Construction of seven-and eight-membered carbocycles by Lewis acid catalyzed C(sp3)-H bond functionalization

Article abstract of DOI:10.1039/c9cc08074k

We achieved a concise construction of seven-and eight-membered carbocycles via Lewis acid catalyzed C(sp³)-H bond functionalization. In these reactions, a quite rare [1,6 (or 7)]-hydride shift/cyclization process proceeded smoothly to afford seven-and eight-membered carbocycles with good chemical yields starting from substrates with high conformational freedom.

Full text of DOI:10.1039/c9cc08074k

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Construction of seven- and eight-membered
carbocycles by Lewis acid catalyzed C(sp3)–H
bond functionalization†
Cite this: Chem. Commun., 2019,
55, 13856
Received 15th October 2019,
Accepted 25th October 2019
Yuna Otawa and Keiji Mori
*
DOI: 10.1039/c9cc08074k
rsc.li/chemcomm
We achieved a concise construction of seven- and eight-membered ring heterocycles,7–12 but also five- and seven-membered ring
carbocycles via Lewis acid catalyzed C(sp3)–H bond functionalization. heterocycles13,14 could be synthesized by the [1,n (n = 4, 5, or 6)]-
In these reactions, a quite rare [1,6 (or 7)]-hydride shift/cyclization hydride shift/cyclization process. These reactions have two impor-
process proceeded smoothly to afford seven- and eight-membered tant characteristics: the reactions can be performed under normal
carbocycles with good chemical yields starting from substrates with reaction concentrations (higher than 0.1 M is acceptable) and
high conformational freedom.
occur intramolecularly. We envisioned that these characteristics
would lead to a novel synthetic method for the construction of
The construction of seven-membered or larger carbocycles is a seven-membered or larger carbocycles without special precautions.
major research topic in modern synthetic organic chemistry. We describe herein a positive answer to this assumption,
Because of their utility in pharmaceuticals and agrochemicals achieving the concise construction of seven- and eight-membered
as well as the difficulty of the construction due to ring strain and carbocycles via [1,n]-hydride shift triggered C(sp3)–H bond functio-
entropic reasons, many organic chemists have devoted much time nalization (n = 6 or 7). When a solution of benzylidene malonates
and effort to the development of an effective method for the having an alkylpropyl ether moiety at the ortho-position in
synthesis of this class of skeleton.1 Such strategies as ring-closing ClCH2CH2Cl was treated with a catalytic amount of Sc(OTf)3, the
metathesis (RCM),2 intramolecular cyclization reaction,3 [5+2]- and desired [1,6]-hydride shift/cyclization process proceeded smoothly
[4+3]-cycloaddition reactions,4 and ring expansion reaction5 have to afford seven-membered carbocycles in good chemical yields.
been developed. Among them, the intramolecular cyclization reaction Interestingly, a synthetically difficult, eight-membered ring for-
is the simplest strategy for the construction of the target structure. mation was also attainable by the [1,7]-hydride shift/cyclization
Most of the reported methods require relatively dilute conditions process starting from benzylidene malonates having an alkylbutyl
(lower than 0.05 M) and special precautions (e.g., dropwise addition ether moiety (Scheme 1).
of substrate) to suppress unwanted intermolecular reactions, thereby
The results of screening for the reaction conditions are illustrated
decreasing the practicality of the synthesis. Thus, the achievement of in Table 1. At first, a solution of benzylidene malonate 3a having
a cyclization reaction for the construction of a carbocyclic skeleton an O-benzylpropyl ether moiety in ClCH2CH2Cl was treated with
using a simple operation and non-high dilution conditions is not 5 mol% of Sc(OTf)3, which has shown excellent catalytic performance
a trivial issue, and there is a strong demand for the development of
a novel and effective method.
Recently, our group has been focusing on the development of
novel C(sp3)–H bond functionalization methods that involve
hydride shift-triggered C(sp3)–H bond functionalization, namely,
the ‘‘internal redox process’’.6–12 A notable feature of this method
is its high synthetic utility: various complex cyclic structures
could be constructed using this method. Not only six-membered
Department of Applied Chemistry, Graduate School of Engineering, Tokyo University
of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan.
E-mail: k_mori@cc.tuat.ac.jp; Fax: +81-42-388-7034; Tel: +81-42-388-7034
† Electronic supplementary information (ESI) available. CCDC 1953402. For ESI
Scheme 1 Formation of seven- and eight-membered carbocycles by
[1,n (n = 6, or 7)]-hydride shift triggered C(sp3)–H functionalization.
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9cc08074k
13856 | Chem. Commun., 2019, 55, 13856--13859
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Table 1 Examination of reaction conditionsa
Yieldb [%]
Entry
Catalyst
Time [h]
4a
3a
1
2
3
4
5
6
7
8
Sc(OTf)3
Yb(OTf)3
Hf(OTf)4
Gd(OTf)3
Mg(OTf)2
Zn(OTf)2
TiCl4
7
