Iron Catalyzed Hydroformylation of Alkenes under Mild Conditions: Evidence of an Fe(II) Catalyzed Process

Article abstract of DOI:10.1021/jacs.8b01286

Earth abundant, first row transition metals offer a cheap and sustainable alternative to the rare and precious metals. However, utilization of first row metals in catalysis requires harsh reaction conditions, suffers from limited activity, and fails to tolerate functional groups. Reported here is a highly efficient iron catalyzed hydroformylation of alkenes under mild conditions. This protocol operates at 10-30 bar syngas pressure below 100 °C, utilizes readily available ligands, and applies to an array of olefins. Thus, the iron precursor [HFe(CO)₄]^-[Ph₃PNPPh₃]⁺ (1) in the presence of triphenyl phosphine catalyzes the hydroformylation of 1-hexene (S2), 1-octene (S1), 1-decene (S3), 1-dodecene (S4), 1-octadecene (S5), trimethoxy(vinyl)silane (S6), trimethyl(vinyl)silane (S7), cardanol (S8), 2,3-dihydrofuran (S9), allyl malonic acid (S10), styrene (S11), 4-methylstyrene (S12), 4-iBu-styrene (S13), 4-tBu-styrene (S14), 4-methoxy styrene (S15), 4-acetoxy styrene (S16), 4-bromo styrene (S17), 4-chloro styrene (S18), 4-vinylbenzonitrile (S19), 4-vinylbenzoic acid (S20), and allyl benzene (S21) to corresponding aldehydes in good to excellent yields. Both electron donating and electron withdrawing substituents could be tolerated and excellent conversions were obtained for S11-S20. Remarkably, the addition of 1 mol % acetic acid promotes the reaction to completion within 16-24 h. Detailed mechanistic investigations revealed in situ formation of an iron-dihydride complex [H₂Fe(CO)₂(PPh₃)₂] (A) as an active catalytic species. This finding was further supported by cyclic voltammetry investigations and intermediacy of an Fe(0)-Fe(II) species was established. Combined experimental and computational investigations support the existence of an iron-dihydride as the catalyst resting state, which then follows a Fe(II) based catalytic cycle to produce aldehyde.

Full text of DOI:10.1021/jacs.8b01286

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Iron Catalyzed Hydroformylation of Alkenes under Mild
Conditions: Evidence of an Fe(II) Catalyzed Process
Swechchha Pandey, K. Vipin Raj, Dinesh R. Shinde, Kumar Vanka, Varchaswal
Kashyap, Sreekumar Kurungot, Chathakudath Prabhakaran Vinod, and Samir H. Chikkali
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01286 • Publication Date (Web): 11 Mar 2018
Downloaded from http://pubs.acs.org on March 11, 2018
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Iron Catalyzed Hydroformylation of Alkenes under Mild Condiꢀ
tions: Evidence of an Fe(II) Catalyzed Process
Swechchha Pandey,a K. Vipin Raj,b Dinesh R. Shinde,c Kumar Vanka,b Varchaswal Kashyap, b Sreekuꢀ
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mar Kurungot, b C. P. Vinod,d Samir H. Chikkalia,e*
aPolymer Science and Engineering Division, CSIRꢀNational Chemical Laboratory, Dr. Homi Bhabha Road, Puneꢀ411008,
India.
bPhysical and Materials Chemistry Division, CSIRꢀNational Chemical Laboratory, Dr. Homi Bhabha Road, Puneꢀ411008,
India.
cCentral NMR Facility, CSIRꢀNational Chemical Laboratory, Dr. Homi Bhabha Road, Puneꢀ411008, India.
dCatalysis Division, CSIRꢀNational Chemical Laboratory, Dr. Homi Bhabha Road, Puneꢀ411008, India.
eAcademy of Scientific and Innovative Research (AcSIR), Anusandhan Bhawan, 2 Rafi Marg, New Delhi 110001, India
ABSTRACT: Earth abundant, first row transition metals offer a cheap and sustainable alternative to the rare and precious metals.
However, utilization of first row metals in catalysis requires harsh reaction conditions, suffers from limited activity, and fails to
tolerate functional groups. Reported here is a highly efficient iron catalyzed hydroformylation of alkenes under mild conditions.
This protocol operates at 10ꢀ30 bars syngas pressure below 100 °C, utilizes readily available ligands and applies to an array of oleꢀ
fins. Thus, the iron precursor [HFe(CO)4][Ph3PNPPh3] (1) in the presence of triphenyl phosphine catalyzes the hydroformylation of
1ꢀhexene (S2), 1ꢀoctene(S1), 1ꢀdecene (S3), 1ꢀdodecene (S4), 1ꢀoctadecene (S5), trimethoxy(vinyl)silane (S6), trimeꢀ
thyl(vinyl)silane (S7), cardanol (S8), 2,3ꢀdihydrofuran (S9), allyl malonic acid (S10), styrene (S11), 4ꢀmethyl styrene (S12), 4ꢀiBuꢀ
styrene (S13), 4ꢀtBuꢀstyrene (S14), 4ꢀmethoxy styrene (S15), 4ꢀacetoxy styrene (S16), 4ꢀbromo styrene (S17), 4ꢀchloro styrene
(S18), 4ꢀvinylbenzonitrile (S19), 4ꢀvinylbenzoic acid (S20), and allyl benzene (S21) to corresponding aldehydes in good to excelꢀ
lent yields. Both electron donating and electron withdrawing substituents could be tolerated and excellent conversions were obꢀ
tained for S11ꢀS20. Remarkably, the addition of 1 mol% acetic acid promotes the reaction to completion within 16ꢀ24 hours. Deꢀ
tailed mechanistic investigations revealed in-situ formation of an ironꢀdihydride complex [H2Fe(CO)2(PPh3)2] (A) as an active cataꢀ
lytic species. This finding was further supported by cyclic voltammetry investigations and intermediacy of an Fe(0)ꢀFe(II) species
was established. Combined experimental and computational investigations support the existence of an ironꢀdihydride as the catalyst
resting state, which then follows a Fe(II) based catalytic cycle to produce aldehyde.
INTRODUCTION
the industry is increasingly being faced with the rocketing
prices of rhodium due to the high demand of this metal in the
automotive industry, which consumes about 80% of this metal.
In addition, the natural abundance of this trace element is posꢀ
ing an even bigger challenge and the search for alternative
metals has already begun.8
Discovered by German chemist Otto Roelen, transition metꢀ
al catalyzed hydroformylation (the “oxo” process) is arguably
the world’s largest homogeneously catalyzed industrial proꢀ
cess with the production of 12 million ton oxoꢀproducts per
annum.1,2 The oxo process is a powerful synthetic tool to conꢀ
vert alkenes into aldehydes with perfect atom economy. It has
been extensively utilized to construct an array of chemical
intermediates.3 The first and second generation catalysts deꢀ
veloped by BASF and ICI were based on cobalt.4 However,
the cobaltꢀcatalyzed process requires harsh conditions such as
100ꢀ350 bars syngas (1:1 mixture of CO:H2) pressure and
around 100ꢀ200 °C temperature. Widespread academic and
industrial research to address this bottleneck led to a rhodium
catalyzed lowꢀpressure oxoꢀprocess (LPO) (10ꢀ60 bars and 80ꢀ
135 °C),5 which was developed by Union Carbide and Celaꢀ
nese in the midꢀ1970s.6 To date, terminal alkenes, internal
alkenes, cyclic olefins and aromatic alkenes have been extenꢀ
sively hydroformylated to pharmaceuticals, fragrances and
agrochemicals using the precious rhodium metal.7 Thus, due to
technical superiority, the rhodiumꢀbased LPO is still the state
of the art process practiced by industry and roughly 70% of
the oxoꢀproducts are produced using this process. However,
Iron (Fe) is an earth abundant element in contrast to preꢀ
cious rhodium, and its usage is justified for reasons of ecoꢀ
nomic and sustainability. Iron catalyzed hydroformylation of
olefins has been reported on few occasions in the past.8 Figure
1 depicts the state of the art in iron catalyzed hydroformylaꢀ
tion. The first example of iron catalyzed hydroformylation was
reported by Reppe and Vetter. At 14% loading of [Fe(CO)5],
ethylene was converted to propanol under 100ꢀ200 bars of CO
pressure.9 Note that no syngas was employed but the water gas
shift reaction was anticipated to deliver the hydrogen. Similar
attempts using 100 bar CO pressure (at 140 °C) were reported
by Palagyi et al. after 30 years, but the conversion remained
very low (30%).10 RuꢀFe cluster catalyzed hydroformylation in
the presence of syngas was
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Page 2 of 20
phosphonite,20 phosphineꢀphosphoramidite21 and phosphineꢀ
A) Prior art
B) This work
CHO
CHO
phosphite22 have been extensively utilized in rhodium cataꢀ
1
2
3
4
5
6
7
8
CHO
lyzed hydroformylation. Thus, given the success of the phosꢀ
phorus ligand in hydroformylation, we zeroed in on readily
accessible σꢀdonor ligands such as phosphines and σꢀdonor πꢀ
acceptor ligands such as phosphites.23 The performance of
precursor 1 in the presence of triphenyl phosphine (L1) and
triphenyl phosphite (L2)
[Fe]
[Fe#] + L
R
3
5
OHC
CHO
CHO
CHO
CO/H2
100-200 bar
CO/H2
10-30 bar
Ar
Low pressure
# High pressure
7
Large substrate scope
Fe(II) mechanism
# Limited substrate scope
# No mechanism
13
OHC
OHC
Table 1. Iron (1) catalyzed hydroformylation of 1ꢀoctene unꢀ
der mild conditons.a
Readily available ligands
Excellent activity
# Ignored ligand effect
# Low activity
R1
9
FG
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Figure 1. State of the art iron catalyzed hydroformylation (left)
and present work (right) {[Fe#]
PPh3/P(OPh)3)}.
