Synthesis and structure of 1-chlorosulfonyl-2-(4′-nitrophenoxy)-5- methylbenzene

Article abstract of DOI:10.1007/s10870-008-9512-x

Diphenyl ethers are structural elements found in medicinally useful antibiotics such as vancomycin as well as in biological toxins in the environment such as dioxins. The purpose of this paper is to report the synthesis and characterization of the previously unreported 1-chlorosulfonyl-2- (4′-nitrophenoxy)-5-methylbenzene, a trisubstituted diphenyl ether derivative. ¹H NMR (CDCl₃) δ 2.45 (3H, methyl ₅, s), 7.07 (1H, H₄, d, J ~8.27 Hz), 7.13 (2H, H _2′, H_6′, d, J ~9.0 Hz), 7.54 (1H, H ₃, d, J ~8.28 Hz), 7.87 (1H, H₆, s), 8.22 (2H, H _3′, H_5′, J ~9 Hz); ¹³C NMR (CDCl₃) δ 20.7, 118.2, 121.9, 126.0, 129.8, 135.2, 135.8, 137.8, 143.8, 150.3, 161.4. An X-ray analysis has provided valuable insight into the effect of steric factors on the three dimensional shape of this compound which serves as a useful advanced intermediate in the synthesis of these biologically active molecules. A multistep synthesis of this molecule has been designed by retrosynthetic analysis as part of an ongoing program aimed at a function-oriented, multi-step economical synthesis of vancomycin lead antibiotics. Crystals are triclinic, space group P-1, a = 7.6537(15), b = 8.976(2), c = 11.050(2) A, α = 67.645(11), β = 79.735(11), γ = 87.798(10)°, V = 690.5(2) A³, Z = 2.

Full text of DOI:10.1007/s10870-008-9512-x

J Chem Crystallogr (2009) 39:504–509
DOI 10.1007/s10870-008-9512-x
ORIGINAL PAPER
Synthesis and Structure of 1-Chlorosulfonyl-2-(40-Nitrophenoxy)-
5-Methylbenzene
Michael H. Silveira Æ Frank R. Fronczek
Received: 4 October 2008 / Accepted: 12 December 2008 / Published online: 7 January 2009
Ó Springer Science+Business Media, LLC 2009
Abstract Diphenyl ethers are structural elements found in
medicinally useful antibiotics such as vancomycin as well as
in biological toxins in the environment such as dioxins. The
purpose of this paper is to report the synthesis and charac-
terization of the previously unreported 1-chlorosulfonyl-2-
(40-nitrophenoxy)-5-methylbenzene, a trisubstituted diphe-
nyl ether derivative. 1H NMR (CDCl3) d 2.45 (3H, methyl5,
Introduction
Diphenyl ethers are structural elements found in biological
toxins in the environment such as dioxins as well as in
medicinally useful antibiotics such as vancomycin. The
latter is a front line antibiotic for the treatment of life-
threatening gram-positive bacterial infections and many
clinical isolates have become resistant to it, focusing interest
on sucessors to vancomycin.
0
0
s), 7.07 (1H, H4, d, J *8.27 Hz), 7.13 (2H, H2 , H6 , d, J
*9.0 Hz), 7.54 (1H, H3, d, J *8.28 Hz), 7.87 (1H, H6, s),
8.22 (2H, H3 , H5 , J *9 Hz); 13C NMR (CDCl3) d 20.7,
118.2, 121.9, 126.0, 129.8, 135.2, 135.8, 137.8, 143.8,
150.3, 161.4. An X-ray analysis has provided valuable
insight into the effect of steric factors on the three dimen-
sional shape of this compound which serves as a useful
advanced intermediate in the synthesis of these biologically
active molecules. A multistep synthesis of this molecule
has been designed by retrosynthetic analysis as part of
an ongoing program aimed at a function-oriented, multi-
step economical synthesis of vancomycin lead antibiotics.
Crystals are triclinic, space group P-1, a = 7.6537(15),
Dioxins are some of the most toxic chemicals known to
science. There are no ‘‘safe’’ levels of exposure to dioxin.
