Water-initiated hydrocarboxylation of terminal alkynes with CO2and hydrosilane

Article abstract of DOI:10.1039/d0cc06320g

This work discloses a Cu(ii)-Ni(ii) catalyzed tandem hydrocarboxylation of alkynes with polysilylformate formed from CO₂and polymethylhydrosiloxane that affords α,β-unsaturated carboxylic acids with up to 93% yield. Mechanistic studies indicate that polysilylformate functions as a source of CO and polysilanol. Besides, a catalytic amount of water is found to be critical to the reaction, which hydrolyzes polysilylformate to formic acid that induces the formation of Ni-H active species, thereby initiating the catalytic cycle.

Full text of DOI:10.1039/d0cc06320g

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Water-initiated hydrocarboxylation of terminal
alkynes with CO2 and hydrosilane†
Meng-Meng Wang,‡ab Sheng-Mei Lu,‡a Kumaraswamy Paridalaa and Can Li
Cite this: Chem. Commun., 2021,
57, 1230
a
*
Received 19th September 2020,
Accepted 16th December 2020
DOI: 10.1039/d0cc06320g
rsc.li/chemcomm
This work discloses a Cu(II)–Ni(II) catalyzed tandem hydrocarbox- series of CO-involving processes using CO2 as a substrate have
ylation of alkynes with polysilylformate formed from CO2 and been developed, providing carboxylic acids, aldehydes, alcohols
polymethylhydrosiloxane that affords a,b-unsaturated carboxylic and carboxylic acid derivatives as products.22–32 Hydrosilanes,
acids with up to 93% yield. Mechanistic studies indicate that poly- especially PMHS (polymethylhydrosiloxane), as a kind of
silylformate functions as a source of CO and polysilanol. Besides, a effective reductant, can reduce CO2 to silyl formate under mild
catalytic amount of water is found to be critical to the reaction, which conditions.33 Silyl formate has been reported as an ideal
hydrolyzes polysilylformate to formic acid that induces the formation carbonyl source for carbonylative transformations.28–30,32
of Ni–H active species, thereby initiating the catalytic cycle.
Moreover, as a kind of formate, silyl formate has similar
properties to methyl formate and phenyl formate,34–37 which
Hydrocarboxylation of alkynes1–4 is one of the effective can be decomposed into CO and a nucleophile. Recently, our
approaches to synthesize a,b-unsaturated carboxylic acids group reported the carboxylation of aryl/vinyl halides with silyl
which are important structural motifs in a myriad of pharma- formate formed from CO2 and PMHS.32 It is confirmed that the
ceuticals. To realize the transformation of alkynes to a,b- reaction proceeds in a silyloxycarbonylation manner and the
unsaturated carboxylic acids, sources of ‘‘H’’ and ‘‘COOH’’ corresponding silanol plays a vital role as a nucleophilic
moieties are formally required. Recent years have witnessed reagent. As part of our continuous work of utilizing CO2, we
an endeavour to use CO2 for the straightforward incorporation report the hydrocarboxylation of alkynes with polysilylformate
of the ‘‘COOH’’ moiety into substrates.5–8 In this context, in situ formed from CO2 and PMHS under neutral conditions
several preeminent catalytic hydrocarboxylation processes of (Scheme 1c). As far as we know, there is no report on the use of
alkynes using CO2 were developed. With different ‘‘H’’ sources silyl formate to realize the hydrocarboxylation of alkynes.
and/or reductants, a variety of alkynes were smoothly trans-
We have demonstrated that silyl formate can work as a
formed into a,b-unsaturated carboxylic acids (Scheme 1a).9–16 carboxylation reagent to introduce a ‘‘COOH’’ moiety into aryl/
However, terminal alkynes are not suitable substrates for most vinyl halides.32 Here, we envisage the possibility of silyl formate
of the CO2-involving hydrocarboxylation reactions, possibly serving as an ‘‘H’’ source like phenyl formate, which has been
due to their hydrogenation propensity in such reductive employed for the hydroesterification of alkynes or alkenes without
environments.9–15
an additional ‘‘H’’ source.34–37 We carried out the hydrocarboxyla-
In addition, palladium or nickel catalyzed carbonylation tion reaction in a one-pot manner, where polysilylformate (PMS-
reactions offered an alternative approach to achieve the hydro- formate) was generated in situ from CO2 and PMHS catalyzed by a
carboxylation of alkynes, where the combination of CO and Cu(OAc)2–dppbz (1,2-bis(diphenylphosphino)benzene) system
acids or formic acid and acid anhydrides was employed through the hydrosilylation reaction in dry 1,4-dioxane and then
(Scheme 1b).17–21 Considering that CO2 can be transformed
into CO or its surrogates in situ with suitable reductants, a
a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China.
