Samarium(0) and 1,1′-dioctyl-4,4′-bipyridinium dibromide: A novel electron-transfer system for the chemoselective reduction of aromatic nitro groups

Article abstract of DOI:10.1021/jo005666q

A mild and efficient electron-transfer method was developed for the chemoselective reduction of aromatic nitro groups using samarium(0) metal in the presence of a catalytic amount of 1,1′-dioctyl-4,4′-bipyridinium dibromide. This method was found to give the product aromatic amine in 79-99% yield with selectivity over a number of other functional and protecting groups such as alkene, azide, benzyl ether, nitrile, amide, halide, p-toluenesulfonamide, t-Boc, tert-butyldiphenylsilyl ether, and aliphatic nitro groups. Our results also indicate that samarium(0) plays an important role in the reduction process and that 1,1′-dioctyl-4,4′-bipyridinium dibromide acts as an electron-transfer catalyst and is essential in the activation of samarium(0) metal. The major active reducing agent responsible for the reduction is believed to be the radical cation species formed from 1,1′-dioctyl-4,4′-bipyridinium dibromide.

Full text of DOI:10.1021/jo005666q

J . Org. Chem. 2001, 66, 919-924
919
Sa m a r iu m (0) a n d 1,1′-Dioctyl-4,4′-Bip yr id in iu m Dibr om id e: A
Novel Electr on -Tr a n sfer System for th e Ch em oselective Red u ction
of Ar om a tic Nitr o Gr ou p s
Chengzhi Yu, Bin Liu, and Longqin Hu*
Department of Pharmaceutical Chemistry, College of Pharmacy, Rutgers, The State University of
New J ersey, 160 Frelinghuysen Road, Piscataway, New J ersey 08854-8020
LongHu@rci.rutgers.edu
Received October 2, 2000
A mild and efficient electron-transfer method was developed for the chemoselective reduction of
aromatic nitro groups using samarium(0) metal in the presence of a catalytic amount of 1,1′-dioctyl-
4,4′-bipyridinium dibromide. This method was found to give the product aromatic amine in 79-
99% yield with selectivity over a number of other functional and protecting groups such as alkene,
azide, benzyl ether, nitrile, amide, halide, p-toluenesulfonamide, t-Boc, tert-butyldiphenylsilyl ether,
and aliphatic nitro groups. Our results also indicate that samarium(0) plays an important role in
the reduction process and that 1,1′-dioctyl-4,4′-bipyridinium dibromide acts as an electron-transfer
catalyst and is essential in the activation of samarium(0) metal. The major active reducing agent
responsible for the reduction is believed to be the radical cation species formed from 1,1′-dioctyl-
4,4′-bipyridinium dibromide.
In tr od u ction
functional groups that are often present in the substrate
such as alkene, azide, benzyl ether, nitrile, amide, halide,
and p-toluenesulfonamide. In addition, reduction of
aromatic nitro compounds often stops at an interme-
diate stage, yielding hydroxylamines,11 hydrazines,12
azoarenes,13 or azoxyarenes.14 Therefore, there is a need
for the development of mild chemical methods for the
selective reduction of aromatic nitro groups.
In our efforts to develop prodrugs for site-specific
activation in tumor cells,15 we were looking for a mild,
selective chemical method to mimick the reduction of
aromatic nitro groups in hypoxic tumor cells or by an
enzyme such as Escherichia coli nitroreductase. It came
to our attention that aliphatic nitro groups could be
converted to hydoxylamines16 and hydrazines17 in moder-
ate to good yields under mild conditions using samarium-
(II) iodide (SmI2). As one of the most versatile single
electron-transfer agents,18 SmI2 is capable of reducing
nitro compounds to their corresponding hydroxylamines,16
Aromatic amines, widely used as intermediates in the
preparation of important chemicals such as dyes, phar-
maceuticals, and agricultural chemicals, can be obtained
easily by the reduction of aromatic nitro compounds using
catalytic hydrogenation1 and a variety of other reduction
conditions. Many reducing agents have been used to
reduce aromatic nitro compounds with the most classic
being zinc, tin, or iron in the presence of an acid.2 Other
reagents used include hydrazine,3 Ru3(CO)12,4 TiCl4-
dialkyl telluride,5 (C2H5O)2PCl,6 metal hydride com-
plexes7 (e.g., NaBH4-NiCl2), and sulfides8 (e.g., sodium
sulfhydrate, ammonium sulfide, or polysulfides). Aro-
matic nitro compounds can also be reduced electrochemi-
cally9 or enzymatically.10 Most chemical methods, how-
ever, lack the desired chemoselectivity over other
* To whom correspondence should be addressed. Tel: (732) 445-
5291; fax: (732) 445-6312.
(1) For a review, see Tafesh, A. M.; Weiguny, J . Chem. Rev. 1996,
96, 2035-2052.
(11) (a) Rondestvedt, C. S.; J ohnson, T. A. Synthesis, 1977, 850-
851. (b) Entwistle, I. D.; Gilkerson, T.; J ohnstone, R. A. W.; Telford,
R. P. Tetrahedron 1978, 34, 213-215. (c) Oxley, P. W.; Adger, B. M.;
Sasse, M. J .; Forth, M. A. Org. Synth. 67, 187-192. (d) Yanada, K.;
Yamaguchi, H.; Meguri, H.; Uchida, S. J . Chem. Soc., Chem. Commun.
1986, 1655-1657. (e) Feuer, H.; Bartlett, R. S.; Vincent, B. F.;
Anderson R. S. J . Org. Chem. 1965, 30, 2880-2882.
(12) (a) Furst, A.; Moore, R. E. J . Am. Chem. Soc. 1957, 79, 5492-
5493. (b) Olah, G. A. J . Am. Chem. Soc. 1959, 81, 3165.
(13) Hutchins, R. O.; Lamson, D. W.; Rua, L.; Milewski, C.; Mary-
anoff, B. J . Org. Chem. 1971, 36, 803-806.
(14) (a) Osuka, A.; Shimizu, H.; Suzuki, H. Chem. Lett. 1983, 1373-
1374. (b) Azoo, J . A.; Grimshaw, J . J . Chem. Soc., Sect. C 1968, 2403-
2405. (c) Ohe, K.; Uemura, S.; Sugita, N.; Masuda, H.; Taga, T. J . Org.
Chem. 1989, 54, 4169-4174. (d) Ogata, Y.; Mibae, J . J . Org. Chem.
