Naphthyl-substituted bisoxazoline and pyridylbisoxazoline-copper(I) catalysts for asymmetric allylic oxidation

Article abstract of DOI:10.1016/j.tetlet.2012.06.010

The synthesis of naphthyl substituted malonyl-derived and pyridine-based bisoxazolines and their applications in the asymmetric allylic oxidation of cyclohexene with t-butyl p-nitroperbenzoate have been performed with much improved reactivity (75% yield)

Full text of DOI:10.1016/j.tetlet.2012.06.010

Tetrahedron Letters 53 (2012) 4518–4521
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Tetrahedron Letters
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Naphthyl-substituted bisoxazoline and pyridylbisoxazoline–copper(I) catalysts
for asymmetric allylic oxidation
Ziniu Zhou a,b, Merritt B. Andrus b,
⇑
a
Zhejiang Pharmaceutical College, 888 Yinxian Avenue Eastern Section, 730 Administration Building, Ningbo, Zhejiang 315100, China
Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT 84602, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of naphthyl substituted malonyl-derived and pyridine-based bisoxazolines and their appli-
cations in the asymmetric allylic oxidation of cyclohexene with t-butyl p-nitroperbenzoate have been
performed with much improved reactivity (75% yield) while maintaining very good enantioselectivity
Received 3 May 2012
Revised 30 May 2012
Accepted 4 June 2012
Available online 12 June 2012
(85% ee). A 1-naphthyl group as the side chain of the oxazoline ligand was found to be optimal. Correla-
tions between the nature of the substituents on the bisoxazolines and the reactivity/selectivity have been
established. Tridentate pyridylbisoxazoline ligands with naphthyl groups were also synthesized and
employed.
Keywords:
Allylic oxidation
Bisoxazoline–copper(I) complex
Naphthyl
Ó 2012 Elsevier Ltd. All rights reserved.
The formation of allylic esters via CH bond oxidation has
attracted considerable interest and has emerged as a viable ap-
tivity. A mechanistic study of alkene hydroperoxidation with per-
oxides catalyzed by copper complexes was also reported using a
racemic version. Unique ligand–substrate combinations were
1,2
2p
proach for the synthesis of complex targets. Among which one
of the most notable examples is the asymmetric Kharasch–Sosnov-
identified that for the first time provided excellent selectivities,
2
2i
sky allylic oxidation of alkenes. Allylic oxidation offers the advan-
94–99% ee, for ester products. For example, 99% ee was obtained
tage of maintaining the alkene functionality in the allylic ester
product, which is complementary to epoxidation and dihydroxyla-
tion. The utility of this approach is seen in the conversion of cyclo-
with cyclopentene using gem-diethyl-diphenyl-bisoxazoline as li-
gand. Currently, investigations of asymmetric allylic oxidation of
alkenes with bisoxazoline catalysts have been limited, especially
from a mechanistic perspective. In this report, we disclose studies
on the asymmetric allylic oxidation of cyclohexene using novel
naphthyl-substituted Box–Cu(I) complexes, which give consider-
ably improved reactivity together with very good selectivity. Based
on previous results with malonyl-derived bisoxazoline–copper(I)
catalysts, correlation between the nature of new bisoxazoline sub-
stituents and the reactivity/selectivity of the catalyst systems can
also be made.
hexenyl benzoate into a key intermediate for the synthesis of
3
leukotriene B
4
and brevetoxins. A major advance with regard to
enantioselectivity (80% ee level) for allylic oxidation was the devel-
2
opment of bisoxazoline–copper(I) catalysts. Since then, a number
of bisoxazoline, trioxazoline ligands have been explored that give,
in the case of cyclopentene, high selectivity. Pyridyl and bipyridyl
2
e,f
bisoxazoline ligands with added phenylhydrazine in acetone
show improved reactivity but with modest selectivity. Aminoinda-
2d
nol-derived bisoxazolines and tridentate pyridine-based bisoxaz-
According to our previously reported mechanistic study using
2
e,f
13
2i
oline (PyBox),
have also been investigated all with moderate
C NMR, copper ligand chelation is favored over the formation
selectivities. While a nine-membered chelate ligand, with biaryl
of a copper–ligand–alkene complex. The selectivity, then, is most
likely a consequence of the Cu(II) benzoate intermediate. Attack
by the allylic radical at the less hindered quadrant of the Cu(II)
complex, away from the flanking t-butyl group, leads to a Cu(III)
intermediate, which rearranges to give S-product and regenerates
the Cu(I)–ligand catalyst.
bis-o-tolyl bisoxazolines shows good selectivity as well as
2
g,h
improved reactivity,
2
a more recent class of C symmetric
pinene-derived bipyridines displayed excellent 96% yield with
cyclohexene but with variable enantioselectivities.2 Non-oxazo-
line-type ligands, bis(imidazolines) and (iminophosphoranyl)
l
4
2
o
ferrocenes, especially the latter, also showed good enantioselec-
It is possible at this point to address the correlation between the
nature of the substituent on the ligand and the reactivity as well as
selectivity. According to studies of bisoxazoline–Cu(II) complexes
2
k
2j
by Jørgensen and Evans, a distorted square planar geometry is
adopted by gem-dimethyl di-t-butyl and diphenyl Box–copper
complexes, with a higher degree of distortion with the di-t-butyl
Box complex. t-Butyl substituents tend to adopt pseudo-axial
⇑
Corresponding author. Tel./fax: +1 801 422 8171.
E-mail address: mbandrus@chem.byu.edu (M.B. Andrus).
0
040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.06.010

