Enantioselective oxidation of olefins catalyzed by chiral copper bis(oxazolinyl)pyridine complexes: A reassessment

Article abstract of DOI:10.1016/j.tet.2006.01.093

Copper complexes of chiral tridentate pybox ligands synthesized using a modified procedure have been studied as catalysts for the enantioselective allylic oxidation of olefins. A variety of olefins have been used in this reaction. Using 5 mol% of a Cu(II)

Full text of DOI:10.1016/j.tet.2006.01.093

Tetrahedron 62 (2006) 3573–3581
Enantioselective oxidation of olefins catalyzed by chiral copper
bis(oxazolinyl)pyridine complexes: a reassessment
*
Sandeep K. Ginotra and Vinod K. Singh
Department of Chemistry, Indian Institute of Technology, Kanpur, UP 208 016, India
Received 23 October 2005; revised 15 December 2005; accepted 26 January 2006
Available online 24 February 2006
Abstract—Copper complexes of chiral tridentate pybox ligands synthesized using a modified procedure have been studied as catalysts for
the enantioselective allylic oxidation of olefins. A variety of olefins have been used in this reaction. Using 5 mol% of a Cu(II) complex of the
tridentate pybox ligand, phenylhydrazine, and tert-butyl perbenzoate as oxidant in acetone, optically active allylic benzoates were obtained
up to 94% ee in few hours. It was also observed that the use of molecular sieves in the reaction did not alter the enantioselectivity.
Temperature was found to be very crucial in rate of the enantioselective allylic oxidation of olefins. Using EPR spectra, it has been shown that
the Cu(II) species is reduced to Cu(I) by phenylhydrazine and phenylhydrazone, but the reduction with the former is faster in comparison to
the latter. It was concluded that the rate enhancement was not specific to the presence of phenylhydrazine or phenylhydrazone, but both were
equally responsible provided acetone was used as a solvent.
q 2006 Elsevier Ltd. All rights reserved.
1. Introduction
copper(II) complex of pyridine 2,6-bis(40-isopropyl-50,50-
diphenyloxazoline) 1a (hence forth this is referred to as ‘ip-
pybox-diph’ ligand) in acetone reduced reaction time to a
great extent without affecting the enantioselectivity
(Scheme 1).
Enantioselective allylic oxidation of olefins catalyzed by
chiral copper complexes continues to be an important area
in asymmetric synthesis.1,2 Early attempts with this reaction
using copper complexes of (C)-a-ethyl camphorate,3a
chiral Schiff bases,3b and L-amino acids4 gave poor
asymmetric induction (5–63% ee). Although important
contributions were made in this area by Pfaltz,5 Andrus,6
Katsuki,7 and Singh8 using peresters and t-butyl peroxide9
as oxidant with copper complexes of chiral bis and
trisoxazoline ligands,10 the longer reaction time was a
universal problem with all these reports. The enantioselec-
tivity using the oxazoline ligands had been reasonably good
(60–93% ee), but the longer reaction time (up to a month)
precluded broader use of these methods in applied synthesis.
We later discovered that the use of phenylhydrazine with
The concept of using phenylhydrazine in the allylic
oxidation of olefins catalyzed by chiral bipyridine–copper
complexes has also been used by other researchers in this
field.11 The reaction was complete in few hours, but the
asymmetric induction was moderate (55% ee in the case of
cyclohexene and 75% ee in the case of cycloheptene).11a
While working on this and related asymmetric reactions
using the 1a, we discovered some discrepancy in the
physical data of this ligand. After some time, we could
detect and solve the problem (vide infra). It was also
discovered that the use of the ‘ip-pybox-diph’ ligand (1a)
OCOPh
1a-Cu(OTf)2, PhNHNH2, PhCO3t-Bu,
acetone, 4Å MS, rt, 24 h
O
S
O
Ph
Ph
N
Ph
Ph
S
S
N
N
73% yield
75% ee
1a
Scheme 1.
Keywords: Enantioselective allylic oxidation; Olefins; Phenylhydrazine; Phenylhydrazone.
*
Corresponding author. Tel.: C91 512 259 7291; fax: C91 512 259 7436; e-mail: vinodks@iitk.ac.in
0040–4020/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.01.093

3574
S. K. Ginotra, V. K. Singh / Tetrahedron 62 (2006) 3573–3581
Ph
Ph
S
O
N
a, X = N; R = Ph
b, X = N; R = Et
c, X = N; R = H
d, X = C; R = Ph
e, X = C; R = H
O
S
O
R
R
X
R
O
S
O
S
N
N
R
S
N
N
O
S
O
R
R
R
S
N
N
1
R
3
2
Figure 1.
MeSO3H, CH2Cl2,
0 ˚C to rt, 7 h
Et3N, CH2Cl2,
0 ˚C to rt, 24 h
O
O
OH
R
HO
N
R
O
O
+
R
R
1a-b
N
R
R
NH
HN
H2N
OH
4a-b
Cl
Cl
5
6a-b
Scheme 2.
Ph Ph
HO
COCl
NH
O
MeSO3H, CH2Cl2,
0 ˚C to rt, 7 h
Et3N, CH2Cl2,
0 ˚C to rt, 24 h
O
O
2
4a
+
O
O
OH
Ph
Ph
HO
Cl
Cl
Ph
Ph
NH
HN
7
Scheme 3.
from a new procedure gave 93% ee as opposed to a 75% ee
reported earlier in the enantioselective allylic oxidation of
cyclohexene.8b To our delight, the reaction was complete in
1 h (67% yield, 91% ee) not 24 h (73% yield, 75% ee;). In
view of this remarkable result, we extended our study
further and report our full results in this paper.
of cyclohexene with tert-butyl perbenzoate in acetone. The
reaction was complete in 1 h and the (S)-allylic benzoate
was obtained in 67% yield and 91% ee (Table 1, entry 1).
