Boron-based diastereomerism and enantiomerism in imine complexes - Determination of the absolute configuration at boron by CD spectroscopy

Article abstract of DOI:10.1002/ejoc.200800787

Boron turns out to be a stable stereogenic center in imine complexes of aryl and alkyl boronates. Diastereomerically pure complexes 7a-c are obtained from chiral imine ligands 5a,b that are derived from the amino alcohol (R)-4. The configuration at the boron atom is determined by crystal structure analyses. Racemic boronates 10a-c, available from a condensation of aryl boronic acids 6 with the achiral imine ligand 9, can be separated into stable enantiomers by HPLC on a chiral column. The racemization barrier ΔG^≠ has been determined to amount to 105-115 kJmol^-1. The comparison of calculated and measured CD spectra permits to assign unambiguously the absolute configuration to boron in the enantiomeric boronate-imine complex 10a. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800787

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200800787
Boron-Based Diastereomerism and Enantiomerism in Imine Complexes –
Determination of the Absolute Configuration at Boron by CD Spectroscopy
Manfred Braun,*[a] Sebastian Schlecht,[a] Marco Engelmann,[a] Walter Frank,[b] and
Stefan Grimme[c]
Keywords: Boron / Chirality / Circular dichroism / Stereochemistry
Boron turns out to be a stable stereogenic center in imine
complexes of aryl and alkyl boronates. Diastereomerically
pure complexes 7a–c are obtained from chiral imine ligands
5a,b that are derived from the amino alcohol (R)-4. The con-
figuration at the boron atom is determined by crystal struc-
ture analyses. Racemic boronates 10a–c, available from a
condensation of aryl boronic acids 6 with the achiral imine
ligand 9, can be separated into stable enantiomers by HPLC
on a chiral column. The racemization barrier ∆G϶ has been
determined to amount to 105–115 kJmol–1. The comparison
of calculated and measured CD spectra permits to assign un-
ambiguously the absolute configuration to boron in the enan-
tiomeric boronate-imine complex 10a.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
pure enantiomers (Scheme 1). In few cases only, the abso-
lute configuration has been determined by crystal structure
analyses.[4c,5]
Introduction
Enantiomerism of tetrahedral main group elements other
than carbon has attracted considerable interest in the past,
and the features of chiral sulfur, phosphorus, nitrogen and
silicon compounds have been studied comprehensively.[1]
Boron, typically adopting a trigonal planar configuration,
has been studied by far less intensively with respect to its
ability to form a stable stereogenic center. In most of these
approaches, boron was embedded in a chiral environment
of enantiomerically pure ligands[2] or counterions.[3] As a
consequence, an inversion of the configuration at boron re-
sults in the formation of epimers. They have been obtained
in a stereoselective manner when the boron atom was incor-
porated in bicyclic systems, the chirality at the boron atom
being induced by the chiral ligands.[2b–2d] When, on the
other hand, the boron is configurationally labile, advantage
has been taken of the readily occurring epimerization of the
boron center in crystallization-induced asymmetric trans-
formations.[2a] Enantiomerism at the boron center, however,
has been investigated only recently in acyloxyboranes 1,
wherein electron-withdrawing substituents X are essential
to avoid racemization.[4] In addition acyclic tetra-coordi-
nated boron compounds 2[5] and 3[6] have been isolated as
Scheme 1. Enantiomeric boron-amine and boron-phosphane com-
plexes.
Results and Discussion
In this communication, complexes of boronates are de-
scribed that are obtained from 2-amino-1,2,2-triphenyle-
thanol 4 and 2-amino-2,2-diphenylethanol 8 and isolated
as pure diastereomers and/or enantiomers. We were able to
determine the absolute configuration at boron in enantio-
meric boronate complexes by CD spectroscopy. The imines
5 derived from (R)-amino alcohol 4[7] were previously used
as ligands in bis-chelated titanium complexes.[8] When p-
chlorophenylboronic acid (6a) was allowed to react with the
imines (R)-5 by heating them in toluene, the new boronates
7a and 7b resulted. In order to find out whether complexes
7 with aliphatic residues at the boron atom can be obtained
as well, n-butyldiisopropoxyborane was treated with the
imine 5b. Indeed, the complexed alkylboronate 7c formed.
