Synthesis of new o-alkyl substituted arylalkylphosphanes: Study of their molecular structure and influence on rhodium-catalyzed propene and 1-hexene hydroformylation

Article abstract of DOI:10.1016/S1381-1169(03)00026-8

A novel set of new phosphane ligands designed to increase the branched/normal ratio of the hydroformylation reaction were prepared in the same way as the previously reported ortho-alkyl substituted arylphosphanes, which have shown increased i/n ratios in the hydroformylation of propylene and 1-hexene. To determine the relationship between the catalytic behavior and stemoelectronic characteristics of the ligands, various functional alkyl groups (methyl, isopropyl, cyclohexyl) were placed on the phosphorus atom directly and in the ortho position of the phenyl ring connected to phosphorus. In the hydroformylation reaction of propylene and 1-hexene a higher i/n ratio lead to nearly all the ligands compared with that of triphenylphosphane. As the ortho-alkyl-substituent became larger, it had a favorable effect on the i-selectivity. The o-alkyl-substituent was orientated outside the ligands' cone angle in the X-ray crystal structures of (2-cyclohexylphenyl)dicyclohexylphosphane and (2,5-dimethylphenyl)bis(4-pyridyl)phosphane, and Rh(acac)(CO)(PR₃) complex of (2-methylphenyl)dicyclohexylphosphane.

Full text of DOI:10.1016/S1381-1169(03)00026-8

Journal of Molecular Catalysis A: Chemical 200 (2003) 81–94
Synthesis of new o-alkyl substituted arylalkylphosphanes: study of
their molecular structure and influence on rhodium-catalyzed
propene and 1-hexene hydroformylation
Helena Riihimäkia,∗, Teija Kangasa, Pekka Suomalainenb, Heidi K. Reiniusc,
Sirpa Jääskeläinenb, Matti Haukkab, A.O.I. Krausec, Tapani A. Pakkanenb,
Jouni T. Pursiainena
a
Department of Chemistry, University of Oulu, P.O. Box 3000, FIN-90014 Oulu, Finland
b
Department of Chemistry, University of Joensuu, P.O. Box 111, FIN-80101 Joensuu, Finland
c
Department of Chemical Technology, Helsinki University of Technology, P.O. Box 6100, FIN-02015 HUT, Finland
Received 6 November 2002; received in revised form 8 November 2002; accepted 11 November 2002
Abstract
A set of new phosphane ligands designed to increase the branched-to-normal ratio of the hydroformylation reaction were
prepared in the same way as the previously reported ortho-alkyl substituted arylphosphanes, which have shown increased
i/n ratios in the hydroformylation of propene and 1-hexene. In order to determine the relationship between the catalytic
behavior and stereoelectronic properties of the ligands, various functional alkyl groups (methyl, isopropyl, cyclohexyl)
were placed on the phosphorus atom directly and in the ortho position of the phenyl ring connected to phosphorus. In
the hydroformylation reaction of propene and 1-hexene a higher i/n ratio resulted with nearly all the ligands compared
with that of triphenylphosphane. Additionally as the ortho-alkyl-substituent became larger, it had a favorable effect on
the i-selectivity. Characterization of the ligands was carried out by NMR spectroscopy (mainly 1H, 31P{ H}, 13C{ H},
HSQC/HETCOR and COSY-90). Properties of the ligands were also studied by quantum mechanical calculations and by syn-
thesizing three Rh(acac)(CO)(PR3) derivatives. The o-alkyl-substituent was orientated outside the ligands’ cone angle in the
X-ray crystal structures of (2-cyclohexylphenyl)dicyclohexylphosphane and (2,5-dimethylphenyl)bis(4-pyridyl)phosphane,
and Rh(acac)(CO)(PR3) complex of (2-methylphenyl)dicyclohexylphosphane.
1
1
© 2003 Elsevier Science B.V. All rights reserved.
Keywords: Hydroformylation; Phosphane; Alkyl groups; NMR spectroscopy; Ab initio calculations
1. Introduction
milder reaction conditions and higher activities are
the significant advantages of phosphane-modified
rhodium-catalysts [1–3]. Both the electronic and
steric properties of the catalysts can be greatly af-
fected by an appropriate choice of phosphane ligands.
Despite numerous studies, however, the demand-
ing question how phosphorus ligands control the
selectivity of hydroformylation still awaits solution
[4].
Catalytic hydroformylation of olefins is an
important process in the petrochemical and the
fine chemicals industries. Improved isomer ratios,
∗
Corresponding author. Tel.: +358-8-553-1647;
fax: +358-8-553-1603.
E-mail address: helena.riihimaki@oulu.fi (H. Riihimäki).
1381-1169/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1381-1169(03)00026-8

