α-Dicationic Chelating Phosphines: Synthesis and Application to the Hydroarylation of Dienes

Article abstract of DOI:10.1021/jacs.7b01441

A series of new P^P-chelating ligands constituted by a dicationic ?[P(H₂Im)₂]⁺² unit (H₂Im = 1,3-dimethyl-4,5-dihydroimidazol-2-ylidene) and a ?PPh₂ group connected through structurally different backbones have been synthesized. Evaluation of their reactivity toward different metal centers provides evidence that the dicationic fragment, otherwise reluctant to coordinate metals, readily participates in the formation of chelates when embedded into such a scaffold. Moreover, it significantly enhances the Lewis acidity of the metals to which it coordinates. This property has been used to develop a Rh catalyst that efficiently triggers the hydroarylation of dienes with electron-rich aromatic molecules. Kinetic studies and deuterium-labeling experiments, as well as density functional theory calculations, were performed in order to rationalize these findings.

Full text of DOI:10.1021/jacs.7b01441

Article
pubs.acs.org/JACS
α‑Dicationic Chelating Phosphines: Synthesis and Application to the
Hydroarylation of Dienes
Lianghu Gu,† Lawrence M. Wolf,‡ Adam Zielinski,† Walter Thiel,§ and Manuel Alcarazo*,†
́
†Institut fur Organische und Biomolekulare Chemie, Georg-August-Universitat Gottingen, Tammannstraße 2, 37077 Gottingen,
̈
̈
̈
̈
Germany
‡Department of Chemistry, University of Massachusetts Lowel, Lowell, Massachusetts 01854, United States
§Max-Planck-Institut fur Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mulheim an der Ruhr, Germany
̈
̈
S
* Supporting Information
ABSTRACT: A series of new P^P-chelating ligands constituted by a
dicationic −[P(H2Im)2]+2 unit (H2Im = 1,3-dimethyl-4,5-dihydroimidazol-2-
ylidene) and a −PPh2 group connected through structurally different
backbones have been synthesized. Evaluation of their reactivity toward
different metal centers provides evidence that the dicationic fragment,
otherwise reluctant to coordinate metals, readily participates in the formation
of chelates when embedded into such a scaffold. Moreover, it significantly
enhances the Lewis acidity of the metals to which it coordinates. This property
has been used to develop a Rh catalyst that efficiently triggers the
hydroarylation of dienes with electron-rich aromatic molecules. Kinetic studies
and deuterium-labeling experiments, as well as density functional theory
calculations, were performed in order to rationalize these findings.
R2P^P(L2)+2, evaluate the influence of the positive charges on
their donor properties by experimental and theoretical
methods, and provide a preliminary demonstration of their
beneficial effects in catalysis.
INTRODUCTION
■
The strategic placement of one or more positively charged
group(s) directly attached to the central phosphorus atom of
phosphines provides a useful entry, complementary to
polyfluorination, for the design of ligands of enhanced π-
acceptor character. In addition, the absence of labile P−halogen
bonds in these α-cationic phosphines significantly enhances
their robustness and facilitates their handling.1 Making use of
these distinctive attributes, we recently developed a series of
new Au(I) and Pt(II) catalysts where the natural π-acidity of
these metal centers is substantially increased and, as a result, an
unmatched activation of alkynes toward nucleophilic attack is
achieved.2 Unfortunately, the use of cationic ligands also faces
an important shortcoming: the strongest π-acceptor phosphines
that might be conceived, namely, di- and tricationic ones, show
very little propensity to form coordination complexes. Only Pt
and Au derivatives are known.3
In an attempt to overcome this intrinsic shortcoming and to
expand the use of polycationic ligands to other transformations,
which might also benefit from the unique electronic properties
that they impart, we envisioned the introduction of an
additional donating atom in the ligand architecture as an
