Rhodium phosphino-enolate complexes as chemo- and regioselective catalysts for the hydroformylation of styrenes

Article abstract of DOI:10.1016/j.jorganchem.2010.04.032

The catalytic utility of [κ²-{3-ⁱPr ₂P-2-O-indene}Rh(COD)] (COD = η⁴-1,5-cyclooctadiene) 1a in the hydroformylation of styrenes was examined. Complex 1a was shown to be an effective pre-catalyst in benzene and tetrahydrofuran, exhibiting good conversions to aldehyde and high branched o-linear selectivity for styrene, 4-chlorostyrene, and 4-methylstyrene under reasonably mild conditions (1000 psi syngas; 1.8 mol% Rh, 45 °C, 2-5 h). Under analogous conditions, the iridium congener of 1a proved inactive for hydroformylation. The synthesis and crystallographic characterization of the new complex [κ²-{2- ⁱPr₂PC₆H₄O}Rh(COD)] 2 is also reported; the catalytic performance of 2 in the hydroformylation of styrene was found to be comparable to that of 1a under similar catalytic conditions. We report herein on the use of rhodium phosphino-enolate complexes as effective pre-catalysts for the hydroformylation of styrene substrates, exhibiting high branched o-linear selectivity under relatively mild conditions.

Full text of DOI:10.1016/j.jorganchem.2010.04.032

Journal of Organometallic Chemistry 695 (2010) 1869e1872
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Rhodium phosphino-enolate complexes as chemo- and regioselective catalysts for
the hydroformylation of styrenes
,
b
Yoon-Seo Uh a, Alaina Boyd a, Vanessa R. Little a, Philip G. Jessop a , Kevin D. Hesp ,
**
*
,
c
Judy Cipot-Wechsler b, Mark Stradiotto b , Robert McDonald
a Department of Chemistry, Queen’s University, Kingston, ON K7L 3N6, Canada
b Department of Chemistry, Dalhousie University, Halifax, NS B3H 4J3, Canada
c X-Ray Crystallography Laboratory, Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada
a r t i c l e i n f o
a b s t r a c t
2-{3-iPr2P-2-O-indene}Rh(COD)] (COD ¼ h4ꢀ1,5-cyclooctadiene) 1a in the
Article history:
The catalytic utility of
[k
Received 12 March 2010
Received in revised form
25 April 2010
Accepted 27 April 2010
Available online 24 May 2010
hydroformylation of styrenes was examined. Complex 1a was shown to be an effective pre-catalyst in
benzene and tetrahydrofuran, exhibiting good conversions to aldehyde and high branched-to-linear
selectivity for styrene, 4-chlorostyrene, and 4-methylstyrene under reasonably mild conditions (1000 psi
syngas; 1.8 mol% Rh, 45 ꢁC, 2e5 h). Under analogous conditions, the iridium congener of 1a proved
inactive for hydroformylation. The synthesis and crystallographic characterization of the new complex
[k
2-{2-iPr2PC6H4O}Rh(COD)] 2 is also reported; the catalytic performance of 2 in the hydroformylation of
Keywords:
Hydroformylation
Catalysis
styrene was found to be comparable to that of 1a under similar catalytic conditions.
Ó 2010 Elsevier B.V. All rights reserved.
P,O-Ligand
Rhodium
Styrene
1. Introduction
conditions [6], we became interested in surveying the utility of 1a
and 1b in the hydroformylation of alkenes. We report herein that
the rhodium complex 1a (but not the iridium analogue 1b) is
a competent pre-catalyst under mild conditions for the hydro-
formylation of styrene substrates, exhibiting high branched-to-
linear selectivity. We also report the synthesis and crystallographic
The hydroformylation of alkenes is an atom-economical CeC
bond-forming reaction that gives rise to synthetically useful alde-
hyde products [1]. As a result, olefin hydroformylation is currently
one of the most important industrialescale reactions, and is
employed in the production of 106 tons of aldehydes annually that
in turn are used in the synthesis of myriad commodity and fine
chemicals, as well as pharmaceuticals [2]. Hundreds of complexes
that combine rhodium (among other metals) with various phos-
phorus-based ligands have been examined as pre-catalysts for
olefin hydroformylation [3]; nonetheless, the quest to establish
increasing levels of catalyst activity, as well as chemo-, regio-, and/
or enantioselectivity, provides the motivation for further explora-
tions of ancillary ligand effects on such catalytic processes [4,5].
