Hf(OTf)4-Catalyzed highly diastereoselective synthesis of 1,3-disubstituted tetralin derivatives via benzylic C(sp3)-H bond functionalization

Article abstract of DOI:10.1039/c7cc01717k

We report a highly diastereoselective synthesis of 1,3-disubstituted tetralin derivatives via Lewis acid catalyzed benzylic C(sp³)-H bond functionalization. The employment of Hf(OTf)₄ as an acid catalyst was crucial in the reaction,

Full text of DOI:10.1039/c7cc01717k

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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DOI: 10.1039/C7CC01717K
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Hf(OTf)4-catalyzed highly diastereoselective synthesis of 1,3-
disubstituted tetralin derivatives by benzylic C(sp3)–H bond
functionalization
Received 00th January 20xx,
Accepted 00th January 20xx
Taira Yoshidaa and Keiji Mori*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
EWG
EWG
EWG
EWG
We report
a highly diastereoselective synthesis of 1,3-
Δ
or
H
EWG
EWG
1
disubstituted tetralin derivatives via Lewis acid catalyzed benzylic
C(sp3)–H bond functionalization. Employment of Hf(OTf)4 as an
acid catalyst was crucial in the reaction, and corresponding
tetralins were obtained in good chemical yields with high to
excellent diastereoselectivities (up to 13.3:1 d.r.).
H
H
+
Acid catalyst
6-endo-
cyclization
[1,5]-H shift
R1
5
X
R1
X
R1
X
1
A
2
X = NR2, O, CH2
EWG = CN, CO2R, etc.
.
Scheme 1. C(sp3)–H functionalization by internal redox process.
The direct functionalization of unreactive C–H bonds has
become a major topic of research interest.1 Because of the
high synthetic potential of the method, i.e., it provides
unprecedented retrosynthetic cleavage and reduces the total
number of steps for the synthesis of complex molecules, many
organic chemists have devoted their efforts to the
development of new strategies. Among them, the C(sp3)–H
bond functionalization mediated by a hydride shift/cyclization
process, namely, the “internal redox process,” has attracted
much attention because of its unique features (Scheme 1).2
The key feature of this transformation is the [1,5]-hydride shift
of the C(sp3)–H bond α to the heteroatom (X). Subsequent 6-
endo cyclization to a cationic species affords heterocycle 2.3–6
Although this method was limited to reactive, heteroatom-
containing substrates (X = NR2 and O), we5c-e,h and Fillion
group4f independently found that the carbon analogue (X =
CH2) was also a viable substrate for this transformation. An
enantioselective variant of the reaction was also realized by
using both chiral metal catalyst and organocatalyst, affording
synthetically useful chiral tetrahydroquinoline derivatives (X =
NR2) with excellent enantioselectivities.7
Despite its useful nature, most of the reactions reported so far
have focused on the construction of a skeleton with a single
stereogenic center, and its application to the construction of a
skeleton with multiple stereogenic centers has been overlooked
until quite recently (Figure 1).2,8 In particular, methods for the
construction of a 2,4-disubstituted skeleton were limited because of
the low reactivity of the substrate with an additional substituent on
the vinylic carbon. Gong and coworkers reported a chiral
phosphoric acid catalyzed asymmetric internal redox reaction with
o-piperidino ketimines to afford 2,4-disubstituted quinazoline
derivatives with excellent diastereoselectivities and good
enantioselectivities (up to 14:1 d.r. and 86% ee).7e Although
excellent diastereoselectivities were achieved in the reaction,
nitrogen-containing substrates (X = NR2) with high reactivities were
the only examples available. An enantioselective variant with less
reactive carbon substrates (X = CH2) for the formation of 1,3-
disubstituted tetralins (benzylic C(sp3)–H bond functionalization)
was reported by Yu and Luo.7m Chiral Cu-BOX complexes well
promoted the desired reaction; however, diastereoselectivity and
enantioselectivity reached only moderate levels (less than 4:1 d.r.
and 69% ee).9
low reactivity
EWG
EWG
EWG
EWG
EWG
4
3
2
R
R1
X
R1
X
R1
X
X
1
many examples
several examples
less studied
providing adducts
providing adducts
with two stereocenters
at 2,3-positions
providing adducts
with two stereocenters
at 2,4-positions
with single stereocenter
Fig. 1 Scope of C(sp3)–H functionalization by internal redox process.