The complexes 7a–c were obtained as pure diastereomers,
[a] Institut für Organische Chemie und Makromolekulare Chemie,
Universität Düsseldorf,
Universitätsstr. 1, 40225 Düsseldorf, Germany
Fax: +49-211-8115079
E-mail: braunm@uni-duesseldorf.de
[b] Institut für Anorganische Chemie und Strukturchemie, Uni-
versität Düsseldorf,
Universitätsstr. 1, 40225 Düsseldorf, Germany
[c] Organisch-Chemisches Institut (Abt. Theoretische Chemie)
Universität Münster,
1
as shown by their H and 13C NMR spectra. Temperature-
Corrensstr. 40, 48149 Münster, Germany
dependent NMR spectroscopy never revealed an epimeri-
Eur. J. Org. Chem. 2008, 5221–5225
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M. Braun, S. Schlecht, M. Engelmann, W. Frank, S. Grimme
SHORT COMMUNICATION
zation at the boron atom.[9] A final proof of structure came
from the crystal structure analyses of 7a and 7c shown in
Figure 1 and Figure 2. Therein, the distance between the
nitrogen and the boron atom of 1.588 to 1.607 Å clearly
indicates the existence of a coordinative bond. In addition,
the crystal structures of the boron complexes 7a and 7c per-
mit to determine the configuration at the boron atom to be
(R). The structure of the related boronate 7b was not only
assigned by analogy but also by essential accordance in the
CD spectra of 7b with those of 7a and 7c (Scheme 2).
Scheme 2. Synthesis of boron complexes 7. Reagents and condi-
tions: i) see ref. [7]; ii) 7a, b: p-Cl-C6H4B(OH)2 (6a), molecular sieves
(3 Å), toluene, reflux; 7c: (iPrO)2B(nBu), toluene, reflux.
The fact that the diastereomers RC,RB-7 are obtained ex-
clusively, whereas the formation of products epimeric at the
boron center are not observed, indicates the higher thermo-
dynamic stability of the former stereoisomers, but does not
answer the question of kinetic stability. This point was ad-
dressed by investigating the stability of analogous enantio-
meric complexes 10 with boron as the only stereogenic cen-
ter. When designing them, it was intended to keep the fea-
ture of the geminal (diphenylamino)methyl moiety in order
to favor the ring closure upon complexation and to enhance
the stability of the complexes. For this purpose, the achiral
imine ligand 9 was synthesized from 2-amino-2,2-diphen-
ylethanol 8[10] and 1-formyl-2-naphthol and converted into
the racemic boron complexes 10a–c by treatment with bo-
ronic acids 6a–c, respectively (Scheme 3).
Figure 1. Diagram of one of the two crystallographically indepen-
dent molecules in the crystal of 7a. Displacement ellipsoids are set
at 30% probability, radii of hydrogen atoms are chosen arbitrarily.
Mean bond lengths (Å) and angles (°): B1/2–O1/4 1.495(5), B1/2–
O2/3 1.439(5), B1/2–N1/2 1.607(5), B1/2–C18/59 1.599(6); O1/4–
B1/2–O2/3 113.1(4), O1/4–B1/2–N1/2 104.1(3), O1/4–B1/2–C18/59
110.8(3), O2/3–B1/2–N1/2 99.4(3), O2/3–B1/2–C18/59 113.3(3),
N1/2–B1/2–C18/59 115.6(3).
Scheme 3. Synthesis of racemic boron complexes 10. Reagents and
conditions: i) methanol/THF (1:1), Na2SO4, reflux, 75%; ii) molec-
ular sieves 3 Å, toluene, reflux 12 to 20 h.
The crystal structure of one representative complex 10b
is shown in Figure 3. Here again, the nitrogen–boron bond
becomes evident from a bond length of 1.58 Å. This is
slightly shorter than the values of amine–boron bonds that
range from 1.62–1.71 Å,[2c,2d,4b,6] and typical for boron-
imine complexes.[2a] The structures of complexes 10a and
10c were confirmed by crystal structure analyses as well.
Figure 2. Molecular structure of 7c in the crystal. Displacement
ellipsoids are set at 30% probability, radii of hydrogen atoms are
chosen arbitrarily. Bond lengths (Å) and angles (°): B1–O1
1.514(4), B1–O2 1.442(4), B1–N1 1.588(4), B1–C4 1.594(5), O1–
B1–O2 113.2(3), O1–B1–N1 102.7(2), O1–B1–C4 108.8(3), O2–B1–
N1 100.8(3), O2–B1–C4 114.4(3), N1–B1–C4 116.4(3).
