Highly Regioselective Isomerizing Hydroformylation of Long-Chain Internal Olefins Catalyzed by a Rhodium Bis(Phosphite) Complex

Article abstract of DOI:10.1002/cctc.201500743

The single-step synthesis, coordination behavior, and application of a bis(phosphite) ligand in the isomerizing hydroformylation of internal olefins was investigated. Interestingly, high-pressure NMR spectroscopy investigations revealed unexpected inequivalency of the two phosphorus nuclei, which display bisequatorial coordination of the bis(phosphite) ligand in a trigonal bipyramidal rhodium complex. Upon employment in the isomerizing hydroformylation of the exceedingly challenging plant oil derived substrate methyl oleate, the bis(phosphite) rhodium complex revealed an unprecedented linear selectivity of 75 %.

Full text of DOI:10.1002/cctc.201500743

DOI: 10.1002/cctc.201500743
Communications
Highly Regioselective Isomerizing Hydroformylation of
Long-Chain Internal Olefins Catalyzed by a Rhodium
Bis(Phosphite) Complex
[a]
[a, b]
Swechchha Pandey and Samir H. Chikkali*
The single-step synthesis, coordination behavior, and applica-
tion of a bis(phosphite) ligand in the isomerizing hydroformy-
lation of internal olefins was investigated. Interestingly, high-
pressure NMR spectroscopy investigations revealed unexpect-
ed inequivalency of the two phosphorus nuclei, which display
bisequatorial coordination of the bis(phosphite) ligand in
a trigonal bipyramidal rhodium complex. Upon employment in
the isomerizing hydroformylation of the exceedingly challeng-
ing plant oil derived substrate methyl oleate, the bis(phos-
phite) rhodium complex revealed an unprecedented linear se-
lectivity of 75%.
thermodynamically unfavorable, which makes terminal func-
tionalization of plant oils a challenging transformation. Very
few attempts have been made to address this bottleneck (see
[
5]
Figure 1). The isomerizing hydroformylation of a fatty acid
[
1]
The primary source of chemicals produced today is crude oil.
However, with increasing demands, limited crude oil reserves,
and increasing environmental constraints, there is a massive
drive to substitute fossil-fuel resources with renewable resour-
ces. In this quest, the scientific community has contributed sig-
nificantly over the last decade, and various renewable resour-
Figure 1. Representative metal-catalyzed isomerizing functionalization of
plant-oil-based methyl oleate.
[2]
ces have been identified. The resurge in the renewable chem-
ical industry has led to the successful launch of several renewa-
ble products such as sugarcane-based polyethylene, polylactic
methyl ester was first reported by Behr et al. but was met with
[3]
[6]
acid, and polyhydroxyalkanoates, to name a few. Among the
various renewable resources, plant oils stand out, as they pro-
vide direct entry to chemical modification. Their easy access
and high H/C ratios (almost equivalent to crude oil) make
very limited success (26% terminal aldehyde). Isomerizing hy-
[
7]
droboration displayed a slight improvement. Isomerizing
[
8]
metathesis has attracted significant attention, and the prod-
[
9]
ucts might hit the market soon. A highly successful example
to date is isomerizing methoxycarbonylation that was first re-
[4]
them substrates of choice. These attractive features are di-
rectly reflected in their 35% share in the renewable chemical
industry, although they contribute only 2% to the total bio-
mass produced.
[
10]
ported by Cole-Hamilton et al. and later developed by Quin-
[
11]
zler and Mecking.
This particular reaction proceeds with
>95% terminal selectivity, and mechanistic investigations, in
[
12]
Chemically, plant oils are long-chain fatty acids with varying
degrees of internal double bonds. The internal double bonds
provide an excellent opportunity to further functionalize plant
oils to useful chemicals and materials. A paradigm shift could
be achieved if the internal double bond could be isomerized
to the terminal position and selectively functionalized. Howev-
er, isomerization of internal double bonds to terminal olefins is
great detail, have been reported. Thus, access to symmetri-
cally functionalized long-chain segments can be obtained by
isomerizing alkoxycarbonylation of plant oils.
Attaining similar terminal selectivity in a one-pot isomerizing
hydroformylation reaction will be of paramount significance, as
it would provide a powerful tool for the heterofunctionaliza-
tion of plant oils along with complete feedstock utilization.
Herein, we present our attempts in the isomerizing hydrofor-
mylation of internal olefins and plant oils by using a bis(phos-
phite)–Rh complex and further functionalization of the resul-
tant linear aldehydes to hydroxy–acid bifunctional molecules.