anchor that should bring the cationic moiety into the vicinity of
the metal and thus induce its coordination. This strategy was
applied with success in the past to coordinate groups that were
reluctant to do so, such as monocationic nitrenium moieties or
boranes and their heavier analogues.4,5 In the following, we
describe the synthesis and coordination chemistry of several
dicationic chelating phosphines of the general structure
RESULTS AND DISCUSSION
■
Synthesis of Dicationic Chelating Phosphines. At the
beginning of our investigation, we decided to prepare
diphosphines 1 and 2, both containing a neutral −PPh2 moiety
and a strong acceptor dicationic [−P(H2Im)2]+2 group but
separated by structurally different linkers. In 1, the adjacent
position of both phosphorus atoms ideally favors their
simultaneous coordination to the incoming metal, whereas in
2, the more flexible 2,2′-biphenylene spacer relaxes geometric
restrictions and serves as a suitable model for future chiral
versions of these systems. Compounds 1a,b were obtained in
good yields through a two-step sequence consisting of the
condensation of known diphosphine 3 with commercially
available 2-chloro-1,3-(dimethyl)imidazolinium chloride 4,
followed by anion exchange (Scheme 1).6 This same synthetic
route was satisfactorily extended to the preparation of 2 once
the necessary primary phosphine 5 was made available from the
already described iodide 6.7
The 31P NMR spectrum of both compounds consists of two
doublets (δP = −8.5 (P2), −43.8 (P1) ppm, JPP = 212 Hz for 1;
and δP = −14.7 (P2), −44.1 (P1) ppm, JPP = 86.7 Hz for 2),
Received: February 10, 2017
© XXXX American Chemical Society
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noteworthy that the HOMO−LUMO gap for 1 is 1.0 eV lower
than that of the neutral analogue 7. In addition, AIM9 and
NBO10 analysis support the presence of a weak P−P bond: the
electron density and its corresponding Laplacian reveal a
closed-shell dative interaction with a Wiberg bond index of
0.124. Remarkably, this interaction cannot be attributed purely
to the proximity induced by the bridging ligand backbone;
when the phenyl backbone was replaced by two hydrogen
atoms, a constraint-free optimization of the fragments resulted
in a structure (1-frag) in which the computed P−P distance
was only slightly larger (3.13 Å) than that in 1 (2.99 Å, Table
1). The M06-L/TZVP binding energy in this intermolecular
Scheme 1. Synthesis and Molecular Structures of 1 and 2 in
the Solid State
a
a
Table 1. Geometries and Bonding Analysis for 1 and 1-frag
1
1-frag
1-frag
ρb(P1−P2)
∇2ρb(P1−P2)
BO
0.033
0.036
0.124
6.7
0.024
0.030
0.155
8.6
ΔEPauli
ΔEelstat
ΔEorb
ΔEdisp
ΔEint
ΔEdist
ΔE
34.2
−26.4
−29.6
−18.0
−39.8
3.5
nP1→σ*P1−C1
−36.3
a
Left: AIM parameters ρb and ∇2ρb, Wiberg bond index BO, and NBO
a
Hydrogen atoms, anions, and solvent molecules were removed for
perturbation energies (kcal/mol) for 1 and 1-frag. Right: Energy
decomposition analysis (kcal/mol) for 1-frag. Results from BP86-D3/
TZ2P//M06-L/TZVP calculations.
clarity; ellipsoids are set at 50% probability.8 Reagents and conditions:
(a) Et3N (2 equiv), THF, 60 °C; then NaSbF6 (excess) in CH3CN,
72%; (b) n-BuLi, ClP(O)(OEt)2, 47%; (c) LiAlH4, 85%; (d) 1 (2
equiv), Et3N (2 equiv); then NaSbF6 or NaBF4 (excess) (1a, 72%; 1b,
65%).
Lewis adduct based on P-donor/P-acceptor fragments was
found to be −31.3 kcal/mol.11 Finally, evaluation of the
physical nature of this interaction through energy decom-
position analysis12 at the BP86/TZ2P//M06-L/TZVP level
indicated that the most prominent contributions to P−P
bonding come from orbital interactions (ΔEorb; 40%), which
are stronger than the electrostatic and dispersion interactions
(see Table 1 and Supporting Information). The presence of this
capto-dative interaction already in the free ligand suggests that
the [P(H2Im)2]+2 moiety should behave as an excellent
acceptor provided that its coordination to metals can be
induced.
