Copper-Catalyzed Direct Twofold C-P Cross-Coupling of Unprotected Propargylic 1,4-Diols: Access to 2,3-Bis(diarylphosphynyl)-1,3-butadienes

Article abstract of DOI:10.1021/acs.orglett.8b03985

The first facile and efficient Cu-catalyzed direct coupling of unprotected propargylic diols with H-phosphine oxides was developed, providing a practical approach to access structurally diverse 2,3-bis(diarylphosphynyl)-1,3-butadienes along with the forma

Full text of DOI:10.1021/acs.orglett.8b03985

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Copper-Catalyzed Direct Twofold C−P Cross-Coupling of
Unprotected Propargylic 1,4-Diols: Access to 2,3-
Bis(diarylphosphynyl)-1,3-butadienes
Fushan Chen, Ying Xia, Rongcan Lin, Yuxing Gao,* Pengxiang Xu,* and Yufen Zhao
Department of Chemistry, College of Chemistry and Chemical Engineering, and the Key Laboratory for Chemical Biology of Fujian
Province, Xiamen University, Xiamen, Fujian 361005, China
*
S Supporting Information
ABSTRACT: The first facile and efficient Cu-catalyzed direct coupling
of unprotected propargylic diols with H-phosphine oxides was
developed, providing a practical approach to access structurally diverse
2
,3-bis(diarylphosphynyl)-1,3-butadienes along with the formation of
2
two new P−C and two new CC bonds under ligand- and base-free
sp
conditions.
5
a
rganophosphorus compounds have attracted continuous
attention for synthetic chemists in recent decades due to
functional group compatibility, thus limiting its application.
O
Additionally, transition-metal-catalyzed addition reaction of a
P(O)−H bond to the triple bond has been one of the most
straightforward methodologies for the preparation of vinyl-
their broad applicability in organic synthesis, medicinal
1
chemistry, and materials science. Among them, 2,3-bis-
6
phosphonates in recent years. Based on this strategy,
(
diarylphosphinyl)-1,3-butadienes are an important class of
Westerhausen’s group reported an example of calcium-
mediated hydrophosphanylation of 1,4-diphenylbuta-1,3-
diyne with diphenylphosphine oxide leading to a 1,3-butadiene
product (Scheme1b), but this transformation suffered from the
commercially unavailable and air-sensitive calcium catalyst and
could not prepare multisubstituted 1,3-butadienes containing
two substituents at the C1 or C4 position, which restricted
their further study. Therefore, developing a general, economic,
and efficient strategy to access highly substituted and novel
butadiene-containing, valuable building blocks with wide
2
applications in synthetic chemistry. Moreover, various useful
3
bidentate chiral ligands could be easily prepared by
asymmetric hydrogenation of these motifs as a type of
chelating diphosphorus system, which were widely used in
4
ligand chemistry. However, to date, the synthetic method for
5b
the synthesis of this class of 2,3-bis(diphenylphosphinyl)-1,3-
5
butadienes is rather rare and only two methods are reported.
One includes using a double [2,3]-sigmatropic rearrangement
from 2-butyne-1,4-diol and air- and moisture-sensitive
chlorodiphenylphosphine as starting materials afforded the
corresponding 2,3-bis(diphenylphosphinyl)-1,3-butadiene
product (Scheme 1a), which was discovered in the early
2
,3-bis(diarylphosphinyl)-1,3-butadienes starting from readily
available substrates under base- and ligand-free conditions
remains highly desirable.
Over the past few years, transition-metal-catalyzed direct
cross-coupling of propargylic 1,4-diols for selective C−C and
C−heteroatom bond formation has emerged as one of the
most promising and significant strategies for the construction
of complex molecular frames in modern organic synthesis
because of its avoidance of the protection of reaction partners
and its prominent advantages of step and atom economy and
1
980s and still used in the synthesis of this motif to date. Yet, it
is greatly limited to the use of unstable and toxic phosphorus
chlorides along with the need for excess bases and poor
Scheme 1. Some Synthetic Strategies to 1,3-Diene
Frameworks
7
environmental sustainability. On the other hand, the stable
and readily available propargylic 1,4-diols as starting substrates
are highly attractive for chemical synthesis from environmental
and economic points of view since the water is the only
8
byproduct in some transformations. Thus, as an appealing
alternative, a more synthetically useful procedure to highly
substituted 2,3-bis(diarylphosphinyl)-1,3-butadienes would
involve direct twofold C−P cross-coupling of unprotected
propargylic 1,4-diols with H-phosphine oxides (Scheme 1c).
