Interception and characterization of alkyl and acyl complexes in rhodium-catalyzed hydroformylation of styrene

Article abstract of DOI:10.1021/ja404799m

Reaction of [Rh(H)(CO)₂(BDP)] (BDP = bis(diazaphospholane)) with styrene at low temperatures enables detailed NMR characterization of four- and five-coordinate rhodium alkyl complexes [Rh(styrenyl)(CO)_n(BDP)] presumed to be intermedi

Full text of DOI:10.1021/ja404799m

Communication
pubs.acs.org/JACS
Interception and Characterization of Alkyl and Acyl Complexes in
Rhodium-Catalyzed Hydroformylation of Styrene
Eleanor R. Nelsen and Clark R. Landis*
Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States
S
* Supporting Information
Scheme 2. General Mechanism for Hydroformylation
ABSTRACT: Reaction of [Rh(H)(CO)2(BDP)] (BDP =
bis(diazaphospholane)) with styrene at low temperatures
enables detailed NMR characterization of four- and five-
coordinate rhodium alkyl complexes [Rh(styrenyl)-
(CO)n(BDP)] presumed to be intermediates in rho-
dium-catalyzed hydroformylation. The five-coordinate acyl
complexes [Rh(C(O)styrenyl)(CO)2(BDP)] are also
observed and characterized. The equilibrium distribution
of these species suggests an inversion of thermodynamic
preference for branched vs linear species from the alkyl to
the acyl stage.
hodium-catalyzed hydroformylation of alkenes (Scheme
R1) has long been important to the production of
commodity chemicals and increasingly shows promise for
fine, and even enantiopure, chemical production.1
Scheme 1. Hydroformylation of Alkenes
A general mechanism was first presented by Breslow and
Heck for cobalt-catalyzed hydroformylation some 50 years ago
(see Scheme 2).2 However, fundamental questions concerning
the origins of regioselectivity and enantioselectivity in rhodium-
catalyzed hydroformylation remain unanswered because key
reaction intermediates have not been characterized structurally,
Figure 1. Structure of (S,S)-3,4-bis(diazaphospholane) (BDP). This
work uses rac-BDP with R = H.
kinetically, or thermodynamically. Herein we report the first
for the enantioselective hydroformylation of various alkene
interception of linear and branched rhodium alkyl intermediates
substrates.8
along a hydroformylation pathway, characterization of their
Previous work from this group on the Rh(BDP)-catalyzed
structures by multinuclear NMR spectroscopy, and estimation
hydroformylation of styrene demonstrated that both the
of their relative thermodynamic stabilities. Furthermore we
branched-to-linear ratio and percent ee of the product increases
demonstrate that the thermodynamic preference for linear vs
branched isomers inverts upon conversion of the rhodium
alkyls to the rhodium acyls.
Synthesis of new phosphine ligands has enabled remarkable
with increasing partial pressure of CO (PCO). These
observations, along with the results of isotopic labeling
experiments and in situ IR monitoring, are consistent with a
kinetic model in which CO dissociation from the resting state 1
progress in controlling the chemo-, regio-, and enantioselectiv-
ity of rhodium-catalyzed hydroformylation.3 Recent design
is required to initiate the cycle. For the kinetically favored (R)-
motifs include scaffolding effects,4,5 secondary coordination
branched alkyl species 4, β-hydride elimination is competitive
sphere hydrogen bonding,6 and supramolecular constructions.7
with trapping by CO to give product. Thus, CO promotes
The 3,4-bis(diazaphospholane) (BDP) class of ligands
introduced in 2004 (see Figure 1) combine high activity
Received: May 20, 2013
greater than one turnover per secondand useful selectivities
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
formation of (R)-branched product but inhibits entry of catalyst
from the resting state, leading to an overall independence of the
rate with respect to [CO]. Production of linear and (S)-
branched aldehydes show inhibition, only, by CO.9
new ligand-containing species. The two major species are the
five-coordinate linear and branched acyl complexes 7l and 7b
(see Chart 1), each of which has two distinct phosphorus
However, the data supporting this kinetic model are indirect;
direct observation of critical intermediates could provide
additional insight into the fundamental thermodynamics and
kinetics that control selectivity. To date the most detailed
characterization of species along the hydroformylation pathway
comes from Brown and Kent’s NMR studies of the reaction of
styrene with [Rh(H)(CO)(PPh3)3] in the absence of H2.10
These studies revealed formation of branched and linear acyls
of the formula Rh(acyl)(PPh3)2(CO)2, with the branched acyl
formed first and followed by equilibration to the more stable
linear acyl. Since that time, however, the few examples of
related intermediates have been limited to slow and/or
unselective catalysts or synthesis of analogous iridium
complexes.11 Because Rh(BDP) catalysts exhibit such high
hydroformylation activities, we speculated that reaction of
styrene with RhH(BDP)(CO)2 at low temperatures and in the
absence of H2 would proceed to alkyl or acyl intermediates at
rates convenient for characterization by NMR.