As part of our ongoing effort to develop highly diastereoselective
internal
redox
reactions
involving
benzylic
C(sp3)–H
functionalizations,5 we recently achieved a highly cis-selective
synthesis of 1,3-disubstituted isoquinoline derivatives starting from
trifluoromethyl-substituted ketimines.5h Herein we report a highly
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52
trans-selective synthesis of 1,3-disubstituted tetralin derivatives,
which are known as the core structure of biologically active
compounds. The reaction started from benzylidene malonate
derived from α-ketoesters. Employment of Hf(OTf)4 as the catalyst
was crucial to achieving excellent diastereoselectivity, and
corresponding tetralin derivatives were obtained in good chemical
yields with high trans selectivities (Scheme 2).
6
7
8
9
Sn(OTf)2
Gd(OTf)3
Zn(OTf)2
Hf(OTf)4
Hf(OTf)4
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
40 (1/3.5)
View Article Online
DOI: 10.1039/C7CC01717K
–
–
90
93
–
–
62 (1/11.5)
85 (1/11.5)
10c
a
Unless otherwise noted, all reactions were performed with 0.10
mmol of triester 5a and 10 mol% catalyst in solvent (1.0 mL) at
refluxing temperature. Diastereomer ratio was determined by
b
Previous work
comparing the integration value of methine proton at C-1 position
for each diastereomer in 1H NMR. c 2.5 mol% catalyst loading.
PMP
N
CF3
1
PMP
Ar
cat. Tf2NH
toluene
CF3
Ar
N
3
R1
R1
up to 5.7/1 d.r.
The substrate scope of this reaction is illustrated in Fig 2. At first,
the effect of the aromatic ring of the phenethyl moiety was
examined, and the results suggested that the reactivity was strongly
governed by the electronic and steric nature of the aromatic ring, as
in the case of our previous findings in the benzylic hydride
shift/cyclization system.5d,e,h An increased catalyst loading (10
mol%) was required to achieve moderate chemical yield in the
synthesis of p-tolyl group substituted products 4b (65%). Phenyl-
type product 4c was obtained in only 30% yield with 10 mol%
Hf(OTf)4.10 The steric effect was also detrimental to the reaction,
and 5 mol% Hf(OTf)4 was essential for the completion of the
reaction when o-methoxyphenyl (OMP) group substituted substrate
3d was employed.
cis selective
This work
MeO2
C
CO2Me
CO2Me
CO2Me
1
cat. Hf(OTf)4
ClCH2CH2Cl
CO2Me
Ar
CO2Me
Ar
3
R1
R1
up to 13.3/1 d.r.
highly trans selective
Scheme 2. Diastereoselective synthesis of 1,3-disubstituted tetralin
derivatives.
Triester 3a derived from methyl phenylglyoxylate derivative was
selected as the substrate because of its potentially high reactivity
and ease of preparation (Table 1). At first, the reaction was
conducted under the optimum conditions used in the
diastereoselective synthesis of 1,3-disubstituted isoquinoline
derivatives (10 mol% Tf2NH in toluene at refluxing temperature).5h
However, the reaction did not proceed to completion and starting
material 3a was recovered (entry 1). In contrast, the desired
reaction proceeded with TiCl4 as catalyst to give 4a in low chemical
yield with low diastereoselectivity (20%, cis/trans = 1/1.1, entry 2).
