Regioselective and rapid hydroformylation of vinyl acetate catalyzed by rhodium complex modified bulky phosphite ligand

Article abstract of DOI:10.1016/j.catcom.2010.01.007

The regioselective hydroformylation of vinyl acetate catalyzed by rhodium complex of monodentate phosphite ligand, tri-1-naphthylphosphite P(ONp)₃, was investigated. The P(ONp)₃ ligand exhibited a considerable impact on the rate and selectivity of hydroformylation of vinyl acetate, notably high turnover frequency (up to 11,520 h^-1) with excellent regioselectivity (99%) to the preferred branched aldehyde and high selectivity to aldehyde (93%). Significant results with the substrate having less reactive character toward hydroformylation and practically easy accessibility of the ligand (synthesized from inexpensive compound, 1-naphthol) make this system very attractive.

Full text of DOI:10.1016/j.catcom.2010.01.007

Catalysis Communications 11 (2010) 616–619
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Regioselective and rapid hydroformylation of vinyl acetate catalyzed
by rhodium complex modified bulky phosphite ligand
Aasif A. Dabbawala a, Raksh V. Jasra b,1, Hari C. Bajaj a,
*
a Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research (CSIR), G.B. Marg,
Bhavnagar 364 021, Gujarat, India
b R&D Centre, Reliance Industries Limited, Manufacturing Division, Vadodara 391 346, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The regioselective hydroformylation of vinyl acetate catalyzed by rhodium complex of monodentate
phosphite ligand, tri-1-naphthylphosphite P(ONp)3, was investigated. The P(ONp)3 ligand exhibited a
considerable impact on the rate and selectivity of hydroformylation of vinyl acetate, notably high turn-
over frequency (up to 11,520 hÀ1) with excellent regioselectivity (99%) to the preferred branched alde-
hyde and high selectivity to aldehyde (93%). Significant results with the substrate having less reactive
character toward hydroformylation and practically easy accessibility of the ligand (synthesized from
inexpensive compound, 1-naphthol) make this system very attractive.
Received 23 November 2009
Received in revised form 31 December 2009
Accepted 6 January 2010
Available online 11 January 2010
Keywords:
Hydroformylation
Vinyl acetate
Ó 2010 Elsevier B.V. All rights reserved.
Monodentate phosphite
High rate
Regioselective
1. Introduction
the past decades, efforts have been made to increase the regiose-
lectivity with high reaction rate and decreased undesired side
Hydroformylation of olefins represents efficient method to pre-
pare aldehydes, and is one of the industrially most important C–C
bond forming reactions catalyzed by Co and Rh metal complexes
[1–3]. Hydroformylation has been mainly studied for terminal ole-
fins. However, in recent year, there has been increased interest in
the hydroformylation of functionalized olefins such as alkyl acry-
late [4], allyl cyanide [5], allyl alcohol [6], enamide [7], and vinyl
acetate [8]. The hydroformylation of such substrates offers the
products having two functional groups widely used in organic syn-
theses. Particularly, vinyl acetate as substrate provides gate way to
valuable building blocks for the preparation of bifunctional inter-
mediate (Scheme 1), which can be further converted into synthet-
ically useful compounds such as 1,2- and 1,3-propanediol, lactic
acid and ethyl lactate.
Vinyl acetate is generally less reactive to syngas relatively to
terminal alkenes [9]. The reaction suffers from a slow rate, forma-
tion of by-products and the high pressure (200 atm) is often re-
quired to achieve high turnover frequency [10]. However, there
are few reports wherein hydroformylation of vinyl acetate is car-
ried out under mild conditions [11–13] wherein good regioselec-
tivity was achieved by sacrificing the reaction rate. Therefore, in
products [14–16]. The high reaction rates for the hydroformylation
of vinyl acetate were obtained using bidentate ligands [15] or with
electron-deficient ligand such as organo phosphite ligand [16] in
comparison with the electron-rich analogues. We have recently re-
ported the synthesis and application of rhodium complex of novel
monodentate bulky phosphite ligand, P(ONp)3 for the hydrofor-
mylation of olefins such as 1-hexene, styrene and cyclohexene
[17]. In this context, we explore the catalytic activity of Rh/
P(ONp)3 system toward vinyl acetate hydroformylation. Herein,
we report the highly regioselective and rapid hydroformylation
of vinyl acetate to the branch aldehyde in the presence of
Rh(CO)2(acac) and P(ONp)3 ligand.
2. Experimental
2.1. General remarks
The ligand synthesis was performed using standard Schlenk
technique under nitrogen atmosphere. THF was distilled from so-
dium/benzophenone prior to use. Toluene was purchased from Sig-
ma–Aldrich as anhydrous grade material and used as received.
Et3N was distilled from sodium and stored under N2. PCl3 and 1-
naphthol were obtained from Merck, India, and used as received.
Rh(CO)2(acac), triphenyl phosphine (PPh3), triphenyl phosphite
P(OPh)3, tricyclohexylphosphine (PCy3), 1,3-bis(diphenylphos-
* Corresponding author. Tel.: +91 278 2471793; fax: +91 278 2566970.
E-mail address: hcbajaj@csmcri.org (H.C. Bajaj).
1
Present address.
1566-7367/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.01.007

