Hydroformylation of unsaturated esters and 2,3-dihydrofuran under solventless conditions at room temperature catalysed by rhodium: N-pyrrolyl phosphine catalysts

Article abstract of DOI:10.1039/c9nj04438h

Rhodium complexes of the type HRh(CO)L₃ (where L is an N-pyrrolyl phosphine, such as P(NC₄H₄)₃, PPh(NC₄H₄)₂, or PPh₂(NC₄H₄)) were applied in the hydroformylation of less reactive unsaturated substrates, namely allyl acetate, butyl acrylate, methyl acrylate, 2,3-dihydrofuran and vinyl acetate. Even at room temperature, these catalysts enabled complete substrate conversion and high chemoselectivity towards the corresponding aldehydes. High conversion of vinyl acetate (88% in 6 h) to the branched aldehyde was obtained with HRh(CO)[P(NC₄H₄)₃]₃ at 25 °C. An increase of the turnover frequency, TOF, up to 2000 mol mol^-1 h^-1 was achieved in this reaction under 20 bar of syngas (H₂/CO = 1) at 80 °C. The introduction of chiral phosphines, BINAP or Ph-BPE, to this system resulted in the production of a branched aldehyde with enantioselectivity, ee, up to 44 and 81%, respectively. High activity combined with high enantioselectivity was achieved due to the formation of the mixed rhodium hydrides HRh(CO)[P(NC₄H₄)₃](BINAP) and HRh(CO)[P(NC₄H₄)₃](Ph-BPE), identified by the NMR method.

Full text of DOI:10.1039/c9nj04438h

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Hydroformylation of unsaturated esters and
,3-dihydrofuran under solventless conditions
2
Cite this: New J. Chem., 2019,
at room temperature catalysed by rhodium
43, 16990
N-pyrrolyl phosphine catalysts†
W. Alsalahi
and A. M. Trzeciak
*
Rhodium complexes of the type HRh(CO)L
PPh(NC , or PPh (NC
3
4 4 3
(where L is an N-pyrrolyl phosphine, such as P(NC H ) ,
4
H )
4
2
2
4
H
4
)) were applied in the hydroformylation of less reactive unsaturated
substrates, namely allyl acetate, butyl acrylate, methyl acrylate, 2,3-dihydrofuran and vinyl acetate. Even
at room temperature, these catalysts enabled complete substrate conversion and high chemoselectivity
towards the corresponding aldehydes. High conversion of vinyl acetate (88% in 6 h) to the branched
aldehyde was obtained with HRh(CO)[P(NC
4
H
4
)
3
]
3
at 25 1C. An increase of the turnover frequency, TOF,
ꢀ
1
ꢀ1
Received 27th August 2019,
Accepted 5th October 2019
up to 2000 mol mol
h
was achieved in this reaction under 20 bar of syngas (H /CO = 1) at 80 1C.
2
The introduction of chiral phosphines, BINAP or Ph-BPE, to this system resulted in the production of a
branched aldehyde with enantioselectivity, ee, up to 44 and 81%, respectively. High activity combined
with high enantioselectivity was achieved due to the formation of the mixed rhodium hydrides
DOI: 10.1039/c9nj04438h
rsc.li/njc
4 4 3 4 4 3
HRh(CO)[P(NC H ) ](BINAP) and HRh(CO)[P(NC H ) ](Ph-BPE), identified by the NMR method.
synthetically useful compounds, such as 1,2- and 1,3-propanediol,
Introduction
9
,10
11,12
lactic acid, ethyl lactate,
amino acid threonine,
and
The hydroformylation of alkenes is one of the widest-ranging 2-hydroxypropanal, a very useful building block for the synthesis
12
processes in the industrial application of homogeneous cata- of steroids, antibiotics, and peptides. Aldehydes obtained from
lysis, which allows for the conversion of alkenes into aldehydes allyl acetate hydroformylation can be easily converted into
6
in a single catalytic step by using hydrogen and carbon 1,3- and 1,4-diols, which have a wide range of applications.
monoxide.
1
–5
The hydroformylation of functionalized alkenes Moreover, isoaldehydes produced by the hydroformylation of
is an attractive synthetic method for the production of bifunc- methyl or butyl acrylate are very important building blocks for
6
tional compounds. Unfortunately, the serious limitation of the synthesis of biologically active compounds, such as malonic
1
3–15
this method is the low reaction rate and low chemoselectivity to acid, 1,4-dicarboxylic acids, and lactones.
aldehydes. Due to the electronic structure, hydroformylation of 2- and 3-carbaldehydes can be synthesized in one step in the
Optically active
1
6
unsaturated esters produced the branched aldehyde as the asymmetric hydroformylation of 2,3-dihydrofuran.
main product. Wilkinson et al. reported that the hydroformy- chiral aldehydes can be utilized as substrates in the preparation
These
7
lation of vinyl acetate catalyzed by HRh(CO)(PPh ) is signifi- of biologically active natural products and pharmaceuticals,
3
3
8
17
cantly less efficient than that of terminal alkenes. Not only such as amprenavir, terazosin, and CCR5 antagonist.
vinyl acetate but also other unsaturated esters are significantly
Rhodium(I) complexes modified with N-pyrrolylphosphine
1
–5
less reactive to CO/H
2
than 1-alkenes.
