Stereogenic boron in 2-amino-1,1-diphenylethanol-based boronate-imine and amine complexes

Article abstract of DOI:10.3762/bjoc.7.72

Racemic boronate-imine and boronate-amine complexes 8 and 10, both featuring a stereogenic boron atom were synthesized from 2-amino-1,1- diphenylethanol (5) and characterized by crystal structure analyses. Proof of enantiomerism at the boron center for th

Full text of DOI:10.3762/bjoc.7.72

Stereogenic boron in 2-amino-1,1-diphenylethanol-
based boronate–imine and amine complexes
Sebastian Schlecht1,§, Walter Frank2 and Manfred Braun*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 615–621.
1Institute of Organic and Macromolecular Chemistry, University of
Düsseldorf, Universitätsstr. 1, D-40225 Düsseldorf, Germany and
2Institute of Inorganic and Structural Chemistry, University of
Düsseldorf, Universitätsstr. 1, D-40225 Düsseldorf, Germany
doi:10.3762/bjoc.7.72
Received: 22 February 2011
Accepted: 24 April 2011
Published: 16 May 2011
Email:
Sebastian Schlecht - sebastian.schlecht@bayer.com; Walter Frank -
wfrank@uni-duesseldorf.de; Manfred Braun* -
braunm@uni-duesseldorf.de
Associate Editor: S. C. Zimmerman
© 2011 Schlecht et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
§ New address: Bayer MaterialScience, 51368 Leverkusen, Germany
Keywords:
boron; chirality; coordination chemistry; crystal structure;
stereochemistry
Abstract
Racemic boronate–imine and boronate–amine complexes 8 and 10, both featuring a stereogenic boron atom were synthesized from
2-amino-1,1-diphenylethanol (5) and characterized by crystal structure analyses. Proof of enantiomerism at the boron center for the
novel boronate–amine complex 10 was established by separation of the enantiomers. Racemization barriers were found to be in the
same range for both amine and imine complexes (100–110 kJ/mol).
Introduction
Enantiomerism of main group hetero elements has been investi- [12], in acyloxyboranes 3 [13-16] with electron with-drawing
gated thoroughly for compounds with stereogenic sulfur [1], substituents X (Scheme 1) that turned out to be crucial to the
phosphorus [2], nitrogen [3] and silicon atoms [4]. By compari- configurational stability at boron, and recently, in boronate
son, stereogenic tetrahedral-coordinated boron has been studied complexes of boradiazaindacene [17]. Pursuing the concept of
to a much lesser extent. This is partly because it was incorpo- tridentate ligands derived from 2-amino-2,2-diphenylethanol we
rated in a chiral environment generated by enantiomerically have been able to obtain boronate–imine complexes 4 with
pure ligands [5-9] or counter-ions [10], although some of these boron as a stable stereogenic center (Scheme 1). Their racem-
complexes have been obtained in a diastereoselective manner ization barrier was measured and the absolute configuration of
[6-9]. On the other hand, enantiomerism at boron has been the isolated enantiomers was determined by a comparison of the
observed in acyclic tetra-coordinated complexes 1 [11] and 2 measured and calculated CD spectra [18]. In this article, we
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Beilstein J. Org. Chem. 2011, 7, 615–621.
describe the synthesis, resolution and characterization of chiral
boronate–imine and boronate–amine complexes 8 and 10, res-
pectively. Their backbone is derived from the regioisomeric
2-amino-1,1-diphenylethanol (5) which features the geminal
diphenyloxymethyl motif [19].
Scheme 1: Stereogenic boron in resolved acyclic and cyclic
complexes 1–4.
Scheme 2: Synthesis of the racemic boronate–imine complex 8 and
boronate–amine complex 10. Reagents and conditions: a) Na2SO4,
MeOH/THF (1:1), 18 h reflux, 87%; b) 4-chlorophenylboronic acid,
molecular sieves 3Å, toluene, 20 h reflux, 17%; c) NaCNBH3, MeOH,
hydrochloric acid, 25 °C, 0.5 h; d), 4-chlorophenylboronic acid,
NaHCO3, toluene, 4 h reflux, 63%.