When Sc(OTf)3 was employed as the catalyst, the chemical yield was
slightly improved (34%) but the diastereoselectivity remained low
(cis/trans = 1/1.1, entry 3). Changing the solvent from toluene to
ClCH2CH2Cl improved the reactivity and 4a was obtained in
quantitative yield although the diastereoselectivity was low (97%,
cis/trans = 1/2.2, entry 4). In this solvent system, moderate
chemical yields were achieved even with other catalysts, such as
The substituents on the aromatic ring (mother nucleus) had an
almost negligible effect on the reaction (phenethyl aromatic group
was fixed to PMP). Substrates with an electron-donating group,
such as a methyl or methoxy group, or an electron-withdrawing
group (F) at various-positions (3e-i) furnished corresponding
tetralins 4e–i in good chemical yields with high
diastereoselectivities (58–77%, cis/trans = 1/7.3-13.3 with 2.5 mol%
Hf(OTf)4). Naphthyl-type substrate 3j was also a good candidate for
the reaction, and product 4j achieved good chemical yields with
high diastereoselectivities (76%, cis/trans = 1/6.7). The relative
stereochemistry of the major isomer was assigned to trans by X-ray
analysis of naphthyl-type compound 4j, and those of others shown
in Table 1 and Fig. 2 were surmised by analogy.11
Yb(OTf)3 and Sn(OTf)2.
However, the diastereoselectivities
remained low (less than cis/trans = 1/3.5, entries 5 and 6). Careful
screening revealed that Hf(OTf)4 was the catalyst of choice, and
excellent diastereoselectivity was achieved with moderate chemical
yield (62%, cis/trans = 1/11.5, entry 9). Fortunately, lowering the
catalyst loading to 2.5 mol% improved the chemical yield
dramatically, and desired 4a was obtained in excellent chemical
yield without sacrificing the diastereoselectivity (85%, cis/trans =
1/11.5, entry 10).
MeO2
C
CO2Me
CO2Me
CO2Me
CO2Me
Ar
CO2Me
cat. Hf(OTf)4
ClCH2CH2Cl
reflux, 24 h
Ar
R1
R1
3
4
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
PMP
Tol
Ph
OMP
4a
85% (2.5 mol%)
cis/trans = 1/11.5
4b
65% (10 mol%)
cis/trans = 1/6.7
4c
30% (10 mol%)
cis/trans = 1/6.3
4d
52% (5 mol%)
cis/trans = 1/10.1
Table 1 Screening for catalysts and reaction conditions.a
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
Me
MeO
F
CO2Me
CO2Me
CO2Me
MeO2
C
CO2Me
CO2Me
CO2Me
PMP
PMP
PMP
1
4e
77% (2.5 mol%)
cis/trans = 1/9.0
4f
4g
75% (2.5 mol%)
cis/trans = 1/7.3
cat. (10 mol%)
CO2Me
PMP
CO2Me
71% (2.5 mol%)
cis/trans = 1/8.1
solvent
reflux, 24 h
3
PMP
3a
4a
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
Me
PMP
MeO
PMP
PMP
yield (%)
4a (cis/trans)b
entry catalyst
solvent
4h
69% (2.5 mol%)
cis/trans = 1/10.1
4i
4j
3a
95
52
53
–
58% (2.5 mol%)
cis/trans = 1/13.3
76% (2.5 mol%)
cis/trans = 1/6.7
1
2
3
4
5
Tf2NH
TiCl4
Sc(OTf)3
Sc(OTf)3
Yb(OTf)3
Toluene
Toluene
Toluene
ClCH2CH2Cl
ClCH2CH2Cl
–
PMP = p-methoxyphenyl, Tol = p-tolyl, OMP = o-methoxyphenyl.
20 (1/1.1)
34 (1/1.1)
97 (1/2.2)
45 (1.3/1)
Fig. 2 Substrate scope.
