Chiral boron-bridged bisoxazoline (borabox) ligands: Structures and reactivities of Pd and Cu complexes

Article abstract of DOI:10.1002/chem.200800822

Anionic boron-bridged bisoxazolines (borabox ligands) have been synthesized and characterized in their protonated forms. The ligands are tuneable over a wide range, allowing either alkyl or aryl substituents at the oxazoline rings and the central bridging boron atom. The structural parameters of this new ligand type have been investigated by X-ray analyses of palladium and copper complexes. Electronic properties have been studied by ¹³C NMR spectroscopy and by DFT calculations on palladium allyl complexes and compared to those of analogous bisoxazoline (box) complexes. Borabox complexes are more electronrich at the metal center than their neutral box congeners, and as a consequence of the longer bonds between the bridging atom and the oxazoline rings, their bite angles are larger. Palladium(II) complexes bearing an unsubstituted allyl ligand and homoleptic copper(II) complexes each possess an almost flat chelate ring. NMR analysis of a (1,3-diphenylallyl)(borabox) palladium complex showed a 92:8 mixture of (syn,syn) and (anti,syn) allyl isomers, in contrast with a previously reported box analogue that existed exclusively in the (syn,syn) form. Comparison of the corresponding crystal structures revealed that the distance between the bisoxazoline and the allyl ligand in the borabox complex is shorter. In the copper-catalyzed allylic oxidation of cyclohexene and cyclopentene with tert-butyl perbenzoate, borabox ligands gave results similar - and in some cases superior - to those obtained with analogous box ligands.

Full text of DOI:10.1002/chem.200800822

DOI: 10.1002/chem.200800822
Chiral Boron-Bridged Bisoxazoline (Borabox) Ligands:
Structures and Reactivities of Pd and Cu Complexes
Valentin Kçhler, ClØment Mazet, AurØlie Toussaint, Klaus Kulicke, Daniel Häussinger,
Markus Neuburger, Silvia Schaffner, Stefan Kaiser, and Andreas Pfaltz*[a]
Abstract: Anionic boron-bridged bisox-
azolines (borabox ligands) have been
synthesized and characterized in their
protonated forms. The ligands are
tuneable over a wide range, allowing
either alkyl or aryl substituents at the
oxazoline rings and the central bridging
boron atom. The structural parameters
of this new ligand type have been in-
vestigated by X-ray analyses of palladi-
um and copper complexes. Electronic
properties have been studied by
13C NMR spectroscopy and by DFT
calculations on palladium allyl com-
plexes and compared to those of analo-
gous bisoxazoline (box) complexes.
Borabox complexes are more electron-
rich at the metal center than their neu-
tral box congeners, and as a conse-
quence of the longer bonds between
the bridging atom and the oxazoline
rings, their bite angles are larger. Palla-
dium(II) complexes bearing an unsub-
stituted allyl ligand and homoleptic
copper(II) complexes each possess an
almost flat chelate ring. NMR analysis
of a (1,3-diphenylallyl)(borabox)palla-
dium complex showed a 92:8 mixture
of (syn,syn) and (anti,syn) allyl isomers,
in contrast with a previously reported
box analogue that existed exclusively
in the (syn,syn) form. Comparison of
the corresponding crystal structures re-
vealed that the distance between the
bisoxazoline and the allyl ligand in the
borabox complex is shorter. In the
copper-catalyzed allylic oxidation of cy-
clohexene and cyclopentene with tert-
butyl perbenzoate, borabox ligands
gave results similar—and in some cases
superior—to those obtained with anal-
ogous box ligands.
Keywords: asymmetric catalysis
·
copper · N ligands · oxazolines ·
palladium
Introduction
The structural motif of the semicorrin ligands 1 has inspired
the development of a variety of related neutral and anionic
N,N ligands such as 2–6.[1–3] C2-Symmetric bisoxazolines of
this type are particularly attractive because they are readily
assembled from commercially available enantiopure amino
alcohols. Among them, the neutral bisoxazoline (box) li-
gands 6 have emerged as the most versatile representatives
of this ligand class, as can be seen in the impressive range of
applications they have found in asymmetric catalysis.[3] As a
further variant of this structure we have recently introduced
anionic bisoxazolines 7 (borabox ligands) with a tetrasubsti-
tuted boron atom connecting the two oxazoline rings.[4] The
borabox ligands would be expected to be weaker electron
donors towards a coordinated metal center than other struc-
turally related anionic ligands such as 1–3, which each pos-
sess an electron-rich p-system with high electron density at
the coordinating N atoms, and in this respect to resemble
more closely the neutral box ligands 6.
[a] Dr. V. Kçhler, Dr. C. Mazet, Dr. A. Toussaint, Dr. K. Kulicke,
Dr. D. Häussinger, M. Neuburger, Dr. S. Schaffner, Dr. S. Kaiser,
Prof. Dr. A. Pfaltz
Department of Chemistry, University of Basel
St. Johannsring 19, 4056 Basel (Switzerland)
Fax : (+41)61-267-1103
E-mail: andreas.pfaltz@unibas.ch
The geometrical properties of borabox and box ligands 6
are very similar, as each possesses a tetrahedral bridging
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800822.
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Chem. Eur. J. 2008, 14, 8530 – 8539

FULL PAPER
atom, in contrast to ligands 1–5 with their rigid, planar p
systems. The anionic charges of borabox ligands could offer
advantages—by enforcing the binding of a metal cation due
to Coulombattraction, for example, or by increasing the sol-
ubilities of metal-borabox complexes in apolar solvents—
relative to complexes derived from neutral ligands, which
are more polar, due to the higher charge of the complex and
the additional external anion.[5] The absence of an additional
external anion that might interfere with catalysis through
coordination could also prove beneficial for certain reac-
tions. We therefore thought that borabox ligands would be a
useful addition to the bisoxazoline ligand family.
ethyl acetate and triethylamine as eluents. When necessary,
the lithium salts 7-Li could be quantitatively regenerated by
treatment of compounds 7-H with n-butyllithium (1 mol
equiv) in diethyl ether or THF at 08C.
Synthesis of palladium complexes: Protonated borabox li-
gands 7ax–az were converted into the corresponding lithium
salts by slow addition at 08C of nBuLi (1 equiv) in THF
(Scheme 2). Subsequent treatment with a solution of [Pd-
A
the desired palladium complexes 10ax–az, which were iso-
lated after removal of lithium chloride by filtration. In the
cases of 10ay and 10az, recrystallization from EtOH/H2O
gave colorless crystals suitable for X-ray analysis. Similarly,
complex 11cx was obtained by treatment of [Pd(1,3-diphe-
nylallyl)Cl]2 (0.5 equiv) with 7cx-H (1 equiv) and K2CO3 in
a CH2Cl2/THF/methanol mixture[7] at 508C. Crystallization
from EtOH/H2O gave single crystals suitable for X-ray anal-
ysis. Crystalline borabox palladium complexes can be han-
dled under air, although they were usually stored under N2
either at room temperature (11cx) or at À208C (10ax–az).