Evaluation of Electronic Properties. The global donor
ability of 1 was estimated through comparative analysis of the
CO stretching frequencies in the Mo complexes 8−11 (Figure
2). The νCO values recorded for dicationic 8 (2043, 1973, 1938
cm−1) are significantly higher than those observed for the other
Mo(0) pincer complexes tested, thus confirming the excep-
tionally high acceptor strength of the [P(H2Im)2]+2 unit, which
surpasses that of strong π-acceptor groups such as −P(C6F5)2
or even −P(1-pyrrole)2 by a wide margin. Moreover, the
isolation of 8 constitutes the first experimental evidence of the
capacity of 1 to act as a chelate in coordination complexes.
Cyclic voltammetry studies also support this conclusion.
Ligands 1 and 2 gave oxidation peaks at 1.050 and 0.974 V,
respectively, which can be assigned to the oxidation of the
−PPh2 groups. Comparison with reference compounds reveals
that the −PPh2 lone pairs in 1 and 2 are more internal than
those in PPh3 (Table 2, entry 3), probably as a result of the
which can be attributed with certainty to the −PPh2 and
[−P(H2Im)2]+2 moieties, respectively. Single crystals suitable
for X-ray structure determination of both compounds were
obtained, and their ORTEP diagrams are depicted in Scheme 1.
Of most interest is the P1−P2 distance (3.076 Å) in 1, which is
significantly shorter than that in its neutral analogue 7 (3.165
Å), suggesting the possibility of a Ph2P→ [P(H2Im)2]+2 dative
interaction in the free ligand as result of the strong Lewis acid
character generated on P2 by the two positively charged
substituents.
This hypothesis was confirmed computationally. Inspection
of the frontier orbitals at the M06-L/TZVP level indicated that
the HOMO is largely localized on the neutral phosphorus
atom, while the LUMO and LUMO+1 orbitals are located
mainly on the dicationic fragment (see Figure 1). It is also
Figure 1. Frontier molecular orbitals of 1 and their energies (in eV)
computed at the M06-L/TZVP level.
B
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both [−P(H2Im)2]+2 groups adopt a distal orientation (see the
Supporting Information for its X-ray structure). Interestingly,
although 14 was isolated as a dimer in the solid state, the
Coulomb repulsion between the components will surely
weaken the strength of the chloride bridges in solution. This
is evident from the trapping of monomeric 15 after addition of
2 equiv of acetonitrile to a dichloromethane solution of 14
(Scheme 2). Analogously, reaction of 2 with [RhCl(cod)]2 in
a
Scheme 2. Syntheses and X-ray Structures of 14−17
Figure 2. Evaluation of the donor ability and X-ray structure of 8.
Hydrogen atoms, anions, and solvent molecules were removed for
clarity; ellipsoids are set at 50% probability.8 Wavenumbers in cm−1.
Table 2. Electrochemical Redox Potentials
a
Hydrogen atoms, anions, and solvent molecules were removed for
clarity; ellipsoids are set at 50% probability.8 Reagents and conditions:
(a) [RhCl(cod)]2 (0.5 equiv), 92%; (b) CH3CN (2 equiv); (c)
[RhCl(cod)]2 (0.5 equiv) in CH3CN, 93%; (d) PtI2(PhCN)2, 95%.
a
Oxidation/reduction potential peaks reported in V and calibrated
b
versus ferrocene/ferrocenium, Bu4NPF6 (0.1 M) in CH2Cl2. No
reduction signal observed from 0 to −2.5 V. No signal detected from
0 to 2 V.
c
acetonitrile afforded the desired Rh complex 16, in which a
biphenyl group serves as the backbone that ties together the
two phosphines. Ligand 1 also demonstrated its ability to form
Pt derivatives. Addition of PtI2(PhCN)2 to DCM solutions of 1
afforded a yellow solid 17 whose 31P NMR spectrum showed
signals (δP = 39.2 (P2), 5.4 (P1) ppm; JPP = 4.6 Hz) shifted
downfield compared to the original resonances and the
appearance of characteristic 195Pt satellites (1JP(2)Pt = 2854
strong electron-withdrawing effect of the dicationic group,
which is markedly stronger in 1 due to the closer proximity of
the two phosphorus atoms imposed by the o-phenylene linker.