In this context, and building on our previous investigations of
the 2-indanone-derived complexes 1a and 1b (Chart 1) in which
these complexes were revealed to be effective pre-catalysts for the
hydrosilylation and hydrogenation of alkenes under mild
characterization of the new and structurally related complex [k2
-
{2-iPr2PC6H4O}Rh(COD)] 2 (COD ¼ h4e1,5-cyclooctadiene), as well
as its use in hydroformylation catalysis.
2. Experimental
2.1. General experimental details pertaining to the synthesis of 2
All manipulations were conducted in the absence of oxygen and
water under an atmosphere of dinitrogen, either by use of standard
Schlenk methods or within a glovebox apparatus, utilizing glass-
ware that was oven-dried (130 ꢁC) and evacuated while hot prior to
use. The tetrahydrofuran (THF) and pentane (both purchased from
VWR) were deoxygenated and dried by sparging with dinitrogen
gas, followed by passage through a double-column solvent purifi-
cation system purchased from mBraun Inc.; THF was purified over
two alumina-packed columns, while pentane was purified over one
alumina-packed column and one column packed with copper-Q5
reactant. The purified solvents were stored over activated 3 Å
* Corresponding author. Tel.: þ1 613 533 3212; fax: þ1 613 533 6669
** Corresponding author. Tel.: þ1 902 494 7190; fax: þ1 902 494 1310
E-mail addresses: jessop@chem.queensu.ca (P.G. Jessop), mark.stradiotto@dal.ca
(M. Stradiotto).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.04.032

1870
Y.-S. Uh et al. / Journal of Organometallic Chemistry 695 (2010) 1869e1872
3
i
i
CH2(COD)), 1.72 (m, 2H, CH2(COD)), 1.03 (dd, JPH ¼ 16.5 Hz,
3JHH ¼ 7.0 Hz, 6H, P(CHMeaMeb)2, 0.96 (dd, 3JPH ¼ 14.0 Hz,
3JHH ¼ 6.5 Hz, 6H, P(CHMeaMeb)2); 13C{1H} NMR (C6D6): 179.2 (d,
J ¼ 25.2 Hz, Ar-quat.), 132.5 (d, J ¼ 1.8 Hz, AreCH), 130.2 (AreCH),
118.3 (dd, J ¼ 10.7, 2.1 Hz, AreCH), 113.5 (d, J ¼ 5.9 Hz, AreCH),
113.7 (d, J ¼ 35.2 Hz, Ar-quat.), 102.9 (dd, J ¼ 10.4, 7.5 Hz, CH(COD)),
64.2 (dd, J ¼ 11.3, 1.4 Hz, CH(COD)), 32.8 (d, J ¼ 2.3 Hz, CH2(COD)),
27.2 (d, J ¼ 1.5 Hz, CH2(COD)), 23.4 (dd, J ¼ 22.4, 2.0 Hz, P(CH
(Me2)2), 17.5 (d, J ¼ 5.4 Hz, P(CHMeaMeb)2), 16.7 (P(CHMeaMeb)2);
Pr
P
Pr
2
2
P
Rh
M
O
O
H
H
1a, M = Rh
1b, M = Ir
2
Chart 1.
31P{1H} NMR (C6D6): 48.6 (d, JRhP ¼ 160 Hz). Crystals suitable for
1
molecular sieves. C6D6 (Cambridge Isotopes) was degassed by using
three repeated freeze-pump-thaw cycles, and dried over 3 Å
molecular sieves for 24 h prior to use. [(COD)RhCl]2 was purchased
from Strem Chemicals and was dried under vacuum for 24 h prior
to use. The characterization of 2 was carried out at 300 K on
a Bruker AV-500 spectrometer operating at 500.1, 125.8, and
202.5 MHz for 1H, 13C and 31P (respectively) with chemical shifts
reported in parts per million downfield of SiMe4 (for 1H and 13C) or
85% H3PO4 in D2O (for 31P).
