Cu(i) catalysed cyclopropanation with enantiopure scorpionate type ligands derived from (+)-camphor or (-)-menthone

Article abstract of DOI:10.1039/c1dt10032g

One-pot syntheses of three new enantiopure heteroscorpionate ligands derived from (+)-camphor or (-)-menthone are described. The ligands are obtained by reacting pyrazoles derived from (+)-camphor or (-)-menthone with sodium hydride and thionyl chloride. Subsequent reactions with pyridine and various aldehydes afford the tripod ligands in multi-gram amounts. Especially the menthopyrazole based ligand 6 showed encouraging ee values up to 69% in the Cu(i) catalysed enantioselective cyclopropanation of styrene with ethyl diazoacetate.

Full text of DOI:10.1039/c1dt10032g

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PAPER
Cu(I) catalysed cyclopropanation with enantiopure scorpionate type ligands
derived from (+)-camphor or (-)-menthone†
Tom Godau,‡ Sascha M. Bleifuß,‡ Astrid L. Mu¨ller,‡ Thomas Roth, Susanne Hoffmann, Frank W. Heinemann
and Nicolai Burzlaff*
Received 9th January 2011, Accepted 12th April 2011
DOI: 10.1039/c1dt10032g
One-pot syntheses of three new enantiopure heteroscorpionate ligands derived from (+)-camphor or
(-)-menthone are described. The ligands are obtained by reacting pyrazoles derived from (+)-camphor
or (-)-menthone with sodium hydride and thionyl chloride. Subsequent reactions with pyridine and
various aldehydes afford the tripod ligands in multi-gram amounts. Especially the menthopyrazole
based ligand 6 showed encouraging ee values up to 69% in the Cu(I) catalysed enantioselective
cyclopropanation of styrene with ethyl diazoacetate.
stereocentre (Fig. 1).12 So far two examples for enantiopure
Introduction
bis(pyrazol-1-yl)acetato ligands based on this concept have
During the last decade the field of bis(pyrazolyl)methane based
been reported by our group, namely the bis(camphorpyrazol-1-
yl)acetic acid Hbpa4cam and bis(menthopyrazol-1-yl)acetic acid
Hbpa4menth (Fig. 1).9,10,13 Recently, we established a one-pot syn-
thesis of enantiopure scorpionate ligands derived from (+)-
camphor using a modified version of the Peterson rearrange-
ment that does not require the cobalt catalyst any more
but pyridine. So far two more enantiopure tripod ligands
have been reported by us following this approach, namely
heteroscorpionate ligands and their application in bioinorganic
chemistry and organometallics has increased tremendously.1 Espe-
cially, several achiral tripodal N,N,O, N,N,S and N,N,N ligands
havebeenreportedby C. J. Carrano and others which are accessible
from 1,1¢-carbonyl-bis(pyrazole) and functionalised aldehydes.2,3
This cobalt-catalysed reaction, in which CO2 is released, is based
on some reports from the 70s by L. K. Peterson and K. I. The`4
and was recently modified by D. L. Reger by substituting 1,1¢-
carbonylbis(pyrazole) for 1,1¢-sulfinyl-bis(pyrazole).5 First appli-
cations of early and middle transition metal complexes bearing
such achiral heteroscorpionate ligands consist of catalytic ethylene
and propylene polymerisation.6
During the last five years various chiral and enantiopure
bis(pyrazol-1-yl)methane based tripod ligands have been studied
either by A. Otero and coworkers or by our group.7–10 Based
on prochirality, a rather simple concept allows us to synthesise
enantiopure facially binding tripod ligands from C2 symmetric
precursors without any additional separation of enantiomers or
diastereomers.9,10 In many C2 symmetric ligands two identical
enantiopure donor groups Y* are connected by a tetrahedral
bridging carbon atom. Several examples based on (+)-camphor
or (-)-menthone derived pyrazoles have been reported by various
groups over the years.11 Any additional group Z at this carbon
atom will form a prochiral centre rather than an additional
Inorganic Chemistry, Department of Chemistry and Pharmacy & Interdisci-
plinary Center for Molecular Materials (ICMM), University of Erlangen-
Nu¨rnberg, Egerlandstr. 1, 91058 Erlangen, Germany. E-mail: Nicolai.
Burzlaff@chemie.uni-erlangen.de; Fax: +49 9131-85-27387
† Electronic supplementary information (ESI) available: Crystallographic
data of ligand 3, 4 and 6 in CIF format. CCDC reference numbers 807802–
807804. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1dt10032g
Fig. 1 Prochiral centre and the homochiral ligands Hbpa4cam, Hbpa4menth
,
9,10
‡ T. G., A. L. M. and S. M. B. contributed equally to this work.
HOPhbpm3cam and HSiprbpm3cam
.
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the phenol N,N,O ligand (2-hydroxyphenyl)bis(camphorpyrazol-
1-yl)methane (HOPhbpm3cam
and the thiol N,N,S lig-
After a scale up of the reaction we were able to obtain up to
seven grams of enantiopure ligand 3 in one run. An analogous
reaction with 2-thiophenecarboxaldehyde afforded the (thien-2-
yl)bis(camphorpyrazol-1-yl)methane (thibpm3cam) ligand 4 in 41%
yield after purification of this new N,N,S ligand by column
chromatography (silica). Due to the loss of C2 symmetry, the
two pyrazole donors in 3 and 4 are no longer spectroscopically
equivalent and exhibit two sets of signals in the 1H and 13C NMR
spectra. This is indicated especially by the six singlet Me 1H NMR
signals at 0.59, 0.63, 0.91, 0.92, 1.23 and 1.25 ppm in the case
of ligand 3 and at 0.63, 0.65, 0.93, 0.93, 1.26 and 1.27 ppm in
the case of ligand 4. The corresponding Me 13C NMR signals are
observed at 10.7, 10.7, 19.0, 19.0, 20.4 and 20.5 ppm (3) or at 10.7,
10.8, 19.0, 19.1, 20.4 and 20.5 ppm (4), respectively. The methyl
group of the imidazolyl donor of 3 is assigned to singlet signals
)
and (2-mercapto-iso-prop-2-yl)bis(camphorpyrazol-1-yl)methane
(HSiprbpm3cam) which both coordinate facially as monoan-
ionic ligands to various transition metals.14,15 Optically active
C3-symmetric tris(pyrazol-1-yl)hydro-borate and tris(pyrazol-1-
yl)phosphine oxide ligands derived from (+)-camphor or (-)-
menthone have been reported by W. B. Tolman before.16 Some
of these ligands have been tested as enantioselective catalysts in
the copper(I) mediated cyclopropanation of styrene and gained
fair enantioselectivities up to 60% ee.