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H. Riihimäki et al. / Journal of Molecular Catalysis A: Chemical 200 (2003) 81–94
Producing linear n-butanal regioselectively has
After the results of our previous study with
o-methyl, o-ethyl, o-isopropyl and o-cyclohexyl sub-
stituted arylphosphanes we have prepared seven
new arylalkylphosphanes: (2-methylphenyl)diisopro-
pylphosphane (1), (2-cyclohexylphenyl)diisopropyl-
phosphane (2), bis(2-methylphenyl)isopropylphos-
phane (3), (2-methylphenyl)dicyclohexylphosphane
(4), (2-cyclohexylphenyl)dicyclohexylphosphane (5),
(2-methylnaphthyl)diisopropylphosphane (6) and (2-
methylnaphthyl)dicyclohexylphosphane (7) [5,15,18].
Moreover, new o-alkyl substituted pyridylphosphanes
(2,5-dimethylphenyl)bis(3-pyridyl)phosphane (8) and
(2,5-dimethylphenyl)bis(4-pyridyl)phosphane (9) have
also been prepared since pyridylphosphane-based cat-
alysts have earlier shown higher reaction rates than
PPh3 [19,20]. Characterization of the ligands was
made by NMR spectroscopy. The ligands were tested
in the hydroformylation of propene and 1-hexene.
Three Rh(acac)(CO)(PR3) complexes: Rh(acac)-
(CO)(3), Rh(acac)(CO)(4) and Rh(acac)(CO)(5) in a
reaction between Rh(acac)(CO)2 and phosphane (PR3)
were prepared and characterized in order to investigate
the coordination properties of the ligands and orienta-
tion of the o-alkyl-substituents. Geometrical arrange-
ment and steric size (cone angle) of the free and coor-
dinated ligands were studied theoretically by ab initio
Hartree–Fock and DFT methods. Crystal structures for
ligands (2-cyclohexylphenyl)dicyclohexylphosphane
(5) and (2,5-dimethylphenyl)bis(4-pyridyl)phosphane
(9), and Rh(acac)(CO)(4) complex were determined
as well.
been the goal in the most important industrial hydro-
formylation process, propene hydroformylation, for a
long time [1–3,5]. Recently, interest in selective for-
mation of the branched form, isobutanal, used in the
production of polyols, such as neopentyl glycol has
increased [5]. Earlier, higher iso-to-normal (i/n) prod-
uct ratios and greater activity in hydroformylation
have been observed as the bulkier ortho-substituted
triarylphosphite (e.g. tris(o-phenoxyphenyl)phosphite
and tris(o-tert-butylphenyl)phosphite) have been used
as ligands [6–10]. It has been suggested that bulky
ligands favor low coordination numbers, and thus, the
coordinative unsaturation of the metal center becomes
greater favoring branched products [7–12].
Similar results have been observed with bulky
arylphosphanes. The activity of the 1-butene hydro-
formylation reaction and i-selectivity of 2-butene
hydroformylation have become greater as catalysts
have been modified with o-substituted arylphos-
phanes [13,14]. Moreover, in the hydroformylation
of propene, we have achieved enhanced selectivity
to branched aldehydes by using o-alkyl substituted
triphenylphosphane with rhodium catalysts [5,15].
The activity and selectivity of these catalysts is at-
tributable to the stereochemistry and large cone angle
of the bulky phosphane ligands [8,9,12].
Rhodium complexes of strongly basic trialkylphos-
phanes (greater σ-electron donor ability) are more sta-
ble and less active low-pressure hydroformylation
catalysts than weakly basic triarylphosphane com-
plexes at comparable temperatures [16,17]. However,
the i/n selectivity of alkylphosphane complexes and
the equilibria between the major carbonylhydride
complexes are very much dependent on the site and the
degree of their branching (i.e. steric crowding in the
vicinity of their P atoms) [16]. In the cobalt-catalyzed
hydroformylation of 1-hexenes bulkier tris(i-propyl)-
phosphane has showed higher i/n ratios than tris(n-
propyl)phosphane [12]. Alkyldiphenylphosphane
ligands branched at their ␣- or ␤-alkyl carbon atoms,
including cyclohexyldiphenylphosphane, have been
found to form bis(phosphane)—rather than tris(phos-
phane)-carbonylhydridorhodium complexes under
simulated hydroformylation conditions. As a conse-
quence, they were more active catalyst ligands for hy-
droformylation and led to the formation of aldehyde
products having higher i/n isomer ratios [16].
2. Experimental section
General comments: The phosphanes are air- and
moisture-sensitive, both as pure compounds and in
solution, and unprotected show observable oxida-
tion within one day. Thus, all reactions were carried
out with standard Schlenk techniques under nitrogen
or argon atmosphere. The new arylalkylphosphanes
(1–9) studied in this work (Scheme 1) were prepared
by modified literature methods from ortho-substituted
brominated aromatics by lithiation with n-butyllithium
and further reaction with chloroalkylphosphanes or
dichloro(2,5-dimethylphenyl)phosphane [21,22]. The
dichloro(2,5-dimethylphenyl)phosphane used in the
synthesis of pyridylphosphanes was prepared by

H. Riihimäki et al. / Journal of Molecular Catalysis A: Chemical 200 (2003) 81–94
83
Scheme 1. Schematic structures and cone angles of reported ligands.
Friedel–Crafts reaction of phosphorus trichloride
with p-xylene in the presence of aluminium chloride
as catalyst [23]. 2-Bromotoluene (99%), 1-bromo-
2-methylnaphthalene (tech., 90%), 3-bromopyridine
(99%), 4-bromopyridine hydrochloride (99%),
chlorodiisopropylphosphane (96%), chlorodicyclo-
hexylphosphane (97%), dichloroisopropylphosphane
(97%), n-butyllithium (2.5 M solution in hexane)
and TMEDA (>99%) were obtained from Aldrich.
1-Bromo-2-cyclohexylbenzene (97%) was from Lan-
caster. Diethyl ether was distilled over sodium-
benzophenone ketyl under nitrogen before use.
Characterization: The characterization of the lig-
1
ands was based on H-, 13C{1H}-, 31P{1H}-, H,H-
correlated COSY-90-, H,C-correlated HSQC- or C,H-
correlated HETCOR-NMR spectroscopy. Long-range
C,H-correlated COLOC-NMR spectra were measured
for ligands 1, 3, 5 and 6. NMR spectra were recorded
on Bruker DPX400 and AM200 spectrometers at
room temperature in CDCl3 (D 99.8%, 0.03% TMS).
NMR spectra of the metal complexes were recorded
on a Bruker AM250 spectrometer. 1H-NMR: refer-
ence SiMe4. 13C{1H}-NMR: CDCl3 set to 77.0 ppm.
31P{1H}-NMR: external standard 85% H3PO4.

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H. Riihimäki et al. / Journal of Molecular Catalysis A: Chemical 200 (2003) 81–94
Table 1
1
13C{ H} NMR (100 MHz) chemical shifts (ppm)a and coupling constants of doublets (Hz) for reported ligands 1–9b
1
2
3
4
5
6
7
8
9
C0–4
C0–3
C0–2
C0–1
3JCP
Cm−1
C10∗
C9∗c
3JCP
C9∗∗c
3JCP
C8∗c
2JCP
C8∗∗c
2JCP
C7∗
–
–
–
26.28
26.80
34.40
40.88
(26.4)
–
–
–
–
–
–
–
–
–
–
–
26.38
26.89
34.58
41.09
(26.1)
–
26.48
27.16
(7.0)
27.31
(11.6)
29.28
(8.4)
30.76
(17.0)
∼34.5e
–
–
–
–
–
–
–
–
–
–
–
–
–
22.04
(24.9)
–
–
–
–
–
–
19.33
(11.1)
20.21
(18.7)
24.10
(12.7)
125.08
128.39
–
130.11
(5.1)
132.35
(2.2)
134.54
(18.9)
144.76
(26.6)
–
21.45
(22.0)
–
–
–
–
–
22.29
(22.2)
–
24.53
(27.4)
–
–
–
–
–
–
21.18
(14.3)
22.96
(28.6)
25.50
(13.8)
128.96
129.73
–
22.61
–
–
21.35
20.74
(22.2)
20.94
–
149.67
(5.0)
–
–
(21.4)
21.65
151.77
–
26.36
27.05
(9.2)
27.16
(12.9)
28.97
(7.3)
30.33
(14.4)
∼33.7d
–
26.36
27.04
(8.4)
26.87
(13.0)
30.49
(10.7)
33.54
(24.8)
35.84
(13.2)
129.25
129.74
–
–
125.49
(4.2)
155.65
(25.4)
142.60
(16.0)
133.07
(14.6)
134.70
131.83
–
132.07
(4.2)
134.51
–
137.37
–
140.40
(26.7)
–
–
–
–
–
–
–
–
19.28
(11.1)
20.28
(19.3)
24.17
(12.6)
124.90
128.65
–
125.94
(3.4)
132.23
–
133.04
(16.6)
154.73
(23.2)
–
–
–
–
–
19.73
(20.6)
–
127.83
(16.5)
–
–
–
145.80
(17.4)
131.14
131.01
–
130.54
(5.9)
133.72
–
136.16
–
140.02
(27.4)
–
25.38
(8.0)
125.75
128.23
–
129.94
(4.6)
131.54
–
136.57
(14.2)
142.97
(25.6)
–
1JCP
C5
125.28
129.04
(3.0)
130.48
(4.8)
132.88
–
124.92
128.74
–
C4
4JCP
C3
126.05
(5.2)
132.69
–
129.25
(5.2)
124.47
–
129.31
–
3JCP
C6
124.52
–
2JCP
C1
∼132.9e
∼132.7e
∼137.0d
∼136.5d
1JCP
C2
–
–
–
–
145.05
(24.2)
155.05
(23.6)
∼145.0d
–
∼146.1d
–
2JCP
C7
–
–
–
–
–
–
–
–
–
–
–
–
125.58
127.13
(9.7)
132.16
(26.9)
132.80
125.64
∼128.9d
–
C8
–
–
–
–
–
–
–
–
–
–
–
–
3JCP
C9
∼129.7e
2JCP
C10
–
–
–
–
132.69
–
a Resonance peaks are singlets if no coupling constant are reported.
b Carbon atoms are numbered as in Scheme 1.
c Carbon atoms in pairs 8∗, 8∗∗ and 9∗, 9∗∗ are diastereotopic and thus non-equivalent with respect to P–C couplings and chemical
shifts.
d Chemical shift can not be interpreted precisely because the resonance peak is broad and thus it is also impossible to interpret the
multiplicity.
e Resonance peak can not be interpreted precisely because of overlapping and thus it is also impossible to interpret the multiplicity.
(2,5-Dimethylphenyl)bis(3-pyridyl)phosphane (8) was
recorded at room temperature in acetone-d6 (D
99.9%) because of solubility problems. 1H-NMR:
CD3COCD3 set to 2.05 ppm and 13C{1H}-NMR:
CD3COCD3 set to 30.5 ppm. 13C{1H}-NMR shifts
of free ligands are presented in Table 1. Exact mass
peaks of the free ligands were determined on a Micro-
mass LCT, using an ESI+ method. IR measurements
of the metal complexes were done with a Nicolet 750
spectrometer in THF. The X-ray diffraction data were
collected with a Nonius KappaCCD diffractometer.
2.1. Synthetic procedure for arylalkylphosphanes
A solution of n-butyllithium was transferred drop-
wise via a canula to a freshly prepared solution of

