Chlorine versus bromine: Facial selectivity in the Diels-Alder reactions of 5-bromo-1,2,3,4,5-pentachlorocyclopenta-1,3-diene

Article abstract of DOI:10.1039/a805355c

Diels-Alder reactions of diene 11 with both electron-poor and electron-rich dienophiles led to approximately 90% addition to the face of 11 syn to the C-5 chlorine. Addition of 4-phenyl-1,2,4-triazoline-3,5-dione was slightly less facially selective. Adduct ratios were obtained by NMR methods; stereochemistry was determined by X-ray crystallography. The results support the view that facial selectivity in Diels-Alder reactions with heteroatom-substituted dienes is mainly controlled by steric, not stereoelectronic, interactions.

Full text of DOI:10.1039/a805355c

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Chlorine versus bromine: facial selectivity in the Diels–Alder
reactions of 5-bromo-1,2,3,4,5-pentachlorocyclopenta-1,3-diene
Lori C. Burry, David O. Miller† and D. Jean Burnell*
Department of Chemistry, Memorial University of Newfoundland, St. John’s, Newfoundland,
Canada A1B 3X7
Received (in Cambridge) 9th July 1998, Accepted 21st September 1998
Diels–Alder reactions of diene 11 with both electron-poor and electron-rich dienophiles led to approximately 90%
addition to the face of 11 syn to the C-5 chlorine. Addition of 4-phenyl-1,2,4-triazoline-3,5-dione was slightly less
facially selective. Adduct ratios were obtained by NMR methods; stereochemistry was determined by X-ray
crystallography. The results support the view that facial selectivity in Diels–Alder reactions with heteroatom-
substituted dienes is mainly controlled by steric, not stereoelectronic, interactions.
3. Conditions for the Diels–Alder reactions of 11 and the facial
Introduction
selectivities are summarized in Table 1. Electron-deficient dien-
ophiles such as N-phenylmaleimide and 1,4-naphthoquinone
were very slow to react, whereas vinylene carbonate, which is
electron-rich, reacted more quickly, although its adducts
decomposed at elevated temperatures. Every Diels–Alder reac-
tion provided only two adducts, and the NMR data indicated
that both adducts were symmetrical. Hence, adducts were not
derived from thermally isomerized dienes, and the similarity of
chemical shifts suggested that adducts were epimeric at the
methano bridge, i.e., not mixtures of endo- and exo-addition
products. Integration of the NMR spectra of every product
mixture revealed that the ratio of the adducts was very similar
(Table 1). The exception was a somewhat lower ratio with 4-
phenyl-1,2,4-triazoline-3,5-dione (PTAD). We had found, both
experimentally5 and computationally,8 that this dienophile can
impart a destabilizing interaction with a syn halogen, similar to
the closed-shell repulsion noted by Coxon et al.9 with another
diene system.
Column chromatography failed to separate the adducts.
Enrichment of any adduct by crystallization was unsuccessful.§
Therefore, the stereochemistry of the adducts from five of the
six Diels–Alder reactions was determined by X-ray crystal-
lography with the crystalline mixtures of the syn–anti adduct
pairs. In order to effect a structural analysis, each mixture was
modelled with both halogens on the methano bridge as brom-
ine, and the proportions of the bromines were varied to obtain
the best refinement values. Thus, the bromine occupancy on
the methano bridge reflected the proportion of chlorine and
bromine on the methano bridge, and the molecular diagrams,
e.g. Fig. 1, showed no obvious distortions. Structures 12, 14, 16,
20 and 22 were assigned from the crystallographic work, and
spectroscopic parallels allowed a confident assignment of 18 as
the major isomer with 3-nitrostyrene. The preferred mode of
attack by the dienophile was syn to the chlorine of 11 in every
instance. Furthermore, from the optimized partial occupancy
data, the proportions of the minor adducts 13, 15, 17 and 23
in the recrystallized samples could be estimated. The adduct
mixture derived from vinylene carbonate contained a signifi-
cant proportion of a third hexachloro component, making
estimation of the amount of 21 difficult. Adduct ratios deter-
mined in this crystallographic manner should be considered
Facial selectivity in the Diels–Alder reactions with plane-
nonsymmetric cyclopentadienes substituted at C-5 by a halogen
has been examined a number of times. Additions to 1 occurred
overwhelmingly syn‡ to fluorine.1 Dienes 2–5, with a C-5
X
H
Cl
Y
X
CH3
CH3
Cl
Cl
H3C
Cl
Cl
H3C
CH3
1
2
6
7
X = F
3
4
Y = H
5
8
9
X = Cl
X = Br
X = I
X = Cl
X = Br
X = I
Y = CH3
10 Y = OCH3
11 Y = Br
25 X = NH2
24 X = OAc
chlorine, all favoured syn‡ addition.2–5 Bromine and, especially,
iodine at C-5, as in 6–9, directed an incoming dienophile anti.4,5
Facial selectivity data had also been reported for two dienes
in which chlorine was pitted against a different heteroatom.
