In Situ High-Pressure 31P{1H} NMR Studies of the Hydroformylation of 1-Hexene by RhH(CO)(PPh3)3

Article abstract of DOI:10.1021/om9907627

The hydroformylation of 1-hexene with syngas (40 bar of 1:1 CO/H₂) in the presence of the catalyst precursor RhH(CO)(PPh₃)₃ has been studied by high-pressure NMR spectroscopy in a 10 mm sapphire tube equipped with a Ti-alloy valve. Except for RhH(CO)₂(PPh₃)₂, no direct observation of labile intermediates involved in the catalytic cycle has been detected; however as many as four rhodium resting states have been seen, and some factors controlling their formation/interconversion/inhibition have been identified. The information gathered from this in situ investigation is particularly relevant to a better understanding of the inhibiting action of the CO pressure as well as the positive action of PPh₃ addition.

Full text of DOI:10.1021/om9907627

Organometallics 2000, 19, 849-853
849
In Situ High -P r essu r e 31P {1H} NMR Stu d ies of th e
Hyd r ofor m yla tion of 1-Hexen e by Rh H(CO)(P P h 3)3
Claudio Bianchini,* Hon Man Lee, Andrea Meli, and Francesco Vizza
Istituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione
(ISSECC)-CNR, Via J . Nardi 39, 50132 Firenze, Italy
Received September 28, 1999
The hydroformylation of 1-hexene with syngas (40 bar of 1:1 CO/H2) in the presence of
the catalyst precursor RhH(CO)(PPh3)3 has been studied by high-pressure NMR spectroscopy
in a 10 mm sapphire tube equipped with a Ti-alloy valve. Except for RhH(CO)2(PPh3)2, no
direct observation of labile intermediates involved in the catalytic cycle has been detected;
however as many as four rhodium resting states have been seen, and some factors controlling
their formation/interconversion/inhibition have been identified. The information gathered
from this in situ investigation is particularly relevant to a better understanding of the
inhibiting action of the CO pressure as well as the positive action of PPh3 addition.
In tr od u ction
new spectroscopic or computational techniques become
available.
Hydridocarbonyltris(triphenylphosphine)rhodium-
(I), RhH(CO)(PPh3)3 (1),1,2 is one of the most com-
mercially important catalysts for the homogeneous
hydroformylation of alkenes, particularly for the low-
pressure conversion of propylene to n-butyraldehyde.3
The first mechanistic studies of the hydroformylation
of alkenes catalyzed by 1 were reported by Wilkinson
and co-workers over thirty years ago.4 Since then,
innumerable studies of the oxo activity of 1 have been
carried out applying a variety of experimental and
theoretical approaches.5-8 The general mechanistic
concepts proposed by Wilkinson and co-workers are still
accepted as being essentially correct; however, new
insights, largely concerning the structure, relative
stability, and exchange processes of either active inter-
mediates or resting states, continue to be discovered as
A relatively new spectroscopic technique for in situ
studies of catalytic reactions at elevated gas pressure
is high-pressure NMR (HPNMR) spectroscopy.7,9 With
the advent of 10 mm-o.d. sapphire tubes equipped with
Ti-alloy valves, the HPNMR technique has further
developed as any commercial spectrometer may be
employed with standard operation; moreover, the head-
space of the tubes is large enough to maintain a constant
concentration of gases into the solution if fast diffusion
takes place. Indeed, mass transfer of reactive gases from
the headspace of 10 mm-o.d. NMR tubes has generally
been found to be efficient enough to replenish the
solution which is being depleted of gaseous reactants
by the catalyst.7,10
Notwithstanding that NMR spectroscopy is featured
by slower time scale and lower sensitivity as compared
to infrared spectroscopy,11 the HPNMR technique is
preferable when the catalytic process under investiga-
tion contains many magnetically active nuclei as well
as many closely related species. No doubt, a reaction
with these characteristics is the hydroformylation of
alkenes promoted by phosphine-modified rhodium cata-
(1) Bath, S. S.; Vaska, L. J . Am. Chem. Soc. 1963, 85, 3500.
(2) J ardine, F. H. Polyhedron 1982, 1, 569.
(3) Frohning, C. D.; Kohlpaintner, C. W. In Applied Homogeneous
Catalysis with Organometallic Compounds; Cornils, B., Herrmann, W.
A., Eds.; VCH: Weinheim, Germany, 1996; p 27.
