Asymmetric Baeyer-Villiger oxidation of prochiral cyclobutanones using a chiral cationic palladium(II) 2-(phosphinophenyl)pyridine complex as catalyst

Article abstract of DOI:10.1055/s-2003-38377

Chiral cationic palladium(II) 2-(phosphinophenyl)pyridine (1a) complex was found to be an effective catalyst for asymmetric Baeyer-Villiger oxidation of prochiral cyclobutanones. For example, good and excellent enantioselectivities (80% and >99% ees) were

Full text of DOI:10.1055/s-2003-38377

LETTER
643
Asymmetric Baeyer–Villiger Oxidation of Prochiral Cyclobutanones Using
a Chiral Cationic Palladium(II) 2-(Phosphinophenyl)pyridine Complex as
Catalyst
Baeyer–Villiger
O
a
xidation of
t
Prochi
s
ra
l
Cyclob
u
utanone
s
ji Ito,*a Ayako Ishii,a Tomomi Kuroda,a Tsutomu Katsuki*b
a
Department of Chemistry, Fukuoka University of Education, CREST, Japan Science and Technology (JST), Akama, Munakata, Fukuoka,
811-4192, Japan
Fax +81(940)351367; E-mail: itokat@fukuoka-edu.ac.jp
b
Department of Chemistry, Faculty of Science, Graduate School, Kyushu University 33, CREST, Japan Science and Technology (JST),
Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
Received 3 February 2003
R
O
R
Abstract: Chiral cationic palladium(II) 2-(phosphinophenyl)pyri-
dine (1a) complex was found to be an effective catalyst for asym-
metric Baeyer–Villiger oxidation of prochiral cyclobutanones. For
example, good and excellent enantioselectivities (80% and >99%
ees) were achieved in the reactions of 3-phenylcyclobutanone and a
tricyclic cyclobutanone, respectively.
O
σ
R'OOH
σ
HO
O
σ*
O
O
*
R
R'
Criegee adduct
Key words: asymmetric Baeyer–Villiger oxidation, asymmetric
catalysis, palladium, chiral P-N ligand
Scheme 1
Criegee adduct is a kind of h2 ligand and the adduct is con-
sidered to make a chelate with a metal ion possessing two
vacant cis-coordination sites. Thus, it was expected that
the conformation of the Criegee adduct could be regulated
to allow the enantiotopos-selective s–s* interaction, if
the adduct makes a chelate with a metal ion bearing an ap-
propriate chiral ligand. Indeed, chiral cationic Co(salen)
complex of cis-b structure and Zr(salen) complex that can
adopt a cis-b structure dynamically have recently be
found to be efficient catalysts for asymmetric B–V reac-
tion using urea–hydrogen peroxide adduct (UHP) as the
oxidant (Scheme 2).7 In these reactions, M(salen)-Criegee
adduct complexes (M = Co, Zr) have been considered to
take octahedral coordination. However, the concept of
stereoelectronic control of B–V oxidation is not confined
to the reaction via an octahedral adduct complex. Thus,
we explored B–V oxidation via metal-Criegee adduct
complex of square planar coordination.
Baeyer–Villiger (B–V) oxidation is a potent tool for trans-
forming carbonyl compounds to lactones or esters, and it
is widely used in organic synthesis.1 Naturally, its asym-
metric version is expected to be a useful method for the
synthesis of chiral lactones that are efficient chiral build-
ing blocks. In the last decade, much effort has been direct-
ed toward study of asymmetric B–V oxidation. In 1994,
Bolm et al. and Strukul et al. independently reported met-
al-mediated asymmetric B–V oxidation.2 Since then, sev-
eral catalytic asymmetric B–V oxidations have been
developed,3–5 but satisfactory enantioselectivity has not
been achieved except for a few examples.4d–4f In contrast,
bio-catalyzed B–V oxidation is well known to proceed
with excellent enantioselectivity.6 B–V oxidation is a two-
step reaction: i) nucleophilic addition of peroxide com-
pound to carbonyl group giving a Criegee adduct and ii)
the rearrangement of the adduct to ester (or lactone) by
migration of the C-C bond to proximal oxygen atom of the
peroxide unit (Scheme 1). The migration that is irrevers-
ible and rate-determining step proceeds smoothly when
the s-orbital of the C-C bond interacts with the s*-orbital
of the O-O bond. Thus, it was expected that highly enan-
tioselective B–V oxidation would be achieved, if one of
the two s-bonds interacts with the s*-bond enantiotopos-
selectively.5e In bio-catalyzed B–V oxidations, the con-
formation of Criegee adduct has been considered to be
controlled by weak bond interactions such as hydrogen
bond between the adduct and enzyme and the migration to
occur enantiotopos-selectively.6
R
*
*
O
N
O
N
O
N M
*
*
*
N
O
*
O
*
O
H
O
chiral salen ligand
N M O
M(salen) of cis-β structure
N
*
*
O
M(salen)-Criegee adduct complex
O
H
O
O
O
O
H
H
H
Zr(salen)
H
Co(salen)
UHP
H
O
UHP
Ph
Ph
87% ee
H
H
98% ee
Synlett 2003, No. 5, Print: 10 04 2003.