The ip-pybox-dieth ligand 1b was equally efficient, but the
enantioselectivity was slightly lower (85% ee). Under the
identical condition, ligand 1c13 gave 65% ee (Table 1, entry
3). We could also reproduce Pfaltz’s result where 71% ee
was reported with this ligand in MeCN.5 As reported earlier,
ligand 1d8b gave racemic product. Along the same line, it
was expected that 1e would not give any enantioselection
It was planned to study the general role of phenylhydrazine
in the enantioselective oxidation of olefins using the ligand
1a (Fig. 1), which was prepared using a procedure
previously reported.12 During the synthesis of 1a, it was
observed that there is an overlapping spot with higher Rf
seen on a silica gel tlc plate as an impurity. In a normal way,
these were seen as a single spot under UV light, but the
impurity becomes clear only when the tlc plate was seen
under UV light after exposure to iodine vapour. The
impurity could be removed only when the compound was
recrystallized from ether. It was observed that the impurity
could be minimized if the cyclization of 6a with
methanesulfonic acid was done at rt (Scheme 2). Ligand
1b was prepared in the same fashion (Scheme 2). A new
chiral ligand 2 was also prepared using the above procedure
from (S)-diphenylvalinol 4a and the corresponding acid
chloride (Scheme 3).
Table 1. Enantioselective allylic oxidation of cyclohexene with different
ligands
OCOPh
L*, Cu(OTf)2, PhNHNH2,
PhCO3t-Bu, acetone, 25 ˚C
S
Entry
Ligand (L*)a
Time (h)
Yield (%)
ee (%)
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
2
01
07
18
02
15
05
37
67
66
85
50
59
42
58
91
85
65b
00
00
00
00
3
a 5 mol% of the chiral ligand was used.
In our initial study, 5 mol% complex of the newly prepared
ligand (S)-1a and Cu(OTf)2, in conjunction with phenyl-
hydrazine, was found to be effective for the allylic oxidation
b The 1c is known (Ref. 5) to give 71% ee under other condition
(1c–CuOTf, MeCN, 3 days).

S. K. Ginotra, V. K. Singh / Tetrahedron 62 (2006) 3573–3581
3575
and this was indeed the case (Table 1, entry 5). These results
indicated that the pyridine nitrogen is important in chelation
with copper in these systems. The results (Table 1, entries 6
and 7) from the ligands 2 and 3 support this hypothesis.
to this reaction, its effect was also studied. To our surprise, it
was found out that as such there was no effect of molecular
sieves on chemical and optical yield of this reaction
(Table 2, entry 7).
As established earlier by us, phenylhydrazine plays an
important role in increasing the rate of the allylic oxidation
of olefins with tert-butyl perbenzoate catalyzed by copper
triflate in acetone.8a In order to ascertain its role with
various Cu salt, the reaction was investigated in different
solvents and the results are summarized in Table 3. We
already know that the high enantioselectivity is obtained by
a complex of 1a with Cu(I) species and not by Cu(II). Thus,
1a–CuOTf complex gave 91% ee and the reaction took 39 h
for completion in acetone (Table 3, entry 2). In the presence
of phenylhydrazine, the same reaction was complete in only
9 h without affecting the enantioselectivity (Table 3, entry
1). If the Cu(I) species was prepared in situ by reduction of
1a–Cu(OTf)2 with phenylhydrazine in acetone, the reaction
was complete in 1 h with 91% ee (Table 3, entry 12). On
changing the solvent to acetonitrile using Cu(I) complex and
phenylhydrazine, the reaction time was not affected as it
took 11–13 days to reach completion with slightly lower
optical yield (Table 3, entries 7 and 8). As observed above,
if a complex of 1a–Cu(OTf)2 and phenylhydrazine were
used in acetonitrile, where Cu(I) species were prepared
in situ by reduction, the reaction time was reduced to
3–4 days without affecting the ee (Table 3, entry 9). A
similar trend was observed in benzene, except the
enantioselectivity was poor (Table 3, entries 10 and 11).
Invariably, lower enantioselectivity (47–60% ee) was
obtained when benzene was used as a solvent in this
reaction. From the results listed in Table 3, it was concluded
that the Cu(I) species prepared by reduction of 1a–Cu(OTf)2
with phenylhydrazine in acetone is more efficient than use
of Cu(I) salt directly in allylic oxidation of olefins.
Having achieved good results in the above reaction at rt with
ligand 1a, the reaction at different temperature was studied
with the hope to increase the enantioselectivity. The
reaction was very sluggish at K20 8C and we could isolate
only 3% of the product with the same level of enantio-
selectivity in 33 days (Table 2, entry 1). Although the
reaction took a little longer time (11–29 h) for completion at
0 and 10 8C, the optical yield of the product was slightly
better (up to 93% ee), although the chemical yield was
similar (Table 2, entries 2 and 3). It was also observed that if
the reaction was initiated at 0 8C and then warmed to rt, the
optical activity of the product was similar (92% ee) but the
reaction time was 20 h (Table 2, entry 4). Thus, it was
concluded that 20–25 8C was the optimal temperature for
this reaction. In order to see whether the reaction was
moisture sensitive, the reaction was carried out in the
presence of water (6 mol%) at 25 8C. To our delight, there
was no erosion in chemical nor optical yield, however, the
reaction time increased from 1 h (67% yield, 91% ee) to
11 h (74% yield, 90% ee) (Table 2, entry 6). In view of our
˚
earlier observation that 4 A molecular sieves are beneficial
Table 2. Effect of temperature and additive on allylic oxidation
OCOPh
1aa, Cu(OTf)2, PhNHNH2,
PhCO3t-Bu, acetone
S
Entry
Additive
Temperature
(8C)
Time
Yield
(%)
ee (%)
1
2
3
4
5
6
7
Nil
Nil
Nil
Nil
Nil
H2Ob
˚
4 A MS
K20
0
10
0 to 25
25
25
33 days
29 h
11 h
20 h
01 h
03
67
62
75
67
79
74
90
93
91
92
91
90
91
In order to determine the optimal amount of catalyst in the
allylic oxidation of cyclohexene, the reaction was tried with
varying amounts of the copper complex (Table 4). Initially,
the reaction was tried with 10 mol% of the complex using
10 equiv of cyclohexene (Table 4, entry 1). The reaction
was complete in 1 h with high yield (86%) and
11 h
04 h
25
a
5 mol% of the complex was used.