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Boron-Based Diastereomerism and Enantiomerism
The tetrahedral character (THC) of the boron atom was
found to be 68% in the complex 10a and 69% in 10b and
10c. It was calculated according to Höpfl’s equation[11] tak-
ing into account the six bond angles θ1–θ6 at the boron
atom [Equation (1)].
(1)
Figure 4. Comparison of experimental (in n-hexane) and theoreti-
cal (BHLYP/TZVP//PBE-D/TZVP) CD spectra of (R)-10a. All
computed excitation energies have been red-shifted by 0.6 eV
(which is the typical error of the TDDFT/BHLYP treatment). The
theoretical intensities were down-scaled by a factor of 0.02.
mization energy is in the same range as that measured for
the acyloxyboranes 1.[4] Nevertheless, the stability toward
racemization is remarkable, as here the stereogenic boron
atom in the complexes 10 forms stable enantiomers, even if
it does not carry electron-withdrawing substituents, which
are a prerequisite to enantiomerism in boron compounds
1–3.
Finally, the absolute configuration had to be assigned to
each of the enantiomers of boron complexes 10. For this
purpose, the CD spectrum of compound 10a was calculated
by density functional methods. All quantum chemical cal-
culations have been performed with the TURBOMOLE
suite of programs.[12] The molecular structures were fully
optimized at the density functional (DFT) level employing
the non-empirical PBE functional[13] with dispersion cor-
rections (DFT-D[14]) and a Gaussian AO basis of valence-
triple-zeta quality(TZVP[15]). These structures were used in
subsequent calculations of the vertical CD spectra by time-
dependent DFT (TDDFT) as described in detail in ref.[16]
To avoid artificial excited electronic states with charge-
transfer character, which often appears for large molecules
when using semi-local density functionals, TDDFT calcula-
tions were carried out by the BH-LYP hybrid functional[17]
with a HF-exchange fraction of 50%. The origin-indepen-
dent velocity-dipole form for the rotary strengths and a full
band-width (at 1/e height) for each electronic transition of
0.5 eV was used in the spectral simulations. The calculated
CD spectrum of (R)-10a is also displayed in Figure 4. When
compared with the measured CD spectra, it becomes evi-
dent that the enantiomer with the positive cotton effect at
highest wavelength is R-configured.
Figure 3. Molecular structure of 10b in the crystal. Displacement
ellipsoids are set at 30% probability, radii of hydrogen atoms are
chosen arbitrarily. Bond lengths (Å) and angles (°): B1–O1
1.503(4), B1–O2 1.437(4), B1–N1 1.583(4), B1–C4 1.607(4), O1–
B1–O2 113.7(2), O1–B1–N1 103.7(3), O1–B1–C4 109.8(2), O2–B1–
N1 101.1(2), O2–B1–C4 112.3(3), N1–B1–C4 115.9(2).
On a chiral HPLC column (Chiracel OD-H), the racemic
mixtures of the boron complexes 10a–c displayed a split
into two peaks of equal area of the two enantiomers. They
were separated, the enantiomeric purity of each of the sepa-
rated samples was checked by chiral HPLC, and their CD
spectra were measured. Figure 4 shows the CD spectrum of
the enantiomerically pure boron complex 10a. Similar CD
were obtained from the separated enantiomers of 10b and
10c, respectively. In all cases, the enantiomeric compounds
ent-10a–c displayed mirror image Cotton effects.
In order to evaluate the configurational stability at the
stereogenic boron center, the rate of racemization was
studied by heating the separated enantiomers of boron
complex 10a in n-decane at 65 °C. The time dependent de-
crease of the optical rotation at λ = 589 nm was used as a
probe of racemization. Thus, the barrier of racemization
was determined for 10a to be ∆G϶ = 109 kJmol–1 (k =
3.010–4s–1). The nitrophenyl-substituted boronate 10b
turned out to be slightly more, the tolyl-substituted com-
plex 10c less stable (∆G϶ = 115 kJmol–1 and 105 kJmol–1,
respectively). Obviously, the racemization increases slightly
Conclusions
In summary, boron has shown itself to be configuration-
with the electron-withdrawing character of the substituents, ally stable in boron complexes with chiral or achiral chelat-
because the stronger electron acceptor enhances the ing ligands 7 and 9. Enantiomerism has been proven in
strength of the boron–nitrogen bond. This effect is under- compounds 10 that form stable stereoisomers, and the first
lined by the B–N bond lengths in the complexes 10a, 10b enantiomeric complexes of boronates with imine ligands
and 10c (1.599, 1.583 and 1.611 Å, respectively). The race- have been resolved and isolated. For the first time, the abso-
Eur. J. Org. Chem. 2008, 5221–5225
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5223

M. Braun, S. Schlecht, M. Engelmann, W. Frank, S. Grimme
SHORT COMMUNICATION
2
2 2
and ∆F synthesis, minimization of Σw(Fo – Fc ) , 353 refined pa-
lute configuration at stereogenic boron could be assigned by
the evident accordance of measured and calculated circular
dichroism.