The analogous box palladium complexes 12a and 12b were
prepared by published procedures.[7]
Results and Discussion
Synthesis of the ligands: Borabox ligands were assembled in
a one-step procedure through coupling of 2H-oxazolines
8a–f with dialkyl- or diarylhaloboranes 9w–z (Scheme 1).[4a]
A library of 15 different ligands was prepared by this ap-
proach. Lithiation of oxazolines 8a–f[6] and subsequent
treatment with the appropriate haloborane (0.5 equiv) at
low temperature in THF led to the lithium salts 7-Li. In gen-
eral, these compounds could be isolated in high purity ac-
cording to NMR spectroscopy (1H, 13C, 11B), as highly hygro-
scopic, white powders in moderate to excellent yields. In
general, for practical reasons, the lithium salts were convert-
ed into the air-stable protonated ligands 7-H by simple chro-
matographic workup on silica gel with mixtures of hexane,
NMR studies and DFT calculations of Pd–borabox com-
plexes: Palladium box and aza-semicorrin complexes cata-
lyze the allylic alkylation of rac-(E)-1,3-diphenylpropenyl
acetate with dimethyl malonate with high efficiency
(2 mol% catalyst precursor, 238C, 48 h, 94–99% yield) and
high enantiomeric excesses (88–
97%).[7] In contrast, borabox
complex 10az does not induce
any reaction between rac-(E)-
1,3-diphenylprop-2-enyl 1-ace-
tate and dimethyl malonate.
Furthermore, complex 11cx,
which is structurally very simi-
lar to the box complex 13, did
not show any decomposition,
nor was any product formation
observed when it was treated
with stoichiometric amounts of
dimethyl
[Bu4N]OAc.
malonate/BSA/
AHCTREUNG
Rapid apparent allyl rotation
was observed by 1D 1H NMR
spectroscopy after the prepara-
tion of box complex 12a b y a
published procedure[7] that in-
volves washing of the filtered
reaction mixture with NaCl so-
lution. Since the promoting ef-
fects of halides on apparent
allyl rotation of Pd complexes
are known,[8] we investigated
the influence of chloride ions
on box complex 12b, prepared
Scheme 1. Synthesis of borabox ligands in their lithiated (7-Li; 34–98%) or protonated (7-H; 5–89%) forms.
a) tBuLi, À788C (À1008C for 7ex, 7ey), THF; b) (R1)2BX (X=Cl, Br), toluene, À788C (À1008C for 7ex,
7ey); c) hexane/ethyl acetate/Et3N, silica gel; d) nBuLi, Et2O or THF, 08C.
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A. Pfaltz et al.
with omission of the NaCl
washing step, and its borabox
analogue 10ay in a NMR titra-
tion experiment. In the absence
of chloride the spectrum shows
the signals of a C1-symmetric
complex, with separate signals
for the four methyl groups.
Upon addition of [Bu4N]Cl
(8 mol%), the signal sets of the
two oxazoline rings and of each
pair of methyl groups in com-
plex 12b merged, whereas the
five protons of the allyl frag-
Scheme 2. Synthesis of box and borabox palladium complexes. a) nBuLi, THF, 08C; b) [Pd
(C13H15)Cl]2, CH2Cl2/THF/methanol, 508C; d) [Pd(C3H5)Cl]2, CH2Cl2, RT; e) AgPF6, THF,
RT. Complexes 13, 14: ref. [7].
G
ment remained distinguishable RT; c) K2CO3, [Pd
G
ACHTREUNG
(Figure 1). The observed dy-
namic effect was clearly caused
by the presence of chloride
ions, since addition of excess
AgPF6 reversed the effect. In
contrast, similar apparent rota-
tion of the allyl fragment could
not be detected when a solution
of the analogous borabox com-
plex 10ay was treated with
[Bu4N]Cl, even when one
equivalent of chloride was
added.
To investigate whether this
contrasting behavior of com-
plexes 12b and 10ay was due to
stronger binding of the anionic
borabox ligand to the cationic
palladium center, a ligand-ex-
change reaction with dppp [1,3-
bis(diphenylphosphino)pro-
pane] was performed. In both
complexes the N,N ligands were
displaced upon addition of di-
phosphine (1 molequiv), lead-
ing in the case of complex 10ay
Figure 1. 1H NMR spectra (500 MHz, CD2Cl2, 295 K) showing a titration experiment of complex 12b (16 mm)
with a solution of [Bu4N]Cl (10 mm in CD2Cl2). Spectrum at top: addition of [Bu4N]Cl (8 mol%) and AgPF6
to a dppp palladium-allyl com- (1.7 equiv).
plex with the borabox ligand
7ay as counter-ion. The most
likely explanation for the different behavior of the two com-
plexes upon addition of chloride is the lower electrophilicity
of the palladium atom in 10ay, which therefore has a
weaker tendency to coordinate a chloride ion.
Further information about the electronic properties of the
borabox-palladium complexes, as well as their differences
from analogous box complexes, was obtained from
13C NMR spectra and density functional theory (DFT) cal-
culations. The 13C chemical shifts of the carbon atoms of the
allyl fragment are known to reflect the electronic properties
of the complex.[7,9] Comparison of the 13C NMR spectra of
complexes 12a, 12b and 10ax–az revealed that allyl C
atoms of the borabox complexes resonate at higher field,
consistent with more electron-rich allyl moieties.
This conclusion was corroborated by DFT calculations.
Natural population analyses (NPA) at the B3LYP level of
theory were carried out for all five palladium complexes.[10]
A correlation between the net atomic charges of the allyl C
atoms and the 13C NMR chemical shifts is given in Figure 2.
The allyl systems in the box-complexes 12a and 12b were
found to be less electron-rich than those in their borabox
congeners 10ax–az. The relative charges on the three
carbon atoms were clearly related to the natures of the sub-
stituents at boron. Electron-withdrawing 3,5-(CF3)2C6H3-
groups resulted in lower electron densities at the allyl frag-
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Chiral Boron-Bridged Bisoxazoline Ligands
FULL PAPER
and 7.7 Hz were observed for
the (anti, syn) isomer. The
latter value is clearly indicative
of a cis relationship of the cen-
tral proton of the allyl system
with one of the protons at the
allyl termini.
