Addition of hypobromous acid to 1-phenylcycloalkenes

Article abstract of DOI:10.1007/s11172-009-0321-y

Reaction of 1-phenylcyclooctene and 1-phenylcycloheptene with N-bromosuccinimide (NBS) in aqueous DMSO gives the corresponding 3-bromo-2-phenylcycloalkenes and 2-phenylcycloalk-2-enols in a ratio of 3: 1. Unlike them, 1-phenylcyclohexene gives a mixture of 3-bromo-2-phenylcyclohexene and 2-bromo-1-phenylcyclohexanol. The oxidation of allylic alcohols with pyridinium chlorochromate afforded the corresponding α,β-unsaturated ketones.

Full text of DOI:10.1007/s11172-009-0321-y

Russian Chemical Bulletin, International Edition, Vol. 58, No. 11, pp. 2299—2303, November, 2009
2299
Addition of hypobromous acid to 1ꢀphenylcycloalkenes
M. Ceylan,a E. Findik,a E. Sahin,b and C. Kazazb
aDepartment of Chemistry, Faculty of Arts and Science, Gaziosmanpasa University,
60250 Tokat, Turkey.
Fax: +90 (356) 252 1585. Eꢀmail: mceylan@gop.edu.tr
bDepartment of Chemistry, Faculty of Arts and Science, Atatürk University,
25240 Erzurum, Turkey.
Fax: +90 (442) 236 0948. Eꢀmail: ertan@atauni.edu.tr
Reaction of 1ꢀphenylcyclooctene and 1ꢀphenylcycloheptene with Nꢀbromosuccinimide
(NBS) in aqueous DMSO gives the corresponding 3ꢀbromoꢀ2ꢀphenylcycloalkenes and
2ꢀphenylcycloalkꢀ2ꢀenols in a ratio of 3 : 1. Unlike them, 1ꢀphenylcyclohexene gives a mixture
of 3ꢀbromoꢀ2ꢀphenylcyclohexene and 2ꢀbromoꢀ1ꢀphenylcyclohexanol. The oxidation of
allylic alcohols with pyridinium chlorochromate afforded the corresponding α,βꢀunsaturated
ketones.
Key words: 1ꢀphenylcycloalkenes, allylic bromides, allylic alcohols, Nꢀbromosuccinimide,
α,βꢀenones.
Nowadays the heterolytic bromination of the С=С
bond is considered as a twoꢀstep process of transꢀaddition
involving the bromonium ion as a key intermediate. Howꢀ
ever, the formation of the bromonium bridge is not genꢀ
eral and mainly determined by the nature of substituents
at the double bond. For instance, in the case of strong
electronꢀdonating substituents (X) capable of stabilizing
the positive charge, along with bridge ions 1, linear ions 2
are formed as intermediates.1,2
Scheme 1
The addition of HOBr to alkene proceeds with the
intermediate participation of the bromonium ion and
affords bromohydrin according to the Markovnikov rule.3
In the present work, we studied the addition of HOBr
(generated by hydrolysis of bromosuccinimide (NBS) in
aqueous DMSO) to 1ꢀphenylꢀsubstituted cycloalkenes
3a—c and the behavior of the products formed during
chromatographic separation on SiO2. The allylic alcohols
obtained were also oxidized to the corresponding
α,βꢀenones.
Results and Discussion
The addition of HOBr to 1ꢀphenylcycloheptene (3b)
and 1ꢀphenylcyclooctene (3с) was accompanied by the
n = 2 (b), 3 (c)
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2230—2233, November, 2009.
1066ꢀ5285/09/5811ꢀ2299 © 2009 Springer Science+Business Media, Inc.

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Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009
Ceylan et al.
evolution of water and hydrogen bromide and afforded
allylic bromide 4b,c and allylic alcohols 5b,с in a ratio
of 3 : 1 (Scheme 1). The products were isolated from
the reaction mixture by chromatography on silica gel. In
addition to bromides 4b,c and alcohols 5b,с, 1,3ꢀdienes
6b,c were obtained. Diene 6с is poorly stable and was
detected only by 1H NMR in a mixture, therefore
we failed to isolate it in a pure form. The reaction of
1ꢀphenylcyclohexene (3а) under similar conditions
gave 4a bromohydrin 7а along with the expected allylic
bromide (4 : 3, overall yield 68%). The formation of
bromohydrin 7а has previously2 been observed in the
reaction of substrate 3а with NBS in aqueous DMSO
at 0 °С.
