Oxidative coupling of alkylated anisole derivatives using MoCl5

Article abstract of DOI:10.1055/s-2004-830870

Despite the Lewis acidic character of MoCl₅ it can be employed for the selective oxidative coupling reaction of alkylated anisole derivatives without transalkylation or de-tert-butylation of the substrates. The spatial demand close to the donor

Full text of DOI:10.1055/s-2004-830870

1
970
LETTER
Oxidative Coupling of Alkylated Anisole Derivatives Using MoCl5
O
D
xidative Couplin
a
g
of
A
lkyla
n
ted Anisole
D
i
erivat
e
ives la Mirk, Birgit Wibbeling, Roland Fröhlich, Siegfried R. Waldvogel*
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstrasse 40, 48149 Münster, Germany
Fax +49(251)8339772; E-mail: waldvog@uni-muenster.de
Received 19 May 2004
mercially available or easily obtained by the Williamson
ether synthesis from the corresponding phenols.
Abstract: Despite the Lewis acidic character of MoCl it can be
employed for the selective oxidative coupling reaction of alkylated
5
10
anisole derivatives without transalkylation or de-tert-butylation of Different 2-alkyl substituted anisoles were subjected to
the substrates. The spatial demand close to the donor function seems
to be beneficial.
the MoCl -mediated reaction (Table 1, entries 1–4). Only
a single biaryl species was detected in the individual reac-
5
Key words: biaryls, coupling, dimerizations, anisole, molybdenum tion mixture.
R1
R1
The oxidative coupling reaction using MoCl as reagent
R2
5
MeO
R4
R3
evokes to a very powerful methodology giving access to
structures which are usually restricted to thallium(III)-
R2
MeO
R4
MoCl5
R4
2
1
mediated transformations. The high reaction rates of the
R3
dehydrodimerization tolerate iodo substituents which are
usually lost in common oxidative coupling procedures
R2
R3
OMe
R1
2
employing e.g. ferrous chloride. Besides simple alkoxy
1–12a
1–12b
groups on the phenolic oxygen, more sophisticated sub-
stituents can be successfully used. However, sometimes
1
2
3
4
R , R , R , R = H, alkyl, aryl
3
Scheme 1 Conditions: MoCl (2 equiv), CH Cl , 0 °C, 30 min.
these side chains can lead, with MoCl , to a selective chlo-
5
2
2
5
4
rination process. Initial investigations revealed that a 1,2-
dialkoxy substitution pattern is beneficial in the dehy- The decreasing bulkiness of the substituent in 2-position
drodimerization reaction since molecular recognition of leads to significantly lower yields. Remarkably, tert-butyl
MoCl by the donor groups on the substrate is anticipat- or iso-propyl moieties give the corresponding biaryls 1b
5
5
ed.
and 2b in excellent isolated yields, whereas ortho-cresol
(
(
3a) is coupled to a lesser extent and the parent anisole
4a) gives only little of the desired biaryl 4b. Since only
In the course of screening for potential substrates in the
MoCl -mediated oxidative coupling reaction it turned out
5
starting material and no other by-products were identified,
a small substituent in the 2-position results in low reactiv-
ity or gives rise to the formation of higher aggregates. The
solid-state structure of 1b clearly visualizes the substitu-
tion pattern, no shift of the tertiary butyl was observed.
The protection of the phenolic oxygen is crucial; the trans-
formation of 2-tert-butyl phenol under these reaction con-
ditions results only in the transalkylation of the tert-butyl
group from the ortho to the para position due to the Lewis
acidity of the molybdenum reagent.
that simple alkylated anisoles are superb substrates. Sur-
prisingly, anisole derivatives exhibiting secondary and
tertiary alkyl groups selectively undergo the dehy-
drodimerization process in good to excellent yields,
whereas these particular alkyl substituents are established
6
as useful protective groups in arene chemistry. Usually,
strong Lewis acids under mild conditions lead to a de-tert-
7
butylation of the arene system. When a donor group is
present, the dealkylation takes place preferrably ortho to
8
it since a metal template is formed. MoCl is considered
5
9
a strong Lewis acid comparable to TiCl . Therefore, de-
4
tert-butylation or transalkylation as the predominant pro-
cess was originally anticipated. With MoCl as the sole re-
5
agent, the oxidative transformation occurs selectively to
give 4,4¢-coupled products. If 4-alkylated anisoles are em-
ployed, the 2,2¢-biaryls are exclusively formed.