Results and Discussion
Following a known procedure [20], the amino alcohol 5 was
readily accessible from methyl glycinate hydrochloride by the
reaction with excess phenylmagnesium bromide. Subsequent hydrochloric acid to give the salt of the amine 9. As the latter
condensation with 1-formyl-2-naphthol (6) gave the imine 7 in decomposed to a considerable extent when stored, it was
87% yield (Scheme 2). In order to generate the boronate–imine converted immediately into the boronate–amine complex 10.
complex 8, the imine 7 was treated with 4-chlorophenylboronic Sodium hydrogen carbonate served as the base to liberate the
acid in boiling toluene in the presence of molecular sieves. The free amine in situ, and complexation with 4-chlorophenyl-
fact that this protocol led to the desired boronate complex 8 in boronic acid in refluxing toluene gave the complex 10 in 63%
only 17% isolated yield is attributed to steric hindrance, which isolated yield. The lack of any absorption at wavelengths higher
originates from the diphenylhydroxymethyl group and has a than 350 nm indicated the absence of the imine chromophore.
disadvantageous effect on complexation. It is remarkable that
considerably higher yields were obtained when regioisomeric Here again the formation of the complex can be deduced from
complexes 4 were prepared via the same protocol. The forma- the 11B NMR shift of 5.5 ppm [21]. Suitable crystals of com-
tion of boronate complex 8 was indicated by its 11B NMR shift pound 10·CH3OH were obtained from methanol/hexane and the
value of 6.6 ppm [21] and confirmed by X-ray crystal structure result of the X-ray analysis is shown in Figure 1. A comparison
analysis as shown in Figure 1. Although the compound crystal- of the boron–nitrogen distance in the imine complex 8 (156.3
lized as a conglomerate, the (S)-enantiomer is shown arbitrarily pm) and the amine complex 10 (159.5 pm) reveals a small elon-
in Figure 1a. The boron–nitrogen distance of 156.3 pm clearly gation of the bond upon saturation of the imine group that
shows the existence of a coordinate bond. The UV spectrum of results from the change in hybridization at the nitrogen from sp2
compound 8 displays a high-wavelength absorption maximum to sp3. The structure determination 10·CH3OH reveals a
at λ = 395 nm, typical for the imine chromophore, as observed hydrogen bridge of the amine hydrogen atom directed to the
for other boronate–imine complexes.
oxygen atom of the alcohol. The fact that the NH group func-
tions as the hydrogen-bond donor is attributed to an enhanced
The imine 7 also served for the preparation of the boro- acidity that is caused by the coordinate bond between nitrogen
nate–amine complex 10. For this purpose, it was reduced [22] and boron. Related hydrogen bonding has been observed in
with cyanoborohydride in methanol in the presence of diastereomerically pure boronate–amine complexes [9].
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Beilstein J. Org. Chem. 2011, 7, 615–621.
Figure 1: Solid-state structure of boronate–imine complex 8 (a) and the chosen asymmetric unit of the crystal structure of boronate–amine-complex
solvate 10·CH3OH (b) showing the hydrogen bonding of the methanol molecule (right). Displacement ellipsoids are drawn at the 30% probability level,
radii of hydrogen atoms are chosen arbitrarily and the hydrogen atom labels are omitted for clarity. Selected bond distances and angles are given in
Table 1.
Table 1: Selected distances (Å) and angles (°) of compounds 8 and
10·CH3OH. (continued)
Table 1: Selected distances (Å) and angles (°) of compounds 8 and
10·CH3OH.