Adducts 4 were promising synthetic building blocks because the
ester moieties could be transformed easily (Scheme 3). When 4a
40
2 | J. Name., 2012, 00, 1-3
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obtained in good chemical yields with good Viteow AreticxlceeOlnlelinnet
was treated with 1.0 equivalent of TfOH in refluxing ClCH2CH2Cl for
24 h, the condensation reaction between the methoxycarbonyl
group at C-1 position and one of the methoxycarbonyl groups at C-2
position syn to C-1 ester moiety occurred to give acid anhydride 5
with three contiguous stereogenic centers.11 Product 5 is a
promising synthetic intermediate for benz[e]isoindole derivatives,
which are potential agents for the symptomatic treatment of
benign prostatic hyperplasia.12 Although the detailed reaction
mechanism is nuclear, this reaction emphasizes the utility of the
present method as a synthetic tool.
DOI: 10.1039/C7CC01717K
diastereoselectivities (up to 13.3:1 trans selectivity).
The
synthetically
condensation reaction leading to
interesting
TfOH-promoted
intramolecular
5
with three contiguous
stereocenters was a good showcase of the synthetic potential of
the present method. Further investigations of other
diastereoselective and enantioselective reactions are underway in
our laboratory and the results will be reported in due course.
We thank Dr. Koya Inomata and Professor Kunio Mochida of
Gakushuin University for X-ray analysis. This work was partially
supported by a Grant-in-Aid for Scientific Research from the Japan
Society for the Promotion of Science, and by grants from The
Uehara Memorial Foundation, The Naito Foundation, and the Inoue
Foundation of Science.
O
O
CO2Me
CO2Me
H
1
1 equiv.
TfOH
O
CO2Me
2
CO2Me
PMP
ClCH2CH2Cl
reflux, 24 h
3
PMP
4a
5
66%
single diastereomer
All hydrogens except those at C-1
and C-3 positions are omitted for clarity.
Notes and references
1
For recent reviews on C–H activation, see: (a) K. Godula, D.
Sames, Science 2006, 312, 67; (b) R. G. Bergman, Nature
2007, 446, 391; (c) D. Alberico, M. E. Scott, M. Lautens, Chem.
Rev. 2007, 107, 174; (d) H. M. L. Davies, J. R. Manning,
Nature 2008, 451, 417; (e) X. Chen, K. M. Engle, D.-H. Wang,
J.-Q. Yu, Angew. Chem. Int. Ed. 2009, 48, 5094; (f) R. Jazzar, J.
Hitce, A. Renaudat, J. Sofack-Kreutzer, O. Baudoin, Chem. Eur.
J. 2010, 16, 2654; (g) T. W. Lyons, M. Sanford, S. Chem. Rev.
2010, 110, 1147; (h) H. M. L. Davies, J. Du Bois, J.-Q. Yu,
Chem. Soc. Rev. 2011, 40, 1855; (i) T. Brückl, R. D. Baxter, Y.
Ishihara, P. S. Baran, Acc. Chem. Res. 2012, 45, 826; (j) Y. Qin,
J. Lv, S. Luo, Tetrahedron Lett. 2014, 55, 551; (k) H. M. L.
Davies, D. Morton, J. Org. Chem 2016, 81, 343.
For recent reviews on internal redox process, see: (a) M.
Tobisu, N. Chatani, Angew. Chem. Int. Ed. 2006, 45, 1683; (b)
S. C. Pan, Beilstein J. Org. Chem. 2012, 8, 1374; (c) M. Wang,
ChemCatChem 2013, 5, 1291; (d) B. Peng, N. Maulide, Chem,
Eur. J. 2013, 19, 13274; (e) L. Wang, J. Xiao, Adv. Synth. Catal.
2014, 356, 1137; (f) M. C. Haibach, D. Seidel, Angew. Chem.
Int. Ed. 2014, 53, 5010.
These types of reactions are classified under the term “tert-
amino effect.” For reviews, see: (a) O. Meth-Cohn, H.
Suschitzky, Adv. Heterocycl. Chem. 1972, 14, 211; (b) W.
Verboom, D. N. Reinhoudt, Recl. Trav. Chim. Pays-Bas 1990,
109, 311; (c) O. Meth-Cohn, Adv. Heterocycl. Chem. 1996, 65,
1; (d) J. M. Quintela, Recent Res. Dev. Org. Chem. 2003, 7,
259; (e) P. Matyus, O. Elias, P. Tapolcsanyi, A. Polonka-Balint,
B. Halasz-Dajka, Synthesis 2006, 2025.
For selected examples of internal redox reactions, see: (a) S.
