Influence of the 4-substituents on the reversal of enantioselectivity in the asymmetric hydroformylation of 4-substituted styrenes with PtCl(SnCl 3)[(2 S, 4 S)-BDPP]

Article abstract of DOI:10.1021/om401104g

The enantioselectivity of the asymmetric hydroformylation of 4-substituted styrenes in the presence of an in situ catalyst, formed from PtCl(SnCl ₃)[(2S,4S)-BDPP] and tin(II) chloride, was influenced by the reaction temperature. The preferred formation of the S and the R enantiomers of the branched aldehyde regioisomers (2a-g) was observed at low and high temperatures, respectively. The electron-donor or electron-acceptor properties of the para substituents of styrene show correlation with the changes in enantioselectivity, especially with the reversal temperature of the enantioselectivity. The reversibility of the formation of the Pt-branched alkyl intermediates, leading to the corresponding R and S enantiomers of 2-arylpropanals, depends on the Hammett constants. The electronic effect of para substituents was investigated by quantum chemical methods employing the simple olefin adducts [HPt(PH ₃)₂(olefin)(SnCl₃)]. Excellent linear correlation was found between the para substituent constants and the electrostatic potential at nuclei of the platinum atom. Equally good correlation has been established for the other atoms as well in the coordination sphere of Pt.

Full text of DOI:10.1021/om401104g

Article
pubs.acs.org/Organometallics
Influence of the 4‑Substituents on the Reversal of Enantioselectivity
in the Asymmetric Hydroformylation of 4‑Substituted Styrenes with
PtCl(SnCl3)[(2S,4S)‑BDPP]
Peter Pongracz, Tamara Papp, Laszlo Kollar, and Tamas Kegl*
́
́
́
́
́
́
́
Department of Inorganic Chemistry, University of Pec
Selective Chemical Syntheses, H-7624 Pecs, Hungary
́
s and Jan
́
os Szentag
́
othai Science Center, MTA-PTE Research Group for
́
S
* Supporting Information
ABSTRACT: The enantioselectivity of the asymmetric hydro-
formylation of 4-substituted styrenes in the presence of an in
situ catalyst, formed from PtCl(SnCl3)[(2S,4S)-BDPP] and
tin(II) chloride, was influenced by the reaction temperature.
The preferred formation of the S and the R enantiomers of the
branched aldehyde regioisomers (2a−g) was observed at low
and high temperatures, respectively. The electron-donor or
electron-acceptor properties of the para substituents of styrene
show correlation with the changes in enantioselectivity, especially with the reversal temperature of the enantioselectivity. The
reversibility of the formation of the Pt-branched alkyl intermediates, leading to the corresponding R and S enantiomers of 2-
arylpropanals, depends on the Hammett constants. The electronic effect of para substituents was investigated by quantum
chemical methods employing the simple olefin adducts [HPt(PH3)2(olefin)(SnCl3)]. Excellent linear correlation was found
between the para substituent constants and the electrostatic potential at nuclei of the platinum atom. Equally good correlation
has been established for the other atoms as well in the coordination sphere of Pt.