In contrast, the analogous
box complex 13 exists exclu-
sively in the (syn,syn) form.[7]
On the other hand, partial syn–
anti isomerization of a 1,3-di-
phenylallyl ligand in palladium–
box complexes bearing phenyl
substituents at C(4) and C(4’)
of the oxazoline rings and addi-
tional substituents at C(5) and
C(5’) has been described by
Pericàs et al.[11]
Assignment of the NMR signals
for complexes 10ax, 10ay, 12a
and 12b: In order to allow com-
parison of the 13C shifts of the
terminal carbon atoms of the
allylic fragments of complexes
10ax, 10ay, 12a and 12b, the
relative positions of the allyl
termini with regard to the oxa-
zoline ligands were determined
Figure 2. Correlation between 13C NMR chemical shifts (500 MHz) and charge densities (DFT, B3LYP, NPA)
of the C atoms of the allyl fragments for box and borabox palladium complexes.
by
2D-NOE
spectroscopy
(Figure 3).
À
ment (charge=À0.543, À0.249 and À0.555 e at C(9), C(10)
and C(11), respectively) than ethyl groups did (charge=
À0.552, À0.251 and À0.567 e at C(9), C(10) and C(11), re-
spectively).
The C(6) H bond preferen-
tially points toward the allyl
ligand and is oriented parallel
À
to the C(4) C(5) bond, as
In addition, complex 11cx exhibited unexpected behavior
in solution. When single crystals of the pure (syn, syn)
isomer were dissolved in CDCl3, partial isomerization of the
1,3-diphenylallyl ligand to form an (anti, syn) isomer was
observed by NMR spectroscopy (Scheme 3). The isomers of
shown by the sizes of 3 Hz of
the coupling constants between
HC(4) and HC(6) in complexes
10ax, 10ay, 10az, 12a and 12b,
the strong NOEs between
HC(4) and HC(6), the strong
differences in intensities of the
NOEs of the spectroscopically
distinguishable methyl groups
Figure 3. Relevant NOEs for
the determination of the rela-
tive positions of the allyl ter-
mini in complexes 10ax, 10ay,
12a and 12b. X=C, B.
3
11cx were distinguished by the different JH,H coupling con-
3
stants in the allyl system. The two JH,H coupling constants
of the two terminal allyl protons are 11.4 Hz and 10.7 Hz in
the (syn, syn) isomer, whereas coupling constants of 11.5 Hz
and HC(4), and the weak—or
complete absence of—NOE sig-
nals between HC(6) and the
protons at C(5). The same ori-
entation is also found for the
À
C(6’) H bonds. Although the
analyses were hampered by
overlapping oxazoline signals
for some of the complexes, and
additionally, in the case of com-
plex 12b, by dynamic processes
Scheme 3. syn–anti Isomerization of complex 11cx.
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A. Pfaltz et al.
(see below), overall the data confirmed the conformation of
the iPr groups depicted in Figure 3 for all complexes.
The NOEs between HC(4’) and HantiC(11) and between
HC(6) and HantiC(9) allowed unambiguous assignment of
the relative positions of C(9) and C(11). Complex 10ax
showed additional signals between HC(6’) and HsynC(11)
and between H3C(7’) and HsynC(11) (Figure 3). Dynamic be-
havior was found for bisoxazoline complex 12b,[12] in which
exchange peaks were observed between HC(6) and HC(6’)
and between H3C(8) and H3C(8’). A combination of ex-
change peaks and NOE signals between HC(6’) and
HantiC(9) and between HC(6) and HsynC(11) indicated rapid
apparent allyl rotation on the timescale of the NOE experi-
ment. Nevertheless, assignment of the two sets of 13C and
1H NMR signals for the spectroscopically distinguishable ox-
azoline rings was also possible in this case by consideration
of all observed NOE signals between the bisoxazoline
ligand and the allyl fragment.
the refinement for complex 10ay. Distances between the Pd
À
center and the allyl termini are very similar: 2.12 Š for Pd
À
C(9) and 2.11 Š for Pd C(11). Likewise, the angles between
the carbon atoms of the allyl termini and the neighbouring
nitrogen atoms at the palladium center differ only marginal-
ly (101.28 for N-Pd-C(9) and 99.78 for N’-Pd-C(11)). The
À
Pd N bonds have nearly the same lengths (10az=
2.0846(18)
and
2.070(2) Š;
10ay=2.0753(16)
and
2.0746(17) Š). Both complexes form flat chelate rings
(Figure 4).
À
Borabox complex 11cx has shorter Pd N bonds than box
analogue 13, and the benzylic C atoms of the ligand are no-
ticeably closer to the ipso carbon atoms of the 1,3-dipheny-
lallyl fragment in the former (closest distance: 3.32 Š for
11cx, 3.67 Š for 13). The resulting larger strain in the bora-
box complex, which destabilizes the (syn, syn) arrangement,
provides an explanation for the observed syn–anti isomeriza-
tion in complex 11cx in solution.
The relative chemical shifts of the 13C signals of the allyl
termini are in agreement with the results of a previous study
of the analogous box complex 14 (see Scheme 2): the allyl
terminus of which the syn proton points toward the substitu-
ent of the oxazoline ring resonates at lower field.[7]
Although the deviation from the ideal planar coordination
geometry is small in both complexes, the allyl fragment of
the borabox complex 11cx is bent in the opposite direction
relative to the ligand, compared to the box complex 13. As
a consequence, the dihedral angles between the plane
formed by the N atoms of the ligand and the Pd atom and
the plane formed by the allyl termini and the Pd atom are
of opposite sign in the two complexes (+48, À48).[15]
Crystal structures of complexes 10ay, 10az and 11cx:[13] The
bonds between the bridging atom and the oxazoline rings
are approximately 0.1 Š longer in the borabox complexes
10ay and 10az than in the reported analogous box complex
14[14] (Figure 4, Table 1), in accordance with the bond
lengths measured in analogous borabox copper complexe-
s.[4a] The bite angles of the ligand are very close to 908, as
expected for square planar metal complexes (90.88 for 10ay
and 10az). The Coxa-B-Coxa angle is close to the ideal tetra-
hedral angle in both structures (110.78 for 10ay and 111.28
for 10az).
Synthesis and x-ray analysis of copper complexes 15ax–az
and 15cx: Copper(II) complexes of borabox ligands were
prepared either by treating the lithium salts (7-Li) of the li-
gands (7ax, 7ay, 7cx) with CuSO4·H2O (1.0 equiv) in a bi-
phasic mixture of water and CH2Cl2, or by treating the pro-
tonated ligand 7az-H with Cu(OAc)2 (1.0 equiv) in
G
MeOH.[16] Crystallization was achieved by layering concen-
trated solutions of the complexes in CH2Cl2 or Et2O with
hexane or pentane. Complex 15ax crystallized upon slow
evaporation of a CH2Cl2 solution (Scheme 4).
Single homoleptic copper
Although the allyl fragments were disordered in both
complexes, analysis of the residual electron density allowed
complexes were found in the
asymmetric units for 15ax and
Table 1. Selected interatomic distances [Š] and angles [8] in palladium complexes.