A.A. Dabbawala et al. / Catalysis Communications 11 (2010) 616–619
617
Aldehydes
the optimum reaction temperature was 90 °C where maximum
TOF and selectivity was achieved.
O
O
CHO
O
The hydroformylation of vinyl acetate catalyzed by Rh/P(ONp)3
was studied at 90 °C under different syngas pressure (2.0–
6.0 MPa). Changes in the syngas pressure were found to affect
the reaction rate and selectivity (Table 1). The results indicate that
the hydroformylation of vinyl acetate proceeds slowly at low syn-
gas pressure. At low syngas pressure, CO insertion may be difficult
into the intermediate species as the ester carbonyl group form
thermodynamically stable five and/or six member rings, resulting
into considerably low TOF. TOF increased considerably with an in-
crease in the syngas pressure (2.0–4.0 MPa) and above that there
was a slight increase in the TOF. For example, when the syngas
pressure was increase from 2.0 to 4.0 MPa, TOF increased by a fac-
tor of ꢀ2.76, whereas with further increase in the syngas pressure
from 4.0 to 6.0 MPa, TOF was increased only by a factor ꢀ1.39 (see
Table 1). From the above discussion one can conclude that mini-
mum 4 MPa syngas pressure is require for achieving reasonable
TOF. In all instances, regioselectivity to branch aldehyde was great-
er then 98%.
The effect of ligand/Rh ratio on the catalytic activity/selectivity
of vinyl acetate hydroformylation was studied at 90 °C, 3.0 MPa
syngas pressure by varying ligand/Rh mmol ratio 3.0–18.0 (Ta-
ble 2). The results indicate that TOF was improved up to ligand/
Rh of 6.0. While TOF was found to decrease as ligand/Rh increased
from 6.0 to 18.0. However, chemo selectivity to aldehyde and reg-
ioselectivity to branched aldehyde were increased. This tendency is
also found in enamides hydroformylation catalyzed by Rh/biphe-
nyl based monodentate phosphite catalytic system [7].
Rh(CO)2acac/L
CHO
+
CO + H2
+
O
O
O
Vinyl acetate
2-acetoxypropanal
b (branched)
3-acetoxypropanal
l (linear)
L =
O
P
O
O
Scheme 1. Rh/tri-1-naphthyl phosphite catalyzed hydroformylation of vinyl
acetate.
phino)propane (dppp), 1,4-bis(diphenylphosphino)butane (dppb),
tris(4-methoxyphenyl)phosphine (p-MeO-C6H4)3P, tris(4-trifluoro-
methylphenyl)phosphine (p-CF3-C6H4)3P and vinyl acetate were
purchased from M/s Sigma-Aldrich Chemicals, USA and used as re-
ceived. The syngas (99.9%) used was from Hydro Gas India Pvt.,
Ltd., India.
The synthesis and characterization of phosphite ligand P(ONp)3
as well as corresponding rhodium complex has been described
elsewhere [17]. The hydroformylation reaction was performed in
a 100 mL stainless steel autoclave (Autoclave Engineers, EZE-Seal
Reactor, USA) and the reaction products were analyzed on a Shima-
dzu GC-17A gas chromatograph equipped with a flame ionization
detector.
When rhodium concentration was increased from 0.16 to
0.46 mM (ligand/Rh ratio 6) at 80 °C, 3.0 MPa syngas pressure, as
expected, the rate of vinyl acetate hydroformylation reaction was
found to increase with an increase in the rhodium concentration
indicating linear dependency with respect to the rhodium concen-
