Cobalt-catalyzed cyclization with the introduction of cyano, acyl and aminoalkyl groups

Article abstract of DOI:10.1039/c9ob00637k

An efficient synthesis of carbo-and heterocycles using CC, CO and CN bonds under cobalt catalysis is described. The substituents on olefins are key for controlling the regio-and chemoselectivity in the initial hydrogen atom transfer step and quaternary carbons are efficiently constructed under mild conditions. Cyclopropane cleavage and tandem cyclization give highly functionalized bicyclic skeletons in a single operation.

Full text of DOI:10.1039/c9ob00637k

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DOI: 10.1039/C9OB00637K
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Cobalt-catalyzed cyclization with the introduction of cyano, acyl and
aminoalkyl groups
Hiroto Hori,a Shigeru Arai*a,b and Atsushi Nishidaa,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
An efficient synthesis of carbo- and heterocycles using C=C, C=O and C=N bonds under cobalt catalysis is described. The
substituents on olefins are key for controlling regio- and chemoselectivity in the initial hydrogen atom transfer step and
quaternary carbons are efficiently constructed under mild conditions. Cyclopropane cleavage and tandem cyclization give
highly functionalized bicyclic skeletons in a single operation.
Introduction
A cyano group is a key functionality for determining the
properties of organic molecules.1 Therefore, its catalytic
introduction to carbon-carbon unsaturated bonds has
been a significant issue in synthetic chemistry.2 Among
various cyanation protocols, metal catalysis is
a
powerful tool for enhancing reaction efficiency and
controlling selectivity. Particularly, nickel catalysis using
simple and unactivated C-C multiple bonds has been an
important hydrocyanation since its discovery (Scheme
1-1),3 and styrene derivatives are representative
substrates for achieving high regioselectivity.4 Recent
applications have successfully expanded the substrate
diversity to the use of olefins,5 terminal alkynes,6 aryl
allenes,7 enynes,8 and allene-ynes. 9
On the other hand, cobalt catalysis has been attractive synthetic
tool,10 and an alternative hydrocyanation mediated by radical
species using simple olefins with TsCN and PhSiH3 is reported
(Scheme 1-2). 11 Carreira and a co-worker proved that it provided a
predictable regiochemistry and functional group tolerance, and
developed new reactions using TsN3,12 TsCl,13 BocN=NBoc,14 and
oximes.15 This radical strategy has great potential as a practical
transformation because the reaction can proceed at room
temperature and does not require the use of any toxic Sn reagents.
These characteristics prompted us to establish new applications,
and herein we report a new cyclization using olefins to connect C=C,
C=O and C=N bonds under cobalt catalysis (Scheme 2).
Results and discussion
Aiming at the development of cobalt-catalyzed hydrocyanative
cyclization and evaluation of its chemo-, regio- and
stereoselectivity, we initially investigated 1a with Co cat. (2 mol%)
using TsCN (1.2 eq) and PhSiH3 (1.2 eq). 11 The reaction proceeded
smoothly to give a mixture of cis- and trans-2a without producing
3a in 47% yield (cis:trans = 2.4:1) after 24 h (Table 1, entry 1). The
reaction efficiency was enhanced when 1b was used instead and
the reaction completed in 6 h to give 2b in 68% yield (cis:trans =
2.6:1) (entry 2). We propose that the catalytic cycle starts with
regioselective hydrogen atom transfer of red olefin (Scheme 3). A
hydride prefers to form a C-H bond at a terminal methylene. The
release of Co(II) gives secondary radical (A1), 10b,c which cyclizes to
A3, and the resulting primary radical would be converted to 2 by
trapping TsCN. 12 Since the conformational preference of A2 would
be favorable for cis-isomer formation, quaternary carbons in a
tether such as in 1c would be effective for stereoselectivity and
cyclization. As expected, 1c was transformed to cis-2c within 4 h in
82% yield, exclusively (entry 3). Chemoselectivity was observed
a. Graduate School of Pharmaceutical Sciences, Chiba University 1-8-1 Inohana,
Chuo-ku, Chiba, Japan 2608675, E-mail:araisgr@chiba-u.jp
b. Molecular Chirality Research Center, Chiba University 1-33 Yayoi-cho, Inage-ku,
Chiba, Japan 2638522
†
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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when non-symmetric dienes were used. For example, mono- and suitable for this cycloaddition and the single products (2k,l) were
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DOI: 10.1039/C9OB00637K
trisubstituted olefins in 1d were effectively differentiated to give exclusively obtained in respective yields of 38% and 31%.