Transition-metal-catalyzed hydroformylation is arguably one
of the largest homogeneously catalyzed reactions. Since its dis-
covery in 1938, numerous aldehydes have been prepared by
[
a] S. Pandey, Dr. S. H. Chikkali
Polyolefin Lab, Polymer Science and Engineering Division
CSIR-National Chemical Laboratory
Dr. Homi Bhabha Road, Pune-411008 (India)
E-mail: s.chikkali@ncl.res.in
[
b] Dr. S. H. Chikkali
Academy of Scientific and Innovative Research (AcSIR)
Anusandhan Bhawan, 2 Rafi Marg, New Delhi-110001 (India)
[
13]
using this transformation. A more recent addition to the fold
of hydroformylation is tandem or isomerizing hydroformylation
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500743.
[
14]
of internal alkenes to linear aldehydes. Isomerizing hydrofor-
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Communications
mylation has attracted increasing attention in recent years, as
it offers a single-window isomerizing-functionalization protocol
It is well established that ligand coordination influences the
[
17]
regioselectivity of a hydroformylation reaction. In our pursuit
[15]
for long-chain internal alkenes such as methyl oleate. We an-
ticipated that a catalyst should meet the following criteria to
ensure successful isomerizing hydroformylation of internal ole-
fins to linear aldehydes:
to identify the catalyst resting state, we treated L1 with [Rh(a-
cac)(CO) ] in the presence of syngas (1.0 MPa) in an NMR tube
2
31
experiment. Analysis by in situ, high-pressure P NMR spec-
troscopy revealed characteristic doublet of doublets at 144.1
1
1
(
J
=240 Hz) and 135.9 ppm ( J
=243 Hz). The observed
RhÀP
RhÀP
1
2
3
) The isomerization of an internal olefin to a terminal olefin
should be faster than the hydroformylation event
) The rate of terminal formylation should be faster than the
rate of internal formylation
chemical shifts and coupling constants are in good agreement
[
18]
with previously reported data. The above splitting pattern
indicates that the two phosphorus nuclei are not identical. This
observation (inequivalent P nuclei) supports the equatorial–
axial coordination of the two phosphorus atoms in a trigonal
) The catalyst should be selective towards terminal formyla-
tion
bipyramidal rhodium [{bis(phosphite)}Rh(CO) H] complex. How-
2
1
ever, the in situ, high-pressure H NMR spectrum displays
The combined effect of the above factors will ensure higher
a broad hydride resonance at À9.86 ppm without a large (>
2
[19]
1
selectivity to a linear aldehyde than an internal aldehyde. The
first challenge is to identify or design a catalytic system that
would meet the above criteria. A quick literature review indi-
cated that the majority of the isomerizing functionalization re-
actions employ bidentate phosphite ligands and in a few cases
30 Hz) J
trans coupling. Thus, the large J
coupling
RhÀP
PÀH
2
constant and a small J
coupling constant (3.5 Hz) supports
PÀH
the unexpected inequivalency of the two phosphorus nuclei
and the formation of an equatorial–equatorial complex of type
[
20]
M2 (Scheme 1) as a catalyst resting state. After having estab-
lished the coordination behavior, we evaluated the per-
formance of L1 in the isomerizing hydroformylation of internal
olefins, and Table 1 summarizes the significant findings.
[
2]
phosphine ligands. Given the above stringent requirements
and literature precedent, we envisioned that bis(phosphite)s
would be the most suitable candidates to provide increased
amount of linear aldehydes.
The anticipated catalytically active species was generated
in situ by mixing ligand L1 and the rhodium [Rh(acac)(CO)2]
precursor in the presence of syngas. To test our ligand, we
started with the isomerizing hydroformylation of the short-
chain olefin 1-octene (S1). Our preliminary screening without
much optimization displayed a linear to branched ratio of
Deprotonation of resorcinol by nBuLi, followed by the addi-
tion of di(naphthalen-1-yl) phosphorochloridite yielded antici-
3
1
pated bis(phosphite) ligand L1 (Scheme 1). The P NMR spec-
6
8:32 at a ligand/metal (L/M) ratio of 2 (Table 1, entry 2). Simi-
larly, slightly more challenging cis-2-octene (S2) displayed
a linear to branched selectivity of 64:36 at L/M=2 (Table 1,
entry 5). These results indicated the preferred formation of the
linear aldehyde. Without further experimentation with S1 and
S2, we set out to evaluate L1 in the isomerizing hydroformyla-
tion of the exceedingly challenging plant oil derived substrate
methyl oleate (S3). Note that the double bond has to isomerize
over nine carbon atoms to reach the terminal position, which
gives a statistical regioselectivity of 11 %. Our initial screening
of the ligand/metal ratio in toluene displayed a very poor se-
lectivity of 15:85 (Table 1, entry 6). Screening of various sol-
vents pointed at a slightly better selectivity (18:82) in dioxane
Scheme 1. Synthesis and coordination behavior of L1. acac=acetylaceto-
nate.