6 mol% of water added.
b
Table 3. Effect of solvent, PhNHNH2, and Cu salt on the reaction
OCOPh
1a-Cu salt,a PhNHNH2,
PhCO3t-Bu, acetone, 25 ˚C
S
Entry
Solvent
PhNHNH2
Cu salt
Time
Yield (%)
ee (%)
1
2
3
4
5
6
7
8
Acetone
Acetone
Acetone
Acetone
MeCN
MeCN
MeCN
MeCN
MeCN
Yes
No
Yes
No
Yes
No
Yes
No
CuOTf$PhH
CuOTf$PhH
[Cu(MeCN)4]PF6
[Cu(MeCN)4]PF6
CuOTf$PhH
09 h
39 h
11 h
43 h
11 days
13 days
10 days
10 days
88 h
22 h
06 days
01 h
73
67
62
90
79
88
75
88
50
59
61
67
91
91
92
91
89
88
86
88
86
60
48
91
CuOTf$PhH
[Cu(MeCN)4]PF6
[Cu(MeCN)4]PF6
Cu(OTf)2
[Cu(MeCN)4]PF6
[Cu(MeCN)4]PF6
Cu(OTf)2
9
Yes
Yes
No
10
11
12
Benzene
Benzene
Acetone
Yes
a5 mol% of the complex was used in the reaction.

3576
S. K. Ginotra, V. K. Singh / Tetrahedron 62 (2006) 3573–3581
Table 4. Effect of amount of the catalyst on the reactiona
OCOPh
1a-Cu(OTf)2, PhNHNH2,
PhCO3t-Bu, acetone, 25˚C
S
Entry
1a–Cu(OTf)2
(mol%)
Time (h)
Yield (%)
ee (%)
1
2
3
4
5
6
7
10
10
05
05
05
2.5
01
01
03
01
11
14
08
39
86
79
67
56
66
82
91
91
91b
91
91c
92d
91
91
Figure 2. EPR spectrum study with phenylhydrazine in acetone. (a) EPR
spectrum of 1a–Cu(OTf)2 in acetone at rt. (b) PhNHNH2 was added
immediately and EPR spectrum was run. (c) EPR spectrum was run after
15 min.
a Usually 10 equiv of cyclohexene was used in all the reactions unless
stated otherwise.
b 5 equiv of cyclohexene was used.
c 2 equiv of cyclohexene used.
d After disappearance of PhCO3t-Bu, 1 equiv of this was added again.
enantioselectivity (91% ee). By reducing the amount of the
catalyst to 5 mol%, there was no change in the reaction rate
(Table 4, entry 3). On further reduction of the amount of the
catalyst to 2.5 and 1 mol%, the similar chemical and optical
yields were obtained, but the reaction time was 8 and 39 h,
respectively (Table 4, entries 6 and 7). Normally, in all the
above reactions, 10 equiv of olefins with respect to tert-
butyl perbenzoate were used. By reducing the amount of
olefin to 5 and 2 equiv, the reaction time was little increased
slightly (3 and 11 h), but there was no change in the
enantioselectivity (Table 4, entries 2 and 4). Since the
reaction was monitored by the disappearance of the perester,
an extra equivalent of the perester was added after a tlc
indicated disappearance of the 1st equivalent of the perester,
but there was no affect on the reaction (Table 4, entry 5).
Figure 3. EPR spectrum study with phenylhydrazone in acetone. (a) EPR
spectrum of 1a–Cu(OTf)2 in acetone at rt. (b) PhNHN]CMe2 was added
and EPR spectrum was run after 30 min. (c) EPR spectrum was run after
1.5 h. (d) EPR was run after 3 h.
Initially, phenylhydrazine was used in the reaction for
reducing the Cu(II) complex of the ligand to Cu(I).14 Since
the rate enhancement was much higher when acetone was
used as a solvent, it was assumed that phenylhydrazone
might be responsible for enhancing the rate. It was further
found that phenylhydrazine and phenylhydrazone were both
reducing Cu(II) to Cu(I) species (green to dark brown), but
the rate enhancement in the reaction could be mainly due to
phenylhydrazone formed in situ from phenylhydrazine and
acetone. In order to confirm and determine the extent of
reduction, EPR spectra were run. EPR study of the complex
1a–Cu(OTf)2 with phenylhydrazine in acetone indicated
that the reduction required 15 min (Fig. 2). For the same
reduction, phenylhydrazone took 3 h (Fig. 3). So, it was
concluded that phenylhydrazone also reduced Cu(II) to a
Cu(I) species, but the reduction process was slower. This is
also clear from the results shown in Table 5. The reaction
was catalyzed by a Cu(II) complex containing chiral ligand
1a, but the enantioselectivity was not good (Table 5, entry
1). By addition of phenylhydrazine, the reaction time was
reduced from 7 days to 1 h and the enantioselectivity was
enhanced from 62 to 91% (Table 5, entry 2).
of phenylhydrazone was added to 1a–Cu(OTf)2 and
stirred for 0.5, 1, 2 and 3.5 h before adding cyclohexene
and perester. As the reduction time of the Cu(II)
complex was increased from 0.5 to 1 h, the time for
completion of the reaction was reduced from 24 to 8 h
and there was very little improvement in the enantios-
electivity (Table 5, entries 3 and 4). However, there was
not much difference on increasing the reduction time to
2–3.5 h (Table 5, entries 5 and 6). This was in accord
with the above fact from EPR spectra that the reduction
of the Cu(II) complex with phenylhydrazone was slower.