cis-2d in 62% yield as a sole product (entry 4). The former olefin is
favored in the initial hydrogen atom transfer due to steric reasons.
Mono- and disubstituted olefins in 1e were also discriminable to
cyclize to 2e as a single product (entry 5). The tertiary carbon
radical center from red olefin in 1e would be favorably generated.
Benzene-fused azacycles were synthesized from 1f-h. All of the
reactions completed within 2 h with high regioselectivity in the
initial C-H bond formation at red olefins and the corresponding
cyclized products were obtained in 58% to 90% yield (entries 6-8).
In the case of indene formation, 1,1- and 1,2-disubstituted olefins in
1j were also differentiable to give a sole bicyclic product (2i) while
controlling three stereogenic centers, which was confirmed by X-ray
crystallographic analysis (entry 9). The cyclization of 1j proceeded
slowly to give a mixture of cis- and trans-2j in respective yields of
55% and 25% (entry 10).
Table 1 Hydrocyanative cyclisation of 1
Since cis-2j can be a useful precursor for a tricyclic system by
controlling three contiguous stereogenic centers, we next
investigated its transformation (Scheme 5). Treatment of cis-2j by
desilylation-bromination gave 3b, which was a precursor for double
alkylation. The first intramolecular alkylation using LDA gave 3c as
an inseparable diastereomixture (1.2:1), which was then
transformed to a single diastereomer (3d) by stereoselective
methylation in 90% yield (2 steps). This strategy
Entry
Substrate
Time
(h)
Products (%)
could be used for the synthesis of
a key
intermediate of Taiwaniaquinone H17 using
modified substrates.
1
2
3
1a: X = NPh
1b: X = NTs
24
6
2a: 47 (cis:trans =
2.4:1)
1c: X = C(CO Et)
4
2b: 68 (cis:trans =
2.6:1)
2
2
Scheme 4. [3+2] Cycloaddition via cyclopropane cleavage
cis-2c: 82
Co cat. (2 mol%)
H
PhSiH3 (1.2 eq)
TsCN (1.2 eq)
4
5
1d: E = CO Et
9
9
cis-2d: 62
H
2
TsN
TsN
Me
CN
Me
EtOH, rt, 2 h
R
R
1e: E = CO Et
2e: 72
2
1k: R = H
1l: R = Me
2k: 38% (single isomer)
2l: 31% (single isomer)
6
7
8
9
1f: R = Me, R = H
2
2
2
3
2f: 71
1
2
H-CoIII
TsCN
H
1g: R = R = Me
2g: 90 (dr = 1.9:1)
2h: 58 (dr = 1.3:1)
2i: 70 (single isomer)
1
2
1h: R = Et, R = H
H
1
2
H
H
TsN
TsN
1i
Me
H
Me
R
R
NTs
B4
B1
NC
H
H
5-exo
H
10
1j
16
cis-2j: 55
H
H
5-exo
TsN
TsN
with trans-2j: 25
Me
Me
R
R
B3
B2
The discrimination of olefins and sp2 carbons observed above was
next used as a radical clock (Scheme 4). If the initial hydrogen atom
transfer occurs on red olefins regioselectively, a [3+2] cycloaddition
sequence16 through cyclopropane-cleavage (B2) followed by 5-exo
cyclization (B3, B4) should be expected. Actually, 1k and 1l were
2l
2k
2 | J. Name., 2012, 00, 1-3
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Scheme 7. Hydroacylative cyclization of acylphosphates
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DOI: 10.1039/C9OB00637K
O
O
Co cat. (5 mol%)
PhSiH3 (1.5 eq)
N
O
N
O
Co
P(O)(OEt)2
t-Bu
t-Bu
EtOH, rt
R
R
H
t-Bu
t-Bu
4a-n
5a-n
Co cat.