(
Table 1, entries 6–9). Therefore, dioxane was used as the sol-
trum of the reaction mixture displayed a single resonance at
vent of choice for the rest of the isomerizing hydroformylation
experiments. Optimization of the ligand to metal ratio suggest-
ed L/M=2 as the most suited combination, although with
a slightly lower conversion (Table 1, entry 7 vs. entries 10–12).
Decreasing the temperature from 120 to 608C resulted in
better conversions, but at the expense of reduced selectivity
(Table 1, entry 7 vs. entries 13–15). Thus, the best linear selec-
tivity was obtained at 1208C, whereas the fully branched alde-
hyde was obtained at 608C. This observation suggests that
high temperature is necessary to facilitate the consecutive b-
hydride elimination and internal-to-terminal double-bond iso-
merization process. Next, the effect of syngas pressure on the
regioselectivity was investigated. A surprisingly higher linear to
branched ratio of 43:57 was observed at a low syngas pressure
1
30 ppm, which suggested the presence of L1. The identity of
L1 was unambiguously ascertained by using a combination of
spectroscopic and analytical tools, and L1 was isolated in 40%
yield. The coordinating ability of L1 towards neutral palladium
complexes was evaluated. THF was added to a mixture of L1
and [(cod)PdCl ] (1:1, COD=1,5-cyclooctadiene), and the reac-
2
tion mixture was stirred at room temperature for 3 h. The vola-
tiles were evaporated, and the residue was washed 2–3 times
with dry pentane to afford M1 as a yellow-colored powder.
31
The appearance of a resonance at 84.5 ppm in the P NMR
spectrum suggested the formation of desired mononuclear
[
16a]
metal complex M1
and ruled out the possibility of a dinu-
[16b]
clear or polynuclear complex.
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the internal aldehyde appears between 9.40 and
9.65 ppm). The NMR spectroscopy findings were fur-
ther supported by GC (Figure S13, Supporting Infor-
mation), which showed a single peak, and GC–MS
analysis, which displayed a base peak at m/z=
Table 1. Isomerizing hydroformylation of internal olefins catalyzed by Rh complexes
of L1.
[a]
3
26.91 Da (Figure S14). Owing to the versatile reac-
tivity of the aldehyde, 1 can be easily transformed
into AA- or AB-type monomers and platform chemi-
cals (Scheme 2). In our pursuit to demonstrate the
synthetic utility of the linear aldehyde, 1 was treated
with KOH in ethanol followed by acidic workup to
yield, as anticipated, 19-hydroxynonadecanoic acid
[
b]
[c]
Entry Substrate L/M Solvent T [8C] CO/H
2
[MPa] t [h] Conversion [%]
L/B
1
2
3
4
5
6
7
8
9
1
S1
S1
S1
S1
S2
S3
S3
S3
S3
S3
S3
S3
S3
S3
S3
S3
S3
S3
S3
S3
S3
S3
S1
S2
1
2
3
4
2
2
2
2
2
3
4
5
2
2
2
2
2
2
2
2
4
–
2
2
toluene 100
toluene 100
toluene 100
toluene 100
toluene 100
toluene 120
dioxane 120
0.5
0.5
0.5
0.5
16
16
16
16
18
16
16
16
16
16
16
16
16
16
16
16
16
16
48
16
16
16
16
16
>99
>99
>99
>99
>99
47
40
53
11
50
50
49
47
93
>99
20
77
95
32
66:34
68:32
63:37
62:38
64:36
15:85
18:82
16:84
5:95
10:90
6:94
5:95
8:92
5:95
[
21]
[
d]
0.5
(
3), which is unprecedented to the best of our
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.1
0.5
1.0
0.1
0.25
0.25
0.5
0.1
0.1
[
22]
1
knowledge. In the H NMR spectrum, a characteris-
tic broad resonance at d=10.94 ppm and a broad
triplet at d=5.36 ppm indicate the presence of
acidic and hydroxy protons (Figure S19). The exis-
tence of 3 was unambiguously ascertained by using
a combination of NMR spectroscopy and mass spec-
trometry. The mass spectrum (ESI) of 3 reveals a posi-
DME
NMP
120
120
0
dioxane 120
dioxane 120
dioxane 120
dioxane 100
dioxane 80
dioxane 60
dioxane 120
dioxane 120
dioxane 120
dioxane 120
dioxane 120
dioxane 120
toluene 100
dioxane 120
dioxane 120
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
3
4
5
6
7
8
9
0
1
2
3
4
0:99
+
tively charged ion at m/z=297.08 [MÀOH] (Fig-
43:57
18:82
1:99
75:25
0:99
NP
0:99
54:46
82:18
[
23]
ure S21). These linear a,w-hydroxy acid functional-
ized molecules can act as AB-type monomers to pro-
duce polyesters. Such linear long-chain polyesters
[
[
[
e]
16
NP
10
>99
>99
[
11b]
f]
have been reported by Mecking et al.;
however,
g]
a diol and diacid was required to prepare the poly-
esters and maintaining the exact stoichiometry is
usually a limiting step to achieve high molecular
weight polymers in these reactions. Reported com-
pound 3 can circumvent this limitation, as it can act
as an AB (single) type monomer to produce the
long-chain polyesters.