Reactions of diene 10 were examined rigorously with a variety
of dienophiles, and addition was shown to be anti to the C-5
chlorine with every dienophile.6 Diene 11 presented a facial
choice between chlorine and bromine. Williamson and Hsu3
stated that 11 was too thermally labile to allow a reasonable
assessment of its facial selectivity. On the other hand, Shesta-
kova et al.7 did report Diels–Alder reactions with 11, but their
assignment of the stereochemistry for the adducts was based on
an interpretation of the 1H NMR chemical shift (60 MHz) data
that was found to be untenable in the study of 10.6 We chose to
re-examine the reactions of 11 with a variety of dienophiles not
only because of uncertainty with regard to the published struc-
tures of the adducts but more importantly because the results
might shed light on how facial selectivity is governed.
Results and discussion
Diene 11 was prepared in good yield by treatment with N-
bromosuccinimide of the cyclopentadienyl anion derived from
† Author to whom correspondence regarding X-ray crystal structures
should be addressed.
‡ In this paper “syn” and “anti” are relative to the chlorine at C-5 of the
diene, or, for dienes without chlorine at C-5, “syn” and “anti” are with
respect to the heteroatom at C-5.
§ The adduct mixture derived from vinylene carbonate also co-
crystallized with some adduct derived from hexachlorocyclopentadiene.
J. Chem. Soc., Perkin Trans. 1, 1998, 3825–3830
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Table 1 Reaction conditions, yields and facial selectivities of the Diels–Alder reactions of diene 11
Dienophile and reaction conditions
Yield (%)
Adduct ratio by NMR
Adduct ratio by X-ray
N-Phenylmaleimide (2 equiv.); toluene reflux, 21 days
1,4-Naphthoquinone (2 equiv.); toluene reflux, 28 days
Styrene (1.5 equiv.); benzene reflux, 6 days
3-Nitrostyrene (1.5 equiv.); toluene reflux, 3 days
Vinylene carbonate (1.2 equiv.); neat, 150 ЊC, 3 h
4-Phenyl-1,2,4-triazoline-3,5-dione (1 equiv.); benzene reflux, 2 days
44
45
63
36
9
12:13 92:8
14:15 89:11
16:17 94:6
18:19 94:6
20:21 88:12
22:23 82:18
12:13 90:10
14:15 95:5
16:17 95:5
—
20:21 93:7
22:23 85:15
83
Y
Y
X
X
Cl
Cl
H
H
H
Cl
Cl
H
O
Cl
Cl
Cl
Cl
O
N
O
O
Ph
14 X = Br, Y = Cl
15 X = Cl, Y = Br
12 X = Br, Y = Cl
13 X = Cl, Y = Br
Y
X
Cl
H
H
Cl
Cl
Cl
H
Z
16 X = Br, Y = Cl, Z = H
17 X = Cl, Y = Br, Z = H
18 X = Br, Y = Cl, Z = NO2
19 X = Cl, Y = Br, Z = NO2
Fig. 1 ORTEP diagram for 22/23.
Y
Y
X
X
Cl
Cl
H
H
approximate, but, as shown in Table 1, these ratios were similar
to those obtained by NMR techniques.