H. Riihimäki et al. / Journal of Molecular Catalysis A: Chemical 200 (2003) 81–94
85
1-bromo-2-alkylbenzene or 1-bromo-2-methylnaph-
thalene in diethyl ether (30 ml) at −10 → 0 ◦C (salted
ice bath). After stirring for 2 h at −10 → 0 ◦C, a
solution of chlorodialkyl- or dichloroalkyl-phosphane
in diethyl ether (25 ml) was slowly added to the bright
mixture of ortho-substituted lithiated aromatics. Stir-
ring was continued for a further 2 h at −10 → 0 ◦C.
After slow warming to room temperature, some solid
material precipitated (inorganic salts). Solid and liq-
uid layers were separated by filtration and the solvent
was removed in vacuo. A crude product was obtained
from the solvent phase and purified by recrystalliza-
tion (from ethanol or hexane) or by column chro-
matography using dichloromethane/hexane (1:2) as
an eluent. The pure product was a bright viscous oil
or a white solid.
(400 MHz, CDCl3, see Scheme 1 for numbering) δH
3
3
(ppm): 0.89 (dd, JHH = 7.2 Hz, JHP = 11.6 Hz,
H8∗, 6 H), 1.14 (dd, 3JHH = 7.2 Hz, 3JHP = 14.8 Hz,
H8∗∗, 6 H), 1.20–1.30 (m, H0–4, 2 H), 1.36–1.48 (m,
H0–2, 4 H), 1.78 (m, H0–3, 4 H), 2.08 (dsep, 3JHH
=
2
7.0 Hz, JHP = 2.0 Hz, H7∗, 2 H), 3.68 (m, H0–1, 1
H), 7.14 (m, H5, 1 H), 7.27–7.30 (m, H3 and H4, 2 H),
7.38 (m, H6, 1 H). 31P{1H} NMR (161 MHz, CDCl3)
δP (ppm): −4.4. Exact mass (Micromass LCT, ESI+):
277.2060 (M + H)+ (calcd. for C18H30P, 277.2085).
2.1.3. Bis(2-methylphenyl)isopropylphosphane (3)
Reactions of 2-bromotoluene (11.8 g, 4.2 ml,
34.5 mmol), n-butyllithium (13.8 ml, 34.5 mmol)
and dichloroisopropylphosphane (2.5 g, 2.1 ml, 17.2
mmol) afforded a bright oily product, which was dis-
solved in ethanol and filtered over silica. The yield
of the pure bright oily product was 3.6 g, 14.0 mmol,
81%. 1H NMR (400 MHz, CDCl3, see Scheme 1
2.1.1. (2-Methylphenyl)diisopropylphosphane (1)
Following the procedure described above, reac-
tions of 2-bromotoluene (3.4 g, 2.4 ml, 20.0 mmol),
n-butyllithium (8.0 ml, 20.0 mmol) and chlorodiiso-
propylphosphane (3.1 g, 3.2 ml, 20.0 mmol) afforded a
crude yellow oily product. The general procedure was
followed except the mixture was stirred over night
to complete reaction. The crude yellow oily prod-
uct was purified by column chromatography using
dichloromethane/hexane (1:2) as an eluent. The yield
of the pure bright oily product was 1.5 g, 7.1 mmol,
35%. 1H NMR (200 MHz, CDCl3, see Scheme 1
3
for numbering) δH (ppm): 1.11 (dd, JHH = 6.8 Hz,
3JHP = 15.9 Hz, H8∗, 6 H), 2.40 (m, H7∗, 1 H), 2.45 (s,
H0–1, 6 H), 7.10–7.20 (m, H3, H4 and H5, 6 H), 7.30
(m, H6, 2 H). 31P{1H} NMR (161 MHz, CDCl3) δP
(ppm): −22.3. Exact mass (Micromass LCT, ESI+):
257.1432 (M + H)+ (calcd. for C17H22P, 257.1459).
2.1.4. (2-Methylphenyl)dicyclohexylphosphane (4)
Reactions of 2-bromotoluene (2.6 g, 1.8 ml,
15.0 mmol), n-butyllithium (6.0 ml, 15.0 mmol) and
chlorodicyclohexylphosphane (3.5 g, 3.3 ml, 15.0
mmol) afforded a white solid product, which was re-
crystallized from ethanol. The yield of the pure white
3
for numbering) δH (ppm): 0.91 (dd, JHH = 7.0 Hz,
3JHP = 11.6 Hz, H8∗, 6 H), 1.12 (dd, 3JHH = 7.0 Hz,
3JHP = 14.8 Hz, H8∗∗, 6 H), 2.11 (dsep, JHH
=
3
7.0 Hz, 2JHP = 2.0 Hz, H7∗, 2 H), 2.57 (s, H0–1, 3 H),
7.16 (m, H5, 1 H), 7.18–7.23 (m, H3 and H4, 2 H),
7.37 (m, H6, 1 H). 31P{1H} NMR (161 MHz, CDCl3)
δP (ppm): −4.8. Exact mass (Micromass LCT, ESI+):
209.1466 (M + H)+ (calcd. for C13H22P, 209.1459).
solid product was 3.6 g, 12.4 mmol, 82%. H NMR
1
(400 MHz, CDCl3, see Scheme 1 for numbering) δH
(ppm): 1.00–2.00 (m, H7∗, H8∗, H8∗∗, H9∗, H9∗∗
,
H10∗, 22 H), 2.56 (s, H0–1, 3 H), 7.15–7.25 (m, H3,
H4 and H5, 3 H), 7.38 (m, H6, 1 H). 31P{1H} NMR
(161 MHz, CDCl3) δP (ppm): −11.6. Exact mass
(Micromass LCT, ESI+): 289.2084 (M + H)+ (calcd.
for C19H30P, 289.2085).
2.1.2. (2-Cyclohexylphenyl)diisopropylphosphane
(2)
Reactions of 1-bromo-2-cyclohexylbenzene (2.8 g,
11.7 mmol), n-butyllithium (4.7 ml, 11.7 mmol)
and chlorodiisopropylphosphane (1.8 g, 1.9 ml,
11.7 mmol) afforded a crude muddy and oily product.
The bright oily product was obtained by washing the
crude product with hexane several times; a white solid
impurity was precipitated. The yield of the pure bright
oily product was 2.7 g, 9.9 mmol, 84%. 1H NMR
2.1.5. (2-Cyclohexylphenyl)dicyclohexylphosphane
(5)
Reactions of 1-bromo-2-cyclohexylbenzene (5.1 g,
21.5 mmol), n-butyllithium (8.6 ml, 21.5 mmol) and
chlorodicyclohexylphosphane (5.0 g, 4.7 ml, 21.5
mmol) afforded a white solid product, which was re-
crystallized from ethanol. The yield of the pure white

