Spin trapping studies on radical cations of styrene

Article abstract of DOI:10.1002/jccs.200100144

The radical cations of styrene (π-type), generated by photosensitized electron-transfer of 1,4-dicyanobenzene (DCB), using 2-methyl-2-nitrosopropane (MNP) as spin trapping agents and nucleophiles as the co-reactant were investigated. The spin-trapped nitr

Full text of DOI:10.1002/jccs.200100144

JournaloftheChineseChemicalSociety, 2001, 48, 983-986
983
Spin Trapping Studies on RadicalCationsofStyrene
Tong-Ing Ho* (
Kai Pan (
), Chiou-Rong Lin (
) and Chi-Wei Hsu (
),
)
DepartmentofChemistry,NationalTaiwanUniversity,Taipei,Taiwan106,R.O.C.
The rad i cal cat ions of sty rene ( -type), generatedbyphotosensitizedelectron-transferof1,4-dicyano-
ben z ene (DCB), us ing 2-methyl-2-nitrosopropane (MNP) as spin trap ping agents and nucleophiles as the
co-reactant were in ves ti gated. The spin-trapped nitroxide rad i cals were sub jected to HPLC-ESR and mass
spec tro scopic stud ies. Long-range hfsc were ob tained by res o lu tion en hance ment. Mass spectralfragmenta-
tion pat terns sup port the regioselective for ma tion of the trapped nitroxides, which is de rived from the
anti-Markovnikov ad di tion of the meth a nol to the -positionofthestyrenecationradicals.
INTRODUCTION
Trapping of Styrene CationRadicals
Typically, 2 mmol of sty rene, 0.003 g (0.035 mmol) of
MNP, and 0.03 g (0.025 mmol) of DCB were mixed with
meth a nol (1.5 mL) and acetonitrile (3.5 mL) in a 10 mL Py rex
tube. Thesolutionwasdegassedwithnitrogenfor5min. then
ir ra di ated with 300 nm light for 2 h. The or ganic sol vents
wereremovedbyarotatoryevaporatortoaffordayellowres-
i due. Then, dry ben zene sol vent was added and the re sul tant
solutionwasthentransferredtoanEPRtube. Thesamplewas
fi nally outgassed with ni tro gen for 20 min. be fore be ing sub-
jectedtofurtherEPRmeasurements.
Thespintrappingtechniquehasbeenappliedtovarious
fieldsofchemistry.1-9 2-Nitro-2-methylpropane (MNP) and
-phenyl-N-tert-butyl-nitrones (PBN) are the spin trap ping
agents com monly used to trap var i ous short-lived free rad i-
cals. In general, the trapped nitroxide radi cals are sta ble
enough for fur ther sep a ra tion by HPLC.10-15 The sig nal ob-
tained from pu ri fied nitroxide rad i cals can then be im proved
by the res o lu tion en hance ment method to ob tain long-range
hyperfine-splitting con stants (hfsc) and the struc tures of the
trapped rad i cals can be elu ci dated by mass spec tros copy.
How ever, up to the pres ent, most of the works on spin trap-
ping are mainly fo cused on -freeradicals;16-20 relativelyfew
re ports in voke trap ping of rad i cal ions. In this let ter, we re-
portthespintrappingontheradicalcationsofstyrene, which
are gen er ated by pho to sen si tized elec tron-transfer of 1,4-
dicanobenzene(DCB).
RESULTS
When sty rene is ir ra di ated in the pres ence of DCB and
MNP with meth a nol and acetonitrile (1:2) sol vent mix ture,
the EPR spec trum (Fig. 1) re vealed the ex is tence of two rad i-
cal spe cies. As the same mix ture was sub jected to HPLC sep-
a ra tion us ing sil ica gel col umn and ethylacetate and hex ane
(1:5) sol vent mix ture as elu ents, two rad i cal spe cies were
alsoobtained. Thefirstradicalwhichexhibitsasimplethree
line EPR spectrum (aN = 15.4 G) can be as signed due to
di-tert-butyl nitroxide,22 aris ing from the photolysis of MNP.
ThesecondradicalobtainedafterHPLCseparation(Fig. 2)
indicatedatripletofdoubletspattern(aH = 2.63 G, aN = 14.69
G, g = 2.0062). The mass spec trum shows m/e vaules at 223
(M++1, 11%); 222 (M+, 3%), 207 (27%), 178 (24%), 149
(92%), 135 (22%), 105 (66%) and 57 (100%).
EXPERIMENTALSECTION
Re search grade sty rene (Merck) is dis tilled be fore use.
Spin trap ping re agent MNP was pre pared ac cord ing to the
pro ce dures re vealed by Stowell.21 The EPR spec tra were
obtained from a spectrometer (Bruker EPR 300X-Band)
equipped with an NMR gaussmeter (ER 035M) op er at ing at a
mod u la tion fre quency of 100 kHz. Typically, the mass spec-
trometer(Finnigan)wasoperatedattheionizationpotentials
of 70 or 20 eV.
The mo lec u lar ion at m/e = 222 and the hyperfine split-
ting con stants (hfsc) in di cate the trapped nitroxide rad i cal
struc ture as1 (Equa tion 1). The pres ence of -hydrogencou-
IrradiationiscarriedoutusingaRayonetphotolysisap-
pa ra tus equipped with 300 and 350 nm lamps.
DedicatedtoProfessorSheng-liehLiuontheoccasionofhisninetiethbirthday.