Cl
Cl
N
O
N
Cl
Cl
Cl
O
Cl
Steric hindrance and torsional effects have been used to
explain the facial selectivity in the Diels–Alder reactions of
some cyclic, plane-nonsymmetric dienes,4,8,10 but additions to
dienes 1,1 2,5 24 and analogues11 and 254 all took place prefer-
entially syn to the C-5 heteroatom. These reactions, and the
reactions of some dienes fused to bicyclic systems,12 led to the
formulation of a number of hypotheses concerning factors that
govern stereoelectronic control of this facial selectivity. The
data in Table 1 indicate that none of these phenomena control
facial selectivity with 11. That the adduct ratios (Table 1) were
the same with N-phenylmaleimide and 1,4-naphthoquinone
(“normal” Diels–Alder reactions) as with vinylene carbonate
(“inverse-electron-demand”) ruled out control of facial selectiv-
ity by a facial bias in the πHOMO 13 or the nucleophilicity14 of the
diene, or of orbital mixing between the heteroatom lone pairs
and the incoming dienophile.15 Control of facial selectivity in
Diels–Alder reactions by a process similar to that originally
hypothesized by Cieplak16 for carbonyl additions would involve
at the transition state σ-donation from the anti substituent at C-
5 of the diene.4,17 σ-Donation by a C–Cl bond should be similar
to that of a C–Br bond,18 so this hypothesis would predict little
facial preference with 11. Williamson and Hsu3 have proposed
that van der Waals/London type forces control facial selectivity
with 3, and he predicted that, since the polarizability of
bromine is greater than that of chlorine, Diels–Alder additions
to 11 should be mainly syn to bromine. Photoelectron spectra
and calculations by Werstiuk and co-workers19 have ruled out
some of these hypotheses, and a recent ab initio computational
study pointed out that steric control of facial selectivity with
5-substituted cyclopentadienes correlated very well with a com-
puted steric factor.8 That the facial selectivity with diene 11
is little affected by the electronic nature of its Diels–Alder
N
O
Ph
O
O
20 X = Br, Y = Cl
21 X = Cl, Y = Br
22 X = Br, Y = Cl
23 X = Cl, Y = Br
reactions is strong experimental corroboration for the proposal
for control of facial selectivity by steric effects.
Experimental
General
Uncorrected mps were determined on a Fisher-Johns appar-
atus. IR spectra were measured as thin films on a Mattson
Polaris FT instrument. A General Electric GE 300-NB spectro-
meter provided the H and 13C NMR spectra. Mass spectra
1
(EI) were obtained on a V. G. Micromass 7070HS instrument or
a Hewlett-Packard 5970 mass selective detector (GC–MS). The
X-ray diffractometer was a Rigaku AFC6S instrument.
5-Bromo-1,2,3,4,5-pentachlorocyclopenta-1,3-diene 11
A 2.5 M solution of n-butyllithium (1.72 ml, 4.31 mmol) in
hexane was added dropwise to a solution of 1,2,3,4,5-
pentachlorocyclopenta-1,3-diene5 (0.790 g, 3.32 mmol) in dry
tetrahydrofuran (THF) (30 ml) at Ϫ78 ЊC. A solution of N-
bromosuccinimide (0.804 g, 4.51 mmol) in THF (15 ml) was
added, and the mixture was allowed to warm slowly to room
temperature. The solution was concentrated under vacuum,
and the orange–brown residue was redissolved in diethyl ether.
The solution was washed with H2O and brine, and dried over
anhydrous MgSO4. Flash chromatography with hexane as the
3826
J. Chem. Soc., Perkin Trans. 1, 1998, 3825–3830

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eluent gave diene 11 (0.834 g, 79%) as an orange–brown oil:
νmax(film)/cmϪ1 1599, 1567, 1231, 1188 and 1136; δC 133.8, 127.3
and 67.7; m/z 322 (1%), 320 (5), 318 (9), 316 (10) and 314 (4) all
Mϩ, 285 (3), 283 (8), 281 (10), 279 (4), 243 (3), 241 (21), 239
(68), 237 (100), 235 (61), 171 (0.2), 169 (5), 167 (15), 165 (16),
134 (3), 132 (14), 130 (23), 97 (10), 95 (31) and 60 (20).
(1R*,4S*,5R*,7R*)- 18 and (1R*,4S*,5R*,7S*)-7-bromo-
1,2,3,4,7-pentachloro-5-(3-nitrophenyl)bicyclo[2.2.1]hept-
2-ene 19
Colourless crystals (mixture of 18 and 19) were obtained from
ether–ethyl acetate–petroleum ether: mp 116–117 ЊC; νmax(film)/
cmϪ1 1601, 1530 and 1349; δH (for 18 from mixture) 8.22 (1 H,
d, J 8.1), 8.02 (1 H, narrow m), 7.58–7.43 (2 H, m), 4.14 (1 H,
dd, J 4.3 and 9.2), 3.05 (1 H, dd, J 9.2 and 12.8) and 2.54 (1 H,
dd, J 4.3 and 12.8); δH (for 19 from mixture) 4.20 (1 H, dd, J 4.8
and 9.8); δC (for 18 from mixture) 148.2, 136.7, 134.7, 133.2,
131.6, 129.6, 123.9, 123.5, 93.9, 84.3, 79.2, 50.8 and 39.9; δC (for
19 from mixture) 51.8 and 41.1; m/z 471 (0.6%), 470 (0.2), 469
(2), 468 (0.5), 467 (3), 466 (0.6), 465 (3), 464 (0.1) and 463 (1) all
Mϩ, 324 (2), 323 (1), 322 (14), 321 (3), 320 (50), 319 (6), 318
(99), 317 (6), 316 (100), 315 (2), 314 (38), 310 (2), 300 (0.8), 299
(10), 298 (2), 297 (25), 296 (2), 295 (31), 294 (1), 293 (14), 243
(3), 242 (1), 241 (19), 240 (3), 239 (57), 238 (5), 237 (88), 236 (3),
235 (55), 220 (2), 219 (0.7), 218 (5), 217 (1), 216 (11), 215 (1),
214 (9), 172 (9), 170 (28), 149 (15), 133 (14), 103 (33) and 77
(34).