Dioxins are cancer hazards. They are fat-soluble, they bio-
accumulate and they are mainly found in meat and dairy
products. Dioxin exposure has been linked to birth defects,
inability to maintain pregnancy, decreased fertility, reduced
sperm counts, endometriosis, diabetes, learning disabilities,
immune system suppression, lung problems, skin disorders,
and lowered testosterone levels [1–4]. Derivatives of
substituted diphenyl ether have been found to show con-
siderable anaesthetic action but were too toxic to be of
practical value. They are useful herbicides and intermedi-
ates for textile protective agents [5]. More recently,
derivatives of diphenyl ethers have been used as advanced
intermediates in the total synthesis of glycopeptide antibi-
otics which includes telcoplanin and vancomycin. The
members of the vancomycin class of glycopeptide antibi-
otics are important agents for the treatment of severe
bacterial infections, particularly those caused by methicil-
lin-resistant Staphylococcus aureus, and the basis of their
activity has been of interest for some time [6–11].
0
0
˚
b = 8.976(2), c = 11.050(2) A, a = 67.645(11), b =
79.735(11), c = 87.798(10)°, V = 690.5(2) A , Z = 2.
3
˚
Keywords Natural products ꢀ Spectroscopy ꢀ Synthesis ꢀ
Diphenyl ether ꢀ Forensic chemistry
M. H. Silveira (&)
Forensic Science Program, Department of Sciences,
Our Lady of the Lake College, 5345 Brittany Drive,
Baton Rouge, LA 70808, USA
The purpose of this paper is to report a multi-step syn-
thesis and characterization of a trisubstituted derivative of
diphenyl ether, 1-chlorosulfonyl-2-(40-nitrophenoxy)-5-
methylbenzene (5). The X-ray structure of the compound
(5) is previously unreported and it serves as a useful
e-mail: silveiramh@aol.com
F. R. Fronczek
Department of Chemistry, Louisiana State University,
Baton Rouge, LA 70803, USA
123

J Chem Crystallogr (2009) 39:504–509
505
0
0
advanced intermediate in the synthesis of these biologically
active molecules.
H3, d, J *8.28 Hz), 7.87 (1H, H6, s), 8.22 (2H, H3 , H5 , d,
J *9 Hz); 13C NMR (CDCl3) d 20.7, 118.2, 121.9, 126.0,
129.8, 135.2, 135.8, 137.8, 143.8, 150.3, 161.4.
Materials and Methods
X-ray Crystallography
Chemicals and Materials
Diffraction data for 5 were collected at 90 K on a Nonius
Kappa CCD diffractometer fitted with an Oxford Cryo-
stream sample chiller and graphite-monochromated MoKa
Glacial acetic acid (CH3COOH), Chlorosulfonic acid
(ClSO3H), Benzene (C6H6), and activated charcoal were
obtained from Sigma–Aldrich Corp. (St Louis, MO).
˚
(a = 0.71073 A) radiation, using a colorless plate crystal.
Data reduction and scaling were carried out using HKL
SCALEPACK [12]. The structure was solved by direct
methods and refined by full-matrix least squares techniques
using SHELXL97 [13]. All non-hydrogen atoms were
refined anisotropically. All hydrogen atoms were visible in
Instruments
1H NMR and 13C NMR were recorded on a Bruker Inova
spectrometer (AM-500) instrument; X-ray data were col-
lected using a Nonius Kappa CCD diffractometer.
difference maps, but were placed in idealized positions
2
˚
˚
with C–H distance 0.95 A for sp C atoms and 0.98 A for
the methyl group. Displacement parameters for H were
assigned as Uiso = 1.2 Ueq for the bonded atom (1.5 for
methyl). Crystal data and details of the data collection and
refinement are given in Table 1.
Experimental Procedure
1-chlorosulfonyl-2-(40-nitrophenoxy)-5-methylbenzene (5).
A cold (0 °C) stirred solution of chlorosulfonic acid
(7.0 mL, 0.1 mol) was treated with small amounts of
crystalline 1-(40-nitrophenoxy)-4-methylbenzene (2.29 g,
0.01 mol) (4). The addition of the solid was completed in
0.5 h. The reaction flask was then stoppered and kept
overnight in a refrigerator at a temperature of 0–5 °C.
Finally, the reaction mixture was poured over crushed ice
with stirring. The precipitated solid was filtered and
washed with cold water till free of acid. The sticky solid
obtained was dissolved in hot benzene and then boiled with
a little activated charcoal. The solution was filtered and
evaporated to give the sulfonyl chloride 5 (70%) after
recrystallization from glacial acetic acid (M. pt., 147 °C).
