Enantioselective Copper-Catalyzed Three-Component Carboboronation of Allenes: Access to Functionalized Dibenzo [b,f][1,4]oxazepine Derivatives

Article abstract of DOI:10.1002/adsc.201900503

A copper-catalyzed enantioselective three-component difunctionalization of allenes with seven-membered cyclic imines and bis(pinacolato)diboron (B₂(Pin)₂) to approach functionalized dibenzo[b,f][1,4]oxazepine derivatives is developed. The chiral products are obtained in up to 81% yield, >20:1 dr, and 98% ee when either a chiral diphosphine ligand or a chiral ferrocenyl-based P,N-ligand is used. Furthermore, the reaction exhibits reversed diastereoselectivities when the chiral diphosphine ligand and the chiral P,N-ligand are used respectively. (Figure presented.).

Full text of DOI:10.1002/adsc.201900503

Title: Enantioselective Copper-Catalyzed Three-Component
Carboboronation of Allenes: Access to Functionalized
Dibenzo[b,f][1,4]oxazepine Derivatives
Authors: Hao Deng, Ziwei Meng, Sifan Wang, Zheming Zhang, Yaqi
Zhang, Yu Shangguan, Fazhou Yang, Dekai Yuan, Hongchao
GUO, and Cheng Zhang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Adv. Synth. Catal. 10.1002/adsc.201900503
Link to VoR: http://dx.doi.org/10.1002/adsc.201900503

10.1002/adsc.201900503
Advanced Synthesis & Catalysis
VERY IMPORTANT PUBLICATION
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Enantioselective Copper-Catalyzed Three-Component
Carboboronation of Allenes: Access to Functionalized
Dibenzo[b,f][1,4]oxazepine Derivatives
Hao Denga, Ziwei Menga, Sifan Wanga, Zheming Zhanga, Yaqi Zhanga, Yu
Shangguana, Fazhou Yanga, Dekai Yuana, Hongchao Guoa, and Cheng Zhanga*
a
Department of Applied Chemistry, China Agricultural University, 2 Yuanmingyuan West Road, Beijing 100193, China.
Email: zhangc9711@cau.edu.cn or cheunggilbert@gmail.com
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropr
reported being suitable substrates to undergo such
three-component reactions.[5] Surprisingly, only
recently have allenes been investigated to generate
the allyl copper species reacting with electrophiles[6],
such as carbonyl compounds[7], imines[8], electron
deficient alkenes[9], and others[10]. Among these,
boron-substituted homoallylic amines were obtained
when imines were employed as electrophiles, and its
asymmetric versions were also achieved when
specific chiral ligands were used.
Abstract. A copper-catalyzed enantioselective three-
component difunctionalization of allenes with seven-
membered cyclic imines and bis(pinacolato)diboron
(B2(Pin)2)
to
approach
functionalized
dibenzo[b,f][1,4]oxazepine derivatives is developed. The
chiral products are obtained in up to 81% yield, >20:1 dr,
and 98% ee when either a chiral diphosphine ligand or a
chiral ferrocenyl-based P,N-ligand is used. Furthermore,
the reaction exhibits reversed diastereoselectivities when
the chiral diphosphine ligand and the chiral P,N-ligand
are used respectively.
Keywords: cross-coupling; difunctionalization of
allenes ; copper-catalyzed; asymmetric catalysis; boron
Homoallylic amines are important organic synthons
to make natural products and biologically active
compounds, and the development of synthetic
methods to obtain such compounds has always been
one of the interested research fields.[1] Boron-
substituted homoallylic amine is of great interest for
synthetic chemists, because they are able to be
converted into diverse valuable motifs concisely.
However, such highly functionalized boron-
substituted homoallylic amines are hard to be
obtained by traditional strategies, and it was not until
2006 that the first example for the synthesis of
enantioselective
boron-substituted
homoallylic
amines via a sequential palladium-catalyzed three-
component method was reported by Morken and
coworkers.[2] In spite of its effectiveness, the method
suffered from complex procedures and high-cost
catalysts.
Scheme 1. Previous work for copper-catalyzed three
component carboboronation of allenes.