(4) (a) Evans, D.; Yagupsky, G.; Wilkinson, G. J . Chem. Soc. (A)
1968, 2660. (b) Evans, D.; Osborn, J . A.; Wilkinson, G. J . Chem. Soc.
(A) 1968, 3133. (c) Yagupsky, G.; Brown, C. K.; Wilkinson, G. J . Chem.
Soc., Chem. Commun. 1969, 1244. (d) Brown, C. K.; Wilkinson, G. J .
Chem. Soc. (A) 1970, 2753. (e) Yagupsky, M.; Brown, C. K.; Yagupsky,
G.; Wilkinson, G. J . Chem. Soc. (A) 1970, 937. (f) Yagupsky, G.; Brown,
C. K.; Wilkinson, G. J . Chem. Soc. (A) 1970, 1392.
(9) (a) Elsevier, C. J . J . Mol. Catal. 1994, 92, 285. (b) Roe, D. C. J .
Magn. Reson. 1985, 63, 388. (c) Bianchini, C.; Frediani, P.; Meli, A.;
Peruzzini, M.; Vizza, F. Chem. Ber. 1997, 130, 1633.
(5) (a) Brown, J . M.; Canning, L. R.; Kent, A. G.; Sidebottom, P. J .
J . Chem. Soc., Chem. Commun. 1982, 721. (b) Brown, J . M.; Kent, A.
G. J . Chem. Soc., Chem. Commun. 1982, 723. (c) Brown, J . M.; Kent,
A. G. J . Chem. Soc., Perkins Trans. 1987, 1597.
(6) (a) Kiss, G.; Mozeleski, E. J .; Nadler, K. C.; VanDriessche, E.;
DeRoover, C. J . Mol. Catal. 1999, 138, 155. (b) Bhanage, B. M.;
Divekar, S. S.; Deshpande, R. M.; Chaudari, R. V. J . Mol. Catal. 1997,
115, 247.
(7) (a) Brown, D. T.; Eguchi, T.; Heaton, B. T.; Iggo, J . A.; Whyman,
R. J . Chem. Soc., Dalton Trans. 1991, 677. (b) Horva´th, I. T.; Millar,
J . M. Chem. Rev. 1991, 91, 1339. (c) Horva´th, I. T.; Kastrup, V.; Oswald,
A.; Mozeleski, J . Catal. Lett. 1989, 2, 85. (d) Moasser, B.; Gladfelter,
W. L.; Roe, D. C. Organometallics 1995, 14, 3832. (e) Moasser, B.;
Gladfelter, W. L. Inorg. Chim. Acta 1996, 242, 125.
(10) (a) Bianchini, C.; Herrera, V.; J ime´nez, M. V.; Meli, A.; Sa´nchez-
Delgado, R. A.; Vizza, F. J . Am. Chem. Soc. 1995, 117, 8567. (b)
Bianchini, C.; Fabbri, D.; Gladiali, S.; Meli, A.; Pohl, W.; Vizza, F.
Organometallics 1996, 15, 4604. (c) Bianchini, C.; Casares, J . A.; Meli,
A.; Sernau, V.; Vizza, F.; Sa´nchez-Delgado, R. A. Polyhedron 1997, 16,
3099. (d) Bianchini, C.; Meli, A.; Patinec, V.; Sernau, V.; Vizza, F. J .
Am. Chem. Soc. 1997, 119, 4945. (e) Bianchini, C.; Meli, A.; Moneti,
S.; Vizza, F. Organometallics 1998, 17, 2636. (f) Bianchini, C.; Lee, H.
M.; Meli, A.; Moneti, M.; Vizza, F.; Fontani, M.; Zanello, P. Macro-
molecules 1999, 32, 2, 4183. (g) Bianchini, C.; Lee, H. M.; Meli, A.;
Moneti, M.; Patinec, V.; Petrucci, G.; Vizza, F. Macromolecules 1999,
32, 3859. (h) Bianchini, C.; Meli, A.; Moneti, S.; Oberhauser, S.; Vizza,
F.; Herrera, V.; Fuentes, A.; Sa´nchez-Delgado, R. A. J . Am. Chem. Soc.
1999, 121, 7071.