1H NMR (CDCl3) d 2.45 (3H, methyl5, s), 7.07 (1H, H4, d,
Results and Discussion
Retrosynthetic Analysis
A retrosynthetic analysis (Fig. 1) approach has been
employed in designing a multistep synthesis of the target
molecule (5). The trifunctionalized derivative of diphenyl
ether contains an aryl chlorosulfonyl element which is the
required retron for an aromatic chlorosulfonation transform
on target (5) to produce the synthetic precursor (4). Thus, a
disconnection of the carbon–sulfur bond on compound (5)
at the C-1 atom bearing a chlorosulfonyl group produces
compound (4) which is a disubstituted diphenyl ether
0
0
J *8.27 Hz), 7.13 (2H, H2 , H6 , d, J *9.0 Hz), 7.54 (1H,
Table 1 Crystal data and
structure refinement
Empirical formula
CCDC deposition no.
fw
C13H10ClNO5S
Z
2
CCDC 698480
327.73
crystal size (mm)
qcalc (g cm-3
l (mm-1
0.07 9 0.30 9 0.32
1.576
)
Crystal system
Space group
Triclinic
P-1
)
0.448
Radiation
MoKa
Cell dimensions
hmax (°)
30.1
˚
a (A)
7.6537(15)
8.976(2)
11.050(2)
67.645(11)
79.735(11)
87.798(10)
690.5(2)
90
Data collected
Independent data
Observed (I [ 2r(I))
Rint
15,885
4,036
˚
b (A)
˚
c (A)
3,415
a (°)
b (°)
c (°)
0.023
R
0.031
wR2 [I [ 2r(I)]
Data/param
0.077
3
˚
V (A )
4,036/192
0.43, -0.46
-3
)
˚
Resistant density (eA
T (K)
123

506
J Chem Crystallogr (2009) 39:504–509
4-methyl-40-nitrodiphenyl ether (4) as a product of Ull-
mann’s coupling reaction [19–22]. The compound (4) was
chlorosulfonated regioselectively in the presence of excess
chlorosulfonic acid at 0 °C to yield the corresponding sul-
fonyl chloride derivative (5). The product obtained (Fig. 2)
was found to be hydrophobic and it has been analyzed by 1H
NMR, and 13C NMR spectroscopy. The compound’s single
crystal structure has been obtained by X-ray analysis. The
sulfonyl chloride (5) was dissolved in boiling glacial acetic
acid and then slowly allowed to cool to room temperature on
its own. The crystals obtained from the supersaturated
solution were filtered, dried under high vacuum and ana-
lyzed by X-ray crystallography.
4
3'
3
2'
4'
5
O N
O
CH
3
2
2
1'
5'
6
6'
1
ClO S
2
5
ClSO H
3
3'
2'
2
3
4
4'
1
1'
1H NMR Analysis
O N
O
CH
3
2
5
6'
6
5'
Nuclear Magnetic Resonance Spectroscopy (NMR) has
been used to determine the structure of compound (5) [23].
Six different types of magnetically unequivalent protons
were identified in the molecule due to the presence of
4
1
proton peaks with six different chemical shifts in the H
ULLMANN'S REACTION
+-
O N
2
Cl
+
K O
CH
3
+ -
K
O
CH
3
O N
2
Cl
+
2
3
2
3
Cu, KI, DMF, reflux
Fig. 1 Retrosynthetic analysis. Structures of compounds 2–5
derivative bearing a methyl group on C-4 and a nitro
substituent on C-40. It is a target molecule containing a
retron for a Ullmann coupling transform to give the syn-
thetic precursors p-cresol (1) and p-chloronitrobenzene (3).
A cleavage of the carbon-oxygen bond at the C-10 atom of
the compound (4) yields p-cresol and p-chloronitrobenzene
which serve as synthons in a multistep synthesis of the
advanced intermediate (5) [14, 15].
