Sequential Transformation of Terminal Alkynes to 1,3-Dienes by a Cooperative Cobalt Pyridonate Catalyst

Article abstract of DOI:10.1021/acs.organomet.9b00486

We describe the cobalt(II) catalyst 1 bearing a phosphinopyridonate ligand for sequential transformation of aryl terminal alkynes to (E,Z)-1,3-dienes with excellent stereoselectivity. By cooperative metal-ligand reactivity, 1 reacts readily with the termi

Full text of DOI:10.1021/acs.organomet.9b00486

Article
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Cite This: Organometallics XXXX, XXX, XXX−XXX
Sequential Transformation of Terminal Alkynes to 1,3-Dienes by a
Cooperative Cobalt Pyridonate Catalyst
Xuewen Zhuang,† Jia-Yi Chen,‡ Zhuoyi Yang,† Mengjing Jia,† Chengjuan Wu,† Rong-Zhen Liao,*,‡
Chen-Ho Tung,† and Wenguang Wang*,†
†Key Lab of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong
University, No. 27 South Shanda Road, Jinan 250100, People’s Republic of China
‡School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan
430074, People’s Republic of China
S
* Supporting Information
ABSTRACT: We describe the cobalt(II) catalyst 1 bearing a
phosphinopyridonate ligand for sequential transformation of aryl
terminal alkynes to (E,Z)-1,3-dienes with excellent stereo-
selectivity. By cooperative metal−ligand reactivity, 1 reacts
readily with the terminal alkynes to afford the alkynyl cobalt
intermediate for dimerization, producing (E)-1,3-enynes, and
then to smoothly catalyze reduction of the acetylene unit to the
olefin by H3N·BH3. A plausible catalytic cycle for both
dimerization and transfer hydrogenation was proposed on the
basis of stoichiometric reactions and DFT calculations, which
suggest the ability of the cobalt catalyst to activate the amine−
borane as well as the terminal alkyne through cobalt pyridonate
cooperation.
Scheme 1. Metal-Catalyzed Formation of Enynes and Their
Reduction to Dienes
INTRODUCTION
■
Sequential catalysis combining two or multiple reactions in a
single reaction vessel can generate the desired targets
efficiently, obviating the need for purification at each step
and meeting the demand for highly atom-economical and
sustainable synthesis.1 The research of sequential catalysis by a
single catalyst is therefore important and of interest.
Representative examples are Grubbs catalysts, which process
sequential olefin metathesis and hydrogenation,2 ring-closing
metathesis and olefin isomerization,3 and sequential alkene
isomerization and oxidation.4 Although interest has extended
to base-metal catalysis,5 sequential reactions performed by a
single metal catalyst are sparse.6,7
1,3-Enynes are practically useful precursors in the synthesis
of conjugated dienes by metal-catalyzed reductive coupling
with various electrophiles.8,9 The produced dienes also find
widespread applications in organic synthesis such as Diels−
Alder reactions,10 and they are important structural motifs in
many biologically active entities11 and polymer materials.12
Dimerization of two terminal alkynes is one of the most
convenient methods to synthesize conjugated enynes.13
Studies of this reaction were originally focused on the
regioselective dimerization catalyzed by d-block and f-block
metals to produce specific Z,14 E,15 or gem stereoisomers16
(Scheme 1a). In particular, the related regioselective catalysis
has been established for abundant metals, represented by the
pincer iron(II) hydrides for Z dimerization,17 a cyclic
(alkyl)amino carbene iron catalyst for E dimerization,18 and
the piano-stool iron(II) catalysts featuring NHC ligands for the
gem dimerization.19 Sequential dimerization of terminal
Received: July 18, 2019
© XXXX American Chemical Society
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alkynes and partial reduction of the resultant 1,3-enynes could
offer a new and straightforward synthetic route to access
conjugated dienes (Scheme 1b). However, the challenge is to
combine the regioselective dimerization with the chemo-
selective reduction of the acetylene unit in tandem using a
single metal catalyst.20
Our group has been particularly interested in incorporating
ligand reactivity in inexpensive metal catalysis for organic
transformations.21,22 With flexible binding modes of either an
O or a N coordination,23 the tautomeric 2-hydroxypyridine/2-
pyridone24 is rich with opportunities to design efficient metal
catalysts for a proton-transfer reaction, hydrogenation, and
transfer hydrogenation.25 Here, we describe a phosphinopyr-
idonate cobalt complex (1), which can activate aryl terminal
alkynes for dimerization and, subsequently, catalytic reduction
of 1,3-enyne to the (E,Z)-1,3-diene.