situ systems formed from platinum−alkyl/aryl complexes and
boron additives was shown.10
INTRODUCTION
■
The importance of the highly selective hydroformylation of
alkenes has mainly been shown in two fully different fields: (i)
the regioselective hydroformylation of propene to give the
linear aldehyde regioisomer n-butyraldehyde, in the presence of
cobalt- and rhodium-containing catalysts, has successful
industrial-scale application and has seen detailed mechanistic
investigations1,2 and (ii) the enantioselective hydroformylation
of vinyl aromatics to 2-arylpropanals in the presence of
rhodium and platinum catalysts has potential applications in the
synthesis of nonsteroidal anti-inflammatory drugs, such as
ibuprofen, naproxen, and suprofen.3,4
As for the platinum-catalyzed hydroformylation, soon after
the discovery of the hydroformylation activity of platinum−
monophosphine−tin(II) halide type in situ systems,5 the
corresponding “preformed” PtCl(SnCl3)(chiral diphosphine)
catalysts and PtCl2(chiral diphosphine) + tin(II) halide in situ
systems were used in enantioselective hydroformylation.6 Even
the first publications have shown the efficiency of these systems
both in the synthesis of chiral 2-arylpropanal derivatives and in
that of the chiral building blocks obtained in the highly enantio-
and regioselective hydroformylation of 1,1-disubstituted
olefins.7 To increase enantioselectivities and catalytic activities
in platinum-based hydroformylation, widespread investigations
were carried out using various types of chiral mono- and
bidentate phosphorus ligands.8 On the other hand, the
applicability of tin(II) halide free hydroformylation catalysts
such as diphenylphosphinous acid platinum complexes9 or in
While great efforts have been made in finding the optimum
performance of the catalysts in terms of activity and chemo-,
regio-, and enantioselectivity, fewer results have been published
on the better understanding of the fine mechanistic details. The
elementary steps of the generally accepted mechanism such as
alkene coordination and its insertion into the Pt−H bond,
carbon monoxide coordination and its insertion into the Pt−
alkyl bond, and the formation of the aldehyde in the
hydrogenolysis of the Pt−acyl complex have been investigated
by both analytical11 and computational12 means. (It is worth
noting that a number of similar studies have been published on
the mechanism of rhodium-catalyzed hydroformylation.13,14
)
The importance of the good leaving properties of the
trichlorostannato ligand forming cationic intermediates with
trichlorostannate counterion was also shown by high-pressure
NMR studies.15
Regarding the fundamental understanding of the enantiose-
lectivity in hydroformylation, and especially the exciting
phenomenon of the reversal of enantioselectivity in the
hydroformylation of styrene8d,e in the presence of PtCl(SnCl3)-
[(2S,4S)-BDPP)] precursor, a seminal work was published by
Casey et al.16 Their investigations on the platinum-catalyzed
deuterioformylation of styrene established that the platinum
hydride addition to styrene, i.e., styrene insertion into the Pt−
Received: November 13, 2013
Published: March 10, 2014
© 2014 American Chemical Society
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31G(d,p) basis set22 was employed. Optimized structures were
identified by the absence of the negative eigenvalues. The electrostatic
potential values were directly obtained from the Gaussian 09 output of
the MESP calculations. Charge decomposition analysis (CDA)
calculations23 have been performed utilizing the AOMix software.24
H bond forming a Pt−alkyl intermediate, is largely irreversible
at low temperature (40 °C) but reversible at higher
temperature (100 °C).
Landis and co-workers reported the Rh-catalyzed hydro-
formylation of terminal and internal aryl alkenes employing
diazaphopholane ligands. 4-Substituted styrenes provided high
regio- and enantioselectivities. Interestingly, in the presence of
(S,S,S)-BisDiazaphos ligand the Hammett constant of the para
substituents showed a linear correlation with the logarithm of
the branched/linear ratio of the aldehydes. Both the
regioisomer and enantiomer ratios were found to be propor-
tional with the carbon monoxide partial pressure but
approximately independent of the H2 pressure. Deuterioformy-
lation studies revealed that the pressure effect arises from the
kinetic competition between the reaction of the branched alkyl
rhodium complex with CO, resulting in the corresponding acyl
complex, and the reversion of the alkyl species to Rh hydride
and olefin by β-elimination.17
RESULTS
■
To determine whether the substituents of styrene have any
influence on the enantioselectivity and the temperature
dependence of enantioselectivity, hydroformylation experi-
ments of various 4-substituted styrenes (1a−g) catalyzed by
an in situ system, formed from PtCl(SnCl3)[(2S,4S)-BDPP]
and tin(II) chloride, were conducted at various temperatures
with 80 bar of a CO/H2 (1/1) mixture (Scheme 1). It is worth
Scheme 1. Hydroformylation of 4-Substituted Styrenes in
the Presence of PtCl(SnCl3)[(2S,4S)-BDPP] + SnCl2
Catalyst
This paper describes the asymmetric hydroformylation of
seven 4-substituted styrenes in the presence of an in situ
catalyst formed from PtCl(SnCl3)[(2S,4S)-BDPP] and tin(II)
chloride. Here we report on the influence of the electron-donor
or electron-acceptor properties of the para substituents of
styrene, which show characteristic changes in the enantiose-
lectivity and, especially, in the temperature of the reversal of the
enantioselectivity. The second goal of this study is to establish a
clear correlation between the electronic effects of the para
substituents and the Hammett substituent constant by means of
a selected quantum chemical descriptor, namely the electro-
static potential at nuclei for all atoms in the coordination sphere
of platinum, including the Pt center itself.