984 J. Chin. Chem. Soc., Vol. 48, No. 6A, 2001
Ho et al.
pling (aH = 2.63 G) and two -hydrogencouplingsfromthe
CH2OCH3 group in the res o lu tion en hance ment spec trum
(Fig. 3)providefurthersupportstotheassignmentofradical
1. The split ting pat terns cor re spond to the fol low ing long-
rangehfsc(asobtainedbycomputersimulations);inaddition
to the -H aH = 2.63 G and aN = 14.69 G, there are two -H
with aH = 0.78 G; two ortho-H with aH = 0.79 G, and one
para- H with aH = 0.72 G. The long-range hfsc ob served for
the phenyl ring pro tons and -hydrogensareinaccordance
with our pre vi ous find ings.10 The ma jor frag men ta tion pat-
tern from the mass spectrum include [M-t-BuO]+ (92%),
[M-t-BuNO]+ (22%), [M-t-BuNO-CH2OCH3]+ (66%), and
[(CH3)3C]+ (100%). These frag men ta tion pat terns are also
sim i lar to that found for the nitroxide rad i cals.10
hv
C6H5CH CH2
CH3OH
+
DCB
+
(2)
C6H5CH2CH2OCH3
2
Inanadditionalexperiment, asolution(0.5mLwater
and 3.5 mL acetonitrile) con tain ing 0.003 g (0.035 mmol)
MNP, 0.03 g (0.025 mmol) DCB and 2 mmole of sty rene was
sealed in a 10 mL Py rex tube. Af ter rou tine de gas sing by ni-
trogenfor20min.,thesolutionwasirradiatedwithaRayonet
300nmfor2h, thentheresidualsolventswereremovedand2
mL of dry ben zene was added. The EPR spec trum (Fig. 4) for
thepurifiednitroxideradicalshowscleartripletsofdoublets
splittingpatterns(aH = 6.63 G, aN = 15.02 G, g = 2.0082). The
massspectrumshowsthefollowingmajorfragmentations:
m/e = 209 (M+1, 39%), 208 (M+, 8%), 178 (M-CH2O, 95%),
149 (M-t-Bu-2H, 20%), 121 (M-t-BuNO, 58%), 122 (M-t-
DCB
C6H5CH=CH2 + CH3OH + MNP
CH3CN
.
(CH3)3CNCH(C6H5)CH2OCH3
O
(1)
1
When photolysis was car ried out in the ab sence of trap-
ping re agent MNP, for ex am ple, sty rene and DCB are ir ra di-
ated in methanol-acetonitrile mixture, the product (35%
yield) is 1-methoxy-2-phenylethane 2 (equa tion 2). 1H-NMR
for com pound2 shows chem i cal shift at ( ) 2.88 (2H, t, J = 7
Hz), 3.35 (3H, s), 3.60 (2H, t, J = 7 Hz), and 7.19-7.28 (4H,
m) ppm. The mass fragmentationsinclude m/e 136 (M+,
11%), 135 (8%), 121 (63%), 104 (47%), 91 (100%), and 77
(30%).
Fig. 2. TheEPRspectrumoftheHPLC-EPRseparated
nitroxide 1.
Fig. 1. The EPR spec trum for the photolysis of sty rene,
meth a nol, and DCB in the pres ence of MNP.
Fig. 3. ResolutionenhancementEPRspectrumforthe
HPLC-separated nitroxide 1.