General procedure for the Diels–Alder reactions:
(3aá,4â,7â,7aá,8s)- 12 and (3aá,4â,7â,7aá,8r)-8-bromo-
4,5,6,7,8-pentachloro-3a,4,7,7a-tetrahydro-2-phenyl-4,7-
methano-1H-isoindole-1,3-(2H)-dione 13
A solution of diene 11 (122 mg, 0.385 mmol) and N-
phenylmaleimide (125 mg, 0.724 mmol) in toluene (8.0 ml) was
heated under reflux for 21 days. The solution was concentrated
under vacuum. TLC (EtOAc–hexanes) showed a single spot, but
1H NMR analysis indicated this was a mixture of 12 and 13, in
a ratio of 11:1, respectively, and unreacted dienophile. The
mixture was redissolved in CH2Cl2 (20 ml) and heated under
reflux. 1,2,3,4-Tetramethyl-5,5-dimethoxycyclopenta-1,3-diene
was added to consume the unreacted dienophile. Flash chrom-
atography (5% EtOAc–hexanes) provided 83 mg (44%) of
colourless crystals composed of both 12 and 13, in the same
ratio (by 1H NMR) as before chromatography. Recrystallization
(acetone–hexane) gave colourless needles with no change in the
12:13 ratio. For these crystals (Found C, 36.75; H, 1.52; N,
2.83. C15H7BrCl5NO2 requires C, 36.74; H, 1.44; N, 2.86%); mp
235–236 ЊC; νmax(film)/cmϪ1 1723; δH (for 12 from mixture)
7.50–7.38 (3 H, m), 7.16–7.09 (2 H, m) and 4.02 (2 H, s); δH (for
13 from mixture) 4.08 (s); δC (for 12 from mixture) 169.1, 132.1,
130.6, 129.5, 126.4, 95.2, 79.9 and 51.4; δC (for 13 from mixture)
94.6; m/z 497 (1%), 495 (6), 493 (21), 489 (38) and 487 (15) all
Mϩ, 458 (2), 456 (4), 454 (4), 452 (2), 313 (2), 311 (11), 309 (29),
307 (35), 305 (15), 267 (4), 265 (12), 263 (18), 261 (12), 241 (2), 239
(7), 237 (11), 235 (7), 173 (100), 119 (19), 91 (15) and 54 (17).
(3aá,4â,7â,7aá,8s)- 20 and (3aá,4â,7â,7aá,8r)-8-bromo-
4,5,6,7,8-pentachloro-3a,4,7,7a-tetrahydro-4,7-methano-
1,3-benzodioxol-2-one 21
Sublimation then recrystallization from ether–petroleum ether
gave colourless crystals (mixture of 20, 21 and the adduct
derived from hexachlorocyclopentadiene): mp 145–155 ЊC
(subl.); νmax(film)/cmϪ1 1822; δH (for 20 from mixture) 5.40 (s);
δH (for 21 from mixture) 5.44 (s); δH (for hexachloro adduct
from mixture) 5.37 (s); δC (for 20 from mixture) 151.3, 132.7,
88.8, 82.0 and 81.1; m/z (GC–MS) 406 (3%), 404 (14) and 402
(11) all Mϩ, 322 (18), 320 (59), 319 (17), 318 (98), 317 (2), 316
(100), 314 (38), 296 (15), 294 (18), 292 (9), 254 (2), 252 (16), 251
(7), 250 (23), 249 (2), 248 (16), 241 (17), 239 (59), 238 (3), 237
(86), 236 (2), 235 (55), 218 (17), 217 (10), 216 (40), 215 (21), 214
(34), 213 (16), 145 (17), 144 (10), 143 (33), 142 (4), 141 (15), 108
(40) and 73 (19).