3'
2'
2
3
4
4'
1
1'
O N
2
O
CH
3
5
6'
6
5'
4
o
ClSO H, 0 C
3
Synthetic Methodology
A function-oriented, multi-step economical synthesis of
vancomycin lead antibiotics designed by antithetic analysis
has created a need for the synthesis and characterization of
the previously unreported compound (5) [15–18]. Com-
mercially available p-cresol (1) was converted into its
potassium salt (2) by heating it with potassium hydroxide
pellets and then reacting it with p-chloronitrobenzene (3) in
the presence of copper and potassuim iodide as catalysts and
DMF as a solvent under refluxing conditions to yield
O N
O
CH
3
2
ClO S
2
5
Fig. 2 Synthetic scheme
123

J Chem Crystallogr (2009) 39:504–509
507
NMR spectrum. Integration of these proton peaks in the
spectrum indicates the presence of seven aromatic protons
and three protons belonging to a methyl group in the
molecule. Information on the connectivity of protons in the
trisubstituted diphenyl ether derivative was obtained from
the coupling constant in order to assign protons to the four
doublets seen in the spectrum. A singlet present upfield at d
(ppm) 2.45 (3H, methyl, s) corresponds to the three methyl
protons. Two doublets present downfield at d 7.07 (1H, H4,
d, J *8.27 Hz) and d 7.54 (1H, H3, d, J *8.28 Hz) have
similar J values showing that the aromatic protons H3 and
H4 are coupled to each other. Another singlet present
downfield at d 7.87 (1H, H6, s) corresponds to the aromatic
proton H6 which is deshielded by the strongly electron
withdrawing chlorosulfonyl group. The presence of two
Fig. 3 ORTEP representation of the trisubstituted diphenyl ether 5
with non-hydrogen atoms shown as anisotropic displacement at the
50% probability level and hydrogen atoms (unlabeled) shown as small
circles of an arbitrary radius
0
0
doublets downfield at d 7.13 (2H, H2 , H6 , d, J *9.0 Hz),
0
0
and d 8.22 (2H, H3 , H5 , J *9 Hz) with identical J values
indicates that a set of magnetically equivalent aromatic
its three dimensional shape. Geometric parameters are
given in Table 2. The dihedral angle formed by the two
phenyl planes is 64.49(4)°. There is a good deal of vari-
ability in the phenyl-phenyl dihedral angle in diphenyl
ethers in the Cambridge Structural Database (CSD, version
5.29, Nov. 2007), but angles slightly less than 90° appear
most common [24]. The two polymorphs of unsubstituted
diphenyl ether have dihedral angles of 87.60(4) and
88.39(2)°. There are no other diphenyl ether structures
having a chlorosulfonyl substituent ortho to the ether O in
the CSD, thus standards for comparison of our dihedral
angle are scarce [25]. However, the structure of 2-phen-
oxybenzenesulfonamide has been reported, and the
dihedral angle formed by its phenyl groups varies from
66.2(1) to 74.3(1)° over four independent molecules, only
slightly larger than that found in (5) [26]. The nitro group is
nearly coplanar with the phenyl ring to which it is attached,
with torsion angle O4-N1-C10-C9-9.52(18)°.
0
0
protons H2 , H6 is coupled to another set of magnetically
0
0
equivalent aromatic protons H3 and H5 . These two sets of
protons are deshielded by the strongly electron withdraw-
ing nitro substituent present in the aryl moiety.
13C NMR Analysis
Carbon spectral analysis of compound (5) confirms the
presence of eleven types of magnetically nonequivalent
carbon atoms [23]. A peak at d (ppm) 20.7 corresponds to
the sp3 hybridized carbon atom in the methyl substituent.
All other peaks correspond to sp2 hybridized carbon atoms
present in the aromatic rings. The peak at d 161.4 corre-
0
sponds to C4 . It is deshielded by a strongly electron
withdrawing nitro group. The peak at d 150.3 corresponds
to C1 which is also deshielded by another strongly electron
withdrawing chlorosulfonyl group. The peak at d 143.8
corresponds to C6. It is deshielded by the methyl group and
also by the strongly electron withdrawing chlorosulfonyl
group. The peaks at d 137.8 and d 135.8 correspond to C2
The chlorosulfonyl group in (5) has normal dimensions,
but its conformation appears slightly unusual. The CSD
contains nine structures containing benzenesulfonyl chlo-
ride moieties determined at low temperature: FAQJOW,
0
and C1 . They are deshielded by the oxygen atom of the
diaryl ether functionality and the former is also deshielded
by the chlorosulfonyl group. In addition, the 13C NMR
˚
Table 2 Selected bond lengths (A), angles (°) and torsion angles (°)
0
spectrum shows the presence of C2 and C6 as a set of
0
Cl1–S1
S1–O2
2.0398(6)
1.4264(10)
1.4275(10)
1.7592(13)
1.3851(15)
1.3808(15)
1.2316(15)
1.2317(16)
1.4614(17)
119.69(6)
106.33(5)
O3–S1–Cl1
106.17(5)
101.69(5)
112.05(6)
119.43(10)
123.27(12)
118.48(12)
118.24(11)
62.95(11)
-125.62(12)
15.29(18)
-9.52(18)
0
0
magnetically equivalent carbon atoms and C3 and C5 as
the other set of magnetically equivalent carbon atoms.
