Catalytic enantioselective C(sp3)H functionalization: Intramolecular benzylic [1,5]-hydride shift

Article abstract of DOI:10.1016/j.tetlet.2014.03.089

The catalytic asymmetric [1,5]-hydride transfer/cyclization sequence involving benzylic C(sp³)H bond was established, providing tetrahydronaphthalene derivatives in moderate to high yield with up to 69% ee, by employing the copper complex of si

Full text of DOI:10.1016/j.tetlet.2014.03.089

Tetrahedron Letters 55 (2014) 2859–2864
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Catalytic enantioselective C(sp3)AH functionalization:
intramolecular benzylic [1,5]-hydride shift
b
b
b,
Jie Yu a, , Nan Li , Dian-Feng Chen , Shi-Wei Luo
⇑
⇑
a Department of Applied Chemistry, Anhui Agricultural University, Hefei 230036, China
b Department of Chemistry, University of Science and Technology of China, Hefei 230026, China
a r t i c l e i n f o
a b s t r a c t
The catalytic asymmetric [1,5]-hydride transfer/cyclization sequence involving benzylic C(sp3)AH bond
was established, providing tetrahydronaphthalene derivatives in moderate to high yield with up to
69% ee, by employing the copper complex of side-armed bisoxazoline as chiral catalyst.
Ó 2014 Elsevier Ltd. All rights reserved.
Article history:
Received 20 January 2014
Revised 11 March 2014
Accepted 19 March 2014
Available online 27 March 2014
Keywords:
Asymmetric catalysis
[1,5]-Hydride transfer
Benzylic C(sp3)AH bond
Side-armed bisoxazoline
Introduction
Results and discussion
Much effort has long been exerted to develop novel C(sp3)AH
bond functionalization owing to its atomic and step economy.1 In
recent years, the C(sp3)AH functionalization via the [1,5]-hydride
shift/cyclization sequence toward rapid buildup of molecular com-
plexity, called the ‘internal redox process’, has received increasing
attention.2 Generally accepted mechanism of the [1,5]-hydride
shift/cyclization demonstrates that the hydride from an appropriate
sp3-C position occurs to migrate with the electronic assistance of
the adjacent heteroatom for the stabilization of the generated car-
bocation, followed by a subsequent 6-endo cyclization to the cation
species, providing structurally diverse nitrogen or oxygen-con-
tained heterocycles 2 (Scheme 1, Eq. 1). The direct enantioselective
processes have also been continuously reported3 since the pioneer-
ing work by Seidel described the first enantioselective catalytic
[1,5]-hydride shift/cyclization reaction.3a More significantly, the
benzylic hydrogen could also participate in the hydride shift with-
out the assistance of an adjacent heteroatom.4 For example, Akiy-
ama and co-workers4e–g have recently established a successful
hydride shift from an aliphatic tertiary position to trigger cycliza-
tion reactions. However, a catalytic enantioselective variant of the
corresponding carbon analogue (3, Scheme 1) remains elusive.
Herein, we report the first asymmetric benzylic [1,5]-hydride
shift/cyclization reaction for the construction of a carbobicyclic
skeleton with two chiral stereogenic centers.
Our initial investigation commenced with the evaluation of
chiral Lewis acid catalysts for the reaction of 2-oxo-2-phenylace-
tate derived benzylidene malonate 3a.5 However, either scandium
triflate or zinc hexafluoroantimonate complex with chiral bisoxaz-
oline ligand 5a6 led to disappointing results (Table 1, entries 1 and
2). Gratifyingly, a chiral complex generated from copper hexafluo-
roantimonate7 and (S,S)-tBu-BOX 5a was able to give 4a in moder-
ate yield with 30% ee (entry 3). However, the stereoselectivity of
this reaction could not be improved by using either (S,S)-Ph-BOX
5b or (S,S)-Bn-BOX 5c (entries 4 and 5). Therefore we tested other
chiral bisoxazoline ligands.8
Compared with the parental molecules, the bisoxazolines with a
side arm have already been widely applied to transition metal-cat-
alyzed asymmetric reactions,9 and generally exhibited somewhat
higher reactivity and better enantiofacial discrimination.10 We
were pleased to find that the transformation of 3a proceeded
smoothly to afford 4a in good yield with 47% ee by employing
the side-arm bisoxazoline 6a (entry 6). The enantioselectivity
could be enhanced to 51% ee when a phenyl ester group was intro-
duced at the C1 position (3b, entry 7). These results encouraged us
to evaluate various side-armed bisoxazoline ligands 6 to improve
the enantioselectivity. Further studies showed that the pendant
groups of side-armed ligands 6 played an extremely important role
in the stereocontrol of product 4b (entries 8–14),5 the use of 4-(t-
butyl)phenyl substituted ligand 6b delivered the best outcomes
comprised of 64% isolated yield and 63% ee (entry 8). However,
⇑
Corresponding authors. Tel.: +86 055165786906 (J.Y.).
E-mail addresses: jieyu@ustc.edu.cn (J. Yu), luosw@ustc.edu.cn (S.-W. Luo).
http://dx.doi.org/10.1016/j.tetlet.2014.03.089
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