tration (Table 3). Similar result was also found for HRh(CO)(PPh3)3
catalyzed hydroformylation of vinyl acetate [18]. Aldehyde selec-
tivity was low (90%) at low 0.16 mM rhodium concentration and
improved up to 0.23 mM rhodium concentration, beyond that
aldehyde selectivity remained unaffected.
3. Results and discussion
The rhodium catalyst of P(ONp)3 was prepared in situ by mixing
P(ONp)3 with Rh(CO)2(acac). This system catalyzed the hydrofor-
mylation of vinyl acetate. The reaction products were 2-acetoxy-
propanal and 3-acetoxypropanal along with acetic acid and ethyl
acetate as side products. The branch aldehyde (2-acetoxypropanal)
was observed as the main product with high turnover frequency.
The selectivity of products in hydroformylation process is
mainly affected by variation in temperature. Temperature was var-
ied from 70 to 100 °C at constant syngas pressure (3.0 MPa). With
an increase of temperature from 70 to 100 °C, the reaction pro-
ceeds faster by a factor of ꢀ3.7 (Table 1) along with slight decrease
in the chemo selectivity toward aldehyde. However, an excellent
branch aldehyde regioselectivity (98%) was obtained in the entire
range of temperatures. At lower reaction temperature (70 °C), high
chemo selectivity toward aldehyde was observed (94.5%), the TOF
was rather low (1800 hÀ1). At 100 °C, the TOF increased up to
6720 hÀ1 with aldehyde selectivity decreased to 89.5%. Therefore,
For comparison, a variety of phosphine ligands were screened
for rhodium catalyzed hydroformylation of vinyl acetate under
Table 2
Effect of ligand/Rh ratio on Rh/tri-1-naphthyl phosphite catalyzed hydroformylation
of vinyl acetate.a
Entry
P/Rh
Conv. (%)
TOF (hÀ1
)
Saldehyde (%)
b/l
1
2
3
4
3.0
6.0
12.0
18.0
20.0
23.5
22.0
21.0
4800
5640
5280
5040
91.0
92.0
94.0
94.0
97/3
98/2
98/2
98/2
Table 1
a
Reaction
conditions:
sub/cat. = 4000,
[Rh(CO)2acac] = 0.23 mmol/L,
Effect of temperature and syngas pressure on Rh/tri-1-naphthyl phosphite catalyzed
temp. = 90 °C, syngas pressure (1:1) = 3.0 MPa, solvent (toluene) = 50 mL, and
reaction time = 10 min.
hydroformylation of vinyl acetate.a
Entry Temp.
Press.
(MPa)
Conv.
(%)
TOFb
(hÀ1
Saldehyde
(%)
b/l
(°C)
)
1c
2
3
4
5
6
7
8
70
80
90
100
90
90
3.0
3.0
3.0
3.0
2.0
4.0
5.0
6.0
15.0
17.0
23.5
28.0
12.5
34.5
42.0
48.0
1800
4080
5640
6720
3000
8280
10,000
11,520
94.5
93.0
92.0
89.5
86.0
93.0
93.0
93.5
98/2
98/2
98/2
98/2
98/2
99/1
99/1
99/1
Table 3
Effect of initial rhodium concentration on Rh/tri-1-naphthyl phosphite catalyzed
hydroformylation of vinyl acetate.a
Entry
[Rh(CO)2acac] (mmol/L)
Rate  103 (m sÀ1
0.17
0.26
0.32
0.40
)
Saldehyde (%)
b/l
90
90
1
2
3
4
0.16
0.23
0.35
0.46
90.0
93.0
93.0
93.0
97/3
98/2
98/2
98/2
a
Reaction conditions: sub/cat. = 4000, [Rh(CO)2acac] = 0.23 mmol/L, P/Rh = 6.0,
and solvent (toluene) = 50 mL.
b
a
Turnover frequency, determined based on GC, reaction time = 10 min.
Reaction time = 20 min.
Reaction conditions: sub/cat. = 4000, P/Rh = 6.0, temp. = 80 °C, syngas pressure
(1:1) = 3.0 MPa, solvent (toluene) = 50 mL, and reaction time = 10 min.
c