Reaction in acetone indicated that phenylhydrazine and
phenylhydrazone both were increasing the rate of the
allylic oxidation reaction, but the former was more
efficient (Table 5, entry 2; 1 h reaction time) than the
latter (Table 5, entry 6; 3 h reaction time). However, on
changing the solvent to acetonitrile, the reaction was
slower in both cases (Table 5, entries 9 and 10) and
there was no difference in the rate (reaction time 88 and
74 h). Thus, it was concluded that the rate enhancement
was not specific to the presence of phenylhydrazine or
phenylhydrazone, but both were equally responsible
provided acetone was used as a solvent. 2,4-Dinitrophe-
nylhydrazine and its corresponding hydrazone had no
effect on the reaction rate (35 days). This observation is
more obvious as the electron transfer rate will be much
more slower due to the nitro-substitution.
Assuming that phenylhydrazone formed in situ in
the above reaction was responsible for the rate
enhancement in the reaction, it was used in place of
phenylhydrazine. Thus, an equivalent amount (5–6 mol%)

S. K. Ginotra, V. K. Singh / Tetrahedron 62 (2006) 3573–3581
Table 5. Effect of reducing agents on the reactiona
3577
OCOPh
S
1a-Cu(OTf)2,a PhCO3t-Bu, 25 ˚C
Entry
Reducing agent
Solvent
Time
yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
10
Nil
PhNHNH2
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
CH2Cl2
Benzene
Acetonitrile
Acetonitrile
07 days
01 h
24 h
08 h
08 h
03 h
18 h
76 h
88 h
74 h
35
67
64
67
69
62
75
65
50
51
62
91
89
PhNHN]CMe2
PhNHN]CMe2
PhNHN]CMe2
PhNHN]CMe2
PhNHN]CMe2
PhNHN]CMe2
PhNHNH2
90b
90c
90d
66
40
86
89
PhNHN]CMe2
O2N
NHNH2
NO2
NHN CMe2
NO2
11
12
Acetone
Acetone
35 days
35 days
40
41
78
66
O2N
a Normally the reaction mixture (RM) was stirred for 30 min after the addition of a reducing agent unless stated otherwise (viz. b,c, and d).
b The RM was stirred for 1 h.
c The RM was stirred for 2 h.
d The RM was stirred for 3.5 h.
Having established optimal conditions for enantioselective
allylic oxidation of cyclohexene, the reaction was extended
to other olefins with tert-butyl perbenzoate using chiral
ligands 1a and 1b. The results are summarized in Table 6.
Cyclopentene and cycloheptene gave a maximum of 70 and
86% ee, respectively. Although cyclooctene gave the
highest enantioselectivity (94% ee), cyclooctadiene gave
only 80% ee.
Although a high ee was obtained for cyclic olefins, acyclic
olefins gave poor selectivity (Scheme 4). During the
oxidation of allylic benzene, 60% yield of a mixture of
benzoates (8a and 9a) with a ratio of 47:53 were obtained.
The optical purity of 8a was 40%. 1-Octene gave a poor
yield of products (8b and 9b) in a ratio of 3:2.
Enantioselectivity in this case was very poor (27% ee).
The allylic oxidation was also attempted on 1-methylcy-
clohexene (Scheme 5). Under optimal conditions, 46% yield
of a mixture of three products (10a, 10b, and 10c) was
obtained. Among these, we could determine the enantio-
selectivity only in case of 10a (7% ee) and 10c (48% ee).
Although 10b was a major product, its enantiomeric excess
could not be determined.
Table 6. Enantioselective allylic oxidation of different olefins with tert-
butyl perbenzoatea
L-Cu(OTf)2, PhNHNH2,
OCOPh
PhCO3t-Bu, acetone, 25 ˚C
S
n
n
Entry
Olefin
L
Time
Yield
(%)
ee (%)
1
2
1a
1b
01 h
07 h
67
66
91
85
3
4
1a
1b
03 h
24 h
76
58
70
65
The transition state model based on p-stacking with the
ligand 1a has been proposed earlier for this reaction.8b The
results detailed in this paper also support the transition state
model proposed earlier. There is an enhancement
of enantioselectivity from 65 to 91% from the ligand
ip-pybox 1c to ip-pybox-diph 1a (Table 1, entries 1 and 3).
This does suggest that the gem-diphenyl groups play an
important role in stabilizing the transition state, and we feel
that it is primarily due to p-stacking.15 gem-Diethyl groups
also enhanced the ee to 85% (Table 1, entry 2), but its effect
is less than that of gem-diphenyl groups. The effect from
5
6
1a
1b
06 h
48 h
47
37
86
60
7
8
1a
1b
06 h
06 d
34
46
94
71
9
10
1a
1b
05 d
05 d
31
40
80
71
a The ee was determined by HPLC on chiral columns as mentioned in
Section 2.
1a-Cu(OTf)2, PhNHNH2,
PhCO3t-Bu, acetone, rt
+
a, R = Ph; time = 11 h; yield = 60%; ratio of
8a & 9a = 47 : 53; 8a = 40% ee
OCOPh
OCOPh
R
R
R
b, R = C5H11; time = 14 d; yield = 24%;
ratioof 8b & 9b = 3 : 2; 8b = 27% ee
8
9
Scheme 4. Enantioselective allylic oxidation of acyclic olefins.

3578
S. K. Ginotra, V. K. Singh / Tetrahedron 62 (2006) 3573–3581
Me
Me
Me
Me OCOPh
1a-Cu(OTf)2, PhNHNH2,
PhCO3t-Bu, acetone, rt, 1 h
PhOCO
+
+
46%
OCOPh
10a
10b
10c
26%
11%
62%
7%ee
48% ee
Scheme 5. Enantioselective oxidation of 1-methylcyclohexene.
diphenyl to diethyl groups in the chiral ligand is quite
consistent in all the olefins studied (Table 6).