(8) (a) Matsubara, T.; Koga, N.; Ding, Y.; Musaev, D. G.; Morokuma,
K. Organometallics 1997, 16, 1065. (b) van der Veen, L.; Boele, M. D.
K.; Bregman, F. R.; Kamer, P. C. J .; van Leeuwen, P. W. N. M.;
Goubitz, K.; Fraanje, J .; Schenk, H.; Bo, C. J . Am. Chem. Soc. 1998,
120, 11616.
(11) (a) Rigby, W.; Whyman, R. W.; Wilding, K. J . Phys. E.: Sci.
Instrum. 1970, 3, 572. (b) Horva´th, I. T. Organometallics 1986, 5, 2333.
(c) Tinker, K.; Morris, D. E. Rev. Sci. Instrum. 1972, 7, 1024. (d)
Die´guez, M.; Clavern C.; Madeu-Bulto`; Ruiz, A.; van Leeuwen, P. W.
N. M.; Schoemaker, G. C. Organometallics 1999, 18, 2107.
10.1021/om9907627 CCC: $19.00 © 2000 American Chemical Society
Publication on Web 01/29/2000

850 Organometallics, Vol. 19, No. 5, 2000
Bianchini et al.
Sch em e 1
purchased from Aldrich. Toluene-d8 for NMR measurements
was dried over molecular sieves. 1H, 13C{1H}, and 31P{1H}
NMR spectra were obtained on a Bruker ACP 200 spectrom-
eter (200.13, 50.32, and 81.01 MHz, respectively). All chemical
shifts are reported in ppm (δ) relative to tetramethylsilane,
referenced to the chemical shifts of residual solvent resonances
(1H, 13C) or 85% H3PO4 (31P). The 10 mm sapphire NMR tube
was purchased from Saphikon, Milford, NH, while the tita-
nium high-pressure charging-head was constructed at the
ISSECC-CNR (Firenze, Italy).15 Note: Since high gas pres-
sures are involved, safety precautions must be taken at all
stages of studies involving high-pressure NMR tubes. GC
analyses of the solutions were performed on a Shimadzu GC-
14 A gas chromatograph equipped with a flame ionization
detector and a 30 m (0.25 mm i.d., 0.25 µm film thickness)
SPB-1 Supelco fused silica capillary column. A UCON-oil LB
550-X packed column (PPG, 2 m, 1/8 in. i.d.) was employed
for the determination of hydrocarbons, 2-ethylpentanal, 2-
methylhexanal, and heptanal. An Al2O3-PLOT capillary col-
umn (50 m, 0.32 mm i.d.) was employed for the determination
of hexane, 1-hexene, cis- and trans-2-hexene, and cis- and
trans-3-hexene. GC/MS analyses were performed on a Shi-
madzu QP 5000 apparatus equipped with a SPB-1 Supelco
fused silica capillary column.
lysts, as shown by the proposed dissociative mechanism
of the catalytic hydroformylation of alkenes mediated
by 1 illustrated in Scheme 1 (dashed box).4,5
Of the various transient intermediates in the catalytic
cycle only the five-coordinate complex RhH(CO)2(PPh3)2
(3) has been characterized spectroscopically as a very
fluxional species due to fast exchange of a phosphine
ligand from equatorial to axial position.5c,8b Some related
derivatives and analogues of the species in the catalytic
cycle have been either isolated or detected in situ,
however. The structure of the square-planar Rh(I)
hydride RhH(CO)(PCy3)2 (PCy3 ) tricyclohexylphos-
phine),12 similar to RhH(CO)(PPh3)2 (2), has been
determined by X-ray methods, while acyl complexes of
the formula Rh(COR)(CO)2(PPh3)2 have been observed
by NMR upon carbonylation of 1 in the presence of
alkenes (R ) alkyl).4e,5b,c An HPNMR study in toluene-
d8 at room temperature of the reaction of 1 with 30 bar
of CO/H2 in the presence of 3 molar excess of PPh3 has
also been carried out.7c Complex 3 was reported to be
the only species detectable by NMR.
In this work is reported a 31P{1H} HPNMR study of
the hydroformylation of 1-hexene catalyzed by 1 in
either the absence or presence of added PPh3. The
combination of experiments in both actual catalytic
conditions and at low temperature has provided for
the first time a complete picture of the equilibria
involving all the species that are visible on the NMR
time scale.