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J. Yu et al. / Tetrahedron Letters 55 (2014) 2859–2864
Enantioselective internal redox process:
H
Y
Y
X
6-endo-
[1,5]-hydride
shift
cyclization
Y
H
R2
H
(eq. 1)
(eq. 2)
R2
R2
R1
X
R1
*
R1
X
2
1
X = NR, O
Y = C(EWG)2, NR',O
This work:
R1
R2
R1
R3
R2
H
R3
R1
H
2
enantioselective
[1,5]-hydride shift
chiral induced
cyclization
*
R3
R2
R4
1
1
*
H
*
R1, R2 = EWG
R4
R4
R4 = Aryl group
3
A
: R = R2 = CO2Me, R3 = CO2Et, R4 = 4-OMeC6H4
1
3a
Scheme 1. The [1,5]-hydride shift/cyclization sequence.
Table 1
Evaluation of the ligands and ester groups in the reactiona
R1O2C
CO2R1
2
1
CO2R2
Metal salt (10 mol%)
Ligand (10 mol%)
C2H4Cl2, reflux, 24 h
1
CO2R1
CO2R1
2
CO2R2
*
*
4
3
OMe
OMe
R3 R3
3 = Me, R5 = t-Bu
3 = Me, R5 = Ph
3 = Me, R5 = Bn
5a
5b
5c
R
R
R
O
O
O
Bn
O
N
N
N
R5
R5
5
6a R3 = Me, R4 = Ph, R5 = Bn
Me
6b R3 = Me, R4 = 4-t-Bu-C6H4, R5 = Bn
R4
O
O
N
3 = Me, R4 = 2-Naph, R5 = Bn
6c
6d
6e
R
R
R
R3
3 = Me, R4 = 2,4,6-Me3-C6H2, R5 = Bn
3 = Et, R4 = 4-t-Bu-C6H4, R5 = Bn
N
O
Bn
Bn
6f R3 = Me, R4 = 4-t-Bu-C6H4, R5 = i-Pr
6g R3 = Me, R4 = 4-t-Bu-C6H4, R5 = s-Bu
6h R3 = Me, R4 = 4-t-Bu-C6H4, R5 = t-Bu
N
N
7
Tang's TOX
R5
R5
6
Entry
Ligand
4
R1
R2
Metal saltb
Yieldc (%)
eed (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
5a
5a
5a
5b
5c
6a
6a
6b
6c
6d
6e
6f
6g
6h
7
6c
6c
4a
4a
4a
4a
4a
4a
4b
4b
4b
4b
4b
4b
4b
4b
4b
4c
4d
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
Et
Et
Et
Et
Et
Et
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Sc(OTf)3
58
3
—
30
5
ZnCl2/AgSbF6
CuCl2/AgSbF6
CuCl2/AgSbF6
CuCl2/AgSbF6
CuCl2/AgSbF6
CuCl2/AgSbF6
CuCl2/AgSbF6
CuCl2/AgSbF6
CuCl2/AgSbF6
CuCl2/AgSbF6
CuCl2/AgSbF6
CuCl2/AgSbF6
CuCl2/AgSbF6
CuCl2/AgSbF6
CuCl2/AgSbF6
CuCl2/AgSbF6
N. R.e
52f
59
39f
76g
65h
64h
56
23
47
51
63
50
37
53
29
24
31l
60
42
48
41f
64
40f,i
54j
70k
23f,j
60
i-Pr
54
a
Unless indicated otherwise, the reaction was carried out on 0.1 mmol scale in DCE (1 mL) and the ratio of MCln/AgSbF6 is 1/n.
The metal salts, such as Cu(OTf)2, Mg(OTf)2, Zn(OTf)2, MgCl2/AgSbF6, InCl3/AgSbF6, and NiCl2/AgSbF6 in combination with ligands 5–7 led to no product formation in the
b
reaction.
c
Isolated yield of major diastereomer.
The ee of major diastereomer was determined by HPLC analysis.
N.R. = no reaction.
d
e
f
The reaction did not proceed to completion.
g
h
i
Diastereomeric ratio was determined by crude 1H NMR and the d.r. = 67/33.
The d.r. = 75/25.
The d.r. = 62/38.
The d.r. = 71/29.
The d.r. = 76/24.
j
k
l
The opposite enantiomer was obtained.