Several other peaks at d 118.2, 121.9, 126.0, 129.8, 135.2
correspond to the other remaining sp2 hybridized carbon
atoms present in the diaryl rings.
C1–S1–Cl1
S1–O3
O2–S1–C1
S1–C1
C7–O1–C2
O1–C2
O4–N1–O5
O1–C7
O4–N1–C10
O5–N1–C10
Cl1–S1–C1–C2
C7–O1–C2–C1
C2–O1–C7–C12
O4–N1–C10–C9
O4–N1
O5–N1
N1–C10
O2–S1–O3
O2–S1–Cl1
X-ray Analysis
The crystal structure of the molecule, (5) shown in Fig. 3,
has provided valuable insight into the effect of steric fac-
tors such as ring strain, torsional strain and steric strain on
123

508
J Chem Crystallogr (2009) 39:504–509
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, by e-mailing data_request@ccdc.cam.
ac.uk, or by contacting The Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
?44(0)1223-336033.
Acknowledgments We thank the following from LSU, Baton
Rouge: Dale Traleaven and Thomas Weldeghiorghis for recording the
1H NMR and 13C NMR spectra and Anne Grove for advice and
helpful discussions. The purchase of the diffractometer was made
possible by grant No. LEQSF (1999–2000)-ENH-TR-13, adminis-
tered by the Louisiana Board of Regents. Financial support by Our
Lady of the Lake College, Baton Rouge, LA and The Board of
Regents, Baton Rouge, LA in the form of Faculty Endowment Pro-
fessorships (2004–2008) awarded to MHS is greatly appreciated.
Fig. 4 Short contacts between nitro groups about the inversion center
at 0, 0, 1
IJEMEP, JAGLIM, NAQLIM, NBZSOC01, OLAVAY,
QQQHJA02, SUTYAH, and TECLUJ. The mean S–Cl
˚
distance is 2.032 A in that set, and the mean S=O distance
˚
is 1.423 A, both similar to values in (5). However, the S–Cl
References
bond in those structures tends to be rotated by nearly the
maximum value out of the phenyl plane, with C–C–S–Cl
torsion angle magnitudes in the range 68.2–89.5°, and
average value 80.6°. The value in (5) is 62.95(11)°, smaller
than in any of the literature structures. This conformation
forms intramolecular distances to the ether O atom O1ꢀꢀꢀO2
1. Steenland K, Piacitelli L, Deddens J, Fingerlhut M, Chang LI
(1999) J Natl Cancer Inst 91:779. doi:10.1093/jnci/91.9.779
2. Hoffman EC, Reyes H, Chu FF, Sander F, Conley LH, Brooks
BA, Hankinson O (1991) Science 252(5008):954. doi:10.1126/
science.1852076
3. Fingerhut MA, Halperin WE, Marlow DA, Piacitelli LA, Hon-
char PA, Sweeney MH, Greife AL, Dill PA, Steenland K, Suruda
AJ (1991) N Engl J Med 324(4):212
4. Manz A, Berger J, Dwyer JH, Flesch-Janys D, Nagel S, Waltsgott
H (1991) Lancet 338(8733):959. doi:10.1016/0140-6736(91)
91835-I
5. Suter C, Oberg E (1931) J Am Chem Soc 53:1566. doi:10.1021/
ja01355a060
6. Evans DA, Katz JL, West TR (1998) Tetrahedron Lett 39:2937.
doi:10.1016/S0040-4039(98)00502-4
7. Evans DA, Dinsmore CJ, Ratz AM (1997) Tetrahedron Lett
38(18):3189. doi:10.1016/S0040-4039(97)00566-2
8. Lorian V (ed) (1986) Antibiotics in laboratory medicine, 2nd edn.
Williams and Wilkins, Baltimore
˚
2.930(1) and O1ꢀꢀꢀCl1 3.231(1) A. There are no previous
examples of phenylsulfonyl chlorides with O-containing
substituents ortho to SO2Cl in the CSD for comparison
purposes.