E-mail: canli@dicp.ac.cn
b University of Chinese Academy of Sciences, Beijing 100049, P. R. China
† Electronic supplementary information (ESI) available: Copies of 1H and 13C
NMR spectra. See DOI: 10.1039/d0cc06320g
Scheme 1 Metal-catalyzed hydrocarboxylation of alkynes.
‡ M.-M. Wang and S.-M. Lu contributed equally.
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Table 1 Optimization of the hydrocarboxylation reaction conditionsa
Table 2 Hydrocarboxylation of different alkynesa
Entry
R1/R2
Yield of 2 and 3b (%)
2/3c
Yield of 2a
1
2
3
4
5
6
7
8
Ph/H (1a)
86
93
80
71
4.7/1
8.8/1
Entry
Catalyst
Solvent
and 3a (%)b
2a/3ac
4-MeC6H4/H (1b)
3-MeC6H4/H (1c)
2-MeC6H4/H (1d)
4-MeOC6H4/H (1e)
4-FC6H4/H (1f)
4-ClC6H4/H (1g)
4-PhenylC6H4/H (1h)
nBu/H (1i)
Ph/TMS (1j)
Ph/Me (1k)
Ph/Et (1l)
Ph/Ph (1m)
16.7 : 1
25.0/1
9.2/1
3.4/1(4.1/1)
9.5/1(9.3/1)
8.2 : 1
12.5/1
6.8/1
1
2
3
4
5
6
7
8
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Toluene
1,4-Dioxane
nButyl ether
Diphenyl ether
Anisole
THF
CH3CN
45(53)d
50
62
39
79
86
81
0
11.0 : 1(10.3 : 1)
9.0 : 1
6.0 : 1
7.5 : 1
5.5 : 1
4.7 : 1
5.7 : 1
/
90
53d(66)e
51 (74)f
50g
9
52g
10
11
12
13
62gh
22
DMF
0.8/1
1.8/1
/
26g
a
Reaction conditions: step 1: Cu(OAc)2ÁH2O (1.0 mol%), dppbz
(1.5 mol%), PMHS (0.165 g, Si–H, 2.5 mmol), CO2 (balloon), 1,4-
dioxane (2 mL), 65 1C, 30 min; step 2 : 1a (1.0 mmol), Ni(acac)2
o10g
a
Reaction conditions: step 1: the same as that in Table 1; step 2 : 1
(1.0 mmol), Ni(acac)2 (5 mol%), dppbz (10 mol%), solvent (8 mL),
100 1C, 24 h; for entries 1, 2 and 11, THF was used as a solvent. All
b
(5 mol%), dppbz (10 mol%), solvent (8 mL), 100 1C, 24 h. Total
c
isolated yield of 2a and 3a based on 1a. Isomer ratio (b : l) determined
by 1H NMR. Step 1: Cu(OAc)2 (1.0 mol%), dry 1,4-dioxane (2 mL); step
d
b
the others used anisole as a solvent. Total isolated yield of 2 and 3
based on 1. Determined by 1H NMR. 120 1C. 120 1C, 48 h. 58 h.
c
d
e
f
2: dry toluene (8 mL) and 10 mol% water; other conditions remain
unchanged. THF = tetrahydrofuran; DMF = N,N-dimethylformamide.
g
h
48 h. TMS group was removed.
dry toluene, phenylacetylene (1a) and Ni(acac)2/dppbz were added. corresponding b : l ratio decreases. The reactivity of the tandem
After the reaction was conducted at 100 1C for 24 hours, no reaction is also affected by the ratio and the amount of two
desired product was obtained. Screening of the reaction condi- solvents (Table S2, ESI†). The reactivity at 80 1C, 100 1C, and
tions remained unsuccessful, which implies that polysilylformate 120 1C indicates that 100 1C is an optimal temperature for the
presumably could not provide the ‘‘H’’ source itself and an reaction (Table S3, ESI†). When the reaction is conducted in a
additional active ‘‘H’’ source is indispensable. Fortunately, when one-time addition manner, no product is obtained, which
10 mol% of water was added in dry toluene, a,b-unsaturated confirms that the tandem method is essential for the hydro-
carboxylic acids 2a and 3a were obtained in a total yield of 53% carboxylation of phenylacetylene (Table S4, ESI†).