J. Yu et al. / Tetrahedron Letters 55 (2014) 2859–2864
2861
Table 2
Evaluation of other reaction parametersa
tBu
O
CuCl2 (10 mol%)
AgX (20 mol%)
MeO2C
CO2Me
CO2Ph
CO2Ph
6b
(10 mol%)
CO2Me
CO2Me
solvent, additive,
reflux, 24 h
Me
N
*
*
O
4b
N
3b
OMe
OMe
6b
Bn
Bn
Entry
Solvent
X
d.r.b
Yieldc (%)
eed (%)
1
2
C2H4Cl2
CH3CCl3
SbF6
SbF6
SbF6
SbF6
PF6
75/25
55/45
64/36
64/36
N.D.f
61/39
75/25
75/25
75/25
64
22
43
41
N.R.
19
40
49
45
63
65
63
60
—
53
67
69
65
3e
4e
5
CHCl2CH2Cl
C2H2Cl4
C2H4Cl2
C2H4Cl2
C2H4Cl2
C2H4Cl2
C2H4Cl2
6
BF4
7g
8h
9i
SbF6
SbF6
SbF6
a
b
c
d
e
f
Unless indicated otherwise, the reaction was carried out on 0.1 mmol scale in solvent (1 mL).
Diastereomeric ratio was determined by crude 1H NMR.
Isolated yield of major diastereomer.
The ee of major diastereomer was determined by HPLC analysis.
The reaction was performed at 85 °C.
N.D. = not determined.
3 Å MS (10 mg) was used; if excess MS was used (>30 mg), the reaction did not work.
4 Å MS (10 mg) was used.
g
h
i
5 Å MS (10 mg) was used.
Table 3
the utilization of Tang’s TOX 79a in combination with the same
copper salt led to slightly diminished ee value but a massive ero-
sion of conversion (entry 15). Variation of malonate moiety of 3
(R1 group) led to a conclusion that the introduction of an ethyl or
isopropyl substituent was not beneficial to stereochemical control
(entries 16 and 17).
The scope of the [1,5]-hydride shift/cyclization reactiona
CuCl2 (10 mol%)
MeO2C
CO2Me
CO2Ph
CO2Me
CO2Me
AgSbF6 (20 mol%)
6b
(10 mol%)
*
C2H4Cl2, reflux
CO2Ph
Ar
*
Ar
4
We next screened other reaction parameters (such as solvents
and additives) using bisoxazoline 6b as the ligand. As shown in
Table 2, 1,1,1-trichloroethane provided a higher enantiomeric ex-
cess than 1,2-dichloroethane (DCE), but the diastereomeric ratio
was nearly 1:1 (entry 2 vs 1). Neither 1,1,2-trichloroethane nor
1,1,2,2-tetrachloroethane led to further improvement of enantiose-
lectivity in this transformation (entries 3 and 4). Substrate 3b was
also treated with 6b and CuCl2 together with either AgBF4 or
3
Entry
4
Ar
Yieldb (%)
d.r.c
eed (%)
1e
4b
4e
4f
4-OMe-C6H4
2-OMe-C6H4
3,4,5-(OMe)3-C6H4
3-OMe-C6H4
74
72
92
N.R.
75/25
66/34
80/20
—
69/18
29/49
32/23
—
2e,f
3g
4h
4g
O
5
4h
86
70/30
45/9
7
AgPF6 (entries 5 and 6). The reaction did not proceed at all in
O
4-Me-C6H4
Ph
6h
4i
4j
4k
61
trace
N.R.
71/29
66/34
—
43/20
N.D.
—
the presence of AgPF6, while significant decreases in both the dia-
stereo- and enantioselectivity were observed in this reaction
involving AgBF4. The following performance of different molecular
sieves (entries 7–9) complemented the optimal conditions we
could achieve as a combination of 10 mol % CuCl2, 20 mol % AgSbF6,
10 mol % 6b, and 4 Å MS (10 mg) in C2H4Cl2 (entry 8).
7h,i
8h
4-Cl-C6H4
a
Unless indicated otherwise, the reaction was carried out on 0.1 mmol scale in
DCE (1 mL) for 72 h. The ees were determined by HPLC analysis.
b
Isolated total yield of major and minor diastereomers.
c
Diastereomeric ratio was determined by crude 1H NMR.
With optimized reaction conditions established, the scope and
generality of this protocol were then explored (Table 3).4f After
the reaction time was prolonged to 72 h, the isolated total yield
of 4b was increased to 74% with maintained ee value (entry 1).
Simply changing the methoxy group from the para- to ortho-posi-
tion dramatically lowered the stereoselectivity (entries 2 and 4).
The reaction of substrate 3e gave the product 4e only with 2:1
d.r. and 29% ee, while substrate 3g containing a meta-methoxy-
phenyl group could barely undergo this process, even under the
circumstance of much higher catalyst loading (50 mol %, entry 4).
Moreover, the 3,4,5-trimethoxy analogue 3f was found to undergo
this transformation with 40 mol % catalytic amount in the absence
of 4 Å MS, affording the desired tetrahydronaphthalene 4f in excel-
lent yield with 4:1 d.r and 32% ee (entry 3), thus indicating that the
substitution pattern of the benzene ring has significant impact on
d
Major/minor diastereomer.
4 Å MS (10 mg) was used.
20 mol % catalytic amount was used.
40 mol % catalytic amount was used.
50 mol % catalytic amount was used.
A messy reaction was always observed for the phenyl-substituted substrate.
e
f
g
h
i
both the diastereo- and enatioselectivity.4f Interestingly, applying
these conditions to the dialkoxy substrate 3h also provided the
product 4h in 86% yield with 45% ee (entry 5). Importantly, the
benzylic product 4i with the p-tolyl group, which has lower elec-
tron-donating ability compared with that of the p-methoxyphenyl
moiety, could also be obtained in moderate yield with 43% ee (en-
try 6). It is noteworthy that the phenyl-substituted substrate 3j