EXPERIMENTAL SECTION
■
noting that some catalytic activities were observed even at room
temperature; however, the evaluation of the catalysts was
carried out in the temperature range 40−120 °C.
Regarding chemoselectivity toward aldehydes, two features
have to be emphasized. (i) The tendency of decreasing
chemoselectivity toward aldehydes with increasing reaction
temperature was observed with all substrates 1a−g. For
example, 92, 90, 86, 80, and 73% chemoselectivities were
obtained using 1b as substrate at 40, 60, 80, 100, and 120 °C,
respectively (Figure 1).
(ii) On comparison of chemoselectivities obtained with
various substrates at the same temperature, small differences
have only been observed at low reaction temperatures, while a
wider range of chemoselectivity was obtained at higher
temperatures. For example, the chemoselectivities obtained
for 1a−g at 60 and 100 °C were varied in the ranges of 89%
(1g) to 94% (1f) and of 79% (1g) to 88% (1f), respectively
(Figure 1).
The regioselectivity toward branched aldehydes (2a−g)
show a characteristic decrease by increasing temperature. While
the two aldehyde regioisomers were formed in nearly equimolar
amounts at 40 °C with all substrates, the highly preferred
formation of the linear aldehydes (3a−g) was observed at 100
°C. An especially low preference for the branched aldehyde
regioisomers was shown with 1b−d, containing the electron-
acceptor substituents 4-trifluoromethyl, 4-chloro, and 4-fluoro,
respectively (Figure 2).
General Considerations. The PtCl2(PhCN)2 precursor was
synthesized from PtCl2 (Aldrich) according to a standard procedure.18
The PtCl2[(2S,4S)-BDPP] and PtCl(SnCl3)[(2S,4S)-BDPP] precur-
sors were synthesized as described in one of our earlier papers.8d,e
Toluene was distilled and purified by standard methods and stored
under argon. 4-Substituted styrenes and tin(II) chloride (anhydrous)
were used as obtained from Sigma-Aldrich without any further
purification. All reactions were carried out under an argon atmosphere
using standard Schlenk techniques.
Mass spectrometry data have been obtained using a GC-MS system
consisting of a Perkin-Elmer AutoSystem XL gas chromatograph. The
GC and chiral GC measurements were run on a Chrom-Card Trace
GC-Focus GC gas chromatograph. The enantiomeric excesses were
determined on a capillary Cyclodex column; (S)-2-arylpropanals
eluted before the R enantiomers.
Hydroformylation Experiments. In a typical experiment, a
solution of PtCl(SnCl3)[(2S,4S)-BDPP] (9.0 mg; 0.01 mmol) and
tin(II) chloride (1.9 mg; 0.01 mmol) in toluene (10 mL) containing
styrene derivatives (1a−g; 2.0 mmol) was transferred under argon into
a 100 mL stainless steel autoclave. The reaction vessel was pressurized
to 80 bar total pressure (CO/H2 = 1/1) and placed in an oil bath of
constant temperature. The mixture was stirred with a magnetic stirrer
for the given reaction time. The pressure was monitored throughout
the reaction. After cooling and venting of the autoclave, the pale yellow
solution was removed and immediately analyzed by GC-MS and chiral
GC.
