Binaphthyl Schiff base complexes of palladium(n). Structures and reactivities toward alkene epoxidation

Article abstract of DOI:10.1039/a908917i

Palladium(n) binaphthyl Schiff base complexes [Pd^II(L)](1: H₂L =(racemic or fl)-2,2′-bis(3,5-dichloro-2-hydroxybenzylideneamino)-l, 1′-binaphthyl(H₂L₂);2: H₂L =(A)-2,2′-bis(3,5-dichloro-2-hydroxylbenzylideneamino)-5,5′,6, 6′,7,7′,8,8′-octahydro-l,1′-binaphthyl(H ₂L³)) were prepared from sequential reactions of H₂L with sodium methoxide and palladium(n) acetate in methanol in about 70% yields. Both complexes 1 and 2 have been characterised by X-ray crystallography as well as ¹H NMR, IR, UV/VIS and MS spectroscopy. The structures of 1 and 2 feature a pseudo planar N₂O₂ arrangement with the Schiff base ligands adopting a stepped conformation, in contrast to the non-planar N₂O₂ geometry usually observed for this type of Schiff base bound to various metal ions. The catalytic behaviour of complexes 1 and 2 toward asymmetric epoxidation of styrenes was investigated. With 2 as a catalyst, a 71% ee was obtained for the epoxidation of p-fiuorostyrene by ¹BuOOH. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/a908917i

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Binaphthyl Schiff base complexes of palladium(II). Structures and
reactivities toward alkene epoxidation
Xiang-Ge Zhou,a Jie-Sheng Huang,a Xiao-Qi Yu,a Zhong-Yuan Zhoub and Chi-Ming Che*a
a Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong.
E-mail: cmche@hkucc.hku.hk
b Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu, China
Received 9th November 1999, Accepted 31st January 2000
Published on the Web 9th March 2000
Palladium() binaphthyl Schiff base complexes [PdII(L)] (1: H2L = (racemic or R)-2,2Ј-bis(3,5-dichloro-2-
hydroxybenzylideneamino)-1,1Ј-binaphthyl (H2L2); 2: H2L = (R)-2,2Ј-bis(3,5-dichloro-2-hydroxylbenzylidene-
amino)-5,5Ј,6,6Ј,7,7Ј,8,8Ј-octahydro-1,1Ј-binaphthyl (H2L3)) were prepared from sequential reactions of H2L with
sodium methoxide and palladium() acetate in methanol in about 70% yields. Both complexes 1 and 2 have been
characterised by X-ray crystallography as well as 1H NMR, IR, UV/VIS and MS spectroscopy. The structures of 1
and 2 feature a pseudo planar N2O2 arrangement with the Schiff base ligands adopting a stepped conformation, in
contrast to the non-planar N2O2 geometry usually observed for this type of Schiff base bound to various metal ions.
The catalytic behaviour of complexes 1 and 2 toward asymmetric epoxidation of styrenes was investigated. With 2
as a catalyst, a 71% ee was obtained for the epoxidation of p-fluorostyrene by tBuOOH.
its salen analogue in the oxygenation of styrene to form 1-
Introduction
phenylethanol.9 Our recent work demonstrated that a Ti–L2
complex catalyzes the trimethylsilylcyanation of aromatic
aldehydes with up to 96% enantiomeric excess (ee)26 but the
epoxidation of unfunctionalized alkenes catalyzed by a Mn–L2
complex28 gives rise to lower ee’s than by Mn–salen catalysts.
In the course of understanding the chiral induction by metal
complexes bearing tetradentate Schiff bases, it is crucial to
ascertain the coordination modes of these ligands. Salens are
known to bind metal ions usually with a planar N2O2 geometry
(inset (a) in Fig. 1), and are often reported to adopt a stepped
conformation (inset (b) in Fig. 1).30–32 These features remain
The C2-symmetric 1,1-binaphthyl unit1–3 and tetradentate Schiff
bases (salen)4–8 constitute two of the most important types of
chiral auxiliaries in metal-mediated asymmetric catalysis.