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1
solid product was 5.8 g, 16.4 mmol, 76%. H NMR
(400 MHz, CDCl3, see Scheme 1 for numbering) δH
(ppm): 1.00–1.95 (m, H0–2, H0–3, H0–4, H7∗, H8∗,
H8∗∗, H9∗, H9∗∗, H10∗, 32 H), 3.66 (m, H0–1, 1 H),
7.14 (m, H5, 1 H), 7.27–7.31 (m, H3 and H4, 2 H), 7.40
(m, H6, 1 H). 31P{1H} NMR (161 MHz, CDCl3) δP
(ppm): −14.6. Exact mass (Micromass LCT, ESI+):
357.2687 (M + H)+ (calcd. for C24H38P, 357.2711).
2.2. Synthetic procedure for pyridylphosphanes
(8) and (9)
n-Butyllithium (12.0 ml, 30 mmol) was slowly add-
ed to dry TMEDA (4.6 ml, 30 mmol). The mixture
was stirred for 15 min at room temperature and then
cooled to −70 ◦C when cold ether (−70 ◦C, 75 ml) was
added. This mixture was then cooled to −115 ◦C and
bromopyridine (30 mmol, Aldrich, 99%) in diethyl
ether (30 ml) was added dropwise. The mixture was
stirred around 5 min, and then dichloro(2,5-dimethyl-
phenyl)phosphane (10 mmol) was added [23]. After
30 min, the rest of dichloro(2,5-dimethylphenyl)phos-
phane (5 mmol) was added, and the yellow mixture
was stirred for 2.5 h at −100 ◦C, before warming to
room temperature overnight. The mixture was ex-
tracted with H2SO4 (2 M), and the aqueous layer was
separated and made alkaline with NaOH. The white
or light yellow solid product was obtained by extrac-
tion of the aqueous phase with diethyl ether. After
extraction, the diethyl ether was removed in vacuo.
The crude product was purified by column chro-
matography using dichloromethane/hexane/methanol
(10:3:1) as an eluent. The yield of the pure white
solid (2,5-dimethylphenyl)bis(3-pyridyl)phosphane
2.1.6. (2-Methylnaphthyl)diisopropylphosphane (6)
Reactions of 1-bromo-2-methylnaphthalene (4.2 g,
2.9 ml, 18.9 mmol), n-butyllithium (7.6 ml, 18.9 mmol)
and chlorodiisopropylphosphane (2.9 g, 3.0 ml,
18.9 mmol) afforded a crude reddish brown oily
product. The crude product was purified by column
chromatography using dichloromethane/hexane (1:2)
as an eluent. The yield of the pure bright oily prod-
1
uct was 2.0 g, 7.8 mmol, 41%. H NMR (400 MHz,
CDCl3, see Scheme 1 for numbering) δH (ppm): 0.73
(dd, 3JHH = 6.8 Hz, 3JHP = 13.8 Hz, H8∗, 6 H), 1.32
3
3
(dd, JHH = 6.8 Hz, JHP = 17.2 Hz, H8∗∗, 6 H),
2.73 (broad, H7∗, 2 H), 2.82 (broad, H0–1, 3 H), 7.29
3
4
(dd, JHH = 8.2 Hz, JHP = 3.4 Hz, H3, 1 H), 7.37
3
3
(t, JHH = 7.4 Hz, H6, 1 H), 7.45 (t, JHH = 7.6 Hz,
H7, 1 H), 7.70 (d, JHH = 8.4 Hz, H4, 1 H), 7.77
3
3
1
(d, JHH = 8.0 Hz, H5, 1 H), 8.24 (broad, H8, 1
(8) was 1.3 g, 4.5 mmol, 30%. H NMR (400 MHz,
H). 31P{1H} NMR (161 MHz, CDCl3) δP (ppm):
6.6. Exact mass (Micromass LCT, ESI+): 259.1632
(M + H)+ (calcd. for C17H24P, 259.1616).
CD3COCD3, see Scheme 1 for numbering) δH (ppm):
2.16 (s, Hm−1, 3 H), 2.34 (s, H0–1, 3 H), 6.64 (m,
H6, 1 H), 7.19 (m, H3, H4, 2 H), 7.42 (m, H9∗∗, 2
H), 7.61 (m, H8∗∗, 2 H), 8.45 (m, H8∗, 2 H), 8.61
(m, H10∗, 2 H). 31P{1H} NMR (161 MHz, CDCl3)
δP (ppm): −25.0. Exact mass (Micromass LCT,
ESI+): 293.1183 (M + H)+ (calcd. for C18H18N2P,
293.1208). The yield of the pure bright solid (2,5-
dimethylphenyl)bis(4-pyridyl)phosphane (9) was
2.1.7. (2-Methylnaphthyl)dicyclohexylphosphane (7)
Reactions of 1-bromo-2-methylnaphthalene (3.2 g,
2.2 ml, 14.3 mmol), n-butyllithium (5.7 ml, 14.3 mmol)
and chlorodicyclohexylphosphane (3.3 g, 3.2 ml,
14.3 mmol) afforded a crude yellowish oily product.
The crude product was purified by column chro-
matography using dichloromethane/hexane (1:2) as an
eluent. The yield of the pure bright oily product was
1
0.5 g, 1.8 mmol, 12%. H NMR (400 MHz, CDCl3,
see Scheme 1 for numbering) δH (ppm): 2.19 (s,
Hm−1, 3 H), 2.40 (s, H0–1, 3 H), 6.59 (m, H6, 1 H),
7.11–7.20 (m, H3, H4, H8∗, 6 H), 8.59 (m, H9∗, 4 H).
31P{1H} NMR (161 MHz, CDCl3) δP (ppm): −15.7.
Exact mass (Micromass LCT, ESI+): 293.1220
(M + H)+ (calcd. for C18H18N2P, 293.1208).
1
3.2 g, 9.4 mmol, 66%. H NMR (400 MHz, CDCl3,
see Scheme 1 for numbering) δH (ppm): 0.88–2.10 (m,
H8∗, H8∗∗, H9∗, H9∗∗ and H10∗, 20 H), 2.52 (m, H7∗,
2 H), 2.84 (s, H0–1, 3 H), 7.31 (d, JHH = 7.6 Hz,
3
3
H3, 1 H), 7.40 (t, JHH = 7.4 Hz, H6, 1 H), 7.48 (t,
3JHH = 7.6 Hz, H7, 1 H), 7.72 (d, JHH = 8.4 Hz,
3
2.3. Synthetic procedure for Rh(acac)(CO)(PR3)
complexes
3
H4, 1 H), 7.79 (d, JHH = 7.2 Hz, H5, 1 H), 8.19
(broad, H8, 1 H). 31P{1H} NMR (161 MHz, CDCl3)
δP (ppm): −8.1. Exact mass (Micromass LCT, ESI+):
339.2251 (M + H)+ (calcd. for C23H32P, 339.2242).
(Acetylacetonato)dicarbonylrhodium(I)
0.19 mmol) and the phosphane ligand (0.19 mmol)
(50 mg,