Computational Details. For all of the calculations the PBEPBE
gradient-corrected functional by Perdew, Burke, and Ernzerhof19 was
selected using the Gaussian 09 suite of programs.20 For platinum and
tin the Stuttgart/Dresden basis set (denoted as SDD) was used, which
is triple-ζ for platinum and double-ζ for tin with contraction patterns
of (8s,7p,6d) → (6s,5p,3d) and (4s,4p) → (2s,2p), treating 60 and 46
electrons as core electrons, respectively.21 For all other atoms the 6-
The diagram of ee vs reaction temperature, obtained in the
asymmetric hydroformylation of 4-substituted styrenes 1a−g
(Figure 3), reflects the same phenomenon as observed
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Figure 1. Effect of temperature on the chemoselectivity toward aldehydes in the hydroformylation of 1a−g.
Figure 2. Effect of temperature on the regioselectivity toward branched aldehyde in the hydroformylation of 1a−g.
Figure 3. Effect of temperature on enantioselectivity in the hydroformylation of 1a−g.
previously with styrene (1a) in the presence of Pt-BDPP-tin(II)
halide8d,e,n and Pt-BDPP-4-NMe2-tin(II) halide8m in situ
catalysts or in the presence of the corresponding PtCl(SnCl3)-
(BDPP) precursor.
with fluoro (σpara = +0.062), chloro (σpara = +0.227), and
trifluoromethyl (σpara = +0.540) substituents, respectively.
Accordingly, a decrease of the reversal temperature was
observed with electron-donor substituents. That is, the reversal
of the enantioselectivity occurred at 95 and 92 °C with methyl
(σpara = −0.170) and methoxy (σpara = −0.268) substituents,
respectively. The only exception that does not fit into this
tendency is the acetoxy substituent (σpara = +0.310), with a
reversal temperature of 87 °C. Its Hammett constant would
predict an increased reversal temperature (above 100 °C), close
to that of the chloro substituent. In contrast to data published
earlier,23 the above catalytic investigations predict a much more
negative Hammett parameter for p-acetoxy substituents. The
re-evaluation of σp for the OAc group has been completed by
That is, the formation of the (S)-2-arylpropanals 2a−g is
favored at low temperatures, while the R enantiomers
predominate at higher temperatures also in the present
investigation. When the temperature of the reversal of the
enantioselectivity, i.e., the temperature of the change of the
absolute configuration of the dominating enantiomer, is plotted
against Hammett constants (σpara) (Figure 4), a correlation was
observed. It was found that the substituents with electron-
acceptor properties increase the reversal temperature. The
reversal of enantioselectivity occurred at 102, 105, and 107 °C
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Figure 4. Reversal temperatures of the enantioselectivity plotted against the Hammett constants.
employing quantum chemical descriptors as well as by
computing the relative acidity of substituted benzoic acids in
the water phase.26 As a result of both approaches a new value of
−0.02 has been suggested, urging the revision of the old
Hammett parameter known from the literature.
It is worth noting that the temperature of the reversal of
enantioselectivity, i.e., the reaction temperature resulting in
racemic mixtures, was determined experimentally as accurately
as possible for 1a−g. Therefore, hydroformylation reactions
were carried out at intermediate temperatures in this range. For
instance, the catalytic experiments were conducted at 92 and 87
°C in the case of 1f (ee 1% (R)) and in the case of 1g (ee 1%
(R)), respectively.
catalyzed hydroformylation containing the (2S,3S)-chiraphos)
ligand, which forms a more rigid chelating ring with platinum in
comparison to BDPP.27 However, the reversibility of the same
step was observed by Casey at higher temperature, resulting in
free styrene which is prone to insert into the platinum−hydride
bond to provide the opposite alkyl enantiomer (Scheme 3).
Scheme 3. Formation of the (R)-2 Branched Formyl
Regioisomers at High Temperature
DISCUSSION
■
We focused our attention on the stereodefining step of the
enantioselective hydroformylation with Pt-BDPP systems, that
is, on the stage, where the stereochemical outcome of the
asymmetric hydroformylation is determined. We aimed at
explaining the characteristic influence of the 4-substituent of
styrene on the reversal of the enantioselectivity as a function of
reaction temperature. In accordance with previous findings, the
preferred formation of (S)-2-arylpropanals have been observed
at low temperature, while the R enantiomers predominate at
higher temperature when the hydroformylation of the
corresponding 4-substituted styrene 1a−f was carried out in
the presence of Pt-(2S,4S)-BDPP-tin(II) chloride catalytic
systems.