containing cis double bonds is challenging, because (E,E)-
dienes are generally more thermodynamically favored than the
(E,Z)-dienes.30 For example, in the case of Ir-catalyzed
dehydrogenative couplings of aryl olefin, (E,Z)-3a is formed
as a minor product.31 In contrast, our sequential catalysis with
a single cobalt(II) catalyst produces the (E,Z)-product
exclusively. Conversion of (E,Z)-3a to its stereoisomers was
not observed in the reaction.
A series of conjugated (E,Z)-dienes were obtained by this
sequential protocol (Table 1). Aryl alkenes with electron-
Table 1. Cobalt-Catalyzed Sequential Transformation of
a
Aryl Alkynes to (E,Z)-1,3-Dienes
RESULTS AND DISCUSSION
■
a
b
R
conversion (%)
product
yield (%)
Synthesis and Characterization of Half-Sandwich
Cobalt Complexes. Complex 1 was synthesized by the
reaction of potassium diphenylphosphinopyridonate26 with 0.5
equiv of [Cp*CoCl]227 in THF at room temperature (Scheme
2). The product isolated as a brown solid has a measured
Ph (2a)
>99
>99
>99
>99
95
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
84
86
80
84
73
50
88
84
89
o-MeC6H4 (2b)
o-FC6H4 (2c)
m-MeC6H4 (2d)
m-FC6H4 (2e)
m-ClC6H4 (2f)
p-MeC6H4 (2g)
p-FC6H4 (2h)
p-MeOC6H4 (2i)
p-F3CC6H4 (2j)
p-tBuC6H4 (2k)
3,5-(CF3)2C6H4 (2l)
2-naphthyl (2m)
3-thienyl (2n)
95
Scheme 2. Synthesis and Solid-State Structure of 1
>99
>99
>99
>99
>99
90
c
81
87
d
83
>99
80
91
63
a
Conditions unless specified otherwise: substrate (0.8 mmol), catalyst
(3 mol %) in THF (1 mL) at 40 °C for 12 h, then H3N·BH3 (0.8
b
c
magnetic moment of 2.24 μB in benzene, consistent with a low-
spin d7 configuration.28 A crystallographic analysis of 1 reveals
that both P and N are coordinated to the metal center, forming
a five-membered ring. The C−O distance of 1.248(3) Å in 1 is
consistent with the reported values for phosphinopyridonate
metal complexes.26
mmol, 2 equiv) was added for 4 h. Isolated yields are provided. 9%
d
yields of Z,Z isomers were also isolated. With 1 equiv of H3N·BH3 at
0 °C.
donating or electron-withdrawing substituents such as a methyl
(2b, 2d, 2g) or fluoro group (2c, 2e, 2d) on the phenyl ring,
are compatible with the transformation and gave the
corresponding (E,Z)-dienes with good yields. Synthesis of
chloro-substituted diarylbutadienes was found to be somewhat
difficult. The sequential transformation of 2f led to formation
of 3f in 50% yield. Although the acidity of terminal alkynes is
sensitive to the electronic nature of the substituent, both
electron-rich and electron-deficient aromatic groups allow the
alkyne to undergo dimerization−reduction under the present
catalytic conditions, providing conjugated dienes such as 3g−k.