2.1.2. N,N0-Bis[10-(S)-isopropyl-2020-diethyl-20-hydroxy-
ethyl]-2,6-pyridine-dicarboxamide (6b). This was pre-
pared as per our procedure. But, the purification was done
by column chromatography over silica gel followed by
recrystallization from CH2Cl2/n-hexane. Yield 50%; mp
141–142 8C; Rf 0.37 (1:1, EtOAc in petroleum ether); [a]D25
In conclusion, we have investigated enantioselective allylic
oxidation of olefins with a copper complex of pybox ligands
with tert-butyl perbenzoate. We have shown that the
reaction time is reduced drastically by using phenyl-
hydrazine in acetone as a solvent. It was also confirmed
that phenylhydrazone formed in situ is also responsible for
the same effect. It was concluded that the shorted reaction
time was not specific to the presence of phenylhydrazine or
phenylhydrazone, but both were equally responsible,
provided acetone was used as a solvent. Under the optimal
conditions using ‘ip-pybox-diph’ ligand (S,S)-1a, we
were able to convert olefins into (S)-allylic benzoate in up
to 94% ee.
1
K8.2 (c 1, CHCl3); [lit.8a [a]D25 K3.6 (c 0.6, CHCl3)]. H
NMR (CDCl3, 400 MHz) d 0.82 (d, JZ7.6 Hz, 6H), 0.90
(d, JZ7.6 Hz, 6H), 0.96 (d, JZ6.8 Hz, 6H), 0.99 (d, JZ
6.8 Hz, 6H), 1.45–1.70 (m, 8H), 2.21 (m, 2H), 4.02 (dd,
JZ10.3, 1.9 Hz, 2H), 8.03 (t, JZ7.8 Hz, 1H), 8.27 (d, JZ
10.0 Hz, 2H), 8.32 (d, JZ7.8 Hz, 2H).
2.2. General procedure for cyclization of amido alcohols
(6) to ip-Pybox-diph (1a) and ip-Pybox-dieth (1b)
Methanesulfonic acid (30 mmol) was added dropwise to a
solution of amido alcohol 6 (5 mmol) in CH2Cl2 (120 mL)
at 0 8C over a period of approximately 10 min and the
reaction mixture was stirred for 7 h in the case of 1a and
18 h in the case of 1b and during the process the cooling
bath warmed to rt (0 to 25 8C). It was diluted with CH2Cl2
(100 mL) and washed with aqueous NaHCO3, water
and brine. The organic layer was dried over anhydrous
NaSO4 and the solvent was evaporated in vacuo to afford 1
as off-white solid, which was purified by column chromato-
graphy and recrystallization.
2. Experimental
2.1. General methods
1H NMR spectra were recorded on 400 MHz spectrometer.
Chemical shifts are expressed in ppm downfield from TMS
as internal standard, and coupling constants are reported in
Hz. Routine monitoring of reactions were performed by
TLC, using precoated silica gel TLC plates obtained by
E-Merck. All the column chromatographic separations were
done by using silica gel (Acme’s, 60–120 mesh). Petroleum
ether used was of boiling range 60–80 8C. Reactions that
needed anhydrous conditions were run under the atmos-
phere of nitrogen or argon using flame-dried glassware. The
organic extracts were dried over anhydrous sodium sulfate.
Evaporations of solvents was performed at reduced
pressure. Tetrahydrofuran (THF) was distilled from sodium
benzophenone ketyl under nitrogen. Benzene, dichloro-
methane, acetonitrile and acetone were distilled from CaH2.
2.2.1. 2,6-Bis[50,50-diphenyl-40-(S)-isopropyloxazolin-20-
yl]pyridine (1a).8 This was prepared from 6a using a
general procedure mentioned above and then recrystallized
with Et2O to afford the product 1a as a white crystal: yield
65%; mp 161–163 8C (lit.8b mp 65–66 8C); Rf 0.37 (1:2,
EtOAc in petroleum ether); [a]2D5 K386.2 (c 1.1, CHCl3)
[lit.8b [a]2D5 K233.0 (c 2.7, CHCl3)]. 1H NMR (CDCl3,
400 MHz) d 0.67 (d, JZ6.6 Hz, 6H), 1.06 (d, JZ6.8 Hz,
6H), 1.93 (m, 2H), 4.87 (d, JZ4.6 Hz, 2H), 7.21–7.30 (m,
9H), 7.34–7.42 (m, 7H), 7.66 (d, JZ6.1 Hz, 4H), 7.89 (t,
JZ6.0 Hz, 1H), 8.22 (d, JZ7.8 Hz, 2H); 13C NMR (CDCl3,
100 MHz) 17.3, 21.9, 30.3, 80.5, 93.5, 125.6, 126.3, 127.0,
127.3, 127.7, 127.8, 128.3, 137.4, 140.5, 145.2, 147.2,
160.6; MS (FAB, m/z) 606 (MCC1).
2.1.1.
N,N0-Bis[10-(S)-isopropyl-2020-diphenyl-20-
hydroxyethyl]-2,6-pyridine-dicarboxamide (6a).8b This
was prepared as per our procedure. But, the purification was
done by column chromatography over silica gel followed by
recrystallization from CH2Cl2/n-hexane. Yield 95%; mp
220–221 8C (lit.8b mp 110–111 8C); Rf 0.27 (1:2, EtOAc in
petroleum ether); [a]2D51 K50.3 (c 1, CHCl3) [lit.8b [a]2D5
K46.2 (c 1, CHCl3)]. H NMR (DMSO-d6, 400 MHz) d
0.83 (d, JZ6.8 Hz, 6H), 1.08 (d, JZ6.6 Hz, 6H), 1.81 (m,
2H), 2.49 (s, 2H, OH), 5.02 (d, JZ10.7 Hz, 2H), 6.07(s, 2H,
NH) 7.02 (t, JZ7.3 Hz, 2H), 7.11–7.19 (m, 6H), 7.32 (t, JZ
7.6 Hz, 4H), 7.52–7.55 (m, 8H), 7.99–8.08 (m, 1H), 8.26 (d,
JZ10.7 Hz, 2H); MS (FAB, m/z) 642 (MCC1).
