Copper-catalysed Csp3-Csp cross-couplings between cyclobutanone oxime esters and terminal alkynes induced by visible light

Article abstract of DOI:10.1039/d0cc00988a

A novel transformation for the construction of Csp³-Csp bonds was achieved via a photo-induced copper-catalysed C-C bond cleavage. This approach was applied to prepare a series of highly functionalized alkynyl nitriles using readily available cyclobutanones and terminal alkynes. Mechanistic exploration showed that the in situ generated copper acetylide complex is an effective photosensitive catalyst to promote the C-C bond cleavage of cycloketoxime esters through a radical process.

Full text of DOI:10.1039/d0cc00988a

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DOI: 10.1039/D0CC00988A
Journal Name
COMMUNICATION
Copper-Catalysed
Csp3-Csp
Cross-Couplings
between
Cyclobutanone Oxime Esters and Terminal Alkynes Induced by
Visible Light
Received 00th January 20xx,
Accepted 00th January 20xx
Binlin Zhao,a, b, d* Yixiao Wu,b,c,d Yu Yuan,c and Zhuangzhi Shib*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel transformation for the construction of Csp3-Csp bond was Scheme 1 Csp3-Csp cross-couplings through C-C bond cleavage.
achieved via a photo-induced copper-catalysed C-C bond cleavage.
a) Mn-catalyzed alkynylation through C-C bond cleavage
This approach was applied to prepare
a series of highly
R1 OH
R3
R4
Mn(OAc)3, Bipy
DMP
O
O
S
functionalized alkynyl nitriles using readily available
cyclobutanones and terminal alkynes. Mechanistic exploration
showed that the in situ generated copper acetylide complex is an
effective photosensitive catalyst to promote the C-C bond
cleavage of cycloketoxime esters through a radical process.
Visible-light-mediated photocatalysis is being recognized as a
powerful and versatile tool for a growing number of organic
transformations.1 However, particular drawbacks of the pervasive
metal complexes based on iridium and ruthenium with respect to
practical application and sustainability could not be ignored. The
investigation of photoredox reactions, copper-based photoactive
complexes have already been attractive and popular photo catalysts
for several single-electron-transfer transformations.2 Recently, the
interesting photo-induced copper-catalysed reactions were
gradually reported, which served as alternative and reliable
strategies for C–C3, C–N4, C–O5 and C–S6 cross-coupling reactions. In
R4
Ph
R2
+
R1
CH3CN, rt
O
R2
R3
b) Photoredox-catalyzed alkynylation through C-C bond cleavage
Me
Me
N
COOH
O
O
R
I
R
O
4ClCzlPN
NC
+
c) This work:
CF3
+
O
R
Cu(OTf)2, L, Base
NC
O
N
R
which, the copper salts could interact with the active nucleophiles of Csp3-Csp bonds via C-C bond cleavage is limited so far. 10 Zhu and
affording the photo-sensitive copper-nucleophilic complexes. The co-workers disclosed a Mn-catalysed Csp3-Csp bond formation
formed copper complexes would tended to absorbed photos upon under oxidative conditions involving a radical process (Scheme 1,
the light irradiation to generate reductive excited state complexes, a).10a Recently, Waser reported a photo-induced oxidative ring
which could activate the oxidative potential substrates to furnish opening and Csp3-Csp coupling by employing cycloketoximes and
the radical species. Finally, the subsequent radical coupling would EBX reagents via radical-mediated C-C bond cleavage process.10b
deliver the desired products.
Pretreated alkynes in these cross-coupling reactions represents a
considerable obstacle.
