Detection of intermediates in cobalt-catalyzed hydroformylation using para-hydrogen-induced polarization

Article abstract of DOI:10.1021/ja0434533

para-Hydrogen-induced polarization methods are shown to enable the in situ detection of linear and branched monophosphine-containing intermediates during hydroformylation when Co(η³-C₃H₅)(CO)₂(PCy₃) is the catalyst precursor. The NMR signal characteristics of the alkyl arms of these species provide direct evidence for the rapid interconversion of linear and branched cobalt alkyls prior to the CO insertion step. The observation of additional para-hydrogen-enhanced signals for the corresponding linear and branched aldehydes enables the reactions selectivity to be rapidly monitored as a function of H₂ and CO pressure or reaction temperature. Copyright

Full text of DOI:10.1021/ja0434533

Published on Web 03/16/2005
Detection of Intermediates in Cobalt-Catalyzed Hydroformylation Using
para-Hydrogen-Induced Polarization
Cyril Godard,† Simon B. Duckett,*,† Stacey Polas,‡ Robert Tooze,‡ and Adrian C. Whitwood†
Department of Chemistry, UniVersity of York, York YO10 5DD, U.K., and Sasol Technology (U.K.) Ltd.,
Purdie Building, North Haugh, St. Andrews 5KY16 9ST, Scotland, U.K.
Received October 29, 2004; E-mail: sbd3@york.ac.uk
The cobalt-catalyzed hydroformylation of alkenes by HCo(CO)4-
based systems is of significant industrial importance.1 When
modified phosphine donors are included, improved linear to
branched product ratios can be obtained.2 Early studies by Heck
and Breslow suggested aldehyde production occurs via a reaction
sequence involving a monohydride precursor that ultimately yields
a metal acyl intermediate which in turn eliminates the aldehyde
after H2 addition.3 The detection and characterization of key species
in this process has proven to be a complex process; examples of
cobalt acyl and alkyl resting states have been isolated.4
We recently demonstrated the benefits of using allyl complexes
as precursors in mechanistic NMR investigations of iridium-
catalyzed hydroformylation reactions.5 Here, we report on inves-
tigations using the cobalt-based allyl complex, Co(η3-C3H5)(CO)2-
(PCy3) 1. We show that NMR spectroscopy, in conjunction with
para-hydrogen,6 provides direct evidence for each of the previously
proposed steps in hydroformylation catalysis3 and allows the
detection of linear and branched cobalt acyl intermediates in
addition to the corresponding aldehydes. These species are visible
through the organic components of the ligands rather than the more
usual hydride signals.
Figure 1. ORTEP diagram for Co(η3-C3H5)(CO)2(PCy3) 1. Ellipsoids
drawn at 50% probability level.
Scheme 1. Transformations Accounting for the p-H2-Enhanced
NMR Signals Seen for the Sites Where the Protons Appear in Red
Complex 1 was obtained by the addition of 10 equiv of allyl-
bromide to Co(CO)4Na at 273 K in THF, followed by the addition
of 1 equiv of the phosphine at 295 K.7,8 Crystals of 1 suitable for
X-ray crystallography were obtained from toluene at room tem-
perature. The structure of this complex, shown in Figure 1, is similar
to that of the previously reported triphenylphosphine analogue8 and
corresponds to a piano-stool with a capping η3-allyl ligand. NMR
and IR data for 1 are provided in the Supporting Information.
When a d8-toluene solution of 1 was placed under 3 atm of pure
1
para-hydrogen (p-H2) at 363 K and the reaction monitored by H
NMR spectroscopy, strong antiphase signals corresponding to the
organic products, propene and propane, were observed. This
confirms that 1 undergoes H2 addition, either by losing a ligand or
undergoing an η3-η1 allyl rearrangement, and that the resultant
dihydride forms a propene hydride complex via transfer of a metal
hydride to the allyl terminus, as shown in Scheme 1; such steps
have been observed previously.5,9 The observation of the p-H2-
enhanced NMR signals for propene, however, reveals that the
Co(CO)2(PCy3)(propene)(H) species undergoes reversible hydride-
alkene insertions to place two hydrogen atoms from a single p-H2
molecule into the alkene; the spectral features of the propene require
exchange to incorporate p-H2 into all of the available proton sites.
The trapping of the corresponding cobalt alkyl intermediate with a
second H2 molecule and subsequent reductive elimination accounts
for the enhanced propane resonances.
enhanced signals for propane, but complex 1 was still rapidly
consumed and the formation of the known complexes Co(H)(CO)3-
(PCy3) and Co(H)(CO)2(PCy3)2 indicated.10 This suggests that
Co(CO)2(PCy3)(propene)(H) is unable to re-form the initial dihy-
dride allyl complex.
To confirm whether the hydride migration to the allyl terminus
is reversible, a sample of 1 was placed under a 1:2 mixture of
propene and p-H2. 1H NMR spectra recorded at 363 K then revealed
Prior to examining the hydroformylation activity of 1, a control
experiment with CO was undertaken. 31P{1H} NMR spectroscopy
† University of York.
‡ Sasol Technology.
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J. AM. CHEM. SOC. 2005, 127, 4994-4995
10.1021/ja0434533 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. 1H spectrum of a d8-toluene solution containing 1 in the presence