Therefore, harsher ligands have been successfully applied for the hydroformyla-
1
8–22
23
conditions and a longer time should be used to transform them tion of different alkenes, such as 1-hexene,
to aldehydes. On the other hand, the hydroformylation of esters 2-pentene, ethane, dicyclopentadiene, a-methylstyrene,
presents an attractive method leading to valuable products 1-butene, and propene. In all cases, high selectivity towards
containing two functional groups. Aldehydes formed in the aldehydes was achieved. These reactions have been carried out
hydroformylation of vinyl acetate can be further converted into in organic solvents
toluene and water.
vinylsilanes,
2
4
25
26
27
2
8
29
1
8–29
and, in a few cases, also in a mixture of
2
8,29
Our previous study evidenced that a solventless procedure
was very efficient in the hydroformylation of olefins and
unsaturated alcohols, providing high conversion and chemo-
University of Wrocław, Faculty of Chemistry, 14 F. Joliot-Curie St., 50-383 Wrocław,
Poland. E-mail: anna.trzeciak@chem.uni.wroc.pl
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
2
2,30–33
c9nj04438h
selectivity towards aldehydes.
The main advantage of the
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solventless procedure is the simplification of the process by the synthesized by the reaction of Rh(acac)(CO)
2
with N-pyrrolyl-
2
elimination of the organic solvent, following the green phosphines under syngas (H /CO) following the procedure
2
8
chemistry rules.
reported earlier for P(NC H ) . The structures of the com-
4 4 3
In this paper, we for the first time report on the hydrofor- plexes were confirmed by IR spectra showing two bands for
ꢀ
1
ꢀ1
ꢀ1
mylation of unsaturated esters, such as vinyl acetate, allyl n(CO) and n(Rh–H) vibrations, at 2073.9 cm and 1987.7 cm
ꢀ
1
ꢀ1
acetate, butyl acrylate, and methyl acrylate, as well as 2,3- for I, at 2052.7 cm and 1970.8 cm for II, and at 2039.7 cm
ꢀ1
dihydrofuran, catalyzed by HRh(CO)L
3
[where L = P(NC
4
H
4
)
3
,
and 1948.1 cm for III, respectively (Fig. S1–S3, ESI†).
PPh(NC , or PPh (NC )] under solventless conditions
4
H
4
)
2
2
4 4
H
and without excess phosphine. We found that these catalysts
were able to efficiently form aldehydes from less active sub-
strates even at room temperature.
Hydroformylation of allyl acetate
The hydroformylation of allyl acetate catalysed by the three hydrido
complexes I, II, and III under solventless conditions in the absence
of any additional excess free ligands was investigated under 20 bar
Experimental
Chemicals
of H /CO = 1. Two aldehydes, 4-acetoxybutanal, linear aldehyde 1,
and 3-acetoxy-2-methyl propanal, isoaldehyde 2, were obtained as
2
the main products with a small amount (about 1–2%) of propyl
acetate (hydrogenation product, 3) (Scheme 1).
3
4
Rhodium complexes Rh(acac)(CO)
I
2
and HRh(CO)[P(NC
were synthesized according to the literature procedure.
Hydrido-rhodium complexes HRh(CO)[PhP(NC H ) ] II and
4 4 3 3
H ) ]
28
The results obtained at 25 and 80 1C are reported in Table 1.
At room temperature the catalytic activity of I, II, and III
differed, as shown in Fig. 1. Complete substrate conversion
was obtained with catalyst I in 2 h, while 94% converted to the
corresponding aldehyde in 8 h when using catalyst II. Only 15%
of the allyl acetate reacted in the case of catalyst III in 14 h. In
all cases, the yield of the branched aldehyde 2 was higher than
that of the linear one, 1. For comparison, we carried out
hydroformylation of allyl acetate with catalyst I at room tem-
perature using toluene as a solvent. In this reaction the con-
version was 59% and the regioselectivity to the linear aldehyde
4
4 2 3
HRh(CO)[Ph P(NC H )] III were prepared by using the same
2
8
4 4 3
2
procedure.
PPh(NC
The N-pyrrolylphosphine ligands P(NC
, and PPh (NC ) were synthesized as described
in the literature. Vinyl acetate was purchased from Fluka; allyl
acetate, 2.3-dihydrofuran, butyl acrylate, BINAP and Ph-BPE were
purchased from Aldrich; methyl acrylate was purchased from
4 4 3
H ) ,
4
H
4
)
2
2
4 4
H
35
International Enzymes Limited; and hydrogen (H , 99.999%)
2
and carbon monoxide (CO, 99.97%) were procured from Air
Products. All chemicals were used without any additional
purification.
(
1/2 ratio) was equal to 0.63 (Table 1, entry 3). Significantly
better results were achieved under solventless conditions, with
00% conversion and the regioselectivity to the linear aldehyde
1/2 ratio) equal to 0.8 (Table 1, entry 2). The high efficiency of
Hydroformylation reactions
1
(
The hydroformylation reactions were performed in 50 ml
stainless steel autoclaves provided with manometers, thermo-
stats, and magnetic stirrers. The required amounts of catalysts
I, II, or III were placed in the autoclave under a nitrogen
atmosphere. The substrate (1 mL) was introduced. The auto-
clave was sealed; hydrogen (5 bar) was passed through three
times; and thereafter the autoclave was pressurized with syngas
catalysts I and II at room temperature for hydroformylation of
allyl acetate is unusual. Good results for this reaction were obtained
6
at 40 1C, but at a higher pressure, 4 MPa (CO : H = 1:1). High
2
conversion and selectivity to linear aldehydes were obtained at
48
1
20 1C in toluene. Allyl acetate was converted to the corres-
ponding aldehydes with yields of 3% and 99% respectively at
0 1C and 80 1C using 30 bar of syngas (CO/H = 1/5) for 4 h in
2
(H /CO = 1) to 20 bar and heated to the desired temperature.