Recently, their “hybridized” forms, i.e. binaphthyl Schiff
bases L1 and L2 (Fig. 1), have been developed.5,9–11 The utility
of L1 in asymmetric catalysis has been a subject of much
investigation.5,12–25 In contrast, there are only a handful of
reports on employing L2 as a chiral auxiliary in asymmetric
catalysis.9–11,26–29 Nishinaga and co-workers reported that a
Co–L2 catalyst gave considerably higher enantioselectivity than
Fig. 1 Tetradentate Schiff base salen and its binaphthyl analogues L1 and L2. The inset shows the coordination modes (a and c) observed for these
ligands in octahedral metal complexes and the stepped conformation (b) in (a). Note that all the metal complexes with the ligand L2 appearing in this
work have R1 = R2 = Cl.
DOI: 10.1039/a908917i
J. Chem. Soc., Dalton Trans., 2000, 1075–1080
This journal is © The Royal Society of Chemistry 2000
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unchanged upon “merging” the salen phenyl rings with binaph-
thyl units to form L1.5 The planar N2O2 arrangements,4,33,34 or
especially the stepped conformations,5 of salen and L1 are con-
sidered as the basis for accounting for the related chiral induc-
tion. “Merging” salen with the binaphthyl unit in the C8–C8Ј
bond region to form L2, however, was found to change its
coordination behaviour, as revealed by the structural determin-
ations of metal complexes with L2, in which the Schiff base
ligand exclusively adopts a non-planar N2O2 arrangement with
a helical conformation (inset (c) in Fig. 1).26–28 These differences
in coordination modes might be responsible for the above-
mentioned lower chiral induction by Mn–L2 than by Mn–salen
catalysts.
H, 2.82; N, 3.45. C34H18Cl4N2O2PdؒH2O requires C, 54.26; H,
2.66; N, 3.72%).
[PdII(L3)] (2). Yield: 65%. 1H NMR (CDCl3, 300 MHz): δ 7.40
(d, J = 2.65 Hz, 2H, 4-H), 7.29 (s, 2H, 7-H), 7.09 (d, J = 8.71
Hz, 2H), 7.08 (d, J = 2.75 Hz, 2H, 6-H), 6.66 (d, J = 7.96 Hz,
2H), 2.83–1.60 (m, 16H, CH2). IR (KBr, cmϪ1): 1607, 1511,
1430, 1304, 1200, 1169, 761. UV/VIS (CHCl3): λmax/nm
(ε × 10Ϫ4/dm3 molϪ1 cmϪ1) 258 (5.89) sh, 296 (2.78) sh, 445
(1.01). FAB MS: m/z 743 (Mϩ) (Found: C, 53.89; H, 3.57; N,
4.02. C34H26Cl4N2O2Pdؒ0.5H2O requires C, 54.33; H, 3.60;
N, 3.73%).
In this work, it is our intention to (i) unearth the reasons why
L2 favours the non-planar N2O2 arrangement in binding metal
ions and (ii) examine the catalytic properties of a metal–L2
complex, [PdII(L2)] (1, R1 = R2 = Cl), that has a stepped con-
formation similar to salen and L1 analogues. The synthesis,
structure and catalytic properties of [PdII(L3)] (2, L3 is a par-
tially hydrogenated L2, see Scheme 1), are also described. Prior
to this work, only one palladium() tetradentate Schiff base
complex was characterized by X-ray crystallography,35 which
contains N3O rather than N2O2 chelating atoms. In addition,
the present work provides the first asymmetric epoxidation of
alkenes catalyzed by a palladium complex.