= (HFe(CO)4¯) (L =
Ru
n
L
Solꢀ
CO/H2
(bars)
Time Conv. L:B b
reported in 1992 for the first time and the TOF of 0.4ꢀ4.0 was
recorded.11 Although it was not clear which metal was responꢀ
sible for the observed hydroformylation activity, a synergistic
effect between the two metals was claimed to be responsible.
The latest report in iron catalyzed hydroformylation was pubꢀ
lished in 2000, utilizing an isolated iron complex [Fe(η6ꢀ
CHT)(η4ꢀCOD)], (CHT: 1,3,5ꢀcycloheptatriene; COD: 1,5ꢀ
cyclooctadiene).12 Iron catalyzed hydroformylation of 1ꢀ
hexene and styrene at 100 °C and 100 bars syngas pressure
was investigated (Fig. 1, left). Thus, the iron catalyzed hydroꢀ
formylation is still in its infancy and suffers from serious limiꢀ
tations, such as high syngas pressure, limited substrate scope,
lack of understanding of ligand effects and low activities. In
addition, no comprehensive picture of the mechanism of Feꢀ
catalyzed hydroformylation exists, beyond the parallels drawn
with Ruꢀcatalyzed hydroformylation.
(equiv.) vent
(h)
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(%)b
47
95
95
92
3
L1 (1)
MeOH 20
73:27
66:34
64:36
64:36
NA
1
L1 (2.5) MeOH 20
2
L1 (3)
L1 (4)
MeOH 20
MeOH 20
3
4
L1 (2.5) THF
L1 (2.5) DXN
L1 (2.5) DCM
L1 (2.5) EtOH
20
20
20
20
5
66
24
17
20
90
76
62
85
3
73:27
63:37
67:33
70:30
60:40
70:30
67:33
67:33
74:26
NA
6
7
8
L1 (2.5) iPrOH 20
L1(2.5) MeOH 30
L1(2.5) MeOH 30
L1(2.5) MeOH 15
L1(2.5) MeOH 20
L1(2.5) MeOH 20
9
10
11
12
13c
14d
15
16
17
18
19
20
21
Herein, we describe iron catalyzed hydroformylation (HF)
of olefins under mild conditions: 10ꢀ30 bars syngas pressure
and below 100 °C, which falls under the purview of LPO. The
generality of the approach has been demonstrated by subjectꢀ
ing various olefins, such as 1ꢀoctene, 1ꢀhexene, 1ꢀdecene, 1ꢀ
dodecene, 1ꢀoctadecene, trimethoxy(vinyl)silane, trimeꢀ
thyl(vinyl)silane, cardanol, 2,3ꢀdihydrofuran, allyl malonic
acid, styrene, 4ꢀmethyl styrene, 4ꢀiBuꢀstyrene, 4ꢀtBuꢀstyrene,
4ꢀmethoxy styrene, 4ꢀacetoxy styrene, 4ꢀbromo styrene, 4ꢀ
chloro styrene, 4ꢀvinylbenzonitrile, 4ꢀvinylbenzoic acid, and
allyl benzene to iron catalyzed hydroformylation. A Fe(II)
based mechanism is proposed as predicated by DFT calculaꢀ
tions and experimental evidence. This methodology relies on
commonly available reagents, does not require harsh condiꢀ
tions, and uses an earth abundant, nonꢀtoxic and cheap metal,
which makes this approach highly suitable for practical hydroꢀ
formylation of industrially important alkenes.
NA
MeOH 35
MeOH 20
0
L2(1)
18
47
27
92
5
68:32
70:30
65:35
63:37
76:24
NA
L2(2.5) MeOH 20
L2(3) MeOH 20
L2(2.5) MeOH 30
L2(2.5) MeOH 30
L2(2.5) MeOH 20
2
22e L2(2.5) MeOH 20
23
68:32
aConditions: 1: 0.0077 mmol, L/M: 2.5, Sub/Fe: 100, Solvent:
1
ml, NA: Not applicable; MeOHꢀMethanol, THFꢀ
Tetrahydrofuran, DXNꢀ1,4ꢀdioxane, DCMꢀDichloromethane,
EtOHꢀEthanol, iPrOHꢀIsopropanol, hardly any (~1%) hydrogenaꢀ
tion product was detected. bDetermined by GC. cPerformed at 120
°C. Performed at 80 °C. L2 was incubated for 24 hours before
addition of 1ꢀoctene.
RESULTS AND DISCUSSION
Hydroformylation of 1ꢀoctene. It is known that the hydroꢀ
formylation reaction proceeds via a metalꢀhydride intermediꢀ
ate.13 In our attempts to meet this criteria, we synthesized an
ironꢀhydride complex [HFe(CO)4][PPN] (1) (where PPN =
Bis(triphenylphosphine)iminium) by following a known proꢀ
cedure.14 The identity of (1) was fully established by using a
combination of spectroscopic and analytical methods. An
overview of iron catalyzed reactions reported in the literature
indicated that iron complexes can be activated in the presence
of suitable ligands.15 Guided by these reports, we anticipated
that the best way to manipulate the reactivity of 1 would be to
offer a competitive ligand to replace carbonyls and activate 1
in situ. Phosphorus ligands such as phosphines,16 phosphites,17
phosphinites,18 diphosphines,19 diphosphites,1f phosphineꢀ
d
e
in the iron catalyzed hydroformylation of 1ꢀoctene was evaluꢀ
ated and the representative catalytic data is summarized in
Table 1. The catalysts were prepared in situ by mixing a suitaꢀ
ble amount of the iron precursor 1 and phosphorus ligands L1
Table 2. Iron (1) catalyzed hydroformylation of alkenes in the
presence of L1. S1: 1ꢀoctene, S2: 1ꢀhexene, S3: 1ꢀdecene, S4:
1ꢀdodecene, S5: 1ꢀoctadecene, S6: trimethoxy(vinyl)silane,
S7: trimethyl(vinyl)silane, S8: cardanol, S9: 2,3ꢀdihydrofuran,
S10: allyl malonic acid, S11: styrene, S12: 4ꢀmethyl styrene,
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Journal of the American Chemical Society
without any hydrogenation side reaction. In our attempts to
S13: 4ꢀiBuꢀstyrene, S14: 4ꢀtBuꢀstyrene, S15: 4ꢀmethoxy styꢀ
1
2
3
4
5
6
7
8
rene, S16: 4ꢀacetoxy styrene, S17: 4ꢀbromo styrene, S18: 4ꢀ
chloro styrene, S19: 4ꢀvinylbenzonitrile, S20: 4ꢀvinylbenzoic
acid, S21: allyl benzene.a
identify the most suitable solvent, various solvents were
screened (Table 1, run 5ꢀ9). Methanol was found to be the
solvent of choice and none of the other solvents were as effecꢀ
tive as methanol. Performing the hydroformylation at lower
and higher syngas pressure indicated an optimal pressure of 20
bars (run 2 vs 10ꢀ12) with 95% conversion within 24 hours,
without jeopardizing the regioselectivity. Increasing the temꢀ
perature to 120 °C led to slightly lower conversion, but deꢀ
creasing the temperature to 80 °C dramatically reduced the
conversion to only 3% (Table 1 runs 13ꢀ14). In a control exꢀ
periment, hydroformylation of 1ꢀoctene using precursor (1), in
the absence of ligand, failed to produce the corresponding
nonanal (Table 1, run 15). This observation clearly indicated
that precursor 1 on its own is not capable of interacting with
alkenes and may not be the actual active species. Thus, the
control experiment accentuates the pivotal role of ligand in
iron catalyzed hydroformylation, without which the iron hyꢀ
dride complex 1 is not active enough. Encouraged by these
results, we evaluated the performance of a readily available, σꢀ
donor πꢀacceptor ligand, triphenyl phosphite (L2).25 Initial
screening in the presence of ligand L2 indicated an optimal
ligand to metal ratio of 2.5 (Table 1, run 16ꢀ18). However, it
should be noted that a longer reaction time was required to
achieve reasonable conversion under identical conditions (Taꢀ
ble 1, run 17).24 Notably, increasing the syngas pressure to 30
bars revealed an improved yield of 92% in 48 hours (Table 1,
run 19). However, performing the reaction at the same syngas
pressure as in run 19 but for a shorter period of time (Table 1,
run 20), lead to only 5% conversion. This anomalous behavꢀ
iour could be due to the weak σꢀdonation and lower coordinatꢀ
ing ability of L2. At a higher ligand to metal ratio, the phosꢀ
phite ligands are known to be slower, leading to longer reacꢀ
tion times, and the behaviour noted here is in line with the
previous reports.26 Incubating L2 for 24 hours and in situ addiꢀ
tion of substrate revealed slight improvement in the activity
(Table 1, run 22).