The MoCl -mediated dehydrodimerization was applied to
5
a variety of differently alkyl substituted anisole deriva-
tives (Scheme 1). All employed substrates are either com- Figure 1 Solid-state structure of 1b.
By employing an aromatic substituent in the 2-position,
the quaterphenyl derivative 5b is obtained in good yield.
When an ortho-cresol derivative with conformationally
SYNLETT 2004, No. 11, pp 1970–1974
Advanced online publication: 06.08.2004
DOI: 10.1055/s-2004-830870; Art ID: G18704ST
0
6
.0
9
.2
0
0
4
©
Georg Thieme Verlag Stuttgart · New York

LETTER
Oxidative Coupling of Alkylated Anisole Derivatives
1971
Table 1 Oxidative Coupling of Anisole Derivatives Using MoCl5
Entry
1
Substrate
Yield biaryl (%)a
Entry
Substrate
Yield biaryl (%)a
83 (7b)11d
91 (1b)
7
8
9
Me
t-Bu
MeO
MeO
Me
1
a
7a
2
3
99 (2b)11a
t-Bu
62 (8b)
i-Pr
MeO
MeO
Me
2
a
8a
62 (3b)11b
83 (9b)11e
Me
Me
MeO
MeO
3
a
Me
9
a
4
MeO
16 (4b)11c
10
96 (10b)11f
i-Pr
MeO
4
a
Me
Me
1
0a
5
6
Ph
78 (5b)
80 (6b)
11
12
67 (11b)
MeO
Me
Me
MeO
1
1a
5
a
a
66 (12b)11e
Me
Me
O
MeO
6
1
2a
a
Isolated yields of products purified by column chromatography.
restricted substituents is applied, which is the case in 2,3- starting material, only higher aggregates or decomposi-
dihydrobenzofuran (6a), a significantly higher yield is tion can be detected. The reaction is not inhibited by two
reached compared to the parent cresol 3a.
substituents adjacent to the coupling position. Conse-
quently substrate 12a, which has an additional substituent
in the ortho position, undergoes the dihydrodimerization
in good yield. This transformation is accompanied by
These findings indicate that a large spatial demand adja-
cent to the donor moiety at the arene seems to be benefi-
cial. Probably, the coordination of a single MoCl5-
fragment is faciliated, and not the formation of higher mo-
1
4% of a chlorinated biaryl 12c.
lybdenum chloride clusters, which may trigger the reac- In all discussed examples (Table 1), the coupling reaction
tivity. When subjecting meta-cresol to this transformation occurs selectively para to the methoxy group; no isomers
5
only oligomers were identified.
or triphenylenes could be observed. The structures of the
biaryls 1b–12b and their substitution patterns have been
proven by NMR techniques. Furthermore, the solid-state
structures of 1b and 6b clearly demonstrate the obtained
coupling pattern (Figure 1 and Figure 2). Interestingly,
despite the very electron-rich nature of both aryl moieties,
in the solid state the biaryl 6b aligns as an almost flat item.
No huge substituents range out of plane and therefore the
In the case of bis-alkylated anisoles, 2,6-, 2,5- and 2,3-
substitution patterns are tolerated, resulting in the corre-
sponding products in 62–96% yield (Table 1, entries 7–
1
1). The importance of a substituent ortho to the methoxy
group becomes clear in the case of 3,5-dialkylated deriv-
atives. 3,5-Dimethyl anisole as well as 3-iso-propyl-5-
methyl anisole do not form any defined product. Besides
Synlett 2004, No. 11, 1970–1974 © Thieme Stuttgart · New York

1
972
D. Mirk et al.
LETTER
Table 2 Oxidative Coupling of para-Substituted Anisoles
Entry
1
Substrate
Yield biaryl (%)a
73 (13b)11g
49 (14b)11a
20 (15b)11h
MeO
t-Bu
i-Pr
Me
1
3a
Figure 2 Solid-state structure of 6b.