6
2
After the reaction was completed, the autoclave was cooled to
room temperature and the residual gases were depressurized.
The organic products were separated by the vacuum transfer
procedure and analyzed by means of GC, GC-MS, and NMR.
49
toluene.
The complete conversion of allyl acetate was obtained at
80 1C in 20 min by using catalysts I or II with the same excellent
selectivity towards aldehydes and high TOF values. Interest-
ingly, the higher temperature promoted better regioselectivity
towards the linear aldehyde and with catalyst I the 1/2 ratio was
Results and discussion
1
.8, while with catalyst II it was 1.04 (Table 1, entries 1 and 3). It
should be mentioned that catalysts I and II preferably formed
4 4 3 3
H ) ] I, HRh- an n-aldehyde, while an isoaldehyde was mainly formed with
Synthesis of rhodium complexes
The hydrido-carbonyl complexes HRh(CO)[P(NC
4 4 3
CO)[PhP(NC II, and HRh(CO)[Ph P(NC H )] III were other catalysts. With catalyst III, a longer time, 50 min, was
6
(
4
H
4
)
2
]
3
2
Scheme 1 Hydroformylation of allyl acetate.
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a
Table 1 Hydroformylation of allyl acetate catalysed by catalysts I, II, and III under solventless conditions
Product distribution
c
d
e
ꢀ1 ꢀ1
Entry
Catalyst
T, 1C
t
Conv., %
1%
2%
1/2 ratio
TOF, mol mol
h
1
2
3
3
4
5
6
I
80
25
25
80
25
80
25
20 min
2 h
2 h
20 min
8 h
50 min
14 h
100
100
59
100
94
100
15
63
43
39
50
33
40
35
56
61
48
66
59
1.8
0.8
0.6
1.04
0.5
0.7
0.4
2376
396
236
2376
93
954
8.6
b
II
III
26.7
73.3
a
b
c
Reaction conditions: allyl acetate (1 mL), [substrate]/[Rh] = 800, P = 20 bar (H
2
/CO = 1). Toluene as the solvent. Selectivity to 4-acetoxybutanal.
d
e
Selectivity to 3-acetoxy-2-methyl propanal. TOF = (mole of products (1 + 2))/(mole of catalyst ꢁ reaction time).
the hydroformylation chemoselectivity was lower mainly
because of the significant contribution of the hydrogenation
process leading to methyl propionate 3. The yield of 3 increased
to 49.8% and 87.5% for catalysts II and III (Table 2, entries 7
and 8). Interestingly, at room temperature only catalyst I
effectively produced aldehydes and the yield of the isoaldehyde
was as high as 98.4% (1) (Table 2, entry 5). The selectivity
to aldehydes increased with an increase of the p–acceptor
properties of the phosphorous ligand. Moreover, the selectivity
to aldehydes was affected by temperature and it decreased
when the temperature was rising.
The hydroformylation of butyl acrylate carried out at 80 1C
with catalyst I was not selective and led to a mixture of products
composed of 77.6% isoaldehyde (1) with 12.2% of the linear
aldehyde (the 1/2 ratio was 6.4) and 10.2% of the hydrogenation
Fig. 1 Hydroformylation of allyl acetate at room temperature.
needed to achieve 100% conversion; however, lower selectivity product (Table 2, entry 2). Similarly, in the case of methyl
and TOF values were noted (Table 1, entry 5).
acrylate hydroformylation, the selectivity was also poor, and
A further increase in the regioselectivity towards the linear the main product (51%) was methyl propionate 3, formed by
aldehyde was achieved upon the addition of excess P(NC
to I (Table S1, ESI†).
4
H
4
)
3
hydrogenation. In this reaction, the regioselectivity towards the
linear aldehyde was better than for butyl acrylate, and the 1/2
ratio was 1.1 (Table 2, entry 6).
Hydroformylation of butyl and methyl acrylate
It is worth noting that the studied catalysts used at room
The typical pattern of acrylate ester hydroformylation is shown temperature allowed the hydroformylation of butyl acrylate
in Scheme 2. Butyl and methyl acrylates mainly produced to perform with high selectivity, although the reaction was
branched aldehyde 1. Smaller amounts of linear aldehyde 2 relatively slow due to the low reactivity of the ester. An increase
and propionic ester were also formed.
in temperature accelerated the reaction, however, the selectivity
Excellent chemoselectivity and regioselectivity towards the decreased.
isoaldehyde were obtained in the hydroformylation of butyl
Hydroformylation of methyl acrylate worked well with
acrylate at room temperature (Table 2, entries 1, 3 and 4). catalyst I at room temperature. A rise in temperature resulted
The isoaldehyde was obtained under these conditions as the in an increase of the hydrogenation efficiency. Catalysts II and
only product with catalyst I. Lower chemoselectivity was III mainly catalysed the hydrogenation of methyl acrylate even
achieved with catalysts II and III, however, the yield of the at room temperature.
isoaldehyde was still higher than that of the linear one with a
/2 ratio of 37.5 and 9.4. Methyl acrylate reacted differently and with high regioselectivity to branched aldehydes was achieved
For comparison, high conversion of up to 95.6% and 69.7%
1
Scheme 2 Hydroformylation of butyl or methyl acrylates.