HP NMR Stu d y of th e Hyd r ofor m yla tion of 1-Hexen e
Ca t a lyzed by 1. Gen er a l P r oced u r e. A 10 mm sapphire
HPNMR tube was charged with a solution of 1 (18.4 mg, 0.02
mmol) in toluene-d8 (2.5 mL) and a 30-fold excess of 1-hexene
1
(75 µL, 0.6 mmol) under nitrogen. After 31P{1H} and H NMR
spectra were recorded, the tube was pressurized with a 1:1
mixture of CO/H2 to 40 bar at room temperature. The reaction
was followed by variable-temperature 31P{1H} and 1H NMR
spectroscopy. After the desired time, the tube was removed
from the probe of the spectrometer and depressurized. NMR
spectra were acquired, and the product composition was
determined by GC and GC/MS analyses.
Exp er im en ta l Section
Gen er a l P r oced u r es. All manipulations were carried out
under an atmosphere of nitrogen by using Schlenk-type
techniques. The catalyst precursor RhH(CO)(PPh3)3 (1)4b and
the dimeric complexes Rh2(CO)4+x(PPh3)4-x (x ) 0, 12; x ) 1,
13; x ) 2, 14) were prepared according to literature meth-
ods.13,14 1-Hexene was percolated through neutral silica prior
to use. All the other reagents and solvents were used as
In some experiments, to avoid initial olefin isomerization,2,4b
1-hexene was added into the toluene-d8 solution of 1 under a
stream of 1:1 CO/H2.
(13) (a) Whyman, J . Chem. Soc., Dalton Trans. 1972, 1375. (b)
Chini, P.; Martinengo, S.; Garlaschelli, G. J . Chem. Soc., Chem.
Commun. 1972, 709.
(14) Chan, A. S. C.; Shieh, H.-S.; Hill, J . R. J . Chem. Soc., Chem.
Commun. 1983, 688.
(12) Freeman, M. A.; Young, D. A. Inorg. Chem. 1986, 25, 1556.

Hydroformylation of 1-Hexene
Organometallics, Vol. 19, No. 5, 2000 851
Markovnikov hydride migration) characterized by a
doublet at δ 27.5 with J (PRh) ) 78.0 Hz.4e,5b,c A triplet
1
at δ 3.28 (J (HH) ) 7.2 Hz) in the H NMR spectrum
(trace b*) appeared contemporaneously to the 31P NMR
doublet at δ 27.5 and was safely assigned to the CH2
group proximal to the CO group in 15 by comparison
with the spectra of related Rh(COR)(CO)2(PPh3)2 com-
plexes.4c,f,5b In terms of the mechanism shown in Scheme
1, the formation of 15 is consistent with the occurrence
of the reaction sequence 1 f 2f 3 f 4 f 5 f 6 f 7 f
1
8. Consistently, no trace of aldehydes was seen by H
NMR at this stage. From a perusal of trace b, however,
we noted that the baseline rose forming a broad hump
just below the signal of 15. As will be shown later in
this work, the broad hump is originated by at least three
fluxional and exchanging dimeric Rh(0) complexes of the
formula Rh2(CO)4+x(PPh3)4-x (x ) 0, 12; x ) 1, 13; x )
2, 14), which at this stage of the reaction are in very
low concentration.
F igu r e 1. 31P{1H} and 1H HPNMR study of the hydro-
formylation of 1-hexene catalyzed by RhH(CO)(PPh3)3 (1)
(sapphire tube, toluene-d8, 40 bar of a 1:1 mixture of CO/
H2, 1-hexene/1 ) 30:1). 31P{1H} NMR spectra: under
nitrogen, before pressurizing with CO/H2, at room temper-
ature (a); under CO/H2 at room temperature (b); after 1 h
under CO/H2 at 60 °C (c); after the NMR probe was cooled
to room temperature (d); after the tube was depressurized
(e). 1H NMR spectrum in the 3.5-3.0 ppm region under
CO/H2 at room temperature (b*).
Increasing the temperature of the NMR probe to 60
°C resulted in extensive broadening of all resonances,
indicating a fast exchange and interaction of ligands
around the rhodium centers (trace c). After 1 h at 60
°C, heptanal and 2-methylhexanal were formed in a ca.