X-ray crystallography were grown from a concentrated solution of
2 in pentane at ꢀ35 ꢁC.
2.3. Experimental details and representative protocol for
hydroformylation experiments
Under inert atmosphere of dinitrogen, solvents were dried via
distillation from the drying agents listed as following e hexane
from sodium metal; dichloromethane and benzene from calcium
hydride. CH3CN (ꢃ99.9%) was purchased from Sigma Aldrich. All air
and moisture sensitive reactions were conducted under an inert
atmosphere (nitrogen or argon passed through activated drierite
and potassium hydroxide) in flame-dried glassware using standard
Schlenk techniques. All catalytic reactions were set up in MBraun or
Vacuum Atmosphere gloveboxes under dinitrogen. High-pressure
experiments were carried out in a Parr stainless steel apparatus
equipped with an oven-dried glass liner. All gaseous reagents H2/
CO (50 ꢄ 5% mix, 99.999% H2, 99.9% CO) and CO2 (99.9999% SFC/SFE
grade) were purchased from Praxair and used directly. All reactions
were performed using 31 mL high pressure steel vessels. Styrene
was passed through a pre-packed aluminium oxide column before
use to remove inhibitor; this substrate was not purified further.
4-Chlorostyrene and 4-methoxystyrene were purified by distilla-
tion under partial vacuum and percolated through a column of
basic alumina. All vessels and glassware were washed, dried at
160 ꢁC overnight, cooled under vacuum and then stored in a glo-
vebox before use. THF was obtained from Fisher Scientific and
passed through a double-column solvent purification system
purchased from Innovative Technologies, Inc. prior to use as solvent
for hydroformylations. The solvent and styrene substrates were
2.2. Synthesis of 2
Using a synthetic protocol similar to that used for the prepa-
ration of the analogous iridium complex [k
2-{2-iPr2PC6H4O}Ir
(COD)] [6], to a magnetically stirred solution of 2-iPr2PC6H4OH
(0.148 g, 0.702 mmol) in THF (2 mL) was added a THF solution of
[(COD)RhCl]2 (0.173 g, 0.351 mol). After 10 min of stirring, NEt3
(98 mL, 0.702 mmol) was added causing the reaction mixture to
turn bright yellow in color. After 3 h of stirring at ambient
temperature, 31P NMR analysis of the reaction mixture indicated
the complete consumption of 2-iPr2PC6H4OH and the presence of
a new phosphorus-containing species. The reaction mixture was
concentrated under vacuum and the remaining solid residue was
extracted into pentane (4 ꢂ 2 mL). The pentane fractions were
combined and removal of the solvent afforded 2 as an analytically
pure orange solid (0.192 g, 0.456 mmol, 65%). Anal. Calcd for
C20H30PORh: C 57.13, H 7.20 N 0.00. Found: C 56.99 H 7.16N < 0.3.
1H NMR (C6D6): 7.20 (m, 1H, AreH), 7.15 (m, 1H, AreH), 6.93 (dt,
3JHH ¼ 8.0 Hz, JHH ¼ 1.5 Hz, 1H, AreH), 6.57 (m, 1H, AreH), 5.58
4
(m, 2H, CH(COD)), 3.67 (m, 2H, CH(COD)), 2.24 (m, 2H, CH2(COD)),
2.12 (m, 2H, CH2(COD)), 1.97e1.84 (m, 4H, P(CHMe2)2 and
Table 1
Hydroformylation of styrene using 1a.a.
O
H
O
1a (catalyst)
H
+
H /CO (1000 psi total)
2
branched
linear
Entry
Solvent
Loading (mol % 1a)
Additive
Temp.