The detailed deuterioformylation experiments of Casey et al.
revealed that the formation of the Pt-alkyl intermediate is
largely irreversible at low temperature: i.e., β-hydride
elimination is not favored (Scheme 2). Therefore, the
predominating enantiomer (S)-2 using (2S,4S)-BDPP) is
determined by the difference in the relative rates of the
formation of the Pt-(S)-alkyl and Pt-(R)-alkyl intermediates.
The olefin insertion as an enantioselectivity-determining step
was also corroborated by computational studies, modeling Pt-
One of the key points of these investigations is the
determination of the relative rates of hydrogenolysis (prod-
uct-forming step): the hydrogenolysis of the Pt-(S)-acyl
enantiomer is slower than that of the corresponding Pt-(R)-
acyl species by a factor of about 6 using Pt-(2S,4S)-BDPP
catalyst.16
Our results have shown that the reversal of the
enantioselectivity occurs in the temperature range 87−107 °C
depending on the electron-donor/electron-acceptor properties
of the 4-substituents of the substrate. Keeping in mind the
above steps which determine the formation of (S)- and (R)-
2a−g, it could be stated that the electron-acceptor substituents
decrease the reversibility of the Pt-alkyl-forming step as a
function of temperature. Since the reversal temperature in the
hydroformylation of 1b is increased relative to that of styrene
(1a), a facile insertion of carbon monoxide but not a β-hydride
elimination resulting instyrene can be supposed.
Scheme 2. Formation of the (S)-2 Branched Formyl
Regioisomers at Low Temperature
According to the present studies, the electron-donor
substituents increase the reversibility of Pt-alkyl formation;
i.e., they decrease the reversal temperature of enantioselectivity.
These substituents, such as CH3 and OCH3, increase electron
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Figure 5. Computed structures of uncoordinated styrene (1a) and model compound HPt(PH3)2(SnCl3)(styrene) (5a).
density on the Pt−carbon bond of the (S)-alkyl intermediate.
In contrast to the above cases involving electron-acceptor
substituents, β-hydride elimination is preferred to carbon
monoxide insertion, resulting in the “re-formed” styrenes 1e,f,
respectively. Their insertion into the Pt−H bond leads to the
favored formation of the corresponding R enantiomers (R)-2e
and (R)-2f, respectively.
EPN of the acidic hydrogen of substituted benzoic acid showed
excellent linear correlation with the para Hammett constants.26
The C1−C2 bond undergoes substantial elongation upon
coordination to the platinum center. The change in bond
strength is also reflected in the C1−C2 Wiberg bond indexes
(WBIs), which are 1.885 and 1.358 for 1a, and 5a, respectively.
The WBI for C2−C3 is also decreased from 1.105 to 1.082
when styrene is coordinated to the Pt center.
The Hammett para substituent constants for 1a−h, as well as
those for complexes 5a−h, are depicted in Figure 6 in the
function of the EPN of the terminal vinylic carbon (C1).
Excellent linear correlation has been obtained in both cases
with σp; somewhat better for substituted styrenes (r2 = 0.978)
than for their corresponding olefin complexes (r2 = 0.961). As
expected, the EPN drops off upon coordination to platinum, in
line with the decrease of electron density around the terminal
carbon due to the electron donation to the HPt(PH3)2(SnCl3)
moiety.
The C2 atom of the vinyl group shows a very similar
behavior with a somewhat smaller, yet still satisfactory linear
correlation (r2 = 0.932) (see Figure S1 in the Supporting
Information). Better linear correlation (r2 = 0.967) have been
achieved between the EPN of platinum and σp (see Figure S2 in
the Supporting Information). The good linear correlation
remains between σp and the phosphorus atoms as well, as the r2
values are 0.958 and 0.961 for the P atom of the axial and
equatorial PH3 ligands, respectively. The equatorial phosphorus
is more negative in comparison to the axial phosphorus, which
may be attributed to the donating effect of the in-plane vinyl
group, resulting in a slightly greater increase in electron density
of the coordinating atoms belonging to the equatorial ligands
(Figure S3 in the Supporting Information). Similarly, the EPN
of the hydrido ligand also correlates well with σp of the para
substituent (r2 = 0.949; Figure S4 in the Supporting
Information). Surprisingly, the best linear correlation has
been achieved for the tin atom of SnCl3, with r2 = 0.972.