618
A.A. Dabbawala et al. / Catalysis Communications 11 (2010) 616–619
Table 4
Thus, (p-MeO-C6H4)3P gave low TOF while P(ONp)3 and (p-CF3-
C6H4)3P gave very high TOF whereas P(OPh)3 exhibiting TOF in be-
tween. Indeed, an excellent regioselectivity to branched aldehyde
with high TOF was obtained only with P(ONp)3 (Table 4). The ob-
served TOF is at least 3.6 times higher than those observed for
the Rh/PPh3 catalyst. The bulky phosphite ligands are known to en-
hance the rate of the hydroformylation reaction [19–23]. The high
activity of P(ONp)3 is explained by the formation of monoligated
rhodium phosphite complex, HRh(CO)3L [23]. P(ONp)3 is bulkier
and sterically hindered ligand as compare to PPh3 (cone angle of
P(ONp)3 is 166° while for PPh3 is 145°) and it gives an active spe-
cies containing only one bulkier phosphite ligand around rhodium
resulting in the formation of HRh(CO)3L, (L = tri-1-naph-
thylphosphite) intermediate species, confirmed by in situ IR and
NMR [17]. This complex easily loses CO which facilitates coordina-
tion of an alkene molecule.
Effect of various ligands on the hydroformylation of vinyl acetate.a
Entry
Ligand
TOFb (hÀ1
)
Saldehyde (%)
b/l
95/5
1
2
3
–
PPh3
PCy3
PCy3
dppp
dppb
(p-MeO-C6H4)3P
P(OPh)3
(p-CF3-C6H4)3P
P(ONp)3
P(ONp)3
480
2240
589
1680
2120
2600
600
4800
8000
8280
10,600
89.0
92.0
83.0
84.0
92.0
93.0
78.0
93.0
87.0
93.0
91.0
93/7
99/1
99/1
98/2
98/2
99/1
92/8
88/12
99/1
99/1
4c
5d
6d
7
8
9
10
11c
a
Reaction conditions: sub/cat. = 4000, [Rh(CO)2acac] = 0.23 mmol/L, P/Rh = 6.0,
temp. = 90 °C, syngas pressure (1:1) = 4.0 MPa, and solvent (toluene) = 50 mL.
b
Turnover frequency, determined based on 30–35% GC conversion.
Temp. = 100 °C.
For bidentate ligand P/Rh = 3.0.
c
d
It is well known that during the hydroformylation of vinyl ace-
tate, the side product, acetic acid reduces the concentration of the
hydridic form of the catalyst by forming the carboxylate rhodium
complex [12]. The coordination of the ester carbonyl group of vinyl
acetate also influences the stability of the transient intermediate
(Scheme 2) and slowdown the carbon monoxide insertion. As a re-
sult the hydroformylation activity of vinyl acetate using rhodium
complex modified by phosphite is lower than that of the hydrofor-
mylation of 1-hexene [17]. However, hydroformylation activity of
vinyl acetate using the bulky phosphite ligand is significantly
attractive as compare to the conventional ligands. Moreover, there
is a possibility of decomposition of phosphite in presence of acetic
acid formed during hydroformylation. In order to check phosphite
stability, 31P NMR spectra of the reaction mixture (sub/cat. = 4000,
[Rh(CO)2acac] = 0.23 mmol/L, P/Rh = 6.0, temp. = 90 °C, syngas
pressure (1:1) = 4.0 MPa, toluene = 50 mL) was recorded after the
completion of the reaction. No decomposition of the phosphite
was observed. In order to ascertain the stability of phosphite in
the presence of added acetic acid, the 31P and 1H NMR of the reac-
tion mixture containing phosphite ligand (0.7 mmol) and acetic
the identical reaction conditions. It was found that at optimum
condition (temp. 90 °C, syngas pressure 4.0 MPa), most of ligands
gave good chemo and regioselectivity but insufficient activity (Ta-
ble 4). With PPh3 ligand, TOF was 2240 hÀ1 and moderate selectiv-
ity (chemo as well as regioselectivity) was achieved at 90 °C and
4.0 MPa syngas pressure. Results clearly indicated that steric nat-
ure of ligands remarkably influences the regioselectivity to
branched aldehyde while electronic nature of ligands greatly influ-
ences catalytic activity. Thus, as shown in entry 4, ligand PCy3, re-
sulted in low activity and low chemo selectivity toward aldehyde.
However, it gave high regioselectivity toward branched aldehyde
(99%). The diphosphines ligand (dppp and dppb) exhibited high
chemo selectivity and favored regioselective formation of
branched aldehyde although TOF was relatively low for the both
diphosphines under identical experimental conditions. Ligands
having more
r-donor ability slow down the reaction considerably
while ligands having
p-acceptor ability enhances the reaction rate.
H
OC
Rh
L
L= P(ONp)3
OC
CO
O
OAc
-CO
OAc
OC
H
H
O
OAc
OC Rh
CO
Rh
L
L
Rh
OC
OC Rh
CO
L
L
CO
CO
OAc
L
O
OC
Rh
H
OAc
CO
O
O
H
O
L
OAc
OC Rh
CO
Scheme 2. Coordination of the ester carbonyl group of vinyl acetate to the rhodium during hydroformylation.

A.A. Dabbawala et al. / Catalysis Communications 11 (2010) 616–619
619
acid (4.9 mmol) in toluene kept at 90 °C and 4 MPa syngas pressure
was recorded by withdrawing the sample at regular interval. The
decomposition products of phosphite, oxide of phosphite/1-naph-
thol were not observed till 3 h (carefully checked by 31P and 1H
NMR) indicating that no decomposition of phosphite take place
within 3 h. The 31P and 1H NMR spectra recoded after 17 h showed
20–25% decomposition of phosphite (indicated by the formation of
oxide of phosphite/1-naphthol in 31P and 1H NMR spectra). As
phosphite system exhibits high reaction rate and hydroformylation
reaction is completed within short time, the influence of side prod-
ucts which may effect the catalyst activity will be less or trivial;
and generally in hydroformylation excess ligand is used.
Network Project (NWP 010). AAD thanks CSIR, New Delhi, for the
award of Senior Research Fellowship.
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Acknowledgements
Authors are thankful to analytical division of the institute for
assistance with analyses and CSIR for financial support under CSIR