Time (h)
Conversion (%)b
B: Lb
1
2
3
4
5
6
7
8
C6H6
C6H6
C6H6
C6H6
C6H6
C6H6
THF
THF
THF
THF
THF
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
0.35
1.8
1.8
1.8
e
80 ꢁ
45 ꢁ
20 ꢁ
45 ꢁ
45 ꢁ
45 ꢁ
C
C
C
C
C
C
5
5
5
2
1.5
1.5
2
1.5
1.5
5
5
5
1.5
1.5
1.5
>99
>99
14
90
81
9
98
93
11
24
37
79
37
32
24
83: 17
95: 5
97: 3
95: 5
95: 5
94: 6
95: 5
95: 5
94: 6
97: 3
96: 4
94: 6
95: 5
95: 5
95: 5
e
e
e
e
1eq PPh3
e
45 ꢁC
45 ꢁC
e
9
1eq PPh3
e
45 ꢁ
C
10
11
12
13
14
15
20 ꢁC
20 ꢁ
45 ꢁC
45 ꢁ
45 ꢁC
45 ꢁ
CO2 (60 bar)
C
THF
e
e
e
e
CH2Cl2
Hexanes
CH3CN
C
C
a
Reactions employing 1000 psi syngas.
b
Conversion to aldehyde and branched-to-linear ratio (B : L) determined on the basis of 1H NMR data using hexamethylbenzene as an internal standard.

Y.-S. Uh et al. / Journal of Organometallic Chemistry 695 (2010) 1869e1872
1871
iPr2
P
separately degassed using standard techniques (freeze-pump-
1. NEt3
PiPr2
thaw), and then stored at ꢀ20 ꢁC in the glovebox until use. The
catalyst (1.8 mol%) and substrate were weighed out in separate 4
dram vials; 2 mL of THF was added to the substrate and then the
resulting solution was transferred to the vial containing the cata-
lyst. The mixture was stirred with a stir bar for 10 min. The entire
vial was then placed into the steel vessel, which was assembled and
tightened before being removed from the glovebox. The vessel
equilibrated in a 45 ꢁC water bath for 40 min. The vessel was then
re-tightened and pressurized to 70 bar with a 1:1 mix of CO and H2.
The reaction was stirred using a stir bar at 650 rpm and 45 ꢁC for
5 h. The vessel was moved to an ice bath for 1 h and then depres-
surized. The solvent was allowed to evaporate slowly to prevent the
loss of product. Samples of the residue were evaluated by use of
NMR spectroscopic methods in CDCl3.
2. 0.5 [(COD)RhCl]2
Rh
2
OH
O
Scheme 1. Preparation of 2.
tetrahydrofuran enabled reactions to be conducted at lower load-
ings than could be achieved in benzene (entry 12); conversely,
dichloromethane, hexanes, and acetonitrile each proved to be a less
suitable reaction medium (entries 13e15).
Having established the catalytic utility of 1a in the hydro-
formylation of styrene, alternative olefinic substrates were exam-
ined (Table 2). While the branched regioselectivity remained high
for reactions employing 4-chlorostyrene, 4-methylstyrene, and
6-methoxy-2-vinylnapthalene, longer reaction times were
required and in the case of the last substrate, low conversions
were achieved. Efforts to extend these highly regioselective
hydroformylations to the aliphatic substrate 1-decene were
unsuccessful; while the conversion was reasonable, the regiose-
lectivity was poor.
3. Results and discussion
The hydroformylation of styrene was selected as a test reaction
for evaluating the catalytic utility of 1a (Table 1). Complex 1a dis-
played good activity and high chemoselectivity for aldehydes, with
the expected branched aldehyde being favored in reactions con-
ducted at 80 ꢁC in benzene (entry 1); the alkene insertion step
leading to the formation of the sterically disfavored branched
product is thought to be promoted by the favorable formation of
In an effort to evaluate the possible influence of the indene
backbone in 1a on catalytic performance, the alternative [(COD)Rh
(k
2-P,O)] complex 2 was prepared in 65% isolated yield from the
corresponding ortho-substituted phenol and [(COD)RhCl]2 in the
presence of triethylamine (Scheme 1). The structure of 2 is in
keeping with the proposed connectivity, and is supported by
single-crystal X-ray diffraction data. An ORTEP [8] diagram of 2 is
presented in Fig. 1, while X-ray experimental data are collected in
Table 3. The overall structural features in 2 compare well with those
observed in both the analogous Ir complex [6], as well as in 1a [9].