10ay[a]
10az[a,b]
14[c]
11cx[d]
13[c]
15cx, whereas two complexes,
with slightly different geome-
tries, occupied the asymmetric
unit in the case of 15az. The
structure of complex 15ay has
an additional C2 axis that
allows superposition of the two
ligand fragments as depicted in
Figure 5. For conciseness, aver-
aged angles and bond lengths
are used in the following discus-
sion.
All complexes adopt a dis-
torted tetrahedral geometry,
and the geometric parameters
of the four complexes are very
similar, the Coxa-B-Coxa angles
[e]
À
C(2) X
1.614(3)
1.612(3)
2.124(2)
2.109(2)
1.619(3)
1.618(4)
–
–
1.499
1.501
1.644(3)
1.635(3)
1.504
1.510
[e]
À
C(2’) X
À
Pd C(9)
Pd C(10)
Pd C(11)
2.110(4)
2.108
2.117(4)
2.075(3)
2.099(3)
1.266
2.1360(18)
2.1119(17)
2.1621(17)
2.0837(15)
2.0966(16)
1.289(2)
1.293(3)
86.22(6)
102.60(15)
99.19(7)
106.17(7)
68.55(7)
2.118(3)
2.100
2.169(3)
2.105(3)
2.130(3)
1.272
À
À
2.114(2)
–
À
N Pd
N’ Pd
C(2)=N
2.0753(16)
2.0746(17)
1.287(4)
2.070(2)
2.0846(18)
1.290(3)
1.286(3)
90.76(7)
111.19(18)
–
À
C(2’)=N’
N’-Pd-N
1.282(3)
1.267
1.268
90.85(11)
110.71(17)
101.20(12)
99.71(12)
68.17(14)
87.6(1)
113.48
101.3(1)
102.8(1)
68.2(2)
84.5(1)
105.98
99.3(1)
108.0(1)
68.3(1)
C(2’)-X-C(2)[e]
N-Pd-C(9)
N’-Pd-C(11)
C(11)-Pd-C(9)
–
–
[a] Data were collected at 173 K. [b] The allyl ligand was disordered and its position relative to the oxazoline
rings could not be determined. Distinctions made between the left and right half of the molecule are therefore
arbitrary. [c] Interatomic distances and angles for these compounds were taken from reference [7] or calculated
with ORTEP-3 (Version 1.08) from the fractional coordinates deposited in the CCDC. Data for these com-
pounds were collected at 250 K. [d] Data for this compound were collected at 123 K. [e] X=B, C.
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Chiral Boron-Bridged Bisoxazoline Ligands
FULL PAPER
vealed that borabox ligand 7ay
with the same substitution pat-
tern gives higher enantioselec-
tivity in the oxidation of cyclo-
hexene. In contrast with box li-
gands,[18] tert-butyl substituents
at the stereogenic centers were
not suitable for this reaction
when borabox ligands were
used.
The observed enantioselectiv-
ities compare well with the best
results obtained with box li-
gands.[19] Cyclohexene reacted
with 79% ee at room tempera-
ture (Table 2) and cyclopentene
with
86% ee
at
À158C
(Table 3) when ligand 7cx
(R1 =Et, R2 =Bn) was em-
ployed. Ligand 7dx, bearing
two additional phenyl substitu-
ents at the 5-positions of the
oxazoline rings, allowed the ox-
idation of cyclopentene in
86% ee even at room tempera-
ture (Table 2).[20,21]
Figure 4. a) Crystal structures of complexes 10ay and 10az (allyl fragment and one CF3 group are disordered);
side view of the chelate rings. b) Comparison between complexes 11cx and 13; side view of the chelate rings
and superposition of relevant fragments (middle). Data for complex 13 are taken from ref. [7].
The reaction of cyclopentene
in the presence of the catalyst derived from borabox ligand
7cx showed only a weak temperature dependence of the ee,
in contrast with the oxidation of cyclohexene (Table 3 and
Figure 6). Thus, at 808C cyclopentene could be fully con-
verted into the benzoate within 1 hour and with a respecta-
ble ee of 76%. The ee values obtained in the oxidation of
cyclopentene are among the best results reported to date for
this reaction.[18,19,22] As with other ligands, relatively high
catalyst loadings and long reaction times are required. In
this respect, further studies of borabox ligands might be re-
warding, in view of the fast rates combined with relatively
high enantioselectivities observed with ligand 7cx at elevat-
ed temperatures (Figure 6).
Scheme 4. Synthesis of the homoleptic borabox copper complexes 72Cu.
a) Cu(SO4), H2O/CH2Cl2 for 15ax, 15ay and 15cx; Cu(OAc)2, MeOH for
15az.
A
ACHTREUNG
being close to the ideal tetrahedral angle, with values be-
tween 108.88 (15ax) and 110.48 (15az). Bond lengths be-
tween boron and the quaternary carbon atoms of the oxazo-
line rings vary from 1.61 Š (15ay, 15cx) to 1.62 Š (15ax,
15az), whereas C=N bond lengths are in the range from
1.28 Š (15ax) to 1.30 Š (15cx). These values are in good
agreement with the corresponding interatomic distances and
angles in the Pd complexes 10ay and 10az discussed
above.[17]
Conclusion
Borabox ligands are readily prepared in a modular fashion
from chiral oxazolines and diaryl- or dialkylhaloboranes.
Variation of the substituents in the oxazoline rings and at
boron allows steric and electronic fine-tuning of the corre-
sponding metal complexes. Borabox ligands are stronger
electron donors than their neutral box counterparts, as
shown by NMR data and DFT calculations. As a conse-
quence of the longer bonds between the bridging atom and
the adjacent oxazoline rings, the Coxa-B-Coxa angles in metal
complexes are smaller, whereas the bite angles are wider
than the corresponding angles in box complexes. With the
exception of sterically overcrowded derivatives (11cx), bora-
box complexes form almost flat chelate rings.
Allylic oxidation of cycloalkenes: Borabox ligands were
evaluated in Cu-catalyzed asymmetric allylic oxidations of
cyclohexene and cyclopentene (Table 2). Initial investiga-
tions showed that enantioselectivities were significantly
higher when the protonated borabox ligands 7-H and
K2CO3 were used for catalyst preparation instead of the lith-
ium salts 7-Li. A direct comparison with box ligand 6b re-
Chem. Eur. J. 2008, 14, 8530 – 8539
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8535

A. Pfaltz et al.
allyl fragments. Borabox li-
gands, on the other hand, are
well suited for copper-catalyzed
allylic oxidation reactions. The
results described here, together
with previous work on enantio-
selective
cyclopropanation[4a]
and acylation of 1,2-diols and
pyridyl alcohols,[4a,b] demon-
strate that borabox ligands are
a useful addition to the previ-
ously developed bisoxazolines,
with different electronic and
geometric properties. Further
applications of borabox ligands
in asymmetric catalysis are cur-
rently under investigation.