(1á,4á,4aâ,9aâ,11r)- 14 and (1á,4á,4aâ,9aâ,11s)-11-bromo-
1,2,3,4,11-pentachloro-1,4,4a,9a-tetrahydro-1,4-methano-
anthracene-9,10-dione 15
(10s)- 22 and (10r)-10-bromo-5,6,7,8,10-pentachloro-5,8-
dihydro-2-phenyl-5,8-methano-1H-[1,2,4]triazolo[1,2-a]-
pyridazine-1,3(2H)-dione 23
Colourless crystals (mixture of 14 and 15) were obtained
from ether–petroleum ether: mp 139–140 ЊC; δH (for 14 from
mixture) 8.02 (2 H, symmetrical m), 7.80 (2 H, symmetrical m)
and 4.09 (2 H, s); δH (for 15 from mixture) 4.16 (s); δC (for 14
from mixture) 189.1, 135.2, 134.7, 132.3, 93.6, 82.3 and 54.2;
m/z 480 (1%), 479 (0.8), 478 (4), 477 (2), 476 (8), 475 (2), 474 (8),
473 (0.7) and 472 (3) all Mϩ, 445 (0.2), 444 (0.1), 443 (2), 442
(1), 441 (4), 440 (1), 439 (5), 438 (0.7), 437 (3), 435 (0.2), 402
(0.5), 401 (1), 400 (0.5), 399 (3), 398 (2), 397 (8), 396 (2),
395 (12), 394 (2), 393 (7), 366 (0.4), 355 (1), 364 (2), 363 (2), 362
(6), 361 (4), 360 (14), 359 (6), 358 (13), 357 (4), 356 (3), 326 (3),
325 (9), 324 (6), 323 (10), 322 (12), 321 (3), 320 (43), 319 (5), 318
(83), 317 (5), 316 (81), 315 (2), 314 (33), 245 (0.7), 238 (9), 237
(100), 236 (6), 235 (61), 169 (22), 167 (64), 158 (12), 104 (66), 76
(98) and 50 (41).
Colourless crystals (mixture of 22 and 23) were obtained from
CH2Cl2–hexane (Found C, 31.69; H, 1.00; N, 8.50. C13H5BrCl5-
N3O2 requires C, 31.71; H, 1.02; N, 8.53%); mp 137–141 ЊC but
turning pink first at 130 ЊC; νmax(film)/cmϪ1 3076, 3046, 3022,
1804, 1749 and 1596; δC (for 22 from mixture) 154.6, 129.6,
129.5, 129.4, 125.5, 92.7 and 87.0; δC (for 23 from mixture)
127.8 and 86.4; m/z 460 (0.4%), 458 (1.5), 456 (1.7) and 454 (0.7)
all Mϩ Ϫ Cl, 324 (0.6), 322 (5), 320 (18), 318 (34), 316 (34), 314
(13), 287 (0.3), 285 (3), 283 (7), 281 (8), 279 (4), 243 (3), 241
(20), 239 (64), 237 (100), 235 (62), 119 (68), 91 (31) and 64 (19).
X-Ray analysis of 12/13
Crystal data. Colourless irregular crystal 0.15 × 0.10 × 0.40
mm; C15H7BrCl5NO2, M = 490.39. Monoclinic, a = 7.762(2),
b = 37.419(2), c = 12.518(2) β = 90.81(2)Њ, V = 3635.4(8) Å3
(from a least-squares refinement using the settings of 16 care-
fully centred reflections in the range 58.52 < 2θ < 59.69Њ,
λ = 1.54178 Å, T = 26.0 ЊC), space group P21/c (#14), Z = 8,
(1R*,4S*,5R*,7R*)- 16 and (1R*,4S*,5R*,7S*)-7-bromo-
1,2,3,4,7-pentachloro-5-phenylbicyclo[2.2.1]hept-2-ene 17
Colourless crystals (mixture of 16 and 17) were obtained from
ether–petroleum ether (Found C, 37.08; H, 1.73. C13H8BrCl5
requires C, 37.06; H, 1.91%); mp 67–68 ЊC; νmax(film)/cmϪ1
1603; δH (for 16 from mixture) 7.33 (3 H, narrow m), 7.10 (2 H,
narrow m), 4.01 (1 H, dd, J 4.3 and 9.1), 2.96 (1H, dd, J 9.1 and
12.9) and 2.51 (1H, dd, J 4.3 and 12.9); δH (for 17 from mixture)
4.07 (1 H, dd, J 4.1 and 9.1); δC (for 16 from mixture) 134.3,
132.3, 132.2, 128.9, 128.5, 94.6, 84.5, 79.4, 51.1 and 39.9; δC (for
17 from mixture) 52.1 and 41.1; m/z 424 (0.5%), 420 (0.5) and
418 (0.1) all Mϩ, 243 (0.2), 241 (1), 239 (3), 238 (0.7), 237 (5),
236 (0.5), 235 (4), 234 (0.8), 233 (3), 127 (16), 125 (55), 104
(100), 103 (10), 78 (11), 77 (7) and 51 (6).