Obviously, tripodal heteroscorpionate ligands based on our
concept are not as sterically crowded as related C3-symmetric
hydrotris(pyrazol-yl)borate or tris(pyrazol-1-yl)phosphine oxide
ligands are, and might therefore provide better enantioselective
properties in catalytic reactions. Thus, we decided to extend
our concept to neutral (+)-camphor or (-)-menthone based
bis(pyrazol-1-yl)methane ligands with either imidazolyl or thienyl
groups as third donors and to study their catalytic properties in
copper(I) mediated enantioselective cyclopropanation reactions.
at 3.51 ppm in the 1H NMR spectrum and at 33.0 ppm in the 13
C
NMR spectrum. Furthermore, both ligands are characterised by
elemental analysis as well as FD+ MS mass spectrometry data and
the a values of the specific rotation have been identified as [a]D29
=
+16.9 (3) and [a]2D8 = +31.2 (4).
The molecular structures of both ligands 3 and 4 (Fig. 2 and
3) have also been confirmed by single crystal X-ray structure
determination experiments in which the absolute configurations
Results
Following the same approach that lead to the successful syn-
theses of HOPhbpm3cam and HSiprbpm3cam before, we tried in a
first attempt a one-pot Peterson-type synthesis with 1-methyl-2-
imidazolecarboxaldehyde and camphorpyrazole 1 [i.e. (4S,7R)-
7,8,8-trimethyl-4,5,6,7-tetrahydro-4,7-methano-1(2)H-indazole].
Pyrazole 1 is an enantiopure pyrazole derived from the chiral
pool that can be obtained easily and in high yield in a two
step synthesis starting from (+)-camphor.16b,17 Camphorpyrazole
(1) was now reacted with sodium hydride and thionyl chlo-
ride. Without any further purification, pyridine and 1-methyl-
2-imidazolecarboxaldehyde were added to the resulting 1,1¢-
sulfinylbis(camphorpyrazole) (2) to yield the (N-methylimidazol-
2-yl)bis(camphorpyrazol-1-yl)methane (mibpm3cam) ligand 3 in
yields up to 81% (Scheme 1).
Fig. 2 Molecular structure of (N-methylimidazol-2-yl)bis(camphor-
pyrazol-1-yl)methane (3); thermal ellipsoids are drawn at the 50%
probability level; hydrogen atoms are omitted for better clarity. Selected
◦
˚
bond lengths (A) and angles ( ): C1–C2 = 1.505(3), C1–N12 = 1.447(2),
C1–N32 = 1.461(2), N12–C1–N32 = 111.31(15), N12–C1–C2 = 108.76(16),
N32–C1–C2 = 111.46(16).
Fig.
3
Molecular structure of (thien-2-yl)bis(camphorpyrazol-1-yl)-
methane (4); thermal ellipsoids are drawn at the 50% probability level;
hydrogen atoms and solvent molecules are omitted for better clarity; only
the major fraction of the disordered thienyl ring is depicted. Selected
◦
˚
bond lengths (A) and angles ( ): C1–C2 = 1.504(2), C1–N12 = 1.446(2),
C1–N32 = 1.454(2), N12–C1–N32 = 111.80(14), N12–C1–C2 = 111.77(15),
N32–C1–C2 = 111.70(15).
Scheme 1 Synthesis of enantiopure scorpionate ligands derived from
(+)-camphor.
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have been assigned according to (+)-camphor. In the case of ligand
4 this assignment is backed by the Flack parameter of 0.01(8).
This is not possible for ligand 3, since the Flack parameter is not
meaningful for structures without third or higher row atoms. In
both molecular structures the (+)-camphor moieties are oriented
towards the potential future metal binding site. This makes both
ligands quite promising candidates for future enantioselective
catalytic applications.
To extend our prochirality based ligand approach to other
chiral pool derived pyrazoles, we then tried an analogous one-pot
synthesis with (R,R)-menthopyrazole (5) [i.e. (4R,7R)-4-methyl-7-
isopropyl-4,5,6,7-tetrahydro-1(2)H-indazole] (Scheme 2).
Fig. 4 Molecular structure of (N-methylimidazol-2-yl)bis(menthopy-
razol-1-yl)methane (6); thermal ellipsoids are drawn at the 50% probability
level; hydrogen atoms are omitted for better clarity and only one out of two
˚
molecules in the asymmetric unit is depicted. Selected bond lengths (A)
and angles (◦): C1–C2 = 1.499(6), C1–N12 = 1.450(5), C1–N32 = 1.449(5),
N12–C1–N32 = 112.1(3), N12–C1–C2 = 110.3(3), N32–C1–C2 = 111.7(3).
Two Cu(I) salts were used in these catalytic tests, [Cu(MeCN)4]BF4
and [(CuOTf)2]C6H6 (Scheme 3). In the case of menthopyrazole
based ligand 6, the reaction of styrene with ethyl diazoacetate,
which is a benchmark for the Cu(I) catalysed cyclopropanation,
gave diastereomeric trans and cis products in a ratio of 60 : 40
and in a reasonable yield up to 75%. This is typical for cyclo-
propanations using ethyl diazoacetate. An encouraging 69% ee
was detected by GC analysis in the case of the [(CuOTf)2]C6H6
precursor (Table 1, entry 4). The R,S assignment was deduced by
a comparative experiment with the bisoxazoline ligand according
to Evans and coworkers.18 Only traces (0.1% to 3.4%) of diethyl
fumarate and diethyl maleate as typical dimerisation byproducts of
ethyl diazoacetate were observed for all catalytic tests (see Table 1).
Scheme 2 Synthesis of enantiopure scorpionate ligands derived from
(-)-menthone.
Enantiomerically pure (R,R)-menthopyrazole 5 is accessible
from (-)-menthone via a three-step synthetic procedure.11b,16b
Similar to the synthesis of 3 and 4, the pyrazole 5 was reacted
with sodium hydride and thionyl chloride. Without any further
purification, pyridine and 1-methyl-2-imidazolecarboxaldehyde
were added to the resulting sulfinylbis(camphorpyrazole) interme-
diate to yield the enantiopure tripod ligand (N-methylimidazol-
2-yl)bis(menthopyrazol-1-yl)methane (mibpm3menth) 6 in a yield of
39% (Scheme 2).
Again, the two menthopyrazole donors in 6 are no longer
spectroscopically equivalent due to the C1 symmetry of the ligand,
1
and exhibit two sets of signals in the H and 13C NMR spectra.