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a
Table 2 Hydroformylation of butyl and methyl acrylate using catalysts I, II, and III under solventless conditions
Product distribution
c
d
e
ꢀ1 ꢀ1
Entry
1
Substrate
Catalyst
t, h
Conv., %
1%
2%
3%
TOF, mol mol
h
Butyl acrylate
I
I
II
III
I
I
II
III
12
1.5
12
12
3.5
1
100
100
100
100
100
100
100
100
100
77.6
0
0
10.2
0
0
1.6
51
67
479
65
b
2
12.2
2.6
9.6
0
3
4
5
97.4
90.4
98.4
26
50.2
12.5
60
Methyl acrylate
225
392
115
29
b
6
23
0
0
7
8
3.5
3.5
49.8
87.5
a
b
c
Reaction conditions: substrate (1 ml), [substrate]/[Rh] = 800, P = 20 bar (H
2
/CO = 1), T = 25 1C. T = 80 1C. For butyl acrylate, selectivity to
d
2
-methyl-2-oxopropionic acid butyl ester; for methyl acrylate, selectivity to 2-methyl-2-oxopropionic acid methyl ester. For butyl acrylate,
e
selectivity to 4-oxo-butyric acid butyl ester; for methyl acrylate, selectivity to 4-oxo-butyric acid methyl ester. TOF = (mole of product (1 + 2))/
mole of catalyst ꢁ reaction time).
(
Scheme 3 Hydroformylation of 2,3-dihydrofuran.
in the hydroformylation of methyl acrylate and butyl acrylate
0
respectively by using a rhodium catalyst with a 2,2 -bis-
0
(
dipyrrolylphosphinooxy)-1,1 -(ꢂ)-binaphthyl ligand at 20 1C
5
0
at 2 MPa for 12 h in toluene.
Fig. 2 Hydroformylation of 2,3-dihydrofuran using catalysts I, II, and III at
room temperature.
Hydroformylation of 2,3-dihydrofuran
,3-Dihydrofuran was hydroformylated under solventless
2
conditions with excellent chemoselectivity towards the corres- with conversions of 95.3–100% in 1 h. The regioselectivity towards
ponding aldehydes, namely 2-formyltetrahydrofuran (1) and 2-formyltetrahydrofuran (1) increased with an increase in the
3
-formyltetrahydrofuran (2) (Scheme 3).
The results summarized in Table 3 and in Fig. 2 evidenced 3.3, and 1.7 for I, II, and III, respectively (Table 3, entries 1, 3 and 5).
that catalysts I, II, and III catalysed the hydroformylation of
For comparison, a significantly lower efficiency in the hydro-
,3-dihydrofuran at room temperature, but with different formylation of 2,3-dihydrofuran was reported for rhodium cata-
p–acceptor properties of the ligands, and the 1/2 ratio was 4.2,
2
16
efficiencies.
lysts modified with diphosphite ligands. After 48 h reaction at
The best results were obtained for catalyst I, which con- 45 1C, the conversion of the substrate was 10–45%. Similar
verted 94.8% of the substrate to aldehydes with a 1/2 ratio of regioselectivity towards 2-formyltetrahydrofuran (1) was obtained
3
1
.5, while the conversion decreased to 67.3% with a 1/2 ratio of under mild conditions with bisdiazaphospholane ligands (a 1/2
44
.4 when using catalyst II. When applying catalyst III, a very low ratio up to 3.9). High conversion with a good 1/2 ratio up to 1.7
was obtained with a phosphine–phosphoramidite ligand using a
The reaction was faster at 80 1C and the three catalysts, I, II, substrate to rhodium ratio = 200 in 16 h, while at room tempera-
conversion, only 7%, was obtained.
45
and III, exhibited high catalytic activities and chemoselectivities ture a very low conversion was obtained, up to only 8%.
a
Table 3 Hydroformylation of 2,3-dihydrofuran using catalysts I, II, and III under solventless conditions
Product distribution
b
c
d
ꢀ1 ꢀ1
Entry
Catalyst
T, 1C
t, h
Conv., %
1%
2%
1/2 ratio
TOF, mol mol
h
1
2
3
4
5
6
I
80
25
80
25
80
25
1
9
1
9
1
95.3
94.8
98
67.3
100
7
80.8
78
76.5
58
63.5
57
19.2
22
23.5
42
36.5
43
4.2
3.5
3.3
1.4
1.7
1.3
953
105.3
980
74.8
1000
5
II
III
14
a
b
Reaction conditions: substrate (1 ml), [substrate]/[Rh] = 1000, P = 20 bar (H
2
/CO = 1), T = 80 1C. Selectivity to 2-formyltetrahydrofuran.
c
d
Selectivity to 3-formyltetrahydrofuran. TOF = (mole of products (1 + 2))/(mole of catalyst ꢁ reaction time).
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Scheme 4 Hydroformylation of vinyl acetate.