Dcalc = 1.79 g cmϪ3, µ(Cu-Kα) = 99.3 cmϪ1
.
Data collection and processing. Graphite-monochromated
Cu-Kα radiation, ω-2θ scans, with 2θ maximum 120.1Њ, ω scan
width (0.94 ϩ 0.14 tan θ)Њ; 5985 reflections collected
(2.5 ≤ θ ≤ 60Њ, 8 h, 40 k, 14 l), of which 5533 (Rint = 0.034)
were unique. Intensities of three representative reflections
were measured after every 150 reflections. No decay correction
was applied. An empirical absorption correction based on
J. Chem. Soc., Perkin Trans. 1, 1998, 3825–3830
3827

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azimuthal scans of several reflections was applied, which resulted
in transmission factors ranging from 0.68 to 1.00. Data were
corrected for Lorentz and polarization effects. A correction for
secondary extinction was applied (coefficient = 6.41292eϪ7).
Calculations used the TEXSAN crystallographic software
package.24
X-Ray analysis of 16/17
Crystal data. Colourless irregular crystal 0.20 × 0.30 × 0.40
mm mounted on a glass fibre; C13H8BrCl5, M = 421.38.
Monoclinic, a = 14.075(2), b = 10.224(3), c = 11.551(3) Å, β =
70.33(2)Њ, V = 1565(1) Å3 (from a least-squares refinement using
the settings of 14 carefully centred reflections in the range
29.45 < 2θ < 32.39Њ, λ = 0.71069 Å, T = 26.0 ЊC), space group
Structure analysis and refinement. The structure was solved
by and expanded using Fourier techniques.20 Non-H atoms
were refined anisotropically; H atoms were included but not
refined. The final cycle of full-matrix least-squares refinement
was based on 3340 observed reflections (I = 2.00σ(I)) and 434
variable parameters and converged (largest parameter shift was
0.00 times its esd). With 90% bromine anti at C-8, the agree-
ment factors were R (Σ(|Fo| Ϫ |Fc|)/Σ|Fo|) = 0.045 and Rw
P21/c (#14), Z = 4, Dcalc = 1.788 g cmϪ3, µ(Mo-Kα) = 34.4 cmϪ1
.
Data collection and processing. Graphite-monochromated
Mo-Kα radiation, ω-2θ scans, with 2θ maximum 50.1Њ, ω scan
width (1.37 ϩ 0.35 tan θ)Њ; 3083 reflections collected
(2.5 ≤ θ ≤ 25Њ, 16 h, 12 k, 13 l), of which 2941 (Rint = 0.050)
were unique. Intensities of three representative reflections were
measured after every 150 reflections. No decay correction was
applied. An empirical absorption correction based on azi-
muthal scans of several reflections was applied, which resulted
in transmission factors ranging from 0.87 to 1.00. Data were
corrected for Lorentz and polarization effects. A correction for
secondary extinction was applied (coefficient = 0.33243eϪ7).
2
([Σw(|Fo| Ϫ |Fc|)2/ΣwFo )]1/2) = 0.046 with GOF = 1.64 (for 85%
bromine anti at C-8, R = 0.047, Rw = 0.048, GOF = 1.71; for
95% bromine anti at C-8, R = 0.047, Rw = 0.049, GOF = 1.76).
The weighting scheme was based on counting statistics and
included a factor (p = 0.022) to downweight the intense reflec-
tions. Plots of Σw(|Fo| Ϫ |Fc|)2 versus |Fo|, reflection order in data
collection, sin θ/λ and various classes of indices showed no
unusual trends. The maximum and minimum peaks on the final
difference Fourier map corresponded to ϩ0.44 and Ϫ0.45
e ÅϪ3, respectively. Neutral atom scattering factors were from
the International Tables for X-ray Crystallography.21a Anomal-
ous dispersion effects were included in Fcalc;
22 the values for ∆f Ј
Structure analysis and refinement. The structure was solved
by direct methods.20,26 Non-H atoms were refined anisotropic-
ally. The final cycle of full-matrix least-squares refinement was
based on 1573 observed reflections (I < 2.0σ(I)) and 197 vari-
able parameters and converged (largest parameter shift was
0.00 times its esd). With 95% bromine anti at C-7, the factors
were R = 0.041 and Rw = 0.040 with GOF = 1.40 (for 90% brom-
ine anti at C-7, R = 0.045, Rw = 0.043, GOF = 1.52; for 100%
bromine anti at C-7, R = 0.043, Rw = 0.043, GOF = 1.50). The
weighting scheme was based on counting statistics and included
a factor (p = 0.030) to downweight the intense reflections. Plots
of Σw(|Fo| Ϫ |Fc|)2 versus |Fo|, reflection order in data collection,
sin θ/λ and various classes of indices showed no unusual trends.