2
rameters, (∆/σ)max = 0.000, R1[Fo Ͼ 2σ(Fo2)] = 0.037, wR2
=
0.079 (all data), w = 1/[σ2(Fo2) + (0.02P)2 + 0.7P] where P =
2
2
(Fo + 2Fc )/3, S = 1.013, ∆ρmax/∆ρmin = +0.256 e/Å3 and
–0.129 e/Å3.
10a: Yellow solid, 268 mg (55%); Rf = 0.58 (chloroform/ethyl ace-
tate, 10:1); m.p. 179 °C. 1H NMR (500 MHz, CDCl3): δ = 4.6, 4.92
(d, J = 9.7 Hz, 2ϫ 1 H), 6.8 (m, 4 H), 7.2–7.34 (m, 10 H), 7.28
(d, Jo = 9.1 Hz, 1 H), 7.31 (m, 1 H), 7.42 (d, Jo = 8.0 Hz, 1 H),
7.58 (d, Jo = 8.0 Hz, 1 H), 7.72 (d, Jo = 8.0 Hz, 1 H), 7.95 (d, Jo
= 9.1 Hz, 1 H), 8.32 (s, 1 H) ppm. 13C NMR (125 MHz, CDCl3):
δ = 77.1, 77.4, 112.7, 120.5, 121.3, 124.6, 126.8, 127.7, 127.8, 128.3–
129.0, 129.0, 129.4, 131.8, 132.8, 138.5, 139.6, 140.5, 155.5, 162.5
ppm. 11B NMR (160 MHz, CDCl3): δ = 7.3 ppm.
Experimental Section
2
General Procedure for the Preparation of Boron Complexes 7 and
10: The corresponding imine, 5 or 9, (1.0 mmol), boronic acid 6
(1.5 mmol) and 1 g of molecular sieves (3 Å) were suspended in
100 mL of dry toluene and refluxed for 20 h. After filtration, the
solvent was removed in a rotary evaporatior and the residue was
purified by column chromatography (Fluka; silica gel 60, chloro-
form/ethyl acetate, 10:1) to deliver boronates 7a, 7b, and 10a–c as
yellow, solid compounds. For the preparation of boronate 7c, the
imine 5b was treated with (iPrO)2B(nBu) in an analogous way, how-
ever, in the absence of molecular sieves.
10b: Yellow solid, 209 mg (42%); Rf = 0.53 (chloroform/ethyl ace-
tate, 10:1); m.p. 240 °C. 1H NMR (500 MHz, CDCl3): δ = 4.6, 4.97
(d, 2J = 9.8 Hz, 2ϫ 1 H), 7.0–7.35 (m, 10 H), 7.29 (d, Jo = 9.1 Hz,
Selected Physical, Spectroscopic and Crystallographic Data
1 H), 7.32 (m, 2 H), 7.35 (m, 1 H), 7.46 (m, 1 H), 7.62 (d, Jo
=
8.1 Hz, 1 H), 7.65 (m, 2 H), 7.75 (d, Jo = 8.1 Hz, 1 H), 7.99 (d, Jo
= 9.1 Hz, 1 H), 8.39 (s, 1 H) ppm. 13C NMR (125 MHz, CDCl3):
δ = 77.1, 77.7, 112.5, 120.5, 121.0, 121.5, 124.9, 127.8, 127.8, 128.5–
129.0, 129.2, 129.5, 131.7, 138.1, 139.6, 140.0, 147.1, 155.6, 162.3
ppm. 11B NMR (160 MHz, CDCl3): δ = 6.9 ppm.