48
48
48
47
47
31
5
75
50
49
Trace
0
0
40
55
0
49
0
87
77
66
0
SnCl4
0
9
TfOH
21
51
2
1
51
3
30
38
56
41
Trace
60
66
34
17
0
0
58
0
10c
11d
12e
13 f
14g
15h
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Fig. 1 Substrate scope for seven-membered ring formation.
3
0
a
Unless otherwise noted, all reactions were conducted with 0.10 mmol of
the substituent effect on the aromatic ring was investigated, which
suggested that various substituents were acceptable in this reac-
tion. Seven-membered ring adducts 4b–h with electron-donating
groups (Me and OMe) and an electron-withdrawing group (F) were
obtained in good chemical yields (53–72%).15 This reaction was
3a in the presence of an acid catalyst (30 mol%) in ClCH2CH2Cl (1.0 mL)
b
c
d
e
at refluxing temperature. Isolated yield. In CH3CN. In toluene. In
f
g
o-xylene at 120 1C. In MeOH. The reaction was conducted under
h
0.50 M. 0.89 mmol scale.
in most of the internal redox reactions we have reported.8 applicable to the formation of seven-membered carbocycle 4i with
Gratifyingly, the desired reaction proceeded smoothly to give a naphthalene core (65% yield). The substituent on the oxygen
desired seven-membered carbocycle 4a in good chemical yield atom was not limited to a benzyl group: both allyl and ethyl ether
(75%, entry 1). Whereas Yb(OTf)3 and Hf(OTf)4 exhibited good derivatives also promoted the reaction effectively.
catalytic performance (50% and 49% yields, respectively, entries 2
Further examination revealed that the formation of eight-
and 3), only the recovery of 3a was observed with Gd(OTf)3 (87%, membered carbocycle 6a by way of the [1,7]-hydride shift process
entry 4). Both Mg(OTf)2 and Zn(OTf)2 were ineffective (entries 5 was also possible by employing benzylidene malonate 5a having
and 6). Versatile strong Lewis acids, such as TiCl4 and SnCl4, an O-benzylbutyl ether moiety. The desired [1,7]-hydride/cycliza-
promoted the reaction effectively, but the chemical yields remained tion process proceeded smoothly even with a low catalytic amount
moderate (40% and 55%, respectively, entries 7 and 8). The of Sc(OTf)3 (5 mol%), and corresponding eight-membered ring
chemical yield of 4a when TfOH (strong Brønsted acid) was used adduct 6a was obtained in good chemical yield (56%). Actually,
was only 30%, which clearly indicated that Lewis acids were the there was a report on C(sp3)–H bond functionalization involving a
effective acid catalysts in this reaction (entry 9).
[1,7]-hydride shift.16 However, this process was only observed in
Solvent screening was conducted with Sc(OTf)3 as the opti- relatively reactive substrates with an adjacent nitrogen atom and
mal catalyst, and the results suggested that ClCH2CH2Cl was in conformationally constrained substrates with a biphenyl link-
the solvent of choice (entries 10–13). In the case of CH3CN, the age. To the best of our knowledge, the present reaction is the first
reaction did not reach completion even with a long reaction time example of [1,7]-hydride shift involved C(sp3)–H bond function-
(4a: 38%, 3a: 17%, and 51 h, entry 10). Complete consumption of alization starting from a less reactive substrate without a nitrogen
3a was observed in some aromatic solvents, such as toluene and atom, and a substrate with high conformational freedom. The
o-xylene; however, the chemical yields of 4a remained moderate substrate scope for this process was sufficiently wide, as shown in
(56% and 41%, respectively, entries 11 and 12). Desired compound Fig. 2. Eight-membered carbocycles 6b–e with various substitu-
4a was not obtained at all when MeOH was employed as the ents on the aromatic ring and ethyl-ether type product 6f were
reaction medium (entry 13).
obtained in moderate to good chemical yields (45–82%).
Deuterium labeling experiments were conducted to clarify the
It should be noted that a high reaction concentration
(0.50 M) was acceptable, affording 4a in good chemical yield mechanism of both reactions (Scheme 2). Observation of the
(60%, entry 14). A scale-up reaction (0.89 mmol) resulted in the primary kinetic isotope effect in both cases (kH/kD = 2.0 for [1,6]-
formation of 4a without sacrificing the chemical yield (66%, hydride shift and kH/kD = 2.3 for [1,7]-hydride shift) suggested
entry 15), which clearly indicates the high practicability of the that the hydride shift process was the rate-determining step, like
present reaction.
most of the internal redox reactions reported so far.8–14
The products thus obtained could be transformed into
With the optimized reaction conditions determined, the
substrate scope of this reaction was examined (Fig. 1). At first, various related molecules, as shown in Scheme 3. Removal of
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more complex, medium-sized carbocycles by the hydride shift/
cyclization is under way.
Conflicts of interest
There are no conflicts of interest.
Notes and references
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C(sp3)–H bond functionalization. This reaction could be per-
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Chem. Commun., 2019, 55, 13856--13859 | 13859