X-Ray structural determinations
Single crystals of H2L3 were grown by vapour diffusion of pen-
tane into a solution of H2L3 in chloroform. Single crystals of
complexes 1ؒ0.5CH2Cl2 and 2ؒAcOEt were obtained by laying
n-hexane on the top of a solution of racemic 1 in dichloro-
methane and by vapour diffusion of hexane into a solution of 2
in ethyl acetate. Data were collected on a Rigaku RAXIS-IIC
imaging-plate diffractometer (H2L3 and 2ؒAcOEt) and a
Siemens P4 four-circle diffractometer (1ؒ0.5CH2Cl2), both with
a Mo-Kα source (λ = 0.71073 Å). For 1ؒ0.5CH2Cl2, the
XSCANS program (Siemens Reference Manual) was applied
to search for diffraction peaks to determine cell parameters,
data collection and reduction. The crystallographic data were
corrected using the LP factor and semi-empirical absorption
corrections were also applied. For H2L3 and 2ؒAcOEt, data
collections were made with 3Њ oscillation (60 images) at 12 cm
distance and 8 min exposure. Absorption corrections were
made by using the ABSCOR program.37 All calculations were
performed on a PC 486 computer with the SHELXTL-PC
program package.38 The structure was solved by direct methods
for H2L3, and by the Patterson superposition method for
1ؒ0.5CH2Cl2 and 2ؒAcOEt. Conventional R-factors R1 and
weighted R-factors wR2 are based on F and F2, respectively. In
all cases, the non-hydrogen atoms were refined anisotropically
by the full-matrix least-squares method, whereas hydrogen
atoms were placed in calculated positions (C–H 0.96 Å),
assigned fixed isotropic thermal parameters and allowed to ride
on their respective parent atoms.
Experimental
General
1H and 13C NMR spectra were recorded on a Bruker DPX 300
FT-NMR spectrometer (300 MHz). 1H–1H COSY and NOESY
NMR spectra were recorded on a Bruker DRX 500 FT-NMR
spectrometer (500 MHz). The chemical shifts (δ) are reported
relative to tetramethylsilane (TMS). UV/VIS spectra were
measured on a Perkin-Elmer Lambda 19 spectrophotometer.
Infrared spectra were recorded on a Shimadzu-470 spec-
trometer. FAB and electrospray (ES) mass spectra were
obtained on Finnigan MAT 95 and LCQ quadruple ion trap
spectrometers, respectively. GC analyses were carried out
on a HP 5890 series II system equipped with a HP 5890A
flame ionisation detector and a HP 3395 integrator. Capillary
columns β- (30 m × 0.25 mm) and γ-cyclodextrin (30 m × 0.32
mm) were used to analyse the epoxides. Elemental analyses
were performed by the Institute of Chemistry, the Chinese
Academy of Sciences. All solvents and substrates were purified
by standard procedures. tert-Butyl hydroperoxide (TBHP)
in toluene was prepared by the method of Sharpless.36
The binaphthyl Schiff bases H2L2 and H2L3 were prepared
according to the published procedures.10,11,26
CCDC reference number 186/1833.
See http://www.rsc.org/suppdata/dt/a9/a908917i/ for crystal-
lographic files in .cif format.
Asymmetric epoxidation of alkenes catalysed by complexes 1
and 2
A typical procedure. To a mixture of activated 4 Å molecular
sieves (200 mg, Aldrich) and complex 1 or 2 (0.01 mmol) in dry
toluene (4 mL) was added styrene (0.05 mmol). Upon cooling
to 0 ЊC, a 3.1 M solution of TBHP in toluene (66.7 µL, 0.2
mmol) was added. The mixture was stirred at 0 ЊC for 6 h, and
then chromatographed on silica gel with ethyl acetate–n-hexane
(1:6) as the eluant. The yield and ee of the products were
determined by GC.