1962, 27, 2048-2052. (e) Hutton, J .; Waters, W. A. J . Chem. Soc. B
1968, 191-195. (f) Porta, F.; Pizzotti, M.; Cenini, S. J . Organomet.
Chem. 1981, 222, 279-281.
(2) (a) Coleman, G. H.; McClosky, S. M.; Suart, F. A. Organic
Syntheses; Wiley: New York, 1955; Collect. Vol. III, p 668-670. (b)
Hartman, W. W.; Dickey, J . B.; Stampfli, J . G. Organic Syntheses;
Wiley: New York, 1943; Collect. Vol. II, pp 175-178. (c) The reaction
with iron is called the Bechamp reduction. For a review of this reaction,
see Hamilton, C. S.; Morgan, J . F. Org. React. 1944, 2, 428-431.
(3) (a) Adger, B. M.; Young, R. G. Tetrahedron Lett. 1984, 25, 5219-
5222. (b) Yuste, F.; Saldana, M.; Walls, F. Tetrahedron Lett. 1982, 23,
147-148.
(4) Alper, H.; Amaratunga, S. Tetrahedron Lett. 1980, 21, 2603-
2604.
(5) Suzuki, H.; Hanazaki, Y. Chem. Lett. 1986, 549-550.
(6) Fischer, B.; Sheihet, L. J . Org. Chem. 1998, 63, 393-395.
(7) Banfi, L.; Narisano, E.; Riva, R. In Handbook of Reagents for
Organic Synthesis; Burke, S. D., Danheiser, R. L., Eds., Wiley Press:
New York, 1999; pp 397-398.
(8) The reaction with sulfides or polysulfides is called the Zinin
reduction. For a review, see Poter, H. K. Org. React. 1973, 20, 455-
481.
(9) Chapuzet, J . M.; Labrecque, R.; LaVoie, M. J . Chim. Phys. PCB
1996, 93, 601-610.
(10) Davidenko, T. I.; Bondarenko, G. I. Appl. Biochem. Microbiol.
(Engl. Transl.) 2000, 36, 63-68.
(15) Hu, L.; Liu, B.; Hacking, D. R. Bioorg., Med. Chem. Lett. 2000,
10, 797-800.
(16) Kende, A. S.; Mendoza, J . S. Tetrahedron Lett. 1991, 32, 1699-
1702.
(17) Sturgess, M. A.; Yarberry, D. J . Tetrahedron Lett. 1993, 34,
4743-4746.
10.1021/jo005666q CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/11/2001

920 J . Org. Chem., Vol. 66, No. 3, 2001
Yu et al.
amines,19 and azo derivatives.19a On the other hand,
samarium(0) metal has rarely been used directly to
reduce nitro groups because the samarium atom on the
accessible solid surface is not sufficiently reactive.20 To
activate samarium(0) metal, additives such as HgCl2,21
TMSCl,22 NH4Cl (aq),23 allyl iodide,24 or iodine25,26 have
to be used. Samarium(0) in the presence of a catalytic
amount of allyl iodide was reported to reduce nitroarenes
to azoxyarenes.24 The only method in the literature for
the reduction of aromatic nitro compounds to aromatic
amines and hydrazines using samarium metal was
carried out in the presence of a catalytic amount of iodine
in a mixed solvent of aqueous ammonium chloride and
tetrahydrofuran (THF).25 This method gave only moder-
ate yields of their corresponding amine products (usually
<74%) with significant formation of hydrazine byproducts
and exhibited selectivity in reducing nitro groups over
aryl halides and aryl amides. This method might also
lack selectivity over common protecting groups of alcohols
and amines. For a similar condition, using samarium(0)
and a stoichiometric amount of iodine in methanol was
reported to selectively and efficiently remove alkyloxy-
carbonyl protecting groups of alcohols and lactams.26
1,1′-Dialkyl-4,4′-bipyridinium halides, sometimes re-
ferred to as viologens, have been used as electron-transfer
catalysts in the photoreduction of protons to produce
hydrogen27 as well as in the reduction of various organic
compounds.28 In one report, 1,1′-dioctyl-4,4′-bipyridinium
dibromide was used as an electron-transfer catalyst in
the sodium dithionite reduction of nitroarenes in the
presence of potassium carbonate in dichloromethane
(CH2Cl2)/water (8:1).26i This method was found to be
selective for the reduction of aromatic nitro groups over
alkene and nitrile.
Sch em e 1
Sch em e 2
samarium(0) metal and that this combination might
provide a mild chemical method for the selective reduc-
tion of aromatic nitro groups. In this paper, we report
the development and the chemical selectivity of such an
efficient electron-transfer system using samarium(0) and
1,1′-dioctyl-4,4′-bipyridinium dibromide.
Resu lts a n d Discu ssion
In a typical experiment, 1,1′-dioctyl-4,4′-bipyridinium
dibromide (5% mol) and samarium(0) powder (2 equiv)29
were added sequentially to a solution of aromatic nitro
substrate (1 mmol) in anhydrous methanol (MeOH). The
reaction mixture was stirred at room temperature, and
the reaction progress was monitored by TLC. Our experi-
ments indicated that the combination of samarium(0)
powder and 1,1′-dioctyl-4,4′-bipyridinium dibromide ef-
fectively reduced at room-temperature aromatic nitro
compounds to their corresponding amines and that the
reduction was selective over a number of common func-
tional groups that might be present in a substrate
(Scheme 1). Other functional groups unaffected by this
process include alkene, azide, benzyl ether, nitrile, amide,
halide, p-toluenesulfonamide, t-Boc, tert-butyldiphenyl-
silyl ether, and aliphatic nitro groups.