Notably, the Rh-alkene distances trans to P (2.198(3) and 2.191(3) Å)
are statistically longer than the Rh-alkene distances trans to O
(2.116(3) and 2.126(3) Å), in keeping with the greater trans-influ-
ence predicted for a phosphine fragment relative to an alkoxy
donor on rhodium [10].
(h
3-benzyl)Rh species [1,3]. Notably, full conversion and increased
regioselectivity were also achieved when employing 1a as a cata-
lyst under similar conditions at 45 ꢁC (entry 2), in keeping with
previously observed trends in rhodium-catalyzed hydro-
formylation [7]. The analogous iridium complex 1b exhibited
negligible catalytic activity for the hydroformylation of styrene
under similar conditions (12 h, 45 ꢁC, 1.8 mol% Ir). While increased
regioselectivity (97:3) was achieved in reactions conducted at 20 ꢁC
using 1a, the conversion to aldehyde under these conditions proved
to be unacceptably low (entry 3); by comparison, Breit and co-
workers [4a] have disclosed a rhodium catalyst system that allows
for the regioselective hydroformylation of alkenes under ambient
temperature and pressure conditions.
Whereas the addition of phosphine and phosphite co-ligands
has been shown to augment the catalytic performance of some
rhodium hydroformylation catalysts [3], the inclusion of triphe-
nylphosphine into the catalytic system resulted in a dramatic loss in
catalytic activity in both benzene and tetrahydrofuran (entry 5 vs.
entry 6; entry 8 vs. entry 9).
Hydroformylation experiments employing 2 as a pre-catalyst
under conditions analogous to those featured in Table 1, entry 7 for
1a afforded catalytic results for 2 that were only slightly more
selective (B:L ratio of 97:3) than those obtained by using 1a, thereby
suggesting that the
k
2-P,O backbone of the phosphino-enolate
ancillary ligand has little influence over the catalytic behavior of the
associated rhodium center. Nonetheless, the desirable catalytic
performance exhibited by 1a and 2 is in contrast to structurally
Solvent effects on the reaction performance were particularly
evident at 20 ꢁC because of the lower conversion observed at that
temperature. Reactions conducted in tetrahydrofuran at 20 ꢁC
afforded improved (albeit modest) conversion to the aldehyde with
high regioselectivity (entry 10) relative to analogous experiments
conducted in benzene (entry 3). The inclusion of liquid CO2 affor-
ded further gains in conversion (entry 11). At 45 ꢁC, the use of 1a in
Table 2
Hydroformylation of alternative alkene substrates using 1a.a
Loading (mol %
1a)
Substrate
Time
(h)
Conversion
(%)b
B: Lb
0.5
0.5
0.5
4-chlorostyrene
4-methylstyrene
6-methoxy-2-
vinylnapthalene
1-decene
12
12
12
71
87
10
95: 5
94: 6
99: 1
1.8
2
53c
1: 1.6
a
Reactions employing 1000 psi syngas in THF at 45 ꢁC.
Conversion to aldehyde and branched-to-linear ratio (B : L) determined on the
b
Fig. 1. The crystallographically determined structure of 2, depicted with 50% thermal
ellipsoids and with the hydrogen atoms removed for clarity. Selected interatomic
distances (Å) for 2: Rh-P 2.2938(8); Rh-O 2.048(2); Rh-C21 2.116(3); Rh-C22 2.126(3);
Rh-C25 2.198(3); Rh-C26 2.191(3).
basis of 1H NMR data using hexamethylbenzene or 1,4-dimethoxybenzene as an
internal standard.
c
Trace amounts of alkene isomerization product observed.

1872
Y.-S. Uh et al. / Journal of Organometallic Chemistry 695 (2010) 1869e1872
Table 3
Crystallographic data for 2.
Graduate Scholarship for K. D. H.). We also thank Dr. Michael
Lumsden (NMR3, Dalhousie) for assistance in the acquisition of
NMR data. P. G. J. also thanks the Canada Research Chairs program.