Preparation of [PdII(L2)] (1) and [PdII(L3)] (2)
To a mixture of Schiff base H2L2 (125 mg, 0.27 mmol ) or H2L3
(127 mg, 0.27 mmol) and NaOMe (35 mg, 0.65 mmol) in
methanol (20 mL) was added palladium() acetate (61 mg, 0.27
mmol). After the mixture was stirred for 2 h, a yellow precipi-
tate was formed. The precipitate was collected by filtration,
washed with methanol and ether, and dried in air.
Results and discussion
[PdII(L2)] (1). The racemic and chiral (R-configuration) com-
plexes of 1 were prepared in ca. 76% yields by employing
racemic H2L2 and (R)-H2L2, respectively. 1H NMR (CDCl3, 500
MHz): δ 8.01 (d, J = 8.58 Hz, 2H, 4Ј-H), 7.97 (d, J = 8.21 Hz,
2H, 8Ј-H), 7.61–7.53 (m, 2H, 7Ј-H), 7.40 (d, J = 2.62 Hz, 2H,
4-H), 7.34 (s, 2H, 7-H), 7.31–7.28 (m, 2H, 6Ј-H), 7.13 (d,
J = 8.55 Hz, 2H, 3Ј-H), 6.99 (d, J = 2.63 Hz, 2H, 6-H), 6.96 (d,
J = 8.52 Hz, 2H, 5Ј-H). IR (KBr, cmϪ1): 1603, 1507, 1430, 1169,
There are a variety of palladium() Schiff base complexes
known in the literature, of which only those bearing bidentate
Schiff base ligands are well documented.39 Although a Pd()-
salen complex was first isolated in 1963,40 tetradentate Schiff
base complexes of this metal ion are still sparse.35,39–42 In this
work, sequential treatment of H2L2 or H2L3 with sodium
methoxide and palladium() acetate in methanol readily
afforded complex 1 or 2 as a pale yellow precipitate (Scheme 1).
By washing with methanol and ether, analytically pure products
of 1 and 2 were obtained in about 70% yields.
949, 864, 821, 801, 774, 748, 488. UV/VIS (CHCl3): λmax
/
nm (ε × 10Ϫ4/dm3 molϪ1 cmϪ1) 265 (4.93) sh, 283 (3.20) sh, 318
(1.70) sh, 441 (0.67). FAB MS: m/z 735 (Mϩ) (Found: C, 53.86;
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Table 1 Crystallographic data for H2L3, [PdII(L2)] (1) and [PdII(L3)] (2)
H2L3
1ؒ0.5CH2Cl2
2ؒAcOEt
Chemical formula
Formula weight
Space group
C34H28Cl4N2O2
638.38
C2/c
C34H18Cl4N2O2Pdؒ0.5CH2Cl2
C34H26Cl4N2O2PdؒAcOEt
777.17
830.87
¯
¯
P1
P1
a/Å
b/Å
c/Å
α/Њ
22.530(5)
10.765(2)
12.843(3)
10.359(2)
13.218(3)
14.597(3)
63.28(3)
71.03(3)
75.74(3)
1676.4(6)
2
10.317(2)
13.332(3)
13.668(3)
98.88(3)
93.85(3)
102.00(3)
1807.3(6)
2
β/Њ
91.78(3)
γ/Њ
V/Å3
3113.5(11)
4
Z
T/K
λ/Å
293(2)
0.71073
1.362
0.414
0.047
0.15
293(2)
0.71073
1.540
0.986
0.07
293(2)
0.71073
1.527
0.852
0.079
ρcalc/g cmϪ3
µ/mmϪ1
R1 a (I > 2σ(I))
wR2 b (I > 2σ(I))
0.19
0.18
2
a R1 = Σ Fo| Ϫ |Fc /|Fc|. b wR2 = {Σ[w(Fo2 Ϫ Fc2)2]/Σ[w(Fo )2]}1/2
.
Fig. 2 ORTEP57 drawing with the atom-numbering scheme for H2L3
(35% probability ellipsoids). Hydrogen atoms are omitted for clarity.
the cluster peak of the parent ion for either 1 (m/z = 735) or 2
(m/z = 743) was observed.