1
This causes six doublet Me H NMR signals at 0.82, 0.84, 0.97,
0.97, 1.11 and 1.11 ppm with 3JH,H couplings of 6.8 to 7.0 Hz. The
corresponding 13C NMR signals are observed at 19.5, 19.5, 21.1,
21.1, 23.1 and 23.1 ppm. The methyl group of the imidazolyl
donor of ligand 6 is assigned to a singlet signal at 3.35 ppm
in the 1H NMR spectrum and a signal at 33.3 ppm in the
13C NMR spectrum. The elemental analysis as well as a [MH+]
peak in the mass spectrum (FD) conform with the suggested
formula of the compound that shows a specific rotation [a]2D9 of
+24.0◦. A single crystal X-ray structure determination revealed
the molecular structure of 6 (Fig. 4).
Scheme
ethyldiazoacetate.
3
Enantioselective cyclopropanation of styrene with
The absolute configurations of this structure model have been
deduced from (-)-menthone. Once again the Flack parameter is
not meaningful here since no third or higher row atoms are present.
Similar to ligands 3 and 4 the (-)-menthone moieties are oriented
towards the potential future metal binding site and thus suitable
for enantioselective inductions.
In an attempt to assess the efficiency of the three new ligands 3,
4 and 6 to effect enantioselective reactions, all three ligands were
employed in Cu(I) catalysed cyclopropanations of alkenes with
diazoacetates, a reaction first reported by Evans and Masamune.18
Usage of [Cu(MeCN)4]BF4 instead of [(CuOTf)2]C6H6 as cata-
lyst precursor resulted in a slightly lower yield and ee value (Table
1, entry 2). Reasonable but lower enantioselectivities (46% ee in
the case of the trans product) and yields were obtained with the
camphor based ligand 3 and [Cu(MeCN)4]BF4 (Table 1, entry
6). In contrast to the menthone based ligand the reaction with
[(CuOTf)2]C6H6 resulted in even lower ee values of 20%. For the
camphor based N,N,S ligand 4, the catalytic test with both Cu(I)
salts led to an increase of the yield, but unfortunately also to almost
a complete loss of the enantioselectivity (Table 1, entries 9 and
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Table 1 Asymmetric cyclopropanation of styrene with EDA and various Cu(I)-complex catalystsa
Catalyst/precursor
Yield [%]d
trans : cise
ee trans%f
ee cis%f
F/M [%]g
b
c
1
2
3
4
5
6
7
8
9
ligand 6/[Cu(MeCN)4]BF4
ligand 6/[Cu(MeCN)4]BF4
65
51
75
30
64
48
73
32
68
90
59 : 41
60 : 40
59 : 41
61 : 39
51 : 49
53 : 47
59 : 41
54 : 46
57 : 43
57 : 43
58.8 0.04 (1S,2S)
65.2 0.26 (1S,2S)
60.1 0.08 (1S,2S)
69.4 0.45 (1S,2S)
40.8 0.33 (1S,2S)
46.3 0.07 (1S,2S)
19.9 0.05 (1S,2S)
43.5 0.03 (1S,2S)
4.6 0.06 (1R, 2R)
6.2 0.10 (1R,2R)
8.2 0.18 (1S,2R)
6.9 0.13 (1S,2R)
8.7 0.08 (1S,2R)
11.4 0.87 (1S,2R)
12.8 0.08 (1S,2R)
13.5 0.16 (1S,2R)
8.3 0.16 (1S,2R)
14.2 0.06 (1S,2R)
1.0 0.06 (1R,2S)
2.4 0.10 (1R,2S)
0.9/0.7
0.6/0.4
1.6/0.7
0.4/0.3
3.4/1.3
2.6/1.0
1.9/1.0
1.9/0.6
0.2/—
0.1/–
b
ligand 6/[(CuOTf)2]C6H6
c
ligand 6/[(CuOTf)2]C6H6
b
c
ligand 3/[Cu(MeCN)4]BF4
ligand 3/[Cu(MeCN)4]BF4
b
ligand 3/[(CuOTf)2]C6H6
c
ligand 3/[(CuOTf)2]C6H6
b
ligand 4/[Cu(MeCN)4]BF4
b
10
ligand 4/[(CuOTf)2]C6H6
a General conditions: [Cu(MeCN)4]BF4 or [(CuOTf)2]C6H6 (1 mol%), ligand (1.5 mol%), styrene (7 equiv.), ethyl diazoacetate (1 equiv.) in CH2Cl2
(12 mL), 18 h. b Ambient temperature. c 0 ◦C. d Isolated yields of a mixture of trans-products, cis-products and dimers after chromatographic purification.
e Determined by GC analysis and 1H NMR. f Determined by chiral GC analysis on a Cyclodex B column (60 m ¥ 0.25 mm, 0.25 mm) g F/M represents
yield of diethyl fumarate/diethyl maleate.
10). We explain this by the two lone pairs of the thienyl S-donor.
Obviously, a bent coordination of the thienyl donor is favored that
Conclusions
In conclusion, the previously published one-pot synthesis of
the homochiral heteroscorpionate ligand (2-hydroxyphenyl)-
minimizes the stereoinduction of the scorpionate ligand.
Finally, we decided to synthesise exemplarily
per(I) based complex as potential pre-catalyst follow-
ing
procedure reported by W. B. Tolman et al.16a
Thus, (thien-2-yl)bis(camphorpyrazol-1-yl)methane (thibpm3cam
a cop-
bis(camphorpyrazol-1-yl)methane HOPhbpm3cam was expanded
to a simple synthetic route to neutral N,N,N and N,N,S het-
eroscorpionate ligands.14 This led to two new (+)-camphor based
chiral ligands (N-methylimidazol-2-yl)bis(camphor-pyrazol-1-
yl)methane (mibpm3cam) (3) and (thien-2-yl)-bis(camphorpyrazol-
1-yl)methane (thibpm3cam) (4). Beside these camphorpyrazolyl
based ligands, the one-pot synthesis was also extended to
a (-)-menthone based ligand. This resulted in a third novel
ligand (N-methylimidazol-2-yl)bis(menthopyrazol-1-yl)methane
(mibpm3menth) (6). All three chiral and enantiopure heteroscorpi-
onate ligands were tested in the Cu(I) catalysed cyclopropanation
of styrene with ethyl diazoacetate. An encouraging enantiomeric
excess of 69% was observed in the case of the menthopyrazole
based mibpm3menth ligand 6. Thus, a proof of concept for the
chiral induction regarding this ligand class was gained, since
this ee value matches well the value reported by W. B. Tolman
a
a
)
(4) was reacted in dichloromethane with (tetrakisacetoni-
trile)copper(I)tetrafluoroborate. Precipitation with n-hexane af-
forded the product [Cu(CH3CN)(thibpm3cam)]BF4 (7) in 84% yield
(Scheme 4).
et al. obtained with
a tris[(4S,7R)-7,8,8-trimethyl-4,5,6,7-
tetrahydro-4,7-methano-2-indazolyl]phosphine oxide ligand.16
Due to the fact that the steric demand of ligands 4 and 6 can
easily be enhanced by the introduction of more bulky imidazolyl
donors [e.g. benzimidazole], we are confident that future ligand
modifications might result in a further increase of the enantiomeric
excess. Furthermore, since these new ligands are easily accessible
in high quantities starting from cheap chiral pool precursors, these
ligands might be promising candidates for some other asymmetric
catalyses in organometallic as well as in coordination chemistry,
such as polymerisation of rac-lactide or the polymerisation of
propylene.