H. Riihimäki et al. / Journal of Molecular Catalysis A: Chemical 200 (2003) 81–94
87
were dissolved in a minimum amount of tetrahy-
drofuran in separate flasks. The solutions containing
equimolar amounts of Rh(acac)(CO)2 and phos-
phane (PR3) were combined and strirred at room
temperature overnight. After that, the solution was
evaporated to dryness in vacuum. Rh(acac)(CO)(3):
IR: υ(CO) = 1971 cm−1
,
31P{1H } NMR: δP
173 Hz. Rh(acac)(CO)(4):
31P{1H} NMR: δP
=
1
54.6 ppm, JP−Rh
=
IR: υ(CO) = 1965 cm−1
,
=
1
47.0 ppm, JP−Rh = 172 Hz, yellow crystals for
X-ray studies were grown from dichloromethane.
Rh(acac)(CO)(5): IR: υ(CO) = 1962 cm−1
,
31P{1H}
1
NMR: δP = 44.1 ppm, JP−Rh = 171 Hz. Attempts
to crystallize Rh(acac)(CO)(3) and Rh(acac)(CO)(5)
species for further studies were not successful, how-
ever.
2.4. Computational details
Gaussian 94 [24] and Sybyl [25] programs were
used in modeling. The initial estimation for phos-
phanes was built on the assumption that the ortho
substituent was situated outside the cone, otherwise
no special geometry around the phosphorus was used.
Ligand structures (Scheme 2) were optimized on
the HF level of theory using the 3-21G∗ basis set.
For the cone angle measurements, the metal(dummy
atom)-phosphorus distance 2.28 Å and van der Waals’
radius of hydrogen 1.2 Å were used. Initial esti-
mations of the geometries of Rh(acac)(CO)(PR3)
species were based on the solid state structure of
Rh(acac)(CO)2 [15,26]. Geometrical optimization
of complexes Rh(acac)(CO)(3), Rh(acac)(CO)(4)
and Rh(acac)(CO)(5) (Scheme 3) were carried out
with the B3PW91 method using the 6-31G∗ basis
set.
Scheme 2. Gas-phase structures of free ligand; 2, 5–7, 3 and 8;
and their calculated cone angles.
[31] was applied to Rh(acac)(CO)(PR3) (Tmax/Tmin
=
0.17355/0.10553). Structural refinements were carried
out with the SHELXL97 program [29]. The crystal-
lographic data for structures 5 (Fig. 1), 9 (Fig. 2) and
Rh(acac)(CO)(4) (Fig. 3): are summarized in Table 2
and selected bond lengths and angles in Table 3. Crys-
tallographic data (excluding structural factors) for the
structure reported in this paper have been consigned to
the Cambridge Crystallographic Data Centre as sup-
plementary publication no. 191946–191948. Copies
of the data can be obtained free of charge by request
to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(fax: +44-1223-336-033; e-mail: deposit@ccdc.cam.
ac.uk).
2.5. X-ray crystallography
The X-ray diffraction data were collected with
a Nonius KappaCCD diffractometer using Mo K␣
radiation (λ = 0.71073 Å) with a Collect [27] data
collection program. The Denzo and Scalepack [28]
were used for cell refinements and data reduction.
All structures were solved by direct methods using
SHELXS97 [29] and the WinGX [30] graphical user
interface. A multi-scan absorption correction based on
equivalent reflections (XPREP in SHELXTL v. 5.1)

88
H. Riihimäki et al. / Journal of Molecular Catalysis A: Chemical 200 (2003) 81–94
Scheme 3. Gas-phase structures of complexes Rh(acac)(CO)(3), Rh(acac)(CO)(4) and Rh(acac)(CO)(5).
2.6. Hydroformylation of propene and 1-hexene
sion, selectivity, and i/n ratios were calculated on a
molar basis. Conversion was calculated with respect
to propene (1-hexene) and selectivity with respect to
aldehydes. The i/n ratio of the aldehydes was defined
as the amount of branched product divided by the
amount of linear product.
Propene hydroformylation: Experiments were car-
ried out in a 250-ml autoclave (Berghof) equipped
with a sampling system and a 230-ml Teflon liner.
The experiments were done in semibatch mode
so that the synthesis gas pressure was kept con-
stant during the experiment. The rhodium precursor
was Rh(NO3)3 (Fluka). In a typical experiment,
The two following procedures were the same for
the two alkenes: A disposable inner Teflon reactor
was used to avoid the accumulation of rhodium on
the reactor walls. The products were analyzed with
a Hewlett Packard 5890 GC equipped with a capil-
lary column (HP-1, 1.0 ␮m × 0.32 mm × 60 m) and
a flame-ionization detector. Products were quantified
by the internal standard method. In addition, the alde-
hydes were identified by GC-MS analysis. Conver-
Fig. 1. X-ray crystal structure of (2-cyclohexylphenyl)dicyclo-
Fig. 2. X-ray crystal structure of (2,5-dimethylphenyl)bis-
hexylphosphane (5).
(4-pyridyl)phosphane (9).