[
2
a] Conditions: [Rh(acac)(CO) ] (2 mg); substrate/metal (S/M)=100, S1=1-octene, S2=
cis-2-octene, S3=methyl oleate; solvent (1 mL); DME=1,2-dimethoxyethane, NMP=
N-methylpyrrolidone. [b] Total conversion determined by H NMR spectroscopy.
c] Linear/branched (L/B) ratio determined by GC. [d] S/M=200. [e] Xanthphos as
ligand. [f] Without [Rh(acac)(CO) ]. [g] Without L1; NP=no product could be detected.
1
[
2
In summary, the synthesis, structural characteriza-
tion, and application of a rhodium complex of bulky
of 0.1 MPa (Table 1, entry 16 vs. entries 17 and 18). We attri-
bute the higher linear selectivity at low syngas pressure to
a decreased rate of internal hydroformylation at low pressure.
Further, a prolonged reaction time at a syngas pressure of
0
.1 MPa provided the highest linear selectivity of 75%, impor-
Scheme 2. Plant oil derived a,w-functionalized monomers/platform chemi-
tantly with enhanced total conversion (Table 1, entry 19 vs. 16).
The three control experiments displayed very poor selectivities
cals.
(
Table 1, entries 20–22) or no reaction. We attribute the dra-
matically enhanced selectivity (Table 1, entry 19) to the re-
duced rate of internal hydroformylation, as the syngas pressure
is further lowered over a period of time (lower than 0.1 MPa,
as it is consumed for the hydroformylation process). The iso-
merizing hydroformylation of cis-2-octene under the best con-
ditions (Table 1, entry 24) led to a linear selectivity of 82%,
which further supplements our hypothesis. To our knowledge,
the linear/branched selectivity of 75:25 is the highest linear se-
lectivity ever reported in the isomerizing hydroformylation of
methyl oleate.
bidentate bis(phosphite) ligand L1 in the isomerizing hydrofor-
mylation of long-chain internal olefins such as methyl oleate
was described. The coordination behavior of L1 was investigat-
ed by using high-pressure NMR spectroscopy. These high-pres-
sure NMR spectroscopy experiments revealed bisequatorial co-
ordination of L1 in a trigonal bipyramidal rhodium complex. In-
terestingly, the two phosphorus nuclei display unexpected in-
1
equivalency, and a J
coupling constant of 240–243 Hz was
RhÀP
observed. Preliminary investigations indicate that the Rh com-
plex of L1 catalyzes the isomerizing hydroformylation of not
only short-chain internal olefins such as cis-2-octene but also
that of highly challenging long-chain plant oil derived internal
olefins such as methyl oleate. Performing the reaction at
1208C at ambient (0.1 MPa) syngas pressure resulted in an un-
In our pursuit to confirm the existence of the linear alde-
hyde, 1 was isolated as a colorless liquid after column chroma-
1
tography. A resonance at 9.77 ppm in the H NMR spectrum
can be ascribed to the terminal aldehyde proton (proton of
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precedented linear selectivity of 75%. In addition, the synthetic
utility of isomerizing hydroformylation in organic synthesis was
demonstrated by transforming a linear aldehyde into 19-hy-
droxynonadecanoic acid, which is a potential AB-type mono-
mer for polyester production. The above observations reveal
that isomerization is favored at higher temperatures and that
terminal formylation is favored at lower syngas pressures.
These findings might guide future developments in the field of
the isomerizing hydroformylation of long-chain internal olefins
to linear selective products. We are currently working on en-
hancing the selectivity, improving the conversion to n-alde-
hyde, and possible polycondensation of 19-hydroxynonadeca-
noic acid.
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Financial support from the Department of Science and Technolo-
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3
1
Keywords: alkenes · bis(phosphite)s · hydroformylation
isomerization · rhodium
·
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[
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[
[
[
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Received: June 29, 2015
Published online on September 17, 2015
ChemCatChem 2015, 7, 3468 – 3471
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