The reduction potentials of 1 (−1.528 V) and 2 (−1.504 V)
that reflect the electrophilic character of the [−P(H2Im)2]+2
fragment are slightly larger (in absolute magnitude) than that of
13 (Table 2, entry 6), implying a marginal loss in acceptor
strength compared with the monodentate analogue. The
bonding Ph2P→[P(H2Im)2]+2 interaction (described above)
may effectively account for the changes in the observed
reduction and oxidation potentials when going from 1 to PPh3
and 13 (Table 2, entries 3 and 6), by considering the mixing of
the lone-pair orbital on the neutral phosphorus with
antibonding orbitals on the dicationic moiety.
Coordination Studies. Encouraged by these results, we
decided to evaluate the ability of 1 and 2 to coordinate different
metals centers. Specifically, given our long-standing interest in
hydroarylation reactions, we analyzed their reactivity toward
Rh(I) and Pt(II) centers. Salt 1 readily displaces cyclooctadiene
in [RhCl(cod)]2, generating a dinuclear tetracation 14 in which
1
Hz; JP(1)Pt = 3457 Hz) which indicate coordination of both P
centers to Pt. The connectivities of 16 and 17 could be
unambiguously confirmed by X-ray analysis.
Catalysis. With complexes 14−16 in hand, we initiated a
catalysis study by revisiting the previously reported Rh(I)-
catalyzed hydroarylation of phenyl-1,3-butadiene 18a with
indole (Chart 1).13 The kinetics of this benchmark trans-
formation have been shown to be remarkably sensitive to the
relative Lewis acidity at the Rh center in the catalyst; thus, it
seems to be an appropriate model to test the impact of our new
ligands.13c
During our first assay, we observed the formation of 19a in
moderate yields in the presence of 5 mol % of 15 in
dichloroethane at 70 °C. Further optimization of the reaction
conditions revealed that the use of 10 mol % of KB(ArF)4
(potassium tetrakis(pentafluorophenyl)borate) is crucial. This
C
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a
engaged in the reaction, though in the latter case the yield was
significantly lower, probably due to the relatively low
nucleophilicity that is characteristic of this system. Identical
results were observed when these experiments were carried out
using a 2:1 mixture of 1 and [RhCl(CO)2]2 instead of complex
15.15
Chart 1. Preliminary Scope of the Hydroarylation of Dienes
It is worth mentioning that less than 5% conversion of 18a to
19a was observed when 1,2-bis(diphenylphosphanyl)benzene 7
or the stronger acceptors 1-(diphenyl-phosphanyl)-2-bis-
(pentafluorophenyl)phosphanylbenzene 12, 1-(diphenylphos-
phanyl)-2-di(1-pyrroyl)phosphanylbenzene 20, and even 2
were used as ancillary ligands under otherwise identical
reaction conditions. The unsatisfactory results obtained with
2 are probably due to the hemilability of this ligand under the
reaction conditions, which is a consequence of the more flexible
bisphenylene backbone.
Density Functional Theory Calculations on the
Reaction Mechanism. To better understand the origin of
the enhanced reactivity observed, we studied the mechanistic
pathway of the transformation of 18a into 19a using density
functional theory (DFT) calculations at the BP86-D3-
(CPCM)/def2-TZVP//BP86-D3/def2-SVP level (see Support-
ing Information for computational methodology and detailed
numerical results). Two reaction pathways were explored to
highlight the importance of the biscationic ligand for the
reactivity observed (Figure 3). The black one includes
biscationic ligand 1, whereas the red path uses a model
monocationic ligand.16 Starting with the dicationic path, the
reaction begins with the coordination of the substrate to the
rhodium complex to form INT1 or INT2, in which the
butadiene side-chain moiety adopts either a η4 or η2
coordination mode, respectively. Both complexes are in
equilibrium, but it is the lower-energy INT2 in which only
the terminal π-system is coordinated to Rh that is attacked by
indole at the internal carbon of the terminal olefin to form
INT3. The free energy of activation for this step is calculated to
be 14.9 kcal/mol. Subsequent rotation of the molecule around
the Rh−C bond forms INT4, which features a weak hydrogen
bonding interaction between the chloride ligand and the
hydrogen at the 3-position of the indole. Proton transfer to the
Rh center takes place through TS3 to form a Rh(III) species,
INT5, which undergoes a facile reductive elimination to afford
the desired product 19a still coordinated through the internal
CC double bond of the original butadiene moiety to Rh in
INT6.