Nevertheless, to the best of our knowledge, there is still no
Received: December 13, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03985
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
example of employing such an attractive and practical approach
to access 2,3-bis(diarylphosphinyl)-1,3-butadiene compounds.
As part of our ongoing efforts to develop environmentally
the yield to 81% (entry 15). Gratifyingly, in the presence of
Cu(OTf) (30 mol %) in DCE (1 mL) at 90 °C for 48 h,
2
increasing the loading of 1a and 2a to 0.6 and 2.4 mmol,
respectively, gave rise to an excellent yield of 91% (entry 16).
With the optimized conditions in hand (footnote a, Table
2), we then investigated the cross-coupling of various
propargylic diols with diphenylphosphine oxide 2a to explore
the generality of this methodology, and the results were
summarized in Table 2. First, various cycloalkyl-containing
propargylic diols were examined, and the substrate 1b carrying
a cyclobutyl substituent could react with 2a to afford the
corresponding product 3b in 65% yield (entry 2). However, a
cyclopentyl-containing substrate 1c provided two isomers 3c
and 3c′, probably due to the occurrence of a 1,3-H shift during
the catalyst process leading to the formation of 3c′ (entry 3).
Interestingly, the substrate 1d having a bulky cyclohexyl gave
no trace of 1,3-butadiene product 3d, but generated 3d′ in a
moderate yield of 52% owing to the effect of steric hindrance
9
friendly new methods for the P−C bond construction, herein,
we disclosed the first example of Cu-catalyzed direct twofold
cross-coupling of propargylic diols with H-phosphine oxides to
construct highly substituted 2,3-bis(diarylphosphinyl)-1,3-
butadienes under ligand- and base-free conditions. This
protocol enables a facile and efficient access to structurally
diverse 2,3-bis(diarylphosphinyl)-1,3-butadiene frameworks
2
along with the formation of two new P−C and two new
sp
CC bonds through a simple one-pot process.
Initially, our efforts focused on the model reaction of 2,5-
dimethylhex-3-yne-2,5-diol (1a) with diphenylphosphine oxide
(2a) to optimize the reaction conditions (Table 1). To our
a
Table 1. Optimization of Reaction Conditions
(
entry 4). It is worth noting that when using various diol
precursors bearing one or two secondary racemic propargylic
alcohols (1e−1q), high selectivity for the E stereoisomers of
the products 2,3-bis(diarylphosphinyl)-1,3-butadienes (3e−
b
entry
catalyst
solvent
temp (°C)
yield (%)
3
q) was observed because of steric hindrance, and the
1
2
3
4
5
6
7
8
9
Cu(OTf)2
Fe(OTf)3
TfOH
CuCl2
CuCl
Zn(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
DCE
DCE
DCE
DCE
DCE
DCE
80
80
80
80
80
80
80
80
80
80
90
100
90
90
90
90
62
21
30
17
10
25
0
3
55
5
75
71
76
54
81
91
corresponding trace Z stereoisomers were not separated by
column chromatography on silica gel. The result revealed that
this novel coupling might undergo an S 1-type reaction
N
mechanism. As shown in Table 2, various propargylic diols
(
1f−1n) having different electron-withdrawing substituents
such as chloro, bromo, iodo, cyano and trifluoromethyl on the
phenyls were all efficiently reacted with 2a via dehydrate cross-
coupling reactions to give the major (E)-2,3-bis(diaryl-
phosphinyl)-1,3-butadiene products (3f−3n) in moderate to
good yields with the stereoselectivity for E-isomers determined
by the coupling constant between the P atom and the C atom
CH CN
3
THF
CH Cl
2
2
1
1
1
1
1
1
1
0
1
2
3
4
5
PhCl
DCE
DCE
DCE
DCE
DCE
DCE
c
d
e
f
13
1
of benzene linked to double bonds on the basis of C and H
6,10
NMR spectroscopic analysis.