1:1 ratio (NMR evidence) together with traces of isomer-
ized alkenes. GC/MS analysis, carried out on a sample
of the solution recovered at the end of the HPNMR
experiment, confirmed the formation of aldehydes in a
normal/iso ratio of 1. The 31P{1H} NMR spectrum at
room temperature (trace d) was similar to that prior to
aldehyde formation (trace b), the only relevant differ-
ences being a major intensity of the acyl signal and of
the hump below. The HPNMR tube was then removed
from the probe and depressurized. The respective 31P
NMR spectrum gave trace e, showing the reappearance
of the precursor 1 and the complete disappearance of
the signal at δ 27.5, in line with the fact that the acyl
derivatives are stable under a CO atmosphere only.4f,5c
Also, appreciable increase in the concentration of the
dimeric species occurred, which is consistent with the
mechanism of their formation (vide infra). Sweeping the
NMR tube with N2 led to the complete disappearance
of the signal due to 3, while the concentration of the
dimeric complexes was apparently unaffected.
HPNMR experiments were carried out adding 20 equiv of
PPh3 (105 mg, 0.4 mmol) or using syngas disproportionately
rich in H2 (H2, 35 bar; CO, 5 bar). HPNMR experiments were
also performed in the absence of H2 using a CO pressure of 20
bar.
Rea ction of Rh 4(CO)12 w ith P P h 3. A 10 mm NMR tube
was charged with a solution of Rh4(CO)12 (4 mg, 0.005 mmol)
in toluene-d8 (2.5 mL) and an 8-fold excess of PPh3 (11 mg,
0.04 mmol) under nitrogen. The reaction was followed by
variable-temperature 31P{1H} NMR spectroscopy.
R ea ct ion of R h 4(CO)12 w it h P P h 3/CO. A 10 mm NMR
tube was charged with a solution of Rh4(CO)12 (4 mg, 0.005
mmol) in toluene-d8 (2.5 mL) and an 8-fold excess of PPh3 (11
mg, 0.04 mmol) under nitrogen, pressurized with CO to 20 bar
at room temperature, and then placed in an NMR probe. The
reaction was followed by variable-temperature 31P{1H}NMR
spectroscopy.
Resu lts a n d Discu ssion
The products that give rise to the broad hump
centered at δ 25 were identified by several experiments,
including their independent synthesis. First of all, a low-
temperature spectrum of the catalytic mixture was
acquired. As is shown in trace a of Figure 2, the broad
hump resolved at -20 °C into several signals spanning
from 30 to 22 ppm, none of which was found to contain
hydride ligands (1H NMR evidence).
The hydroformylation of 1-hexene in the presence of
1 has been studied in a 10 mm HPNMR tube with 40
bar of 1:1 CO/H2. The concentration of 1 was ca. 8 mM
in all experiments in order to have a satisfactory signal-
to-noise ratio as well as maintain the system in the
conditions of the dissociative mechanism and first-order
rate in catalyst concentration.4d
A
31P NMR spectrum of a mixture of 1, CO (20 bar),
A sequence of selected 31P{1H} NMR spectra is shown
in Figure 1.
and 1-hexene (30 equiv) was then acquired in the
absence of H2 (trace b). Three defined resonances at
identical chemical shifts with those of trace a but with
increased intensity were clearly visible at -20 °C (again
only a broad hump was observed at room temperature).
In these conditions, only a very small amount of the acyl
complex 15 was detected due to the competing formation
of the dimeric Rh(0) complexes. In other words, in the
absence of H2, the acyl complex 15 cannot accumulate
during the carbonylation reaction.4f,5c,6 In a third experi-
ment (trace c), 1 was reacted with 20 bar CO in the
In the absence of syngas, no appreciable transforma-
tion of 1 was observed (trace a, doublet at δ 41.3, J (PRh)
) 153.9 Hz). Only in the presence of CO/H2 did 1
immediately transform into several products at room
temperature (trace b). The carbonylation of the precur-
sor was revealed by the formation of 3 (doublet at δ 37.3,
J (PRh) ) 138.7 Hz) and free PPh3 (singlet at δ -4.8),
while the occurrence of alkene coordination/hydride
migration was indirectly shown by the formation of the
acyl complex Rh(CO(CH2)5CH3)(CO)2(PPh3)2 (15) (anti-

852 Organometallics, Vol. 19, No. 5, 2000
Bianchini et al.