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Scope of iron catalyzed hydroformylation. With optiꢀ
mized reaction conditions in hand, the scope of the iron cataꢀ
lyzed hydroformylation was examined (Table 2) and about 20
substrates were evaluated. Both aliphatic and aromatic subꢀ
strates were hydroformylated with good to excellent converꢀ
sion to aldehydes. The aromatic substrates exhibited slightly
lower reactivity. A short chain alkene, 1ꢀhexene, was hydroꢀ
formylated under further milder conditions with 50% excluꢀ
sive conversion to heptanal (Table 2, P2) along with 72% lineꢀ
ar selectivity. Hydroformylation of longꢀchain alkenes is even
more challenging, as their reactivity decreases with increasing
carbon number and the possibility of internal isomers and corꢀ
responding aldehyde products increases.27 With increasing
chain length of the olefin, the reactivity was found to deꢀ
crease.28 Thus, at 15 bars syngas pressure and 100 °C, a C10
olefin S3 led to only 47% yield, whereas increasing the CO/H2
pressure to 30 bars led to an improved yield of 97% (Table S3,
run 2 vs 3).29 Along the same lines, 1ꢀdodecene (S4) and 1ꢀ
octadecene (S5) displayed 97% and 87% yield respectively
under identical conditions (Table S3, run 4ꢀ5).30,31
aConditions: 1: 0.0077 mmol, L/M: 2.5, Sub/Fe: 100, Solvent:
1 ml methanol, L:B = Linear:Branched, Yield (in bracket, %)
determined by GC/1H NMR spectroscopy, hardly any (~1%) hyꢀ
drogenation product was detected.
With this initial success, the resilience of the catalyst was
examined by subjecting functional olefins to iron catalyzed
hydroformylation. The catalyst was found to tolerate triꢀ
methoxy group without any hindrance and 75% conversion to
aldehyde was observed (Table 2, P6). A slight change in the
silane to trimethyl(vinyl)silane led to 49% conversion to aldeꢀ
or L2 in the presence of syngas. Preliminary screening indiꢀ
cated an optimal ligand to metal ratio of 2.5 (Table 1, run 1ꢀ
4).24 Remarkably, addition of triphenyl phosphine to iron preꢀ
cursor 1 catalyzed the hydroformylation of 1ꢀoctene to nonaꢀ
nal with excellent conversion (90ꢀ95%) (Table 1, run 2ꢀ4),
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Journal of the American Chemical Society
hyde. A notoriously difficult cardanol (S9), which is a nonꢀ
Page 4 of 20
formylation of allyl aromatics.37 The resultant aldehydes are
high value pharamaceutical intermediates.38 As a representaꢀ
tive case, iron catalyzed hydroformylation of allyl benzene
(S21) was investigated. Precursor 1 in the presence of L1 cataꢀ
lyzed the hydroformylation of allylbenzene to yield the linear
selective (64%) product (P21) with a moderate conversion of
22% (Table S3, run 25).
1
2
3
4
5
6
7
8
edible plant oil derived substrate, was tested in the iron cataꢀ
lyzed hydroformylation. Although only 11% aldehyde product
could be observed, the fact that such a mixture (cardanol is
mixture of three different internal olefins) could be hydroꢀ
formylated indicates the potential that the iron catalyst holds.
A highly challenging heterocyclic olefin, 2,3ꢀdihydrofuran
(S9), was hydroformylated to yield (62%) a highly regioseꢀ
letive 3ꢀcarbaldehyde with 97% selectivity (Table 2, P9). Hyꢀ
droformylation of 1,1ꢀdisubstituted difunctional olefin S10
(allyl malonic acid) lead to reduced activity and only 10%
aldehyde could be observed, clearly indicating the limited
functional group tolerance of the current catalytic system. On
an average, aliphatic olefins were hydroformylated in 24
hours, whereas aromatic substrates required 48 or more hours.
Styrene was chosen as a representative benchmark substrate
and iron catalyzed hydroformylation was examined.32 Under
Acetic acid promoted iron catalyzed hydroformylation.
Having established iron catalyzed hydroformylation of various
olefins, we pondered about the role of the solvent. It is to be
noted that hydroformylation was found to take place in alcoꢀ
holic solvents, suggesting active participation of the solvent.
In this context, we postulated that the alcoholic solvents might
be delivering a proton to precursor 1, to generate the active
species. To test our hypothesis, we investigated the effect of
acidic additives on the hydroformylation activity. To our deꢀ
light, the addition of 1 mol% (AcOH:[Fe] = 1:1) acetic acid
was found to dramatically promote the hydroformylation reacꢀ
tion.39 Initial additive screening suggested that 1 equivalent (as
compared to iron precursor 1) of acetic acid is sufficient to
promote the reaction (Table 3, run 1ꢀ3). Under optimized conꢀ
ditions, hydroformylation of S2 led to 49% conversion within
16 hours, which otherwise required 48 hours for similar conꢀ
version without the additive (Table 3, run 1 versus Table S3
run 1). Remarkably, six fold increased activity was observed
in the hydroformylation of styrene, which was completed
(94% conversion) within 24 hours (Table 3, run 4), instead of
the earlier 16% conversion (Table S3, run 11). Along the same
lines, accelerated hydroformylation of S12, S15 and S17 was
observed in the presence of acetic acid. Table 3 lists the imꢀ
portant experiments. Thus, addition of 1 mol% (1 equivalent
compared to iron precursor 1) acetic acid promotes the hydroꢀ
formylation of 1ꢀhexene, styrene and styrene derivatives and
accelerated conversion could be obtained within 16ꢀ24 hours.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
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44
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46
47
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59
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Table 3. Acetic acid promoted iron (1) catalyzed hydroꢀ
formylation of 1ꢀhexene, styrene and styrene derivatives.a
Run Subꢀ
strate
AcOH Time Conv.
L:B b
(h)
16
16
16
24
24
24
24
(%)b
49
25
1
1c
2c
3c
4
S2
1
2
5
1
1
1
1
72:28
73:27
NA
S2
S2
S11
S12
S15
S17
94
32
64
80
14:76
16:84
16:84
8:92
5
6
7
aConditions: 1: 0.077 mmol, L/M: 2.5, Sub/Fe: 100, Solvent: 1
ml methanol, CO/H2: 20 bars, Temp.: 100 °C, S2: 1ꢀhexene, S11:
styrene, S12: 4ꢀmethyl styrene, S15: 4ꢀmethoxy styrene, S17: 4ꢀ
b
c
bromo styrene, NA: Not applicable; Determined by GC. Temp.:
80 °C.
Mechanistic investigations.