Scheme 4 Synthesis of the copper(I) complex 7.
Formation of the complex has been confirmed by the NMR
spectra and elemental analysis. The six methyl singlets are shifted
slightly in the 1H NMR spectrum to 0.61, 0.67, 0.93, 0.94, 1.24 and
1.24 ppm due to the coordination of the ligand. The corresponding
13C NMR signals are observed at 10.3, 10.6, 18.8, 18.9, 20.3
1
and 20.5 ppm. A singlet in the H NMR spectrum at 2.36 ppm
integrates to three protons and has been assigned to the acetonitrile
ligand. The corresponding 13C NMR signal appears at 2.6 ppm.
A similar reaction with (N-methylimidazol-2-yl)bis(camphor-
pyrazol-1-yl)methane (mibpm3cam) (3) resulted in a mixture of the
intended complex [Cu(CH3CN)(mibpm3cam)]BF4 and a bisligand
complex [Cu(mibpm3cam)2]BF4 as indicated by ESI MS data with
m/z(%) peaks at 951 (98) [M+ + 3], 548 (83) [M+] and 507 (71)
[M+ - CH3CN]. Attempts to isolate analytically pure complex
[Cu(CH3CN)(mibpm3cam)]BF4 have not been successful so far.
Experimental section
General remarks
All reactions were carried out under an inert gas atmosphere using
conventional Schlenk techniques. Solvents were freshly distilled or
purchased in absolute grade and stored with appropriate drying
agents. 1H and 13C NMR spectra were recorded on a Bruker
DPX300 AVANCE instrument. The values of the chemical shift d
6550 | Dalton Trans., 2011, 40, 6547–6554
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1
are relative to tetramethylsilane (TMS) or the deuterated solvent.
Signals of the ligands 3, 4 and 6 were assigned according to
DEPT and COSY experiments. Mass spectra were recorded with
a JOEL JMS-700 spectrometer (FD) and with a Bruker Esquire
6000 mass spectrometer with an electrospray ion source (ESI)
and a quadrupole ion trap (QIT) as mass analyser. The elemental
analyses were carried out on an Euro ES 3000 instrument (Euro
Vector) and an EA 1108 instrument (Carlo Erba). Infrared
spectra were prepared as KBr pellets and were recorded on a
Varian EXCALIBUR FTS-3500 FT-IR spectrometer. Optical
rotations were recorded with a Perkin Elmer 241 polarimeter
under the following standard conditions: room temperature,
wavelength 589.3 nm (sodium D line), cell length 1 dm, solvent
and sample concentration (in g mL1) are given with the individual
experiment. A Bruker SMART APPEXII diffractometer was used
for X-ray structure determination. Chiral GC experiments were
carried out with a Shimadzu 17A gas chromatograph (Shimadzu
Europa, Duisburg, Germany) equipped with a flame ionization
detector (FID). Separation of the enantiomers was accomplished
using a Cyclodex B column (Agilent Technologies, Waldbronn,
Germany), consisting of 10.5% b-cyclodextrin in DB-1701, of
60 m length, 0.25 mm inner diameter and 0.25 mm film thickness.
Diethyl furmarate and diethyl maleate were quantified by gas chro-
matography according to a calibration curve. Camphorpyrazole
(1) and menthopyrazole (5) were synthesised according to the
literature.16 All other reagents were commercially available and
used as received without further purification.
Yield: 7.22 g (16.3 mmol, 81%). H NMR (300 MHz, CDCl3,
25 ◦C): d = 0.59 (s, 3H, H12 or H12¢, CH3), 0.63 (s, 3H, H12 or H12¢,
CH3), 0.91 (s, 3H, H11 or H11¢, CH3), 0.92 (s, 3H, H11 or H11¢, CH3),
1.14 (m, 2H, H5b,5b¢ or H6b,6b¢, CH2), 1.28 (m, 2H, H5b,5b¢ or H6b,6b¢
,
CH2), 1.23 (s, 3H, H13 or H13¢, CH3), 1.25 (s, 3H, H13 or H13¢, CH3),
1.80 (m, 2H, H6a and H6a¢, CH2), 2.02 (m, 2H, H5a and H5a¢, CH2),
2.70 (d, JH,H = 3.96 Hz, 2H, H4 and H4¢, CH), 3.51 (s, 3H, C17,
3
CH3), 6.87 (s, 1H, H15 or H16, CHim), 7.03 (s, 1H, H3 or H3¢, CHpz),
7.04 (s, 1H, H3 or H3¢, CHpz) 7.11 (s, 1H, H15 or H16, CHim), 7.61
(s, 1H, H1, CH) ppm. 13C NMR (75 MHz, CDCl3, 25 ◦C): d =
10.7 (C13 and C13¢, CH3), 19.0, 19.0 (C11 and C11¢, CH3), 20.4, 20.5
(C12 and C12¢, CH3), 27.4, 27.5 (C5 and C5¢, CH2), 33.0 (C17, CH3),
33.5, 33.5 (C6 and C6¢, CH2), 47.1 (C4 and C4¢, CH), 50.2, 50.2
(C7 and C7¢, Cquart), 60.1, 60.3 (C8 and C8¢, Cquart.), 70.8 (C1, CH),
120.9 (C3 or C3¢, CHpz), 121.4 (C15 or C16, CHim), 122.2 (C15 or C16,
CHim), 127.6 (C3 or C3¢, CHpz) 127.9, 128.1 (C10 and C10¢, Cpz), 142.5
(C14, Cim), 167.3, 167.5 (C9 and C9¢, Cpz) ppm. FD MS (CH2Cl2):
m/z (%) = 445 (100) [MH ]. IR (KBr): n = 3141 w (n(C C–H)
pyrazole), 2956 vs. (ns(CH3)), 2869 m (nas(CH3)), 1583 m (n(C N)
or n(C C) pyrazole)), 1489 s (n(C C) pyrazole), 1443 m, 1387
m, 1349 m cm-1. Anal. Calc. for C27H36N6 (444.61 g mol-1): C 72.94,
H 8.16, N 18.90; found C 73.36, H 8.19, N 19.22. [a]2D9 = +16.9 (c =
1 ¥ 10-3, CH2Cl2).