The maximum and minimum peaks on the final difference
Fourier map corresponded to ϩ0.33 and Ϫ0.44 e ÅϪ3, respect-
ively. Neutral atom scattering factors and the values for ∆f Ј and
∆f Љ were from the International Tables for X-ray Crystal-
and ∆f Љ and for the mass attenuation coefficients were from the
International Tables for Crystallography.23a,b Calculations used
the TEXSAN crystallographic software package.24
X-Ray analysis of 14/15
Crystal data. Colourless irregular crystal 0.20 × 0.05 × 0.40
mm mounted on a glass fibre; C15H6BrCl5O2, M = 475.38. Tri-
clinic, a = 13.562(5), b = 14.154(5), c = 11.188(2) Å, α =
108.29(2), β = 95.32(2), γ = 116.92(3)Њ, V = 1746.8(13) Å3 (from
a least-squares refinement using the settings of 21 carefully
centred reflections in the range 20.08 < 2θ < 25.10Њ, λ = 0.71069
Å, T = 26.0 ЊC), space group P-1 (#2), Z = 4, Dcalc = 1.81 g
cmϪ3, µ(Mo-Kα) = 31.3 cmϪ1
.
Data collection and processing. Graphite-monochromated
Mo-Kα radiation, ω ϩ 2θ scans, with 2θ maximum 50.1Њ, ω
scan width (1.15 ϩ 0.35 tan θ)Њ; 6459 reflections collected
(2.5 ≤ θ ≤ 25Њ, 16 h, 16 k, 13 l), of which 6174 (Rint = 0.051)
were unique. Intensities of three representative reflections were
measured after every 150 reflections. No decay correction was
applied. An empirical absorption correction based on azi-
muthal scans of several reflections was applied, which resulted
in transmission factors ranging from 0.74 to 1.00. Data were
corrected for Lorentz and polarization effects.
lography.21a,b Anomalous dispersion effects were included in
22
Fcalc
.
Calculations used the TEXSAN crystallographic soft-
ware package.27
X-Ray analysis of 20/21
Crystal data. Colourless irregular crystal 0.30 × 0.25 × 0.40
mm mounted on a glass fibre; C8H2Br0.7Cl5.3O3, M = 389.94.
Monoclinic, a = 12.453(2), b = 23.838(2), c = 8.853(5) Å,
β = 84.84(3)Њ, V = 2617(2) Å3 (from a least-squares refinement
using the settings of 20 carefully centred reflections in the range
23.85 < 2θ < 31.07Њ, λ = 0.71069 Å, T = 26.0 ЊC), space group
Structure analysis and refinement. The structure was solved
by direct methods and expanded Fourier techniques.20,25 Non-H
atoms were refined anisotropically. The final cycle of full-matrix
least-squares refinement was based on 2811 observed reflections
(I = 2.00σ(I)) and 415 variable parameters and converged
(largest parameter shift was 0.00 times its esd). With 95% brom-
ine anti at C-11, the agreement factors were R = 0.049 and
Rw = 0.047 with GOF = 1.41 (for 90% bromine anti at C-11,
R = 0.050, Rw = 0.047, GOF = 1.44; for 100% bromine anti at C-
11, R = 0.051, Rw = 0.050, GOF = 1.52). The weighting scheme
was based on counting statistics and included a factor
(p = 0.027) to downweight the intense reflections. Plots of
Σw(|Fo| Ϫ |Fc|)2 versus |Fo|, reflection order in data collection, sin
θ/λ and various classes of indices showed no unusual trends.
The maximum and minimum peaks on the final difference
Fourier map corresponded to ϩ0.47 and Ϫ0.45 e ÅϪ3, respect-
ively. Neutral atom scattering factors were from the Inter-
P21/c (#14), Z = 8, Dcalc = 1.979 g cmϪ3, µ(Mo-Kα) = 32.8 cmϪ1
.
Data collection and processing. Graphite-monochromated
Mo-Kα radiation, ω-2θ scans, with 2θ maximum 50.1Њ, ω scan
width (1.15 ϩ 0.35 tan θ)Њ; 5081 reflections collected
(2.5 ≤ θ ≤ 25Њ, 14 h, 28 k, 10 l), of which 4763 (Rint = 0.044)
were unique. Intensities of three representative reflections were
measured after every 150 reflections. No decay correction was
applied. An empirical absorption correction based on azi-
muthal scans of several reflections was applied, which resulted
in transmission factors ranging from 0.89 to 1.00. Data were
corrected for Lorentz and polarization effects.