a
Isomeric ratios, shown in parentheses, were determined by GC-MS
b
1
or H NMR analysis of crude reaction mixtures. Obtained as a 9:1
mixture of the 2- and 4-regiomers. With 10 mol % of catalyst used. All
c
Our calculations indicate that TS3 leading to the formation
of INT5 is the highest-energy transition state of the complete
catalytic cycle and therefore governs the kinetics of the
complete process.17 Finally, product release from INT6 with
association of a new substrate is only slightly uphill by 3.4 kcal/
mol.
The analogous monocationic path (red) requires significantly
more energy; the difference in the activation barriers between
the two pathways is predicted to be 14.4 kcal/mol. Hence, it is
clear that the second positive charge on the ligand is critical for
sufficiently activating the diene toward nucleophilic attack of
the indole.
Two of the steps from the catalytic cycle deserve further
discussion because they are fundamental to understanding the
role of the ancillary ligand 1. The nucleophilic attack of indole
to the coordinated olefin via TS1 is not an easy step, and it can
only take place because of the high acceptor strength of 1. This
becomes evident by comparing the reactant complex INT2 to
yields are isolated.
additive substantially increases the solubility of the catalyst in
the reaction medium by anion exchange and thus significantly
improves the obtained conversions. Under these conditions, a
range of indoles with different substitution patterns were found
to undergo the desired hydroarylation in good to excellent
yields and regioselectivities (19a−h). These results encouraged
us to test the reaction with more challenging arenes such as
dimethylresorcinol derivatives, which are characterized by a
significantly lower nucleophilicity parameter than indole on
Mayr’s scale (N = 5.75 and 2.48 for 1-methylindole and
dimethoxyresorcinol, respectively).14 Again in these cases, high
conversions and excellent regioselectivities were uniformly
observed (19i−k). In addition, the transformation is compat-
ible with halogens, as demonstrated by the isolation of 19c,
19g, and 19j. Finally, azulene and benzofurane could also be
D
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Figure 3. Gibbs free energy profile for the Rh-catalyzed hydroarylation of phenyl-1,3-butadiene with indole and precatalyst 15, calculated at the
BP86-D3(CPCMDCE)/def2-TZVP//BP86-D3/def2-SVP level of DFT. Biscationic path is shown in black, and the monocationic one in red.
the analogous intermediates in which 1 has been formally
exchanged with neutral 7 (INT2n) and the model mono-
cationic ligand INT2′ (Figure 4). In the addition step, the
transfer of 0.21 and 0.08e, respectively, from the diene to the
Rh complex. The presence of a significantly lower acceptor
orbital in INT2 and the accumulation of positive charge in the
diene fragment account for the enhanced reactivity observed for
ligand 1 relative to dppbz.
Also relevant is the nature of the highest-energy step, which
is predicted to be the proton transfer from the substrate to Rh
(via TS3), yielding an effective activation barrier of 24.6 kcal/
mol for the whole cycle. This step involves the oxidation of the
metal center from Rh(I) to Rh(III) and should thus be
facilitated by a strong donor rather than an acceptor ligand;
however, it occurs smoothly. Hence, the acceptor strength of 1
is large enough to allow the attack of the nucleophile to the
coordinated diene, a step that seems to be energetically
prohibitive for the other ligands tested but not too large to
hinder the subsequent metal protonation.
Finally, the experimentally observed primary kinetic isotope
effect of 2.1 is consistent with a proton transfer during the rate-
determining step occurring through a nonlinear transition state;
thus, it additionally supports our mechanistic proposal in which
TS3 is rate-determining.
Figure 4. Relevant unoccupied orbitals of INT2 (right), INT2′
(middle), and the neutral analogue INT2n (left). The orbital energies
(ε) and the NBO charge transfers (Δ) are also given.