Sch em e 2
Figu r e 2. 31P{1H} HPNMR spectra (sapphire tube, toluene-
d8, 20 bar of CO, -20 °C): RhH(CO)(PPh3)3 (1) and 30
equiv of 1-hexene (b); 1 and 30 equiv of 2-hexene (c); 1 (d);
1, 20 equiv of PPh3, and 30 equiv of 1-hexene (e). Trace a,
reported for comparative purposes, shows the spectrum,
acquired at -20 °C, of the final solution of the HPNMR
study of the hydroformylation of 1-hexene catalyzed by 1
(sapphire tube, toluene-d8, 40 bar of a 1:1 mixture of CO/
H2, 1-hexene/1 ) 30).
presence of 2-hexene: the 31P NMR spectrum was
substantially similar to that with 1-hexene, but no trace
of acyl complex was observed in agreement with a much
higher energy barrier to 2-alkene insertion into the
Rh-H bond as compared to 1-alkenes.4d Consistently,
the carbonylation of 1 in the absence of alkene gave an
identical 31P NMR spectrum (trace d). Finally, 1 was
reacted with CO (20 bar) and 1-hexene in the presence
of a 20-fold excess of PPh3. The 31P NMR spectrum
(trace e) showed the formation of the acyl complex 15
and of the dimers Rh2(CO)4+x(PPh3)4-x (x ) 0, 12; x )
1, 13), therefore indicating that the excess PPh3 inhibits
the formation of the dimer with the highest content of
CO, i.e., Rh2(CO)6(PPh3)2 (14).
Figu r e 3. 31P{1H} HPNMR spectra (sapphire tube, toluene-
d8, -20 °C): Rh4(CO)12 and 8 equiv of PPh3 under nitrogen
(a); Rh4(CO)12 and 8 equiv of PPh3 under 20 bar of CO (b);
RhH(CO)(PPh3)3 under 20 bar of CO (c).
To unambiguously establish the nature of the prod-
ucts formed upon carbonylation of 1, the Rh(0) dimers
were generated following a known procedure.13,14 To a
solution of Rh4(CO)12 in toluene-d8 in an HPNMR tube
was added an excess of PPh3 (8 equiv) under nitrogen.
The respective 31P NMR spectrum at -20 °C is reported
as trace a in Figure 3. Under these conditions, 12
(doublet at ca. δ 23) and 13 (doublet at ca. δ 25) were
largely predominant over 14. Pressurizing the HPNMR
tube with 20 bar of CO increased the concentration of
14 at the expense of those of 12 and 13 (trace b).
A comparison with the spectrum of 1 under 20 bar of
CO at -20 °C (trace c) confirms that a high pressure of
CO is needed to form an appreciable amount of 14.
From this study it is reasonable to conclude that the
formation of the Rh(0) dimers largely depends on the
CO pressure, although a certain influence is exerted also
by the concentration of free PPh3.4d A quite similar
situation takes place in actual hydroformylation condi-
tions: almost no trace of dimers was spectroscopically
detected by treatment of a toluene solution of 1 and
1-hexene with syngas disproportionately rich in H2 (H2,
35 bar; CO, 5 bar). Similarly, scarce production of
dimers was observed by NMR when the hydroformyla-
tion of 1-hexene with 1 was carried out in the presence
of an excess of free phosphine as actually occurs in
industrial conditions.3
The first evidence of the formation of dimeric Rh(0)
complexes upon carbonylation of 1 was reported in 1968
by Wilkinson et al., who discovered that the action of
CO on toluene solutions of 1 involved rapid conversion
into 3 and the yellow complex Rh2(CO)4(PPh3)4 (12).4a,f
The formation of the latter dimer has been reported to
proceed by reversible thermal elimination of H2 from
3, as shown in Scheme 2. Once formed, 12 is in
equilibrium with 13 and 14 via phosphine displacement
by CO,13 and with 3 via H2 uptake.4a,f Obviously, the
equilibrium concentration of the various species depends
on the CO and H2 pressures as well as the PPh3
concentration. X-ray structural analyses are available
for 1314 and for the model compound, analogous to 12,
Rh2(CO)2[P(O-i-C3H7)3]4.16 A detailed 31P NMR analysis
has been reported for Rh2(CO)6[P(OPh)3]2, which exhib-
its a second-order AA′XX′ 31P NMR spectrum.7a
A
similar second-order NMR pattern is shown by the PPh3
derivative 14 (doublet of multiplets centered at δ 28.5)
that indeed was found to disappear by addition of PPh3
to the reaction mixture (trace e in Figure 2).
(15) CNR (Bianchini, C.; Meli, A.; Traversi, A.) It. Pat. FI A000025,
1997.