Interestingly, the reaction of p-trifluoromethylacetylene (2j)
not only provides (E,Z)-product but also produces the (Z,Z)-
isomer (9% yield) as a minor product. Further studies indicate
that the production of the (Z,Z)-isomer arises from the
hydrogenation of the (Z)-enyne product generated at the
dimerization step, as is described below. Increasing the acidity
of the alkyne appears to accelerate the reduction process. In
the reaction of 2l, the reduction can proceed to completion
within 2 h at 0 °C to produce exclusively the (E,Z)-diene (3l)
in 83% yield. This catalyst also displays excellent activity in
transformation of alkynes with other aromatic groups such as
thienyl and naphthyl groups (2m, 2n), providing the
corresponding (E,Z)-products in reasonable yields.
Transformation of Terminal Alkynes to Conjugated
Dienes. Our studies began with the evaluation of activity of 1
in promotion of the dimerization of the simplest terminal aryl
alkyne, phenylacetylene (2a), and subsequent reduction of the
resulting 1,3-enyne to 1,4-diphenyl-1,3-dibutadiene (3a) by
H3N·BH3, which behaves as a convenient dihydrogen source
for transfer hydrogenations.29 A solution in THF of 2a with 3
mol % of 1 was stirred at 40 °C to conduct the dimerization,
which was followed by addition of solid H3N·BH3 into the
system. The progress of the reactions was monitored by GC-
MS, which revealed full conversion of the substrate after 12 h.
1
1
Analysis of the isolated product by H NMR and H−1H
COSY spectroscopy established the formation of 3a in an
(E,Z) configuration (eq 1). Synthesizing conjugated dienes
B
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a
Dimerization of Aryl Alkynes. To gain mechanistic
insight into the catalysis, we investigated the stoichiometric
reaction of 1 with alkynes in the first step of the dimerization.
By cooperative Co−N reactivity, 1 reacts readily with aryl
alkynes to afford the alkynyl cobalt species. For instance,
reaction of 1 with an equimolar quantity of 2i ((PMP)CCH,
in Scheme 3) was indicated by an instant color change from
Table 2. Dimerization of Terminal Alkynes
a
b
R
conversion (%)
product yield (%) (E:Z)
Ph (2a)
o-MeC6H4 (2b)
o-FC6H4 (2c)
>99
>99
>99
>99
93
5a
5b
5c
5d
5e
5f
5g
5h
5i
96:0
93:0
96:0
95:0
96:0
88:5
95:0
96:0
98:0
86:11
93:0
87:0
95:0
70:0
57:19
Scheme 3. Dimerization of 2i by 1 Proceeding through an
Alkynyl Cobalt Intermediate
m-MeC6H4 (2d)
m-FC6H4 (2e)
m-ClC6H4 (2f)
p-MeC6H4 (2g)
p-FC6H4 (2h)
p-MeOC6H4 (2i)
p-F3CC6H4 (2j)
p-tBuC6H4 (2k)
3,5-(CF3)2C6H4 (2l)
2-naphthyl (2m)
3-thienyl (2n)
95
>99
>99
>99
>99
>99
87
>99
73
80
5j
5k
5l
5m
5n
c
triethylsilyl (2o)
5o
a
Conditions unless specified otherwise: substrate (0.4 mmol), catalyst
b
(3 mol %), in 0.5 mL of THF at 40 °C for 12 h. The conversions and
yields were determined by H NMR analysis using tetraethylsilane as
internal standard. The reaction was carried out at 60 °C.
1
c
brown to deep green. Production of the alkynyl cobalt
intermediate (4) was initially recognized from ESI-MS studies
of the reaction solution. When (PMP)CCD was used, a
peak at m/z 619.202 appeared, in comparison to m/z =
618.195 for 2i with 1.
Indeed, the cobalt complex is capable of catalyzing the
dimerization of alkynes without addition of an external base
(Table 2). With 3 mol % of 1 in THF at 40 °C, 2a dimerizied
to (E)-5a with an excellent yield. All of the alkyne substrates
subjected to the sequential transformation were converted to
the corresponding (E)-enynes in similar yields. Although the
reaction for 2l is comparably slower and delivered 87%
conversion, only the (E)-enyne was formed. A larger scale
preparation (510 mg, 92% yield) was conducted for 5m, whose
structure was confirmed crystallographically (Figure S31). In
the dimerization of 2j, both E and Z isomers of 5j were
detected with yields of 86% and 11%, respectively. A small
amount of the Z isomer was also observed in the reaction of 2f.