In 2006, Sadighi made a seminal report that a boryl
copper(I) complex could insert into alkenes and form
boron-substituted alkyl copper species[3] which would
undergo further nucleophilic addition or substitution
reaction with electrophiles.[4] Alkynes are also
In 2016, Procter group disclosed a highly
chemoselective and diastereoselective N-heterocyclic
carbene
(NHC)/Cu-catalyzed
three-component
borylcarbonation of allenes with B2(Pin)2 and
1
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10.1002/adsc.201900503
Advanced Synthesis & Catalysis
imines.[8a] Latterly, they achieved an enantioselective
version of the reaction by employing a chiral NHC-
ligand combined with copper iodide, and it afforded
the chiral homoallylic amines with moderate to high
reactivity and low selectivity. As a result, expanding
the substrate scope to phenyl allenes and cyclic
imines, as well as the developement of diverse
effective chiral ligands are highly desired. Given the
prevalence of the scaffold of dibenzo-1,4-oxazepines
in therapeutic drug development,[13] we are interested
to incorporate such chiral boron-substituted
homoallylic amines into the dibenzo-1,4-oxazepines
by using seven-membered heterocyclic imines as
electrophiles. Herein, we report a diastereo- and
enantioselective copper-catalyzed three-component
difunctionalization of allenes with B2(Pin)2 and
seven-membered heterocyclic imines in the presence
of a chiral diphosphine- or P,N-ligand ， and the
diastereoselectivity for the reaction is reversed when
diastereoselectivities
and
enantioselectivities
(Scheme 1a).[11] Very recently, Hoveyda et al
developed an effective method to synthesize
homoallylic primary amine with exclusive
diastereoselectivities and high enantioselectivities via
three-component coupling reactions of B2(Pin)2, alkyl
allenes, and NH-ketimine salts (Scheme 1c).[12]
However, among these, only the type of chiral NHC-
ligands was employed to control diastereoselectivities
and enantioselectivities for the homoallylic amines.
In addition, the substrate scope is confined to acyclic
imines and mostly alkyl allenes, while cyclic
imines[8b] and phenyl allenes[11] showed sluggish
the
two
chiral
ligands
are
switched.
Table 1. Optimization of reaction conditions[a]
Entry
1
2
3
4
5
6
7
8
[M]/L (x mol %)
CuCl/L1 (10)
CuCl/L2 (10)
CuCl/L3 (10)
CuCl/L4 (10)
CuCl/L5 (10)
CuCl/L6 (10)
CuCl/L7 (10)
CuCl/L8 (10)
CuBr/L5 (10)
CuI/L5 (10)
CuCl/L5 (10)
CuCl/L5 (10)
CuCl/L5 (10)
CuCl/L5 (10)
CuCl/L5 (10)
CuCl/L5 (10)
CuCl/L5 (15)
CuCl/L5 (5)
Base
Solvent
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
THF
Yield (%)[b]
Dr (4aa:5aa)[c] Ee (%, major)[d]
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
LiOtBu
NaOtBu
KOMe
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
14
57
91
78
77
54
32
26
15
10
70
57
59
trace
trace
73
57
27
52
-
0
-
0
2:1
>20:1
5:1
1:4
-
67
72
87
96
3
-
0
9
-
-
10
11
12
13
14
15
16
17
18
19[e]
-
-
4:1
4.2:1
4:1
-
49
87
86
-
1,4-dioxane
Toluene
PhCF3
PhCF3
PhCF3
-
-
5:1
4:1
-
86
80
52
88
CuCl/L5 (10)
5:1
[a]
Unless otherwise indicated, all reactions were carried out with 1a (0.1 mmol), 2a (0.3 mmol), 3 (0.11 mmol), and base
(0.1 mmol) with copper salt/ligand in 1 mL of solvent at 25 C for 12 hours. [b]Total isolated yield. Determined by H
o
[c]
1
NMR. [d] Determined by HPLC analysis using a chiral stationary phase. [e] Reaction was carried out at 10 oC.
2
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10.1002/adsc.201900503
Advanced Synthesis & Catalysis
At the outset of the experiment, phenyl allene 1a,
4
5
6
7
8
4-MePh (2d) 4ad
4-EtPh (2e)
4ae
4-CF3Ph (2f) 4af
2-MePh (2g) 4ag
3-MePh (2h) 4ah
57
50
65
65
68
3:1
85
88
81
92
88
seven-membered heterocyclic imine 2a, and B2(pin)2
were subjected to various copper salts, ligands, bases,
and solvents, to probe optimal reaction conditions
(Table 1).[14] First, CuCl, PhCF3, and KOtBu were
selected as the copper salt, solvent, and base
respectively, and various chiral ligands are subjected
to the reaction to investigate their catalytic efficacy
(Table 1, entries 1-8). Monophosphoramidite ligands
(L1 and L2) gave sluggish to moderate yields of 4aa
but they did not exhibit any activity for the chiral
induction of products (Table 1, entries 1-2). To great
delights, the diphosphine ligand L3 affords the final
product 4aa in 91% total yields, 2/1 dr, and 67% ee
(Table 1, entry 3). Ferrocene-based chiral P,N-ligand
L4 afforded the product with improved yield,
diastereoselectivity and enantioselectivity (Table 1,
entry 4), and replacement of L4 with L5 made the best
performance when it furnished product 4aa in 77%
yield, 5/1 dr, and 87% ee (Table 1, entry 5).