(16) Burch, R. R.; Muetterties, E. L.; Schultz, A. J .; Gebert, E. G.;
Williams, J . M. J . Am. Chem. Soc. 1981, 103, 5517.

Hydroformylation of 1-Hexene
Organometallics, Vol. 19, No. 5, 2000 853
in nice agreement with all previous mechanistic studies
of the oxo chemistry of 1. In particular, the information
gathered from this in situ investigation is indeed
relevant to a better understanding of the inhibiting
action of the CO pressure as well as the positive action
of PPh3 addition. Experimental evidence has been
obtained for the effect of the CO pressure that increas-
ing the concentration of inactive species such as the five-
coordinate acyl complex 15 and the Rh(0) dimers 12,
13, and 14, actually reduces the rate of reaction. Ana-
logously, the addition of PPh3 to the reaction mixture
has been shown to have the desirable effect of inhibiting
dimerization reactions.
An increased normal/iso ratio of aldehydes in the
presence of excess PPh3 has been found to occur also in
the HPNMR reactions. Although neither 2 nor 4 has
been detected spectroscopically, it has been found that
the presence of PPh3 reduces the rate of formation of 3
and consequently that of 4. In other words, the presence
of PPh3 increases the equilibrium concentration of the
more sterically demanding square-planar intermediate
2 which favors the occurrence of the anti-Markovnikov
addition and hence the production of the linear alde-
hyde. Indeed, previous mechanistic studies and HPNMR
experiments in water using RhH(CO)[P(m-C6H4SO3-
Na)3]3 have shown that the normal/iso ratio of the
aldehydes is largely controlled by the competitive reac-
tions of the alkene with intermediates 2 and 4, resulting
in high or low normal/iso ratio, respectively.7c
F igu r e 4. 31P{1H} HPNMR study of the hydroformylation
of 1-hexene catalyzed by RhH(CO)(PPh3)3 (1) in the pres-
ence of PPh3 (sapphire tube, toluene-d8, 40 bar of a 1:1
mixture of CO/H2, 1-hexene/1 ) 30, PPh3/1 ) 20). 31P{1H}
NMR spectra: under nitrogen, before pressurizing with
CO/H2, at room temperature (a); under CO/H2 at room
temperature (b); after 1 h under CO/H2 at 60 °C (c); after
the NMR probe was cooled to room temperature (d).
In Figure 4 are displayed selected 31P{1H} HPNMR
spectra of the hydroformylation of 1-hexene in toluene-
d8 in the presence of a 20-fold excess of PPh3.
Before pressurizing with 40 bar of 1:1 CO/H2 syngas,
1 and free phosphine were the only species visible on
the NMR time scale (trace a). Intermediate 3 started
to form upon addition of syngas. At room temperature
the conversion of 1 to 3 was much slower than observed
with no added phosphine, however (trace b was actually
taken after the same reaction time of trace b in Figure
1). With time, the concentration of 3 increased and the
acyl complex 15 slowly accumulated. No trace of alde-
Con clu sion s
The hydroformylation of 1-hexene in the presence of
RhH(CO)(PPh3)3 has been studied in actual hydro-
formylation conditions using high-pressure NMR spec-
troscopy. Scheme 1 reports a simplified hydroformyla-
tion mechanism promoted by the catalyst precursor
RhH(CO)(PPh3)3 and highlights all the species that have
been characterized by HPNMR spectroscopy.
Although no direct observation of labile intermediates
involved in the catalytic cycle has been detected [except
for RhH(CO)2(PPh3)2], as many as four rhodium resting
states have been seen and some factors controlling their
formation/inhibition have been identified.
1
hyde products was seen by H NMR at room tempera-
ture. Heating the NMR probe to 60 °C resulted in a
rapid equilibration of all Rh species (trace c) as well as
hydroformylation of 1-hexene to give a normal/iso ratio
of ca. 2, consistent with the presence of an excess of
phosphine.4d After cooling to room temperature, the first
spectrum showed the presence of 3 and of traces of 15
(trace d). A 31P NMR spectrum at -20 °C showed a very
low concentration of dimers.
Ack n ow led gm en t. Thanks are due to Shell Inter-
national Chemicals B. V. (Amsterdam) for financing a
postdoctoral program to H.M.L. at ISSECC-CNR.
The overall picture of the hydroformylation of 1-hex-
ene with 1 provided by the present HPNMR studies is
OM9907627