Alkynes are important functional moieties in organic synthesis
and it can be applied as versatile building blocks in pharmaceutical
Few reports shows that, the available copper acetylide complex
chemistry, materials, and the chemical industry through having high reducing ability (E1/2= -1.77 V vs. SCE)11 served as an
cycloaddition, metathesis and cross-coupling reactions.7,8 However, effective photosensitizer for photoinduced copper-catalysed
the construction of Csp3-Csp bonds has remained a considerable alkylations12 and other functionalizations.13 Inspired from previous
challenge in organic chemistry.9 Therefore, reports on construction
discoveries and our interest in single-electron-transfer (SET)-
induced C-C bond cleavage of cycloketoxime esters,14 we
envisioned that the photoexcitation of copper acetylide complex
could promote the reductive SET process of cycloketoxime esters to
accomplish a Csp3-Csp cross-coupling reactions in the absence of
additional photoredox catalyst. Herein, we report the first photo-
induced copper-catalysed Csp3-Csp cross-coupling reaction through
both in situ generation of copper acetylide complex and C-C bond
cleavage furnishing the alkynyl nitriles under mild and practical
reaction conditions.15
a.
Department of Chemistry and Materials Science, College of Science, Nanjing
Forestry University, Nanjing 210037, China, E-mail: zhaobinlin@njfu.edu.cn
State Key Laboratory of Coordination Chemistry, School of Chemistry and
b.
Chemical Engineering, Nanjing University, Nanjing, 210093, China, E-mail:
shiz@nju.edu.cn
c. College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou
225002, China
d. These authors contributed equally.
†Electronic Supplementary Information (ESI) available: Detailed experimental
procedures, characterization of starting materials and products and
crystallographic data. See DOI: 10.1039/x0xx00000x
Initial experiments were investigated by employing
cyclobutanone O-(4-(trifluoromethyl)benzoyl) oxime (1a) and
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Table 1 Optimization studiesa
Table 2 Substrate Scopea
View Article Online
DOI: 10.1039/D0CC00988A
CF3
CF3
Cu(OTf)2 (10 mol%), L1 (20 mol%)
K2CO3 ( 3 equiv), TBAI (5 mol%)
O
N
O
N
Cu(OTf)2 (10 mol%), L1 (20 mol%)
R
CN
O
K2CO3 ( 3 equiv), TBAI (5 mol%)
DMF : CH3OH (3 : 1), Ar, RT
Blue LEDs
O
Ph
NC
X
DMF : CH3OH (3 : 1), Ar, RT
Blue LEDs
2a
(
)
Ph
Ph
R
1a
3aa
X
2a
(
)
Ph
Yield
(%)b
3
1
Variation from the “standard conditions”
none
Entry
NC
NC
NC
NC
83 (73)c
1
2
3
4
5
6
7
8
9
Ph
Bn
Ph
Ph
Ph
3ca
Ph
3aa
3ba
, 82%
, 73%
, 51%
, 71%
, 37%
without Cu(OTf)2, L, base or light
without TBAI
0
57
59
0
NC
NC
NC
OBn
Ph
COOEt
CN
Ph
Ph
L2 instead of L1
L3 or L4 instead of L1
b
3da
3fa
3ea
, 47%
Me
NC
NC
Cs2CO3 or KOTf instead of K2CO3
CuCl instead of Cu(OTf)2
without DMF or MeOH
CH3CN instead of DMF
Ph
0
O
Br
65
trace
75
Ph
Ph
N
Boc
tBu
3ga
3ha
3ia
, 85%
, 77%
, 59%
R
N
Me
Bn
NC
NC
NC
NC
R
R
N
N
NC
NC
N
N
Ph
Ph
Ph
Ph
N
N
Ph
3ja
3ka
3la
, 77%
Ph
, 78%
, 43%
, 55%
H
L
R = H,
R = tBu,
L3
L4
1
Ph
L
H
2
O
a1a (0.2 mmol), 2a (0.24 mmol), Cu(OTf)2 (10 mol%), L1 (20 mol%), K2CO3 (3
equiv) and TBAI (5 mol%) in a mixture of DMF : CH3OH (3 : 1, 0.1 M), under
Ar, rt, blue LEDs, 3 h. bGC yield using dodecane as internal standard.
cisolated yield.