+
˜
Synthesis of (thien-2-yl)bis(camphorpyrazol-1-yl)methane
(thibpm3cam) (4)
Sodium hydride (60.0 mmol, 2.40 g) was suspended in THF
(200 mL) and cooled in an ice/water bath. Camphorpyrazole
(1) (50.0 mmol, 8.81 g) was added and the resulting yellow
mixture was stirred at 0 ◦C for 30 min. After adding dropwise
thionyl chloride (30.1 mmol, 2.18 mL) the cooling was removed
and the reaction mixture was stirred for an additional 30 min.
2-thiophenecarboxaldehyde (50.0 mmol, 4.68 mL) and pyridine
(50.0 mmol, 4.03 mL) were added and the mixture was heated
to 50 ◦C over 22 h. The pyridine was distilled off at 105 ◦C
under vacuum. After cooling down to ambient temperature, the
mixture was dissolved in CH2Cl2 (250 mL) and washed with
water (3 ¥ 200 mL). The organic layer was dried (Na2SO4),
filtered and the solvent was evaporated in vacuo. Afterwards the
brown residue was purified by column chromatography (SiO2, l =
25 cm, ∅ = 7 cm, dichloromethane/ethylacetate 30 : 1 v/v). The
product elutes as the third fraction (Rf 0.37) and was received
as both pure (b) and mixed fractions (a). Both fractions (a) and
(b) were evaporated and filtered through aluminium oxide (l =
8 cm, ∅ = 4 cm, dichloromethane/ethylacetate 30 : 1 v/v) to
remove camphorpyrazole (1) from the reaction mixture. Fraction
(b) shows the product after additional purification by column
chromatography. The product was received as a light yellow
powder. Appropriate crystals for X-ray structure determination
were received by recrystallisation from methanol/water mixture.
Synthesis of (N-methylimidazol-2-yl)bis(camphorpyrazol-1-
yl)methane (mibpm3cam) (3)
Sodium hydride (40.0 mmol, 0.96 g) was suspended in THF
(200 mL) and cooled in an ice/water bath. Camphorpyrazole (1)
(40.0 mmol, 7.05 g) was added and the resulting yellow mixture
was stirred at 0 ◦C for 30 min. After adding dropwise thionyl
chloride (20.0 mmol, 1.45 mL) the cooling was removed. 1-Methyl-
2-imidazolecarboxaldehyde (22.7 mmol, 2.50 g) and pyridine
(40 mmol, 3.22 mL) were added and the mixture was heated under
reflux over 16 h. The pyridine was distilled off at 105 ◦C under
vacuum. After cooling down to ambient temperature, the mixture
was dissolved in CH2Cl2 (200 mL) and washed with water (2 ¥
200 mL). The organic layer was dried (Na2SO4), filtered and the
solvent was evaporated in vacuo. The residue was dissolved in a
small amount of diethyl ether, layered with pentane and stored in
the refrigerator. After 12 h the product mibpm3cam (3) was filtered
off as a white precipitate, washed with cold diethyl ether and dried
in vacuo. Further fractions can be obtained after the filtrate has
been concentrated in vacuo.
1
Yield: 4.56 g (10.3 mmol, 41%). H NMR (300 MHz, CDCl3,
25 ◦C): d = 0.63 (s, 3H, H12 or H12¢, CH3), 0.65 (s, 3H, H12 or H12¢,
CH3), 0.93 (s, 6H, H11 and H11¢, CH3), 1.15 (m, 2H, H6b and H6b¢,
CH2), 1.26 (s, 3H, H13 or H13¢, CH3), 1.27 (s, 3H, H13 or H13¢, CH3),
1.32 (m, 2H, H5b and H5b¢, CH2), 1.81 (m, 2H, H6a and H6a¢, CH2),
2.04 (m, 2H, H5a and H5a¢, CH2), 2.72 (m, 2H, H4 and H4¢, CH),
6.81 (d, 3JHH = 3.3 Hz, 1H, H15, CHthi), 6.94 (dd, 3JHH = 3.75 Hz,
4.95 Hz, 1H, H16, CHthi), 7.00 (s, 1H, H3 or H3¢, CHpz), 7.07 (s, 1H,
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3
6.8 Hz, 3H, CH3), 0.97 (d, JH,H = 7.0 Hz, 3H, CH3), 0.97 (d,
3JH,H = 7.0 Hz, 3H, CH3), 1.11 (d, 3JH,H = 7.0 Hz, 3H, CH3), 1.11
3
(d, JH,H = 7.0 Hz, 3H, CH3), 1.60 (m, 8H, H5, H5¢, H6 and H6¢,
CH2), 2.08 (m, 2H, H11 and H11¢, CH), 2.49 (m, 2H, H7, H7¢, CH),
2.75 (m, 2H, H4 and H4¢, CH), 3.35 (s, 3H, H16, CH3), 6.90 (d,
3JH,H = 1.0 Hz, 1H, H15, CHim), 7.01 (s, 1H, H16, CHim), 7.45 (d,
3JH,H = 5.1 Hz, 2H, H3 and H3¢, CHpz), 7.57 (s, 1H, H1, CH). 13C
NMR (75 MHz, (CD3)2CO, 25 ◦C): d = 19.5, 19.5, 21.1, 21.1 (C12,
C12¢, C13 and C13¢, CH3), 22.5, 22.7 (C6 and C6¢, CH2), 23.1 (C10
and C10¢, CH3), 27.2, 27.2 (C11 and C11¢, CH), 30.7 (C5 and C5¢,
CH2), 31.5, 31.6 (C4 and C4¢, CH), 33.3 (C17, CH3), 40.8, 40.9 (C7
and C7¢, CH), 72.5 (C1, CH), 123.9, 124.3 (C9 and C9¢, Cpz), 124.1
(C16, CHim), 127.2 (C3 and C3¢, CH), 127.9 (C15, CHim), 143.0 (C14,
CHim), 152.5, 152.7 (C8 and C8¢, Cpz). FD MS (CH2Cl2): m/z (%) =
H3 or H3¢, CHpz), 7.28 (d, 3JHH = 5.1 Hz, 1H, H17, CHthi), 7.68 (s,
1H, H1, CH) ppm. 13C NMR (100 MHz, CDCl3, 25 ◦C): d = 10.7,
10.8 (C13 and C13¢, CH3), 19.0, 19.1 (C11 and C11¢, CH3), 20.4, 20.5
(C12 and C12¢, CH3), 27.5, 27.6 (C5 and C5¢, CH2), 33.5, 33.6 (C6
and C6¢, CH2), 47.1, 47.2 (C4 and C4¢, CH), 50.2, 50.3 (C7 and C7¢,
Cquart.), 60.2, 60.4 (C8 and C8¢, Cquart.), 74.1 (C1, CH), 120.9, 121.5
(C3 and C3¢, CHpz), 126.5, 126.6, 126.9 (C15, C16 and C17, CHthi),
127.5 (C10 and C10¢, Cpz), 141.0 (C14, Cthi), 167.1, 167.8 (C9 and
C9¢, Cpz) ppm. FD MS (CH2Cl2): m/z (%) = 447 (100) [MH+]. IR
449 (40) [MH+], 272 (100) [MH+ - pz3menth]. IR (KBr): n = 3132
˜
w (n(C C–H) pyrazole), 2951 vs. (ns(CH3)), 2931 vs. (ns(CH3)),
2868 m (nas(CH3)), 1560 m (n(C N) or n(C C) pyrazole), 1500
m (n(C C) pyrazole), 1457 m, 1368 m, 1346 m cm-1. Anal. Calc.