Experimental Section
Figure 5. Crystal structures of four different homoleptic borabox copper complexes. CF3 groups in 15az were
disordered.
General: All reactions were carried
out under argon or nitrogen unless
otherwise noted. Solvents were puri-
fied by standard procedures or were
used as purchased from FLUKA
(puriss., absolute, over molecular
sieves). NMR spectra were recorded
on 400 and 500 Bruker Avance spec-
trometers at 295 K, if not otherwise
noted. NOE spectra for 10ax–z and
11cx were acquired at 295 K with use
of a mixing time of 350 ms and 256
points in the F1 dimension and 2048
points in the F2 dimension with corre-
sponding acquisition times of 26 and
205 ms. In the case of 12a, two NOE
spectra were acquired at 275 K and
295 K, with use of a mixing time of 1 s
and 128 and 2048 points with acquisi-
tion times of 13 ms and 205 ms in the
F1 and F2 dimensions, respectively. In-
frared spectra were obtained on a Shi-
Table 2. Allylic oxidation of cyclopentene and cyclohexene in the presence of different Cu catalysts.[a]
L*
R1
R2
R3
n
t [d]
Yield [%][b]
ee [%][c]
7ay-Li
7aw-Li
7bw-Li
7az-Li
7ay-Li
7ay-H
7ax-H
7cy-H
7cx-H
7cz-H
7dx-H
7ey-H
7 fy-H
7ay-H
7bx-H
7cx-H
7dx-H
7ey-H
6b
Ph
Cy
Cy
iPr
iPr
tBu
iPr
iPr
iPr
iPr
Bn
Bn
Bn
Bn
Ph
CH2iPr
iPr
tBu
Bn
Bn
Ph
H
H
H
H
H
H
H
H
H
H
Ph
H
H
H
H
H
Ph
H
H
2
2
2
2
1
2
2
2
2
2
2
2
2
1
1
1
1
1
2
19
18
19
24
34
8
2–3
5
10
14
5
2
4
2
1.5
2
3
2
59
56
63
78
53
60
31
rac.
71
71
74
64
78
79
70
78
65
72
75
8
3,5-(CF3)2C6H3
Ph
Ph
Et
Ph
Et
53
71[d]
69[d]
69[d]
76
3,5-(CF3)2C6H3
madzu
FTIR-8400S
spectrometer
Et
Ph
Ph
Ph
Et
Et
Et
Ph
Ph
20[d]
80
fitted with
a
Specac Golden Gate
Mk II ATR unit, with use of neat sam-
ples. Optical rotations were measured
77
70
65
79
87
75
75
on
a Perkin–Elmer 341 polarimeter
fitted with a Na lamp. Elemental anal-
yses were obtained from the Micro-
Analytical Laboratory of the Depart-
ment of Chemistry of the University
of Basel. Ph2BCl,[23a] Et2BBr,[23] bis-
82
86
78
48
iPr
19
A
[a] Reactions were performed on a 0.5 mmol scale with [Cu
K2CO3 (M=H, 15 mol%). [b] After column chromatography. [c] ee and absolute configuration determined by
HPLC by literature procedures; see Experimental Section. [d] Reaction performed on a 0.25 mmol scale.
G
R
chloride,[5d,23c,24] oxazolines 8a–c, 8e
and 8 f,[25] box ligands 6a and 6b (R1 =
Ph, R2 =iPr[4a,23c] and R1 =Me, R2 =
iPr[26]) and [Pd(1,3-diphenylallyl)Cl]2
were prepared by literature proce-
dures. All other reagents were avail-
able from commercial suppliers and were used without further purifica-
tion. For conciseness the abbreviation “oxa” has been used for “oxazo-
line”.
In contrast with Pd–box complexes, Pd–borabox com-
plexes do not catalyze allylic substitution reactions, which
can be explained by the higher electron densities in their
8536
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 8530 – 8539

Chiral Boron-Bridged Bisoxazoline Ligands
FULL PAPER
Table 3. Temperature effect on the allylic oxidation in the presence of
purifiable by chromatography. If necessary, the lithium salts can be readi-
ly prepared from the protonated derivatives (see below).
catalyst Cu
(7cx).[a]
A
Compound 7aw-H: Yield: 70%; Rf =0.30 (hexane/ethyl acetate/Et3N
10:1:0.5); [a]2D0 =À63 (c=0.11 in CH2Cl2); 1H NMR (500 MHz, CDCl3):
d=13.09 (brs, 1H; NHN), 4.37 (m, 2H; oxa), 4.00 (m, 2H; oxa), 3.75 (m,
2H; oxa), 1.61–1.48 (m, 6H; Cy), 1.43 (brd, 3JH,H =9.5 Hz, 4H; Cy),
1.21–0.28 (m, 12H; Cy), 1.15 (m, 2H; CH
6H; CH3), 0.92 ppm (d, 3JH,H =7.0 Hz, 6H; CH3); 13C NMR (126 MHz,
CDCl3): d=197.5 (N=C), 71.3 (oxa), 67.1 (oxa), 32.9 (CH(CH3)2), 31.5
G
n
T [8C]
t [h]
Yield [%][b]
ee [%][c]
(Cy), 31.3 (Cy), 29.1 (Cy), 27.9 (Cy), 19.1 (CH3), 18.9 ppm (CH3);
11B NMR (161 MHz, CDCl3): d=À12.9 ppm (s); IR: n˜ =2962, 2908, 2839,
1573, 1465, 1442, 1411, 1311, 1203, 1018, 956, 933, 864 cmÀ1; MS (FAB):
m/z (%): 403 [M+H]+ (100), 208 [MÀoxaÀCy+H]+ (18); elemental
analysis calcd (%) for C24H43BN2O2 (402.43): C 71.63, H 10.77, N 6.96;
found: C 71.53, H 10.75, N 6.90.