Scheme 1
Spectral features of complexes 1 and 2
Structure determinations of H2L3, [PdII(L2)] (1) and [PdII(L3)] (2)
Complexes 1 and 2 are diamagnetic, exhibiting well-resolved 1H
NMR signals of the coordinated Schiff bases at normal fields.
In each case, only one set of naphthyl or benzylideneamine
proton resonances has been observed, suggesting that both 1
and 2 have a C2 symmetry in solution. The signal assignment
Table 1 summarises the crystallographic data for the three
compounds, whose structures are shown in Fig. 2–4. Selected
bond distances and angles are listed in Table 2.
The compound H2L3 (Fig. 2) is the first structurally charac-
terised example of partially hydrogenated H2L2. The compon-
ent atoms of each half of H2L3 are basically co-planar (mean
deviation from the least-squares plane: 0.0637 Å), with the
least-squares planes of the two halves making a dihedral angle
of 85.9Њ. These features are similar to those observed for H2L2
by Meunier and co-workers.10 However, partial hydrogenation
of H2L2 does cause some appreciable structural changes. For
example, for the partially hydrogenated naphthyl groups in
H2L3, the bond distances involving the sp3 carbon atoms aver-
age 1.460(3) Å, which is significantly longer than the mean dis-
tance (1.392(2) Å) between the sp2 carbon atoms; the angles
C(11)–C(12)–C(13) and C(14)–C(15)–C(16) of 114.0(2)Њ are
considerably smaller than the angles (120Њ) expected in an
idealized phenyl ring.
1
for 1 was made on the basis of H–1H COSY and NOESY
NMR measurements. The spectrum of complex 2 is more
complicated, featuring several multiplets in the aliphatic region.
However, the 4-, 6-, and 7-H chemical shifts are similar to those
observed for complex 1. Compared with H2L2 and H2L3, which
give 7-H resonances at 8.5527 and 8.426 ppm, respectively, both
1 and 2 exhibit the 7-H signals (7.34 and 7.29 ppm, respectively)
upfield shifted by more than 1.1 ppm. For comparison, binding
of L2 to a ruthenium() ion to form the diamagnetic complex
[RuII(L2)(NO)Cl]26 shifts the 7-H signal upfield by less than
0.74 ppm.
A comparison of the UV/VIS spectra between 1 and H2L2 or
between 2 and H2L3 showed that both ligands lost almost all the
original bands upon binding to the palladium() ion, indicating
the occurrence of a major structure adjustment, such as a
severe breakage of the π-electron delocalization over the entire
halves of L2 or L3 (vide infra). The IR spectra of 1 and 2 exhibit
intense bands at 1603 and 1607 cmϪ1, respectively, ascribable to
The unit cell of racemic complex 1 consists of a pair of (R)-
and (S)-enantiomers, whose structures feature an almost planar
N2O2 arrangement (Fig. 3) with a mean deviation of 0.11 Å
from the least-squares plane. This is different from the struc-
tures of [CuII(L2)],27 [MIII(L2)(acac)] (M = Fe, Mn),28 and
[RuII(L2)(NO)Cl]26 with the same ligand, all of which have a
the C᎐N stretching vibrations. In the FAB MS measurements,