Spin Trapping Studies on Rad i cal Cat ions of Sty rene
J. Chin. Chem. Soc., Vol. 48, No. 6A, 2001 985
Table 1. Spectral Data for the Spin-trapped Nitroxide Radicals
BuNO+H, 84%), 103 (M-t-BuNO-H2O, 61%), 91 (M-t-
BuNO-CH2O, 58%), and 57 ( t-Bu+,100%).Thesefragmenta-
tions are con sis tent with the spin-trapped nitroxide rad i cal3,
which is de scribed in equa tion 3.
nitroxide g value
hfsc
MS fragmentations
1
2.0062 aH = 2.63 G m/e = 223 (M++1, 11%);
aN = 14.69 G 222 (M+, 3%), 207 (27%),
178 (24%), 149 (M-t-BuO+,
92%), 135 (M-t-BuNO+,
DCB, hv
C H CH=CH2 + H2O + MNP
22%), 105 (M-t-BuNO-
(CH3)3CNCH(C6H5)CH2OH
O
6
5
+
CH3CN
CH2OCH3 , 66%) and 57
((CH3)3C+, 100%).
2.0082 aH = 6.63 G m/e = 209 (M+1, 39%), 208
aN = 15.02 G (M+, 8%), 178 (M-CH2O,
95%), 149 (M-t-Bu-2H,
(3)
3
3
If the nucleophile is re placed by so dium azide, the
20%), 121 (M-t-BuNO,
trappedradicalbecomes4 as shown in equa tion (4). The EPR
spec trum for the nitroxide rad i cal4 shows one -hydrogen
(aH = 3.35 G) and one ni tro gen (aN = 14.49 G) and with the g
value of 2.0065. The mass spec trum of the nitroxide 4 shows
majorfragmentationsincludingm/e = 233 (M+, 2%), 208
(M-N2+H2, 2%), 149 (M-N2-t-Bu, 9%), 119 (M-t-BuNO-N2,
100%), and 57 (t-Bu, 46%). The spec tral data for the nitro-
xides 1, 3 and 4 are summarized in Ta ble 1.
58%), 122 (M-t-BuNO+H,
84%), 103 (M-t-BuNO-
H2O, 61%), 91 (M-t-BuNO-
CH2O, 58%), and 57 (t-Bu+,
100%).
4
2.0065 aH = 3.35 G m/e = 233 (M+, 2%), 208
aN = 14.49 G (M-N2+H2, 2%), 149 (M-
N2-t-Bu, 9%), 119 (M-t-
BuNO-N2, 100%), and 57
(t-Bu, 46%).
DCB, hv
C6H5CH=CH + NaN3 + MNP
(CH3 )3CNCH(C6H5)CH2N3
O
DMSO
the prod ucts are the Markovnikov ad di tion of the nucleo-
philes to the -positionofthe -bondanionradicalsystems.24
In contrast, if the electron acceptor sensitizers (such as
9,10-dicyanoanthracene or 1,4-dicyanobenzene) were used,
cationradicalsystemswereformedinstead. Thenucleophile
additionfollowstheanti-Markovnikov rule that meth a nol is
added to the -po si tion of the sty rene cat ion rad i cal. Our re-
ac tion of the meth a nol ad di tion (equa tion 2) is con sis tent
withapreviousreport.23
4
(4)
DISCUSSION
Methanoladditiontothestyrenecationradicalisregio-
selective, the opposite of the styrene anion radical sys-
tems.23-25 When 1-phenylcyclohexene or 1,1-diphenyl ethyl-
enewereirradiatedinthepresenceof1-methoxynaphthalene
or 1,4-dimethoxynaphthalene (elec tron-donor sensitizers),
The fact that all EPR spec tra of the spin-trapped nitro-
xideradicals( 1, 3, and 4)revealedonlyone -hydrogenindi-
cate the trap ping of -styrylradical.Theresolutionenhance-
mentresultsobtainedfromthetrappednitroxidealsoindicate
that there are two -hy dro gen hfsc along with other long-
range hfsc. The regioselective for ma tion of the -styryl rad i-
calwasfurtherconfirmedbymassspectroscopicfragmenta-
tionanalysis.Them/e105(66%)fragmentationfornitroxide
1 can be as signed to [M-t-BuNO-CH2OCH3]+, which clearly
in di cates that meth a nol was added to the -positionofthe
styrene cation rad i cal. Sim i lar framentation was also ob-
served for the trapped nitroxide 3. In this case, the ob served
m/e = 91 (58%) rep re sents [M-t-BuNO-CH2O]+ frag ment.
For nitroxide 4, the m/e 119 (100%) rep re sents the [M-t-
BuNO-N2]+ fragment.Thus,themassspectralfragmentation
patterns along with long-range -hy dro gen hfsc and EPR
spec tra (only one -hydrogensplitting),allindicatedthatthe
nucleophiles at tacked the -po si tion of the sty rene cat ion
Fig. 4. The EPR spec trum for the HPLC-EPR pu ri fied
nitroxide 3.