2
3
a
MeO
molecules in the crystal are arranged in parallel stacks, al-
lowing p-p-interactions between the layers.
1
4a
It is known that Lewis acidic additives can improve the
MeO
scope and performance of MoCl -mediated oxidative cy-
5
1
2
clization processes, but in the discussed transformations
an addition of TiCl has no significant influence on the re-
4
1
5a
action outcome. 1-Methoxy naphthalene forms the de-
sired product only with poor yields of 22%. For the
Isolated yields of products purified by column chromatography.
synthesis of the corresponding binaphthyl compounds,
hypervalent iodine reagents will give better results.13
ferrous chloride. In this case, FeCl activated by sulfuric
3
acid, leads to a complex mixture of products. Besides the
loss of the tertiary butyl groups, transalkylation resulting
in 4-tert-butyl anisole is the dominant process. Without
acidic activation, the dealkylation process is suppressed,
but instead we obtained a mixture of biarylic isomers due
to transalkylation processes that still occur. Therefore,
The oxidative ortho-coupling of anisole derivatives is
barely described in literature. Besides processes mediated
1
4
by highly toxic thallium(III) reagents, electrochemical
1
5,16
oxidations were reported.
Applying the MoCl meth-
5
odology on 4-substituted anisoles we were able to obtain
the desired biaryls (Scheme 2). Again, anisoles with bulky
substituents, e.g. tert-butyl and iso-propyl, form the
corresponding biaryls in higher yields than para-cresol
MoCl is superior in selectivity and tolerance of substitu-
5
ents. Oxidative coupling of 10a using a hypervalent io-
dine compound, phenyliodine(III) bis(trifluoroacetate)
(
15a) (Table 2). Unfortunately, the extension of this pro-
(
PIFA), in combination with BF -etherate as the strong
3
tocol to para-substituted anisoles with additional alkyl
groups failed. The transformation of these substrates re-
sults only in decomposition or formation of higher aggre-
gates. The yields are significantly lower compared to the
corresponding ortho-substituted substrates, but the direct
and selective coupling of 13a and 14a without shifting or
loss of the alkyl moieties is remarkable.
Lewis acid, resulted in the desired biaryl 10b in 39%
yield, significantly lower than in the MoCl -mediated pro-
5
cess.
In conclusion, the MoCl -mediated dehydrodimerization
5
of anisole derivatives leads directly and selectively to bi-
aryls wherein the coupling occurs in the free position para
to the methoxy moiety. The transformation allows a large
variety in the substitution pattern and results in the desired
products with yields up to 99% when bulky substituents
are present ortho to the methoxy-group. Even tertiary
alkyl groups undergo this procedure without loss or trans-
alkylation of these particular substituents. When 4-substi-
R
OMe
OMe
MoCl5
2
tuted anisoles were subjected to MoCl the 2,2¢-dianisoles
OMe
5
R
were exclusively obtained.
R
1
3–15a
13–15b
R = Me, i-Pr, t-Bu
Scheme 2 Conditions: MoCl (2 equiv), CH Cl , 0 °C, 30 min.
Oxidative Coupling by MoCl : General Procedure
5
MoCl (4.00 mmol) was dissolved in anhyd CH Cl (25 mL) under
5
2
2
5
2
2
an inert atmosphere and chilled to 0 °C. Substrate (2.00 mmol) was
added and the reaction mixture was stirred for 30 min at 0 °C. After
The high reaction rate may explain why even tert-butylat-
ed substrates are successfully converted by MoCl . When
quenching with sat. NaHCO solution (25 mL) the aqueous layer
3
5
was extracted with EtOAc (3 × 50 mL). Subsequent treatment with
treating either 1a or 13a with MoCl at elevated tempera- brine, anhyd MgSO , and concentration of the organic fraction pro-
5
4
tures, predominantly decomposition occurs indicating vided the crude product, which was purified by column chromato-
graphy on silica with cyclohexane–EtOAc or pentane–Et O as
eluents. All biarylic compounds formed colorless or light yellow
solids upon evaporation of the solvents. The products were dried in
that the side reactions are still present but are tremendous-
ly slower than the desired oxidative coupling process.