for C27H40N6 (448.65 g mol-1): C 72.28, H 8.99, N 18.73; found C
71.96, H 8.94, N 18.39. [a]2D9 = 24.0 (c = 1 ¥ 10-3, CH2Cl2).
˜
(KBr): n = 3106 w, 3073 w (n(C–H) thienyl), 2958 vs. (ns(CH3)),
2927 sh, 2869 m (nas(CH3)), 1590 (n(C N) or n(C C) pyrazole))
Synthesis of [Cu(CH3CN)(thibpm3cam)]BF4 (7)
cm-1. Anal. Calc. for C27H34N4S (446.65 g mol-1): C 72.60, H 7.67,
N 12.54, S 7.18; found: C 72.40, H 7.51, N 12.52, S 6.92. [a]D29
+31.2 (c = 1 ¥ 10-3, CH2Cl2).
=
Thibpm3cam (4) (326 mg, 0.729 mmol) was dissolved in
dichloromethane (5 mL) and added to
a suspension of
[Cu(CH3CN)4]BF4 (215 mg, 0.663 mmol) in dichloromethane
(4 mL). After 30 min a clear yellow solution was obtained, and
this solution was stirred for an additional 10 h. By adding n-
hexane a brown oil was deposited from this solution. The product
precipitated from this brown oil after treatment in a ultrasonic
bath for 30 min. The received white solid was filtered, washed with
n-hexane (2 ¥ 5 mL) and dried in vacuo.
Synthesis of (N-methylimidazol-2-yl)bis(menthopyrazol-1-
yl)methane (mibpm3menth) (6)
Sodium hydride (45.4 mmol, 1.82 g) was suspended in THF
(180 mL) and cooled in an ice/water bath. Menthopyrazole (5)
(45.4 mmol, 8.1 g) was added and the resulting colourless mixture
was stirred at 0 ◦C for 30 min. After adding dropwise thionyl
chloride (22.7 mmol, 1.65 mL) the cooling was removed. 1-Methyl-
2-imidazolecarboxaldehyde (45.4 mmol, 5.0 g) and pyridine
(45.4 mmol, 3.7 mL) were added and the mixture was heated
under reflux over 16 h. The pyridine was distilled off at 105 ◦C
under vacuum. After cooling down to ambient temperature, the
mixture was dissolved in CH2Cl2 (200 mL) and washed with water
(2 ¥ 200 mL). The organic layer was dried (Na2SO4), filtered and
the solvent was evaporated in vacuo. The residue was dissolved in
a small amount of diethyl ether, layered with pentane and stored in
the refrigerator. After 12 h the product mibpm3menth (6) was filtered
off as a white precipitate, washed with cold diethyl ether and dried
in vacuo. Further fractions can be obtained after the filtrate has
been concentrated in vacuo.
Yield: 355 mg (0.556 mmol, 84%). 1H NMR (300 MHz, CDCl3,
◦
25 C): d = 0.61 (s, 3H, H12 or H12¢, CH3), 0.67 (s, 3H, H12 or
H12¢, CH3), 0.93 (s, 3H, H11 and H11¢, CH3), 0.94 (s, 3H, H11 and
H11¢, CH3), 1.21 (m, 4H, H5b, H5b¢, H6b and H6b¢, CH2), 1.24 (s,
6H, H13 or H13¢, CH3), 1.85 (m, 2H, H6a and H6a¢, CH2), 2.07 (m,
3
2H, H5a and H5a¢, CH2), 2.36 (s, 3H, H19, CH3), 2.79 (t, JHH
=
3.3 Hz, 2H, H4 and H4¢, CH), 6.58 (s, 1H, H15, CHthi), 6.83 (t,
3JHH = 4.3 Hz, 1H, H16, CHthi), 7.58 (d, JHH = 4.9 Hz, 1H, H17,
3
CHthi),7.76 (s, 2H, H3 and H3¢, CHpz), 7.76 (s, 1H, H1, CH) ppm.