NMR spectroscopy. Unfolding the elementary steps in the
iron catalyzed hydroformylation will be of great significance
for understanding the reactivity of the iron catalyst and might
unlock the synthetic potential of this earth abundant metal in
hydroformylation. Primitive reports on iron catalyzed HF eiꢀ
ther refer to the ruthenium based mechanism40 or cite the
Reppe process, which proposes the CO deficient [H2Fe(CO)3]
as an active intermediate.41 However, direct experimental or
theoretical evidence for iron catalyzed HF is largely missing.
optimized conditions, a quantitative conversion was observed
at 20 bars syngas pressure at 100 °C, with the preferred
branched aldehyde formed with 92% selectivity (Table 2,
P11). The reversal of regioselectivity is very commonly obꢀ
served in styrenic substrates and monodentate phosphine ligꢀ
ands are known to preferably deliver the branched product.33,34
Both electron donating and electron withdrawing substituents
were tolerated (Table 2, P12ꢀ20). The electron donating subꢀ
strates 4ꢀmethyl styrene (S12),35,36 4ꢀtertbutyl styrene (S14)
demanded 30 bars syngas pressure for 45ꢀ50% conversion. To
demonstrate the practical significance of this methodology,
hydroformylation of S13 was performed to yield aldehyde P13
which can be eventually oxidized to yield ibuprofen (Table 2),
an antiꢀinflammatory drug. Whereas the electron withdrawing
substituents fared better and 4ꢀbromo styrene (S17) led to full
conversion at 20 bars syngas pressure (Table S3, runs 21 vs
15). S17 also revealed a high regioselectivity of 96% branched
aldehyde (P17) with an excellent yield of 97%. Acetoxy, niꢀ
trile and carboxyl groups could be tolerated but at the cost of
slightly reduced conversion to aldehyde (Table S3, run 20, 23ꢀ
24).
In our attempts to trap key intermediates, precursor 1 was
treated with triphenyl phosphine at 45 °C and the progress of
the reaction was monitored by phosphorus NMR spectroscopy.
The 31P resonances at 82.3 and 71.5 ppm indicated coordinaꢀ
tion of L1 to the metal and the presence of intermediate (i)
(Scheme1) (SI Fig. S27). The above phosphorus chemical
shifts fall within the range of mononuclear ironꢀphosphine
complexes reported earlier. Beller and coꢀworkers in their
investigation on iron catalyzed hydrogen production using
formic acid reported that the coordinated phosphorus in
[Fe(CO)3(PPh3)2] complex appear at 82.5 and 70.6 ppm.42
Therefore, the resonance at 82.3 and 71.5 ppm observed in our
investigation can be assigned to coordinated L1. It should be
noted that the phosphine coordination was observed at an eleꢀ
vated temperature of 45 °C. In an ideal situation, addition of
acetic acid at 45 °C would lead to the generation of intermediꢀ
ate A. However, addition of acetic acid to the above NMR
Thus, the above observations indicate that electron poor styꢀ
rene derivatives are relatively easy to hydroformylate, whereas
electron rich styrenics are slightly difficult to access. While a
significant amount of literature deals with the hydroformylaꢀ
tion of vinyl aromatics, very little is known about the hydroꢀ
ACS Paragon Plus Environment

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Journal of the American Chemical Society
by following this route.44 In a third protocol, formation of A
tube and recording proton NMR did not show any hydride
1
2
3
4
5
6
7
8
resonance. This is most likely due to release of H2
was accessed by synthesizing the known ironꢀphosphine comꢀ
plex (iii). Isolated complex (iii) was treated with hydrogen gas
in a high pressure NMR tube and the tube was heated to 60 °C
for 16 hours. The progress of the reaction was monitored by
proton and phosphorus NMR spectroscopy. 31P NMR of the
resultant mixture revealed a resonance at 83.1 ppm (SI Fig.
S36) and the corresponding proton NMR displayed two broad
singlets at ꢀ8.91 and ꢀ9.02 ppm (SI, Fig. S35). These resoꢀ
nances can be tentatively assigned to an iron complex that
would be similar to species A; perhaps with a different spatial
arrangement of the two hydrides. Thus, the above spectroscopꢀ
ic investigations suggest formation of species A, which might
be the potential active species for hydroformylation. The inꢀ
termediate thus prepared is highly unstable and only in situ
characterization has been attempted.
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60
Cyclic voltammetry. The interconversion of Fe(0) to Fe(II)
species was further supported by cyclic voltammetry (CV)
investigations. The cyclic voltamogram (CVs) of the iron preꢀ
cursor (1) was recorded before and after the addition of acetic
acid in the electrolyte medium (Fig. 2).45 Initial reduction poꢀ
tential for the compound (1) was found to be 0.43 V (vs Pt),
which was shifted to 0.33 V (vs Pt) after acetic acid addition.
Similarly, a shift in the oxidation peak of the precursor 1 was
noted from 0.55 to 0.40 V(vs Pt). The above shift in the peak
0.06
0.05
0.04
0.03
0.02
0.01
0.00
-0.01
Scheme 1. Proposed catalytic cycle for iron catalyzed hydroꢀ
formylation. The orientation of ligands around the metal is only
for the sake of understanding and does not mean that this is the
final spatial arrangement of the ligands.
-0.02
Compound (1)
Compound (1)+Acetic acid
from intermediate (A) at elevated temperature. To arrest the
hydrogen release, the NMR tube with added L1 was first heatꢀ
ed for 16 hours at 45 °C, and then the tube was cooled to 0 °C.
At this temperature, acetic acid was added to the NMR tube
and a proton and 31P NMR was recorded. Thus, the addition of
acetic acid led to the appearance of a very weak hydride signal
at ꢀ12.28 ppm (SI Fig. S30), but the intensity of the signal was
so weak (even after large number of scans) that it demanded
further support. In the hope of capturing intermediate A, comꢀ
pound 1 was treated with triphenyl phosphine and acetic acid
was added at room temperature without heating the reaction
mixture. Immediately a proton NMR was recorded, which
revealed a doublet centered at ꢀ9.50 ppm (SI Fig. S33). This
chemical shift can be assigned to complex A, which is conꢀ
sistent with literature reports.43 Similar results were obtained
when deuterated acetic acid (CD3COOD) was used (SI Fig.
S33R). This is most likely due to fast HꢀD exchange between
added deuterated acetic acid and the protic solvent (methanol).
In a second route to trap intermediate species A (scheme 1), 1
was treated with acetic acid to reveal a hydride resonance at ꢀ
15.3 ppm (SI Fig. S34). Observation of the hydride resonance
confirmed the formation of species (ii). However, addition of
L1 did not show any coordination at room temperature and
heating the sample to obtain the desired coordination led to
elimination of H2. Therefore, species A could not be generated
-0.03
-0.04
0.0
0.2
0.4
0.6
0.8
1.0
Poential(V vs Pt)
Figure 2. Cyclic voltamograms of iron precursor (1) (black line)
and (1) + acetic acid (red line), recorded at 50mV/s with an elecꢀ
trode rotation rate of 900 rpm in 0.1 M LiClO4 (solution in methꢀ
anol
potential clearly indicates that the initial Fe(0) species in preꢀ
cursor 1 is being converted to the ironꢀdihydride species after
the addition of acetic acid. To verify these results further, conꢀ
trol experiments were conducted under analogous conditions.
Ferrocene, ferrocene carboxylic acid, [Fe(CO)3(PPh3)2] and
[Fe(CO)5] were selected as control samples. As can be seen in
Figure S41, the ferrocene and ferrocene carboxylic acid,
wherein iron is in the +2 oxidation state, displayed separate
peak positions for oxidation and reduction. The shift in the
peak position is related to the carboxylic group attached to the
cyclopentadienyl ring.
Further, we recorded the CVs of the iron(0) compounds
such as [Fe(CO)3(PPh3)2] and [Fe(CO)5]. The CV of
[Fe(CO)3(PPh3)2] revealed an oxidation and reduction peak at
higher potential against the iron(+2) ferrocene and ferrocene
carboxylic acid, whereas no peak for [Fe(CO)5] could be idenꢀ
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Journal of the American Chemical Society
Page 6 of 20
tified in the analogous potential range (SI Fig. S42). The dehyde 7b. The energy barriers for this particular step (inserꢀ
1
2
3
4
5
6
7
8
anomalous behavior of the [Fe(CO)5] could be because of the
homogenous ligand surrounding the metal. As evident from
the control experiments outlined above, it is most likely that
the Fe(0) precursor 1 is converted to Fe(II) after the addition
of acetic acid. The reduction peak of the acetic acid treated
compound 1 was in close resemblance with Ferrocene (0.26 V
vs Pt), i.e. iron(2+). Whereas precursor 1 (0.43 V vs Pt) withꢀ
out acetic acid treatment resembles [Fe(CO)3(PPh3)2] (0.46 V
vs Pt) , i.e. Fe(0).
tion or hydride transfer) suggested that the 1,2ꢀinsertion is
more favorable (by 1.3 kcal/mol) (Fig. 4, bottom) for 1ꢀ
hexene. However, an opposite trend is seen in the case of styꢀ
rene, where the 2,1ꢀinsertion is favored (by 1.7 kcal/mol) (Fig.