7a: Yellow solid, 238 mg (38%); Rf = 0.3 (chloroform/n-hexane,
1:1). H NMR (200 MHz, CDCl3): δ = 1.30 (s, 9 H), 1.37 (s, 9 H),
6.25 (s, 1 H), 6.27–6.31 (m, 2 H), 6.78–6.86 (m, 2 H), 6.92–6.98 (m,
2 H), 7.05–7.23 (m, 11 H), 7.36–7.40 (m, 3 H), 7.58 (s, 1 H), 7.65
(d, Jm = 2.5 Hz, 1 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 29.9,
31.7, 34.7, 35.9, 83.0, 87.4, 119.5–141.6, 159.2, 161.6 ppm.
1
Crystallographic data: C31H23BN2O4, Mr = 498.32, orthorhombic,
space group Iba2, a = 12.5991(9) Å, b = 18.5482(10) Å, c =
21.6143(12) Å, V = 5051.1(5) Å3, Z = 8, Dx = 1.311 g cm–3,
Crystallographic data: C41H41BClNO2, Mr = 626.01, monoclinic,
space group P21, a = 18.3345(9) Å, b = 10.8254(6) Å, c =
µ
=
0.087 mm–1,
T
=
291 K, crystal dimensions:
18.9903(10) Å, β = 110.593(6) °, V = 3528.3(3) Å3, Z = 4, Dx
=
0.4 mmϫ0.2 mmϫ0.2 mm, STOE-IPDS, Mo-Kα radiation (λ =
0.71073 Å), θmax = 25.00°,11612 measured, 2276 unique, and 1430
observed reflections with IϾ2σ(I), LP correction, direct methods
1.178 g cm–3, µ = 0.144 mm–1, T = 291 K, crystal dimensions:
0.3 mmϫ0.2 mmϫ0.15 mm, STOE-IPDS, Mo-Kα radiation (λ =
0.71073 Å), θmax = 26.07°, 39602 measured, 13815 unique, and
4471 observed reflections with IϾ2σ(I), LP correction, direct
2
2 2
and ∆F synthesis, minimization of Σw(Fo – Fc ) , 343 refined pa-
rameters, (∆/σ)max = 0.000, R1[Fo2 Ͼ 2σ(Fo2)] = 0.029, wR2 = 0.053
2
2 2
methods and ∆F synthesis, minimization of Σw(Fo – Fc ) , 841
(all data), w = 1/[σ2(Fo2) + (0.018P)2] where P = (Fo + 2Fc )/3, S
= 0.965, ∆ρmax/∆ρmin = +0.095 e/Å3 and –0.099 e/Å3.
2
2
2
refined parameters, (∆/σ)max = 0.001, R1[Fo Ͼ 2σ(Fo2)] = 0.042,
wR2 = 0.085 (all data), w = 1/σ2(Fo2), S = 0.828, ∆ρmax/∆ρmin
+0.166 e/Å3 and –0.141 e/Å3.
=
10c: Yellow solid, 294 mg (63%); Rf = 0.48 (chloroform/ethyl ace-
1
tate, 10:1); m.p. 192 °C. H NMR (500 MHz, CDCl3): δ = 2.06 (s,
7b: Yellow solid, 304 mg (54%); Rf = 0.61 (chloroform/ethyl ace-
3 H), 4.65, 4.89 (d, 2J = 9.5 Hz, 2ϫ 1 H), 6.7 (d, Jo = 7.8 Hz, 2
1
tate, 10:1); m.p. 214 °C. H NMR (500 MHz, CDCl3): δ = 6.2 (s, 1
H), 6.8 (d, Jo = 7.8 Hz, 2 H), 7.2–7.35 (m, 10 H), 7.27 (d, Jo
=
H), 6.87 (t, Jo = 7.7 Hz, 2 H), 6.9 (d, Jo = 8.1 Hz, 2 H), 7.1 (d, Jo
= 8.0 Hz, 2 H), 7.1–7.3 (m, 13 H), 7.3 (d, Jo = 9.0 Hz, 1 H), 7.32
(t, Jo = 7.6 Hz, 1 H), 7.39 (t, Jo = 7.6 Hz, 1 H), 7.5 (d, Jo = 8.1 Hz,
1 H), 7.74 (d, Jo = 8.0 Hz, 1 H), 7.98 (d, Jo = 9.1 Hz, 1 H), 8.0 (s,
1 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 82.8, 87.2, 113.5,
120.6, 121.4, 124.7, 126.7, 126.8–129.0, 127.7, 127.8, 129.1, 129.4,
131.8, 133.1, 135.2, 138.7, 139.0, 139.6, 156.2, 163.6 ppm. 11B
NMR (160 MHz, CDCl3): δ = 6.82 ppm.