CONCLUSIONS
■
leading orbital interaction with the nucleophile will involve the
π*-system of the diene, which is the LUMO in INT2n and
INT2′ and the LUMO+3 in INT2 (where the LUMO, LUMO
+1, and LUMO+2 orbitals are localized elsewhere but are all
similar in energy).
The LUMO+3 orbital of INT2 is significantly lower in
energy than the LUMOs of both neutral INT2n (by ∼5 eV)
monocationic INT2′ (by ∼2 eV). Furthermore, the computed
NBO charge transfer upon complexation reveals a net donation
of 0.02e from the metal to the diene in the neutral complex,
whereas complexation in INT2 and INT2′ results in a net
We present the first synthesis of bidentate α-dicationic
phosphines containing one dicationic and one neutral
phosphine unit connected by different backbones. This
architecture induces the coordination to metals of the dicationic
group, which otherwise is reluctant to react. Infrared studies as
well as theoretical calculations indicate that once these ligands
are in the coordination sphere of the desired metal, they show
pronounced acceptor characteristics which surpass those of
their neutral counterparts. Making use of these distinct
properties, we have developed a Rh catalyst that shows
E
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(3) For the few known examples, see ref 1d and (a) Coles, M. P.;
remarkable activity in the hydroarylation of dienes with
electron-rich hetero- and homoarenes. Ongoing research in
our group is focused on the application of this ligand platform
to other catalytic processes and on the development of
asymmetric versions of this reaction.
̌
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b01441.
■
S
X-ray data for 1 (CIF)
X-ray data for 2 (CIF)
X-ray data for 8 (CIF)
X-ray data for 14 (CIF)
X-ray data for 15 (CIF)
X-ray data for 16 (CIF)
X-ray data for 17 (CIF)
X-ray data for 19n (CIF)
Experimental procedures, spectral and analytical data,
computational methodology, and detailed numerical
results from the DFT calculations (PDF)
AUTHOR INFORMATION
Corresponding Author
*malcara@gwdg.de
■
ORCID
Walter Thiel: 0000-0001-6780-0350
Manuel Alcarazo: 0000-0002-5491-5682
Notes
The authors declare no competing financial interest.
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(b) Ziegler, T.; Rauk, A. Inorg. Chem. 1979, 18, 1755−1759.
(c) Ziegler, T.; Rauk, A. Inorg. Chem. 1979, 18, 1558−1565.
(13) (a) Wang, M. Z.; Wong, M. K.; Che, C. M. Chem. - Eur. J. 2008,
14, 8353−8364. (b) Peng, J.; Du, D. M. Eur. J. Inorg. Chem. 2012,
2012, 4042−4051. (c) Roberts, C. C.; Matías, D. M.; Goldfogel, M. J.;
Meek, S. J. J. Am. Chem. Soc. 2015, 137, 6488−6491.
ACKNOWLEDGMENTS
■
Generous financial support from the Fonds der Chemischen
Industrie (Dozentenstipendium to M.A.), the European
Research Council (ERC Starting Grant to M.A.), the Deutsche
Forschungsgemeinschaft (AL 1348/5-1), the Chinese Scholar-
ship Council (doctoral fellowship to L.G.), and the DAAD
(doctoral fellowship to A.Z.) is gratefully acknowledged. We are
also grateful to C. Wille and H. Tinnermann for cyclic
voltammetry measurements, and to Prof. C.W. Lehmann and
(14) Mayr, H.; Kempf, B.; Ofial, A. R. Acc. Chem. Res. 2003, 36, 66−
77.
(15) Pt compound 17 did not perform satisfactorily, probably fast
decomposition takes place after abstraction of one of the iodide anions
with Ag[B(ArF)4].
(16) No transition state could be localized for the attack of indole to
the diene substrate coordinated to [7·RhCl].
Dr. R. Goddard (MPI-Kohlenforschung, Mulheim an der Ruhr)
for solving the X-ray structures.
̈
(17) Kozuch, S.; Shaik, S. Acc. Chem. Res. 2011, 44, 101−110.
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