Figure 7 depicts the relationship between the EPN of tin and σp
of the para substituents of styrenes.
COMPUTATIONAL STUDIES
■
In order to elucidate the effect of the para substituent on
styrene, the simple model compounds HPt(PH3)2(SnCl3)(4-
substituted styrene) (see Figure 5), denoted as 5a−h, have
been selected, providing a straightforward description for Pt-
phosphine-SnCl2 systems containing a coordinated styrene. In
comparison to experiments, the set of substituents has been
extended with the uncoordinated and coordinated 4-phenyl-
styrene, which are denoted as 1h and 5h, respectively. The
metal-hydrido-olefin adduct is usually high in terms of free
energy, but it is not available as a separate complex to study
spectroscopically, although it plays a vital role in the key step of
olefin activation.
The molecular electrostatic potential (MESP) is a function of
the electron density. The three-dimensional MESP is a
fundamental quantity, extensively applied for chemical and
biological reactivity.28 The MESP, for instance, in the form of
an isosurface, can delineate negative regions in a molecule
which are subject to electrophilic attack. Lone pair regions in P-
donor ligands, for example, reveal negative MESP in the lone
pair region and can readily be characterized by the minimum of
the most negatively valued point (Vmin).29 For the coordinating
power of NHC ligands the MESP at the carbene carbon
revealed even better correlation with Tolman electronic
30
parameters in comparison to Vmin
.
For quantum chemical
descriptors in the present study the electrostatic potential at
nuclei (EPN) has been chosen, which is a very frequently used
electronic property in chemistry for describing substituent
effects. The EPN can be calculated using eq 1, where ZA is the
ZA
ρ(r′)
|RY − r′|
VY ≡ V(RY) =
−
dr′
∑
Thus, the para substituent of styrene has an effect not only
on the electron density of the coordinating vinyl group but also
on the platinum center as well as on the other atoms in the
coordination sphere of Pt. The effect of σp on the electrostatic
potential in the nucleus of Sn (VSn) is depicted in Figure 7. The
linear correlation between the σp and all atoms in the platinum-
hydrido-olefin complex, which may play a crucial role
∫
|RY − RA|
(1)
A≠Y
nuclear charge of atom A with radius vector RA and ρ(r′) is the
electron density of the molecule. It was found recently that the
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Figure 6. Relationship between the Hammett para substituent constants and the electrostatic potential in the nucleus of the terminal carbon atom of
uncoordinated 4-substituted styrenes (top) and of those in the corresponding complexes [HPt(PH3)2(4-substituted styrene)(SnCl3)] (bottom).
Figure 7. Relationship between the Hammett para substituent constants and the electrostatic potential in the nucleus of tin in the complexes
[HPt(PH3)2(olefin)(SnCl3)].
influencing the outcome in every elementary step in Pt-
catalyzed hydroformylation, may provide an explanation why
measurable quantities of the catalytic reaction, such as the
enantioselectivity, show correlation with the σp of the
substituent on styrene. In accord with our recent studies26 on
para-substituted benzoic acids and substituted benzenes the
computed σp of the OAc group reveals a remarkable difference
from the value known from the literature (+0.31).25
It should be noted that the reason for the use of PH3 as
ligand was to eliminate the steric and other effects of the
phosphine, since in our case only the electronic influence of the
4-substituted styrene was in question. Nonetheless, the
calculations were repeated for the cis-[HPt(PPh3)2(SnCl3)(4-
substituted styrene) complexes as well, in order to inspect the
effect of triphenylphosphine (see Figures S5−S7 in the
Supporting Information). Again, for Pt and Sn good linear
correlation was obtained with σp (0.972, and 0.916,
respectively); however, much worse correlation was found for
the hydride ligand (0.569), which might be attributed to the
combined steric and electronic effects of the triphenylphos-
phine.