9.1 Hz, 1 H), 7.27 (t, Jo = 8.0 Hz, 1 H), 7.39 (t, Jo = 8.0 Hz, 1 H),
7.56 (d, Jo = 8.2 Hz, 1 H), 7.68 (d, Jo = 8.0 Hz, 1 H), 7.91 (d, Jo
= 9.1 Hz, 1 H), 8.29 (s, 1 H) ppm. 13C NMR (125 MHz, CDCl3):
δ = 21.1, 77.1, 77.2, 112.9, 120.5, 121.5, 124.4, 127.5, 127.7, 127.7–
128.7, 128.8, 129.3, 131.9, 132.3, 136.3, 138.9, 139.2, 141.5, 147.1,
155.6, 162.6 ppm. 11B NMR (160 MHz, CDCl3): δ = 4.6 ppm.
CCDC-689995 (for 7a), -689999 (for 7c), -689997 (for 10a·n-
C6H12), -689996 (for 10b), and -689998 (for 10c·0.5Et2O) contain
the supplementary crystallographic data. These data can be
obtained free of charge from The Director, CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK (Fax: +44-1223-336033; E-mail:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
7c: Yellow solid, 469 mg (92%); Rf = 0.69 (chloroform/ethyl ace-
1
tate, 10:1); m.p. 197 °C. H NMR (500 MHz, CDCl3): δ = 0.7 (t,
3J = 7.1 Hz, 3 H), 1.2 (m, 6 H), 6.1 (s, 1 H), 7.0–7.3 (m, 15 H), 7.2
(d, Jo = 9.1 Hz, 1 H), 7.28 (t, Jo = 7.1 Hz, 1 H), 7.38 (t, Jo
=
7.7 Hz, 1 H), 7.5 (d, Jo = 8.2 Hz, 1 H), 7.7 (d, Jo = 7.9 Hz, 1 H),
7.86 (s, 1 H), 7.9 (d, Jo = 9.0 Hz, 1 H) ppm. 13C NMR (125 MHz,
CDCl3): δ = 14.2, 26,1, 27.4, 82.2, 88.1, 113.9, 120.6, 121.8, 124.2,
127–130, 127.5, 128.6, 129.2, 132, 136.1, 138.5, 139.1, 139.3, 155,
163.4 ppm. 11B NMR (160 MHz, CDCl3): δ = 8.8 ppm.
Acknowledgments
This work was supported by the Deutsche Forschungsgemeinschaft
(DFG). We are grateful to Sabine Houben for her assistance with
the preparation of several compounds.
Crystallographic data: C35H32BNO2, Mr = 509.43, orthorhombic,
space group P212121,
c = 18.275(4) Å, V = 2882.8(10) Å3, Z = 4, Dx = 1.174 g cm–3,
0.071 mm–1,
293 K, crystal dimensions:
a = 11.019(2) Å, b = 14.315(3) Å,
µ
=
T
=
[1] For reviews, see: M. R. Barbachyn, C. R. Johnson, in Asym-
metric Synthesis, vol. 4 (Eds.: J. D. Morrison, J. W. Scott), Aca-
demic Press, San Diego, 1984, chapter 2; D. Valentine Jr., in
Asymmetric Synthesis, vol. 4 (Eds.: J. D. Morrison, J. W. Scott),
0.5 mmϫ0.1 mmϫ0.05 mm, STOE-IPDS, Mo-Kα radiation (λ =
0.71073 Å), θmax = 25.00°, 37818 measured, 2860 unique, and 1970
observed reflections with IϾ2σ(I), LP correction, direct methods
5224
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Eur. J. Org. Chem. 2008, 5221–5225

Boron-Based Diastereomerism and Enantiomerism
Academic Press, San Diego 1984, chapter 3; F. A. Davis, R. H.
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C. A. Maryanoff, B. E. Maryanoff, in Asymmetric Synthesis,
vol. 4 (Eds.: J. D. Morrison, J. W. Scott), Academic Press, San
Diego, 1984, chapter 5.
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[11]
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1
[9] In a low-temperature H NMR spectrum (223 K) of 7a (H in-
Received: August 12, 2008
Published Online: September 17, 2008
stead of Cl), one of the geminal phenyl rings displayed a set of
Eur. J. Org. Chem. 2008, 5221–5225
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5225