Empirical formula
Formula weight
Crystal dimensions
Color, habit
Crystal system
Space group
a (Å)
C20H30O1P1Rh1
420.32
0.39 ꢂ 0.18 ꢂ 0.08 mm3
yellow, prism
monoclinic
P21/n
Appendix. Supplementary data
Crystallographic Solution and Refinement Details for 2. Crys-
tallographic data were obtained at 193(ꢄ2) K on a Bruker PLAT-
9.9409(10)
15.9331(15)
12.0772(12)
94.5317(12)
1906.9(3)
4
b (Å)
c (Å)
FORM/SMART 1000 CCD diffractometer using
a
graphite-
b
(deg)
monochromated Mo K
a
(l
¼ 0.71073 Å) radiation, employing
V (Å3)
a sample that was mounted in inert oil and transferred to a cold gas
stream on the diffractometer. Programs for diffractometer opera-
tion, data collection, data reduction, and multi-scan absorption
correction (including SAINT and SADABS) were supplied by Bruker.
The structure was solved by use of a Patterson search/structure
expansion, and the refinement was carried out employing full-
matrix least-squares procedures on F2. Anisotropic displacement
parameters were employed throughout for the non-hydrogen
atoms, and all hydrogen atoms were added at calculated positions
Z
r
m
calcd (g cmꢀ3
(mmꢀ1
)
1.464
0.982
)
Range of transmission
limit (deg)
0.9256e0.7007
54.94
ꢀ12 ꢅ h ꢅ 12
ꢀ20 ꢅ k ꢅ 20
ꢀ15 ꢅ l ꢅ 15
14 626
4354
0.0412
3459
4354/0/208
1.042
2
q
Total reflections
Independent reflections
R(int)
and refined by use of
a riding model employing isotropic
Observed reflections
Data/restraints/parameters
Goodness-of-fit
displacement parameters based on the isotropic displacement
parameter of the attached atom. Crystallographic data for the
structural analysis has been deposited with the Cambridge Crys-
tallographic Data Centre, CCDC No. 767857 for compound 2.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2010.04.032
R1 [Fo2 ꢃ 2
s
(Fo2)]
0.0346
0.0840
0.773, ꢀ0.465
wR2 [Fo2 ꢃꢀ3
s
(Fo2)]
Largest peak, hole (eÅꢀ3)
related complexes of the type [Rh(PO)(CO)L] (PO ¼
k
2-{2-
Ph2PC6H4O}; L ¼ HOC6H4PPh2 or PPh3), which proved inactive in
the hydroformylation of 1-alkenes [10]. Given that the addition of
PPh3 to 1a resulted in diminished catalytic performance (vide
supra) and that 1a [present work] and [Rh(PO)(P{OPh}3)2] [10] are
capable of mediating the hydroformylation of 1-alkenes, it is
plausible that the poor performance of the [Rh(PO)(CO)L]
complexes reported by Trzeciak et al. [10] can be attributed to the
presence of the triphenylphosphine co-ligand, rather than to the
phosphino-enolate ancillary ligand as proposed by the authors.
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2-P,O) catalysts under
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15;
hydroformylation conditions reveal that Rh-O cleavage [11], Rh-P
cleavage [12], and complete loss of the P,O-ligand [10] are all
feasible under catalytic hydroformylation conditions; in the
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4. Summary and conclusions
The results reported herein confirm that the known rhodium
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complex 1a, as well as the newly reported complex [k
2-{2-iPr2P-
C6H4O}Rh(COD)] 2, are competent pre-catalysts for the hydro-
formylation of styrene substrates under mild conditions, exhibiting
high branched-to-linear selectivity under relatively mild condi-
tions. In contrast, the iridium analogue of 1a (i.e. 1b) proved inef-
fective in mediating the hydroformylation of styrene under
comparable conditions. Encouraged by the performance of the
rhodium complexes featured herein, future work will target chiral
variants of 1a and 2, for use in mediating asymmetric olefin
hydroformylation reactions.
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