1
1
1
1
1
1
2
2
2
À15
5
360
141
48
8
2
1
219
86
11
30
80
79
78
70
69
69
72
61
86
82
82
81
78
76
79
68
65
25
40
60
80
25
60
80
Conversion of the protonated ligands into their lithium salts: nBuLi
(0.14 mL, 0.23 mmol) was added at 08C to a solution of 7ay-H (84 mg,
0.22 mmol) in THF (10 mL). After 2 h of additional stirring at room tem-
perature, the volatiles were removed under reduced pressure and 7ay-Li
[a] Reactions were performed on a 0.5 mmol scale in the presence of [Cu-
(CH3CN)4][PF6] (5 mol%), (7cx)-H (7.5 mol%) and K2CO3 (15 mol%).
was isolated as a white solid (82.0 mg, 0.207 mmol, 96%). [a]2D0 =À49 (c=
G
ACHTREUNG
3
0.11 in CH2Cl2); 1H NMR (500 MHz, (CD3)2CO): d=7.31 (brd, JH,H
=
[b] After column chromatography. [c] ee and absolute configuration de-
6.9 Hz, 4H; arom CH ortho), 6.98 (m, 4H; arom CH meta), 6.90 (m, 2H;
arom CH para), 3.86–3.73 (m, 4H; oxa), 3.64 (“t”, JH,H =7–8 Hz, 2H;
termined by HPLC by literature procedures; see Experimental Section.
oxa), 1.79 (m, 2H; CH
(CH3)2), 0.85 (d, 3JH,H =6.9 Hz, 6H; CH3),
G
0.73 ppm (d, 3JH,H =6.9 Hz, 6H; CH3); 13C NMR (126 MHz, (CD3)2CO):
arom C ipso to B: not detected, d=209.4 (N=C), 134.5 (arom CH), 125.7
(arom CH ortho), 123.1 (arom CH para), 71.1 (oxa), 65.4 (oxa), 31.6
(CH
(CD3)2CO): d=À12.7 ppm (s); IR: n˜ =2962, 2877, 2360, 1635, 1589, 1465,
1388, 1265, 1157, 1103, 1033, 964, 864, 732 cmÀ1
A
.
For analytical data for ligands 7aw-Li, 7bw-Li, 7bx-Li, 7by-Li, 7bz-Li,
see Supporting Information of reference [4a].
General procedure for the synthesis of palladium complexes 10ax–az: A
solution of 7ay-H (54 mg, 0.14 mmol) in THF (2 mL) was cooled to 08C,
and nBuLi (1.6m in hexane, 95 mL, 0.15 mmol) was slowly added with
stirring. After 30 min the reaction mixture was allowed to warm up to
room temperature, and a solution of [Pd(C3H5)Cl]2 (25.3 mg, 69.2 mmol)
U
in THF (1 mL) was slowly added. After a further 20 minutes, the vola-
tiles were removed in vacuo, benzene (5 mL) was added, and the turbid
solution was filtered. After removal of volatiles, the solid was redissolved
in hot EtOH, and the minimum possible amount of water was added.
Upon cooling to room temperature and standing overnight the product
recrystallized as colorless needles,
Figure 6. Temperature dependence of ee in the copper-catalyzed allylic
oxidation of cyclopentene (conditions, see Table 2).
which were suitable for X-ray analysis.
Crystallization was repeated twice
from the mother liquors (65 mg,
88%).
Synthesis of protonated borabox ligands 7-H: tBuLi (1.7m in hexane,
1.78 mL, 3.03 mmol) was added at À788C over a 10 min period to a solu-
tion of oxazoline (2.75 mmol) in THF (100 mL), resulting in a pale
yellow solution. After the system had been stirred for 30 min at this tem-
perature, a solution of R2BCl (1.38 mmol) in toluene (5 mL) was added
by cannula, and the cooling bath was removed immediately after addi-
tion. After 4 to 12 h, the reaction was complete, and volatile compounds
were removed under vacuum. The remaining foamy residue was redis-
solved in benzene and filtered to remove LiCl, and the solvent was
evaporated again. The crude product was washed with pentane (3”
20 mL) and dried. The lithium salt 7-Li was then dissolved in the mini-
mum possible amount of the eluent mixture (typically: hexane/ethyl ace-
tate/Et3N 10:1:0.5 unless otherwise mentioned) and directly transferred
to a silica gel column (h=10–12 cm; 1=2 cm). After elution and evapo-
ration of the solvents, the product was isolated in its protonated form.
M.p. 1648C; [a]2D0 =À24 (c=0.42 in
CH2Cl2);
1H NMR
(500 MHz,
CD2Cl2): d=7.25 (d, 3JH,H =6.9 Hz,
2H; arom CH), 7.16–7.01 (m, 8H;
arom CH), 5.54 (“ddt”, 3JH,H =12.4,
12.0, 6.9 Hz, 1H; HC(10)), 4.07 (dd,
2JH,H =9.0 Hz,
3JH,H =4.3 Hz,
1H;
HC(5) trans to HC(4)), 4.05 (m, 2H;
H2C(5’)), 3.99 (dd, 3JH,H =9.6 Hz, 2J=9.0 Hz, 1H; HC(5) cis to HC(4)),
3.94 (td, 3JH,H =6.9, 3.1 Hz, 1H; HC(4’)), 3.83 (ddd, 3JH,H =9.6, 4.3,
3.1 Hz, 1H; HC(4)), 3.74 (“dd”, 3JH,H =6.9 Hz, 4JH,H =2.2 Hz, 1H;
HC(11) syn), 3.57 (“dd”, 3JH,H =6.9 Hz, 4JH,H =2.2 Hz, 1H; HC(9) syn),
2.93 (m, 3JH,H =12.4 Hz, 1H; HC(11) anti), 2.85 (m, 3JH,H =12.0 Hz, 1H;
HC(9) anti), 2.01 (qqd, 3JH,H =7.1, 6.9, 3.1 Hz, 1H; HC(6)), 1.89 (qqd,
3JH,H =7.1, 6.9, 3.1 Hz, 1H; HC(6’)), 0.86 (d, 3JH,H =7.1 Hz, 3H; H3C(7)),
Lithium salts 7ax-Li and 7bx-Li were extracted from the crude product
with dichloromethane instead of benzene. Lithium salts of entirely alkyl-
substituted ligands (7aw-Li, 7ax-Li, 7bw-Li, 7bx-Li) are soluble in alka-
nes, and the amount of pentane used to wash the crude products should
be reduced in these cases to avoid loss of product.
3
3
0.85 (d, JH,H =7.1 Hz, 3H; H3C(7’)), 0.49 (d, JH,H =6.9 Hz, 3H; H3C(8)),
0.44 ppm (d, 3JH,H =6.9 Hz, 3H; H3C(8’)); 13C NMR (126 MHz, CD2Cl2):
d=193.2 (br, 2C; C(2), C(2’)), 152.2 (br, 2C; arom C ipso), 134.6, 127.2,
127.1, 125.1, 125.0 (10C; arom CH), 115.1 (C(10)), 74.5 (2C; C(4),
C(4’)), 67.1, 67.0 (C(5), C(5’)), 59.9 (C(11)), 56.7 (C(9)), 31.3 (2C; C(6),
C(6’)), 19.6, 19.5 (C(7), C(7’)), 14.3 (H3C(8’)), 14.2 ppm (H3C(8));
Note: Although the lithium salts 7-Li can be isolated as highly hygroscop-
ic white solids, it was generally preferable to convert them directly into
the protonated ligands, which are less hygroscopic, easier to handle and
Chem. Eur. J. 2008, 14, 8530 – 8539
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8537