3ma
3na
3oa
, 74%
, 52 %
a1 (0.2 mmol), 2a (0.24 mmol), Cu(OTf)2 (10 mol%), L1 (20 mol%), K2CO3 (3
equiv) and TBAI (5 mol%) in a mixture of DMF : CH3OH (3 : 1, 0.1 M), under
Ar, rt, blue LEDs, 3 h; isolated yield. bDMF : EtOH (3 : 1, 0.1 M).
phenylacetylene (2a) with the participant of Cu(OTf)2 as catalyst
and 2,2':6',2''-terpyridine as the ligand in the presence of K2CO3 (3
equiv) and tetrabutylammonium iodide (TBAI) (5 mol%) in the
mixture of DMF and MeOH under the irradiation of blue LEDs
affording the desired product 6-phenylhex-5-ynenitrile (3aa) in 83%
yield (entry 1). Notably, no desired product was observed in the
absence of Cu(OTf)2, ligand, base or light (entry 2). Furthermore,
the alkyl nitrile 3aa was detected in 57% yield without the addition
of TBAI (entry 3), which suggested that TBAI, as a common phase
transfer catalyst, promoted the dissolution of copper acetylide
complex and base. Meawhile, the coupling was optimized by
varying ligands, when the ligand was changed to p-tBu-substituted
terpyridine (L2), lower activity was observed furnishing the product
3aa in 59% yield (entry 4). However, 2,2'-bipyridine (L3) and 1,10-
phenanthroline (L4) showed worse performance, no desired
reaction was observed (entry 5). It is worthy to mention that no
desired product 3aa was formed when Cs2CO3 or KOTf was used
instead of K2CO3 (entry 6). The encouraging conversion was
observed when the reaction was subjected with CuCl afforded 3aa
in 65% yield (entry 7). Delightfully, the solvents played important
roles in this transformation, only trace amount of 3aa was obtained
if DMF or MeOH was absent in the reaction system (entry 8). In
addition, the yield of alkynyl nitriles (3aa) was slightly decreased to
75% with replacement of DMF to CH3CN (entry 9).
could be prepared conveniently in satisfactary yields from the
corresponding ketones, which are commercial available or
syntesized from alkenes according to the known procedures.16
Cyclobutanone oximes bearing Bn, Ph and OBn groups on the
cyclobutanone rings afforded the corresponding products in
moderate to good yields (3ba-3da). Although the ester and cyano
group substituted cyclobutanone oximes (1e-1f) were totally
consumed, lower yield of products (3ea-3fa) were obtained owing
to the alcoholysis process under the basic conditions. Meanwhile,
ether substituted substrate performed well to yield the product 3ga
in 77% yield. The disubstituted cyclobutanone oxime 1h and spiro
substrate 1i reacted smoothly giving 3ha and 3ia in 59% and 85%
yield, respectively. To our delight, alfa- (Me, Bn, Allyl and Ph)-
substituted cyclobutanone oximes reacted with phenylacetylene 2a
under the irradiation of blue LEDs generating the coupling products
(3ja-3ma) in moderate to good yields via a selective C-C bond
cleavage. Interestingly, 2-((3-phenylprop-2-yn-1-yl)oxy)acetonitrile
3na could be prepared in modest yield under the optimized
conditions. Moreover, bicycle cyclobutanone oxime 1o could be
converted into the desired product 3oa in 74% yield.