Z. Zhou, M. B. Andrus / Tetrahedron Letters 53 (2012) 4518–4521
4519
positions, while one phenyl group is pseudo-axial and the other as-
sumes a pseudo-equatorial position.
NHBOC
OH
O
4 mol% K2OsO2(OH)4
Cl
Cl
In perester based allylic oxidation, attack of the intermediate al-
lyl radical on the Cu(II)–ligand–benzoate should be faster with the
gem-dimethyl di-t-butyl than with the corresponding diphenyl li-
gand. This is due to the phenyl group occupying a more relaxed
pseudo-equatorial position with less distortion from the square pla-
nar geometry. A disfavored steric interaction between the phenyl
groups and the incoming allyl radical is consistent with lower yields
obtained with both cyclopentene and cyclohexene (49% and 44% in
Table 1). In addition, this disfavored interaction could also
contribute to a certain degree of reversibility for the allyl–Cu(III)
intermediate formation, causing lower catalyst turnover. On the
other hand, this type of interaction presumably produces stronger
enantio-differentiation of the Cu(III) intermediate which leads to
higher enantioselectivity. With an incoming relaxed allyl radical
of larger steric size, interaction with a bulky t-butyl is expected to
be stronger, causing slower Cu(III) formation, thus leading to lower
yields for cycloheptene and 1,5-cyclooctadiene (3% and 13%). On the
other hand, interaction between a larger allyl radical and a more ri-
gid Cu(II)–ligand framework with a bulky t-butyl may contribute to
higher enantio-differentiation of the Cu(III) intermediate, which
irreversibly affords higher selectivity for medium-sized ring sub-
strates (95% ee and 94% ee). Among the malonyl-derived bisoxazo-
line ligands used in the reactions discussed above, the phenyl
substituted oxazoline ligands generally produce higher selectivities
for a range of substrates. In addition, phenyl oxazoline complexes
O
O
N
N
5 mol% (DHQ)2PHAL
2
o
Na
n-PrOH/H2O, 0 C
1
1
6
0 %
BOCHN
OH
O
4 mol% K2OsO2(OH)4
5
mol% (DHQ)2PHAL
n-PrOH/H2O, 0 C
o
3
Na
60 %
O
O
NHBOC
OH
NH2
HCl/dioxane
00%
OH
O
Cl
Cl
1
Et3N, CH2Cl2, 0 C
30 mins, 30%
o
2
4
O
N
N
TsCl, Et3N, DMAP
CH2Cl2, RT, 15h
O
O
5
0%
N
H
N
H
OH
OH
6
5
H2N
O
O
OH
NC
CN
N
N
ZnCl2, PhCl, reflux, 2d
61%
7
8
5
are more flexible compared to the corresponding t-butyl ligands.
Therefore by introducing a larger aryl group, with naphthyl in place
of phenyl, it was envisioned that a stronger directing effect than the
previous phenyl based ligands could be imposed with enhanced
reactivity while maintaining high enantioselectivity. In addition,
naphthyl based catalysts may provide additional mechanistic in-
sight. Therefore, the synthesis of naphthyl bisoxazolines is reported
together with their application for catalytic cyclohexene oxidation
using t-butyl p-nitroperbenzoate as oxidant.
Scheme 1. Synthesis of naphthyl-substituted bisoxazoline ligands.
thalene with a mixture of t-butyl carbamate and tert-butylhypo-
chlorite in basic alcoholic solution (1 is formed in situ), followed
by the addition of catalytic (4 mol %) K
5 mol % of phthalazine dihydroquinine (DHQ)
PrOH/H O at 0 °C. Reasonable yields were acquired with subse-