Bromide 4а was isolated by chromatography of the
reaction mixture using a short (5 cm) column with silica
gel and hexane as an eluent. This bromide was also
obtained by the reaction of alcohol 5а with PBr3 in benꢀ
zene. Along with bromide 4а, alcohol 5а, bromohydrin
7a, and biphenyl (8) were isolated from the reaction mixꢀ
ture by repeated column chromatography (see Scheme 1).
The formation of products 4с and 5с was due to the
elimination of Н2О and НBr from bromohydrins 7с and 9
formed at the initial step of the process. Diene 6с is
formed, probably, due to the SiO2ꢀassisted elimination of
the bromide anion from bromo compound 4с followed by
proton elimination from allylic cation 10 (Scheme 2).
The structures of the products were determined by IR,
1Н and 13С NMR spectroscopy, and by comparison with
the published data.4,5 The structures of products 4а, 5а,
7а, and 8 were confirmed by their chemical transformaꢀ
tions, in particular, by the pꢀTsOHꢀcatalyzed dehydraꢀ
tion of bromohydrin 7а to allylic bromide 4а and the
transformation of the latter into allylic alcohol 5а by the
action of AgNO3 and water and by the inverse transforꢀ
mation of alcohol 5а into bromide 4а on the treatment
with PBr3. Biphenyl (8) is formed from bromide 4а due to
the SiO2ꢀinduced elimination of the bromide anion from
the molecule of 4a followed by proton elimination from
allylic cation 10а and diene 6а aromatization (Scheme 3).
The synthesized allylic alcohols 5а—с were converted
to 2ꢀphenylcycloalkꢀ2ꢀenones 11а—с by the oxidation
Scheme 2
Scheme 3

Bromination of phenylcycloalkenes
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009 2301
Scheme 4
addition, 2ꢀphenylcycloalkꢀ2ꢀenones 11а—с were synꢀ
thesized in 60—79% yields by the oxidation of the
obtained 2ꢀphenylcycloalkꢀ2ꢀenols 5а—с with chloroꢀ
chromate.
Experimental
1ꢀPhenylcyclohexene (3а),4 1ꢀphenylcycloheptene (3b),7
and 1ꢀphenylcyclooctene (3с)8 were synthesized using known
1
procedures. The Н and 13С NMR spectra of the solutions in
n = 1 (a), 2 (b), 3 (c)
CDCl3 were recorded on Varian 200 and Varian Mercury 400
spectrometers using SiMe4 and residual protons of CDCl3 as
references. IR spectra were recorded with a Jasco FT/IRꢀ430
instrument (solutions in CCl4). Elemental analysis was carried
out on a Leco CHNS 932 analyzer. Melting points were measured
on an Electrothermal 9100 apparatus. Column chromatography
was carried out on silica gel (Merck, 60—230 mesh).
Reaction of 1ꢀphenylcyclohexene (3а), 1ꢀphenylcycloheptene
(3b), and 1ꢀphenylcyclooctene (3с) with NBS (general procedure).
A mixture of NBS (2.4 g, 13.4 mmol), 1ꢀphenylcycloalkene 3
(5.4 mmol), and water (0.2 g, 11 mmol) in DMSO (25 mL) was
stirred at ~20 °C for 30 min, the solid materials formed were
filtered off, the filtrate was extracted with ether (2×25 mL), and
the combined ethereal extracts were washed with water, dried
over Na2SO4, and evaporated. The obtained residue was chromꢀ
atographed on silica gel (layer height 5 cm) using nꢀhexane as
eluent. After one to two fractions were collected, the products
were eluted with an nꢀhexane—СНСl3 (7 : 3) mixture. 3ꢀBromoꢀ
2ꢀphenylcycloalkꢀ1ꢀenes 4а—с were isolated from the collected
first fractions.
3ꢀBromoꢀ2ꢀphenylcyclohexꢀ1ꢀene (4а). The yield was 28%,
needleꢀlike crystals, m.p. 49—51 °С (cf. Ref. 2: m.p. 50—52 °С).