986 J. Chin. Chem. Soc., Vol. 48, No. 6A, 2001
Ho et al.
radicaltrappedbyMNP.
Received August 13, 2001.
The mech a nism for the for ma tion of the trapped nitro-
xides can thus be sum ma rized in Scheme I. The DCB is ex-
cited to the ex cited state DCB*. Then elec tron-transfer from
styrenetoDCB*leadstoastyrenecationradical-DCBanion
rad i cal pair. Meth a nol is added to the -po si tion of the sty-
renecationradicalaffordingthe -styryl rad i cal which can
be trapped by MNP. In the ab sence of MNP, back elec tron
transfer form DCB anion radical to the methanol added
-styryl rad i cal can oc cur to form the 1-phenyl-2- methoxy-
ethane 2. The anti-Markovnikovadditionofmethanoliscon-
sistentwiththeobservationinapreviousreport.23
Key Words
Styrene; EPR; Cation radical; Spin trapping.
REFERENCES
1. Janzen, E. G. Acc. Chem. Res. 1971, 4 , 31.
2. Perkins, M. J. Adv. Phys. Org. Chem. 1980, 17, 1.
3. Hair, D. L.; Oehler, U. M.; Krygsman, P. H.; Janzen, E. G.
J. Org. Chem. 1988, 53, 4535.
4. Stolze, K.; Duling, D. R.; Ma son, R. P. J. Chem. Soc.,
Chem. Commun. 1988, 268.
5. Kaur, H.; Leung, K. H. W.; Perins, M. J. J. Chem. Soc.,
Chem. Commun. 1981, 142.
Scheme I The mech a nism for the for ma tion of sty rene
cat ion rad i cal spin-trapped by MNP in meth-
anol
6. Rockenbauer, A.; Gyor, M.; Tudos, F. TetrahedronLett.
1985, 27, 3425.
7. Ohto, N.; Niki, E.; Kamiya, Y. J. Chem. Soc., Perkin
Trans. 2 1977, 1770.
8. Chandra, H.; Davidson, I. M. T.; Symons, M. C. R. J.
Chem. Soc., Far a day Trans. 1 1983, 79, 2705.
9. Mackor, A.; Wajer, A. J.; De Boer, Th. J. Tetrahedron
1968, 24, 1623.
*
-
CN
CN
CN
CN
+
hv
SET
+
CN
CN
-
CN
OCH CN
3
OCH
3
CH OH
3
H
+
+
10. Pan, K.; Lin, C. R.; Ho, T. I. Magn. Reson. Chem. 1993,
31, 632.
CN
CN
2
11. Pan, K.; Ho, T. I. Magn. Reson. Chem. 1996, 34, 1007.
12. Lin, C. R.; Wang, C. N.; Ho, T. I. J. Org. Chem. 1991, 56,
5025.
MNP
13. Pan, K.; Ho, T. I. J. Chin. Chem. Soc. 1997, 44, 243.
14. Lin, R. R.; Pan, K.; Ho, T. I. Appl. Spectro. 1997, 51,874.
15. Ho, T. I.; Nozaki, K.; Naito, A.; Okazaki, S.; Hatano, H.
J. Chem. Soc., Chem. Commun. 1989, 206.
16. Lin, C. R.; Lin, Y. C.; Ho, T. I. J. Chin. Chem. Soc. 1993,
40, 47.
C(CH
N
)
3 3
CN
CN
O
OCH
3
+
1
17. Ho, T. I.; Pank, K.; Chen, C. P. J. Chin. Chem. Soc. 1998,
45, 27.
18. Pank, K.; Cheng, K. L.; Ho, T. I. Magn. Reson. Chem.
1995, 33, 518.
19. Wang, S. R.; Cheng, C. P.; Ho, T. I. J. Chem. Soc.,Dalton
Trans. 1988, 2695.
20. Ho, T. I.; Chang, C. M.; Wang, S. R.; Cheng, C. P. J.
Chem. Soc., Dal ton Trans. 1988, 123.
21. Stowell, J. C. J. Org. Chem. 1971, 36, 3055.
22. Rosenthal, I.; Mossoba, M. M.; Riesz, P. Can. J. Chem.
1982, 60, 1486.
23. Hixson, S. S. J. Am. Chem. Soc. 1972, 94, 2505.
24. Maroulis, A. J.; Ar nold, D. R. Synthesis 1979, 819.
25. Mizuno, K.; Nakanishi, I.; Ichinose, N.; Otsuji, Y. Chem
Lett. 1989, 1059.
Insummary, wehavedemonstratedthatthestyrenecat-
ionradicalwhichisphoto-sensitizedbyanelectron-deficient
sensitizer (DCB) can be regioselectively trapped by nucleo-
phile (meth a nol) and spin traps (MNP) si mul ta neously lead-
ing to the sta ble nitroxide rad i cals1, 3 and 4. EPR spec tra for
all these nitroxides shows only one -hy dro gen hfsc. The
spin-trapped nitroxides can be sep a rated by HPLC-EPR tech-
niques. Resolutionenhancementstudyindicatedthatnitro-
xide 1 has two -hydrogenhfsc(aN = 0.78G) along with other
long-rangehfsc.Massspectralfragmentationstudiesprovide
fur ther ev i dence for the regioselective for ma tion of nitro-
xides. Thus, we have shown that spin trap ping is also pos si ble
forthestyrenecationradicals.