2.2.2. 2,6-Bis[50,50-diethyl-40-(S)-isopropyloxazolin-20-
yl]pyridine (1b).8a This was prepared from 6b using the
above general procedure: yield 85%; mp 82–84 8C; Rf 0.4
(2:3, EtOAc in petroleum ether); [a]2D5 K36.4 (c 1.1,
CHCl3) [lit.8b [a]D25 K37.1 (c 1.4, CHCl3)]. 1H NMR
(CDCl3, 400 MHz) d 0.95 (t, JZ7.6 Hz, 6H), 1.02–1.06 (m,
12H), 1.16 (d, JZ6.6 Hz, 6H) 1.70 (m, 4H), 1.86–2.0 (m,
6H), 3.74 (d, JZ7.8 Hz, 2H), 7.82 (t, JZ7.8 Hz, 1H), 8.03
(d, JZ7.8 Hz, 2H).

S. K. Ginotra, V. K. Singh / Tetrahedron 62 (2006) 3573–3581
3579
2.2.3. N,N0,N00-Tris[10-(S)-isopropyl-2020-diphenyl-20-
hydroxyethyl]-1,3,5-benzene-tricarboxamide (7). A sol-
ution of 1,3,5-benzenetricarboxylic chloride (4.8 mL, 1.0 M
solution in CH2Cl2) was added slowly to a mixture of (S)-
(K)-2-amino-3-methyl-1,1-diphenylbutan-1-ol 4a8 (3.63 g,
14.2 mmol), Et3N (5 mL, 36 mmol) and CH2Cl2 (50 mL) at
0 8C over a period of 15 min and the mixture was stirred for
24 h (0 8C to rt). The reaction mixture was diluted with
CH2Cl2 (50 mL) and washed with aqueous NaHCO3, water,
and brine. The organic layer was dried over anhydrous
NaSO4, and the solvent was evaporated in vacuo to afford
white solid, which was purified by column chromatography
over silica gel and recrystallised from CH2Cl2/hexane to get
pure product 7 (3.10 g, yield 80%): mp 178–180 8C; Rf 0.22
(1:4, EtOAc in petroleum ether); [a]2D5 K82.0 (c 2, CHCl3).
1H NMR (CDCl3, 400 MHz) d 0.93 (d, JZ6.8 Hz, 9H), 0.97
(d, JZ6.8 Hz, 9H), 1.96 (m, 3H), 6.74 (d, JZ10 Hz, 3H),
7.09 (t, JZ7.6 Hz, 3H), 7.19–7.26 (m, 11H), 7.35 (t, JZ
7.6 Hz, 6H), 7.51 (t, JZ7.6 Hz, 13H), 7.88 (s, 3H); 13C
NMR (CDCl3, 100 MHz) 18.0, 22.9, 29.1, 58.9, 76.7, 77.0,
77.3, 82.2, 125.2, 125.3, 126.9, 127.8, 128.3, 128.4, 135.4,
145.2, 146.0, 166.3. Anal. Calcd for C60H63N3O6: C, 78.15;
H, 6.89; N, 4.56. Found: C, 78.0; H, 6.97; N, 4.48.
left at rt until the reaction was complete (disappearance of
perester by TLC) during which time the color of the reaction
mixture again changed (dark brown for 1a and 1b; light
brown for 1c; green for 1d; yellow for 1e and 3; fluorescent
reddish brown for 2). After the reaction was over, the
solvent was removed in vacuo and the crude product was
purified by column chromatography using silica gel.
2.4. General procedure for enantioselective allylic
oxidation of cyclohexene with tert-butyl perbenzoate
using a complex of chiral ligand (1a) and (CuOTf)2$PhH
or [Cu(CH3CN)4]PF6
A solution of 1a (0.06 mmol) and Cu(I) salt (0.05 mmol) in
appropriate solvent (4 mL) was stirred at rt for 1 h. To this
brown solution was added cyclohexene (10 mmol), and the
mixture was stirred for 5 min. Then, tert-butyl perbenzoate
(1 mmol) was added dropwise under N2 atmosphere. After
few minutes, the color of the solution turned green. The
reaction mixture was left at rt until the reaction was
complete (disappearance of perester by TLC) during which
time the color of the reaction mixture turned dark brown
again. After the reaction was over, the solvent was removed
in vacuo and the crude product was purified by column
chromatography using silica gel.
2.2.4. 1,3,5-Tris[50,50-diphenyl-40-(S)-isopropyloxazolin-
20-yl]benzene (2). Methanesulfonicacid (571 mL, 8.8 mmol)
was added dropwise to a solution of amido alcohol 7 (900 mg,
0.98 mmol) inCH2Cl2 (40 mL) at0 8C overa periodof10 min
and the mixture was stirred for 18 h (0 8C to rt). The reaction
mixture was diluted with CH2Cl2 (30 mL) and washed with
aqueous NaHCO3, water and brine. The organic layer was
dried over anhydrous NaSO4 and the solvent was evaporated
in vacuo to afford 2 as a off-white solid, which was purified by
column chromatography (650 mg, yield 77%): mp 121–
124 8C; Rf 0.56(1:4, EtOAc inpetroleum ether); [a]2D5 K376.3
2.5. General procedure for enantioselective allylic
oxidation of cyclohexene with tert-butyl perbenzoate
using a complex of chiral ligand (1a) and (CuOTf)2$PhH
or [Cu(CH3CN)4]PF6 in the presence of PhNHNH2
A solution of a chiral ligand (0.06 mmol) and Cu(I) salt
(0.05 mmol) in appropriate solvent (4 mL) was stirred at rt
for 1 h. To this brown solution was added phenylhydrazine
(6 mL, 0.06 mmol), and the mixture was stirred for 30 min.