Although the catalytic dimerization performed by 1 has not
been observed for common alkyl alkynes such as 1-hexyne and
1-butyne, the reaction of trimethylsilylacetylene (2o) at 60 °C
for 48 h resulted in a 80% conversion, providing a mixture of
dimerizied products (E)-5o and (Z)-5o in a ratio of 57:19.
Interestingly, the cobalt catalyst can perform selective cross-
dimerization of 2o with aryl terminal alkynes. For an instance,
the reaction of 2m with excess of 2o (5 equiv) at 40 °C
achieved cross-coupling to provide the desired enyne 5p with
63% isolated yield (eq 2). Besides the cross-dimerizied
product, the 2-ethynylnaphthalene homodimerizied product
5m was isolated in 18% yield, whereas the formation of 5o
arisen from homocoupling of 2o was not observed. With
3 mol % catalyst loading, further transformation of 5p to the
Crystallographic analysis confirms that 4 is a 17-electron
alkynyl cobalt(II) complex. The RCCH group in the
structure is activated by the Co−N site, resulting in formation
of the CoII−CCR moiety with a detached pyridone group.
The Co−C distance is 1.88(1) Å, about 0.08 Å shorter than
that in TpiPr2Co−CC−R.32 Since the CC length is not
sensitive to the electronic effect of the substituents, the bond
length of 1.19(1) Å in 4 is thought to be in accord with the
mean value (ca. 1.20 Å) of alkynes.33 In comparison to 1, the
C−O distance of 1.230(1) Å is slightly decreased, by 0.01 Å. In
comparison to the νCO band at 1624 cm−1 observed for 1, the
IR spectrum of 4 appears as two bands for νCO in THF, one at
1624 cm−1 and the other at 1606 cm−1, suggesting
tautomerism of the detached pyridone group.34 The CC
stretching vibration for alkynyl cobalt(II) complexes is weak,32
but the band at 2106 cm−1 is displayed.
As expected, the further reaction of 4 with 2i provided a
mixture of 4 with (PMP)CCD, suggesting that the
organocobalt product is 1. The isolated (E)-enyne product
was identified by NMR spectroscopy and GC-MS analysis. On
2
the basis of the H NMR spectrum, the alkene protons at the
enyne moiety were found to be partially labeled (Figure S9),
indicating intermolecular H/D exchange between the
pyridinone site of 4 and d-2i during the dimerization.
C
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(E,Z)-diene by H3N·BH3 underwent smoothly, giving 3o
nearly quantatively. Note that selective cross-dimerization is
more challenging than the homodimerization.17a,19 The
present sequential dimerization-semireduction strategy provide
a potential alternative approach to synthesize the conjugated
vinylsilanes, which are valuable synthetic intermediates and
found versatile applications in organic chemistry and material
science.35
The stereochemical relationship between the 1,3-enyne and
the corresponding 1,3-diene product provides mechanistic
insight into this semireduction process. In the sequential
transformation of 2j, both (E,Z)- and (Z,Z)-dienes were
obtained, and Z dimerization as well as E dimerization was
observed in the initial homocoupling process. Examinations of
the kinetics show that the interconversion of the E and Z
isomers of 5j is not possible and the ratio of (E,Z)- and (Z,Z)-
2j is almost retained, at 86:11 during the reduction. The
stereoisomers (E)-5j and (Z)-5j were isolated, and the
subsequent reductions by H3N·BH3 were conducted separately
(eqs 3 and 4). It was found that the (E)-enyne was essentially
unambiguously show that the cobalt-catalyzed reduction of
the acetylene unit proceeds through cis addition.