H2…O1a
—
—
—
2.19
8
10·CH3OH
O3–H2…O1a
O3…O1a
2.897(5)
144.1
B1–O1
1.496(3)
1.455(3)
1.563(3)
1.595(3)
1.337(2)
1.433(2)
1.460(2)
1.282(2)
1.550(3)
1.429(3)
1.397(3)
111.4(2)
106.54(19)
109.59(19)
101.08(19)
113.9(2)
113.93(19)
—
1.458(6)
1.427(5)
1.598(6)
1.607(6)
1.349(5)
1.418(5)
1.481(5)
1.480(5)
1.520(5)
1.484(6)
1.382(6)
112.2(4)
105.9(4)
111.4(4)
101.2(4)
113.9(4)
111.5(4)
0.95(2)
B1–O2
a−x+1, −y+2, z+0.5.
B1–N1
B1–C4
O1–C11
O2–C1
Upon complexation of the boron, a typical pyramidalization
occurs that can be measured in terms of the tetrahedral char-
acter (THC). For the complexes 8 and 10, this has been calcu-
lated by taking into account the six bond angles θ1–θ6 using the
equation of Höpfl (Equation 1) [23].
N1–C2
N1–C3
C1–C2
C3–C10
C10–C11
O1–B1–O2
O1–B1–N1
O1–B1–C4
O2–B1–N1
O2–B1–C4
N1–B1–C4
N1–H1
(1)
The corresponding values obtained for the boron and the
H1…O3
—
1.99(3)
nitrogen atoms in the complexes 8 and 10 are given in Table 2.
N1–H1…O3
N1…O3
—
143(3)
—
2.807(5)
0.82
Thus, the tetrahedral character is essentially identical in both
complexes for the boron atom. The pyramidalization of nitrogen
O3–H2
—
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Beilstein J. Org. Chem. 2011, 7, 615–621.
were obtained for the separated enantiomers of the imine com-
plex 8. Previously, we have been able to assign the absolute
configuration to the complex 4a, a regioisomer of 8, by com-
parison of the measured and calculated CD spectra [18]. The
(R)-enantiomer of 4a is characterized by an intense, positive
Cotton effect at high wavelength. As a similar effect is observed
in the CD spectrum of the enantiomer of 8 that is eluted at
lower retention time, the (R)-configuration is attributed to this
Table 2: Bond length and tetrahedral character (THC) of boronates 8
and 10.
Complex
BN distance
THC boron THC nitrogen
8
156.3 ppm
159.5 ppm
75%
75%
—
10
71%
that occurs upon saturation of the imine bond is evident from enantiomer by analogy. As we were interested in whether or not
the values of the tetrahedral character of the nitrogen atom in the configurational stability differs in boronate–imine
the amine complex. The saturation of the carbon–nitrogen bond complexes 4 and 8 on the one hand and boronate–amine com-
has a considerable impact on the structure of both of the plex 10 on the other hand, the racemization barrier of the latter
complexes 8 and 10. This becomes evident from Figure 2, was determined. For this purpose, a sample of the enantiomeri-
which shows a superposition of the skeletons of complexes 8 cally pure complex 10 was heated in n-decane at 60 °C, and the
(red) and 10 (green). As a consequence of the saturation and the decay of the optical purity was followed by chiral HPLC. Thus,
rehybridization at nitrogen, the five-membered rings are the racemization rate at 325.4 K was measured to be 4.1·10−4
connected in a more concave, folded manner in the amine com- s−1 and the racemization barrier ΔG‡ amounted to 101.0
plex 10 than in the rather flat junction of both rings in the imine kJ·mol−1 [24]. The corresponding values for boronate–imine
complex 8. It is possible that steric repulsion is more severe in complexes 4 varied from 105–110 kJ·mol−1, indicating that the
the amine complex 10 that could slightly weaken the change from the imine to the amine ligand does not alter the
boron–nitrogen bond.
configurational stability at boron to any considerable degree.