O
O
O
O
O
Br
H
H
H
O
OMe
5a: 65% (4 h)
5b: 59% (3 h)
5c: 69% (10 h)
O
O
O
To clarify the synthetic utility of the above cyclization under cobalt
catalysis, we next examined the use of activated C=O bonds as
electrophiles for hydroacylation.18,19 Despite the broad generality of
metal-mediated carbonylation, many challenges still remain in
radical carbonylation because radical addition to C=O is a reversible
process. Therefore, the choice of nucleophilic carbon radicals and
cleavable C-X bonds is a key issue for promoting this transformation
(Scheme 6).20 However, previous studies used harsh conditions and
toxic Sn reagents, which are unsuitable for practical synthesis and
prevented the use of various electrophilic C-X bonds. To solve these
problems, a new acylation protocol was next examined using simple
olefins and acylphosphates21 as alternative radical precursors
(Scheme 7).
H
H
H
R
N
N
N
Ts
O2S
O2S
5d (R = H): 57% (4 h)
5e (R = Me): 57% (10 h)
F
OMe
5g: 46% (1 h)
5f: 56% (1 h)
H
O
EtO2C
EtO2C
EtO2C
EtO2C
O
O
H
H
Me
N
Ts
5i: 81% (18 h)
5j: 54% (9 h)
5h: 30% (3 h)
Ph
CO2Et
O
O
Ph
Ph
O
O
H
O
EtO2C
EtO2C
H
H
H
5n: 52% (6 h)
5m: 37% (15 h)
5k: 50% (10 h)
5l: 53% (8 h)
Since 1,1- and 1,2-disubstituted olefins were discriminable in the
reaction of 1g,i, carboacylative cyclization was examined next. Red
olefin in 4o was more favorable for initial and regioselective
hydrogen atom transfer,10b and the subsequent cyclization
sequence on trans-olefin gave tricyclic products (5o) (Scheme 8).
The structure of the major diastereomer was established to be
trans-5o by X-ray crystallographic analysis.
Initially, benzene-fused substrates (4a-h) with Co cat. (5 mol%)
were used for hydroacylative cyclization to construct a 6-membered
ring with oxygen and nitrogen atoms (5a-h). The olefinic carbons in
4a were effectively differentiated to cyclize at the more-substituted
sp2 carbon and the corresponding adduct (5a) was obtained as a
sole isomer in 65% yield. A bromo substituent on the benzene ring
was inert under this radical cyclization and gave 5b in 59% yield. A
methoxy group as well as various sulfonamides instead of oxygen
showed similar reactivity to give the cyclized products (5c-g) in 46%
to 57% yield, and a spiro ring-system in 5h could also be used for its
construction. In the formation of cyclopentenones using linear
aliphatic substrates, quaternary carbons in precursors increased the
cyclization efficiency. For example, malonate derivative (5i) was
obtained in 81% yield and other cyclopentenones (5j-l) were
obtained in moderate yield. However, formation of a 6-membered
ring decreased the yield of 5m to 37% even after 15 h. In lactone
formation, a similar hydroacylative cyclization proceeded to give 5n
as a single product in 52% yield.
Scheme 8. Tandem cyclization of 4o
O
H
H
Me
NSO2Ar
O
H
Co cat. (5 mol%)
PhSiH3 (1.5 eq)
TsN
(EtO)2(O)P
TsN
trans-5o: 28%
NSO2Ar
+
EtOH, rt, 2 h
Me
O
H
H
Me
NSO2Ar
4o: Ar = 4-FC6H4
H
TsN
cis-5o: 16%
X-ray crystallographic
analysis of trans-5o
To our delight, C=N bonds could also be used for this Sn-free radical
protocol under cobalt catalysis (Table 2). Keto-oximes22 are
attractive precursors for facile access to quaternary carbons,
however only limited methods for their production, using Sn
reagents,23 Rh catalysis,24 and photocatalysis,25 are known. We
expected that stable and readily available oximes could be used for
our protocol as radical acceptors.