Then, an olefin (10 mmol) was added followed by a
dropwise addition of a perester (1 mmol) under N2
atmosphere. After few minutes, the color of the solution
started changing towards green. The reaction mixture was
left at rt until the reaction was complete (disappearance of
perester by TLC) during which time the color of the reaction
mixture turned dark brown again. After the reaction was
over, the solvent was removed in vacuo and the crude
product was purified by column chromatography using
silica gel.
1
(c 1, CHCl3). H NMR (CDCl3, 400 MHz) d 0.67 (d, JZ
6.6 Hz, 9H), 1.06 (d, JZ6.6 Hz, 9H), 1.87 (m, 3H), 4.85 (d,
JZ4.6 Hz, 3H), 7.23–7.38 (m, 24H), 7.59 (d, JZ7.6 Hz, 6H),
8.90 (s, 3H); 13C NMR (CDCl3, 100 MHz) 17.1, 21.9, 30.5,
80.2, 93.2, 126.3, 127.0, 127.3, 127.7, 127.8, 128.3, 128.9,
130.6, 140.5, 145.2, 160.4. Anal. Calcd for C60H57N3O3: C,
83.01; H, 6.62; N, 4.84. Found: C, 82.90; H, 6.71; N, 4.76.
2.3. General procedure for enantioselective allylic
oxidation of olefins with tert-butyl perbenzoate in the
presence of PhNHNH2 using a complex of chiral ligands
and Cu(OTf)2
2.6. General procedure for enantioselective allylic
oxidation of cyclohexene with tert-butyl perbenzoate
using (1a)–Cu(OTf)2 in the presence of hydrazones
A solution of a chiral ligand (0.06 mmol) and Cu(OTf)2
(0.05 mmol) in appropriate solvent (4 mL) was stirred at
appropriate temperature (see Tables) for 1 h. To this colored
(green for 1a; blue for 1b, 1c, 1d, 1e, and 3; colorless for 2,)
solution was added phenylhydrazine (6 mL, 0.06 mmol), and
the mixture was stirred for 30 min. During this time, the
color of the solution changed (dark brown for 1a and 1b;
light brown for 1c; yellow for 1d, 1e, and 3; fluorescent
orange for 2) giving an indication for reduction of Cu(II) to
Cu(I) species. Then, an olefin (2 mmol to 10 mmol
depending upon the experiment) was added followed by a
dropwise addition of a tert-butylperbenzoate (1 mmol)
under N2 atmosphere. After few minutes, the color of the
solution changed (green for 1a, 1b, 1c, 1d, 1e, and 3;
fluorescent reddish brown for 2). The reaction mixture was
A solution of a 1a (0.06 mmol) and Cu(OTf)2 (0.05 mmol)
in appropriate solvent (4 mL) was stirred at rt for 1 h. To this
green solution was added appropriate arylhydrazone
(0.06 mmol), and the mixture was stirred for appropriate
time (Table 5). During this time, the color of the solution
changed (brown for phenylhydrazone and yellow for 2,4-
dinitrophenylhydrazone) giving an indication for reduction
of Cu(II) to Cu(I) species. Then, an olefin (10 mmol) was
added followed by a dropwise addition of a perester
(1 mmol) under N2 atmosphere. After few minutes, the
color of the solution turned green in the case of
phenylhydrazone. The reaction mixture was left at rt until
the reaction was complete (disappearance of perester by

3580
S. K. Ginotra, V. K. Singh / Tetrahedron 62 (2006) 3573–3581
TLC) during which time the color of the reaction mixture
again changed to dark brown in the case of phenylhy-
drazone. After the reaction was over, the solvent was
removed in vacuo and the crude product was purified by
column chromatography using silica gel.
(R), 14.01 min (S). 1H NMR (CDCl3, 400 MHz) d 5.30 (td,
JZ10.5, 1.2 Hz, 1H), 5.40(td, JZ17.3, 1.2 Hz, 1H), 6.13 (s,
JZ5.8 Hz, 1H), 6.52 (d, JZ5.8 Hz, 1H), 7.29–7.47 (m, 8H),
8.10 (dd, JZ8.3, 1.2 Hz, 2H).
2.6.7. (S)-1-Octenyl-3-benzoate (8b).6a It was obtained in a
maximum of 27% ee. The optical purity was determined by
HPLC on chiralcel OD column [hexane/2-propanol
99.9:0.1]; flow rate 0.5 mL/min; tRZ20.91 min (R),
2.6.1. (S)-2-Cyclohexenyl-1-benzoate (Table 2, entry 2).8b
It was obtained in a maximum of 93% ee. The optical purity
was determined by HPLC on chiralpak AD-H column
[hexane/2-propanol 99.7:0.3]; flow rate 0.5 mL/min; tRZ
11.52 min (R), 12.63 min (S); [a]2D5 K167.2 (c 4.4, CHCl3)
1
22.78 min (S). H NMR (CDCl3, 400 MHz) d 0.88–1.84
(m, 11H), 5.20 (d, JZ6.5 Hz, 1H), 5.32(d, JZ17.0 Hz, 1H),
5.46–5.51 (m, 1H), 5.85–5.93 (m, 1H), 7.44 (t, JZ7.3 Hz,
2H), 7.55 (t, JZ7.3 Hz, 1H), 8.06 (d, JZ8.3 Hz, 2H).
[lit.8b (86% ee); [a]D25 K157.0 (c 0.45, CHCl3)]. H NMR
1
(CDCl3, 400 MHz) d 1.68–2.17 (m, 6H), 5.51 (m, 1H), 5.81–
5.85 (m, 1H), 5.99–6.03 (m, 1H), 7.43 (t, JZ7.8 Hz, 2H),
7.54 (t, JZ7.3 Hz, 1H), 8.06 (dd, JZ6.8, 1.3 Hz, 2H).