The racemization process involves the breaking of the co-
ordinate boron–nitrogen bond to produce a trigonal boron atom
that is then attacked by the nitrogen from the opposite face. As
the nitrogen–boron distances were similar in all the boronate
complexes 4, 8, and 10, it is plausible that the racemization
barrier is also in the same range for all of these compounds.
Conclusion
In summary, it was shown that boron based enantiomerism is
not only possible in boronate–imine complexes 4 and 8, but also
in amine complexes as well as exemplified by complex 10 that
differs from 8 in the saturation of the imine bond to an amine
bond. Enantiomers of boronate complexes 8 and 10 are acces-
sible and were shown to be stable at room temperature. The
racemization barriers are of similar magnitude in the amine and
imine complexes.
Experimental
General: Melting points (uncorrected) were determined with a
Büchi 540 melting point apparatus. NMR spectra were recorded
with a Bruker DXR 500 spectrometer. Mass spectra were
recorded on an ion-trap API mass spectrometer Finnigan LCQ
Deca (ESI), triple-quadrupole-mass spectrometer Finnigan TSQ
7000, and sector field mass spectrometer Finnigan MAT 8200
Figure 2: Superposition of the skeletons of boronate complexes 8
(red) and 10 (green).
In order to prove the enantiomerism based upon stereogenic (EI, 70 eV). High resolution mass spectra were carried out on
boron, the racemates of the complexes 8 and 10 were resolved Bruker FT-ICR APEX III (7.0 T) (MALDI) at the University of
by HPLC on a chiral column. Both complexes displayed peaks Bielefeld. Column chromatography was performed with Fluka
of identical intensity resulting from the enantiomers. The differ- silica gel 60 (230–400 mesh) and thin layer chromatography
ences in the retention times permitted isolation of the pure enan- was carried out on Merck TLC Silicagel 60 F254 aluminium
tiomers on a semi-preparative scale. Mirror-image CD spectra sheets. HPLC was performed with the chiral column
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Beilstein J. Org. Chem. 2011, 7, 615–621.
CHIRALPAK IB. Toluene was freshly distilled from sodium C31H23NO2BCl, [M + Na]+ 509.14389; found, 509.14438, [M
under a nitrogen atmosphere.
+ H]+ 487.16195; found, 487.16219.
1-{[(2-Hydroxy-2,2-diphenylethyl)imino]methyl}naph- Boronate–amine complex 10
thalen-2-ol (7)
The imine 7, (0.367 g, 1.00 mmol) and sodium cyanoborohy-
A mixture of amino alcohol 5 (0.427 g, 2.00 mmol), naph- dride (0.19 g, 3.0 mmol) were dissolved in absolute methanol
thaldehyde 6 (0.379 g, 2.20 mmol), and sodium sulfate (2.0 g, (75 mL). After adding 5 mL of hydrochloric acid (10%), the
14 mmol) was suspended in absolute methanol (25 mL) and dry yellow solution gradually became colorless on stirring at room
toluene (25 mL) under a nitrogen atmosphere. The yellow mix- temperature for 1 h. Distilled water (50 mL) was added and the
ture was heated under reflux with stirring for 18 h. After solution extracted three times with chloroform. The combined
cooling to room temperature, the sodium sulfate was removed organic layers were dried with sodium sulfate and the solvent
