αN Ј/mT αH /mT αF /mT Remark
4-NO2PBN (10)
4-NO2PBN (10)
4-NO2PBN (10)
PBN (1)
Thermal
UV
Thermal, [HF] ≈ 7 m
Thermal
Thermal, [HF]] ≈ 7 m
Thermal, [HF] ≈ 7 m 
Photolysis (UV)
Thermal; [AcOH] = 1.2 m
1.24
1.22
1.22
1.27
1.27
1.27
1.27
1.25
0.11
0.12
0.11
0.11
0.11
—
4.77
4.70
4.71
4.52
4.54
4.59
4.54
4.52
None
1.35
1.33
None
1.38
1.39
Weak signal
0.38
0.39
0.21
0.20
0.21
0.20
ϩNO2PBNOx (0.78 mT)
ϩNO2PBNOx (0.78 mT)
ϩPBNOx (0.80 mT)
See Fig. 4
PBN (1)
0.36
0.354
0.36
0.20
—
0.19
0.19
0.18
0.18
[α-2H]PBN
PBN (1)
PBNOx (0.81 mT)
0.11
0.11
1.36
None
PBN (1)
ϩPBNOx (0.80), AcO–
ؒ
PBN (1.33 mT, 0.13)
[α-2H]PBN
DMPO (2)
Thermal; [AcOH] = 2.3 m
1.26
1.18
—
4.53
None
1.42
ϩPBNOx (0.80), AcO–
ؒ
PBN (1.33 mT)
ؒ
Thermal
0.616 5.46
0.286 1.98
—
F–DMPO : also αH Ј, αH Љ
0.200, 0.139
t
ؒ
(Bu 2NO ), (ii) 3 × 2 lines with αN = 1.50 and αH = 0.44 mT
ؒ
(assigned to PhSO2–PBN ; see above) and 3 × 4 lines with
αN = 1.36, αN Ј = 0.3–0.4, αH = 0.70 and αH Ј = 0.18 mT.
The 4-tolylsulfonamido spin adduct, 4-CH3C6H4SO2NH–
ؒ
PBN , has been characterized in aqueous medium, αN = 1.56,
αN Ј = 0.175 and αH = 0.338 mT, from the acidification of a solu-
tion of chloramine-T (N-chlorotoluene-p-sulfonamide) and
PBN in water.23 This species was not stable, being converted
into a new, unidentified radical with αN = 0.71 and αN Ј = 0.42
mT within a few minutes. The same unstable species, accom-
panied by other radicals, was detected during irradiation of
analkaline solution of PBN and chloramine-T.
An experiment in which a low concentration (2.3 mmol
dmϪ3) of a weak acid, acetic acid, was added, supported the
idea that acids of different kinds can be entered in eqn. (11) and
promote the formation of spin adducts.19 Fig. 6 shows the time
ؒ
dependence of the F–PBN and PBNOx signals and that
ؒ
[F–PBN ]max was now obtained after ca. 30 min. A low concen-
ؒ
tration of AcO–PBN also developed, the time to reach [AcO–
Fig. 6 Time dependence of F–PBN᭹ (3) (᭞), PBNOx (18) (᭢) and
AcO–PBN (ᮀ) when formed from a solution of [α- H]PBN (0.10 mol
dmϪ3), 8 (0.09 mol dmϪ3) and acetic acid (2.3 mol dmϪ3) in dichloro-
methane under thermal conditions
ؒ
PBN ]max being ca. 30 min.
The thermal reaction of DMPO 2 and 8 in HFP gave two
superimposed spectra, one belonging to F–DMPO (Fig. 1)
and one to what had the appearance of a spectrum of an
adduct of an N-centred radical (Table 2). The photolysis (light
of λ > 400 nm) of DMPO with (PhSO2)2NH in dichloro-
methane in the presence of TAPT gave an EPR spectrum of
what presumably is (PhSO2)2N–DMPO according to its coup-
ling constants. This spectrum matched the second spectrum
obtained from DMPO and 8.
2
ؒ
ؒ
ؒ
to PhSO2–PBN ; an almost identical spectrum was obtained by
TAPT–hv treatment of PBN on dichloromethane in the pres-
ence of 4-toluenesulfonic acid, αN = 1.52 and αH = 0.476 mT)
and (iii) a 3 × 4 line spectrum with αN = 1.39, αN Ј = 0.3–0.4,
αH = 0.72 and αH Ј = 0.176 mT. The large α-hydrogen coupling
constant of 0.72 mT was verified by using [α-2H]PBN as the
substrate (loss of this coupling and appearance of a 1:1:1 trip-
let with αD = 0.12 mT). The 3 × 4 line spectrum is preliminarily
ؒ
Coupling constants of the spin adducts obtained in the vari-
ous reactions of spin traps with 8 in dichloromethane are given
in Table 2.
ؒ
assigned to PhSO2NH–PBN , in which one of the PhSO2
groups has been split off. In the present context, note that the
spectrum disappeared within 10 s after the light source had
been shut off and thus the species corresponding to this spec-
trum cannot contribute to the spectrum of the N-centred spin
adduct recorded during the thermal HF-promoted reaction of
PBN and 8 in Fig. 3.