1Н NMR, δ: 1.83—1.68 (m, 2 H); 2.46—2.12 (m, 4 H);
5.33—5.36 (m, 1 H); 6.19 (t, 1 H, J = 3.6 Hz); 7.25—7.63 (m,
5 H). 13С NMR, δ: 141.47, 140.00, 129.00, 128.67, 127.41,
126.01, 50.70, 33.84, 25.99, 17.46. IR, ν/cm–1: 3069, 3044,
3017, 2935, 2862, 1588, 1496, 1448, 1244, 1224, 758, 694, 536.
Found (%): C, 60.48; H, 5.22. C12H13Br. Calculated (%):
C, 60.78; H, 5.53.
(Z)ꢀ3ꢀBromoꢀ2ꢀphenylcycloheptꢀ1ꢀene (4b). The yield was
36%, colorless oil. 1Н NMR, δ: 1.70—1.67 (m, 1 H); 2.08—1.95
(m, 3 H); 2.32—2.19 (m, 2 H); 2.49—2.41 (m, 2 H); 5.31—5.27
(m, 1 H); 6.17 (t, 1 H, J = 7.2 Hz); 7.43—7.26 (m, 5 H).
13С NMR, δ: 143.95, 143.33, 136.31, 128.37, 127.11, 126.11,
57.36, 35.51, 28.53, 27.14, 26.69. IR, ν/cm–1: 3062, 3029, 2937,
2883, 1602, 1455, 1448, 1303, 1016, 752, 702. Found (%):
C, 62.97; H, 5.88. C13H15Br. Calculated (%): C, 62.77; H, 6.02.
(Z)ꢀ3ꢀBromoꢀ2ꢀphenylcyclooctꢀ1ꢀene (4с). The yield was
31%, colorless oil. 1Н NMR, δ: 1.46—1.39 (m, 2 H); 1.60—1.49
(m, 1 H); 1.95—1.63 (m, 3 H); 2.47—2.28 (m, 2 H); 2.59—2.48
(m, 2 H); 5.50—5.46 (dd, 1 H, J = 5.5 Hz, J = 11.7 Hz); 5.94
(t, 1 H, J = 8.6 Hz); 7.39—7.32 (m, 5 H). 13С NMR, δ: 142.20,
140.78, 132.35, 128.11, 127.86, 127.32, 51.19, 41.41, 29.54,
27.89, 27.28, 26.62. IR, ν/cm–1: 3079, 3054, 3019, 2925, 2852,
1598, 1492, 1446, 1249, 1222, 763, 698, 534. Found (%):
C, 63.22; H, 6.68. C14H17Br. Calculated (%): C, 63.41; H, 6.46.
2ꢀPhenylcycloalkꢀ2ꢀenols 5а—с, biphenyl (8), and 2ꢀbromoꢀ
1ꢀphenylcyclohexanol (7а) were isolated from the second and
third fractions obtained by the elution of the reaction products
of substrates 3а—с with NBS with an nꢀhexane—СНСl3 (7 : 3)
mixture.
with pyridinium chlorochromate (РСС) in dichloroꢀ
methane at room temperature (3 h) (Scheme 4).
The Xꢀray study of crystals of ketone 11а showed that
its molecules form centrosymmetric space group Р21/с
(Z = 4) (Fig. 1). It contains the cyclohexꢀ2ꢀenone
and phenyl moieties linked by the single С—С bond
(C(6)—C(7), 1.489(3) Å). The cyclohexene ring has
the halfꢀchair conformation with the parameters
Q = 0.361(4) Å, θ = 53.9(5)°, and φ = 192/1(6)° (see
Ref. 6). The phenyl and cyclohexene moieties are
considerably twisted relatively to each other about the
C(6)—C(7) bond, and the dihedral angle formed by
the phenyl and plane of the С(8)—С(7)—С(12) atoms is
49.7(1)°. Ketone 11а exists in the strained conformation,
which is facilitated by the participation of the carbonyl O
atom in hydrogen bonding with the adjacent molecules.