At room temperature, the reaction rate with catalysts II and
III was slower, and as a result 80.1% and 70.5% of the substrate
was converted in 8 h and 24 h with high regioselectivity towards
the isoaldehyde (Table 4, entries 4 and 6). When the tempera-
ture was increased to 80 1C, catalysts II and III showed similar
catalytic activity to catalyst I. Complete conversion of the
substrate was obtained in 20 min at 80 1C with almost the
same regioselectivity and TOF (Table 4, entries 3 and 4).
For comparison, the hydroformylation of vinyl acetate without
2
solvent has been reported for rhodium supported on TiO nano-
9
tubes; however, a conversion of only 19.5% was reached at 6 MPa.
Low conversion, up to 35%, with good chemo- and regioselectivity
to the branched aldehyde was obtained by using different ligands
Fig. 3 The effect of the catalyst on the hydroformylation of vinyl acetate
at room temperature.
10
at 90–100 1C and 4 MPa of syngas in toluene.
ꢀ1
ꢀ1
The TOF values of ca. 2000 mol mol
h
obtained for
catalysts I, II, and III at 80 1C exceeded more than twice those
Hydroformylation of vinyl acetate
reported for vinyl acetate hydroformylation in biphasic systems
3
8
The hydroformylation of vinyl acetate produced 2-acetoxypropanal with water-soluble phosphines at 30 bar. Higher TOF values
1) as a major product, along with acetic acid (2) and propanal (3) as were achieved with a bulky phosphite ligand P(ONp) (tris-1-
(
3
3
9
40
side products formed by the decomposition of 3-acetoxypropanal naphtylphosphite) and with a diphenylphosphinite, under
36,37
(
linear aldehyde) (Scheme 4).
At room temperature, it is clearly seen that the catalytic
activity depends on the kind of N-pyrrolyl phosphine coordinated
conditions harsher than ours, namely at 4 MPa and 90 1C.
Asymmetric hydroformylation of vinyl acetate
to rhodium (Fig. 3). A decrease in the vinyl acetate conversion, The high catalytic activity of catalysts I, II and III allowed them
the reaction rate, and the TOF values with a decrease in the to be considered as candidates for asymmetric hydroformyla-
number of pyrrolyl groups was observed, in the following tion under solventless conditions. In order to achieve asym-
order: HRh(CO)[P(NC
HRh(CO)[Ph P(NC )]
Catalyst I is very active and provided total conversion of vinyl
4
H
H .
4 4 3
4
)
3
]
3
4 HRh(CO)[PhP(NC
4
H
4
)
2
]
3
4
metric induction, a chiral ligand, (R)-BINAP, was used together
with complexes I, II and III.
2
The effect of (R)-BINAP on the reaction course is strongly
acetate in 20 min at 80 1C and in 40 min at 60 1C or 50 1C. dependent on its amount (Table 5). At a [(R)-BINAP]/[I] ratio of
A longer time, 4 h, was needed to achieve 100% conversion at 1, complete conversion of vinyl acetate with excellent regio-
3
0 1C. It should be pointed out, however, that catalyst I selectivity towards the isoaldehyde was obtained; however,
converted vinyl acetate to aldehydes at 30 1C and even at the ee was as low as 1% (Table 5, entry 1). Increasing the
5 1C. The regioselectivity towards the isoaldehyde increased [(R)-BINAP]/[I] ratio to 2 and 3, the reaction rate and conversion
from 82.5 to 89.5% with the temperature lowered from 80 1C to decreased to 56 and 41%, but at the same time the enantio-
2
ꢀ
1
ꢀ1
30 1C, while the TOF decreased from 2000 to 179 mol mol
h
selectivity increased to 25 and 44%, respectively (Table 5,
entries 2 and 3). A slightly lower ee, 38%, was obtained using
(Table 4 and Table S2, ESI†).
a
Table 4 Hydroformylation of vinyl acetate using catalyst I under solventless conditions
b
c
ꢀ1 ꢀ1
Entry
Catalyst
T, 1C
t
Conv., %
1%
TOF, mol mol
h
1
2
3
4
5
6
I
80
25
80
25
80
25
20 min
6 h
20 min
8 h
20 min
24 h
100
88
100
80
100
71
82.5
91
82
89
81
2000
107
1988
72
1964
21
II
III
90
a
b
c
Reaction conditions: vinyl acetate (1 ml), [substrate]/[Rh] = 800, P = 20 bar. Selectivity to 2-acetoxypropanal. TOF = (mole of product 1)/(mole of
catalyst ꢁ reaction time).
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Table 5 Asymmetric hydroformylation of vinyl acetate using catalyst I with the addition of (R)-BINAP under solventless conditions
b
c
ꢀ1 ꢀ1
Entry
Catalyst
[L]/[Rh]
Conv., %
1%
ee%
TOF, mol mol
h
a
1
I
1
2
3
3
3
3
3
100
56
41
80.4
75
73
70
76
81
79
1
25
44
23
10
38
34
643.2
74.8
38.4
62.4
96.1
38.7
48.1
2
3
4
5
6
7
II
III
67
95.5
35.8
45.8
Rh(acac)(CO)
Rh(acac)(CO)
2
+
2
P(NC
4
H
4
)
3
a
b
Reaction conditions: vinyl acetate (1 ml), [substrate]/[Rh] = 800, T = 80 1C, P = 20 bar (H
2
/CO = 1), L = (R)-BINAP, t = 6 h. t = 1 h. Selectivity to
c
2
-acetoxypropanal. TOF = (mole of product 1)/(mole of catalyst ꢁ reaction time).