Mechanistic Insights. DFT calculations36 were performed
in an attempt to elucidate the mechanism of the entire
reaction. Complex 1 is a 17-electron complex, and it was
calculated to be agreeing with the established magnetic
moment. The dimerization reaction is initiated by the
formation of a catalyst−alkyne complex. The calculations
showed that the triplet bond of the alkyne does not favor side-
on coordination to Co, which would generate a 19-electron
complex (Figure S17). Instead, the alkyne forms a hydrogen
bond with the pyridonate oxygen atom (O−H distance of 1.97
Å at Int1). This is followed by a concerted metalation−
deprotonation37 reaction through TS1, generating a Co−alkyl
intermediate (Int2, Figure 1), with the proton delivered to the
pyridonate oxygen atom. This step has a barrier of 9.3 kcal/
mol in the quartet state (21.2 kcal/mol in the doublet). Spin
crossing from doublet to quartet is thus required during the
reaction. During the formation of the Co−C bond, the
pyridonate nitrogen atom dissociates from the Co center;
therefore, Int2 retains a 17-electron configuration. Tautomeric
proton transfer from the pyridonate oxygen atom to the
pyridonate nitrogen atom is almost barrierless (Figure S18).
This results in the formation of Int3, which is 6.9 kcal/mol
more stable than Int2 and in line with the crystal structure of
the alkynyl cobalt intermediate.
From Int3, the binding of the second alkyne molecule to
form a 19-electron side-on coordinated Co−alkyne complex is
not favorable. Instead, the alkynide migratory 1,2-insertion38
into a second molecule of alkyne takes place from the van der
Waals either to the terminal carbon of alkyne (TS2B1) or to the
α-carbon of alkyne (TS2B2). The barriers for TS2B1 and TS2B2
were calculated to be 26.8 and 30.3 kcal/mol relative to Int3
plus an alkyne molecule, suggesting that migratory insertion to
the α-carbon is energetically less favorable. Finally, proton
transfer from the NH to the alkene anion leads to the
converted to (E,Z)-2j and the (Z)-enyne precursor was
transformed only to the (Z,Z)-isomer. These results
Figure 1. Gibbs free energy diagram at the M06-D3/SDD-6-311+G(2df,2p)//B3LYP-D3/SDD-6-31G(d,p) level (in kcal/mol) for the
dimerization of phenylacetylene catalyzed by 1. Schematic representations of all intermediates are shown. Energies in the quartet states are given in
parentheses. Cp* is omitted for clarity. For optimized structures, see the Supporting Information.
D
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Figure 2. Gibbs free energy diagram at the M06-D3/SDD-6-311+G(2df,2p)//B3LYP-D3/SDD-6-31G(d,p) level (in kcal/mol) for the
semireduction of 5a by H3N·BH3 catalyzed by 1. Schematic representations of all intermediates are shown. Energies in the quartet states are given
in parentheses. Cp* is omitted for clarity. For optimized structures, see the Supporting Information.
formation of the product and regeneration of the catalyst. TS2
was calculated to be the turnover-limiting step with a total
barrier of 26.8 kcal/mol using the M06-D339 functional, which
can be considered as being in excellent agreement with the
experimental condition. For steric reasons, the corresponding
Markovnikov product is 5.1 kcal/mol higher in energy.
Importantly, the total barrier for the formation of the
Markovnikov product is 3.5 kcal/mol higher.
Finally, the proton is delivered from the amino group to the
substrate almost without a barrier, producing 3a. The second
step is calculated to be turnover limiting, with an energy barrier
of 23.9 kcal/mol.
To verify the Z-reduction route, transfer hydrogenations of
1,3-enyne with deuterated amine boranes were performed. For
instance, reduction of 5a either by H3N·BD3 or D3N·BH3
2
produces deuterated-3a (eqs 5 and 6). H NMR spectrum
Catalyst 1 thus preferentially mediates the anti-Markovnikov
addition of the alkynide to the alkyne. It should be pointed out
that the B3LYP*-D340 functional (energy diagram shown in
Figure S18) gave a total barrier of 29.8 kcal/mol, which is
somewhat overestimated. Importantly, B3LYP*-D3 also favors
the anti-Markovnikov addition pathway (29.8 kcal/mol vs 31.0
kcal/mol). Therefore, both reproduce the regioselectivity well.