A. Pfaltz et al.
11B NMR (161 MHz, CD2Cl2): d=À12.8 ppm (s); IR: n˜ =3070, 3058,
2JH,H =8.8 Hz, 3JH,H =8.8 Hz, 1H; oxa CHH (B)), 3.70 (dd, 2JH,H =8.5 Hz,
3JH,H =3.3 Hz, 1H; oxa CHH (C)), 3.52 (overlaid signal, JH,H =8–9 Hz,
1H; oxa CHH (C)), 3.13 (dddd, 3JH,H =9.8, 8.8, 4.2, 3.3 Hz, 1H; oxa CH
(C)), 3.07 (dd, 2JH,H =13.6 Hz, 3JH,H =5.5 Hz, 1H; Ph(A)CHH), 2.70
(overlaid signal, 1H; Ph(D)CHH), 2.68 (overlaid signal, 1H;
Ph(A)CHH), 2.24 (dd, 2JH,H =14.2 Hz, 3JH,H =9.8 Hz, 1H; Ph(D)CHH),
3041, 2989, 2954, 2923, 1577, 1558, 1481, 1188, 1172, 1114, 974, 960, 948,
788, 742, 727, 700 cmÀ1 MS (FAB): m/z (%): 536 [M]+ (22), 495
;
[MÀC3H5]+ (13), 459 [MÀC6H5]+ (58), 391 [MÀPd
N
G
(%) for C27H35N2O2B1Pd1 (536.8): C 60.41, H 6.57, N 5.22; found: C
60.59, H 6.56, N 5.23.
3
3
0.63 (t, JH,H =7.6 Hz, 3H; CH3), 0.54 (t, JH,H =7.7 Hz, 3H; CH3), ꢀ0.32,
ꢀ0.26 ppm (overlaid signals, assignment by HMQC, CH2CH3); the re-
maining aromatic proton signals of the (anti, syn) isomer were overlaid
by the signals of the (syn, syn) isomer and were not assigned; 13C NMR
(126 MHz, CD2Cl2, 275 K): d=138.2, 137.6, 137.5 (arom C ipso), 130.0,
129.6, 129.6, 126.5, 129.3, 129.1, 128.8, 128.4, 128.2, 127.8, 127.1 (arom
CH), 106.7 ((PhCH)2CH), 78.7 (Ph(E)CH), 71.4 (Ph(F)CH), 70.6 (oxa
CH2 (B)), 70.2 (oxa CH (B)), 69.3 (oxa CH2 (C)), 64.2 (oxa CH (C)),
43.1 (Ph(A)CH2), 41.3 (Ph(D)CH2), 17.0, 16.7 (assignment by HMQC,
CH2CH3), 12.6, 12.4 ppm (CH3); no further signals were detected;
11B NMR (161 MHz, CD2Cl2): d=À15.9 ppm (s); only one signal was de-
tected, although both isomers were present according to 1H NMR; IR:
n˜ =3060, 3027, 2934, 2894, 2872, 2857, 1600, 1568, 1497, 1489, 1453, 1190,
1182, 1171, 1074, 1035, 1028, 948, 942, 836, 756, 735, 694 cmÀ1; MS
(FAB): m/z (%): 689 [M+H]+ (10), 659 [MÀC2H5]+ (42), 528 [MÀoxa]+
Synthesis of palladium complex 11cx: Ligand 7cx-H (230 mg, 589 mmol),
(1,3-diphenylallyl)palladium chloride dimer (196 mg, 292mmol)[7] and
K2CO3 (81.6 mg, 590 mmol) were stirred at 508C for 80 min in a mixture
of CH2Cl2 (12 mL), THF (10 mL) and methanol (10 mL) in a sealed
Schlenk flask. After the system had cooled to room temperature, the so-
lution was filtered, and the volatiles were removed. The residue was dis-
solved in CH2Cl2 (50 mL) and washed with water (2”20 mL). The organ-
ic phase was separated and dried over Na2SO4. After removal of volatiles
the remaining solid was recrystallized from hot EtOH/H2O by slow cool-
ing to room temperature. During the first crystallization the formation of
a black precipitate was observed, which made another filtration and
evaporation of solvents necessary. The product finallyrecrystallized as
yellow blocks (179 mg), which were washed with a small amount of
MeOH. Another batch (49 mg) was obtained by crystallization from the
mother liquor. The crystals obtained were suitable for x-ray diffraction
(228 mg, 56%); m.p. 156–1588C.
(6), 299 [Pd
(C3H3Ph2)]+ (33); elemental analysis calcd (%) for
A
C39H43BN2O2Pd (689.0): C 67.98, H 6.29, N 4.07; found: C 68.14, H 6.39,
N 4.09.
NMR data for the (syn,syn)-isomer:
The ratio of (syn, syn) to (anti, syn)
isomer in the sample was 91:9.
Synthesis of copper complexes: CuSO4·H2O (21.0 mg, 0.132 mmol) in
H2O (10 mL) was combined with
a solution of 7ay-Li (105 mg,
1H NMR (500 MHz, CD2Cl2, 275 K):
d=7.53–7.48 (m, 2H; arom CH (E)
ortho), 7.46–7.41 (m, 4H; arom CH
(F) ortho, arom CH (D) meta), 7.36–
7.20 (m, 10H; arom CH), 7.16 (m,
2H; arom CH (D) ortho), 6.87 (m,
2H; arom CH (A) ortho), 5.86 (dd,
0.265 mmol) in CH2Cl2 (20 mL) with vigorous stirring at room tempera-
ture. After 25 minutes a saturated aqueous solution of NaHCO3 (5 mL)
was added. Stirring was continued for an additional 25 minutes, during
which the organic phase changed from blue to green. The organic phase
was separated and filtered through a plug of Celiteꢁ, and volatiles were
evaporated. The remaining green crystalline solid was dissolved in the
minimum possible amount of CH2Cl2/Et2O (1 mL) and layered with
hexane. After 2 days, blue-green plates had started to grow. They were
collected and subjected to single-crystal analysis. Compound 15ay:
[a]2D0 =À931 (c=0.11 in CH2Cl2); MS (MALDI-TOF, 2,5-dihydrobenzoic
acid): m/z: 842 [M+H]+; elemental analysis calcd (%) for
C48H60B2CuN4O4 (842.19): C 68.46, H 7.18, N 6.65; found: C 68.78, H
7.23, N 6.60.