The addition of samarium(0) powder to a solution of
1,1′-dioctyl-4,4′-bipyridinium dibromide in methanol gave
instantly a deep blue suspension, a phenomenon similar
to mixing sodium dithionite (Na2S2O4) with a biphasic
solution of 1,1′-dioctyl-4,4′-bipyridinium dibromide in
CH2Cl2-H2O.28i The deep blue color of the suspension is
also similar to that of SmI2 in THF. Both solutions of
dithionite-1,1′-dioctyl-4,4′-bipyridinium and SmI2 in THF
were believed to contain a blue radical species that acts
as a reducing agent. On the basis of our observation of
the formation of the blue suspension, we concluded that
the reaction occurring on the surface of samarium(0)
metal was an electron-transfer reaction. As shown in
Scheme 2, electron transfer from samarium(0) to the
pyridine rings in 1,1′-dioctyl-4,4′-bipyridinium dibromide
led to the formation of a hyperconjugated radical cation
(1) that is responsible for the blue color. This is consistent
with an earlier mechanistic study by Okawara et al. that
demonstrated the immediate formation of a similar blue
On the basis of these earlier reports, we decided to test
the hypothesis that 1,1′-dialkyl-4,4′-bipyridinium dibro-
mide might act as an electron-transfer catalyst to activate
(18) (a) Namy, J . L.; Girard, P.; Kagan, H. S. New J . Chem. 1977,
1, 5-7. (b) Molander, G. A. Chem. Rev. 1992, 92, 29-68. (c) Molander,
G. A. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I.,
Eds.; Pergamon: Oxford, 1991; Vol. 1, pp 251-283. (d) Soderquist, J .
A. Aldrichim. Acta 1991, 24, 15-23. (e) Kagan, H. B. New J . Chem.
1990, 14, 453-436. (f) Kagan, H. B.; Sasaki, M.; Collin, J . Pure Appl.
Chem. 1988, 60, 1725-1728. (g) Kagan, H. B.; Namy, J . L. Tetrahedron
1986, 42, 6573-6614. (h) Hasegawa, E.; Curran, D. P. J . Org. Chem.
1993, 58, 5008-5010 and references therein.
(19) (a) Souppe, J .; Danon, L.; Namy, J . L.; Kagan, H. B. J .
Organomet. Chem. 1983, 250, 227-236. (b) Zhang, Y.; Lin, R. Synth.
Commun. 1987, 17, 329-332.
(20) Hou, Z.; Taniguchi, H.; Fujiwara, Y. Chem. Lett. 1987, 305-
308.
(21) (a) Molander, G. A.; Harring, L. S. J . Org. Chem. 1989, 54,
3525-3532. (b) Clive, D. L. J .; Daigneault, S. J . Org. Chem. 1991, 56,
3801-3814.
(22) (a) Wang, L.; Zhang, Y. Tetrahedron 1998, 54, 11129-11130.
(b) Lautens, M.; Ren, Y. J . Org. Chem. 1996, 61, 2210-2214.
(23) Wang, L.; Zhang, Y. Tetrahedron Lett. 1998, 39, 5257-5260.
(24) Hou, Z.; Fujiwara, Y.; Taniguchi, H. J . Org. Chem. 1988, 53,
3118.
(25) Wang, L.; Zhou, L.; Zhang, Y. Synlett 1999, 1065-1066.
(26) Yanada, R.; Negoro, N.; Bessho, K.; Yanada, K. Synlett 1995,
1261-1263.
(27) Endo, T.; Saotome, Y.; Okawara, M. J . Am. Chem. Soc. 1984,
106, 1124-1125.
(28) (a) Tomioka, H.; Ueda, K.; Ohi, H.; Izawa, Y. Chem. Lett. 1986,
1359-1362. (b) Maidan, R.; Goren, Z.; Becker, J . Y.; Willner, I. J . Am.
Chem. Soc. 1984, 106, 6217-6222. (c) Endo, T.; Saotome, Y.; Okawara,
M. Tetrahedron Lett. 1985, 26, 4525-4526. (d) Saotome, Y.; Endo, T.;
Okawara, M. Macromolecules 1983, 16, 881-884. (e) Endo, T.; Ageishi,
K.; Okawara, M. J . Org. Chem. 1986, 51, 4309-4310. (f) Park, K. K.;
Lee, C. W.; Oh, S.-Y.; Park, J . W. J . Chem. Soc., Perkin Trans. 1 1990,
2356-2357. (g) Park, K. K.; J oung, W. K.; Choi, S. Y. Bull. Korean
Chem. Soc. 1993, 14, 461-465. (h) Park, K. K.; Oh, C. H.; J oung, W.
K. Tetrahedron Lett. 1993, 34, 7445-7446.
(29) It should also be noted that excess samarium(0) (up to 3-7
equiv) did not adversely affect the reduction and could help offset the
samarium(0) loss caused by oxidation during storage. Therefore, a little
excess samarium(0) could increase the yield of product isolated if old
samarium(0) powder was used.

Chemoselective Reduction of Aromatic Nitro Groups
J . Org. Chem., Vol. 66, No. 3, 2001 921
Ta ble 1. Ch em oselective Red u ction of Nitr o Com p ou n d s
to Cor r esp on d in g Am in esa
were reduced slowly relative to the other substrates
because of their poor solubility in MeOH. When a less
polar solvent such as THF, diethyl ether (Et2O), or CH2-
Cl2 was added as a cosolvent to increase the solubility of
these substrates, the formation of the deep blue species
was rather slow, and the reduction of nitro compounds
was adversely affected. While mixed solvents of THF and
MeOH were often the solvent of choice in reductions
employing SmI2,19a the addition of a less polar solvent
such as THF inhibited the electron-transfer process
between samarium(0) and 1,1′-dioctyl-4,4′-bipyridinium
dibromide.
As shown in Table 1, our method selectively reduced
aromatic nitro compounds to their corresponding amines
in satisfactory yields (79-99%) in the presence of other
functional groups such as cyano (6), amide (10, 12, and
26), alkenyl (14), azido (16), benzyl ether (18), halide (20,
22, and 24), and aliphatic nitro (28), groups which might
otherwise be reduced using other reduction conditions.
The chemoselectivity in the reduction of aromatic nitro
group versus cyano group in compound 6 using our
process is the opposite of that reported using NaBH4-
AlCl3 complex in diglyme.30 Protecting groups, such as
tert-butyldiphenylsilyl (TBDPS) (in 8), tert-butoxycar-
bonyl (t-Boc) (in 12), and p-toluenesulfonyl (Tos) (in 26),
are unaffected under the slightly basic condition (pH 7-8)
in this procedure. The cleavage of TBDPS that was
reported under basic conditions31 was not observed using
our method. It should also be noted that t-Boc was not
affected under our condition while Sm(0)-I2/MeOH could
remove this protecting group in almost quantitative
yield.26 The p-toluenesulfonamides that could be removed
using samarium(II) iodide32 was stable under our condi-
tion. Furthermore, the fact that reduction of 1-nitro-2-
(2-nitroethyl)benzene 28 affords only the 2-(2-nitroethyl)-
aniline product 29 in good yield (83%) indicates that our
reduction condition is selective for the aromatic over the
aliphatic nitro groups. To compare the mildness and
selectivity of our method with classical hydrogenation,
we examined reductions of compounds 22 and 28 using
atmospheric hydrogenation at room temperature in the
presence of 5% Pd-C. This hydrogenation procedure was
found to produce a complex product mixture within 1.5
h for each of the two compounds examined. This result
indicates that our condition is superior to hydrogenation
and should be useful in the selective reduction of aro-
matic nitro groups over halides or aliphatic nitro groups.