Reactions of spin traps and 8 in HFP
As stressed above, HFP as a solvent strongly suppresses all
kinds of nucleophilic reactivity.13 It is therefore expected that
the Forrester–Hepburn mechanism should not operate in HFP,
since the addition of HA to a nitrone presumably should pro-
ceed in a nucleophilic addition–protonation sequence. This
prediction was shown to be correct: spin traps 1, 10 and 12 did
not show any paramagnetic activity upon treatment with 8 in
HFP under conditions otherwise similar to the ones used for
reactions given in Table 2. This was true for both thermal and
photo reactions (UV). However, 4-Me2NPBN (13) underwent
one-electron oxidation to give its persistent (many hours) rad-
ical cation as expected in view of its low oxidation potential of
0.64 V.
When 4-NO2PBN, a stronger electrophile, was used in the
same reaction a more distinct EPR spectrum of two over-
lapping spectra was obtained [Fig. 5(b)]. The more intense 3 × 4
lines spectrum has αN = 1.33, αN Ј = 0.3–0.4, αH = 0.64 and
ؒ
αH Ј = 0.325 mT, and thus should correspond to 4-NO2PBN –
N(H)SO2Ph. The less intense spectrum could not be analysed
with certainty but might have αN = 1.29, αN Ј = 0.12 and
ؒ
αH = 0.39 mT and then possibly correspond to 4-NO2PBN –
N(SO2Ph)2.
Finally, 4-FPBN, a spin trap of reactivity in between those
of PBN and 4-NO2PBN on irradition with (PhSO2)2NH and
TAPT gave a spectrum which was similar to that of Fig.
5(α), being composed of three spectra of (i) αN = 1.56 mT
Conclusions
Two mechanisms of generating fluoro spin adducts from
J. Chem. Soc., Perkin Trans. 2, 1997
897

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nitrones have been characterized. The first involves the use of a
strong oxidant, XeF2 and proceeds via initial formation of the
nitrone radical cation, followed by its reaction with fluoride ion
(‘inverted spin trapping’). The second one uses an N-fluoro
reagent, (PhSO2)2NF, which is a weak oxidant and exerts its
action by oxidizing the hydroxylamine formed by addition of
an acid HA to the nitrone (the Forrester–Hepburn mechanism).
If HA is equal to HF, the fluoro spin adduct is formed. In a
parallel reaction, spin adducts of N-centred radicals are
Council and the Knut and Alice Wallenberg Foundation is
gratefully acknowledged. We thank Professor E. Steckhan,
University of Bonn, for a gift of TAPT.
References
1 E. G. Janzen and J. I. Liu, J. Magn. Reson., 1973, 9, 510.
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4 Unless otherwise stated, potentials were taken from: L. Eberson,
Electron Transfer Reactions in Organic Chemistry, Springer-Verlag,
Heidelberg, 1987.
ؒ
formed, such as PhSO2–N(F)–PBN .
5 L. Eberson, M. P. Hartshorn and O. Persson, J. Chem. Soc., Perkin
Trans. 2, 1997, 195.
Experimental
6 W. Sümmerman and U. Deffner, Tetrahedron, 1975, 31, 593.
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Materials
PBN (1), DMPO (2) and 4-NO2PBN (10) were purchased from
Aldrich and used as received. 4-MeOPBN (12) and 4-Me2-
NPBN (13) were available from an earlier study5 and F5PBN
(11) was prepared from pentafluorobenzaldehyde and 2-methyl-
2-nitropropane according to ref. 24. Xenon() fluoride and
N-fluorodibenzenesulfonamide (8) were from Aldrich and di-
benzenesulfonamide from Fluka AG. Bu4NFؒ3H2O was from
Merck AG, and tris(4-methoxyphenyl)amine (15), tris(4-tolyl)-
amine 16 and 2,3-dimethyl-1,4-dimethoxybenzene (17) were
available from earlier studies.16,17 All solvents used were of
highest commercial quality available. TAPT was a gift from
Professor E. Steckhan, University of Bonn. Epa of F5PBN (11)
was measured in dichloromethane–Bu4NPF6 by differential
pulse voltammetry (BAS-100 instrument, Pt button anode).
EPR spectral experiments
These were performed as described in earlier work. The HF
containing solutions used for promotion of the reaction
20 P. Carloni, L. Eberson, L. Greci, P. Sgarabotti and P. Stipa, J. Chem.
Soc., Perkins Trans. 2, 1996, 1297.
21 M. Martiny, E. Steckhan and T. Esch, Chem. Ber., 1993, 126, 1671.
22 L. Eberson and O. Persson, J. Chem. Soc., Perkin Trans. 2, in the
press.
23 J. C. Evans, S. K. Jackson and C. C. Rowlands, Tetrahedron,
1985, 41, 5191, 5195.
24 R. D. Hinton and E. G. Janzen, J. Org. Chem., 1992, 57, 2646.
3
᎐
between PBN and 8 were made up by adding 10 µl (1 µl ᎐ 1mm )
᎐
of methanesulfonic acid to a solution of a 10–15% excess of
Bu4NFؒ3H2O in 20 cm3 of dichloromethane and stirring for 15
min. Stock solutions of known concentrations of XeF2 were
made up in dichloromethane or HFP, but could only be kept for
a limited period (3–4 h) while still keeping their activity.
Paper 6/07509F
Received 4th November 1996
Accepted 22nd January 1997
Acknowledgements
Financial support from the Swedish Natural Science Research
898
J. Chem. Soc., Perkin Trans. 2, 1997