Next, we continued to explore the scope of the terminal alkynes
shown in Table 3. Varieties of phenylacetylene derivatives bearing
both electron-donating and electron-withdrawing groups at the
ortho, meta and para-positions of phenyl rings (2b-2k) were
selected as good candidates for this Csp3-Csp coupling, furnishing
the products (3ab-3ak) in moderate to good yields. What’s more,
With the optimal conditions in hand, we examined the scope and
the limitation of visible-light-induced copper-catalysed Csp3-Csp
coupling reaction. Firstly, varieties of cyclobutanone oximes were
investigated to furnish a range of alkynyl nitriles under the
optimized reaction conditions (Table 2). The cyclobutanone oximes
2 | J. Name., 2012, 00, 1-3
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heteroaromatic terminal alkynes (2l-2n) were all successfully experiment in the presence of equal amount of 1aVaienwdAr1ticjleuOnndliener
applied in the reaction system to form the alkynyl nitriles (3al-3an) standard conditions. After 30 mins 3aa and 3ja were detected in
DOI: 10.1039/D0CC00988A
in 52%-70% yields. Furthermore, 1-ethynylcyclohex-1-ene (2o) was similar yields, which conformed that the radical generation and
also a suitable substrate under the optimized condition. It is trapping process were similar between primary and secondary
noteworthy that this methodology could also suitable for the radical intermediates (Scheme 2, b). Moreover, the desired product
functionalization of alkyl terminal alkynes. Evidentlly, range of alkyl 3aa was formed in 43% yield when cyclobutanone oxime 1a was
alkyne coupling partners underwent the visible-light-induced coulped with stoichiometric amout of copper acetylide complex
copper-catalysed coupling process to generate the corresponding 2a'17 in the presence of L1 (20 mol%) and TBAI (5 mol%) under the
products in moderate yields (3ap-3au). Ether (2q, 2r), chloro (2s), visible light irradiation for 3 hours (no desired product was
silicon (2t) and indole (2u) groups were all compatible with this observed without the addition of L1) (Scheme 2, c). This control
method. Importantly, the terminal alkynes having structurally experiment suggested that the in situ formed copper acetylide
complex molecular scaffolds derived from the typical natural complex 2a' was an effective photosensitive catalyst and reactive
products could also survived and converted into the corresponding intermediate, which could be transformed into the product.
functionalized complex alkynyl nitriles in moderate yields under the
modified conditions (3av-3az).
Scheme 2 Mechanistic experiments.
Me
Me
Table 3 Substrate Scopea
N
NOBz (p-CF3)
"Standard conditions"
2a
NC
O
CF3
a)
Me Me
(
)
Ph
O
N
Cu(OTf)2 (10 mol%), L1 (20 mol%)
K2CO3 ( 3 equiv), TBAI (5 mol%)
TEMPO 1.0 equiv
4
O
81% (1H NMR yield)
NC
1a
DMF : CH3OH (3 : 1), Ar, RT
Blue LEDs
R
NC
2
(
)
R
NOBz (p-CF3)
1a
3
Ph
3aa
NC
46% (1H NMR yield)
1a
"Standard conditions"
NC
NC
b)
2a
(
)
Ph
30 mins
Me
R2
NOBz (p-CF3)
N
3ab
R2 = p-Me,
R2 = p-Ph,
, 71%
, 71%
, 61%
, 72%
O
NC
Ts
3ac
Me
Ph
3al
3am
, 52%
, 70%
3ad
3ae
R2 = p-Cl,
R2 = p-Br,
1j
3ja
47% (1H NMR yield)
3af
R2 = p-OMe,
3ag
, 46%
NC
NC
R2 = m-Me,
, 65%
NOBz (p-CF3)
L1 (20 mmol), TBAI (5 mol%)
3ah
R2 = m-F,
R2 = o-Cl,
, 68%
c)
NC
S
3ai
, 81%
3aj
DMF : CH3OH (3 : 1), Ar, RT
Blue LEDs
Ph
R2 = p-CN,
, 66%
3aa
3an
3aq
3ao
, 54%
3ak
, 69%
R2 = p-COOMe,
, 80%
Cu
Ph
1a
43% (1H NMR yield)
2a'
without L1, ND
NC
NC
NC
OPh
OBn
Ph
To get more information of the photosensitive catalyst of this
reaction, we examined the UV-Vis absorption and emission of
Cu(OTf)2, copper acetylide complex and copper acetylide complex