3.2:1
4.6:1
16:1
4.6:1
2-MeOPh
(2i)
9
64
64
6.6:1
4:1
92
89
4ai
3,4-Me2C6H4
(2j)
10
4aj
2,6-Me2C6H4
(2k)
11
81
65
50
10:1
94
98
76
4ak
12[e]
Cy (2l)
PhCH2CH2
(2m)
>20:1
10:1
4al
13
4am
[a]
Unless otherwise indicated, all reactions were carried
out with 1a (0.1 mmol), 2 (0.3 mmol), 3 (0.11 mmol), and
KOtBu (0.1 mmol) with CuCl/L5 (10 mmol %) in 1 mL of
PhCF3 at 25 C for 12 hours. [b]Total isolated yield.
o
[c]
Determined by 1H NMR. [d] Determined by HPLC analysis
using a chiral stationary phase. [e] L6 was used as the ligand.
Interestingly,
the
reaction
when
gave
the
reversed
stereo-bulky
diastereoselectivity
diphosphine ligand L6 was used, and product 5a was
obtained in 54% yield and 96% ee (Table 1, entry 6).
However, N,N-ligand L7 and P,N-ligand L8 were not
suitable ligands when they gave poor yields of
products and low enantioslectivities (Table 1, entries
7-8). Replacing CuCl with CuBr or CuI greatly
impaired the reaction when the product was obtained
in extremely low yield (Table 1, entries 9-10). The
reason for this phenomenon might be that NHC/Cu
complexes formed from CuBr and CuI are prone to
be affected by trace of water in PhCF3. Various bases
were then examined as well (Table 1, entries 11-13),
and tBuOK still gave the best performance, and other
After the determination of optimal reaction
conditions, we then evaluated the efficiency of the
catalytic method by expanding the substrate scope of
allenes. Allenes bearing various substituents as well
as diverse substituted patterns were subjected to the
optimal reaction conditions (Table 2). With respect to
phenyl allenes, no matter electron-withdrawing
groups or electron-donating groups substituted phenyl
allenes, corresponding products are obtained in
moderate to good yields and diastereoselectivities,
along with high enantioselectivities (Table 2, entries
1-11). Strong electron-withdrawing group CF3 on 4-
position of the phenyl ring showed corrosive effect
on the enantioselectivity of corresponding product
(Table 2, entry 6). Regarding substituted positions on
the phenyl ring, 2-substituted phenyl allene gave
much higher diastereoselectivity than the 3- or 4-
substituted counterparts (Table 2, entries 4, 7 and 8).
Interestingly, the 2,6-dimethylphenyl allene afforded
the corresponding product with much better yields
t
t
bases, such as BuOLi, BuONa, and MeOK, gave
decreased yields, diastereoselectivities, and
enantioselectivities of the product. Finally, different
kinds of solvents were screened (Table 1, entries 14-
16), and it showed that THF and 1,4-dioxane were
unfavorable solvents when trace of product 4aa was
detected. Toluene showed relatively inferior result to
PhCF3. Increasing or decreasing the loading amounts
of the copper salt and the ligand gave harmful results
to the reaction (Table 1, entries 17-18). Lowered
temperature did not show better diastereoselectivity
and enantioselectivity, but afforded the product in
much lower yield (Table 1, entry 19).
and
diatereoselectivity, along with
higher
enantioselectivity than the 3,4-dimethylphenyl allene
(Table 2, entry 11). The alkyl allene 2l is a suitable
substrate for the reaction since it formed the
corresponding
product
4al
in
excellent
diastereoselectivity and enantioselectivity albeit in
moderate yield (Table 2, entry 12). However, when
phenylethyl allene 2m is subjected to the reaction,
both the diastereoselectivity and enantioselectivity of
the product were greatly decreased (Table 2, entry
13). In addition, the absolute configuration of product
4ac was determined by X-ray crystallography.[15]
Table 2. Substrate scope expansion for allenes[a]
Table 3. Substrate scope expansion for imines[a-d]
Yield Dr
Ee
Entry R1
4
(%)[b] (4:5)[c]
(%)[d]
87
1
2
3
Ph (2a)
4-BrPh (2b)
4-ClPh (2c)
77
76
75
5:1
3.4:1
4:1
4aa
4ab
4ac
85
87
3
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10.1002/adsc.201900503
Advanced Synthesis & Catalysis
imines with electron-donating groups gave better
results (4da-4ha). Position of substituent on the
phenyl ring of imines did not affect the
diastereoselectivity and enantioselectivity too much
for the reaction.