᎐
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Table 2 Selected bond distances (Å) and angles (Њ) for H2L3, [PdII(L2)]
(1), and [PdII(L3)] (2)
H2L3
C(11)–C(12)
C(13)–C(14)
C(15)–C(16)
C(17)–C(17A)
C(8)–N(1)
1.512(2)
1.312(3)
1.520(2)
1.505(3)
1.415(2)
C(12)–C(13)
C(14)–C(15)
C(11)–C(16)
C(7)–N(1)
1.481(3)
1.475(3)
1.399(2)
1.279(2)
1.733(2)
C(2)–Cl(1)
C(12)–C(11)–C(16)
C(11)–C(12)–C(13)
C(13)–C(14)–C(15)
C(13)–C(12)–C(17)
121.4(2)
114.0(2)
123.0(3)
121.6(2)
C(15)–C(16)–C(11)
C(12)–C(13)–C(14)
C(14)–C(15)–C(16)
C(12)–C(17)–C(16)
120.5(1)
121.7(2)
114.0(2)
121.5(2)
[PdII(L2)] (1)
Pd(1)–N(1)
Pd(1)–N(2)
Pd(1)–O(1)
Pd(1)–O(2)
1.975(3)
1.989(3)
2.004(3)
1.990(2)
N(1)–C(21)
N(2)–C(28)
C(27)–O(1)
C(34)–O(2)
1.302(4)
1.291(4)
1.299(4)
1.300(4)
N(1)–Pd(1)–N(2)
N(2)–Pd(1)–O(2)
Pd(1)–O(1)–C(27)
Pd(1)–N(1)–C(2)
Pd(1)–N(2)–C(12)
94.1(1)
89.9(1)
120.9(2)
116.9(2)
117.5(2)
O(2)–Pd(1)–O(1)
N(1)–Pd(1)–O(1)
Pd(1)–O(2)–C(34)
Pd(1)–N(1)–C(21)
Pd(1)–N(2)–C(28)
86.7(1)
89.9(1)
121.5(2)
124.0(2)
125.3(3)
[PdII(L3)] (2)
Pd(1)–N(1)
Pd(1)–N(2)
Pd(1)–O(1)
Pd(1)–O(2)
2.006(2)
1.993(2)
2.000(2)
1.998(2)
N(1)–C(21)
N(2)–C(28)
C(23)–O(1)
C(30)–O(2)
1.249(3)
1.280(3)
1.294(3)
1.285(3)
Fig. 3 ORTEP drawing with the atom-numbering scheme for the (R)-
enantiomer of complex 1 (35% probability ellipsoids). Hydrogen atoms
are omitted for clarity.
N(1)–Pd(1)–N(2)
N(1)–Pd(1)–O(1)
Pd(1)–O(1)–C(23)
Pd(1)–N(1)–C(20)
Pd(1)–N(2)–C(2)
94.46(7)
89.41(7)
120.4(1)
114.5(1)
115.6(1)
O(1)–Pd(1)–O(2)
O(2)–Pd(1)–N(2)
Pd(1)–O(2)–C(30)
Pd(1)–N(1)–C(21)
Pd(1)–N(2)–C(28)
86.97(6)
90.05(7)
121.1(1)
124.3(1)
124.3(1)
tate acac ligand, does not unquestionably signify an unusual
coordination behaviour of L2 as compared to salen,43 such a
geometry found for [CuII(L2)]27 and [RuII(L2)(NO)Cl]26 sug-
gests that the binaphthyl Schiff base L2 is different from salen in
chelating metal ions, since both [CuII(salen)]44 and [RuII(salen)-
(NO)Cl]45,46 have been found to adopt basically planar N2O2
arrangements. The questions naturally arise as to what causes
such a difference and why complexes 1 and 2 show an almost
planar N2O2 geometry. To address the first question, the follow-
ing possibilities are to be considered: (i) the σ-bonds linking
naphthyl and phenoxide rings might experience a significantly
larger strain when L2 adopts a planar N2O2 arrangement; (ii)
the rigidity of the binaphthyl unit,3 i.e. the propensity of two
naphthyl planes to maintain a relatively large dihedral angle,
possibly makes it hard for L2 to accommodate a planar N2O2
geometry; and (iii) the propensity of the π-electrons in L2 to
have maximum delocalisation may favour the non-planar N2O2
geometry. It seems that the possibilities (i) and (ii) could be
excluded, since the key bond distances and angles, and the
binaphthyl dihedral angles of L2 or L3 in the complexes with
planar and non-planar N2O2 moieties are found to be similar.