Rh(acac)(CO)
Table 5, entry 6). The addition of P(NC
resulted in an increase in the conversion by 10% at a similar ee next selected as the chiral ligand.
of 34% (Table 5, entry 7). carried out at 60–80 1C and 20 bar of syngas (H
2
with (R)-BINAP under the same conditions
To improve the results of the rhodium catalysed asymmetric
(
4 4 3
H )
to this reaction hydroformylation of vinyl acetate, (R,R)-Ph-BPE phosphine was
4
7,54
A series of reactions were
/CO = 1) with
2
For comparison, higher enantioselectivity (an ee of 78%) was the vinyl acetate to Rh molar ratio varying from 800 to 3000. It
reported for the hydroformylation of vinyl acetate with a was found that the conversion and enantio- and regioselectivities
rhodium catalyst modified with a chiral phosphine–phosphite decreased with an increase in the substrate amount and a
4
1
ligand, however, the reported conversion was only 15%.
substrate to Rh ratio equal to 800 was determined to be optimal
Bisphosphite chiral ligands, bearing naphthyl substituents, and used in further studies. The change of the (R,R)-Ph-BPE to I
enabled the preparation of a branched aldehyde with an ee ratio from 1.2 to 2 caused a slight decrease in the vinyl acetate
from 10 to 26%. The enantioselectivity increased to as much as conversion from 96 to 93%, however, the enantioselectivity
9
9% after the introduction of polyether binders as regulation increased from 71 to 81% (Table 6, entries 1 and 2). Further
4
2
agents. High enantioselectivity accompanied by excellent increases of the ligand amount only affected the conversion, while
regioselectivity and low conversion was obtained using the the enantioselectivity remained the same (Table 6, entry 3).
BettiPhos ligand at a low temperature. Unfortunately, an Similarly, high conversion and high enantioselectivity were
increase in the reaction temperature led to lower enantioselec- obtained by using (S,S)-Ph-BPE (Table 6, entry 7). A significantly
tivity although the reaction was faster.
Hydroformylation reactions performed with catalysts II to 60 1C (Table 6, entry 4). Interestingly, remarkably lower ee
and III and a threefold excess of (R)-BINAP were efficient values, 53 and 51%, were obtained by utilising Rh(acac)(CO) as a
4
9
lower conversion was obtained when the temperature decreased
2
considering the yield of branched aldehyde 1, however, the catalyst (Table 6, entries 6 and 7). Comparing these results clearly
enantioselectivity, ee, 23% and 10%, was lower than with indicated the advantage of catalyst I over the in situ generated
2
catalyst I (Table 5, entries 4 and 5). Thus, surprisingly, the HRh(CO) (Ph-BPE) in terms of enantioselectivity. While the con-
non-chiral pyrrolyl phosphine coordinated to the rhodium version and TOF values were similar in both catalytic systems, the
catalyst affected the reaction enantioselectivity.
difference in ee was ca. 30% in favour of precursor I.
The use of BINAP for the rhodium catalysed asymmetric
hydroformylation of vinyl acetate resulted in moderate enantio-
Mechanistic studies
selectivity and yield of the branched aldehyde. On the other The catalytic results collected for the asymmetric hydro-
hand, this ligand has demonstrated excellent efficiency for formylation of vinyl acetate indicated the influence of non-
4
6
asymmetric hydrogenation and isomerisation reactions.
chiral pyrrolyl phosphine ligands on the catalytic reaction, in
particular on its enantioselectivity. In order to get closer insight
into the mechanism of the asymmetric hydroformylation
of vinyl acetate, the NMR method was employed. First, the
Table 6 Hydroformylation of vinyl acetate under solventless conditions
using catalyst HRh(CO)[P(NC
Ph-BPE
4 4 3 3
H ) ] , I, modified with chiral ligand (R,R)-
reaction of HRh(CO)[P(NC H ) ] with (R)-BINAP was performed
4
4 3 3
in an H
two P(NC
4 4 3
with the formula HRh(CO)[P(NC H )
2
/CO atmosphere. It was found that (R)-BINAP substituted
ligands forming a new hydrido-rhodium complex,
](BINAP). As expected,
d
e
ꢀ1 ꢀ1
Entry
[L]/[Rh]
Conv., %
1%
ee%
TOF, mol mol
h
4 4 3
H )
1
2
3
1.2
2
3
1.2
1.2
2
96
93
88
18
98
98
89
97
98
99
98
94
93
99
ꢀ71
ꢀ81
ꢀ81
ꢀ78
ꢀ53
ꢀ51
80
742
720
696
136
736
720
704
BINAP occupied ee (bisequatorial) positions in a trigonal
bipyramidal complex (Scheme 5). This was evidenced by the
a
4
b
31
5
6
presence in the P NMR of a doublet of triplets in the region of
b
c
109 ppm assigned to P(NC H ) and a doublet of doublets at
4
4 3
7
2
3
(
8.15 ppm from two equivalent phosphorus atoms of BINAP
Fig. S12 and S10, ESI†). A similar mixed complex was formed as a
product of the partial substitution of PPh in HRh(CO)(PPh by a
Reaction conditions: vinyl acetate (1 ml), [Sub]/[Rh] = 800, P = 20 bar
a
b
(
H
(
2
/CO = 1). T = 80 1C, t = 1 h. T = 60 1C. Rh(acac)(CO)
2
as the catalyst.