One of them (C(5)—H...O(1)) is directed along the а axis
(C(5)—H, 0.93 Е; C(5)...O(1)i, 3.608(4) Å; C(5) H...O(1),
169°; i: x + 1, +y, +z), whereas another bond is directed
along the b axis (C(9)—H, 0.97 Å; C(9)...O(1)ii, 3.536(4) Å;
the C(9)—H...O(1) angle is 156°; ii: –x, +y – 1/2,
–z + 1/2). In addition the noncovalent interaction
С(8)—Н...π(phenyl) contributes, as a whole, to the stabiꢀ
lization of the structure of ketone 11a. In this case, the
hydrogen atom is stretched almost directly to the cyclic
centroid (X) (С(8)...Х, 3.701(4) Å; the С—Н...Х angle
is 139°).
To sum up, the reaction of 1ꢀphenylꢀsubstituted
cycloheptene and cyclooctene (3b,с) with NBS in aqueꢀ
ous DMSO affords allylic bromides 4b,c and allylic
alcohols 5b,c in a ratio of 3 : 1. 1ꢀPhenylcyclohexene (3а)
forms bromohydrin 7а along with allylic bromide. In
O(1)
C(1)
C(12)
C(11)
C(2)
C(3)
C(6)
C(7)
C(8)
C(10)
C(4)
C(5)
C(9)
Fig. 1. Molecular structure of 2ꢀphenylcyclohexꢀ2ꢀenone (11а).

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Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009
Ceylan et al.
2ꢀPhenylcyclohexꢀ2ꢀenol (5а). The yield was 14%, colorꢀ
less solid substance, m.p. 55—57 °С (cf. Ref. 9: m.p. 58—59 °С).
1Н NMR, δ: 1.89—1.75 (m, 2 H); 2.05—1.93 (m, 2 H);
2.32—2.28 (m, 2 H); 4.79—4.77 (m, 1 H); 6.24 (t, 1 H,
J = 3.9 Hz); 7.45—7.32 (m, 5 H). 13С NMR, δ: 143.08, 142.02,
130.67, 130.53, 129.09, 128.01, 67.51, 33.57, 28.06, 19.36.
IR, ν/cm–1: 3419 3056, 3023, 295, 2863, 1683, 1494, 1446,
1220, 1058, 937, 758, 698. Found (%): C, 82.43; H, 8.45.
C12H14О. Calculated (%): C, 82.72; H, 8.10.
13С NMR, δ: 198.22, 148.22, 140.65, 136.78,128.86, 128.23,
127.80, 39.30, 26.84, 23.17. IR, ν/cm–1: 3054, 3023, 2938, 2865,
1680, 1628, 1610, 1502, 1460, 1430, 1390, 1160, 1130, 850,
702. Found (%): C, 83.98; H, 7.37. C12H12О. Calculated (%):
C, 83.69; H, 7.02.
(Z)ꢀ2ꢀPhenylcycloheptꢀ2ꢀenone (11b). The yield was 79%,
1
viscous oil (cf. Ref. 11: m.p. 6—7 °С). Н NMR, δ: 1.96—1.75
(m, 4 H); 2.57—2.48 (m, 2 H); 2.77—2.71 (m, 2 H); 6.76 (t,
1 H, J = 6.43 Hz); 7.36—7.25 (m, 5 H). 13С NMR, δ: 206.11,
144.71, 141.77, 138.90, 128.14, 128.10, 127.44, 43.19, 28.07,
24.86, 22.16. IR, ν/cm–1: 3060, 3025, 2935, 2861, 1704, 1598,
1494, 1448, 1278, 1070, 939, 756, 698, 578. Found (%):
C, 83.44; H, 7.69. C13H14О. Calculated (%): C, 83.83; H, 7.58.
(Z)ꢀ2ꢀPhenylcyclooctꢀ2ꢀenone (11с). The yield was 60%,
(Z)ꢀ2ꢀPhenylcycloheptꢀ2ꢀenol (5b). The yield was 16%,
viscous oil. 1Н NMR, δ: 2.86—1.57 (m, 9 H); 4.83—4.77
(dd, 1 H, J = 2.2 Hz, J = 7.3 Hz); 6.03 (t, 1 H, J = 6.7 Hz);
7.37—7.25 (m, 5 H). 13С NMR, δ: 146.51, 143.11, 132.14, 128.3,
126.84, 126.79, 72.81, 33.85, 27.73, 26.79, 25.06. IR, ν/cm–1
:
1
3428, 3058, 3025, 2929, 2856, 1683, 1598, 1492, 1446, 1274,
1078, 757, 700. Found (%): C, 82.74; H, 5.74. C13H16О.