of CO and p-H2 at 373 K with key resonances indicated (b ) branched and
l ) linear aldehyde).
Figure 3. Plot of the relative p-H2-based NMR signal intensities for species
3, 4, and the corresponding aldehydes as a function of temperature for
samples of 1 reacting in toluene with 3 atm of a 2:1 mixture of CO and H2.
(Note, blue ( data obtained for a 1H signal, measured using normal methods
when the thermally polarized state was visible.)
revealed the liberation of PCy3, while the 1H NMR spectrum
confirmed the formation of a new η3-allyl complex, which was
subsequently identified as Co(η3-C3H5)(CO)3.11 At 363 K, 15% of
1 was converted to Co(η3-C3H5)(CO)3.
These reactions were then repeated at 378 K, and the CO to H2
ratio varied across the series 4:1, 1:1, 1:2, and 1:3. The relative
strength of the NMR signals of 3 relative to those of 4 now proved
to be minimized at a CO:H2 ratio of 1:2; this also corresponded to
the point of highest linear aldehyde selectivity.
Here, we have shown that the utilization of the PHIP effect allows
the mapping of the hydroformylation reaction of a cobalt catalyst
modified with PCy3. The detection of reaction intermediates that
do not contain a hydride ligand via enhanced signals within the
organic ligand framework proved possible. In addition, the high
signal strengths enable the hydroformylation products themselves
to be monitored at very early reaction times. We expect this
extension of the para-hydrogen approach to become applicable to
the study of a wide range of catalytic reactions. We are currently
using it to probe how the ligand sphere influences the reactions of
modified and unmodified cobalt hydroformylation catalysts.
When a d8-toluene solution of 1 was placed under 3 atm of a
1
1:2 mixture of CO and H2 at 363 K, the corresponding H NMR
spectrum showed much weaker p-H2-enhanced signals for propene
and propane. In addition, five new p-H2-enhanced signals were also
detected in clear regions of the spectrum at δ 3.57, 3.22, 0.92, 0.80,
and 0.72 (Figure 2). The corresponding COSY spectrum connected
the δ 3.22 signal (t, JHH ) 7 Hz) to the δ 0.92 (t, JHH ) 7 Hz)
signal and a new resonance at δ 1.67 (sextet, JHH ) 8 Hz), which
was masked by the phosphine. These data confirm that a linear
CH2CH2CH3 group has been formed with protons from p-H2 being
placed on all three of the carbon centers. When a 1H{31P} spectrum
was recorded, the signal at δ 3.22 simplified due to removal of a
single JPH coupling of 3 Hz from a 31P center which resonates at δ
62.0. COSY confirmed that the additional 1H signal at δ 3.57
(septet, JHH ) 8 Hz) coupled to a signal at δ 1.20 (d, JHH ) 8 Hz),
which indicated that a CH(CH3)2 group was formed. When this
reaction was repeated with 13CO, both of the terminal alkyl proton
signals showed additional 13C splittings that arose from 13C signals
that appeared at δ 235.6 and 240.9, respectively. Since both the
latter resonances exhibited doublet multiplicities due to 31P-13C
couplings (JPC ) 39.3 and 40.5 Hz, respectively), their origin as
metal acyl groups, trans to a single phosphine ligand, was con-
firmed. The corresponding species are, therefore, Co(COCH2CH2-
CH3)(CO)3(PCy3) 3 and Co(COCH(CH3)2)(CO)3(PCy3) 4. In view
of the fact that 3 and 4 contain PCy3, it is possible to conclude that
1 reacts initially with H2 via CO loss. The remaining p-H2-enhanced
resonances at δ 0.80 and 0.72 were shown to arise from the
aldehydes CH(O)CH(CH3)2 and CH(O)CH2CH2CH3, respectively.
The p-H2-enhanced NMR signals of 3 and 4 can be accounted
for by the reversible hydride transfer processes illustrated in Scheme
1; the signals seen for site B correspond to directly formed linear
product, while those seen for site A arise via relinearized branched
intermediates which can also react to give 4.
When this reaction was observed at different temperatures using
a constant concentration of 1 and a CO:H2 ratio of 1:2, the size of
the signals due to the aldehydes, propene, and propane increased
with increase in temperature. Figure 3 summarizes these variations.
A series of 1D EXSY experiments were then completed to test for
magnetization transfer from 3 or 4 into the corresponding aldehydes.
None was observable on the NMR time scale. The enhanced NMR
signals seen for the two aldehydes, therefore, require the corre-
sponding 16-electron intermediates, Co(COCH2CH2CH3)(CO)2-
(PCy3) and Co(COCH(CH3)2)(CO)2 (PCy3), to be trapped by H2
(which leads to rapid aldehyde elimination) rather than CO (which
leads to 3 or 4). The variation in the observed product ratios reflects
a higher linear aldehyde turnover at higher temperature.
Acknowledgment. Sasol Technologies (UK) Ltd funded this
work.
Supporting Information Available: X-ray data for 1, NMR data
and experimental procedures (CIF, PDF). Crystal data for 1: C23H38-
CoO2P, FW 436.43, yellow blocks, crystal dimensions 0.32 × 0.24 ×
0.14 mm, monoclinic, P21/n, a ) 10.4726(8) Å, b ) 15.4811(12) Å,
c ) 13.8229(11) Å, â ) 92.511(2)°, V ) 2238.9(3) Å3, Z ) 4, µ(Mo
KR) ) 0.853 mm-1, T ) 115(2) K, 17 426 reflections collected, R(int)
) 0.0247. This material is available free of charge via the Internet at
http://pubs.acs.org.
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