2
The advantage of using MoCl as an oxidative agent be- high vacuum.
5
comes clear when comparing the results of the transfor-
mation of 2-tert-butyl anisole (1a) with another typical
transition metal reagent for oxidative coupling reactions: (s, 6 H, OCH ), 6.92 (d, 2 H, J = 8.6 Hz, 5-H), 7.35 (dd, 2 H,
17
3
(
,3¢-Di-tert-butyl-4,4¢-dimethoxybiphenyl (1b): Mp 124 °C
1
EtOH). H NMR (300 MHz, CDCl ): d = 1.43 (s, 18 H, CH ), 3.86
3
3
3
3
5,6
Synlett 2004, No. 11, 1970–1974 © Thieme Stuttgart · New York

LETTER
Oxidative Coupling of Alkylated Anisole Derivatives
1973
3
= 8.6 Hz, 4J2,6 = 2.4 Hz, 6-H), 7.46 (d, 2 H, 4J2,6 = 2.4 Hz, 2-H).
,6
J
(2) Waldvogel, S. R.; Aits, E.; Holst, C.; Fröhlich, R. Chem.
Commun. 2002, 1278.
(3) Kramer, B.; Fröhlich, R.; Bergander, K.; Waldvogel, S. R.
Synthesis 2003, 91.
(4) Mirk, D.; Kataeva, O.; Fröhlich, R.; Waldvogel, S. R.
Synthesis 2003, 2410.
5
1
3
C NMR (75 MHz, CDCl ): d = 29.78 [C(CH ) ], 34.93 [C(CH ) ],
5.15 (OCH ), 111.85 (C5), 125.23 (C6), 125.57 (C2), 133.78 (C1),
38.35 (C3), 157.63 (C4). MS (EI, 70 eV): m/z (%) = 326 (100)
M ], 311 (36) [M – CH ], 57 (29) [C H ]. Anal. Calcd for
3
3
3
3 3
5
1
[
3
+
+
+
3
4
9
C H O (326.47): C, 80.94; H, 9.26. Found: C, 80.68; H, 9.06.
2
2
30
2
(
5) Waldvogel, S. R. Synlett 2002, 622.
4
,4¢-Dimethoxy-3,3¢-diphenyl-biphenyl (5b): Mp 130 °C (EtOH).
1
(6) Tahiro, M. Synthesis 1979, 921.
(7) Lewis, N.; Morgan, I. Synth. Commun. 1988, 18, 1783.
(8) Sartori, G.; Bigi, F.; Maggi, R.; Porta, C. Tetrahedron Lett.
H NMR (300 MHz, CDCl ): d = 3.82 (s, 6 H, OCH ), 7.03 (d, 2 H,
3
3
3
1
J
= 8.4 Hz, 5-H), 7.33–7.58 (m, 14 H, 2-H, 6-H and phenyl-H).
5
,6
3
C NMR (75 MHz, CDCl ): d = 55.74 (OCH ), 111.66 (C5),
3
3
1994, 35, 7073.
1
26.68, 127.00 and 129.36 (C2, C6 and C10), 127.99 (C8), 129.56
(9) Kobayashi, S.; Busujima, T.; Nagayama, S. Chem.–Eur. J.
(
C9), 131.05 (C3), 133.44 (C1), 138.51 (C7), 155.72 (C4). MS (EI,
+
2000, 6, 3491.
7
0 eV): m/z (%) = 366 (100) [M ]. Anal. Calcd for C H O
26 22 2
(
10) All synthesized substrates were employed in analytically
pure manner, their data correspond to the literature. (a) 1a:
Kamitori, Y.; Hojo, M.; Masuda, R.; Izumi, T.; Tsukamoto,
S. J. Org. Chem. 1984, 49, 4161. (b) 2a: Paquette, L. A.;
Doehner, R. F. J. Org. Chem. 1980, 45, 5105. (c) 5a:
Lukeman, M.; Wan, P. J. Am. Chem. Soc. 2002, 124, 9458.