4, bottom). These computational findings complement
PPh3
H
R
CHO
CHO
7a
OC
OC
7b
R
Fe
H
PPh3
A
H2
H2
9
G Sty:-12.0
G Hex:-14.0
G Sty:-14.2
G Hex:-15.2
Computational and additional experimental evidence.
The existence of A was investigated by computational methꢀ
ods and complex A was found to be the lowest energy isomer
(SI, Table S8) among the six possible geometrical isomers.46
Based on the computational investigations, it is proposed that
the in situ generated active species A forms a πꢀbond with the
olefin to yield species B (Scheme 1), which inserts into the Feꢀ
H bond to give the metalꢀalkyl complex C. The next step is the
formation of the acyl intermediate D. Upon addition of H2, D
releases the aldehyde and regenerates the active species A. In
our attempts to trap the acyl intermediate, a reaction between 1
and 1ꢀhexene was conducted at 0 °C to obtain the acyl interꢀ
mediate D. The acyl complex D was then treated with iodine
in methanol to obtain methyl heptanoate, which was characterꢀ
ized by NMR (SI, Fig. S37), ESIꢀMS (SI, Fig. S38) and GCꢀ
MS (SI, Fig. S39). The observation of methyl heptanoate conꢀ
firms the intermediacy of species D. Similar evidence was
presented for the cobalt catalyzed hydroformylation mechaꢀ
nism by Heck and Breslow.47 Thus, formation and intermediaꢀ
cy of A has been demonstrated by various experimental methꢀ
ods and it is therefore reasonable to believe that complex A is
the actual active species. The observation of the methyl hepꢀ
tanoate further supports the proposed mechanism.
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PPh3
PPh3
OC
CO
OC
CO
Fe
R
Fe
H
H
R
PPh3
PPh3
6b
O
O
6a
R
G Sty:17.4
G Hex:16.5
G Sty:-9.7
CO
G Sty:-18.9
G Hex:-20.5
CO
G Hex:-11.9
PPh3
CO
PPh3
H
OC
OC
OC
R
Fe
Fe
H
R
PPh3
Path A
PPh3
Path B
5b
5a
PPh3
G Sty:-10.9
G Hex:-10.4
OC
OC
H
G Hex:-2.5
G Sty:-3.3
Fe
H
R
B
Figure 3. The reaction mechanism for the hydroformylation
of olefins calculated at the PBE/TZVP level of theory. ∆G (in
kcal/mol) represents Gibbs free energy of reaction.
1-Hexene: Path A
1-Hexene: Path B
15.8
PPh3
PPh3
14.5
OC
OC
H
H
OC
H
CO
H
Fe
Fe
TSpath_A
TSpath_B
C4H9
A radical mechanism with a potential radical species
[•Fe(CO)4¯] has also been evoked in the past.9,48 However, the
existence of the radical mechanism seems unlikely based on
three experimental pieces of evidence. (a) Two control experꢀ
iments were performed in the presence of excess (150 times)
C4H9
0.0
0.0
PPh3
PPh3
radical
scavengers
2,2,6,6ꢀTeramethylpiperidinyloxy
OC
OC
H
H
OC
OC
H
H
-4.4
Fe
(TEMPO) and galvinoxyl (SI, section 6). In both these cases,
the aldehyde product was obtained. If a radical mechanism had
been operating, the radical would have been scavenged by
TEMPO or galvinoxyl and there would not have been any
aldehyde formation. Thus, the formation of aldehyde in the
presence of the radical scavenger rules out the possibility of a
radical mechanism. (b) In situ NMR investigations could be
performed without any paramagnetic NMR resonances. The
absence of a paramagnetic species further supports the absence
of a radical mechanism. (c) CV experiments indicated two
electron processes and, therefore, intermediacy of one electron
transformations is unlikely.
Fe
-6.3
PPh3
OC
CO
PPh3
41-hexene
Fe
C4H9
41-hexene
C4H9
OC
OC
H
H
H
Fe
H
Intpath_B
C4H9
Intpath_A
PPh3
C4H9
14.2
PPh3
CO
Styrene: Path A
Styrene: Path B
OC
OC
OC
H
12.5
Fe
Fe
H
H
H
TSpath_A
TSpath_B
Ph
Ph
To obtain detailed insight into the iron catalyzed hydroꢀ
formylation reaction, full quantum chemical calculations have
been performed with density functional theory (DFT) at the
PBE/TZVP level of theory. It is well known that an octahedral
complex with a general formula MA2B2C2 has six different
stereoisomers,49 and from the calculations, it is found that A is
the most favorable stereoisomer for [Fe(CO)2(H)2(PPh3)2]
(Fig. 3). The insertion of olefin from the πꢀcomplex intermediꢀ
ate B can follow two pathways: 1,2ꢀinsertion (Path A) or 2,1ꢀ
insertion (Path B) (Fig. 3). The former would lead to the linear
aldehyde 7a, while the latter would produce the branched alꢀ
0.0
0.0
PPh3
PPh3
-3.5
PPh3
Fe
OC
OC
H
H
OC
OC
H
H
-3.8
Fe
Fe
PPh3
OC
OC
H
OC
H
CO
Fe
H
Ph
H
4styrene
Ph
4styrene
Intpath_A
Ph
Intpath_B
Ph
Figure 4. Free energy profile for the hydride transfer step in path
A and path B. The free energy values are in kcal/mol.
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Journal of the American Chemical Society
the experimental observations, wherein higher amount of
gas, generates the catalytically active species (which is beꢀ
lieved to be A) and delivers hydroformylation of 1ꢀhexene, 1ꢀ
octene, 1ꢀdecene, 1ꢀdodecene, 1ꢀoctadecene, triꢀ
1
2
3
4
5
6
7
8
branched product was observed for styrene (Table 2, run 12)
and the linear product was seen to be the major one for 1ꢀ
hexene (Table S3, run 1).
methoxy(vinyl)silane, trimethyl(vinyl)silane, cardanol, 2,3ꢀ
dihydrofuran, allyl malonic acid, styrene, 4ꢀmethyl styrene, 4ꢀ
iBuꢀstyrene, 4ꢀtBuꢀstyrene, 4ꢀmethoxy styrene, 4ꢀacetoxy styꢀ
rene, 4ꢀbromo styrene, 4ꢀchloro styrene, 4ꢀvinylbenzonitrile,
4ꢀvinylbenzoic acid, and allyl benzene. The reaction operates
under relatively mild conditions of 100 °C and 10ꢀ30 bars
syngas pressure within 24ꢀ48 hours. Initial optimization studꢀ
ies with 1ꢀoctene indicated an optimal ligand to metal ratio of
2.5, methanol as the most suitable solvent, and a temperature
of 100 °C. Short chain 1ꢀhexene could be hydroformylated at
10 bars syngas pressure and 80 °C. While longꢀchain olefins
S3ꢀS5 (C10ꢀC18) required slightly higher syngas pressure of
30 bars to achieve excellent conversion to corresponding aldeꢀ
hydes. The scope of iron catalyzed hydroformylation was exꢀ
tended to functional olefins, cyclic olefins, vinyl aromatics and
the hydroformylation S1ꢀS21 was examined. Compared to the
aliphatic olefins, vinyl aromatics required longer reaction
times. The catalyst tolerated electron donating as well as elecꢀ
tron withdrawing functional groups and displayed good to
excellent yields. Notable branched selectivity was observed
for styrenic substrates (S11ꢀS20), along with significant
yields. The addition of 1 mol% of acetic acid was found to
promote the hydroformylation reaction and the reaction time
for vinyl aromatics could be reduced to 24 hours.
Is it Iron or Rhodium?