H. Riihimäki et al. / Journal of Molecular Catalysis A: Chemical 200 (2003) 81–94
89
the autoclave was charged with the rhodium pre-
cursor (0.02 mmol calculated as rhodium), acetone
(310 mmol, Merck, >99%), internal standards decane
(7 mmol, Fluka, >98%) and hexane (12 mmol, Riedel
de Häen, >99%), and the phosphane. In the case of
(2-methylphenyl)dicyclohexylphenylphosphane (4)
and (2,5-dimethylphenyl)bis(4-pyridyl)phosphane (9),
THF was used instead of acetone because of solubility
problems. The result of (2,5-dimethylphenyl)bis(3-
pyridyl)phosphane (8) may not be directly compa-
rable with other modified ligands because of differ-
ent device adjustments. If not otherwise stated, the
ligand-to-rhodium ratio was 10:1 on a molar basis.
The system was flushed with nitrogen, and pressurized
with propene (2 bar, Aga, 99.8%), heated to the re-
action temperature (100 ◦C) with continuous stirring,
and then pressurized to the reaction pressure (10 bar)
with a 1:1 molar ratio of H2 and CO (MG, 99.997%).
Fig. 3. X-ray crystal structure of complex Rh(acac)(CO)(4).
Table 2
Crystallographic data for structures 5, 9 and Rh(acac)(CO)(4)
5
9
Rh(acac)(CO)(4)
Empirical formula
Fw (g/mol)
Colour, habit
Crystal size (mm)
Temperature (K)
Crystal system
Space group
a (Å)
C24
356.51
Bright
H
37P
C18
292.31
Bright
H
17N2P
C25H37O3PRh
519.43
Yellow
0.40 × 0.40 × 0.30
0.20 × 0.20 × 0.20
0.20 × 0.20 × 0.20
150 (2)
150 (2)
120 (2)
Monoclinic
P2/c
Triclinic
¯
P1
Monoclinic
P21/n
10.0838 (2)
13.1270 (3)
18.3891 (4)
90
90.2814 (8)
90
2434.14 (9)
4
9.1387 (2)
12.4453 (3)
18.9759 (5)
90
96.6355 (9)◦
90
8.8204 (2)
9.9813 (2)
10.5157 (3)
107.0300 (10)
111.6510 (10)◦
104.928 (2)◦
749.87 (3)
2
b (Å)
c (Å)
α (◦)
β (◦)
γ (◦)
Volume (Å3)
Z
2143.75 (9)
4
ρ (calcd.) (g/ml)
1.105
0.132
1.295
0.178
1.417
0.790
µ (mm−1
)
Theta range
Index ranges
2.71–25.50
−11 ≤ h ≤ 11, −15
≤ k ≤ 15, −22 ≤ l
≤ 22
4.18–26.41
−11 ≤ h ≤ 10, −12
≤ k ≤ 12, −13 ≤ l
≤ 13
2.77–26.00
−12 ≤ h ≤ 10, −15
≤ k ≤ 16, −20 ≤ l
≤ 22
No. collected reflections
No. unique reflections
Rint
7708
5445
16017
3979
0.0202
3018
0.0191
4771
0.0653
R1 (I ≥ 2σ)
0.0396
0.0931
0.1008
1.045
0.0377
0.0936
0.0989
1.062
0.0473
0.1094
0.1107
1.227
wR2 (I ≥ 2σ)
wR2 (all reflections)
Goof

90
H. Riihimäki et al. / Journal of Molecular Catalysis A: Chemical 200 (2003) 81–94
Table 3
ortho-alkyl-substituted aryl rings were combined with
alkyl or pyridyl groups instead of phenyl rings. In the
preparation of pyridylphosphanes 8 and 9, we used
a combination of low reaction temperatures and the
addition of N,N,Nꢀ,Nꢀ-tetramethylethylenediamine
(TMEDA) to prevent addition/coupling reactions dur-
ing the formation of the pyridyllithium and also to
promote a greater reactivity of the pyridyllithium
towards the chlorophosphane [22]. Despite this
the yields remained rather low especially that of
(4-pyridyl)phosphane (12%). Reasonably good yields
(76–84%) were obtained for the other ligands stud-
ied except for (2-methylphenyl)diisopropylphosphane
(35%). The technical grade of 1-bromo-2-methylna-
phthalene also lowered the yield of naphthylphos-
phane ligands (44 and 66%). The ligands are presented
in Scheme 1.
Selected bond lengths (Å) and angles (◦) for structures 5, 9 and
Rh(acac)(CO)(4)
5
9
Rh(acac)(CO)(4)
Bond
P(1)-C(1)
P(1)-C(7)
1.865(2)
1.857(2)
1.855(2)
1.520(2)
1.832(2)
1.836(2)
1.835(2)
1.844(4)
1.848(4)
1.832(4)
1.509(7)
P(1)-C(13)
C(18)-C(19)
C(8)-C(19)
C(11)-C(20)
P(1)-Rh(1)
Rh(1)-O(1)
Rh(1)-O(3)
Rh(1)-C(25)
C(25)-O(2)
1.511(2)
1.508(2)
2.2510(10)
2.052(3)
2.084(3)
1.795(5)
1.158(5)
Angle
C(1)-P(1)-C(7)
103.71(7) 103.35(7) 105.0(2)
102.52(7) 100.90(7) 102.9(2)
102.42(7) 102.68(7) 105.4(2)
C(1)-P(1)-C(13)
C(7)-P(1)-C(13)
C(25)-Rh(1)-P(1)
O(1)-Rh(1)-C(25)
O(3)-Rh(1)-P(1)
Rh(1)-C(25)-O(2)
3.2. Characterization of free ligands
89.50(13)
177.25(15)
175.83(9)
179.1(4)
The structural characterization of the ligands was
done by NMR methods. Generally, assignment of
the 13C{1H} and 1H chemical shifts were possi-
ble. However, due to the overlapping of the 1H
resonances’ multiplicities of cyclohexyl groups, sub-
stituted phenyl rings and pyridyl rings, the exact peaks
remained uninterpreted. The unconfirmed 1H NMR
shifts were assigned on the basis of the information
from two-dimensional NMR spectra: H,C-correlated
HSQC or C,H-correlated HETCOR, H,H-correlated
COSY 90 and long-range C,H-correlated COLOC.
Two-dimensional spectra were also necessary in the
assignment of the 13C{1H} chemical shifts of car-
bons 3–5 (Table 1, Scheme 1) and the carbons of the
naphthyl ring, dicyclohexyl and diisopropyl groups.
According to previous literature, magnetic non-
equivalency of protons arises when a group is at-
tached to a center that is asymmetric with respect
to the group, although not necessarily with respect
to the molecule as a whole, and is manifested as
a chemical shift difference between protons that
appear to be chemically equivalent [32–34]. The
protons may be non-equivalent even if the energy
barrier to rotation about the P–C bond is low [32].
We have made the same observation for the o-alkyl
substituted diisopropylphosphanes (1, 2, 6) and di-
cyclohexylphosphanes (4, 5, 7) studied. In these
diisopropylphosphanes and dicyclohexylphosphanes,
At least nine samples were taken for analysis in each
experiment: one of the fresh reaction, one immedi-
ately after pressurizing with H2 and CO, which was
considered as the starting point of the reaction, six
during the experiment, and one after the reaction.
1-Hexene hydroformylation: Experiments were con-
ducted in a 100-ml autoclave (Berghof) with a 60-ml
Teflon liner. The experiments were done in batch mode
with rhodium precursor Rh(acac)(CO)2. The reactor
was charged under a nitrogen purge with substrate,
rhodium catalyst, ligand (L/Rh 5), and internal stan-
dard, cyclohexane. The autoclave was then sealed and
pressurized using a 1:1 mixture of H2 and CO (MG,
99.997%) to 20 bars and heated at reaction tempera-
ture 100 ◦C for 4 h; the autoclave was then cooled and
brought to normal atmospheric pressure.
3. Results and discussion
3.1. Synthesis of free ligands
All the ligands prepared had these essential fea-
tures. The ortho-substituents in the aromatic parts
were changed (methyl or cyclohexyl), and the