(d) 7a: Okada, Y.; Ishii, F.; Kasai, Y.; Nishimura, J.
Tetrahedron 1994, 50, 12159. (e) 8a: Menting, K.-H.;
Eichel, W.; Riemenschneider, K.; Schmand, H. L. K.; Boldt,
P. J. Org. Chem. 1983, 48, 2814. (f) 9a: Alonso, F.; Barba,
I.; Yus, M. Tetrahedron 1990, 46, 2069. (g) 10a:
(
366.45): C, 85.22; H, 6.05. Found: C, 85.13; H, 5.84.
1
8
2
,2¢,3,3¢-Tetrahydro-[5,5¢]bibenzofuranyl (6b): Mp 149 °C
1
3
(
Et O). H NMR (300 MHz, CDCl ): d = 3.21 (t, 4 H, J
= 8.7
2
3
CH2CH2
3
Hz, ArCH ), 4.57 (t, 4 H, J
= 8.7 Hz, OCH ), 6.80 (d, 2 H,
2
CH2CH2
2
4
3
3
J5,6 = 8.1 Hz, 5-H), 7.24 (dd, 2 H, J = 8.1 Hz, J = 1.8 Hz, 6-
5,6
13
2,6
4
H), 7.33 (d, 2 H, J2 = 1.8 Hz, 2-H). C NMR (75 MHz, CDCl3):
d = 29.75 (ArCH ), 71.30 (OCH ), 109.27 (C5), 123.44 (C6),
1
eV): m/z (%) = 238 (100) [M ]. Anal. Calcd for C H O (238.28):
C, 80.65; H, 5.92. Found: C, 80.26; H, 5.82.
,6
2
2
26.56 (C2), 127.42 (C3), 134.16 (C1), 159.12 (C4). MS (EI, 70
+
1
6
14
2
Schleusener, E.; Eugst, C. H. Helv. Chim. Acta 1972, 55,
986. (h) 11a: Eskildsen, J.; Christensen, T.; Reenberg, T.;
Larsen, U.; Christensen, J. B. Org. Prep. Proced. Int. 2000,
32, 398. (i) 12a: Andriamialisoa, Z.; Valla, A.; Cartier, D.;
Labia, R. Helv. Chim. Acta 2002, 85, 2926. (j) 13a: van
Walree, C. A.; Roest, M. R.; Schuddeboom, W.; Jenneskens,
L. W.; Verhoeven, J. W. J. Am. Chem. Soc. 1996, 118, 8395.
(k) 14a: Kim, S. S.; Kim, C. S. J. Org. Chem. 1999, 64,
3
,3¢-Di-tert-butyl-4,4¢-dimethoxy-5,5¢-dimethylbiphenyl (8b):
1
Mp 93–94 °C (Et O). H NMR (300 MHz, CDCl ): d = 1.44 [s, 18
2
3
H, C(CH ) ], 2.38 (s, 6 H, CH ), 3.80 (s, 6 H, OCH ), 7.20 (d, 2 H,
3
3
3
3
4
4
13
J2,6 = 2.4 Hz, 6-H), 7.31 (d, 2 H, J = 2.4 Hz, 2-H). C NMR (75
2,6
MHz, CDCl ): d = 17.49 (CH ), 31.06 [C(CH ) ], 35.10 [C(CH ) ],
6
1
3
3
3 3
3 3
0.60 (OCH ), 123.75 (C5), 128.42 (C6), 131.36 (C2), 136.31 (C1),
3
42.51 (C3), 157.64 (C4). MS (EI, 70 eV): m/z (%) = 354 (100)
+
+
[
M ], 339 (14) [M – CH ]. Anal. Calcd for C H O (354.53): C,
3
24 34
2
9261. (l) 15a: Yoon, J.; Sheu, C.; Houk, K. N.; Knobler, C.
8
1.31; H, 9.67. Found: C, 81.19; H, 9.64.