Iron catalyzed crossꢀcoupling reactions such as “arylation”
were reported to be influenced by presence of copper impuriꢀ
ties in the Iron precursor FeCl3.50 In this context; we pondered
about the possible role of rhodium impurities in our iron preꢀ
cursor.51 Thus, a three prong approach was used to establish if
it is Iron or Rhodium that is catalyzing the hydroformylation
reaction. i) In the first approach, commercially available iron
precursor [Fe2(CO)9] was evaluated in the hydroformylation of
1ꢀoctene, in presence and absence of phosphine ligand under
identical conditions and the results are presented in table 4. As
evident, no hydroformylation was observed with [Fe2(CO)9]
and a meager 1% hydroformylation product (Fig. S46) was
observed in presence of triphenyl phosphine ligand. Had there
been any rhodium impurity in the commercial precursor
[Fe2(CO)9], it would have catalyzed the HF in presence of
triphenyl phosphine ligand. Thus, the lack of hydroformylaꢀ
tion in the above experiment suggests that the impurities in the
commercial precursor do not catalyze hydroformylation. ii) In
the catalyst preparation step, [Fe2(CO)9] was converted into
[Fe(CO)5], which is subsequently utilized for the synthesis of
complex 1. In order to further investigate the role of rhodium
impurity in this intermediate, [Fe(CO)5] catalyzed hydroꢀ
formylation of 1ꢀoctene with and without triphenyl phosphine
under identical conditions was examined. In this case as well,
almost no hydroformylation was observed (Fig. S47). These
observations rule out the possible hydroformylation by rhodiꢀ
um impurities and support the assumption that the reaction is
most likely catalyzed by Iron. iii) In our attempts to detect the
rhodium, the iron precursor [Fe2(CO)9] and complex 1 were
subjected to bulk analyses using atomic absorption/emission
spectroscopy and surface analysis using XPS. Both of these
analyses revealed absence of rhodium in the precursor (see
supporting information section 8). Thus, it is most likely that
the hydroformylation is catalyzed by iron and it is highly unꢀ
likely that the rhodium impurity that is beyond the detection
limits (0.01 ppm) is responsible for the observed HF.
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47
48
49
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54
55
56
57
58
59
60
Combined experimental and computational investigations
indicate that the diꢀhydride species A is the actual active cataꢀ
lytic species. The identity of species A was established by
multiple NMR experiments, which indicated coordination of
ligand L1 and formation of the ironꢀdihydride complex. Cyclic
voltammetry results revealed
a Fe(0) to Fe(II) interꢀ
conversion, explaining the accelerating effect of acetic acid.
Control experiments with externally added radical scavengers
ruled out the possibility of a radical or an Fe(I) to Fe(III)
mechanism. The experimental findings were further corroboꢀ
rated by DFT calculations. Among the six possible stereoꢀ
isomers of ironꢀdihydride complex, species A was found to be
the most favorable. Transition state calculations for 1ꢀhexene
insertion revealed that 1,2ꢀinsertion was favored by 1.3
kcal/mol, whereas styrene preferred 2,1ꢀinsertion by 1.7
kcal/mol. Thus, experimental and computational investigations
establish that the iron catalyzed hydroformylation follows a
Fe(II) catalytic cycle, as depicted in Scheme 1. The role of
rhodium impurities in catalyzing the hydroformylation of alꢀ
kenes was investigated. These studies established that the reꢀ
ported hydroformylation is catalyzed by iron and it is highly
unlikely that the rhodium impurities are responsible for the
observed hydroformylation.
Table 4. Hydroformylation of 1ꢀoctene with iron in presence
and absence of ligand.a
Run Feꢀ
precursor
Ligand
L:M
Conv.
(%)b
[Fe2(CO)9]
[Fe2(CO)9]
[Fe(CO)5]
[Fe(CO)5]
NA
2.5
2.5
2.5
2.5
0
1
2
3
4
PPh3
NA
~1
0
PPh3
0
EXPERIMENTAL SECTION
aConditions: [Fe2(CO)9]: 0.0109 mmol, [Fe(CO)5]: 0.0204
mmol, Sub/Fe: 100, Solvent: 1 ml methanol, CO/H2: 20 bars,
Temp.: 100 °C, Time : 24 hours; NA: Not added; bDetermined by
GC.
Materials and methods. Unless noted otherwise, all maꢀ
nipulations were carried out under an inert atmosphere of arꢀ
gon using standard Schlenk line techniques or MꢀBraun glove
box.
Tetrahydrofuran
was
distilled
from
sodiꢀ
um/benzophenone under argon atmosphere. Methylene chloꢀ
ride was distilled on calciumꢀhydride. Methanol, ethanol, and
isopropanol were dried on magnesium cake. Magnesium turnꢀ
ing, triphenyl phosphine, triphenyl phosphite were purchased
from SigmaꢀAldrich and used without further purification. 1ꢀ
hexene, 1ꢀoctene, 1ꢀdecene, 1ꢀdodecene, 1ꢀoctadecene, styꢀ
CONCLUSIONS
In summary, the current work unveils a new iron catalyst for
the hydroformylation of aliphatic and aromatic olefins under
mild conditions. An iron hydride precursor 1, in combination
with readily available phosphine or phosphite ligand and synꢀ
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Journal of the American Chemical Society
Page 8 of 20
rene, 4ꢀchloro styrene, trimethoxy(vinyl)silane, trimeꢀ many, modelꢀARCOS, Simultaneous ICP Spectrometer (IIT
1
thyl(vinyl)silane, allyl malonic acid, 4ꢀtBuꢀstyrene, 4ꢀacetoxy
styrene, 4ꢀvinylbenzonitrile, 4ꢀvinylbenzoic acid and allyl
benzene were purchased from SigmaꢀAldrich and used after
passing through a plug of neutral alumina followed by distillaꢀ
tion. Cashew nut shell liquid was received from Sunshield
Chemicals Limited (subsidiary of Solvay) and was further
purified to obtain cardanol. 4ꢀiso-butylꢀstyrene was prepared
by following a known procedure.52 [Fe2(CO)9], 4ꢀmethoxy
styrene, 4ꢀmethyl styrene, 4ꢀbromo styrene were obtained
from Alfa Aesar. Complex [HFe(CO)4][PPN] (1) was syntheꢀ
sized by modifying literature procedure.13 [(PPh3)2Fe(CO)3]
(iii) was prepared by following a reported procedure.42 Other
chemicals like methylene chloride, methanol, ethanol, isoproꢀ
panol, 1,4ꢀdioxane, tetrahydrofuran, toluene, chloroform, calꢀ
cium hydride, 2,3ꢀdihydrofuran, etc. were purchased from
local suppliers. Acetic acid (glacial) was purchased from
Qualigens Fine Chemicals and was dried by adding acetic
anhydride (7:3). The syngas and hydrogen gas were supplied
by Ms. Vadilal Chemicals Ltd, Pune, India. The hydroꢀ
formylation of αꢀolefins was run in an Amar Equipment Pvt.
Ltd. high pressure reactor equipped with pressure regulators
and safety rupture valve. The hydroformylation of 1ꢀoctene
with incubation was run in an Amar Equipment Pvt. Ltd. high
pressure reactor equipped with additional high pressure liquid
charging chamber, pressure regulators and safety rupture
valve. NMR spectra were recorded on Bruker 200, 400, and
500 MHz instruments. Chemical shifts are referenced to exterꢀ
nal reference TMS (1H and 13C) or 85% H3PO4 (Ξ =
40.480747 MHz, 31P). Coupling constants are given as absoꢀ
lute values. Multiplicities are given as follows s: singlet, d:
doublet, t: triplet, m: multiplet. In-situ high pressure NMR was
recorded in Wilmad quick pressure valve NMR tube. Mass
spectra were recorded on Thermo scientific QꢀExactive mass
spectrometer; with Hypersil gold C18 column (150 x 4.6 mm
diameter 8 ꢁm particle size mobile phase used is 90% methaꢀ
nol + 10 % water + 0.1 % formic acid). IR spectrum was recꢀ
orded on Bruker alphaꢀT spectrometer in liquid state. Sample
was dissolved in chloroform and spectrum was obtained using
sodium chloride window. GC analyses for 1ꢀhexene, 1ꢀoctene,
1ꢀdecene, 1ꢀdodecene, 1ꢀoctadecene, styrene, 4ꢀmethoxy styꢀ
rene, 4ꢀbromo styrene, 4ꢀmethyl styrene, other styrenic subꢀ
strates and allyl benzene were carried out on an Agilent 7890B
GC system. GCꢀMS analysis was carried out on a Varian 3800
GCꢀMS (Saturn 2000MS) with VFꢀ5 capillary column (5%
phenyl, 95% dimethyl polysiloxane). The column oven proꢀ
gram used is same as that for GC analysis. Headspace GC
analysis was performed on an Agilent 7890A GC system
equipped with Porapak column and thermal conductivity deꢀ
Bombay), and Agilent modelꢀ4100 MPꢀAES.