However, propargylic diols
bearing electron-donating substituents including methyl,
alkoxyl, and mercapto on the benzene ring provided none of
the expected 1,3-butadiene products, indicating that the
electronic effect is evident in this transformation. Notably,
the propargylic diol moiety showed a higher chemoselectivity
over the chloro, bromo, and iodo atoms as leaving groups
under the present conditions and 1,3-butadiene products (3f−
6
a
Conditions: 1a (0.30 mmol), 2a (1.20 mmol), catalyst (30 mol %),
solvent (2 mL), 48 h, under Ar. Isolated yield. Using 50 mol % of
Cu(OTf) . Using 20 mol % of Cu(OTf) . Solvent (1 mL). Using
a (0.60 mmol), 2a (2.40 mmol), Cu(OTf) (0.18 mmol), solvent (1
b
c
d
e
f
2
2
1
2
mL).
3
h) could be obtained by this coupling, which demonstrated
the potential of this new approach for the construction of more
complex molecules through the next coupling of these halide
products. The bulky substrates containing a naphthyl moiety
delight, 1a could react with 2a to give the desired product 3a
in 62% yield in the presence of Cu(OTf) (30 mol %) in DCE
2
(
2 mL) at 80 °C for 48 h under an argon atmosphere (entry
). Encouraged by this result, other catalysts were further
examined, and we found they were either poorly effective
(
1o) and a tert-butyl group (1q) could all undergo the
1
coupling to produce the desired products (3o and 3p). The
heterocyclic propargylic diols having a thiophene ring (1r and
(
Fe(OTf) , CuCl , CuCl, and TfOH; entries 2−5) or entirely
3
2
1
s) were also compatible with the present reaction conditions,
ineffective (Zn(OTf) ; entry 6). To advance the process
2
affording the desired products 3r and 3s as a mixture of Z/E
further, a subsequent survey on the role of various solvents for
the aforementioned transformation revealed that DCE is the
isomers in good total yields. In addition, 1-cyclohexyl-4-
1
methylpent-2-yne-1,4-diol 1t only gave 3t′ determined by H
most suitable solvent, and other solvents such as CH CN,
3
1
3
and C NMR spectroscopic analysis in 67% yield and the
expected 1,3-butadiene product was not observed. Fortunately,
the product 3a was recrystallized from ethyl acetate/n-hexane
as colorless crystals, and the molecular structure of 3a was
THF, CH Cl , and PhCl were less effective (entries 7−10).
2
2
Note that increasing the temperature to 90 °C could improve
the product yield up to 75%, but further enhancing the
reaction temperature to 100 °C led to the yield reduction
11
(
entries 11−12). Subsequently, the loading of Cu(OTf) was
confirmed by X-ray diffraction analysis (CCDC 1873031).
2
also investigated, yet 20 and 50 mol % of Cu(OTf) did not
The result clearly revealed that the phosphoryl moiety was
preferentially installed at the C2- and C3-positions of 1a in this
coupling reaction.
2
significantly promote this reaction (entries 13 and 14).
Interestingly, reducing the solvent to 1 mL could improve
B
DOI: 10.1021/acs.orglett.8b03985
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Organic Letters
Letter
a
Table 2. Direct Twofold Cross-Coupling of Propargylic Diols with Diphenylphosphine Oxide
a
b
c
Reaction conditions: 1 (0.60 mmol), 2 (2.40 mmol), and Cu(OTf) (0.18 mmol) in DCE at 90 °C for 48 h under argon. Isolated yield. The
2
d
31
ratio of 3c/3c′ was determined by the isolated yield. E/Z ratio was obtained by P NMR spectroscopic analysis.
To further extend the scope of this reaction, some other H-
phosphine oxides were also evaluated. As demonstrated in
Scheme 2, apart from 2a, other H-phosphine oxides such as 2b,
Notably, we found that the propargylic diol 1u bearing a
diphenyl group only gave a trace of the desired 1,3-butadiene
product 3y under the standard conditions, but it could react
with 2a to generate the major product 4a containing a β-
ketophosphine oxide framework, which is of interest for the
application in the well-known HWE (Horner−Wadsworth−
2
c, 2d, and 2e were all suitable coupling partners, and the
corresponding products (3u−3x) were generated in 62%, 66%,
1%, and 71% yields, respectively.
12
Emmons) reaction and also exhibits wide-ranging biological
7
13
14
activities and eminent metal-complexing abilities.
Similarly, the substrate 1v could also afford the β-
ketophosphine oxide 4b in 43% yield along with the generation
of the 1,3-butadiene product 3z in 40% yield (Scheme 3). The
results indicated that the formation of β-ketophosphine oxide
Scheme 2. Direct Twofold Cross-Coupling of 1a with H-
Phosphine Oxides
a
might be attributed to the attack of H O onto the key allenic
2
intermediate during the reaction process.