B.; Cram, D. J. J. Org. Chem. 1996, 61, 9323.
4
,4¢-Dimethoxy-2,2¢,3,3¢-tetramethylbiphenyl (11b): Mp 166 °C
(
11) Analytical data for known biaryls correspond to the
1
(
(
Et O). H NMR (300 MHz, CDCl ): d = 1.96 (s, 6 H, 2-CH ), 2.21
s, 6 H, 3-CH ), 3.85 (s, 6 H, OCH ), 6.75 (d, 2 H, J = 8.4 Hz, 5-
16
2
3
3
literature: (a) See ref. (b) Ronlán, A.; Bechgaard, K.;
3
3
3
5,6
Parker, V. D. Acta Chem. Scand. 1973, 27, 2375.
3
13
H), 6.92 (d, 2 H, J5 = 8.4 Hz, 6-H). C NMR (75 MHz, CDCl3):
d = 12.05 (2-CH ), 16.96 (3-CH ), 55.50 (OCH ), 107.33 (C5),
1
MS (EI, 70 eV): m/z (%) = 270 (100) [M ]. Anal. Calcd for
C H O (270.37): C, 79.96; H, 8.20. Found: C, 80.06; H, 8.21.
,6
(
(
c) Denmark, S. E.; Ober, M. H. Org. Lett. 2003, 8, 1357.
d) Rathore, R.; Bosch, E.; Kochi, J. K. Tetrahedron 1994,
3
3
3
24.79 (C3), 127.48 (C6), 135.06 (C1), 136.28 (C2), 156.37 (C4).
50, 6727. (e) McKillop, A.; Turrell, A. G.; Young, D. W.;
+
Taylor, E. C. J. Am. Chem. Soc. 1980, 102, 6504. (f)Miura,
Y.; Yamaguchi, F.; Oga, Y. Bull. Chem. Soc. Jpn. 1992, 65,
1
8
22
2
1
4
3
-Chloro-4,4¢-dimethoxy-2,2¢,3¢,5,6,6¢-hexamethylbiphenyl
1159. (g) See ref. (h) Koenig, K. E.; Lein, G. M.; Stuckler,
P.; Kaneda, T.; Cram, D. J. J. Am. Chem. Soc. 1979, 101,
3553.
1
(
12c): Mp 125–126 °C (CH Cl ). H NMR (300 MHz, CDCl ):
2
2
3
d = 1.96 (s, 3 H, CH ), 1.94 (s, 3 H, CH ), 1.95 (s, 3 H, CH ), 2.17
3
3
3
(s, 3 H, CH ), 2.26 (s, 3 H, CH ), 2.41 (s, 3 H, CH ), 3.27 (s, 3 H,
(12) (a) Kramer, B.; Fröhlich, R.; Waldvogel, S. R. Eur J. Org.
Chem. 2003, 3549. (b) Kramer, B.; Waldvogel, S. R. Angew.
Chem. Int. Ed. 2004, 43, 2446.
(13) Tohoma, H.; Morioka, H.; Takizawa, S.; Arisawa, M.; Kita,
Y. Tetrahedron 2001, 57, 345.
(14) Eberson, L.; Hartshorn, M. P.; Persson, O. Acta Chem.
Scand. 1995, 49, 640.
(15) Fichter, R. Helv. Chim. Acta 1924, 7, 810.
3
3
3
1
3
OCH ), 3.85 (s, 3 H, OCH ), 6.65 (s, 1 H, arom-H). C NMR (75
MHz, CDCl ): d = 11.89 (CH ), 13.48 (CH ), 17.22 (CH ), 17.48 (2
×
1
1
3
3
3
3
3
3
CH ), 20.74 (CH ), 55.42 (OCH ), 59.53 (OCH ), 109.45 (CH),
3
3
3
3
22.06, 128.58, 129.64, 130.57, 133.10, 133.32, 133.95, 134.56,
35.98 (quat-C), 154.20, 156.45 (C4, C4¢). MS (EI, 70 eV): m/z
+
(
(
%) = 332, 334 (100, 33) [M ]. HRMS: m/z calcd for C H ClO
M ): 332.1543. Found: 332.1552.