S,S)-Ph-BPE. Selectivity to 2-acetoxypropanal. TOF = (mole of
product 1)/(mole of catalyst ꢁ reaction time).
c
d
e
3 3
)
3
25
chelating phosphine.
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Scheme 5 Reaction of HRh(CO)(P(NC
4
H
)
4 3
)
3
with (R)-BINAP.
Scheme 6 Reaction of HRh(CO)[P(NC
4
H
4
)]
3
with (R)-BINAP or Ph-BPE under hydroformylation conditions.
31
Fig. 4
P{H} NMR spectra in the region of pyrrolyl phosphine measured after vinyl acetate hydroformylation with (R)-BINAP and catalysts I, II and III
(Fig. S5, S7 and S8 respectively, ESI†).
It is worth noting that the coordination mode ee (bisequatorial) with two non-equivalent P atoms of (R)-BINAP. The signals
or ea (equatorial–apical) of the chelating phosphine ligand in the assigned to BINAP were observed as two multiplets in the
catalytic rhodium species has an important influence on the region of 24–28 ppm (Fig. S5, ESI†).
1
,43
hydroformylation selectivity.
On the other hand, the exchange
The most interesting finding of these studies is the presence
of the P(NC H ) ligand in the coordination sphere of rhodium
25,51
of an equatorial and an apical CO ligand is very fast.
4
4 3
The next experiment was performed with the same sub- hydride during the entire catalytic process (Scheme 6). The
strates in the presence of vinyl acetate, simulating hydroformy- mixed rhodium complex was identified in solution after the
lation conditions. The signals assigned to the P(NC H ) ligand hydroformylation of vinyl acetate despite the presence of a
4 4 3
were shifted to the 80 ppm region and presented two doublets threefold excess of (R)-BINAP over rhodium (Fig. 4). Our data
of doublets originating from the coupling with rhodium and indicated the presence of three phosphorus atoms bonded to
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31
Fig. 5
8 4 4 3 3 2
P NMR (toluene-d ) spectra of the reaction HRh(CO)[P(NC H ) ] + (R,R)Ph-PBE + CO + H at different temperatures.
rhodium in the catalytic process. This is in contrast with the quintet at d = ꢀ10.3 ppm which can be assigned to HRh[(R,R)-
structure of the rhodium hydroformylation catalyst most Ph-BPE] (Fig. S28, ESI†). When the same reaction was carried
accepted in the literature, namely the dicarbonyl complex out under H /CO, the mixed hydride complex was formed,
2
2
2
HRh(CO) L with only two Rh–P bonds.
containing both phosphines. Due to the dynamic processes
2
2
3
1
It is seen in Fig. 4 that under hydroformylation conditions occurring in the solution, analysis of the P NMR spectrum
with (R)-BINAP, catalysts II and III presented similar behaviour was difficult, however, at lower temperatures the resolution was
to catalyst I. Due to the dynamic process of ligand exchange, the better (Fig. 5).
signals assigned to pyrrolyl phosphines were broadened and
only the JRh–P coupling was observed but not JP–P. On the other (at d = 112.5 ppm) originating from diphosphine and two
hand, the spectra indicated the same coordination mode of pseudo quartets from P(NC (at d = 81.5 ppm) confirmed
R)-BINAP in all three mixed hydrido complexes (Scheme 6). the coordination of both phosphines, P(NC H ) and (R,R)-Ph-
For example, at ꢀ40 1C, the two doublets of doublets
4
4 3
H )
(
4
4 3
Faster exchange of ligands in the coordination sphere of BPE, to rhodium. Accordingly, the signal of the hydride ligand
rhodium facilitated the hydroformylation rate and increased presented two multiplets at d = ꢀ10.84 ppm. The doublet at
18
the conversion of vinyl acetate, but decreased the enantioselec- d = 109 ppm (JRh–P 211 Hz) represents the unreacted precursor I.
tivity (Table 5, entries 4 and 5).
In contrast, rhodium complexes containing only the (R,R)-Ph-PBE
52,53
The reaction of catalyst I with (R,R)-Ph-BPE was first studied ligand were not detected under these conditions.
3
1
in the absence of H
2
/CO. According to P NMR, a mixture of
A similar pattern was also detected in the presence of vinyl
different complexes was formed, however, most probably the acetate, indicating the similar structure of rhodium complexes
P(NC H ) ligand was completely removed from the coordina- in both systems with the ee position of the chelating phosphine
4
4 3
1
tion sphere of rhodium. The H NMR spectrum presented a (Fig. 6). Different coordination modes, in which the Ph-BPE
31
Fig. 6
8 4 4 3 3 2
P NMR (toluene-d ) spectra of the reaction HRh(CO)[P(NC H ) ] + (R,R)-Ph-PBE + CO + H + vinyl acetate at different temperatures.
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ligand occupied ea positions in the HRh(CO)
Paper
2
(Ph-BPE) complex,
3 A. M. Trzeciak and J. J. Ziolkowski, Coord. Chem. Rev., 1999,
190–192, 883–900.
were confirmed by a variable temperature experiment.