Art Id.1437-2096,E;2003,0,05,0643,0646,ftx,en;U01803ST.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214
Scheme 2

644
K. Ito et al.
LETTER
In 1994, Strukul et al. reported an enantiomer-differenti- what improved the enantioselectivity and catalytic
ating B–V oxidation of racemic 2-substituted cycloal- activity (entry 4). Use of aqueous hydrogen peroxide as a
kanones with hydrogen peroxide in the presence of chiral terminal oxidant again diminished enantioselectivity (en-
cationic platinum(II)BINAP complex as the chiral cata- try 5). Compound 1b bearing bulky 1-(t-butyldimethylsi-
lyst. This reaction has been proposed to proceed through loxy)-1-methylethyl group was a less efficient auxiliary in
a platinum-Criegee adduct complex possessing a square terms of reactivity and enantioselectivity (entry 6). We
planar coordination, and the mechanism of asymmetric in- also examined the reaction using chiral 2-(phosphinophe-
duction has been explained by steric factor.4e Several oth- nyl)oxazolines 2a,b, which are efficient chiral auxiliaries
er catalytic4c,d,f and stoichiometric5 B–V reactions using for palladium-catalyzed allylic alkylation10a–c and Heck
chiral bidentate ligand have been reported, but the de- reaction,10d but only moderate enantioselectivities were
tailed mechanism of their asymmetric induction has not observed (entries 7 and 8). Reactions using C2-symmetric
been given.
ligands such as bipyridines 3a,b, bis(oxazoline)s 4a,b and
BINAP 5 (Figure 2) were also studied but their enantiose-
lectivities were low (entries 9–14).
In order to realize highly enantioselective B–V oxidation
via square planar metal-Criegee adduct complex, it was
considered that two factors, geometry of the complex (A
or B) and the conformation of the chelate of the adduct (A
or B, the conformers of B are not given here), had to be
regulated strictly (Figure 1).8
Table 1 Asymmetric B–V Oxidation of 3-Phenylcyclobutanonea
Entry Metal
Ligand Time
(h)
Yield
(%)
% eeb
Confign.c
1
Pt
1a
1a
1a
1a
1a
1b
2a
2b
3a
3b
4a
4b
5
24
24
20
2
79
84
7
3
R
R
R
R
R
R
R
R
–
Ph
Ph
H
2d
3
Pt
*L1
O
O
*L1
O
O
M
O
*L1
*L2
O
M
*L2
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
87
41
47
26
8
*L1
*L2
O
O
*L2
M
O
H
M
O
O
O
H
A
4e
5d
6
100
88
H
Ph
Ph
B
20
45
17
18
–
(geometric isomers)
conformation of A ent-conformation of A
52
Figure 1 Schematic draws of M-Adduct complexes
7
94
10
38
–
We have reported that the chiral bipyridines, in which the
substituent at the chiral center protrudes in the vicinity of
the metal center, are efficient chiral auxiliaries for copper-
catalyzed asymmetric cyclopropanation and ring-enlarge-
ment reactions.9a–d We have also revealed that chiral 2-
(phosphinophenyl)pyridine ligand bearing 7-substituted
dihydropyrindine unit are efficient chiral auxiliaries for
palladium-catalyzed allylic substitution of both acyclic
and cyclic alkenyl substrates, which proceeds through a
square planar p-allyl palladium complex.9e,f Thus, we ex-
pected that the chiral auxiliary bearing 7-substituted dihy-
dropyrindine unit would be an efficient auxiliary for
controlling the geometry and the conformation of metal-
Criegee adduct complex of square planar coordination.