Structure analysis and refinement. The structure was solved
by direct methods.20,26 Non-H atoms were refined anisotropic-
ally. The final cycle of full-matrix least-squares refinement
was based on 2180 observed reflections (I < 2.0σ(I)) and 307
national Tables for X-ray Crystallography.21a Anomalous dis-
22
persion effects were included in Fcalc
.
The values for ∆f Ј and
∆f Љ were from the International Tables for Crystallography.23a
3828 J. Chem. Soc., Perkin Trans. 1, 1998, 3825–3830

View Article Online
variable parameters and converged (largest parameter shift was
0.00 times its esd). With 65% bromine anti at C-8 (20), 5% syn at
C-8 (21) and 30% of the hexachloro-adduct, the agreement
factors were R = 0.047 and Rw = 0.047 with GOF = 1.46 (for
70% 20 and 30% of the hexachloro-adduct R = 0.049,
Rw = 0.048, GOF = 1.51; for 65% 20, 8% 21 and 27%
hexachoro-adduct R = 0.049, Rw = 0.048, GOF = 1.51). The
weighting scheme was based on counting statistics and included
a factor (p = 0.030) to downweight the intense reflections. Plots
of Σw(|Fo| Ϫ |Fc|)2 versus |Fo|, reflection order in data collection,
sin θ/λ and various classes of indices showed no unusual trends.
The maximum and minimum peaks on the final difference
Fourier map corresponded to ϩ0.45 and Ϫ0.40 e ÅϪ3, respect-
ively. Neutral atom scattering factors and the values for ∆f Ј
and ∆f Љ were from the International Tables for X-ray Crystal-
request to the CCDC for this material should quote the full
literature citation and the reference number 207/265.
Acknowledgements
This work was supported by a grant from the Natural Sciences
and Engineering Research Council of Canada.
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22
Fcalc
.
Calculations used the TEXSAN crystallographic soft-
ware package.27
X-Ray analysis of 22/23
Crystal data. Colourless irregular crystal 0.30 × 0.30 × 0.30
mm mounted on a glass fibre; C13H5BrCl5N3O2, M = 492.37.
Monoclinic, a = 9.019(6), b = 13.339(9), c = 14.876(4) Å,
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Data collection and processing. Graphite-monochromated
Mo-Kα radiation, ω-2θ scans, with 2θ maximum 50.1Њ, ω scan
width (0.94 ϩ 0.35 tan θ)Њ; 3435 reflections collected
(2.5 ≤ θ ≤ 25Њ, 10 h, 15 k, 17 l), of which 3222 (Rint = 0.023)
were unique. Intensities of three representative reflections were
measured after every 150 reflections. No decay correction was
applied. An empirical absorption correction based on azi-
muthal scans of several reflections was applied, which resulted
in transmission factors ranging from 0.92 to 1.00. Data were
corrected for Lorentz and polarization effects. A correction for
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were refined but their isotropic B’s were held fixed. The final
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observed reflections (I < 2.0σ(I)) and 233 variable parameters
and converged (largest parameter shift was 0.00 times its esd).
With 85% bromine anti at C-10, the agreement factors were
R = 0.038 and Rw = 0.038 with GOF = 1.87 (for 75% bromine
anti at C-10, R = 0.048, Rw = 0.052, GOF = 2.58; for 95%
bromine anti at C-10, R = 0.045, Rw = 0.050, GOF = 2.49). The
weighting scheme was based on counting statistics and included
a factor (p = 0.020) to downweight the intense reflections. Plots
of Σw(|Fo| Ϫ |Fc|)2 versus |Fo|, reflection order in data collection,
sin θ/λ and various classes of indices showed no unusual trends.
The maximum and minimum peaks on the final difference
Fourier map corresponded to ϩ0.41 and Ϫ0.40 e ÅϪ3, respect-
ively. Neutral atom scattering factors were from the Inter-
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22
dispersion effects were included in Fcalc
.
The values for ∆f Ј
and ∆f Љ and the values for the mass attenuation coefficients
were from the International Tables for Crystallography.23a,b
Calculations used the TEXSAN crystallographic software
package.24
Full crystallographic details, excluding structure factor
tables, have been deposited at the Cambridge Crystallographic
Data Centre (CCDC). For details of the deposition scheme, see
‘Instructions for Authors’, J. Chem. Soc., Perkin Trans. 1, avail-
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3829

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Paper 8/05355C
3830
J. Chem. Soc., Perkin Trans. 1, 1998, 3825–3830