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conditions. Various unsaturated bonds such as C=C, VCie=wOAraticnledOCn=linNe
Table 2 Cyclisation of 6
DOI: 10.1039/C9OB00637K
act as radical acceptors to give a wide variety of hetero- and
carbocycles and multi-ring systems in a single operation. These
achievements may lead to new applications in synthetic chemistry
and further studies are currently underway.
Entry
Substrate
Time (h)
Products (%)
Experimental
1
2
3
4
5
6
(Z)-6a: R1 = H
8
7a: 0
Typical procedure for Co-catalyzed hydrocyanative
cyclization, synthesis of cis-2c: To a solution of diene (66.1
mg, 0.28 mmol), TsCN (59.9 mg, 0.33 mmol) and PhSiH3 (40.6
L, 0.33 mmol) in EtOH (1.38 mL) was added Co complex
(3.33 mg, 0.006 mmol, 2 mol%) at room temperature. After
being stirred for 4 h under argon, the solvent was removed
under reduced pressure. The resulting residue was charged
on a silica gel pad. Column chromatography (hexane:AcOEt =
20:1) gave cis-2c (60.3 mg, 0.23 mmol, 82%) as a colorless oil.
(Z)-6b: R1 = Bn
(Z)-6c: R1 = Bz
24
4
7b: 0
7c: 35
7d: 52
7d: 60
7e: 61
7f: 66
(Z)-6d: R1 = TBS
(E)- and (Z)-6d: R1 = TBS1)
(E)- and (Z)-6e: R1 = Ac2)
6f3)
5
5
8
7
5
8
9
6g: E = CO Et4)
5
6
7g: 96
7h: 83
2
6h: E = CO Bn5)
2
Conflicts of interest
There are no conflicts to declare.
10
11
6i6)
7
7i: 0
7j: 0
6j7)
24
Acknowledgements
This work was supported by the Grant-in-Aid from Japan
Society for the Promotion of Science (JSPS) (Grant No. 17K08205)
1) A mixture (E:Z = 1:3.1) was used. 2) E:Z = 1:1.5. 3) E:Z = 1:7.3.4)
E:Z = 1:5.7. 5) E:Z = 1:2.6. 6) E:Z = 4.8:1. 7) E:Z = 1:1.
First, we investigated the effect of OR groups using (Z)-6a-d.26
Although hydrogen and a benzyl group were inert, a benzoyl group
promoted the cyclization and 7c was obtained in 35% yield (entries
1-3). This reaction gave contiguous quaternary carbons on a
pyrrolidine ring and the yield was improved to 52% by TBS
protection of oxime (6d) (entry 4). To evaluate the effect of the
stereochemistry of a C=N bond, a mixture of (E)- and (Z)-6d was
next used and the corresponding cycloadduct (7d) was obtained in
60% yield (entry 5). This result indicates that cyclization efficiency
was not influenced by the stereochemistry of a C=N bond. An acetyl
group was also suitable for enhancing the cyclization efficiency and
7f was obtained in 66% yield (entry 6). The oxime derived from an
aliphatic ketone such as 6f also cyclized to give 7f as a single isomer
in 66% yield (entry 7). Malonate derivatives as cyclization
precursors were efficiently converted to 7g and 7h in respective
yields of 96% and 83% (entries 8,9). The cyclization efficiency was
strongly dependent on ring size, and 6i was not transformed to 7i
because of a messy reaction (entry 10). In addition, aldoxime 6j was
ineffective for achieving cyclization (entry 11).
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DOI: 10.1039/C9OB00637K