2
OsO
2
4
(OH) along with
2
PHAL ligand in n-
2
The key step for the naphthyl oxazoline ligand synthesis was
the preparation of the naphthyl-substituted amino alcohols. The
reported approach of Sharpless for naphthyl-substituted amino
alcohol synthesis using asymmetric aminohydroxylation6a–d of
vinylnaphthalenes was found to be the most efficient and reliable.
quent BOC deprotection of 2 and 3 using 4 N HCl in dioxane at
room temperature, giving the corresponding amino alcohols in
quantitative yields with high purity (Scheme 1). The crude amino
alcohols were subsequently coupled with 2,2-dimethylmalonyl
dichloride. Compound 4 gave the desired bis-hydroxymalonodia-
mide 5 in only 30 min, with low yield. Treatment of the bis-hydro-
xy malonodiamide 5 under standard cyclization conditions using
p-toluenesulfonyl chloride and triethylamine in the presence of
catalytic DMAP afforded the bis-(2-naphthyl)-substituted gem-di-
methyl bisoxazoline (Box) 6 in moderate yield. Low yield (37%)
of bis-hydroxymalonodiamide was also observed when amino
alcohol 7 was reacted with 2,2-dimethylmalonyl dichloride in the
presence of triethylamine at low temperature, À20 °C. This prob-
lem of producing the bis-hydroxymalonodiamide in low yield
was addressed by an alternative approach using 2,2-dimethylmal-
ononitrile to couple with the amino alcohol 7. In the presence of
6
6
g
t-Butylhypochlorite was made according to a reported procedure
using t-butyl alcohol and commercial bleach. The t-butylhypochlo-
rite obtained was dried over anhydrous CaCl prior to use. As
shown in Scheme 1, BOC-protected chiral amino alcohols (2 and
) were obtained by treating 1-vinylnaphthalene and 2-vinylnaph-
2
3
Table 1
Allylic oxidation of cycloalkenes using gem-dimethyl bisoxazolines copper(I)
complex2
i
O
S
7
O
O
excess anhydrous zinc chloride in refluxing chlorobenzene for
O
O
two days the corresponding bisoxazoline (Box) 8 was afforded in
61% yield (Scheme 1). In addition, this new bis-nitrile route is able
to give the target ligands in only three steps from the vinylnaphth-
lenes, which is superior to previous literature approaches.
O
N
N
O
+
R
CuPF6
R
NO2
x
O2N
CH3CN, - 20 o
5
eq.
1 eq.
C
x
The pyridine-based naphthyl-substituted bisoxazolines (PyBox
11a and 11b) have not been reported previously. The goal for our
synthesis is to extend the scope of bisoxazoline ligands applied in
asymmetric allylic oxidation of alkenes. The same synthetic strategy
for the preparation of known bisoxazolines was employed for the
preparation of these compounds. The naphthyl-substituted amino
alcohols (4 and 7) were prepared and coupled with commercially
available 2,6-pyridine-dicarbonyldichloride to give the correspond-
ing bis-hydroxy pyridinediamide products (9 and 10, Scheme 2).
Timea (d)
Yieldb (%)
% eec
Entry
Cycloalkenes
R
1
2
3
4
Cyclohexene
Cyclopentene
Cycloheptene
Ph
Ph
t-Bu
t-Bu
17
10
10
10
44
49
3
96
82
95
94
1,5-Cyclooctadiene
13
a
b
c
Time in days, using a sealed vial placed in the freezer with stirring.
Yields are for isolated, chromatographed materials based on the perester.
Enantiomeric excess determined by chiral phase HPLC.