8
100
–
9
10
89
12
70
3
35
9
S
11
12
13
14d
88
14
15
18
10
S
44
R
R
R
100
100
5
3
a Reactions were carried out at room temperature in 1,2-dichloroet-
hane using UHP as a terminal oxidant, unless otherwise mentioned.
b Enantiomeric excess was determined by HPLC analysis using chiral
stationary phase column (Daicel Chiralpak AD-H; hexane–i-PrOH,
95:5).
B–V oxidation of 3-phenylcyclobutanone using urea–hy-
drogen peroxide adduct (UHP) as a terminal oxidant7 in
the presence of a cationic platinum(II) complex of 2-
(phosphinophenyl)pyridine ligand 1a was first examined
(Table 1). The platinum complex was prepared by mixing
(COD)PtCl2 and 1a in situ in 1,2-dichloroethane and sub-
sequent treatment with silver tetrafluoroborate. The B–V
oxidation proceeded smoothly at room temperature but
was low enantioselective (7% ee, entry 1). Use of aqueous
hydrogen peroxide as a terminal oxidant vitiated the enan-
tioselectivity (entry 2). However, we were pleased to find
that the cationic palladium(II) complex of 1a showed
moderate enantioselectivity of 41% ee (entry 3). Replac-
ing tetrafluoroborate with hexafluoroantimonate some-
c Configuration was determined by comparison of the elution order
with the authentic samples (ref.7c).
d Aqueous hydrogen peroxide (30%) was used as a terminal oxidant,
instead of UHP.
e Silver hexafluoroantimonate was used in the preparation of the cat-
alyst, instead of silver tetrafluoroborate.
2+
-
(5 mol%)
2BF4
O
(Chiral ligand)M
M= Pt, Pd
O
Ph
O
*
1,2-dichloroethane, r.t.
Ph
Scheme 3
Synlett 2003, No. 5, 643–646 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Baeyer–Villiger Oxidation of Prochiral Cyclobutanones
645
9 and 11–13) and high enantioselectivity of 80% ee
was obtained when the reaction was conducted in THF
at –60 °C.11 However, lowering the reaction temperature
to –80 °C decreased enantioselectivity to considerable
extent (entry 14).
O
N
N
N
R
PPh2
N
PPh2
R
R
R
1a: R= i-Pr
1b: R= CMe2(OTBS)
2a: R= Ph
2b: R= t-Bu
3a: R= CH2(OTBS)
3b: R= CMe2(OTBS)
Under the optimized reaction conditions, we next exam-
ined B–V oxidation of several other 3-substituted cyclo-
butanones (Table 3). The reactions of 3-aryl-substituted
cyclobutanones showed good to high enantioselectivities
(entries 1–3), while that of 3-octylcyclobutanone exhibit-
ed moderate selectivity (entry 4).
O
O
N
N
PPh2
PPh2
R
R
Table 3 Asymmetric B–V Oxidation of Several 3-Substituted
Cyclobutanones Using 1a as the Chiral Sourcea
4a: R= CH2Ph
4b: R= t-Bu
5
Entry 3-Substituent (R)
Time (h) Yield
% ee
73b
73b
83b
60e
Confign.
Figure 2
1
2
3
4
p-Chlorophenyl
208
76
52
94
65
Rc
d
Table 2 Optimization of the Reaction Conditions for Asymmetric
B–V Oxidation of 3-Phenylcyclobutanone Using Palladium(II)-1a
Hexafluoroantimonate Complex as the Catalyst
p-Methoxyphenyl 208
–
d
2-Naphthyl
211
211
–
d
Entry Solvent
Temp (°C) Time (h) Yield (%) % eea
n-Octyl
–
a Reactions were carried out in THF at –60 °C using UHP as a terminal
oxidant.
1
2
CH2Cl2
1,4-Dioxane
Et2O
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
–40
rt
10
19
18
100
95
50
47
44
53
51
51
48
57
73
59
73
b Enantiomeric excess was determined by HPLC analysis using chiral
stationary phase column (Daicel Chiralpak AD-H; hexane–i-PrOH,
95:5).