The hydrido dicarbonyl complex 1 (made with rac-BDP, R =
H) cleanly forms upon exposure of [Rh(acac)(BDP)] to 140
psi 1:1 CO/H2 overnight at 60 °C in dichloromethane. The 1H
and 31P NMR spectra of 1 are consistent with a fluxional
trigonal bipyramidal species with axial−equatorial coordination
of the bisphosphine, as has been seen for other small-bite-angle
ligands.12
Under a narrow range of conditions (ca. 1 atm CO, 0 to −30
°C), the reaction of this rhodium hydride with 5 equiv of
styrene can be monitored directly by 1H and 31P NMR
spectroscopy. A representative set of 31P{1H} spectra, showing
the course of the reaction at −20 °C under a CO atmosphere, is
presented in Figure 2. The phosphorus signal of the hydride
complex 1 (δ 83 ppm) disappears in a roughly first-order
fashion, with a half-life of about 30 min. It is replaced by several
Chart 1. Rhodium Alkyl and Acyl Complexes Generated
from the Reaction of [Rh(H)(CO)2(BDP)] with Styrene
a
a
Chiral centers denoted by (*).
resonances at δ 69.1 and 59.3 ppm and δ 69.8 and 62.4 ppm,
respectively. Each peak is a doublet of doublets; the larger
1
2
coupling is JPRh, and the smaller is JPP.
The branched (7b) and linear (7l) regioisomers are
distinguished by 31P−1H multiple-bond correlation experiments
which show through-bond coupling from the ligand phospho-
rus atoms of 7b to a methine quartet and methyl doublet, and
from those of 7l to methylene protons. Acyl carbonyl signals,
characterized by their far-downfield chemical shifts, confirm the
assignment as acyl rather than alkyl complexes (see Figure 3).
The chemical shifts and coupling constants are consistent with
those reported by Brown for the five-coordinate acyl complexes
of triphenylphosphine.10b
The other diastereomer of the branched acyl complex, 7b′,
cannot be distinguished in the relatively crowded 31P NMR
Figure 2. 31P{1H} NMR spectra (202.5 MHz) monitoring the reaction
of [Rh(H)(CO)2(BDP)] (1; 83 ppm; 60 mM) with styrene under a
CO atmosphere at −20 °C in CH2Cl2. Spectra show acyl complexes
7b, 7l and alkyl complexes 4b, 4b* growing in over time. Conversion
to aldehydes <1%.
Figure 3. Phosphorus (left) and acyl carbon (right) signals for 7l
generated using 13CO. Coupling of both Pax and Peq to two distinct
carbonyl ligands establishes coordination number. 31P{1H} NMR,
202.5 MHz; 13C{1H} NMR, 125.8 MHz; both in CH2Cl2 at −20 °C.
Labeled carbons are denoted in the figure by (*).
B
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Journal of the American Chemical Society
Communication
spectrum; however, the 13C NMR spectrum shows a doublet 1
ppm upfield from the methine resonance of 7b which is
consistent with 7b′.
complex is kinetically favored (consistent with the conclusion of
earlier studies that insertion to form a branched alkyl is fastest),
but isomerizes to the thermodynamically favored linear acyl
complex ([7l]:[7b] ≈ 35:1).17 Moreover, 7b appears to be
favored over diastereomeric 7b′ by a ratio of 18:1. Equilibration
between the acyls 7b and 7l must occur via the alkyls 4b and 5l.
Thus, 4b and 5l must also be equilibrated under conditions
where the acyls are in equilibrium; they appear in a ratio of
[4b]:[5l] ≈ 9:1. Although monocarbonyl 4b and dicarbonyl 5l
differ in the number of terminal CO ligands, these data
demonstrate a thermodynamic preference for the branched alkyl
regioisomer18 (and facile coordination of CO to the minor
species 4l).
While both four-and five-coordinate acyl complexes are
possible under these conditions, the coupling pattern of the acyl
complexes generated using 13CO rules out four-coordinate
species. Figure 3 shows that PA is coupled to three 13C nuclei,
demonstrating that species 7b,l are five-coordinate trigonal
bipyramidal species with two terminal CO ligands.13
Similar acyl species have been observed in the reactions of 1
with allyl cyanide and vinyl acetate; details will be reported in a
future publication.