Calculated (%): C, 82.94; H, 5.57.
colorless crystals, m.p. 91—93 °С. Н NMR, δ: 1.73—1.67 (m,
4 H); 1.98—1.93 (m, 2 H); 2.41—2.36 (m, 2 H); 2.58—2.55
(m, 2 H); 6.24 (t, 1 H, J = 5.8 Hz); 7.32—7.30 (m, 5 H).
13С NMR, δ: 212,94, 142.46, 140.22, 131.47, 128.75, 127.76,
126.27, 45.63, 29.75, 29.10, 23.55, 22.89. IR, ν/cm–1: 3058,
3020, 2929, 2856, 1714, 1455, 1216, 1062, 1037, 771, 698.
Found (%): C, 83.72; H, 8.38. C14H16О. Calculated (%):
C, 83.96; H, 8.05.
(Z)ꢀ2ꢀPhenylcyclooctꢀ2ꢀenol (5с). The yield was 19%, viscous
oil. 1Н NMR, δ: 1.48—1.39 (m, 2 H); 1.59—1.50 (m, 1 H);
1.89—1.66 (m, 4 H); 2.08—1.99 (m, 1 H); 2.32—2.27 (m,
2 H); 5.05—5.02 (dd, 1 H, J = 5.4 Hz, J = 9.2 Hz); 5.79 (t, 1 H,
J = 8.9 Hz); 7.32—7.25 (m, 5 H). 13С NMR, δ: 143.26, 140.27,
130.07, 128.44, 128.33, 127.19, 70.36, 39.17, 30.26, 27.41, 27.05,
24.54. IR, ν/cm–1: 3390, 3080, 3060, 2923, 2852, 1652, 1558,
1448, 1218, 1052, 979, 771, 698. Found (%): C, 82.96; H, 9.18.
C14H18О. Calculated (%): C, 83.12; H, 8.97.
Biphenyl (8). The yield was 25%, needleꢀlike crystals.
(1R*,2S*)ꢀ2ꢀBromoꢀ1ꢀphenylcyclohexanol (7а). The yield
was 27%, colorless solid, m.p. 52 °С (cf. Ref. 2: m.p. 48—49 °С).
1Н NMR, δ: 1.94—1.64 (m, 5 H); 2.19—2.11 (m, 1 H);
2.61—2.48 (m, 1 H); 2.77—2.65 (m, 1 H); 4.53—4.51 (m, 1 H);
7.48—7.32 (m, 3 H); 7.61—7.55 (m, 2 H). 13С NMR, δ: 148.33,
130.14, 129.78, 127.59, 76.36, 62.02, 33.42, 33.23, 22.93, 22.38.
IR, ν/cm–1: 3421, 3089, 3058, 3027, 2939, 2862, 1496, 1446,
1213, 1145, 1033, 1008, 966, 761, 698, 559. Found (%):
C, 56.22; H, 5.74. C12H15BrО. Calculated (%): C, 56.49;
H, 5.93.
Xꢀray diffraction study was carried out on a Rigaku RꢀAXIS
RAPIDꢀS diffractometer equipped with a twoꢀdimensional area
IP detector at 293 K (λ(MoꢀKα) radiation, graphite monoꢀ
chromator) using monoclinic crystals of compound 11а, C12H12О,
space group Р21/с; unit cell parameters: a = 6.089(2) Å,
3
b = 12.369(5) Å, c = 12.745(5) Å, β = 101.89(2)°; V = 939.3(2) Å ;
Z = 4; dcalc = 1.22 mg m–3; absorption coefficient 0.076 mm–1
;
F(000): 368; θ = 2.3—30.6°; θ range from 30.6° to 99.7%. The
reflections of the substrate under study were obtained by the
variation of the ω parameter with three sets of various χ and φ
values. In each of six experiments performed, 108 reflections
were obtained, which cover ~99.7% of the crystal surface of
compound 11с. The lattice parameters were determined taking
into account reflections with F2 > 2σ(F2). The calculations were
performed using the СristalClear software.12 The structure was
solved by a direct method using the SHELXSꢀ97 program13 and
refined in the anisotropic approximation. The final R factors are
Diene 6c was isolated from the second fraction obtained by
the elution with nꢀhexane of the reaction products of substrate
3с with NBS.