2
0
25
2
+
(
(
16) Nilsson, A.; Ahlberg, E. Acta Chem. Scand. 1988, 42, 378.
17) X-ray crystal structure analysis for 1b: formula C H O ,
2
2
30
2
Acknowledgment
M = 326.46, colorless crystal 0.35 × 0.15 × 0.10 mm,
a = 10.711 (1), b = 15.276 (1), c = 11.784 (1) Å, = 94.77
The work was supported by the Fonds der Chemischen Industrie
and by the Sonderforschungsbereich 424. A donation of MoCl5
from H. C. Starck (Goslar, Germany) was very helpful.
3
–3
–1
(
1), V = 1921.4 (3) Å , r = 1.129 g cm , m = 5.42 cm ,
calc
empirical absorption correction (0.833T0.948), Z = 4,
monoclinic, space group C2/c (No. 15), l = 1.54178 Å,
T = 223 K, w and j scans, 5262 reflections collected (h, k, l),
–
1
[
1
(sinq)/l] = 0.59 Å , 1568 independent (R = 0.041) and
References
int
262 observed reflections [I2 s(I)], 113 refined parameters,
R = 0.042, wR = 0.124, max. residual electron density 0.14
–0.13) e Å , hydrogens calculated and refined as riding
(
1) (a) Kramer, B.; Averhoff, A.; Waldvogel, S. R. Angew.
Chem. Int. Ed. 2002, 41, 2981. (b) Waldvogel, S. R.;
Fröhlich, R.; Schalley, C. A. Angew. Chem. Int. Ed. 2000,
2
–3
(
20
atoms.
39, 2472.
Synlett 2004, No. 11, 1970–1974 © Thieme Stuttgart · New York

1
974
D. Mirk et al.
LETTER
(18) X-ray crystal structure analysis for 6b: formula C H O ,
SHELXS-97,19e structure refinement SHELXL-97 (G. M.
Sheldrick, Universität Göttingen, 1997), graphics VMD 1.8
(W. Humphrey, A. Dalke, K. Schulten, University of
Illinois) and POV-ray 3.5 (b) Otwinowski, Z.; Minor, W.
Methods Enzym. 1997, 276, 307. (c) Blessing, R. H. Acta
Crystallogr., Sect. A 1995, 51, 33. (d) Blessing, R. H.
J. Appl. Cryst. 1997, 30, 421. (e) Sheldrick, G. M. Acta
Crystallogr., Sect. A 1990, 46, 467.
1
6
14
2
M = 238.27, light yellow crystal 0.50 × 0.40 × 0.05 mm,
a = 5.799 (1), b = 12.856 (1), c = 7.673 (1) Å, = 98.54 (1),
3
–3
–1
V = 565.7 (1) Å , r = 1.399 g cm , m = 7.27 cm , no
calc
absorption correction (0.713T0.965), Z = 2, monoclinic,
space group P2 /c (No. 14), l = 1.54178 Å, T = 223 K, w/2q
1
scans, 2469 reflections collected (-h, k, l), [(sinq)/l] = 0.62
–1
Å , 1152 independent (R = 0.042) and 1011 observed
int
reflections [I2 s(I)], 83 refined parameters, R = 0.047,
(20) Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication CCDC-239234 and 239235. Copies of the data
can be obtained free of charge on application to The
Director, CCDC, 12 Union Road, CambridgeCB2 1EZ, UK
[fax: int. code +44 (1223)336-033, e-mail:
2
wR = 0.135, max. residual electron density 0.43 (–0.19) e
–3
Å , hydrogens calculated and refined as riding atoms.
19) (a) Data set was collected with Nonius KappaCCD
diffractometer, equipped with a rotating anode generator
Nonius FR591. Programs used: data collection COLLECT
(
1
9b
(
Nonius B.V., 1998), data reduction Denzo-SMN,
1
9c,d
absorption correction SORTAV,
structure solution
deposit@ccdc.cam.ac.uk].
Synlett 2004, No. 11, 1970–1974 © Thieme Stuttgart · New York