In comparison with the method using Sm(0)-I2(cat.)/
THF-NH4Cl (aq) for the reduction of aromatic nitro
compounds, our method gave better product yields for the
same substrates (78-85% versus 56-74%). The differ-
ences in yield and selectivity could be attributed to a
unique mechanism of action where the hyperconjugated
radical cation (1) formed by Sm(0) and 1,1′-dioctyl-4,4′-
bipyridinium dibromide is the real, active reducing agent
rather than Sm(II) species. This was further supported
by the reduction of 26 and 28, where both the tosyl
protecting group in 26 and aliphatic nitro group in 28
radical cation when zinc powder was mixed with 1,1′-
dipropyl-4,4′-bipyridinium dibromide in acetonitrile/
water (49:1).28c In our method, the strong reduction
potential of Sm(0) (E° (Sm2+/Sm0) ) -2.68 V, E° (Sm3+
/
Sm0) ) -2.30 V) and Sm(II) (E° (Sm3+/Sm2+) ) -1.55 V)
favors the electron-transfer process from samarium(0) to
bipyridinium ion, leading to the formation of the radical
cation.19a
The scope of this reduction reaction is illustrated in
Table 1, where we examined a series of aromatic nitro
compounds with a variety of substituents. Electron-
withdrawing groups such as cyano (in 6), amide (in 10,
12, and 26), alkenyl (in 14), chloro (in 20 and 22), and
bromo (in 24) did not adversely affect the reduction of
nitro groups. In general, the time required for the
reduction as monitored by TLC ranges from 30 min to
about 28 h depending on the substrate. The rate of
reduction is limited by the solubility of the substrate in
MeOH, which is the solvent of choice in this process. As
shown in Table 1, tert-butyl-(2-nitrobenzyloxy)diphenyl-
silane (8) and 1-(2-benzyloxyethyl)-4-nitrobenzene (18)
(30) Brown, H. C.; Rao, B. C. S. J . Am. Chem. Soc. 1956, 78, 2582-
2588.
(31) (a) Torisawa, Y.; Shibasaki, M.; Ikegami, S. Chem. Pharm. Bull.
1983, 31, 2607-2615. (b) Wood, W. W.; Rashid, A. Tetrahedron Lett.
1987, 28, 1933-1936. (c) Malik, A. A.; Cormier, R. J .; Sharts, C. M.
Org. Prep. Proced. Int. 1986, 18, 345-352. (d) Shekhani, M. S.; Khan,
K. M.; Mahmood, K.; Shah, P. M.; Malik, S. Tetrahedron Lett. 1990,
31, 1669-1670.
(32) Vedejs, E.; Lin, S. J . Org. Chem. 1994, 59, 1602-1603.

922 J . Org. Chem., Vol. 66, No. 3, 2001
Yu et al.
Ta ble 2. Effect of Meta l in th e Red u ction of Nitr o
Com p ou n d 8a
Sch em e 3
metal
equivalentsb
time (h)
yield (%)c
Sm
Zn
Zn-Cu
Mg
In
2
3
3
3
3
3
3
16
24
18
14
24
24
24
92
0
61
45
5
Fe
Sn
0
10
Sch em e 4
a
All the reactions were carried out with tert-butyl-(2-nitroben-
zyloxy)diphenylsilane 8 (0.2 mmol), 1,1′-dioctyl-4,4′-bipyridinium
dibromide (5 mol %) and the given metal in MeOH (10 mL) at
b
room temperature. Equivalents of metal were calculated based
on the substrate 8. c Isolated yield of amine product 9.
example, the Sm(II) species could be formed in the
activation process of samarium(0) metal. Rather than
transferring its electron to dioctyl bipyridinium ion, the
Sm(II) formed could reduce directly the nitro groups or
their reduced intermediates to the hydrazine byproducts
as has been reported in the literature.17
were not affected using our method but could be removed
or reduced by Sm(II) reducing agents.16,32
To study the role of samarium(0) in our method, we
examined the effect of several other metals in the
reduction of tert-butyl-(2-nitrobenzyloxy)diphenylsilane
8 under the same condition. As shown in Table 2, using
samarium(0) metal in the presence of 1,1′-dioctyl-4,4′-
bipyridinium dibromide, compound 8 was reduced to the
amine product 9 in 92% yield. Among the other metals
tested, only magnesium turning and zinc-copper couple
were sufficiently reactive to reduce compound 8 to give
the amine product 9 in 45% and 61% yield, respectively.
While the other metals including zinc, indium, iron, and
tin failed to reduce 8 in the presence of 1,1′-dioctyl-4,4′-
bipyridinium dibromide. This result suggests that sa-
marium(0) metal plays an important role in this reduc-
tion system. Also, we found that samarium(0) in MeOH
in the absence of 1,1′-dioctyl-4,4′-bipyridinium dibromide
would not effect the reduction of aromatic nitro com-
pounds, suggesting that a catalytic amount of 1,1′-dioctyl-
4,4′-bipyridinium dibromide is essential in the activation
of samarium(0) metal.
Reduction of N-t-Boc-5-nitro-o-toluidine (12) not only
afforded the desired amine 13 as the major product, but
also gave as a minor product symmetrical hydrazine 30
in 11% yield. Similarly, reduction of 3-nitrostyrene (14)
gave the desired amine 15 as the major product and N,N′-
bis(3-vinylphenyl)hydrazine 31 as the minor product in
8% yield (Scheme 3).