13C NMR (75 MHz, CDCl3, 25 ◦C): d = 2.6 (C19, CH3), 10.2, 10.5
(C13 and C13¢, CH3), 18.7, 18.9 (C11 and C11¢, CH3), 20.3, 20.4 (C12
and C12¢, CH3), 26.9, 27.2 (C5 and C5¢, CH2), 33.1, 33.4 (C6 and
C6¢, CH2), 46.8, 46.9 (C4 and C4¢, CH), 51.3 (C7 and C7¢, Cquart.),
Yield: 3.97 g (8.85 mmol, 39%). 1H NMR (300 MHz, CD3CN,
◦
3
3
25 C): d = 0.82 (d, JH,H = 6.8 Hz, 3H, CH3), 0.84 (d, JH,H
=
6552 | Dalton Trans., 2011, 40, 6547–6554
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Table 2 Structure determination details of ligands 3, 4 and 6
mibpm3cam (3)
thibpm3cam (4)
mibpm3menth (6)
Empirical formula
Formula weight [g mol-1]
Crystal colour/habit
Crystal system
C27H36N6
444.62
colourless block
Monoclinic
C2, 4
25.4805(12)
7.7565(5)
17.8684(7)
90.00
133.701(3)
90.00
2553.1(2)
2.85–28.52
-34, 34
-10, 10
-24, 24
960
0.071
0.23 ¥ 0.21 ¥ 0.16
1.157
150(2)
39319
3470
2971
C27H34N4S ¥ H3COH
478.68
colourless needle
Monoclinic
C2, 4
26.0261(6)
7.9220(2)
16.4224(5)
90.00
127.099(4)
90.00
2 ¥ C27H40N6
448.65
colourless plate
Monoclinic
P21,4
8.8408(3)
11.2369(4)
27.0450(9)
90.00
93.068(2)
90.00
2682.89(16)
1.51–25.35
-9, 10
-13, 13
-32, 32
976
0.068
0.50 ¥ 0.19 ¥ 0.10
1.111
230(2)
18702
5177
3904
Space group, Z
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
2700.62(19)
1.55–27.44
-31, 33
q/◦
h min, max
k min, max
-10, 10
l min, max
-21, 21
F(000)
1032
m(Mo-Ka)/mm-1
Crystal size/mm
Dcalcd [g cm-3]
T/K
0.146
0.42 ¥ 0.15 ¥ 0.14
1.177
220(2)
16574
6038
4927
Reflections collected
Indep. reflections
Obs. reflections (>2sI)
Parameter
298
373
622
Restrains
1
91
0.0555
0.0724
0.01(8)
0.0437, 0.1007
0.0580, 0.1088
0.193/- 0.194
1.028
10
0.0890
0.1669
—
0.0533, 0.1328
0.0753, 0.1485
0.395/- 0.227
1.021
wt. parameter a
wt. parameter b
Flack parameter
R1, wR2 (obsd.)
R1, wR2 (obsd.)
0.0699
0.6097
—
0.0423, 0.1074
0.0541, 0.1141
0.241/- 0.213
1.066
a
a
3
˚
Diff. peak/hole [e/A ]
Goodness of fit on F2
a Flack parameter not meaningful for structures without third or higher row atoms.
60.8, 61.2 (C8 and C8¢, Cquart.), 70.7 (C1, CH), 125.6 (C3 and C3¢,
CHpz), 126.4, 126.5, 126.8 (C15, C16 and C17, CHthi), 127.3 (C10
and C10¢ or C17, Cpz or C N), 127.4 (C10 and C10¢ or C17, Cpz or
removing the solvent in vacuo, the residue was purified over a
short silica plug (hexane/ethyl acetate, 9 : 1). After evaporation
of solvents, the cyclopropane products were characterized by
1H and 13C NMR spectroscopy. The enantiomeric excess of the
products was determined by chiral GC experiments carried out
with a Shimadzu 17A gas chromatograph equipped with a flame
ionization detector (FID). Separation of the enantiomers was
accomplished using a Cyclodex B column (60 m ¥ 0.25 mm,
0.25 mm). The GC oven was set isothermal at 110 ◦C with a
total analysis time of 215 min, while nitrogen was used as carrier
gas. The injector was operated in split mode, and the injector
C
N), 140.4 (C14, Cthi), 169.4, (C9 and C9¢, Cpz) ppm. IR (KBr):
˜
n = 3115 w, (n(C–H) thienyl), 2970, 2931, 2873 vs. (ns(CH3)),
1590 m (n(C N) or n(C C) pyrazole), 1085, 1067, 1048, 1036 m
(n(BF4)) cm-1. Anal. Calc. for C29H37N5SCuBF4 (638.05 g mol-1):
C 54.59, H 5.84, N 10.98, S 5.03; found: C 54.83, H 5.80, N 10.83,
S 4.60.
General procedure for cyclopropanation reactions and
chromatographic conditions
◦
temperature was set to 250 C. Furthermore, the temperature of
◦
the FID was set to 200 C. The column flow was set to 1.20 mL
min-1, total flow 54 mL min-1, purge pressure 100 kPa, purge
flow 3.00 mL min-1 and septum purge 6.90 mL min-1. Resulting
from these parameters the column head pressure was 181 kPa and
the split ratio 38 : 1. The identification of cis and trans products
in the chromatogram was made by a reference measurement of
commercially available trans-ethyl-2-phenylcyclopropanecarboxy-
late.
A typical experiment of styrene (4.06 mL, 35.0 mmol) with ethyl
diazoacetate (0.530 mL, 5.00 mmol) catalyzed by [(CuOTf)2]C6H6
(11.8 mg, 0.050 mmol) in combination with ligand 6 (33.6 mg,
0.075 mmol) resulted in ethyl-2-phenyl-cyclopropanecarboxylate
in a yield of 717 mg (3.77 mmol, 75.4%).
A solution of ligand 3, 4 or 6 (0.075 mmol, 1.5 mol%)
in dichloromethane (2 mL) was added to a solution of the
Cu(I)-precursor (0.050 mmol, 1.0 mol %) in dichloromethane
(1.5 mL). After stirring for 1 h under nitrogen atmosphere,
styrene (4.064 mL, 35.0 mmol) was added to the catalyst, and
the solution was stirred for another 30 min. Ethyl diazoacetate
(0.530 mL, 5.00 mmol) in dichloromethane (8 mL) was added
dropwise over a period of 3 h. After stirring for an additional
18 h at ambient temperature the solvent was removed under
reduced pressure and the crude product was submitted to 1H NMR
analysis to determine the cis : trans ratio. To remove the excess of
styrene, methanol was added and the solution was filtered. After
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cis-Ethyl-2-phenylcyclopropanecarboxylate
3 (a) T. C. Higgs, D. Ji, S. R. Czernuszewicz, K. Spartalian, C. J.
O’Connor, C. Seip and C. J. Carrano, J. Chem. Soc., Dalton Trans.,
1999, 807–813; (b) B. S. Hammes and C. J. Carrano, J. Chem. Soc.,
Dalton Trans., 2000, 3304–3309; (c) B. S. Hammes and C. J. Carrano,
Chem. Commun., 2000, 1635–1636; (d) B. S. Hammes and C. J. Carrano,
Inorg. Chim. Acta, 2000, 300–302, 427–433; (e) B. S. Hammes and C. J.
Carrano, Inorg. Chem., 2001, 40, 919–927; (f) E. Galardon, M. Giorgi
and I. Artaud, Dalton Trans., 2007, 1047–1052.
4 (a) K. I. The´, L. K. Peterson and E. Kiehlmann, Can. J. Chem., 1973,
51, 2448–2451; (b) K. I. The´ and L. K. Peterson, Can. J. Chem., 1973,
51, 422–427; (c) L. K. Peterson, E. Kiehlmann, A. R. Sanger and K. I.
The´, Can. J. Chem., 1974, 52, 2367–2374.