Synthesis of [HFe(CO)4]¯[PNP]+ (1). The desired iron preꢀ
cursor [HFe(CO)4]¯[PNP]+ was prepared by following a literaꢀ
ture reported procedure.13
1H NMR (500 MHz in CDCl3): δ = ꢀ8.52 (s, 1H, FeꢀH). 13C
NMR (125 MHz in CDCl3): δ = 161.5 (C=O), 135.0, 133.6,
130.8, 129.2, 128.3. 31P NMR (500 MHz in CDCl3): δ =
21.02. IR (cmꢀ1) = 1870 (C=O). ESIꢀMS (ꢀve mode): m/z =
168.91 [M]¯.
General procedure for hydroformylation.53 In a typical
hydroformylation experiment, a stainless steel autoclave (450
mL) equipped with 50 ml high pressure liquid charging chamꢀ
ber, pressure regulator and a safety valve was used. Individual
vials were charged with metal precursor [HFe(CO)4]¯[PNP]+
(5.5 mg, 0.0077 mmol)}, ligand (as in Tables 1ꢀ3), solvent (1
ml), substrate (100 equiv.) and stirring bars in a glove box.
The vials were transferred to autoclave and the autoclave was
purged three times with syngas (CO: H2 = 1:1) before pressurꢀ
izing it to the desired pressure. Suitable temperature and presꢀ
sure was maintained during the reaction. After completion of
the reaction, the autoclave was cooled to 0 °C, and excess gas
was vented off in a wellꢀventilated fumeꢀhood. The conversion
and regioꢀselectivity were determined by using gas chromaꢀ
tography (GC) and proton NMR.
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1ꢀhexene. GC analysis for 1ꢀhexene was carried out on an
Agilent 7890B GC system using HPꢀ05 column (30 m × 320
ꢁm × 0.25 ꢁm), split ratio 30:1, column pressure 10 psi, injecꢀ
tor temperature of 260 °C, detector temperature of 330 °C,
argon carrier gas. Temperature program: Initial temperature 50
°C, hold for 1 min.; ramp 1: 4 °C/min. to 120 °C; ramp 2: 20
°C/min. to 250 °C; ramp 3: 20 °C/min. to 320 °C, hold for 2
min. Retention time for 1ꢀhexene = 2.05 min hydrogenated
product (nꢀhexane) = 2.07 min.; branched aldehydes = 4.74
min.; linear aldehyde = 5.65 minute (SI Fig. S7).
1ꢀoctene. Temperature program: Initial temperature 70 °C,
hold for 1 min.; ramp 1: 4 °C/min. to 120 °C; ramp 2: 10
°C/min. to 250 °C; ramp 3: 20 °C/min. to 320 °C, hold for 2
min. Retention time for 1ꢀoctene = 2.7 min.; hydrogenated
product (nꢀoctane) = 2.8 min.; branched aldehydes = 7.02
min.; linear aldehyde = 8.1 minute (SI Fig. S6).
1ꢀdecene, 1ꢀdodecene, 1ꢀoctadecene. Temperature proꢀ
gram: Initial temperature 70 °C, hold for 1 min.; ramp 1: 4
°C/min. to 120 °C; ramp 2: 10 °C/min. to 250 °C; ramp 3: 20
°C/min. to 320 °C, hold for 2 min. Retention time for 1ꢀdecene
= 5.4 min. hydrogenated product (nꢀdecane) = 5.7 min.;
branched aldehydes = 13.1 min.; linear aldehyde = 14.4 miꢀ
nute (SI Fig. S8). Retention time for 1ꢀdodecene = 9.9 min.
hydrogenated product (nꢀdodecane) = 11.2 min.; branched
aldehydes = 18.0 min.; linear aldehyde = 18.8 min. (SI Fig.
S9). Retention time for 1ꢀoctadecene = 22.4 min.; branched
aldehydes = 26.0 min.; linear aldehyde = 26.5 min. (SI Fig.
S10).
°
tector. Inlet temperature was maintained at 150 C, column
flow = 14 ml/min. Detector temperature was maintained at 200
°C. Temperature program: starting at 60 °C with hold time of 5
mins. Ramp 1: @ 10 °C to 100 °C, hold for 21 mins. Retention
time for H2 = 1.5 mins. The XPS measurements were carried
out using Thermo Scientific Kalpha+ spectrometer using a
monochromatic Al Kα (1486.6 eV) xꢀray source. The base
pressure of the spectrometer was 2 × 10ꢀ9 mbar. The wide area
and narrow region scans were acquired using 100 eV and 50
eV pass energy respectively. The bulk analyses for the detecꢀ
tion of rhodium impurity was carried out at three different
places using; Varian atomic absorption spectrometer, modelꢀ
220fs (NCL), SPECTRO Analytical Instruments GmbH, Gerꢀ
Styrene, 4ꢀmethyl styrene. GC analysis for styrene and 4ꢀ
methyl styrene was carried out on an Agilent 7890B GC sysꢀ
tem using Supelco βꢀdex 225 (30 m* 0.25 mm * 0.25 µm),
split ratio 30:1, column pressure 10 psi., injector temperature
of 220 °C, detector temperature of 300 °C, argon carrier gas.
Temperature program: Initial temperature 100 °C, hold for 2
min.; ramp 1: 2 °C/min. to 160 °C; ramp 2: 20 °C/min. to 210
°C; hold for 2 min. Retention time Rt for styrene = 7.3 mins.
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1
for hydrogenated product (Ethyl benzene) = 6.3 mins, nꢀ
(25 °C): 31P NMR (500 MHz, CDCl3): δ = 71.5 (s). H NMR
(500 MHz, CDCl3): δ = ꢀ9.50 (d, 2JPꢀH = 43 Hz).
1
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4
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7
8
dodecane = 14.7 min. (internal standard), for branched aldeꢀ
hydes = 17.0 mins. for linear aldehyde = 23.2 mins. (SI Fig.
S16). Retention time Rt for 4ꢀmethyl styrene = 10.3 mins. for
branched aldehydes = 22.0 mins. for linear aldehyde = 22.7
mins. (SI Fig. S17).
Formation of ironꢀdihydride complex (ii). The iron precurꢀ
sor [HFe(CO)4][PPN] (1) (0.016 g, 0.000022 moles) was disꢀ
solved in 0.6 ml CDCl3:MeOH (3:1) mixture in an NMR tube
and the tube was cooled to liquid nitrogen temperature. Excess
(50 µL) (0.00087 moles) amount of acetic acid was added to
above solution at ꢀ196 °C. An immediate colour change from
light brown to dark red was observed, indicating the change in
oxidation state of the metal. The NMR tube was taken out
from the liquid nitrogen bath just before inserting it in the
magnet and recording the NMR spectrum.
4ꢀmethoxy styrene, 4ꢀbromo styrene, 4ꢀiBuꢀstyrene, 4ꢀ
tBuꢀstyrene, 4ꢀacetoxy styrene, 4ꢀbromo styrene, 4ꢀchloro
styrene, 4ꢀvinylbenzonitrile and allyl benzene. GC analyses
for above styrenic substrates was carried out on an Agilent
7890B GC system using Supelco βꢀdex 225 (30 m* 0.25 mm *
0.25 µm), split ratio 30:1, column pressure 10 psi., injector
temperature of 220 °C, detector temperature of 300 °C, argon
carrier gas. Temperature program: Initial temperature 100 °C,
hold for 2 min.; ramp 1: 2 °C/min. to 160 °C; ramp 2: 10
°C/min. to 210 °C; hold for 2 min. Retention time Rt for 4ꢀ
methoxy styrene = 20.5 mins. for branched aldehydes = 33.3
mins. for linear aldehyde = 36.0 mins. (SI Fig. S20). Retention
time Rt for 4ꢀbromo styrene = 19.3 mins. for branched aldeꢀ
hydes = 35.4 mins. for linear aldehyde = 38.5 mins. (SI Fig.