by filtration and the filtrate concentrated in a rotary evaporator. was removed in a rotary evaporator. Immediately, the resulting
The residue was purified by column chromatography (chloro- crude colorless product, 4-chlorophenyl boronic acid (0.122 g,
form/ethyl acetate, 10:1) to give product 7 as a solid (0.640 g, 1.00 mmol) and sodium hydrogen carbonate (0.13 g, 1.5 mmol)
87%). Rf 0.1 (chloroform/ethyl acetate, 10:1); mp 178 °C; were suspended in 100 mL of dry toluene and heated under
1H NMR (500 MHz, CDCl3) δ 1.90 (s, 1H, aliphatic OH), 3.30 reflux for 4 h. After the addition of distilled water, the layers
(d, J = 2.6 Hz, 2H, CH2), 6.90 (d, J = 9.3 Hz, 1H, 3-H), 7.18 were separated and the aqueous phase was extracted with chlo-
(m, 1H, 6-H), 7.2–7.3 (m, 10H, phenyl H), 7.36 (t, J = 7.0 Hz, roform. The combined organic layers were dried with sodium
1H, 7-H), 7.57 (d, J = 7.9 Hz, 1H, 5-H), 7.66 (d, J = 9.3 Hz, 1H, sulfate and the solvent was removed in a rotary evaporator. The
4-H), 7.74 (d, J = 8.4 Hz, 1H, 8-H), 8.57 (d, J = 6.7 Hz, 1H, residue was purified by column chromatography (chloroform/
N=CH), 14.8 (broad s, 1H, phenolic OH); 13C NMR (125 MHz, ethyl acetate, 10:1) to yield boronate 10 as a colorless solid
CDCl3) δ 59.9 (CH2), 73.8 (CPh2), 107.0 (C-1), 118.1 (C-8), (0.210 g, 63%). Rf 0.76 (chloroform/ethyl acetate, 10:1), mp
122.9 (C-6), 123.9 (C-3), 126.3 (C-9), 126.9–130.6 (phenyl C), 157 °C; 1H NMR (500 MHz, CDCl3) δ 3.53 (t, J = 11.5 Hz, 1H,
129.5 (C-5), 133.3 (C-10), 136.0 (phenyl ipso-C), 137.0 (C-4), Ph2C–CHH), 3.83 (dd, J = 11.4 Hz, J = 5.5 Hz, 1H,
160.2 (C-11), 174.2 (C-2); MS (ESI) m/z (%): 368 [M + H]+ Ph2C–CHH), 3.95 (dd, J = 15.3 Hz, J = 4.3 Hz, 1H, napththyl
(40), 350 (100), 196 (30).
CHHN), 4.30 (d, J = 15.4 Hz, 1H, naphthyl CHHN), 4.34
(quint, J = 5.6 Hz, 1H, NH), 6.92 (d, J = 9.0 Hz, 1H, naphthyl
3-H), 6.94 (d, 2H, o-chloro H), 7.0–7.25 (m, 10H, phenyl H),
Boronate–imine complex 8
The imine 7, (0.367 g, 1.00 mmol), 4-chlorophenylboronic acid 7.23 (d, J = 8.2 Hz, 2H, m-chloro H), 7.29 (t, J = 8.3 Hz, 1H,
(0.183 g, 1.50 mmol) and 1.0 g of molecular sieves (3 Å) were naphthyl 8-H), 7.54 (d, J = 9.2 Hz, 1H, naphthyl 4-H), 7.64 (d,
suspended in 100 mL of dry toluene and heated under reflux for J = 8.0 Hz, 1H, naphthyl 5-H); 13C NMR (125 MHz, CDCl3) δ
20 h. After filtration, the solvent was removed in a rotary evap- 43.6 (naphthyl CH2), 58.5 (Ph2C-CH2), 82.0 (Ph2C), 104.6
orator and the residue purified by column chromatography (naphthyl C-1), 120.0 (naphthyl C-8), 121.5 (naphthyl C-3),
(chloroform/ethyl acetate, 10:1) to afford boronate 8 as a yellow 122.7 (naphthyl C-6), 125.8–128.5 (phenyl C), 128.0 (o-chloro
solid (80 mg, 17%). Rf 0.76 (chloroform/ethyl acetate, 10:1); C), 128.1 (naphthyl C-9), 128.5 (naphthyl C-5), 130.0 (naph-
mp 126 °C; 1H NMR (500 MHz, CDCl3) δ 4.52 (d, J = 11.0 Hz, thyl C-4), 131.6 (naphthyl C-10), 133.8 (m-chloro C), 145.7
1H, CHH), 4.61 (d, J = 11.0 Hz, 1H, CHH), 6.99 (t, J = 7.4 Hz, (phenyl ipso-C), 146.4 (phenyl ipso-C), 152.9 naphthyl C-2);
1H, phenyl H), 7.03 (d, J = 7.4 Hz, 2H, o-chloro H), 7.1–7.54 11B NMR (160 MHz, CDCl3) δ 5.5; MS (ESI) m/z (%): 512 [M