It is noteworthy that a gram-scale experiment was conducted
by employing 1a (15 mmol, 2.13 g) with 2a (60 mmol, 12.12
g) under the optimal reaction conditions. The desired product
3
a was obtained in a high yield of 82%, showing that the
present method could be easily adopted for the large-scale
synthesis with high efficiency (Scheme 4).
Based on the above results and previous reports,
15
a
plausible mechanism for the 2-fold cross-coupling is presented
in Scheme 5. First, intermediate A was formed by the
coordination of a copper cation to the triple bond and OH
group. Next, the critical propargylic carbocation intermediate
B could be easily gained through the elimination of the OH
a
Reaction conditions: 1a (0.60 mmol), 2 (2.40 mmol), and
Cu(OTf) (0.18 mmol) in DCE at 90 °C for 48 h under argon.
Isolated yield.
2
C
DOI: 10.1021/acs.orglett.8b03985
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. 1u and 1v Reacted with 2a To Afford β-
Ketophosphine Oxides
structurally diverse array of more highly substituted 2,3-
bis(diarylphosphinyl)-1,3-butadiene compounds along with
2
the construction of two new P−C and two new CC
sp
bonds. Importantly, this novel twofold cross-coupling could
facilitate the creation of unique phosphorus-containing 1,3-
buta-diene molecules bearing P−C−C−P backbones, which
might further transform to a new type of more useful
diphosphine ligands by ready reduction. Most attractively,
the coupling reaction only uses inexpensive and commercially
available Cu(OTf) as the catalyst without the need for a base
2
a
and a ligand, and various special 2,3-bis(diphenylphosphinyl)-
Reaction conditions: 1a (0.60 mmol), 2a (2.40 mmol), and
1
,3-butadienes could be conveniently obtained in a simple one-
Cu(OTf) (0.18 mmol) in DCE at 90 °C for 48 h under argon.
Isolated yield.
2
step process, which represents a prominent advantage of this
method. In addition, the use of easily accessible propargylic
diols prepared from a broad range of aldehydes and ketones,
producing water as the only byproduct in this transformation,
the operational simplicity, and the high step and atom
economy mean this strategy will find widespread application
in the preparation of important 2,3-bis(diarylphosphinyl)-1,3-
butadiene frameworks in modern organic synthesis and
coordination chemistry. Further mechanistic investigations
and application research are currently underway.
Scheme 4. Gram-Scale Preparation of 3a
Scheme 5. Possible Mechanism for the Twofold Cross-
Coupling Reaction
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03985.
General experimental procedures, characterization data
of all products (PDF)
Accession Codes
CCDC 1873031 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
15a−c
group with the assistance of a Lewis acid, Cu(OTf)2.
*
E-mail: gaoxingchem@xmu.edu.cn.
E-mail: xpengxiang@xmu.edu.cn.
Then, in view of the least sterically hindered possibility, the
*
nucleophile 2 (in the form of the trivalent phosphine oxide 2′)
attacked the C3-position of B via an S 1-type substitution to
ORCID
N
provide the key product C of the first cross-coupling cycle.
Subsequently, the second cross-coupling proceeded by a
Yuxing Gao: 0000-0002-6420-0656
Yufen Zhao: 0000-0002-8513-1354
Notes
similar coordination of Cu(OTf) to the allenyl and OH
2
15d
group, forming intermediate D, followed by the removal of
the OH group to produce intermediate E. Finally, nucleophilic
substitution of 2 onto the least sterically demanding C2-
position of E afforded the desired product 3 along with the
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
regeneration of Cu(OTf) as a catalytically active species (path
2
This work was supported by the Chinese National Natural
Science Foundation (21202135, 21642010, 41576081) and the
Fundamental Research Funds for the Central Universities
(20720180083, 20720160034).
a). However, using some propargylic diols bearing bulky
electron-donating substituents on the benzene ring of C4-
position of 1 as substrates, the nucleophile H O instead of
2
bulky 2′ attacked the C2-position of D (path b), resulting in
the generation of β-ketophosphine oxide 4 owing to the effect
of the steric hindrance.
In summary, we have developed the first efficient and
practical Cu-catalyzed direct twofold C−P cross-coupling of
various unprotected propargylic diols with H-phosphine oxides
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