The formation of these hydrazine intermediates is often
observed in the reduction of nitro groups. Nitro group
reduction is believed to involve several intermediates
including nitroso compounds and hydroxylamines.33 Simi-
larly, under our condition using samarium(0) powder and
1,1′-dioctyl-4,4′-bipyridinium dibromide, a nitro com-
pound (32) could be reduced to the corresponding amine
(35) through the nitroso (33) and the hydroxylamine (34)
intermediates as shown in Scheme 4. Condensation of
the reduced intermediates and the amine product fol-
lowed by subsequent reduction could explain the forma-
tion of the hydrazine byproduct under our condition as
well as conditions reported by others.11a,19a
In conclusion, we developed a novel electron-transfer
system using a combination of samarium(0) and 1,1′-
dioctyl-4,4′-bipyridinium dibromide in the chemoselective
reduction of aromatic nitro compounds. In comparison
with SmI2 reduction of nitro groups, our method is
superior in terms of both chemoselectivity and efficiency.
SmI2 is known to reduce alkyne, alkene,18b and halides34
and cleave p-toluenesulfonamide and t-Boc protecting
groups. In addition, SmI2 is difficult to handle because
of its high sensitivity to air and moisture and its poor
solubility in THF. Our method using samarium(0) as a
reducing agent not only avoids the cumbersome prepara-
tion required in the case of SmI2, but also takes full
advantage of the reduction potential of samarium(0)
metal. Furthermore, the use of 1,1′-dioctyl-4,4′-bipyri-
dinium dibromide as an electron-transfer catalyst makes
our method more selective and mild; protecting groups
such as silyl ether, amide, Tos and t-Boc are not cleaved
in our condition. Furthermore, our method is selective
for the reduction of aromatic, not aliphatic, nitro groups.
Thus, the method that we developed represents a mild,
As outlined in Scheme 4, the product amine (35) could
condense under certain conditions with nitrosoarene
intermediate (33) to produce azoarene intermediate (37),
which could be further reduced to the hydrazine byprod-
uct (38).11a Alternatively azoarene (37) could be produced
from the reduction of azoxyarene intermediate (36),19a
which is formed by condensation of nitrosoarene (33) with
N-arenylhydroxylamine (34). The fact that we did not
isolate any azoxyarene intermediate (36) suggests that
the latter pathway is less likely to occur or that the
azoxyarene intermediate formed is readily reduced fur-
ther. However, prolonged treatment of the hydrazine
compounds 30 and 31 with samarium(0) and 1,1′-dioctyl-
4,4′-bipyridinium dibromide failed to convert them to
their corresponding amine products. These results indi-
cate that once the N-N bond is formed, it is stable under
our reduction condition. This is consistent with our
observation that the azido group in compound 16 could
survive the reduction condition we developed. It should
also be noted that we could not exclude the formation of
the byproduct hydrazines through other mechanisms. For
(34) Molander, G. A.; Harris, C. R. In Handbook of Reagents for
Organic Synthesis; Burke, S. D., Danheiser, R. L., Eds., Wiley Press:
New York, 1999; pp 354-358.
(33) March, J . Advanced Organic Chemistry; Wiley-Interscience:
New York, 1992; pp 1216-1217.

Chemoselective Reduction of Aromatic Nitro Groups
J . Org. Chem., Vol. 66, No. 3, 2001 923
compounds and their spectroscopic data and melting points
matched those of commercially available material.
efficient, and economical condition that should prove
useful in functional group transformation in organic
synthesis.
ter t-Bu tyl-(2-n itr oben zyloxy)d ip h en ylsila n e 8. To a
solution of 2-nitrobenzyl alcohol (5.32 g, 33.6 mmol) in
anhydrous DMF (60 mL) were added imidazole (2.74 g, 40.3
mmol) and TBDPSCl (8.6 mL, 33.6 mmol). The reaction
mixture was stirred at room temperature for 2 h, quenched
with water (50 mL), and extracted with Et2O (3 × 60 mL).
The combined organic phase was washed with brine (50 mL),
dried over anhydrous MgSO4, filtered through a pad of cotton,
and concentrated in vacuo. The residue was purified by flash
column chromatography on silica gel with EtOAc and hexanes
as the eluent (hexanes:ethyl acetate ) 11:1 f 10:1 f 9:1) to
give 8 as a white solid (12.86 g, 98%). Rf ) 0.35 (hexanes:ethyl
Exp er im en ta l Section
Gen er a l Meth od s. All glassware used in our experiments
was evacuated, flame-dried, and flushed with argon before use.
Reactions were performed under argon, unless otherwise
noted. Solvents were either ACS reagent grade or HPLC grade
and were subjected to the following purification procedures
before use. Et2O and THF were freshly distilled from sodium/
benzophenone under argon. MeOH was redistilled from sodium
methoxide and stored over 4 Å molecular sieves. CH2Cl2 was
freshly distilled from calcium hydride. Triethylamine (Et3N)
was distilled from calcium hydride and stored over potassium
hydroxide. CDCl3 was stored over magnesium turning and
filtered through a short column packed with basic aluminum
oxide. N,N-Dimethylformamide (DMF) was stored over anhy-
drous 4 Å molecular sieves for at least one week before use.
Pyridine was dried over potassium hydroxide for 24 h at room
temperature followed by distillation over CaH2 under nitrogen
and then stored over 4 Å molecular sieves in the darkness.
p-Toluenesulfonyl chloride was recrystallized from hexanes
before use. 1,1′-Dioctyl-4,4′-bipyridinium dibromide was pur-
chased from TCI America (Portland, OR). Samarium(0) powder
and other commercially available compounds were purchased
from Aldrich Chemical Co. (Milwaukee, WI) and used without
further purification.
1
acetate ) 9:1); mp 75-77 °C; H NMR (CDCl3) δ 8.16-8.10
(m, 2H), 7.79-7.71 (m, 5H), 7.52-7.28 (m, 7H), 5.21 (s, 2H),
1.19 (s, 9H); 13C NMR (CDCl3) δ 138.0, 135.7, 134.1, 133.1,
130.1, 128.2, 128.0, 127.7, 124.8, 63.1, 27.1, 19.6; IR (KBr):
3056.4, 2964.1, 2923.1, 1517.9, 1333.3 cm-1; FABMS (in
3-nitrobenzyl alcohol matrix) m/z (relative intensity) 392 (6,
MH+), 334 (100), 314 (29), 197 (42).