(Table 1, parentheses in entry 1). Taking into account the facile
Having optimized the reaction conditions, we began to
hydrolysis tendency of silyl formate, we deduce that the added investigate the scope of the phenylacetylene derivatives
catalytic amount of water is to hydrolyze the polysilylformate into (Table 2). For various terminal phenylacetylenes, the corres-
formic acid which provides the ‘‘H’’ source to initiate the ponding phenylacrylic acid products are obtained with the
reaction.38 Also, we find that the reaction can afford the products dominant product being a branched a-phenylacrylic acid.
with 45% yield in normal 1,4-dioxane and toluene (solvents for Phenylacetylenes bearing an electron-donating substituent on
step 1 and step 2) (Table 1, entry 1). For convenience, the following the phenyl ring afford the hydrocarboxylation products in good
reaction conditions were optimized with the normal solvents.
to excellent yields (entries 2–5). The position of the substituent
The effect of ligands on the reaction shows that the reactivity on the phenyl ring has some effect on the reaction and the
markedly depends on the nature of the ligands (Table S1, ESI,† hydrocarboxylation of para-methyl phenylacetylene 1b provides
entries 1–5). Among all the screened mono- and bidentate a higher yield of 93% (entry 2) than that of the ortho- or
ligands, the biphosphine ligand dppbz appears to be the meta-methyl substituted phenylacetylene (entries 3 and 4). In
optimal ligand for the reaction. The reaction results of other addition, the position of the substituent on the phenyl ring
nickel precursors Ni(OAc)2, Ni(PPh3)2Br2 and Ni(PPh3)2Cl2 are affects the b : l ratio of the product (entries 2–4). The reaction
inferior to that of Ni(acac)2 (Table S1, ESI,† entries 6–8). The with ortho-methyl substituted phenylacetylene affords the high-
solvent effect indicates that ether solvents (Table 1, entries 2–6), est b : l ratio (25.0/1, entry 4), which indicates that the steric
including 1,4-dioxane, diphenyl ether, nbutyl ether, anisole and effect controls the regioselectivity. Substrate 1e with a 4-
THF are all effective for the reaction and THF is the most methoxyl group on the phenyl ring performs quite well afford-
striking one to deliver products in 86% yield (Table 1, entry 6). ing a yield of 90% (entry 5). When 1f and 1g bearing a fluoro or
Besides, acetonitrile is also an excellent solvent for the reaction chloro group on the phenyl ring are hydrocarboxylated under
(Table 1, entry 7). However, DMF can hardly afford the desired the same conditions, lower yields of 53% and 51% are achieved,
products (Table 1, entry 8). Through the analysis of the relation- respectively (entries 6 and 7), indicating that the electron-
ship between the yield and the b : l ratio, it can be basically withdrawing substituent on the phenyl ring has a negative
concluded that as the yield of the reaction increases, the effect on the reactivity. When anisole is used as a solvent
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instead of THF, the reactions with 1f and 1g afford slightly
higher yields of 66% and 74%, respectively, at a higher tem-
perature or with a longer reaction time (parentheses in entries 6
and 7). For the substrate 1h with a phenyl substituent, a
moderate yield of 50% is achieved (entry 8). This catalyst system
is also effective for aliphatic 1-hexyne 1i, affording the product
in a moderate yield with high branched-selectivity (entry 9). It is
worth noting that terminal alkynes are not favourable sub-
strates for most of the CO2-involving direct hydrocarbo-
xylation.9–15 Our strategy making use of polysilylformate
generated in situ from CO2 and PMHS provides an alternative
Fig. 1 (a) Effect of water on the reaction; (b) the effect of formic acid on
the reaction. Reaction conditions: For (a): step 1: the same as that in
Table 1, only Cu(OAc)2 and dry 1,4-dioxane were used; step 2: 1a
(1.0 mmol), Ni(acac)2 (5 mol%), dppbz (10 mol%), dry anisole (8 mL), and
route for the hydrocarboxylation of terminal alkynes. When water (0–100 mol%) were added, 100 1C, 24 h. For (b): the same as that in
(a), only formic acid (0–100 mol%) was added instead of water. PA =
phenylacetylene; acids = a-phenylacrylic acid and trans-cinnamic acid;
ST = styrene.