(m, 9H, phenyl H), 7.17 (d, J = 9.0 Hz, 1H, naphthyl 3-H), 7.29 + Na]+ (97), 490 [M + H]+ (32), 370 (32), 196 (100); HRMS
(d, J = 7.4 Hz, 2H, m-chloro H), 7.31 (m, 1H, naphthyl 6-H); calcd for C31H25NO2BCl, [M + K]+ 527.13348; found,
7.42 (d, J = 8.0 Hz, 1H, naphthyl 7-H), 7.61 (d, J = 8.0 Hz, 1H, 527.13353; [M + Na]+ 511.15954, found, 511.15935.
naphthyl 8-H), 7.65 (d, J = 8.1 Hz, 1H, naphthyl 5-H), 7.85 (d,
J = 9.1 Hz, 1H, naphthyl 4-H), 8.5 (s, 1H, N=CH); 13C NMR Crystal structure determination
(125 MHz, CDCl3) δ 65.1 (CH2), 82.4 (CPh2), 109.8 (naphthyl Crystals of compounds 8 and 10·CH3OH suitable for X-ray
C-1), 119.7 (naphthyl C-8), 121.2 (naphthyl C-3), 125.8–128.9 analysis were selected by means of a polarization microscope
(phenyl C), 126.1 (o-chloro C), 127.5 (naphthyl C-9), 129.3 and investigated with a STOE imaging plate diffraction system
(naphthyl C-5), 131.6 (naphthyl C-10), 133.1 (m-chloro C), using graphite monochromatic MoKα radiation (λ = 0.71073
139.2 (naphthyl C-4), 145.2 (phenyl ipso-C), 146.1 (phenyl Å). To avoid loss of methanol, and deterioration, crystals of
ipso-C), 154.1 (N=CH), 162.7 (naphthyl C-2); 11B NMR (160 compound 10·CH3OH were enclosed in thin walled glass capil-
MHz, CDCl3) δ 6.6; MS (ESI) m/z (%): 510 [M + Na]+ (10), laries. Unit cell parameters were determined by least-squares
489 [M + H]+ (20), 376 (35) 368 (100); HRMS calcd for refinements on the positions of 8000 and 3769 reflections in the
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Beilstein J. Org. Chem. 2011, 7, 615–621.
range 2.25° < θ < 25.25° and 1.95° < θ < 19.70°, respectively. ters were applied for all atoms heavier than hydrogen, and,
Space group type no. 61 was uniquely determined in the case of using the subsequently mentioned restraints and constraints,
compound 8. For crystals of compound 10·CH3OH Laue class refinements [26] by full-matrix least-squares calculations on F2
4/m and serial systematic extinctions were consistent with the converged (max. shift/esd: 0.000 and 0.001, respectively). For
enantiomorphic pair P41–P43. In the course of refinement P41 the NH group of 10·CH3OH, weak N–H bond length restraints
was shown to be the correct choice for the crystal under investi- were applied. With bond lengths and angles constrained to
gation. Data sets were corrected for Lorentz and polarization idealized values for the CH3, CH2, CH, and OH groups of the
effects in both cases. Crystal data, as well as details of data boron complexes and for the methanol molecule of 10·CH3OH,
collection and structure refinement are listed in Table 3.
the riding model was applied for all the other H atoms. In add-
ition, the H atoms of the CH3 group of the methanol molecule
The structures were solved by direct methods [25] and subse- were allowed to move collectively around the neighbouring
quent ΔF-syntheses and approximate positions of all the C–C axis, and the H atom of the OH group was also allowed to
hydrogen atoms were found. Anisotropic displacement parame- rotate around the neighbouring O–C axis. Isotropic displace-
Table 3: Summary of crystal data, details of intensity measurements and structure refinements of 8 and 10·CH3OH.