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J. Yu et al. / Tetrahedron Letters 55 (2014) 2859–2864
CO2Ph
CO2Me
HO
OH
LiAlH4, Et2O
75%
OH
CO2Me
major-4b
69% ee
OMe
OMe
8 (73% ee)
99% ee after
recrystalization
Scheme 2. Dermination of the absolute configuration of the stereogenic centers in major-4b.
TS-1,5-Hs-SR(0.00, 0.00)
TS-1,5-Hs-SS(3.07, 2.57)
TS-1,5-Hs-RS(-0.08, 0.99)
TS-1,5-Hs-RR(1.54, 3.61)
Figure 1. Optimized transition state structures at the level of B3LYP with basis set 6-31G⁄ for C, H, O, N, and lanl2dz for Cu atom, relative energies in enthalpy (blue) and
Gibbs free energy (red), distances in angstrom.
could not undergo the internal redox process even with 50 mol %
catalyst loading smoothly, demonstrating that the electronic nat-
ure of the aromatic ring changed the reactivity significantly (entry
7 vs 1 and 6). On the other hand, the desired product was not
obtained in the case of 3k that had an electron-withdrawing
substituent (entry 8).
The absolute configuration of the product was accessed by
X-ray crystallography analysis. As all the tetrahydronaphthalene
compounds 4 failed to grow crystals, we made necessary derivati-
zation of major-4b to get a qualified crystal sample. On exposure of
major-4b (69% ee) to LiAlH4/Et2O reductive system, chiral triol 8
was obtained in 75% yield with 73% ee. The X-ray structure of 8