The steric bulk of phenyl groups could be the reason no
apparent relationship could be found between the σp values of
the para substituents and the binding energy of styrenes to the
platinum center. In the presence of PH3 ligands, however, a
linear correlation of r2 = 0.908 has been found for these two
quantities, and even better correlation, r2 = 0.933, has been
obtained between the electrostatic potential in the nucleus of Pt
and the ZPE-corrected binding energies of the 4-substituted
styrenes (Figure 8). Moreover, invoking charge transfer analysis
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Figure 8. Relationship between the electrostatic potential in the nucleus of Pt in complexes [HPt(PH3)2(olefin)(SnCl3)] and the ZPE-corrected
binding energies of para-substituted styrenes.
(CDA), a linear correlation of r2 = 0.934 has been also found
between the σp and the amount of back-donation (BD) from Pt
ACKNOWLEDGMENTS
This paper is dedicated to Professor Las
■
́
zlo
́
Marko
́
on the
to the coordinated olefins. The range of BD is between 0.258
electron for the OMe substituent and 0.279 electron for the
most withdrawing CF3.
These results also emphasize the importance of the thorough
examination of substrate electronic effects as well, which is
sometimes underrated in comparison to the electronic effect of
phosphines and other P-donor ligands.
occasion of his 85th birthday. The authors thank the
Supercomputer Center of the National Information Infra-
structure Development (NIIF) Program. The support of the
“Synthesis of supramolecular systems, examination of their
physicochemical properties and their utilization for separation
and sensor chemistry” project (SROP-4.2.2.A-11/1/KONV-
2012-0065) is also gratefully acknowledged. P.P. acknowledges
the support of the “National Excellence Program” in the
framework of SROP-4.2.4.A/2-11/1-2012-0001.
SUMMARY
■
As a summary it could be stated that the electronic properties
of the 4-substituents of the parent styrene influence the reversal
temperature of the enantioselectivity in platinum-(2S,4S)-
BDPP-tin(II) chloride-catalyzed asymmetric hydroformylation.
When the electron-donating character of the para substituent is
increased, and hence the electron density of the aromatic ring
of the substrate, a reversal of enantioselectivity has been
observed at lower temperature. Moreover, the presence of
electron-donating substituents results in higher branched
regioselectivities at higher temperatures in comparison to
styrenes containing electron-withdrawing para substituents.
These experiments provide further evidence for the reversibility
of the Pt-alkyl intermediate formation at higher temperature.
On examination of the electronic structure of the model
platinum complex with substituted styrenes, it is found that not
only are the electrostatic potentials at nuclei (EPN) of carbons
of the coordinating vinyl group directly influenced by the para
substituent but also the linear correlation is established between
the σp and the EPN of all atoms in the coordination sphere of
platinum, including the Pt center itself. The consequence of this
computational study calls attention to the importance of
substrate electronic effects, which may play a role in structure−
selectivity relationships equally important as the electronic and
steric effects attributed to P-donor ligands.
REFERENCES
■
(1) For reviews and book chapters see: (a) Ojima, I.; Tsai, C.-Y.;
Tzamarioudaki, M.; Bonafoux, D. The Hydroformylation Reaction. In
Organic Reactions; Overman, L., et al., Eds.; Wiley: Hoboken, NJ, 2000;
pp 1−354. (b) Claver, C.; van Leeuwen, P. W. N. M. Rhodium
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ASSOCIATED CONTENT
■
S
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
(4) For recent research papers in rhodium-catalysed asymmetric
hydroformylation see: (a) Jouffroy, M.; Semeril, D.; Armspach, D.;
Matt, D. Eur. J. Org. Chem. 2013, 27, 6069. (b) Fernandez-Perez, H.;
Benet-Buchholz, J.; Vidal-Ferran, A. Org. Lett. 2013, 15, 3634.
(c) Adint, T.; Wong, G.; Landis, C. J. Org. Chem. 2013, 78, 4231.
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X.; Cao, B.; Liu, T.; Zhang, X. Adv. Synth. Catal. 2013, 355, 679.
AUTHOR INFORMATION
■
Notes
The authors declare no competing financial interest.
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