The dihedral angles between benzylideneamine and the
attached naphthyl plane in complexes 1 and 2 were then exam-
ined, which are ca. 93 and 91Њ, respectively, both considerably
larger than the corresponding angles found in the complex [RuII-
(L2)(NO)(Cl)] with non-planar N2O2 geometry (64Њ).26 This
indicates that maintaining a pseudo planar N2O2 coordination
mode would almost fully break the π-electron delocalisation
from benzylideneamine to the attached naphthyl groups, caus-
ing L2 or L3 to suffer from a significant strain. Therefore, the
propensity for the π-electrons in L2 to have maximum delocal-
isation seems to favour the non-planar N2O2 geometry.
Fig. 4 ORTEP drawing with the atom-numbering scheme for complex
2 (35% probability ellipsoids). Hydrogen atoms are omitted for clarity.
non-planar N2O2 geometry as shown in inset (c) of Fig. 1,
except for [CuII(L2)] having a typical distorted tetrahedral N2O2
arrangement. The structure of the chiral complex 2 (Fig. 4)
resembles that of complex 1, which again features an almost
planar N2O2 geometry, with the mean deviation from the least-
squares plane of 0.12 Å. The bond distances involving the sp3
carbon atoms of the partially hydrogenated naphthyl groups
average 1.486(5) Å, similar to the mean value of these distances
found in H2L3.
Rationalisation of the coordination behaviour of L2 or L3
The pseudo planar N2O2 geometry observed for complexes 1
and 2 could be rationalised by a strong preference of the pal-
ladium() ion for a square planar configuration.39,47,48 It is likely
Although the non-planar N2O2 geometry in the structures of
[MIII(L2)(acac)] (M = Fe, Mn),28 both bearing an extra biden-
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that the enhanced stability resulting from the pseudo square
planar coordination of the palladium() ion to L2 or L3 com-
pensates the strain due mainly to forcing the benzylideneamine
planes to be almost perpendicular to the attached naphthyl
rings. Interestingly, either complex 1 or 2 adopts a typical
stepped conformation, like metal–salen30–32 and metal–L1 5
complexes. The phenoxide rings of 1 or 2 constitute a dihedral
angle of 10.3 or 10.6Њ, respectively, both smaller than the
corresponding angle of 17.3Њ found in [CrIII(salen)(H2O)2]Cl.49
styrene after 6 h, affording styrene epoxide and benzaldehyde in
37 and 50% yield (based on the consumed styrene), respectively,
with a 17% ee obtained for the epoxide. Under the same condi-
tions except with p-fluorostyrene as substrate, a 27% conver-
sion, 51 and 32% yields of p-fluorostyrene epoxide and
p-fluorobenzaldehyde, respectively, were obtained, with the ee
of the epoxide reaching 71%. This is to be compared with the
[CuII(L2)]/TBHP system,27 which affords epoxides in moderate
yields but gives no asymmetric induction. Since complex
[CuII(L2)] has a typical distorted tetrahedral N2O2 arrangement
with a helical conformation, it might be the case that the
stepped conformations of catalysts 1 and 2 are indeed beneficial
to the chiral induction.
Worthy of note is that p-methylstyrene, which has an
electron-donating para substituent, was almost unreactive
toward reaction 3 (<2% conversion after a 6 h reaction). This,
together with the results described above, probably suggests
that a less electron-rich alkene is more reactive toward reaction
3, a feature to some extent similar to the alkene epoxidations by
metalloporphyrin peroxo complexes reported by Valentine and
co-workers.55,56 The epoxidations of cis- and trans-β-methyl-
styrene via reaction 3 resulted in poor enantioselectivity (<5%
ee), and were non-stereospecific for the cis alkene, like the
epoxidation of cis alkenes through reactions 1 and 2. This
implies that a stepwise mechanism is operative in the case of
this substrate.