J. Yu et al. / Tetrahedron Letters 55 (2014) 2859–2864
2863
revealed an assignment of the configuration of the stereogenic cen-
ters to be (1S,3R)11 (Scheme 2).
Foundation funded project (2011M501048), and FRFCU
(WK2060190017).
To investigate the stereochemical control of this catalytic intra-
molecular [1,5]-hydride shift, the DFT calculation12 was performed
on the model reaction of 3b–4b with 6b as catalyst.2s,3g The located
transition state (TS) structures are shown in Figure 1. The coordina-
tion of Cu(II) in 6b with two carbonyl oxygen of the homo-biester
groups of C@C in 3b should lower the electron density of the C@C
double bound significantly and thereby facilitated the [1,5]-hydride
transferring process. The internal migrated hydride may take two
path ways to approach the destination carbon (C1 position) from
its Si- or Re-face, and there are two hydrogen atoms as candidate
to be shifted, respectively. DFT calculations, on the TS of [1,5]-H
transferring process, indicated that the steric repulsion interactions
between the chiral bisoxazoline side-chains and the coordinated
homo-biester groups of C@C result in particular interaction be-
tween the p-methoxyphenyl group (C2 position in 3b) and the phe-
nyl ester group of C@C, furthermore, lead to a different stability of
TSs in hydride shifting. The located TS structure, TS-1,5-Hs-SR, Re-
facial approaching destination carbon C1 of pro-R hydrogen atom
in C2, corresponding to the major product, was predicted to be
the most stable TS. Due to the repulsive interaction between the
p-methoxyphenyl group of C2 and the phenyl ester group of C1,
the located TS of Re-facial approaching destination carbon C1 of
pro-S hydrogen atom of C2, TS-1,5-Hs-SS was predicted to be less
stable ꢀ3 kcal/mol than TS-1,5-Hs-SR. In other way, for the Si-facial
approaching of pro-R hydrogen atom transfer, the located TS struc-
ture TS-1,5-Hs-RS, corresponding to the enantiomer of major prod-
uct, was slightly less stable than TS-1,5-Hs-RS, about 1 kcal/mol in
Gibbs free energy. For the same reason as in TS-1,5-Hs-SS, the lo-
cated TS of Si-facial approaching destination carbon C1 of pro-S
hydrogen atom, TS-1,5-Hs-RR was predicted to be less stable
ꢀ2 kcal/mol than TS-1,5-Hs-RS. The calculated results consist with
the experimental observations and implied that the coordination of
Cu(II) in 6b with two carbonyl oxygen of the homo-biester groups
in 3b activated the [1,5]-hydride shift process and the match and
mismatch interacting among the chiral bisoxazoline side-chains,
the ester groups of destination double bond and the p-methoxy-
phenyl attached on the carbon atom the migrated hydride leaving
from, plausibly contributed to the moderate stereochemical
controls.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.03.089.
References and notes
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Conclusions
5. See the Supporting information for details.
In summary, we have developed the first example of catalytic
asymmetric [1,5]-hydride transfer/cyclization sequence involving
benzylic C(sp3)AH bond, employing the copper complex of side-
armed bisoxazoline 6b as chiral catalyst. The reaction provided
an easy access to optically active tetrahydronaphthalene deriva-
tives in moderate to high yield with up to 69% ee. DFT calculation
indicated that the complex bisoxazoline Cu(II) further coordinated
to the carbonyl oxygen of homo-biester groups of C@C, improved
the electrophilicity of C@C double bound significantly, thereby,
promoted the hydride transferring process, and simultaneously,
the steric repulsion among the chiral bisoxazoline side-chains,
the ester groups of double bond, and the p-methoxyphenyl group
on C2 resulted in the particular stereoselectivity in the [1,5]-hy-
dride transferring process.
6. For reviews on bisoxazolines in asymmetric catalysis, please see: (a) Ghosh, A.
K.; Mathivanan, P.; Cappiello, J. Tetrahedron: Asymmetry 1998, 9, 1; (b)
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À
7. The weakly coordinating anions SbF6 was used to accelerating the hydride
shift because the presence of
a counteranion which is more weakly
coordinating than OTfÀ would lead to the unsaturated alkene with enhanced
electrophilicity. For selected effective catalytic reactions, see: (a) Van Seggen,
D. M.; Hurlburt, P. K.; Noirot, M. D.; Anderson, O. P.; Strauss, S. H. Inorg. Chem.
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8. Full details are available in the Supporting information.
Acknowledgments
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H.; Bellemin-Laponnaz, S. Chem. Eur. J. 2008, 14, 4142; (c) Hargaden, G. C.;
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10. For recent uses of sidearm-modified BOX ligands, see: (a) Rendina, V. L.;
Moebius, D. C.; Kingsbury, J. S. Org. Lett. 2011, 13, 2004; (b) Castillo, M. R.;
We sincerely thank Prof. Liu-Zhu Gong for his support. We are
grateful for financial support from NSFC (21202155), Anhui
Agricultural University (Yj2013-9), China Postdoctoral Science

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Castillón, S.; Claver, C.; Fraile, J. M.; Gual, A.; Martín, M.; Mayoral, J. A.; Sola, E.
11. CCDC 880868 (8) contains the supplementary crystallographic data for this
Letter. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif.
12. The calculations were carried out with the Gaussian 03: Frisch, M.J. et al.;
Gaussian03, revision D.03; Gaussian, Inc.: Wallingford, CT., 2004 (for complete
Ref. 12, see the Supporting information).
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