4
520
Z. Zhou, M. B. Andrus / Tetrahedron Letters 53 (2012) 4518–4521
H
H
N
NH
4
2
N
HO
2 2
Cl
N
OH
OH
Et
3
N, CH
°C, 1 h
0%
Cl
Cl
+
O
O
N
0
O
O
4
9
H
2
N
H
H
OH
N
N
Et
3
N, CH
2
Cl
2
HO
N
OH
Cl
Cl
+
N
O
O
-
10 °C, 25 min
4%
O
O
6
7
1
0
H
N
H
N
TsCl, Et N, DMAP
3
HO
N
OH
O
N
O
R
O
O
R
N
N
0
°C to rt, 4 to 5 h
67% to 75%
11
9
(R = 2-naphthalene)
0 (R = 1-naphthalene)
R
R
1
a
b
R = 1-nap
R = 2-nap
Scheme 2. Synthesis of naphthyl-substituted pyridine-based bisoxazoline ligands.
Table 2
to the case of di-t-phenyl–Box-Cu(II)–benzoate, catalytic turnover
is facilitated. The 1-naphthyl substituent, therefore, can be con-
sidered to be even more rigid than the t-butyl substituent, while
the less constrained 2-naphthyl group should have more flexibil-
ity upon formation of the copper–ligand benzoate complex. How-
ever, in this case the 1-naphthyl-Box copper catalyst 8 provides
higher selectivity and yield (Table 2).
Two novel tridentate pyridine bisoxazoline ligands were also
synthesized (Scheme 2), and employed in allylic oxidation of cyclo-
hexene. Low selectivity with moderate yields was obtained in this
case (Table 2). It was also found that the newly prepared ligands 11
tended to decompose at ambient temperature, being especially se-
vere for the di-2-naphthyl PyBox ligand 11b. The solubility for both
ligands in acetonitrile is very low, as is the case with gem-dimethyl
dinaphthyl-Box 6 and 8. This problem hampered both the reactiv-
ity and selectivity of allylic oxidation.
Allylic oxidation of cyclohexene using gem-dimethyl bisoxazolines copper(I) complex
O
O
O
O
N
N
O
O
O
S
+
R
CuPF6
R
NO2
O2N
5
eq.
1 eq.
Solvent, - 20 o
C
Entry
Ligand
Solvent
CHCl /CH
CN/CH
Cl
Timea (d)
Yieldb (%)
% eec
1
2
3
1-Nap 8
2-Nap 6
Ligand 11a
3
2
Cl
Cl
2
(3:1)
2
(1:1)
5
7
6
75
40
41
85
80
27
CH
CH
3
2
2
2
a
b
c
Time in days, using a sealed vial placed in the freezer with stirring.
Yields are for isolated, chromatographed materials based on the perester.
Enantiomeric excess is determined by chiral phase HPLC.
In conclusion, the synthesis of naphthyl substituted malonyl-
derived and pyridine-based bisoxazolines as well as their applica-
tions as ligands in copper catalyzed asymmetric allylic oxidation
of cyclohexene with t-butyl p-nitroperbenzoate were performed.
The key reaction in the synthesis is the preparation of naphthyl
amino alcohols employing Sharpless aminohydroxylation of
vinylnaphthlenes. At low temperature, 1-naphthyl substituted
malonyl-derived bisoxazoline 8–Cu(I) complex with t-butyl
p-nitroperbenzoate produces allylic ester product in good yield
while maintaining excellent enantioselectivity for the asymmetric
allylic oxidation of cyclohexene. Correlations between the nature
of the substituents on the ligand and the observed reactivity/
selectivity have been established and correlated with previously
reported catalysts.
Treatment of the bis-hydroxy pyridinediamide products with p-tol-
uenesulfonyl chloride and triethylamine in the presence of catalytic
DMAP afforded the desired pyridine bisoxazoline (PyBox) ligands
1
1a and 11b in good yields (67% and 75%).
A preliminary study of the oxidation of cyclohexene showed
(Table 2) that both ligand 6 and 8 copper complex catalysts
(
8
8
15 mol %) gave ester products in good enantioselectivities, at
0% ee and 85% ee, respectively.8 More importantly, the ligand
copper complex showed improved reactivity at 75% isolated
yield after five days reaction at low temperature, and the perester
was consumed completely, leaving a single product spot visible
by TLC. In contrast, with previous cases using malonyl-derived
bisoxazoline ligands, complete consumption of the perester was
typically not observed. It was also found that when acetonitrile,
the previously preferred solvent, was used alone, it was not able
to dissolve the ligands. Thus, a combined solvent with methylene
chloride was used for 6, while 8 could only be dissolved using a
mixed solvent system of chloroform–methylene chloride (3:1).
The difference in reactivity for these two ligands reflects the
structural constraints of the ligand–Cu(II)–benzoate intermediate.
Presumably, the 1-naphthyl ligand is more rigid conformationally
when the 8–Cu(II)–benzoate is formed. In other words, as 2-naph-
thyl is more flexible than the 1-naphthyl group, in similar manner
Acknowledgments
We are grateful for the support provided by the National Sci-
ence Foundation (Career Award to M.B.A., CHE-9501867) and Brig-
ham Young University.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.06.

Z. Zhou, M. B. Andrus / Tetrahedron Letters 53 (2012) 4518–4521
4521
0
10. These data include MOL files and InChiKeys of the most
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by chiral HPLC with comparison to racemic material. The ligands were
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results. In all cases the S,S-ligands generated (S)-ester product. Levorotatory allyl
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