8
10·CH3OH
Empirical formula
Mr
C31H23BClNO2
487.76
C32H29BClNO3
521.82
tetragonal
P41
Crystal system
Space group
Z
orthorhombic
Pbca
8
4
Temperature [K]
Unit cell parameters
a [Å]
291(2)
291(2)
10.7877(6)
18.0573(11)
25.9072(13)
5046.6(5)
1.284
15.1310(11)
b [Å]
c [Å]
11.8802(8)
2719.9(5)
1.274
Volume [Å3]
Dcalcd
Absorption coefficient
F(000)
0.181
0.175
2032
1096
Crystal size [mm3]
Crystal color
Diffractometer type
Scan mode
θ range for data collection
Limiting indices
0.12 × 0.12 × 0.12
yellow
0.17 × 0.13 × 0.13
colorless
Stoe-IPDS
φ
Stoe-IPDS
φ
2.26-25.00
−12<h<12
−21<k<21
−30<l<30
62403
2.18-24.99
−17<h<17
−17<k<17
−13<l<13
30038
Reflections collected
Reflections unique
Reflections observed
Criterion for observation
Completeness
4441
4761
1731
1274
I>2σ(I)
I>2σ(I)
1.000
0.996
Refined Parameters
R1a, observed
325
347
0.036
0.035
wR2b, all data
0.070
0.050
Goodness-of-Fit, Sc
x(Flack)
0.90
0.82
−0.03(8)
0.10/−0.09
809031
Largest diff. peak/hole
CCDC-identifier
0.26/−0.17
809030
aR1 = || Fo|-|Fc||/|Fo|; bwR2 = [w(Fo2-Fc2)2/w(Fo2)2]1/2, where w = 1/[σ2(Fo2)+(aP2)] and P = (Fo2+2Fc2)/3; cS = [w(Fo2-Fc2)2/(n-p)]1/2.
620

Beilstein J. Org. Chem. 2011, 7, 615–621.
17.Haefele, A.; Zedde, C.; Retailleau, P.; Ulrich, G.; Ziessel, R. Org. Lett.
2010, 12, 1672–1675. doi:10.1021/ol100109j
ment parameters of H atoms were constrained to 120% of the
equivalent isotropic displacement parameters of the parent N
and C atoms for the NH, CH and CH2 groups and to 150% of
the parent C and O atoms for the CH3 group and the OH group
of the methanol molecule in 10·CH3OH. CCDC-809030 (com-
pound 8) and CCDC-809031 (compound 10·CH3OH) contain
the supplementary crystallographic data (excluding structure
factors) for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via http://
www.ccdc.cam.ac.uk/data_request/cif.
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doi:10.1016/S0957-4166(01)00199-9
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Acknowledgements
This work was supported by the Deutsche Forschungsgemein-
Slightly different values resulted, when the tetrahedral character was
calculated according to Oki’s equation, which is based on the three
angles between boron and the covalently bound neighbouring atoms: 8
(boron): 74%, 10 (boron) : 77%, 10 (nitrogen): 74%; cf. reference [16].
24.For reasons of control, the racemization of compounds 4 and 10 was
followed by measurement of the optical rotation. Based on this, the
experimental error was assessed to amount to ±0.5 kJ·mol−1.
25.Sheldrick, G. M. SHELXS97, Program for the Solution of Crystal
Structures; University of Göttingen: Germany, 1990.
schaft (Br 604/15-1,2).
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