2.6.8. 1-Methyl-1-cyclohexen-3-yl benzoate (10c).5,9
It was obtained in a maximum of 47.6% ee. The optical
purity was determined by HPLC on chiralpak AD-H column
[hexane/2-propanol 99.7:0.3]; flow rate 0.5 mL/min;
tRZ11.71 min (minor), 12.19 min (major) d 1.77 (s,
3H), 1.46–2.06 (m, 6H), 5.50 (bs, 1H), 5.60 (bs, 1H), 7.42
(t, JZ7.6 Hz, 2H), 7.54 (t, JZ7.3 Hz, 1H), 8.05 (d, JZ
7.3 Hz, 2H).
2.6.2. (S)-2-Cyclopentenyl-1-benzoate (Table 6, entry
11).7b It was obtained in a maximum of 70% ee. The optical
purity was determined by HPLC on chiralcel OD column
[hexane/2-propanol 99.9:0.1]; flow rate 0.5 mL/min; tRZ
26.82 min (S), 32.35 min (R). [a]2D5 K136.2 (c 0.9, CHCl3)
[lit.7b (93% ee); [a]D25 K179.0 (c 0.37, CHCl3)]. H NMR
1
(CDCl3, 400 MHz) d 1.93–2.01 (m, 1H), 2.34–2.45 (m, 2H),
2.55–2.64 (m, 1H), 5.92–5.97 (m, 2H), 6.16 (p, 1H), 7.25–
7.44 (m, 2H), 7.53 (tt, JZ7.6, 1.2 Hz, 1H), 8.03 (m, 2H).
Acknowledgements
2.6.3. (S)-2-Cycloheptenyl-1-benzoate (Table 6, entry
14).8b It was obtained in a maximum of 86% ee. The optical
purity was determined by HPLC on chiralpak AD-H column
[hexane/2-propanol 99.7:0.3]; flow rate 0.5 mL/min; tRZ
15.70 min (R), 16.37 min (S); [a]2D5 K57.8 (c 0.6, CHCl3)
[lit.8b (82% ee) [a]D25 K38.2 (c 1, CHCl3)]. 1H NMR
(CDCl3, 400 MHz) d 1.43–2.30 (m, 8H), 5.65 (d, JZ9.8 Hz,
1H), 5.81–5.92 (m, 2H), 7.44 (t, JZ7.8 Hz, 2H), 7.55 (t, JZ
7.3 Hz, 1H), 8.06 (d, JZ7.3 Hz, 2H).
This work has been supported by a grant from the
Department of Science and Technology. S.K.G. thanks
CSIR for the award of a junior research fellowship.
References and notes
2.6.4. (S)-2-Cyclooctenyl-1-benzoate (Table 6, entry 17).8b
It was obtained in a maximum of 94% ee. The optical purity
was determined by HPLC on chiralpak AD-H column
[hexane/2-propanol 99.7:0.3]; flow rate 0.5 mL/min; tRZ
21.79 min (S), 12.63 min (R); [a]2D5 C86.5 (c 1.2, CHCl3)
1. (a) Andrus, M. B.; Zhou, Z. J. Am. Chem. Soc. 2002, 124,
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1
[lit.8b (82% ee) [a]D25 C72.68 (c 1.25, CHCl3)]. H NMR
(CDCl3, 400 MHz) d 1.43–1.75 (m, 7H), 2.04–2.40 (m, 3H),
5.60–5.75 (m, 2H), 5.87–5.92 (m, 1H), 7.43 (t, JZ7.6 Hz,
2H), 7.54 (t, JZ7.3 Hz, 1H), 8.05 (d, JZ7.3 Hz, 2H).
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2.6.5. (S)-2-Cyclooctadienylbenzoate (Table 6, entry 20).16
It was obtained in a maximum of 80% ee. The optical purity
was determined by HPLC on chiralpak AD-H column
[hexane/2-propanol 99.7:0.3]; flow rate 0.5 mL/min; tRZ
17.71 min (S), 19.01 min (R): [a]2D5 K132.0 (c 0.6, CHCl3).
1H NMR (CDCl3, 400 MHz) d 1.47–1.54 (m, 1H), 1.64–1.72
(m, 1H), 1.85–1.96 (m, 2H), 2.07 (m, 1H), 2.37 (m, 1H), 5.65
(m, 2H), 5.75–5.81 (m, 1H), 5.93–6.00 (m, 2H), 7.43 (t, JZ
7.8 Hz, 2H), 7.55 (t, JZ7.6 Hz, 1H), 8.05 (d, JZ8.6 Hz, 2H);
13C NMR (CDCl3, 100 MHz) 21.6, 28.3, 30.0, 74.4, 125.8,
126.1, 128.2, 129.5, 130.7, 130.9, 132.8, 133.1, 165.8. Anal.
Calcd for C15H16O2: C, 78.92; H, 7.06. Found: 78.78; H, 7.21.
3. (a) Denney, D. B.; Napier, R.; Cammarata, A. J. Org. Chem.
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obtained in a maximum of 40% ee. The optical purity was
determined by HPLC on chiralpak AD-H column [hexane/
2-propanol 99.7:0.3]; flow rate 0.5 mL/min; tRZ13.28 min
5. Gokhale, A. S.; Minidis, A. B. E.; Pfaltz, A. Tetrahedron Lett.
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14. (a) We have confirmed by UV–vis and EPR methods that 1a–
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presence of phenylhydrazine. For details, see Ref. 10. (b) For
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15. (a) For a review on p-stacking in asymmetric synthesis, see:
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catalyzed by copper complexes of bisoxazoline, see: Schultz,
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9, 4341–4360.
10. For allylic oxidation with chiral dinuclear copper complexes,
see Fahrni, C. J. Tetrahedron 1998, 54, 5465–5470.
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16. The absolute stereochemistry is unknown, but it was
temporarily assigned (S) based on a similar case.