J. Pastine, K. M. McQuaid, D. Sames, J. Am. Chem. Soc. 2005,
127, 12180; (b) A. Polonka-Bálint, C. Saraceno, K. Ludányi, A.
Bényei, P. Mátyus, Synlett 2008, 2846; (c) K. M. McQuaid, D.
Sames, J. Am. Chem. Soc. 2009, 131, 402; (d) C. Zhang, S.
Murarka, D. Seidel, J. Org. Chem. 2009, 74, 419; (e) D. Shinkai,
H. Murase, T. Hata, H. Urabe, J. Am. Chem. Soc. 2009, 131,
3166; (f) J. C. Ruble, A. R. Hurd, T. A. Johnson, D. A. Sherry, M.
R. Barbachyn, P. L. Toogood, G. L. Bundy, D. R. Graber, G. M.
Kamilar, J. Am. Chem. Soc. 2009, 131, 3991; (g) K. M.
McQuaid, J. Z. Long, D. Sames, Org. Lett. 2009, 11, 2972; (h)
M. J. Mahoney, D. T. Moon, J. Hollinger, E. Fillion,
Tetrahedron Lett. 2009, 50, 4706; (i) G. Zhou, J. Zhang, Chem.
Commun. 2010, 46, 6593; (j) J. Cao, X. Yang, X. Hua, Y. Deng,
G. Lai, Org. Lett. 2011, 13, 478; (k) I. D. Jurberg, B. Peng, E.
Wöstefeld, M. Wasserloos, N. Maulide, Angew. Chem. Int. Ed.
2012, 51, 1950; (l) T. Sugiishi, H. Nakamura, J. Am. Chem. Soc.
2012, 134, 2504; (m) X. Gao, V. Gaddam, E. Altenhofer, R. R.
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Scheme 3 Derivatization to acid anhydride 5 with three contiguous
stereogenic centers.
There are two possible mechanisms for achieving the high
diastereoselectivity in the reaction (Figure 3): (1) highly
stereoselective cyclization after the hydride shift process (kinetic
control), and (2) thermodynamic shift to
a more stable
diastereomer through the ring-opening and ring-closing equilibrium
between 4a and B (thermodynamic control). Importantly, the
reaction was completed within 1 h to give desired adduct 4a in
excellent chemical yield albeit with low diastereoselectivity (90%,
cis/trans = 1/3.3). Subjecting low-selectivity adduct 4a to the
optimum reaction conditions (2.5 mol% Hf(OTf)4 in refluxing
ClCH2CH2Cl for 24 h) markedly improved the diastereoselectivity
(cis/trans = 1/3.3 →1/11.1). These results suggested that both
kinetic control and thermodynamic control were involved in the
reaction, and the latter was the major factor for the excellent
diastereoselectivity.13
2
3
[Hf]
O
O
O
O
E
Me
O
E
MeO
OMe
MeO
OMe
[Hf]
O
E
H
O
Hf(OTf)4
H
+
OMe
E = CO2Me
kinetic control
OMe
OMe
OMe
B
3a
4a
4
thermodynamic
control
CO2Me
CO2Me
2.5 mol%
Hf(OTf)4
2.5 mol%
Hf(OTf)4
CO2Me
3a
4a
ClCH2CH2Cl
ClCH2CH2Cl
PMP
reflux, 1 h
reflux, 24 h
4a
90%, cis/trans = 1/3.3
84%, cis/trans = 1/11.1
Fig. 3 Two possible mechanisms for high diastereoselectivity.
In summary, we have achieved a highly diastereoselective
synthesis of 1,3-disubstituted tetralin derivatives by Lewis acid
catalyzed benzylic C(sp3)–H bond functionalization. After careful
screening for the acid catalyst, we found that the use of Hf(OTf)4
was the key to achieving the high diastereoselectivity. Various
substituents, such as electron-donating and electron-withdrawing
groups on the aromatic ring, had a negligible effect on the reaction,
and a wide variety of 1,3-disubstituted tetralin derivatives were
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9628.