3
94
c Configuration was determined by comparison of the elution order
with the authentic samples (ref.7c).
4
AcOEt
EtOH
1.5
0.5
19
23
0.5
100
100
100
98
d Configuration has not been determined.
5
e Enantiomeric excess was determined by HPLC analysis using chiral
stationary phase column (Daicel Chiralcel OD-H; hexane–i-PrOH,
90:10) after converted into the corresponding g-hydroxy benzylamide
according to the reported procedure (ref.7c).
6
Acetone
DMF
7
8
DME
100
93
The B–V oxidation of tricyclic cyclobutanone was also
examined and excellent enantioselectivity of >99% ee
was achieved.
9
DME
46
23
18
10
11
THF
92
THF
–20
100
O
O
O
Palladium 1a complex (5 mol%), UHP
H
H
H
H
7
H
H
1
12
13
14
THF
THF
THF
–40
–60
–80
24
100
91
78
80
60
10
THF, –40 °C, 44 h
4
214
186
H
H
98
(1S, 4R, 7R, 10S)
a Enantiomeric excess was determined by HPLC analysis using chiral
stationary phase column (Daicel Chiralpak AD-H; hexane–i-PrOH,
95:5).
89%, >99% ee
Scheme 4
In conclusion, we were able to demonstrate that 2-(phos-
phinophenyl)pyridine 1a was an effective chiral control-
ler for palladium(II)-catalyzed B–V oxidation, which
possibly proceeds through a square planar palladium(II)-
Criegee adduct complex. To the best of our knowledge,
this is the first success example of the B–V oxidation us-
ing chiral palladium(II) complex as catalyst. Further stud-
ies on metal-mediated B–V oxidation are underway in our
laboratory.
With these results at hand, we next attempted the optimi-
zation of the reaction conditions for the B–V oxidation us-
ing palladium(II)-1a hexafluoroantimonate complex as
the chiral catalyst (Table 2). Although strong solvent ef-
fect on enantioselectivity was not observed (entries 1–10),
use of 1,2-dimethoxyethane (DME) or tetrahydrofuran
(THF) enhanced enantioselectivity up to 57% and 59%
ees, respectively (entries 8 and 10). Lowering reaction
temperature further improved enantioselectivity (entries
Synlett 2003, No. 5, 643–646 ISSN 0936-5214 © Thieme Stuttgart · New York

646
K. Ito et al.
LETTER
University Press: Oxford, 2001, 214. (h) Mihovilovic, M.
D.; Muller, B.; Stanetty, P. Eur. J. Org. Chem. 2002, 3711.
(7) (a) Uchida, T.; Katsuki, T. Tetrahedron Lett. 2001, 42,
6911. (b) Watanabe, A.; Uchida, T.; Ito, K.; Katsuki, T.
Tetrahedron Lett. 2002, 43, 4481. (c) Uchida, T.; Katsuki,
T.; Ito, K.; Akashi, S.; Ishii, A.; Kuroda, T. Helv. Chim.
Acta. 2002, 85, 3078.
References
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(11) Typical experimental procedure
To a solution of bis(benzonitrile)palladium(II) chloride (1.9
mg, 5.0 mol) in THF (0.4 mL) was added ligand 1a (2.3 mg,
5.5 mol) under nitrogen and stirred at room temperature for
1 h. Silver hexafluoroantimonate (3.4 mg, 10 mol) was
placed another flask under nitrogen and to this flask was
added the above palladium(II)-1a dichloride solution. After
being stirred for 1 h at room temperature, the mixture was
filtered through a pad of Celite under nitrogen. To the filtrate
was added 3-phenylcyclobutanone (15.2 mg, 0.1 mmol) and
then cooled to –60 °C. To the cooled solution was added
UHP (12.2 mg, 0.13 mmol) and the mixture was further
stirred at the temperature for 214 h. The mixture was
concentrated and the residue was chromatographed on silica
gel (hexane–EtOAc, 9:1) to give dihydro-4-phenylfuran-
2(3H)-one (14.8 mg, 91%). The enantiomeric excess of the
product was determined to be 80% ee by HPLC analysis
using chiral stationary phase column (Daicel Chiralpak AD-
H; hexane–i-PrOH, 95:5).
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Synlett 2003, No. 5, 643–646 ISSN 0936-5214 © Thieme Stuttgart · New York