Under conditions of low [CO], three species with 31P
resonances at δ 50 and 79 ppm, δ 73.7 and 74.4 ppm, and δ 57
and 78 ppm become more prominent. We have characterized
these as the branched rhodium−alkyl monocarbonyl complex
4b (see Figure 4),14 the η3-benzyl complex 4b* resulting from
CO dissociation from 4b, and the linear alkyl−rhodium
dicarbonyl complex 5l, respectively (see Chart 1).
The isomerization of 7b to 7l must involve β-hydride
elimination to give an olefin hydride complex (3 in Scheme 2).
Does isomerization proceed with styrene dissociation9 or is it
intramolecular? To address this question, we added p-CF3-
styrene to an equilibrated mixture of 7b and 7l (see Figure 5).
Figure 4. Phosphorus (left) and methine carbon (right) signals for 4b
generated using 13CO. PA and PB are coupled to only one carbonyl
ligand. 31P{1H} NMR, 202.5 MHz; 13C{1H} NMR, 125.8 MHz; both
in CH2Cl2 at −20 °C. Labeled carbons are denoted in the figure by
(*).
Figure 5. Reaction of 1 with styrene at −20 °C in CD2Cl2, followed by
reaction (time = 12500 s) of the equilibrated mixture of 7b and 7l with
p-CF3-styrene. Concentrations calculated from 31P{1H} NMR spectra.
Multinuclear NMR and judicious use of 13C-labeled styrene
(in the α and β positions) and 13CO provide definitive
characterization.
For 4b the methyl and methine carbons appear at δ 37.7 and
20.3 ppm, respectively. The characterization of 4b* as a
formally 3-coordinate η3-benzyl species is supported by the
observation of strong (38 Hz) 31P−13C coupling when α-13C-
labeled styrene is used, the nearly identical 31P chemical shifts
of the two ligand phosphorus atoms as seen in rhodium allyl
bis(phosphine) complexes,15 and the absence of additional
splitting in the 31P NMR spectrum when 13CO is used. The
methine carbon shows a measurable (9.8 Hz) carbon−rhodium
coupling in addition to carbon−phosphorus coupling. In 5l, the
β-methylene carbon (trans to phosphorus) is upfield-shifted to
δ 10.3 ppm, and is coupled to phosphorus (60.4 Hz) and
rhodium (13.8 Hz). To the best of our knowledge, this is the
first direct observation of alkyl intermediates for any rhodium
hydroformylation catalyst.16 (See Supporting Information for
selected spectroscopic data for 7b, 7l, 4b, 4b*, and 5l.)
The characterization of linear and branched acyl and alkyl
complexes provides unique insights into kinetic and thermo-
dynamic influences in hydroformylation. The branched acyl
The rate of formation of the CF3-containing linear acyl complex
(which requires olefin dissociation, followed by branched acyl
formation and subsequent isomerization) is comparable to the
rate of isomerization of 7b to 7l. Such results are consistent
with a pathway for 7l to 7b isomerization that proceeds with
olefin dissociationthat is, through rhodium hydrido carbonyl
species 2.
In conclusion, we have characterized five-coordinate rhodium
acyl and four- and five-coordinate alkyl complexes of Rh(BDP)
catalysts. To the best of our knowledge, this is the first example
of characterization of an alkyl intermediate for any rhodium
hydroformylation catalyst and the first example of detailed
characterization of an acyl complex for an enantioselective
hydroformylation catalyst. All data point to a strong kinetic
preference for formation of the branched alkyl and acyl species.
In contrast, thermodynamics appear to favor the linear isomer
at the acyl stage but the branched isomer at the alkyl stage.
These observations indicate that a delicate balance of rates and
thermodynamics leads to the common observation of strong
C
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Journal of the American Chemical Society
Communication
(5) (a) Grunanger, C. U.; Breit, B. Angew. Chem., Int. Ed. 2008, 47,
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dependence of regioselectivity upon reaction conditions for
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ASSOCIATED CONTENT
* Supporting Information
■
̌
(6) (a) Smejkal, T.; Breit, B. Angew. Chem. 2007, 120, 317−321.
S
̌
(b) Smejkal, T.; Breit, B. Angew. Chem. 2008, 120, 4010−4013.
̌
Experimental details and NMR spectra for 1, 4b,b*, 5l, and
7b,l. This material is available free of charge via the Internet at
http://pubs.acs.org.
(c) Smejkal, T.; Gribkov, D.; Geier, J.; Keller, M.; Breit, B. Chem.
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AUTHOR INFORMATION
Corresponding Author
landis@chem.wisc.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Dr. Charlie Fry for generous assistance with
NMR experiments, Prof. Charles P. Casey for helpful
discussions, and the NSF for funding (DGE-1256259 and
CHE-1152989).
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