The most notable intermolecular interaction is between
nitro groups related by inversion (symmetry operation i =
-x, -y, 2-z), illustrated in Fig. 4. It is similar to antiparallel
carbonyl-carbonyl contacts described by Allen et al. [27]
i
i
˚
with O5ꢀꢀꢀO5 2.952(1), O5ꢀꢀꢀN1 2.994(1) A, 0.09 and
˚
0.08 A less than van der Waals sums, respectively.
In conclusion, a multistep synthesis of 1-chlorosulfonyl-
2-(40-nitrophenoxy)-5-methylbenzene, a trisubstituted deriv-
ative of diphenyl ether has been accomplished. The synthetic
method converts a disubstituted derivative of diphenyl ether
(4) into a trisubstituted (5) chlorosulfonated one in a
chemoselective and regioselective manner. A single crystal
X-ray structure has provided valuable insight into the effect
of steric factors such as ring strain, torsional strain and steric
strain on the three dimensional shape of this molecule (5). In
9. Nagarajan R (ed) (1994) Glycopeptide antibiotics. Marcel Dekker,
New York
10. Groves P, Searle MS, Waltho JP, Williams DH (1995) J Am
Chem Soc 117:7958. doi:10.1021/ja00135a014
11. Walsh CT, Fischer SL, Park I-S, Prahalad M, Wu Z (1996) Chem
Biol 3:21. doi:10.1016/S1074-5521(96)90079-4
12. Otwinowski Z, Minor W (1997) Macromolecular crystallography,
part A, vol 276. New York Academic Press, New York
13. Sheldrick GM (ed) (1997) SHELX97 A program for crystal
¨
structure solution and refinement. University of Gottingen,
Germany
1
addition, it has been characterized spectroscopically by H
NMR, and 13C NMR. The function-oriented, multi-step
economical synthesis of vancomycin lead antibiotics has
been designed by retro synthetic analysis. The synthesized
molecule (5) is a useful advanced intermediate in the total
synthesis of these biologically active molecules.
14. Wender PA, Verma VA, Paxton TJ (2008) Acc Chem Res
41(1):40. doi:10.1021/ar700155p
15. Corey EJ, Cheng X-M (eds) (1989) The logic of chemical syn-
thesis. Wiley, New York
16. Walsh CT (2008) Acc Chem Res 41(1):4. doi:10.1021/ar7000414
17. Lu H, Tonge PT (2008) Acc Chem Res 41(1):11. doi:10.1021/
ar700156e
18. Rama Rao AV, Gurjar MK, Reddy LR, Rao AS (1995) Chem Rev
95(6):2135. doi:10.1021/cr00038a014
19. Ullmann F, Meyer GM, Loewenthal O, Emilo G (1904) Justus
Liebigs Ann Chem 332:38. doi:10.1002/jlac.19043320104
20. Ullmann F (1896) Ber Dtsch Chem Ges 29:1878. doi:10.1002/
cber.189602902145
21. Kornblum N, Kendall D (1952) J Am Chem Soc 74:5782. doi:
10.1021/ja01142a518
Supporting Information Available
Magnetic measurements for the trisubstituted diphenyl
ether (5). This material is available in online for download-
ing. CCDC-698480 contains the CIF for this paper, which
123

J Chem Crystallogr (2009) 39:504–509
509
22. Vogel A (1956) Practical organic chemistry, 3rd edn. E.L.B.S.
and Longman Group, Essex, England
26. Glidewell C, Low JN, Skakle JMS, Wardell JL (2004) Acta
Crystallogr C60:o364
23. Pavia DL, Lampman GM, Kriz GS, Vyvyan JR (2001) Intro-
duction to spectroscopy. Brooks/Cole, California
24. Allen FH (2002) Acta Crystallogr B58:380
27. Allen FH, Baalham CA, Lommerse JPM, Raithby PR (1998) Acta
Crystallogr B54:320
25. Choudhury AR, Islam K, Kirchner MT, Mehta G, Guru Row TN
(2004) J Am Chem Soc 126:12274. doi:10.1021/ja046134k
123