H. Riihimäki et al. / Journal of Molecular Catalysis A: Chemical 200 (2003) 81–94
91
the two alkyl groups are equivalent to each other,
and the carbon atoms that are symmetrically ar-
ranged around the plane of symmetry of each alkyl
unit are diastereotopic. The experimental results
(Table 1) of ligands 1, 5 and 6 COLOC-NMR spectra
show a long-range 13C-1H (3JCH) coupling between
non-equivalent carbons 8∗ and 8∗∗ and also 9∗ and 9∗∗,
which is possible only if the two nuclei belong to the
same alkyl group. Assignment of 13C chemical shifts
of the isopropyl and cyclohexyl carbons agree with
the shielding order δ(C7∗) > δ(C8∗), δ(C8∗∗) > δ(C9∗),
δ(C9∗∗) > δ(C10∗) [33]. Comparison of 31P-13C cou-
pling constants reveals that the diastereotopic carbons
are also non-equivalent with respect to P–C coupling
constants [32–34]. Both the JCP and JCP values of
the bond of carbons 8∗∗ and 9∗∗ are considerably
larger than those of carbons 8∗ and 9∗. According
to previous literature, the two bonds have the same
dihedral angle with the P–C bond, only the different
average distance to the phosphorus lone pair controls
the magnitude of these couplings [33,34]. The phos-
phorus lone pair is much closer in motional average
to the 8∗∗ and 9∗∗ bond than to the 8∗ and 9∗ bond
in these compounds [33].
lphosphanes (1, 2 and 6) led to opposite confor-
mations where ortho-alkyl-substituents were located
inside the cone. The geometrical arrangement of
(2,5-dimethylphenyl)bis(3-pyridyl)phosphane (8) re-
sembled dicyclohexylphosphanes. The ortho-methyl-
substituent lay outside the cone (Scheme 2).
Measurements of the coordinated ligands gave
lower cone angle values than the free ligands. The
difference was largest in the case of the bis(2-methyl-
phenyl)isopropylphosphane (3) ligand. In the free
ligand state, the isopropyl group bent inside the cone
leaving both methyl groups outside the cone. In the
coordinated state, one o-methyl substituent was very
close to the rhodium center. Consequently, the other
methyl group rotated inside the cone, at the same
time, the isopropyl group was rotated partly away
from the cone. There was a huge difference between
the cone angles of the coordinated (163◦) and free
(198◦) states.
In the case of 4 and 5 ligands, methyl and cyclo-
hexyl substituents were similarly bent outside the cone
towards rhodium in the free and coordinated state. The
substituted benzene ring was only rotated a bit, leaving
space for alkyl substituents as the P–Rh bond formed.
Consequently, the cone angles of Rh(acac)(CO)(4) and
Rh(acac)(CO)(5) complexes were around 10◦ smaller
than those of the corresponding free ligands.
2
3
3.3. Synthesis of complexes
Reaction between equimolar amounts of Rh(acac)-
(CO)2 and phosphanes (PR3) afforded rhodium
complexes Rh(acac)(CO)(3), Rh(acac)(CO)(4) and
Rh(acac)(CO)(5). The isolation of pure complexes
was difficult, since the product seems to be in equi-
librium with the starting materials. Consequently,
only the synthesis of Rh(acac)(CO)(4) complex gave
a few single crystals after several straight purification
operations.
3.5. Crystal structures of complexe Rh(acac)(CO)(4)
and ligands 5 and 9
The structures of both ligands, 5 and 9, and complex
Rh(acac)(CO)(4) were determined by single-crystal
X-ray diffraction analysis. Crystalographic data are
summarized in Table 2 and selected bond lengths and
angles are given in Table 3. The crystal structures are
shown in Figs. 1–3.
3.4. Ab initio calculations of free ligands
and complexes
The conformation of (2-cyclohexylphenyldicyclo-
hexylphosphane (5) given by ab initio Hartree–Fock
calculation (Scheme 1) was similar to that ob-
tained by the X-ray crystal structure measurement
(Fig. 1), where the cyclohexyl substituent was lo-
cated outside the cone. In addition, the conforma-
tion of (2,5-dimethylphenyl)bis(3-pyridyl)phosphane
(8) (Scheme 2) obtained by using the ab initio
Hartree–Fock calculation resembled the X-ray crystal
structure of (2,5-dimethylphenyl)bis(4-pyridyl)phos-
phane (9) (Fig. 2). Therefore, ab initio Hartree–Fock
The cone angles of the free ligands are given
in Scheme 1. Compared with triphenylphosphane
(149◦), the cone angles increased from 165◦ for 1 to
211◦ for 5. Owing to the higher steric repulsion in-
duced by cyclohexyl rings bonded to the phosphorus
atom directly (4, 5 and 7), conformations resulted in
which the ortho-substituents were located outside the
cone (see Scheme 2). Optimization of diisopropy-