2-(ter t-Bu t yld ip h en ylsila n yloxym et h yl)p h en yla m in e
9. Yield, 92%; Rf ) 0.3 (hexanes:ethyl acetate ) 3:1); 1H NMR
(CDCl3) δ 7.80-7.76 (m, 4H), 7.52-7.41 (m, 6H), 7.18 (t, J )
7.1 Hz, 1H), 6.96 (dd, J ) 7.3, 1.5 Hz, 1H), 6.79-6.70 (m, 2H),
4.80 (s, 2H), 4.30 (br s, 2H, N-H), 1.13 (s, 9H); 13C NMR
(CDCl3) δ 146.2, 135.8, 133.4, 130.0, 128.8, 128.7, 127.9, 125.0,
118.1, 115.8, 65.6, 27.0, 19.4; IR (neat): 3456.4, 3374.3 cm-1
;
FABMS (in 3-nitrobenzyl alcohol matrix) m/z (relative inten-
sity) 361 (32, M+), 304 (100); HRMS (FAB) (in 3-nitrobenzyl
alcohol matrix) m/z calcd for C23H27NOSi (M+) 361.1862, found
361.1862.
Unless otherwise stated, all reactions were magnetically
stirred and monitored by thin-layer chromatography (TLC)
using 0.25 mm Whatman precoated silica gel plates. TLC
plates were visualized using either 7% (w/w) ethanolic phos-
phomolybdic acid or 1% (w/w) aqueous potassium permagnate
containing 1% (w/w) NaHCO3. Flash column chromatography
was performed using silica gel (Merck 230-400 mesh). Yields
were based on the chromatographically and spectroscopically
pure compounds.
(2-Meth yl-5-n itr op h en yl)ca r ba m ic Acid ter t-Bu tyl Es-
ter 12. A solution of 5-nitro-o-toluidine (350 mg, 2.3 mmol) in
MeOH (20 mL) was charged with NaHCO3 (396 mg, 4.6 mmol)
and (t-Boc)2O (510 mg, 2.3 mmol). The reaction mixture was
sonicated for 4 h and evaporated in vacuo to remove most of
the solvent. The residue was diluted with EtOAc (200 mL) and
brine (60 mL). The organic phase was dried over anhydrous
Na2SO4, filtered through a pad of cotton, and concentrated in
vacuo. The residue was purified by flash column chromatog-
raphy on silica gel with EtOAc and hexanes as the eluent
(hexanes:EtOAc ) 7:1 f 6:1 f 5:1) to give 12 as a brownish
crystal (487 mg, 85%). Rf ) 0.45 (hexanes:EtOAc ) 3:1); mp
143-144 °C;1H NMR (CDCl3) δ 8.81 (d, J ) 2.6 Hz, 1H), 7.82
(dd, J ) 16.4, 2.6 Hz, 1H), 7.30-7.26 (m, 2H), 6.51 (br s, 1H),
2.35 (s, 3H), 1.55 (s, 9H); 13C NMR (CDCl3) δ 152.5, 147.2,
137.6, 133.7, 130.9, 118.0, 114.9, 81.8, 28.4, 18.2; IR (KBr):
3342.5, 1697.5 cm-1; FABMS (in 3-nitrobenzyl alcohol matrix)
m/z (relative intensity) 252 (47, M+), 197 (100), 180 (9).
(5-Am in o-2-m eth ylp h en yl)ca r ba m ic Acid ter t-Bu tyl
Ester 13. Yield, 79%; Rf ) 0.25 (hexanes:ethyl acetate ) 3:1);
1H NMR (CDCl3) δ 7.34 (d, J ) 1.6 Hz, 1H), 6.89 (d, J ) 8.2
Hz, 1H), 6.33 (dd, J ) 2.6, 8.0 Hz), 6.28 (br s, 1H), 3.58 (br s,
2H), 2.13 (s, 3H), 1.52 (s, 9H); 13C NMR (CDCl3) δ 153.0, 145.5,
137.2, 131.0 116.2, 110.3, 107.2, 80.5, 28.6, 17.0; IR (KBr):
3425.6, 3333.3, 3312.8, 1692.3 cm-1; FABMS (in 3-nitrobenzyl
alcohol matrix) m/z (relative intensity) 222 (80, M+), 166 (100);
HRMS (FAB) (in 3-nitrobenzyl alcohol matrix) m/z calcd for
Melting points were determined using a Mel-Temp ap-
paratus and are uncorrected. Infrared spectra were recorded
with a Perkin-Elmer model 1600 series FTIR spectrometer
using polystyrene as an external standard. Infrared absor-
1
bance is reported in reciprocal centimeters (cm-1). All H and
13C NMR spectra were recorded on a Varian Gemini 200 MHz
spectrometer at ambient temperature. Chemical shifts (200
MHz for 1H and 50 MHz for 13C) are reported in parts per
1
million (δ) relative to CDCl3 (δ 7.24 for H and 77.0 for 13C) or
DMSO-d6 (δ 2.49 for 1H and 39.5 for 13C). Coupling constants
(J values) are given in hertz (Hz). Mass spectral data were
obtained from the University of Kansas Mass Spectrometry
Laboratory (Lawrence, KS).
Gen er a l P r oced u r es for th e Red u ction of Nitr o Com -
p ou n d s. A flame-dried, 50 mL, round-bottomed, two-neck
flask equipped with a Teflon-coated magnetic stirring bar was
flushed with dried argon and charged with substrate (1 mmol)
in anhydrous MeOH (30 mL). To the well-stirred clear solution
were added sequentially a catalytic amount of 1,1′-dioctyl-4,4′-
bipyridinium dibromide (22 mg, 0.05 mmol) and Sm (301 mg,
2 mmol). The deep blue suspension formed was stirred
vigorously at room temperature until TLC showed disappear-
ance of the starting material. At the end of reaction, saturated
aqueous NH4Cl (50 mL) and Et2O (100 mL) were added to
partition the reaction mixture. The aqueous phase was sepa-
rated and extracted with Et2O (2 × 50 mL). The combined
ether phase was washed with brine (50 mL) and dried over
anhydrous MgSO4. After filtration and removal of organic
solvent under reduced pressure, the residue was purified by
flash column chromatography using ethyl acetate (EtOAc)/
hexanes (0.5% Et3N) to give the desired aromatic amine
product.35 Compounds 3, 5, 7, 11, 21, 23, and 25 are known
C
12H18N2O2 (M+) 222.1368, found 222.1368; N, N′-Bis-[3-(t-
Boc-a m in o)-4-m et h ylp h en yl]h yd r a zin e 30. Yield, 11%;
Rf ) 0.5 (hexanes:ethyl acetate ) 3:1); mp 114-116 °C; 1H
NMR (CDCl3) δ 8.30 (s, 1H), 7.73 (dd, J ) 15.8, 2.2 Hz, 1H),
7.41 (d, J ) 8.0 Hz, 1H), 7.29 (s, 1H), 6.45 (s, 1H), 2.35 (s,
3H), 1.55 (s, 9H); 13C NMR (CDCl3) δ 166.1, 152.7, 137.7, 135.1,
131.1, 118.1, 111.3, 81.5, 28.5, 18.5; IR (neat): 3333.3, 2974.3,
1692.3 cm-1; FABMS (in 3-nitrobenzyl alcohol matrix) m/z
(relative intensity) 442 (3, M+), 327 (21), 316 (50), 222 (2), 181
(100); HRMS (FAB) (in 3-nitrobenzyl alcohol matrix) m/z calcd
for C24H34N4O4 (M+) 442.2580, found 442.2608.