(1Е,3Z)ꢀ2ꢀPhenylcyclooctaꢀ1,3ꢀdiene (6с). The yield was
21%, colorless oil. 1Н NMR, δ: 1.54—1.43 (m, 4 H); 2.26—2.16
(m, 2 H); 2.32—2.27 (m, 2 H); 6.03—5.93 (m, 2 H); 6.11 (t,
1 H, J = 8.1 Hz); 7.24—7.18 (m, 1 H); 7.36—7.25 (m, 2 H);
7.42—7.37 (m, 2 H). 13С NMR, δ: 141.29, 137.83, 133.73,
128.86, 128.39, 127.05, 126.72, 126.63, 28.70, 28.60, 24.37,
22.62. IR, ν/cm–1: 3054, 2923, 2852, 1652, 1635, 1558, 1488,
1457, 750, 696. Found (%): C, 91.46; H, 8.98. C14H16. Calculꢀ
ated (%): C, 91.25; H, 8.75.
Oxidation of 2ꢀphenylcycloalkꢀ2ꢀenols 5а—с with pyridinium
chlorochromate (PCC) (general procedure). A solution of alcohol
5 (0.54 mmol) in СН2Сl2 (5 mL) was added to a stirred solution
of PCC (0.12 g, 0.56 mmol) in СН2Сl2 (10 mL) at 0 °С for
30 min. The reaction mixture was stirred at ~20 °C for 3 h and
then filtered. The organic layer was washed water (30 mL),
and dried (Na2SO4), and the solvent was evaporated. Ketones
11а—с were obtained in the residue.
Table 1. Selected bond lengths, bond angles, and torꢀ
sion angles for structure 11с
Parameter
Bond
Value
d/Å
O(1)—C(12)
C(7)—C(8)
C(7)—C(12)
C(7)—C(6)
C(12)—C(11)
1.213(3)
1.342(3)
1.485(3)
1.489(3)
1.505(4)
Bond angle
ω/deg
C(8)—C(7)—C(12)
C(8)—C(7)—C(6)
C(12)—C(7)—C(6)
119.7(2)
121.1(2)
119.20(19)
Torsion angle
θ/deg
2ꢀPhenylcyclohexꢀ2ꢀenone (11а). The yield was 73%, colorꢀ
less crystals, m.p. 78 °С (cf. Ref. 10: m.p. 67—69 °С). 1Н NMR,
δ: 2.11 (t, 2 H, J = 6.2 Hz); 2.54 (m, 2 H); 2.59 (t, 2 H,
J = 6.6 Hz); 7.06 (t, 1 H, J = 4.3 Hz); 7.25—7.36 (m, 5 H).
C(8)—C(7)—C(6)—C(5)
C(12)—C(7)—C(6)—C(5)
C(8)—C(9)—C(10)—C(11)
48.6(3)
–130.9(2)
–40.5(4)

Bromination of phenylcycloalkenes
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 11, November, 2009 2303
R1 = 0.076 with I > 2σ(I), wR2 = 0.226 for 2874 reflections with
F2 = 1.058. The obtained bond lengths and selected bond and
torsion angles are listed in Table 1.
7. M. Ceylan, Y. Budak, M. Ulukaya, M. B. Gürdere, E. Findik,
Turk. J. Chem., 2006, 30, 663.
8. M. Ceylan, Y. Budak, M. B. Gürdere, Turk. J. Chem., 2006,
30, 635.
9. H. Hock, M. Siebert, Chem. Ber., 1954, 87, 554.
10. B. Miller, H. S. Wong, Tetrahedron, 1972, 28, 2369.
11. D. Gýnsburg, R. Pappo, J. Am. Chem. Soc., 1953, 75, 1094.
12. Rigaku, Crystal Clear, Version 1.3.6, Rigaku American
Corporation 9009 New Trails Drive, The woodlands, TX
77381—5209, USA, 2005.
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3. D. R. Dalton, D. G. Jones, Tetrahedron Lett., 1967, 30,
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13. G. H. Sheldrick, SHELXSꢀ97 and SHELXLꢀ97, University
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Received March 11, 2008;
in revised form June 9, 2009