92
H. Riihimäki et al. / Journal of Molecular Catalysis A: Chemical 200 (2003) 81–94
Table 4
Results for hydroformylation of propene by rhodium catalysts with reported phosphane ligands
Ligand
θ (◦)
δP (ppm)
X (%)
Initial rate mol/(molRh s)
Sisobutanal (%)
i/n
1
2
3
4a
5
6
7
8b
9a
PPh3
165
172
198
181
211
199
185
181
–
−4.8
−4.4
52
47
22
5
35
23
22
50
53
98
17
15
5
0
12
5
46
48
52
46
49
35
33
47
48
36
0.9
0.9
1.1
0.9
1.0
0.5
0.5
0.9
0.9
0.6
−22.3
−11.6
−14.6
6.6
−8.1
5
−25.0
−15.7
−3.3
15
12
45
149
Reaction conditions: 10 bar (CO/H2 = 1), T = 100 ◦C, X: conversion = 2 h, propene/Rh = 512, L/Rh = 10. Initial rate for aldehyde
formation.
a THF was chosen as a solvent, because of solubility problems. PPh3: X(2 h) = 81%, ri = 37, Si = 36%, THF as solvent.
b Different adjustments of the device. PPh3: X(2 h) = 100%, ri = 55, Si = 37%.
calculations were performed for all the ligands
studied.
of the coordinated state, and thus assisted in the
prediction of catalytic activity and selectivity trends.
The X-ray crystal structure of Rh(acac)(CO)(4)
(Fig. 3) was in agreement with the ab initio calcula-
tion of the species (Scheme 3). The bond lengths of
the X-ray crystal structure and the calculated structure
were almost identical: Rh–P bond length, 2.25 Å for
crystal structure and 2.30 Å for calculated structure,
Rh–C, 1.80 Å for crystal structure and 1.82 Å for cal-
culated structure and the measured C–O bond length
was 1.16 Å for both structures. The C–Rh–P angles
obtained by using single-crystal X-ray diffraction
analysis (89.50◦) and by using ab initio calculations
(95.36◦) were in the same range. The calculated com-
plex structures containing corresponding phosphane
ligands seemed to give a reliable qualitative picture
3.6. Hydroformylation of propene and 1-hexene
Results from the hydroformylation of propene and
1-hexene are displayed in Tables 4 and 5. The widely
used triphenylphosphane was chosen as a reference
ligand because in the hydroformylation of propene the
unmodified rhodium catalyst had very low initial activ-
ity and conversion even though isobutanal was formed
in a roughly equal amount with n-butanal [5].
In the case of 1-hexene hydroformylation, high
isomerization activity (>50%) was associated with all
ligands, and some hydrogenation activity (<20%) was
found as well. In every case, the major isomerization
Table 5
Results for hydroformylation of 1-hexene by rhodium catalysts with reported phosphane ligands
Ligand
θ (◦)
δP (ppm)
X (%)
S(2−ep) (%)
S(2−mh) (%)
S(1−hep) (%)
i/n
3
4
5
6
7
8
198
181
211
199
181
181
149
–
−22.3
−11.6
−14.6
6.6
−8.1
−25.0
−3.3
–
98
98
98
99
98
98
96
99
1
0
3
5
2
3
0
5
6
6
10
15
7
12
4
15
9
10
12
15
9
14
7
16
0.8
0.6
1.1
1.3
0.9
1.1
0.5
1.2
PPh3
No ligand
Reaction conditions: 20 bar, T = 100 ◦C, X = 4 h, 1-hexene 15.5 mmol, Rh(acac)(CO)2 1.55 × 10−6 mol, L/Rh = 5. S(2−ep): selectivity
to 2-ethylpentanal; S(2−mh): selectivity to 2-methylhexanal; S(1−hep): selectivity to 1-heptanal.

H. Riihimäki et al. / Journal of Molecular Catalysis A: Chemical 200 (2003) 81–94
93
product was 2-hexene. Nevertheless, the tested ligands
1–8 gave higher total aldehyde selectivity and sim-
ilar conversions comparable with the PPh3-modified
reaction. The main products were 1-heptanal and
2-methylhexanal, respectively.
lower initial rates compared with the corresponding
(2,5-dimethylphenyl)diphenylphosphane although the
pyridylphosphane-modified complexes are known to
obtain higher reaction rates than their phenyl ana-
logues [15,19,20].
Generally, the modified phosphanes prepared gave
considerably higher i/n ratios compared with triph-
enylphosphane. i-Selectivity was slightly higher as
the size of the o-alkyl-substituent became larger
with the structure maintained otherwise unmodi-
fied. In propene hydroformylation, the enhancement
was 2% between diisopropylphosphanes 1 and 2,
and in 1-hexene hydroformylation, 4% between di-
cyclohexylphosphanes 4 and 5. In the case of ster-
ically bulkier naphthylphosphane structures 6 and
7, i-selectivity slightly lessened as isopropyls were
switched over to cyclohexyl groups; however, with
phenylphosphanes, 1–5, no considerable changes ap-
peared. In propene hydroformylation, the i/n ratios
observed with ligands 6 and 7 were even lower than
that of the PPh3 reference. In any case, the best
selectivity to total aldehydes (35%) was given by (2-
methylnaphthyl)diisopropylphosphane (6) in 1-hexene
hydroformylation; its regioselectivity to the branched
products, 2-methylhexanal and 2-ethylpentanal (i/n =
1.3), which was slightly better than when no ligand
(1.2) was present. On the whole, the selectivities
of the ligands used in this study to form isobutanal
were comparable with the previously tested reaction
modified with o-methyl, o-ethyl, o-isopropyl and
o-cyclohexyl substituted phenylphosphanes [5,15].
In propene hydroformylation, the initial activities
and conversions of all the o-alkyl substituted lig-
ands (5–17 mol molRh−1 s−1 and 22–52%) studied
were considerably lower than that of the PPh3 ligand
(45 mol molRh−1 s−1, 98%). Relative to the PPh3,
the initial activity of naphthylphosphanes (6 and
7) and bis(2-methylphenyl)isopropylphosphane was
markedly lower, while that of (2-methylphenyl)diiso-
propylphosphane was the highest of all synthesized
ligands. In the propene hydroformylation reaction, the
initial activity and conversion decreased with greater
steric crowding of the ligands. With all these ligands,
the initial rates and conversions in aldehyde forma-
tion were even lower than those with the previously
tested phenylphosphanes containing one o-methyl, o-
ethyl, o-isopropyl or o-cyclohexyl substituted phenyl
ring [5,15]. Even pyridylphosphanes 8 and 9 showed
The 31P{1H} NMR chemical shifts give informa-
tion about the stereoelectronic state of the phosphane
ligands; whereas, the cone angles are commonly used
as a good, but simple measure for steric stress [35]. In
addition to the electronegativity, the angles between
substituents affect the 31P{1H} NMR chemical shifts.
It was observed that cone angles became larger and the
31P{1H} NMR chemical shifts of the ligands studied
became smaller mainly because of the steric stress of
the alkyl groups. Closely related ligands showed this
approximate order: the i-selectivity became greater
and initial activity lessened as the 31P{1H} NMR shift
of the free ligand became less.
4. Conclusions
In the hydroformylation of propene, the conver-
sions and initial rates were negatively affected as the
steric demands of the ligands, generated by the o-
alkyl-substituent of naphthyl or phenyl rings and by
alkyl groups bonded to the phosphorus atom directly,
became greater. On the other hand, the selectivity
in forming isobunal was higher than that of triph-
enylphosphane. It seemed according to the ab initio
HF calculations and single-crystal X-ray studies that
one side of the complex was blocked by the alkyl sub-
stituent; and thus, the steric properties of the active
complex steered the regioselectivity and activity of
hydroformylation to the extent that the replacement
of isopropyl or cyclohexyl groups with electron-
withdrawing pyridyl groups did not change the hydro-
formylation results notably. On the whole, the o-alkyl
substituted arylalkylphosphane ligands used in this
study were inferior modifiers of rhodium-catalyst to
the corresponding o-alkyl substituted arylphosphane
ligands.
Acknowledgements
We gratefully acknowledge the financial support
from Neste Oxo AB and the National Technology

94
H. Riihimäki et al. / Journal of Molecular Catalysis A: Chemical 200 (2003) 81–94
Agency of Finland (Tekes). We wish to thank Ms.
Päivi Joensuu and Ms. Sari Ek for the mass spectra
and Ms. Merja Harteva for performing the propene
hydroformylation tests and Ms. Liisa Saharinen for
assistance in the 1-hexene hydroformylation tests and
preparation of rhodium complexes.
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