Alkene epoxidation catalyzed by complexes 1 and 2
Palladium-mediated catalysis in organic synthesis has received
much attention.50,51 Nevertheless, there are only two reports in
the literature on alkene epoxidation catalyzed by palladium
complexes,52,53 neither is enantioselective. In 1980, Foote and
Ryang reported that PhC(O)C(N2)Ph reacted readily with oxy-
gen in the presence of palladium() acetate to give an inter-
mediate which can transfer an oxygen atom to alkenes to give
epoxides under very mild conditions (reaction 1 in Scheme 2).52
In attempting to identify the active intermediates in reaction
3, we treated complex 1 with 20 equiv. of TBHP, a molar ratio
identical to that for the catalytic reactions, in dichloromethane
at room temperature for 0.5 h and obtained a homogeneous
solution. The ES-MS of the solution shows a peak at m/z 735
corresponding to the parent ion of complex 1 and a more
intense peak at m/z 823 which could be ascribed to [Pd(L2)-
(OOBut)]ϩ or {[Pd(L2)(HOOBut)] Ϫ H}ϩ. No peaks due to
demetallation were observed under the ES-MS conditions.
Conclusions
Scheme 2
Palladium() tetradentate binaphthyl Schiff base complexes,
[PdII(L2)] (1) and [PdII(L3)] (2), could be readily prepared
through reaction of palladium() acetate with sodium
methoxide-treated H2L2 and H2L3, respectively. Both complexes
have a pseudo planar N2O2 arrangement with the ligands
adopting a typical stepped conformation, a coordination mode
unusual for metal–L2 complexes but attributable to the strong
preference of palladium() ion for a square planar geometry.
Comparison of the key metrical parameters of 1 and 2 with
those of related metal complexes having the usual non-
planar N2O2 arrangements reveals that the propensity of the
π-electrons in L2, and L3 as well, to have maximum delocalis-
ation is most probably a dominant factor in controlling the
coordination mode of such Schiff bases. The chiral complexes 1
and 2 can catalyse the asymmetric epoxidation of unfunctional-
ised alkenes with up to 71% ee.
Thereafter, Saito and co-workers observed that reaction of
alkenes with α-silyloxyalkyl peroxybenzoates in the presence of
palladium() acetate also afforded epoxide products (reaction 2
in Scheme 2).53 Noticeably, although a number of palladium
Schiff base complexes have been reported,35,39–42 their reactivity
toward alkene epoxidation remains unexplored.
We investigated the reactions of styrene and substituted
styrenes with several oxidants, namely TBHP, PhIO54a and Cl2-
PyNO (2,6-dichloropyridine N-oxide), in dichloromethane or
toluene in the presence of a catalytic amount of chiral com-
plexes 1 or 2. Cl2PyNO was found to be unreactive toward the
epoxidation reactions. In the case of PhIO, severe degradation
of the catalysts occurred, and the yields of epoxides were very
low. For example, with a 1:PhIO:styrene molar ratio of
1:10:50, only <8% yield of styrene oxide (based on the amount
of starting PhIO) was obtained after a 6 h reaction in toluene.
The oxidation by TBHP (reaction 3 in Scheme 2) afforded
epoxides in better yields (vide infra), although catalyst degrad-
ation was still a problem. The major side products were alde-
hydes. This is different from the reactions between terminal
alkenes (such as cyclohexene and styrene) and Pd(RCO2)-
(ButOO) (obtained by treating Pd(RCO2)2 with TBHP)
reported by Mimoun and co-workers,54b which resulted in the
formation of methyl ketones in ≥98% yields. Preliminary
studies revealed that complex 2, bearing the partially-
hydrogenated binaphthyl Schiff base, is superior to complex 1
as the epoxidation catalyst, and toluene is a better solvent than
dichloromethane. The results described below were all obtained
in toluene.
Acknowledgements
This work was supported by The University of Hong Kong and
Hong Kong Research Grants Council.
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