Table 4. Substrate scope for reversed diastereoselectivities
using L6 as the ligand[a]
Yield
(%)[b]
54
58
50
Dr
Ee
(%)[d]
96
97
93
94
-
Entry R1
5
(5:4)[c]
4:1
1
2
3
4
5
Ph (2a)
5aa
5ab
5ac
5ad
5ae
4-BrPh (2b)
4-ClPh (2c)
4-MePh (2d)
Cy
7:1
>20:1
10:1
<1:20
56
-
[a] All reactions were carried out with 1a (0.1 mmol), 2 (0.3
mmol), 3 (0.11 mmol), and KOtBu (0.1 mmol) with
o
CuCl/L6 (10 mmol %) in 1 mL of PhCF3 at 25 C for 12
hours. [b]Total isolated yield. [c] Determined by 1H NMR. [d]
Determined by HPLC analysis using a chiral stationary
phase.
Since the reaction with ligand L6 gave reversed
diastereoselectivity during optimization of reaction
conditions (Table 1, entry 6), we then tried to obtain
the diastereoisomers for other substrates by switching
the ligand from L5 to L6 (Table 4). Results showed
that phenyl allenes bearing substituents with different
electronic properties (Br, Cl, and Me) were all
suitable substrates, and the corresponding
diastereoisomers are obtained in moderate yields,
good to excellent diastereoselectivities, along with
excellent enantioselectivities (Table 4, entries 2-4).
However, when alkyl allene 2l is subjected to the
same conditions, the reaction did not show reversed
diastereoselectivity (Table 4, entry 5 and Table 2,
entry 12). Reactions underwent sluggishly when
imines bearing substituents on the phenyl ring are
employed in the reaction, and the corresponding
diastereoselectivity-reversed products 5 are obtained
in low yields.[14]
[a] All reactions were carried out with 1 (0.1 mmol), 2a (0.3
mmol), 3 (0.11 mmol), and KOtBu (0.1 mmol) with
o
CuCl/L5 (10 mmol %) in 1 mL of PhCF3 at 25 C for 12
hours. [b]Total isolated yield. [c] Determined by 1H NMR. [d]
Determined by HPLC analysis using a chiral stationary
phase.
We then examined the substituent effects of the
seven-membered heterocyclic imines on the reaction
(Table 3). Typically, imines bearing electron-
withdrawing group on both sides of phenyl rings
afforded higher yields of corresponding products
(4ba, 4ca, and 4ja) than that with electron-donating
groups (4da-4fa, and 4ha), except for 4ga. The
reason might be that electron-withdrawing groups
make the imines more electrophilic to undergo
nucleophilic attack by allyl copper intermediates.
With respects to diastereoselectivities, however,
4
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10.1002/adsc.201900503
Advanced Synthesis & Catalysis
mmol) in 0.5 ml PhCF3 was added and the
resulting mixture was stirred at 25 ºC for 12 h.
After the reaction is finished, the mixture was
filtered through a short silica column and
concentrated, and diastereoselectivities of
1
products were determined by H-NMR analysis.
Finally,
boron-subsituted
dibenzo[b,f][1,4]oxazepine derivative 4 or 5 was
obtained after the mixture was purified by silica
gel column chromatography (hexane
EtOAc=10:1).
:
Acknowledgements
We are grateful to the National Key Research and Development
Plan of China (2017YFD0200504). This work is also partly
supported by the Start-Up Funds from CAU. Support from
College of Science @CAU is also gratefully acknowledged.
Scheme 2. Scale-up synthesis of product 4ac and 5ac.
References
In the end, scale-up syntheses of 4ac and 5ac were
also performed (Scheme 2). Under optimal reaction
conditions by using L5 as the ligand, product 4ac is
obtained in 80% yield, 4.3:1 dr, along with 80% ee.
As expected, replacing ligand L5 with Ligand L6
resulted in product 5ac in 40% yield, >20:1 dr, and
90% ee.
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three-
component coupling reaction of allenes with
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diphosphine ligand or a chiral ferrocenyl-based
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exhibited reversed diastereoselectivities when the
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Experimental Section
General procedure for asymmetirc
carboboronation of allenes: CuCl (1 mg, 0.01
mmol, 10 mol %), a ligand (0.01 mmol, 10
mol %), KOtBu (11.2 mg, 0.1mmol) ,
dibenzo[b,f][1,4]oxazepine 1 (0.1mmol) and
allene 2 (0.3mmol) were placed in an oven dried
20 mL Schlenk flask. The flask was evacuated
for 0.5 h and backfilled with argon. PhCF3 (0.5
mL) were added at room temperature under
argon atmosphere. Then, B2(pin)2 (28 mg, 0.11
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Advanced Synthesis & Catalysis
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