For examples of enantioselective internal redox reactions,
see: (a) S. Murarka, I. Deb, C. Zhang, D. Seidel, J. Am. Chem.
Soc. 2009, 131, 13226; (b) Y. K. Kang, S. M. Kim, D. Y. Kim, J.
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Chem. Asian J. 2012, 7, 2569; (h) Z.-W. Jiao, S.-Y. Zhang, C. He,
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K. S. Prakash, T. Bach, J. Am. Chem. Soc. 2006, 128, 966; (d) R.
View Article Online
Webstar, C. Böing, M. Lautens, J. Am. DCOheI:m10..1S0o3c9./C270C0C90,1171371K,
444.The low chemical yield of 4c was ascribed to the
formation of stilbene 6 (30%), which was produced via [1,5]-
hydride shift followed by the elimination of benzylic
hydrogen instead of cyclization.
[Hf]
O
O
O
O
MeO
OMe
MeO
OMe
E
Hf(OTf)4
H
+
E
3c
E = CO2Me
H
5
D
6
10 The structures of 4j and 5 were unambiguously established
by single-crystal X-ray analysis. CCDC-1533723 and 1533724
contain the supplementary crystallographic data of 4j and 5.
These data can be obtained free of charge from The
Cambridge
Crystallographic
Center
via
www.ccdc.cam.ac.uk/data_request. Data
11 M. D. Meyer, R. J. Altenbach, F. Z. Basha, W. A. Carroll, S.
Condon, S. W. Elmore, J. F. Kerwin Jr., K. B. Sippy, K. Tietje, M.
D. Wendt, A. A. Hancock, M. E. Brune, S. A. Buckner, I. Drizin
J. Med. Chem. 2000, 43, 1586.
12 Epimerization at C-1 position adjacent to methoxycarbonyl
group was another scenario for diastereomerization. This
possibility was excluded by the reaction of 4b having p-tolyl
group (poor electron donating group compared to PMP
group). Subjection of 4b (cis/trans = 1/1.7) under the
optimized reaction conditions (2.5 mol% Hf(OTf)4 in refluxing
ClCH2CH2Cl for 20 h) resulted in almost no change of
diastereomer ratio (cis/trans = 1/2.1).
6
MeO2
C
CO2Me
CO2Me
CO2Me
30 mol%
Sc(OTf)3
1
2.5 mol%
Hf(OTf)4
CO2Me
Tol
CO2Me
4b
ClCH2CH2Cl
reflux, 24 h
ClCH2CH2Cl
reflux, 20 h
PMP
3b
4b
84%, cis/trans = 1/1.7
81%, cis/trans = 1/2.1
7
8
9
For examples of construction of polycyclic framework with
contiguous stereogenic centers at 2,3-positions, see: (a) S.
Pastine, D. Sames, Org. Lett. 2005, 7, 5429; (b) S. Murarka, C.
Zhang, M. D. Konieczynska, D. Seidel, Org. Lett. 2009, 11,
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Org. Lett. 2012, 14, 4054; (d) P.-F. Wang, C.-H. Jiang, X. Wen,
Q.-L. Xu, H. Sun, J. Org. Chem. 2015, 80, 1155. See also, refs
7a, 7b, and 7h-l.
Some related diastereoselective reactions, see: (a) M.
Lautens, S. Hiebert, J. Am. Chem. Soc. 2004, 126, 1437; (b) F.
Mühlthau, O. Schuster, T. Bach, J. Am. Chem. Soc. 2005, 127,
9348; (c) F. Mühlthau, D. Staudier, A. Goeppert, G. A. Olah, G.
4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C7CC01717K
Highly diastereoselective synthesis of 1,3-disubstituted tetralin derivatives was achieved
by Hf(OTf)4-catalyzed benzylic C(sp3)–H bond functionalization.