3-Vin ylp h en yla m in e 15.36 Yield, 82%; Rf ) 0.35 (hexanes:
1
ethyl acetate ) 4:1); H NMR (CDCl3) δ 7.13 (t, J ) 18 Hz,
1H), 6.83 (d, J ) 8 Hz, 1H), 6.73 (s, 1H), 6.70-6.55 (m, 2H),
5.70 (d, J ) 18 Hz, 1H), 5.20 (d, J ) 11 Hz, 1H), 3.85 (br s,
2H); 13C NMR (CDCl3) δ 146.4, 138.1, 136.7, 128.9, 116.2,
(35) Because aromatic amine products are known to be unstable
upon exposure to air and light, all products should be handled with
care and stored under argon at low temperature.

924 J . Org. Chem., Vol. 66, No. 3, 2001
Yu et al.
114.3, 113.1, 112.2; IR (neat): 3435.9, 3353.8, 3210.2, 1620.5,
4-Meth yl-N-(4-n itr op h en yl)ben zen esu lfon a m id e 26.40
To a solution of p-nitroaniline (138 mg, 1 mmol) in dry pyridine
(10 mL) were added p-toluenesulfonyl chloride (191 mg, 1
mmol) and 4-(dimethylamino)pyridine (12 mg, 0.1 mmol) at
room temperature. The reaction mixture was heated to 80 °C
and stirred at this temperature for 2 h. Ethyl ether (100 mL)
and saturated aqueous CuSO4 solution (50 mL) were added
to partition the reaction mixture. The organic phase was
washed with saturated aqueous CuSO4 solution (2 × 50 mL)
and brine (2 × 50 mL), dried over anhydrous MgSO4, filtered
through a pad of cotton, and concentrated in vacuo. The crude
product was pure without any further purification. mp (etha-
nol): 189-190 °C; [lit. 189-190 °C (ethanol)]; 1H NMR
(DMSO-d6) δ 11.21 (br s, 1H), 8.15-8.10 (m, 2H), 7.76 (dd,
J ) 10.0, 1.8 Hz, 2H), 7.40-7.28 (m, 4H), 2.33 (s, 3H).
N-(4-Am in op h en yl)-4-m eth ylben zen esu lfon a m id e 27.40
Yield, 96%; Rf ) 0.25 (hexanes:ethyl acetate ) 3:2); mp
1600 cm-1; N, N′-Bis-(3-vin ylp h en yl)h yd r a zin e 31.37 Yield,
1
8%; Rf ) 0.5 (hexanes:ethyl acetate ) 4:1); mp 95-97 °C; H
NMR (CDCl3) δ 7.20 (t, J ) 8.0 Hz, 1H), 6.95-6.92 (m, 2H),
6.81-6.77 (m, 1H), 6.74-6.60 (m, 1H), 5.76 (d, J ) 1.0 Hz,
1H), 5.68-5.66 (m, 1H), 5.23 (dd, J ) 11.6, 0.8 Hz, 1H); 13C
NMR (CDCl3) δ 149.3, 138.9, 137.1, 129.7, 118.3, 114.1, 112.1,
110.2; IR (KBr): 3343.6, 3087.2, 1605.1, 1574.3.
1-(2-Azid oeth yl)-4-n itr oben zen e 16.38 To a solution of
4-nitrophenethyl alcohol (1.23 g, 7.4 mmol) in anhydrous DMF
(50 mL) were added with stirring sodium azide (1.91 g, 29.6
mmol) and 4-(dimethylamino)pyridine (899 mg, 7.4 mmol)
followed by bis(2,4-dichlorophenyl) chlorophosphate (2.99 g,
7.4 mmol). The reaction mixture was stirred for 24 h at room
temperature and quenched with water (60 mL). The aqueous
reaction mixture was extracted with Et2O (3 × 60 mL). The
combined organic phase was dried over anhydrous MgSO4,
filtered through a pad of cotton, and concentrated in vacuo.
The residue was purified by flash column chromatography on
silica gel with EtOAc and hexanes as the eluent (hexanes:ethyl
acetate ) 6:1 f 5:1 f 4:1) to give 16 as an oil (1.17 g, 83%).
Rf ) 0.2 (hexanes:ethyl acetate ) 6:1); 1H NMR (CDCl3) δ
8.20-8.15 (m, 2H), 7.41 (dd, J ) 17.6, 0.8 Hz, 2H), 3.60 (dd,
J ) 15.2, 6.4 Hz, 2H), 3.00 (t, J ) 6.8 Hz, 2H); 13C NMR
(CDCl3) δ 147.1, 146.1, 129.9, 124.0, 51.9, 35.3; IR (neat):
2092.3, 1517.9 cm-1; FABMS (in 3-nitrobenzyl alcohol matrix)
m/z (relative intensity) 193 (17, MH+), 165 (7), 154 (100).
4-(2-Azid oeth yl)p h en yla m in e 17. Yield, 92%; Rf ) 0.3
(hexanes:ethyl acetate ) 4:1); 1H NMR (CDCl3) δ 7.03 (dd,
J ) 12.8, 2.0 Hz, 2H), 6.71-6.65 (m, 2H), 3.64 (br s, 2H), 3.46
(t, J ) 7.4 Hz, 2H), 2.81 (t, J ) 7.1 Hz, 2H); 13C NMR (CDCl3)
δ 145.3, 129.8, 128.0, 115.5, 53.0, 34.7; IR (neat): 3446.2,
3364.1, 2102.6 cm-1; FABMS (in 3-nitrobenzyl alcohol matrix)
m/z (relative intensity) 162 (85, M+), 120 (19), 106 (100); HRMS
(FAB) (in 3-nitrobenzyl alcohol matrix) m/z calcd for C8H10N4
(M+) 162.0905, found 162.0905.