3JH,Ph(E)CH =11.3 Hz,
3JH,Ph(F)CH
=
10.7 Hz, 1H; (PhCH)2CH), 4.01 (d,
3JH,H =10.7 Hz, 1H; Ph(F)CH), 3.90 (m, overlaid signal, 1H; oxa CH
2
3
(B)), 3.86 (“t”, JH,H =8.0 Hz, JH,H =8.0 Hz, 1H; oxa CHH (B)), 3.75 (dd,
2JH,H =8.5 Hz, 3JH,H =4.1 Hz, 1H; oxa CHH (C)), 3.50 (“t”, 2JH,H =9 Hz,
3JH,H =9 Hz, 1H; oxa CHH (C)), 3.46 (“t”, JH,H =8.0 Hz, 1H; oxa CHH
(B)), 3.01 (d, 3JH,H =11.3 Hz, 1H; Ph(E)CH), 2.81 (“tdd”, 3JH,H =9.3, 5.5,
4.1 Hz, 1H; oxa CH (C)), 2.68 (dd, 2JH,H =13.8 Hz, 3JH,H =4.1 Hz, 1H;
Ph(A)CHH), 2.53 (dd, 2JH,H =13.1 Hz, 3JH,H =5.5 Hz, 1H; Ph(D)CHH),
2.45 (dd, 2JH,H =13.1 Hz, 3JH,H =9.3 Hz, 1H; Ph(D)CHH), 1.35 (dd,
2JH,H =13.8 Hz, 3JH,H =10.3 Hz, 1H; Ph(A)CHH), 0.78 (m, 6H; CH3),
0.77 (m, 2H; CH2CH3), 0.33 ppm (m, 2H; CH2CH3); 13C NMR
(126 MHz, CD2Cl2, 275 K): d=196.9 (b, N=C), 140.9 (arom C (E) ipso),
140.2 (arom C (F) ipso), 139.1 (arom C (D) ipso), 138.5 (arom C (A)
ipso), 130.1 (2C; arom CH (D) ortho), 129.4 (2C; arom CH), 129.3 (2C;
arom CH), 129.2 (2C; arom CH), 128.7 (2C; arom CH), 128.6 (2C; arom
CH), 128.3 (4C, arom CH (E+F) ortho), 127.7 (2C; arom CH (E+F)
para), 126.8 (arom CH (D) para), 126.6 (arom CH (A) para), 109.2
((PhCH)2CH), 74.1 (Ph(E)CH), 72.1 (oxa CH2 (C)), 71.3 (oxa CH2 (B)),
71.1 (Ph(F)CH), 66.9 (oxa CH (B)), 63.8 (oxa CH (C)), 43.4
(Ph(D)CH2), 41.2 (Ph(A)CH2), 18.8,
Copper-catalyzed allylic oxidation of alkenes—Procedure A with Li salts
7-Li: In a glovebox, a ligand 7-Li (37.6 mmol) and [Cu(CH3CN)4]PF6
G
(9.3 mg, 25 mmol) were placed in a 10 mL Youngꢁ tube, fitted with a mag-
netic stirring bar. CH3CN (2 mL) was added, and the tube was sealed
and removed from the glovebox. The mixture was stirred for 1 h at room
temperature. The olefin (2.0 mmol) was then added, followed by tert-
butyl perbenzoate (95 mL, 0.50 mmol) under a stream of argon. The flask
was sealed, and stirring was continued at the desired temperature. Reac-
tion progress was monitored by TLC (hexane/Et2O 15:1; Rf =0.30 (tert-
butyl peroxybenzoate) and either 0.41 (cyclohex-2-enyl benzoate) or 0.50
(cyclopent-2-enyl benzoate). When the reaction was complete, volatiles
were removed in vacuo, and the crude product was purified by column
chromatography (SiO2, 1=2 cm, h=14 cm, hexane/Et2O 15:1) to give a
colorless oil.
12.3 (b, CH2CH3), 12.1, 11.7 ppm
(CH3).
Procedure B with protonated ligands 7-H: In a glovebox, a ligand 7-H
(37.6 mmol), [Cu(CH3CN)4]PF6 (9.1 mg, 24 mmol) and K2CO3 (10.7 mg,
R
77.4 mmol) were mixed in a 10 mL Youngꢁ tube, fitted with a magnetic
stirring bar. CH3CN (2 mL) was added, and the tube was sealed and re-
moved from the glovebox. The mixture was stirred for 1 hour at room
temperature. The reaction and workup were carried out as described in
procedure A.
NMR data for the (anti,syn) isomer
1H NMR (500 MHz, CD2Cl2, 275 K):
d=7.51 (m, overlaid signal, 2H; arom
CH (F)), 7.40–7.36 (m, 6H; arom
CH), 7.31 (fully overlaid signal, as-
signed by 2D-NMR, arom CH (A)
ortho), 7.12–7.09 (m, 3H; arom CH),
The complexation reaction with ligand 7dx-H was performed at 868C to
circumvent solubility problems.
6.61 (m, 2H; arom CH (D) ortho),
5.62 (d, 3JH,H =7.7 Hz, 1H; Ph(E)CH),
HPLC conditions
5.48 (dd, 3JH,H =11.5, 7.7 Hz, 1H; (PhCH)2CH), 4.56 (d, 3JH,H =11.5 Hz,
1H; Ph(F)CH), 4.44 (“tdd”, 3JH,H =8.8, 5.5, 3.3 Hz, 1H; oxa CH (B)),
4.16 (dd, 2JH,H =8.8 Hz, 3JH,H =3.3 Hz, 1H; oxa CHH (B)), 4.08 (“t”,
Cyclohex-2-enyl benzoate: Chiracel OD-H; heptane/iPrOH 99.8:0.2;
208C; 0.5 mLminÀ1; l1 =210, l2 =230 nm; retention time (minutes): 17.7
(major, S), 19.2 min (minor, R).
8538
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Chem. Eur. J. 2008, 14, 8530 – 8539

Chiral Boron-Bridged Bisoxazoline Ligands
FULL PAPER
Cyclopent-2-enyl benzoate: Chiracel OD-H; heptane/iPrOH 99.8:0.2;
208C; 0.5 mLminÀ1; l1 =210, l2 =230 nm; retention time (minutes): 17.2
(major, S), 20.4 min (minor, R).
Chem. Soc. 1990, 112, 4587–4588; e) M. Sjçgren, S. Hansson, P. O.
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[12] Complex 12a also showed dynamic behavior at 295 K by NOE spec-
troscopy. Another NOE spectrum recorded at 275 K, at which the
dynamic process was slowed down considerably, was used for the as-
signment of the relative position of the allyl fragment. The NOE
spectra for complexes 10ax–az and 12b were recorded at 295 K.
[13] CCDC 664549 (15ax), 664550 (15cx), 664551 (15az), 664552 (11cx),
664553 (10ay) and 664554 (10az) contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif
Acknowledgements
Support of this work by the Swiss National Science Foundation is grate-
fully acknowledged. We thank Prof. Dr. Markus Meuwly for his help with
the DFT calculations and Dr. Balamurugan Ramalingam for helpful sug-
gestions concerning the synthesis of ligand 8d.
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Received: April 30, 2008
Published online: August 7, 2008
Chem. Eur. J. 2008, 14, 8530 – 8539
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8539