coordianted with L1, respectively (Figure 1). It was shown that only
copper catalyst dissolved in the solvent did not show any apparent
absorption signal above λ = 400 nm. In contrast, the copper
acetylide complex exhibits a strong absorption at λ = 475 nm,
which was almost the same with the absorption of copper acetylide
complex coordianted with L1. When the copper acetylide complex
3ap
3ar
, 60%
, 63%
, 58%
NC
NC
NC
N
Cl
TBS
3au
3as
3at
, 55 %
, 37 %
, 34 %
Late-stage modification:
O
Me
H
O
O
H
O
Me
Me
H
N
O
H
O
Me
O
Me
CN
Me
CN
from Diacetone-D-glucose
CN
from Estrone
3av
from Pargyline
3ax
3aw
, 61%
, 43%
, 57%
Me
Abs [Cu (OTf)2]
O
Abs [Copper acetylide complex]
Abs [Copper acetylide complex + L1]
Me
Me Me
Me
Em [Copper acetylide complex]
O
Me
Em [Copper acetylide complex + L1]
Me
H
Me
O
O
Me
H
H
CN
O
from Fenofibrate
3ay
from Cholesterol
b
CN
, 50%
3az
, 60%
a1a (0.2 mmol), 2 (0.24 mmol), Cu(OTf)2 (10 mol%), L1 (20 mol%), K2CO3 (3
equiv) and TBAI (5 mol%) in a mixture of DMF : CH3OH (3 : 1, 0.1 M), under
Ar, rt, blue LEDs, 3 h; isolated yield. b1a (0.2 mmol), 2z (0.1 mmol)
400
500
600
700
Wavelength (nm)
To gain mechanistic insight into this photo-induced copper-
catalysed Csp3-Csp cross-coupling reaction, radical inhibitor λ = 454 nm)
Figure 1 UV-Vis absorption spectra and emission spectra (excited at
experiment was conducted with an addition of TEMPO (1.0 equiv)
and copper acetylide complex coordianted with L1 were excited at
λ = 454 nm, similar emission band centered at around λ = 500 nm
were observed with emission lifetime of 10.4 nanoseconds and 7.2
to the optimized reaction system. The formation of 3aa was totally
suppressed with the formation of γ-cyanoalkyl-TEMPO product in
81% yield (Scheme 2, a). In addition, we carried out a competitive
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nanoseconds, respectively. Based on these results, we concluded
that both the copper acetylide complex and copper acetylide
complex coordianted with L1 might be the important light-
absorption photoredox catalysts displaying nearly good potential
for the activation of cyclobutanone oximes under the irradiation of
blue LEDs.
Based on these mechanistic results, we proposed a mechanism
summarized in scheme 3. Visible-light irradiation of in situ
generated copper acetylide complex 2' delivered an excited state
copper complex with a high reductive potential, which underwent a
reductive SET process to give γ-cyanoalkyl radical 1' as well as
complex 2''. Then the radical 1' was captured imediatlly by complex
2'' producing alkynyl nitriles 3 as well as regenerating the copper
salt.12b Ligand (L1) might play an essential role during this process.
5
6
7
Scheme 3 Proposed catalytic cycles.
R
CuLn
2'
R
H
8
2
base
*
CuL1
L1CuX
R
CF3
2''
9
O
N
NC
O
CN
R
1'
3
1
*
X
R
CuL1
2''
In conclusion, we reported the first photo-induced copper-
catalysed Csp3-Csp cross-coupling affording varieties of alkynyl
nitriles through selective C-C bond cleavage. This method is general
and efficient with good functional group compatiblity. The primary
mechanistic experiments suggested that the in situ generated
copper acetylide complex is an effective photosensitive catalyst for
of this radical-mediated transformation. It is also envisaged that the
interesting reactivity displyed herein would inspire the future
expedient devolopment of CC bond forming reactions.
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Acknowledgement:
We thank the “1000-Youth Talents Plan”, and the “Jiangsu
Specially-Appointed Professor Plan”. This work was also supported
by a program A for Outstanding PhD candidate of Nanjing
University.
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