5 (a) D. L. Reger, T. C. Grattan, K. J. Brown, C. A. Little, J. J. S. Lambda,
A. L. Rheingold and R. D. Sommer, J. Organomet. Chem., 2000, 607,
120–128; (b) D. L. Reger, J. R. Gardinier and M. Smith, J. Organomet.
Chem., 2005, 690, 1901–1912; (c) D. L. Reger, R. P. Watson, M. D.
Smith and P. J. Pellechia, Organometallics, 2006, 25, 743–755.
6 (a) S. Milione, C. Cuomo and A. Grassi, Top. Catal., 2006, 40, 163–172;
(b) J. Zhang, P. Braunstein and T. S. A. Hor, Organometallics, 2008, 27,
4277–4279; (c) J. Zhang, A. Li and T. S. A. Hor, Organometallics, 2009,
28, 2935–2937.
◦
1H NMR (300 MHz, CDCl3, 25 C): d = 0.95 (t, JHH = 7.2 Hz,
3H, OCH2CH3), 1.31 (m, 1H, CH2), 1.70 (m, 1H, CH2), 2.06 (m,
1H, CHCO2Et), 2.54 (m, 1H, PhCH), 3.86 (q, 3JHH = 7.2 Hz, 2H,
OCH2CH3), 7.17 (m, 5H, Ph) ppm. 13C NMR (75 MHz, CDCl3,
3
◦
25 C): d = 11.0 (CH3, OCH2CH3), 13.9 (CH2, CH2), 21.7, (CH,
CHCO2Et), 25.3 (CH, PhCH), 60.01 (CH2, OCH2CH3), 126.5,
127.8, 129.2 (CH, Ph), 136.5 (C, Ph), 170.8 (C4, C, CHCO2Et)
ppm. Retention times (GC): tR = 161.24 min (major), tR = 170.86
min (minor).
X-Ray structure determinations
Single crystals of 3, 4 or 6 were mounted with perfluorinated
ether or glue on a glass fibre. A Bruker-Nonius KappaCCD
diffractometer and a Bruker SMART APEXII diffractometer
were used for data collection (graphite monochromator, Mo-Ka
7 A. Otero, J. Ferna´ndez-Baeza, A. Lara-Sa´nchez, J. Tejeda and L. F.
Sa´nchez-Barba, Eur. J. Inorg. Chem., 2008, 5309–5326.
˚
radiation, l = 0.71073 A). The structures were solved by using
8 (a) A. Otero, J. Ferna´ndez-Baeza, A. Antin˜olo, J. Tejeda, A. Lara-
Sa´nchez, L. Sa´nchez-Barba, M. Sanchez-Molina, S. Franco, I. Lopez-
Solera and A. M. Rodr´ıguez, Eur. J. Inorg. Chem., 2006, 707–710;
(b) A. Otero, J. Ferna´ndez-Baeza, A. Antin˜olo, J. Tejeda, A. Lara-
Sa´nchez, L. Sa´nchez-Barba, M. Sanchez-Molina, S. Franco, I. Lopez-
Solera and A. M. Rodr´ıguez, Dalton Trans., 2006, 4359–4370; (c) A.
Otero, J. Ferna´ndez-Baeza, A. Antin˜olo, J. Tejeda, A. Lara-Sa´nchez, L.
Sa´nchez-Barba, M. Sa´nchez-Molina, S. Franco, I. Lo´pez-Solera and
A. M. Rodr´ıguez, Inorg. Chem., 2007, 46, 8475–8477.
9 I. Hegelmann and N. Burzlaff, Eur. J. Inorg. Chem., 2003, 409–411.
10 L. Peters and N. Burzlaff, Polyhedron, 2004, 23, 245–251.
11 (a) D. A. House, P. J. Steel and A. A. Watson, Aust. J. Chem., 1986, 39,
1525–1536; (b) M. Bovens, A. Togni and L. M. Venanzi, J. Organomet.
Chem., 1993, 451, C28–C31; (c) D. L. Jameson and R. K. Castellano,
Inorg. Synth., 1998, 32, 55–57; (d) C. Kashima, Y. Miwa, S. Shibata and
H. Nakazono, J. Heterocycl. Chem., 2003, 40, 681–688.
12 H. Hirschmann and K. R. Hanson, Tetrahedron, 1974, 30, 3649–3656.
13 To avoid extraordinarily long abbreviations the abbreviation system
for Tp and benzopyrazolylborates TpBo ligands introduced by S. Trofi-
menko, was transferred to this problem. A superscript of 3 preceding
‘cam’ indicates a 3,4-fusion of the camphor group to the pyrazole. This
resulted in the abbreviations Hbpa4cam, Hbpa4menth HOPhbpm3cam and
HSiprbpm3cam. For further details see: S. Trofimenko, Scorpionates -
The Coordination Chemistry of Polypyrazolylborate Ligands, Imperial
College Press, London, 1999, pp. 5.
direct methods and refined with full-matrix least-squares against
F2 {Siemens SHELX-97}.19 A weighting scheme was applied in
2
the last steps of the refinement with w = 1/[s (Fo2) + (aP)2 + bP]
and P = [2Fc2 + max(Fo2,0)]/3. Hydrogen atoms were included
in their calculated positions and refined in a riding model. The
molecular structure of ligand 4 exhibits a disordered thienyl group
that was successfully resolved in the refined structure model. The
asymmetric unit contains also methanol that is disordered and was
included in the structure model on two positions in 50% portions.
In the case of compound 6 the asymmetric unit consists of two
molecules of 6. In one of these molecules an isopropyl group is
disordered but was successfully resolved in the structure model.
All details and parameters of the measurements are summarised
in Table 2. The structure pictures were prepared with the program
Diamond 2.1e.20
Acknowledgements
This work was supported by the Deutsche Forschungsgemein-
schaft (BU 1223/5-1).
14 J. Elflein, F. Platzmann and N. Burzlaff, Eur. J. Inorg. Chem., 2007,
5173–5176.
15 T. Godau, F. Platzmann, F. W. Heinemann and N. Burzlaff, Dalton
Trans., 2009, 254–255.
Notes and references
16 (a) C. J. Tokar, P. B. Kettler and W. B. Tolman, Organometallics, 1992,
11, 2737–2739; (b) D. D. LeCloux, C. J. Tokar, M. Osawa, R. P. Houser,
M. C. Keyes and W. B. Tolman, Organometallics, 1994, 13, 2855–2866.
17 D. L. Jameson and R. K. Castellano, Inorg. Synth., 1998, 32, 55–57.
18 (a) D. A. Evans, K. A. Woerpel, M. M. Hinman and M. M. Faul, J. Am.
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