S22). Retention time Rt for 4ꢀisoꢀbutyl styrene = 20.7 mins.
for branched aldehydes = 32.6 mins. for linear aldehyde = 37.1
mins. (SI Fig. S18). Retention time Rt for 4ꢀtertbutyl styrene =
19.9 mins. for branched aldehydes = 33.3 mins. for linear alꢀ
dehyde = 36.2 mins. (SI Fig. S19). Retention time Rt for 4ꢀ
acetoxy styrene = 30.0 mins. for branched aldehydes = 32.9
mins. for linear aldehyde = 38.44 mins. (SI Fig. S21). Retenꢀ
tion time Rt for 4ꢀchloro styrene = 14.4 mins. for branched
aldehydes = 31.4 mins. for linear aldehyde = 36.0 mins. (SI
Fig. S23). Retention time Rt for 4ꢀvinylbenzonitrile = 28.3
mins. for branched aldehydes = 37.0 mins. for linear aldehyde
= 38.5 mins. (SI Fig. S24). Retention time Rt for allyl benzene
= 8.4 mins. for branched aldehydes = 24.3 mins. for linear
aldehyde = 29.2 mins. (SI Fig. S25).
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Key resonances of (ii).
1H NMR (500 MHz, CDCl3): δ = ꢀ15.38.
[Fe(CO)3(PPh3)2] to iron dihydride complex (A). 0.010 g
(0.0000150 moles) of [Fe(CO)3(PPh3)2] was taken in a high
pressure NMR tube and tolueneꢀd8 (0.3 ml) was added to it.
The above NMR tube was pressurized with hydrogen gas (10
bars) and an NMR was recorded.
Mechanistic investigations. Coordination of L1 followed
by generation of ironꢀdihydride complex A. In a dried and
argon cooled Schlenk tube 0.010 g (0.0000138 moles) of 1
was dissolved in 0.4 ml CDCl3 and 2 equivalent (0.0073 g,
0.0000277 moles) triphenyl phosphine (L1) was added to
above mixture and the resultant solution was transferred to a
high pressure NMR tube. The above mixture was heated to 50
°C for 45 minutes and 45 °C for 16 hours, and the NMR was
recorded at 45 °C (SI Fig. S27).
31P NMR (500 MHz, Tolueneꢀd8): δ = 83.1 (s). 1H NMR (500
MHz, Tolueneꢀd8): δ = ꢀ8.91 (m), ꢀ9.02 (m).
ASSOCIATED CONTENT
Supporting Information. Synthetic procedures, detailed screenꢀ
ing tables, characterization data, computational details, NMR
spectra, GC chromatograms, control experiments, cyclic voltamꢀ
metry. This material is available free of charge via the Internet at
http://pubs.acs.org.
H
PPh3
H
PPh3
PPh3
OC
OC
H
OC Fe
CO
AcOH
2PPh3
CO
CO
Fe
OC Fe
AUTHOR INFORMATION
H
PPh3
CO
1
Corresponding Author
i
( )
A
* Email: s.chikkali@ncl.res.in
Key resonances of (i).
31P NMR (500 MHz, CDCl3): δ = 82.4 (s), 71.6 (s).
ACKNOWLEDGMENT
Having observed L1 coordination, the NMR tube was allowed
to freeze to liquid nitrogen temperature. Now acetic acid (50
ꢁL) was added to above NMR tube under frozen condition and
a proton NMR was recorded at 0 °C immediately. A very
weak hydride resonance appeared at ꢀ12.2 ppm. The new hyꢀ
dride signal may be attributed to a diꢀhydride species A. Room
temperature treatment of 1 with L1 and acetic acid revealed a
doublet at ꢀ9.50 ppm (SI, Fig. S33).
We gratefully acknowledge the financial support from DSTꢀ
SERB (SR/S2/RJNꢀ11/2012 and EMR/2016/005120) India, AvH
foundation, Bonn and CSIRꢀNCL, India. SP would like to thank
UGC for the senior research fellowship. We thank SAIFꢀIIT Boꢀ
maby, Venture Center, Pune and Dr. Tejas Gaydhankar, NCL
Pune for Atomic Absorption/Emission Spectroscopy analysis.
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X. ACS Catal. 2015, 5, 6828ꢀ6837. (c) Chikkali, S. H.; van
Key resonances of A.
1
At 45 °C: 31P NMR (500 MHz, CDCl3): δ = 81.07 (s). H
NMR (500 MHz, CDCl3): δ = ꢀ12.28. At room temperature
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Journal of the American Chemical Society
Page 10 of 20
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(23) Many of the commercial plants for hydroformylation use theꢀ
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(36) For recent representative example of hydroformylation of 4ꢀ
methyl styrene, see: Doro, F.; Reek, J. N. H.; van Leeuwen, P.
W. N. M. Organometallics 2010, 29, 4440ꢀ4447.
(37) Kollar, L.; Farkas, E.; Batiu, J. J. Mol. Catal. A: Chem. 1997,
115, 283ꢀ288.
(38) Siegel, H.; Himmele, W. Angew. Chem. Int. Ed. 1980, 19,
178ꢀ183.
(39) Additive effect was evaluated with an additive to [Fe] ratio of
1, 2, 5. 1 equivalent acetic acid was found to promote the hyꢀ
droformylation, while excess acetic acid seems to have deleteꢀ
rious effect, see supporting information.
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2
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8
9
(40) See reference 9, 10 and SanchezꢀDelgado, R. A.; Bradley, J.
S.; Wilkinson, G. J. Chem. Sci. Dalton Trans. 1976, 399ꢀ404.
(41) Bellagamba, V.; Ercoli, R.; Gamba, A. J. Organomet. Chem.
1982, 235, 201ꢀ214.
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(42) Boddien, A.; Loges, B.; Gärtner, F.; Torborg, C.; Fumino, K.;
Junge, H.; Ludwig, R.; Beller, M. J. Am. Chem. Soc. 2010,
132, 8924ꢀ8934.
(43) The observed chemical shifts are consistent with those reportꢀ
ed
for
[H2Fe(CO)2(dppe)],
[H2Fe(CO)2(PPh3)2],
[H2Fe(CO)2{P(OR)3}2], see: (a) Schubert, U.; Knorr, M. In-
org. Chem. 1989, 28, 1765ꢀ1766. (b) Cenini, S.; Porta, F.;
Pizzotti, M. Inorg. Chima. Acta. 1976, 20, 119ꢀ126. (c) Porta,
F.; Cenini, S.; Giordano, S.; Pizzotti, M. J. Organomet. Chem.
1978, 150, 261ꢀ271. (d) Brunet, J. J.; Kindela, F. B.; Labroue,
D.; Neibecker, D. Inorg. Chem. 1990, 29, 4152ꢀ4153.
(44) In this step, heating to 50 °C is involved, so as to obtain reaꢀ
sonable phosphine coordination. It is most likely that due to
heating, H2 gas is released leaving no hydride on the metal.
(45) It was observed that addition of acetic acid accelerates the
hydroformylation reaction; therefore acetic acid was chosen
to investigate the change in oxidation state of iron precursor
(1).
(46) See supporting information, section 7 for energy optimization,
transition state calculations and computational details.
(47) Heck, R.; Breslow, D. J. Am. Chem. Soc. 1961, 83, 4023ꢀ
4027.
(48) An organoꢀcatalytic hydroformylation with a radical intermeꢀ
diate has been recently reported, see: Huang, H.; Yu, C.;
Zhang, Y.; Zhang, Y.; Mariano, P. S.; Wang, W. J. Am.
Chem. Soc. 2017, 139, 9799ꢀ9802.
(49) M. North, in Principles and applications of stereochemistry,
Stanley Thornes (publishers) Ltd, Cheltenham, UK, 1998.
(50) (a) Buchwald, S. L.; Bolm, C. Angew. Chem. Int. Ed. 2009,
48, 5586ꢀ5587; (b) Plenio, H. Angew. Chem. Int. Ed. 2008,
47, 6954ꢀ6956.
(51) The influence of impurities in a catalytic reaction was pointed
out by one of the reviewer and the editor during the reviewing
process; we thank both of them for the constructive comꢀ
ments.
(52) Movahhed, S.; Westphal, J.; Dindaroglu, M.; Falk, A.;
Schmalz, H.ꢀG. Chem. Eur. J. 2016, 22, 7381ꢀ7384.
(53) A patent has been filed, see: Chikkali, S. H.; Pandey, S. Indiꢀ
an Patent Application: IN2017 11015470.
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Iron Catalyzed Hydroformylation of Alkenes under Mild Conditions: Evidence of an Fe(II) Catalyzed Process
Swechchha Pandey,a K. Vipin Raj,b Dinesh R. Shinde,c Kumar Vanka,b Varchaswal Kashyap,b Sreekumar Kurungot,b Samir
H. Chikkalia,d*
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Table 2
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