internal alkynes 1j bearing the TMS group at one end of the
alkyne react with polysilylformate, the TMS group is removed
during the reaction, leaving a-phenylacrylic acid and trans-
cinnamic acid as the products (entry 10).39,40 However, this
catalyst system is inefficient for the hydrocarboxylation of
internal alkynes such as 1k, 1l, and 1m (entries 11–13). For
these substrates, low yields of 22%, 26%, and o10% are
obtained even after prolonging the reaction time or by increas-
ing the reaction temperature in anisole.
the hydrocarboxylation of phenylacetylene. The effect of the
formic acid amount on the reaction was also investigated and
is shown in Fig. 1b. According to Fig. 1b, the reaction conversion
increases with the increase of formic acid added and 10 mol% or
more formic acid achieves complete conversion of phenylacetylene
while styrene increases continually. When formic acid addition
reaches 50 mol%, the yield of styrene also becomes very high.
A similar trend to that of the effect of water on the reaction is
observed, and the suitable amount of formic acid for the
reaction is found to be in the range of 10–20 mol%. Obviously,
the catalytic amount of formic acid present in the actual
reaction system is enough for the reaction.
Based on the above results, a possible reaction mechanism
is proposed as shown in Scheme 2. In the presence of a catalytic
amount of water, the catalytic amount of polysilylformate is
hydrolyzed to formic acid and the remaining polysilylformate
was decomposed into CO and the corresponding polysilanol.
Then, Ni(acac)2 reacts with formic acid and produces
[Ln(HCOO)Ni–H] (L = dppbz) species A accompanied by the
release of CO2 and acetylacetone. Insertion of A into the pheny-
lacetylene produces intermediate B. Polysilanol as a nucleophile
that existed in the reaction system attacks B and replaces the
formate on B to form intermediate C. Subsequently, CO inserts
into the Ni–C bond of C, forming the acyl nickel species D, and
To understand the catalytic reaction mechanism, several
control experiments were performed. Firstly, through monitoring
the amount of CO simultaneously under the reaction conditions
with (Fig. S1, ESI,† condition A) or without phenylacetylene 1a
(Fig. S1, ESI,† condition B), we found that: (1) the amount of
CO under both conditions A and B increased as the reaction
proceeded; (2) the amount of CO under condition B was a
multiple of that under condition A at the same measuring time.
These results indicate that CO is produced rapidly and a large
proportion of it is further consumed during the reaction, which
implies that polysilylformate is decomposed into CO and a
carbonylation process is involved. (For more details see Fig. S2
in the ESI.†)
As mentioned in the former part, a catalytic amount of water
is essential in this system. It is proposed that the role of water is
to hydrolyze polysilylformate to formic acid, which provides an
active ‘‘H’’ source to initiate the catalytic cycle. If it is so, a
catalytic amount of formic acid should promote the reaction
similarly. To confirm this assumption, we performed the reaction
with 10 mol% water or 10 mol% formic acid using dry 1,4-dioxane
and dry anisole as solvents. As expected, a similar result was
obtained providing the product with 71% or 81% yield, which is
in agreement with our assumption.
To figure out the optimized amount of water for the reaction,
a series of experiments with different amounts of water in a dry
solvent were implemented and the results are summarized in
Fig. 1a. As shown in Fig. 1a, the conversion of phenylacetylene
increases with the increase of water, and 15 mol% or more water
achieves complete conversion of phenylacetylene. However, with
the increase of the amount of water added, a side reaction of
phenylacetylene hydrogenation to styrene begins to emerge and
it becomes more serious when the amount of water increases
further. Strikingly, when the amount of water added reaches
50 mol%, the yield of styrene becomes as high as 45%. As a
result, 15–20 mol% of water seems to be a favourable range for
Scheme 2 Possible catalytic cycle for the hydrocarboxylation of alkynes.
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Oxidative addition of formic acid to E regenerates Ni(II)
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excellent yields with high branched-selectivity. The reaction
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