1
(ethanol): 185-186 °C; [lit. 185-186 °C (ethanol)]; H NMR
(DMSO-d6) δ 9.37 (br s, 1H), 7.50 (d, J ) 8.0 Hz, 2H), 7.29 (d,
J ) 8.4 Hz, 2H), 6.65 (d, J ) 8.8 Hz, 2H), 6.36 (d, J ) 8.8 Hz,
2H), 4.90 (br s, 2H), 2.32 (s, 3H); 13C NMR (DMSO-d6) δ 147.1,
143.3, 137.9, 130.1, 127.6, 126.6, 125.3, 114.8, 21.8.
1-Nitr o-2-(2-n itr oeth yl)ben zen e 28.41 To a cooled solution
of â,2-dinitrostyrene (843 mg, 4.34 mmol) in methanol (20 mL)
in ice-water bath was added sodium borohydride (0.4 g, 10.57
mmol) in one portion. The reaction mixture was stirred at room
temperature for 45 min before ethyl ether (100 mL) and
saturated aqueous NH4Cl solution (30 mL) were added to
quench the reaction. The aqueous phase was extracted by ethyl
ether (2 × 30 mL). The combined organic phase was washed
with brine (40 mL), dried over anhydrous MgSO4, filtered
through a pad of cotton, and concentrated in vacuo. The
residue was purified by flash column chromatography on silica
gel with EtOAc and hexanes as the eluent (hexanes:ethyl
acetate ) 5:1 f 4:1) to give 28 as an oil (638 mg,75%). Rf )
0.5 (hexanes:ethyl acetate ) 2:1); 1H NMR (CDCl3) δ 8.09 (dd,
J ) 8.1, 1.5 Hz, 1H), 7.63-7.40 (m, 3H), 4.80 (t, J ) 6.7 Hz,
2H), 3.63 (t, J ) 6.7 Hz, 2H).
1-(2-Ben zyloxyeth yl)-4-n itr oben zen e 18.39 Rf ) 0.4 (hex-
1
anes:ethyl acetate ) 9:1); mp 46-48 °C; H NMR (CDCl3) δ
8.17 (d, J ) 8.8 Hz, 2H), 7.44-7.28 (m, 7H), 4.54 (s, 2H), 3.76
(t, J ) 6.4 Hz, 2H), 3.05 (t, J ) 6.4 Hz, 2H); 13C NMR (CDCl3)
δ 147.4, 138.1, 129.9, 128.5, 127.8, 127.7, 123.6, 73.2, 70.0, 36.3;
IR (neat): 2851.3, 1600.0, 1512.8 cm-1; FABMS (in 3-nitroben-
zyl alcohol matrix) m/z (relative intensity) 258 (49, MH+), 212
(2), 165 (6), 150 (31), 154 (100).
2-(2-Nitr oeth yl)p h en yla m in e 29. Yield, 83%; Rf ) 0.3
1
(hexanes:ethyl acetate ) 2:1); H NMR (CDCl3) δ 7.17-7.03
(m, 2H), 6.76 (dd, J ) 13.3, 7.5 Hz, 2H), 4.65 (t, J ) 7.6 Hz,
2H), 3.71 (br s, 2H), 3.25 (t, J ) 7.7 Hz, 2H); 13C NMR (CDCl3)
δ 144.5, 130.2, 128.9, 120.1, 119.6, 116.7, 74.1, 29.6; IR
(neat): 3446.2, 3382.4, 1628.3, 1550.8, 1498.6, 1379.8 cm-1
;
4-(2-Ben zyloxyeth yl)p h en yla m in e 19. Yield, 80%; Rf )
0.3 (hexanes:ethyl acetate ) 4:1); 1H NMR (CDCl3) δ 7.37-
7.29 (m, 5H), 7.05 (d, J ) 8.4 Hz, 2H), 6.66 (dd, J ) 6.6, 1.8
Hz, 2H), 4.56 (s, 2H), 3.67 (t, J ) 7.2 Hz, 2H), 3.60 (br s, 2H),
2.86 (t, J ) 7.4 Hz, 2H); 13C NMR (CDCl3) δ 144.8, 138.7, 129.9,
129.0, 128.5, 127.8, 127.7, 115.4, 73.1, 71.9, 35.7; IR (neat):
3446.2, 2253.8, 1620.5, 1512.8 cm-1; FABMS (in 3-nitrobenzyl
alcohol matrix) m/z (relative intensity) 227 (100, M+), 210 (10),
136 (78); HRMS (FAB) (in 3-nitrobenzyl alcohol matrix) m/z
calcd for C15H17NO (M+) 227.1310, found 227.1311.
MS (ESI) m/z (relative intensity) 167.14 (100, MH+); HRMS
(FAB) (in 3-nitrobenzyl alcohol matrix) m/z calcd for C8H10N2O2
(M+) 166.0742, found 166.0736.
Ack n ow led gm en t. We gratefully acknowledge the
financial support of grant SNJ -CCR 700-009 from the
State of New J ersey Commission on Cancer Research.
Su p p or t in g In for m a t ion Ava ila b le: 1H NMR and 13C
NMR spectra for compounds 8, 9, 12, 13, 16, 17, 19, 29, 30,
and 31. This material is available free of charge via the
Internet at http://pubs.acs.org.
(36) Atkinson, R. N.; Moore, L.; Tobin, J .; King, S. B. J . Org. Chem.
1999, 64, 3467-3475.
(37) Wiley, R. H.; Smith, N. R. J . Am. Chem. Soc. 1948, 70, 2295-
2296.
J O005666Q
(38) Yu, C.; Liu, B.; Hu, L. Org. Lett. 2000, 2, 1959-1961. Ca u -
tion : azides are potentially explosive and should be handled with care.
The procedure should be performed in a well-ventilated hood.
(39) Lo´pez-Calahorra, F.; Ballart, B.; Hombrados, F.; Marti, J .
Synth. Commun. 1998, 28, 795-800.
(40) Morgan, G. T.; Micklethwait, F. M. G. J . Chem. Soc. 1905, 87,
1302-1310.
(41) Baxter, I.; Swan, G. A. J . Chem. Soc., Sect. C 1968, 468-470.