The spectroscopic studies supported the observation that both
chiral and non-chiral ligands coordinated to rhodium influenced
the catalytic activity. In particular, the presence of P(NC H )
4 Hydroformylation. Fundamentals, processes and Applications
in Organic Syntheses, ed. A. B o¨ rner and R. Franke, Wiley-
VCH, Verlag GmbH KGaA, 2016.
4
4 3
increased the enantioselectivity originating from Ph-BPE.
5 P. J. Thomas, A. T. Axtell, J. Klosin, W. Peng, C. L. Rand,
T. P. Clark, C. R. Landis and K. A. Abboud, Org. Lett., 2007,
9
, 2665–2668.
Conclusions
6
7
8
9
A. G. Panda, P. J. Tambade, Y. P. Patil and B. M. Bhanage,
React. Kinet., Mech. Catal., 2010, 99, 143–148.
C. Botteghi, R. Ganzerla, M. Lenarda and G. Moretti, J. Mol.
Catal., 1987, 40, 129–182.
Hydrido-rhodium complexes of the type HRh(CO)L
an N-pyrrolyl phosphine ligand, such as P(NC , PPh(NC
or PPh (NC )) were successfully applied in the solventless
3
(where L is
4
H
4
)
3
4 4 2
H ) ,
2
4 4
H
C. K. Brown and G. Wilkinson, J. Chem. Soc. A, 1970,
hydroformylation of unsaturated esters (such as allyl acetate, butyl
acrylate, methyl acrylate, or vinyl acetate) and 2,3-dihydrofuran
under 20 bar of syngas (H /CO = 1) at room temperature. The
presence of the pyrrolyl phosphine ligand enabled the selective
formation of the desired aldehydes with high efficiency. The
strongest p–acceptor phosphine gave the best results, which
were remarkably better than those obtained for these substrates
in other catalytic systems. As expected, the hydroformylation
reactions were faster at 80 1C than at 25 1C. At this temperature,
vinyl acetate was hydroformylated with a high reaction rate
2753–2764.
Y. Shi, X. Hu, B. Zhu, S. Wang, S. Zhang and W. Huang, RSC
Adv., 2014, 4, 62215–62222.
2
1
1
1
0 A. A. Dabbawala, R. V. Jasra and H. C. Bajaj, Catal. Commun.,
010, 11, 616–619.
1 C. F. Hobbs and W. S. Knowles, J. Org. Chem., 1981, 46,
422–4427.
2
4
2 C. Botteghi, S. Paganelli, A. Schionato and M. Marchetti,
Chirality, 1991, 3, 355–369.
13 G. Fremy, Y. Castanet, R. Grzybek, E. Monflier, A. Mortreux,
A. M. Trzeciak and J. J. Ziolkowski, J. Organomet. Chem.,
1
4 A. G. Panda, M. D. Bhor, S. R. Jagtap and B. M. Bhanage,
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7 X. Zheng, K. Xu and X. Zhang, Tetrahedron Lett., 2015, 56,
ꢀ
1
ꢀ1
(TOF up to 2000 mol mol
h ), and excellent regioselectivity
towards the isoaldehyde.
995, 505, 11–16.
A moderate enantioselectivity, up to an ee of 44%, was
obtained with the addition of a threefold excess of the chiral
ligand (R)-BINAP in the hydroformylation of vinyl acetate by
using catalyst I. Excellent chemo- and regioselectivity with high
enantioselectivity, up to an ee of 81%, were obtained when
another chiral phosphine, (R,R)-Ph-BPE, was applied.
Spectroscopic studies ( P and H NMR) of the reaction
mixture revealed the presence of mixed HRh(CO)L(P–P) complexes
containing pyrrolyl phosphine (L) and chiral diphosphine (P–P) in
both systems. In most catalytic systems used in hydroformylation,
HRh(CO) P species with only two Rh–P bonds have been assumed
2 2
to be catalytically active. We earlier proposed the presence of three
phosphorus atoms in the coordination sphere of rhodium in the
active catalysts containing pyrrolyl phosphine ligands. In these
studies we shown that the coordination of both ligands to rhodium
positively influenced the hydroformylation rate and selectivity. This
is especially evident in the hydroformylation of vinyl acetate
catalysed by I and Ph-BPE, which occurred with much better
1
1
1
1
1
30–134.
3
1
1
1
149–1152.
8 A. M. Trzeciak, T. Glowiak, R. Grzybek and J. J. Ziolkowski,
J. Chem. Soc., Dalton Trans., 1997, 1831–1838.
2
19 M. P. Magee, W. Luo and W. H. Hersh, Organometallics,
002, 21, 362–372.
2
18
20 A. M. Trzeciak, B. Borak, Z. Ciunik, J. J. Ziolkowski,
C. F. M. Guedes da Silva and L. J. A. Pombeiro, Eur. J. Inorg.
Chem., 2004, 1411–1419.
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Ziolkowski, J. Organomet. Chem., 2005, 690, 3260–3267.
2 W. Gil, A. M. Trzeciak and J. J. Ziolkowski, Organometallics,
2
2
2
2
2
2
2
enantioselectivity than that with a Rh(acac)(CO) precursor.
2
008, 27, 4131–4138.
3 A. M. Trzeciak, J. J. Ziolkowski and B. Marciniec, C. R. Acad.
Sci., Ser. IIc: Chim., 1999, 2, 235–239.
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Conflicts of interest
There are no conflicts to declare.
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