Other plausible pathways from Int2 (alkynide migratory 1,2-
insertion) and Int4 (converted metalation and hydrogen atom
transfer) have been considered (see Figure S19), but both
were associated with higher barriers, ruling out their
possibilities.
The reduction process was also calculated to involve four
key steps (Figure 2). At the outset, 1 reacts with H3N·BH3 via
a nucleophilic attack of the pyridonate oxygen on the boron
moiety (TS4), which is coupled with the coordination of one
of the boron hydrides to the metal center. The barrier for this
step was calculated to be 22.1 kcal/mol in the quartet state,
and the resulting intermediate Int6 lies at −3.7 kcal/mol
relative to 1 plus H3N·BH3. In the next step, the NH3 molecule
that is released performs a nucleophilic attack (TS5) on the
boron moiety of Int6, leading to the formation of a critical
Co−H intermediate (Int7). TS5 was calculated to be 23.9
kcal/mol relative to Int6. Then, the substrate alkyne carbon
could react with the Co−H moiety of Int7 though a hydride
transfer reaction.
The calculations showed that the hydride transfer to the α-C
adjacent to the phenyl group (TS6A) is slightly more favorable
than that to the β-C (TS6B), with a barrier difference of only
0.5 kcal/mol. The barriers for these two transition states are
21.0 and 21.5 kcal/mol, respectively, relative to Int6 plus
substrate. Owing to the small difference in the energy barrier,
both pathways could compete with each other. This hydride
transfer generates a transit ionic pair intermediate (Int8).
studies indicate that the D atom was added to both the α-
carbon and β-carbon of the alkyne moiety for the reaction of
5a with either H3N·BD3 or D3N·BH3. These results support
the DFT predictions that the two pathways involving Int8A
and Int8B compete with each other.
CONCLUSIONS
■
On the basis of experimental and calculated results, a catalytic
cycle is proposed for the sequential dimerization−reduction of
2a (Scheme 4). Complemented by the pyridonate site, our
cobalt(II) complex is able to activate the terminal alkyne,
affording the isolable alkynyl cobalt intermediate (Int3). The
alkynide insertion into a second molecule of alkyne gives
Int5B1. Subsequently, proton transfer from the NH to the
alkene anion leads to the formation of the (E)-enyne product
(5a) and regeneration of the catalyst. It was predicted that the
cobalt catalyst reacts with H3N·BH3 through cobalt−
pyridonate cooperation (Int6), and the formation of the
cobalt hydride intermediate (Int7) is thermodynamically
favorable. The insertion of 5a to the cobalt hydride Int7
generates the intermediates Int8A and Int8B, which are two
E
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Scheme 4. Proposed Cycle for the Sequential Transformation of 2a to the (E,Z)-1,3-Diene on the Basis of Calculated Full
Energy Diagrams
Notes
competing pathways. Finally, the proton delivery from the
amino group to the substrate is essentially exergonic.
The authors declare no competing financial interest.
Through metal−ligand cooperation, the cobalt complex not
only can activate the terminal alkyne for dimerization but also
reacts with amine−borane for the subsequent transfer
hydrogenation. Using a single cobalt catalyst, we realized the
two distinct transformations in tandem to harvest 1,3-
butadienes with excellent stereoselectivity. Such a sequential
strategy portends the potential utility of inexpensive metals for
these significant organic transformations.41
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the National
Natural Science Foundation of China (21871166, 21873031,
and 91427303).
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00486.
Experimental details, characterization of the products,
crystallographic data, and computational details (PDF)
Accession Codes
CCDC 1893491−1893493 contain the supplementary crys-
tallographic 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
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for R.-Z.L.: rongzhen@hust.edu.cn.
*E-mail for W.W.: wwg@sdu.edu.cn.
■
ORCID
Chengjuan Wu: 0000-0002-8552-2654
Rong-Zhen Liao: 0000-0002-8989-6928
Chen-Ho Tung: 0000-0001-9999-9755
Wenguang Wang: 0000-0002-4108-7865
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DOI: 10.1021/acs.organomet.9b00486
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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DOI: 10.1021/acs.organomet.9b00486
Organometallics XXXX, XXX, XXX−XXX