Neutral and Cationic Palladium Complexes of P-Stereogenic Phosphanes Bearing a Heterocyclic Substituent

Article abstract of DOI:10.1002/ejic.201600608

The coordination chemistry of 13 optically pure P-stereogenic diarylmonophosphanes P(Het)PhR [Het = 4-dibenzofuranyl (DBF), 4-dibenzothiophenyl (DBT), 4-dibenzothiophenyl S,S-dioxide (DBTO₂) and 1-thianthrenyl (TA); R = OMe, Me, iPr, Fc (ferroc

Full text of DOI:10.1002/ejic.201600608

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DOI: 10.1002/ejic.201600608
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P-Chiral Ligands
Neutral and Cationic Palladium Complexes of P-Stereogenic
Phosphanes Bearing a Heterocyclic Substituent
Pau Clavero,[a] Arnald Grabulosa*[a] Mercè Rocamora,[a] Guillermo Muller,[a] and
Mercè Font-Bardia[b]
Abstract: The coordination chemistry of 13 optically pure P- of the type [Pd{η3-(2-methylallyl)(κ2P,S-P)}]PF6 after chloride ab-
stereogenic diarylmonophosphanes P(Het)PhR [Het = 4-di- straction with TlPF6. The crystal structure of the complex [Pd(η3-
benzofuranyl (DBF), 4-dibenzothiophenyl (DBT), 4-dibenzothio- 2-methylallyl)(κ2P,S-PPh(OMe)(1-TA)]PF6 is reported. The neutral
phenyl S,S-dioxide (DBTO2) and 1-thianthrenyl (TA); R = OMe, Pd complexes were found to be highly active in the hydrovinyl-
Me, iPr, Fc (ferrocenyl)] to Pd-allyl moieties is described. Both ation of styrene after activation with AgBF4, except for the TA-
neutral [PdCl(η3-(2-methylallyl)(κP-P)] and cationic [Pd{η3-(2- based phosphanes. The cationic Pd complexes were evaluated
methylallyl)(κP-P)2}]PF6 complexes have been prepared. Coordi- in allylic alkylation and amination with the model substrate rac-
nation of the heteroatom of the heterocycle was only possible trans-1,3-diphenylprop-2-enyl acetate (rac-I), achieving total
in the case of TA-based phosphanes; these furnished complexes conversions and up to 70 % ee.
Introduction
In these processes, it is thought that secondary (hemilabile)
interactions can play a crucial role, improving the activity and
Although chiral diphosphanes are the most successful type of selectivity of the reaction. Several elegant examples have been
ligands of transition-metal homogeneous catalysis, for certain provided by RajanBabu and co-workers[2] and by Franciò, Leit-
reactions or under certain conditions monophosphorus ligands ner and co-workers,[3] who convincingly demonstrated the im-
can give better results or they can even be required, due to portance of secondary interactions in Ni-catalysed hydrovinyl-
mechanistic restrictions. One example is the Ni- or Pd-catalysed ation of olefins. The design of the ligand, however, is not easy,
hydrovinylation of activated olefins.[1]
because it has to contain the appropriate Lewis base suitably
Scheme 1. P-stereogenic phosphanes containing a heterocyclic substituent.
[a] Departament de Química Inorgànica i Orgànica, Secció de
Química Inorgànica, Universitat de Barcelona,
Martí i Franquès, 1-11, 08028 Barcelona, Spain
E-mail: arnald.grabulosa@qi.ub.es
http://www.ub.edu/inorgani/ca/index.html
[b] Departament de Cristallografia, Mineralogia i Dipòsits Minerals, Universitat
de Barcelona,
located in the scaffold of the ligand to interact with the metal
centre during the catalytic reaction.
Very recently,[4] we described a series of monophosphorus,
P-stereogenic ligands containing a heterocyclic substituent [4-
dibenzofuryl (DBF), 4-dibenzothiophenyl (DBT), 4-di-
benzothiophenyl (DBTO2) and 1-thianthrenyl (TA)] designed
with the aim of disposing the heteroatom of the heterocycle in
a suitable position allowing it to interact with the metal atom
(Scheme 1).
Martí i Franquès, s/n, 08028 Barcelona, Spain
E-mail: arnald.grabulosa@qi.ub.es
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201600608.
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The coordination chemistry towards Ru–η6-arene moieties scribed in previous reports,[5,7] they were obtained by splitting
and the application of the complexes to transfer hydrogenation dimer D with slightly more than four equivalents of phosphane
was also described. It was found that the sulfur atoms of DBT- in the presence of an excess of ammonium hexafluorophos-
and TA-containing ligands were able to act in conjunction with phate (Scheme 3).
phosphorus as bidentate ligands in a κ2P,S-coordinated fashion.
In this paper we describe the coordination of these ligands
to Pd–η3-allylic moieties and the application of the obtained
complexes to catalytic hydrovinylation and allylic substitution
reactions.
Results and Discussion
Scheme 3. Preparation of cationic Pd complexes Pd1′–13′.
Neutral Complexes
The complexes were obtained as stable brown solids after
extractive workup with water to remove inorganic salts. They
As previously described for other monophosphanes,[5] treat-
ment of the well-known Pd-dimer D with slightly more than
two equivalents of phosphane in dichloromethane yielded the
expected neutral complexes Pd1–13, of the type [PdCl(η3-2-
methylallyl)(P)] (Scheme 2).
were characterised by the usual techniques. The NMR spectra
showed that a single species was present in solution, as previ-
ously found for analogous compounds.[5,7] The presence of the
allyl group and the chirality of the phosphane makes it the case
that the atoms in the molecule are all different. Therefore, two
sharp doublets in the 31P{1H} NMR spectra corresponding to
the two coupled phosphorus atoms (2JP, P = 30–57 Hz) could be
observed. In the 13C NMR spectra the two terminal allyl carbon
atoms appeared as doublets or doublets of doublets, due to the
coupling with the P atoms of the phosphanes. The differences
between the 13C chemical shifts of these two atoms are small
(<2 ppm), as found in similar complexes.[7] In the 1H NMR spec-
tra, the four resonances of the allylic H atoms appeared as two
broad singlets, corresponding to the syn protons and two dou-
blets, corresponding to the anti protons (2JH,P = 9–12 Hz). The
preparation of Pd4′ in pure form was not possible because it
was always contaminated with around 25 % of neutral Pd4.
We next moved to study of the coordinative interactions be-
tween the heterocycle and the Pd centre. Following our previ-
ous report with ruthenium,[4] we treated the neutral complexes
Pd7 and Pd10 with thallium hexafluorophosphate in dichloro-
methane, and the solid obtained after the filtration of TlCl and
removal of the solvent was analysed by NMR (Scheme 4).
In the case of Pd7, no peaks appeared in the 31P NMR spec-
Scheme 2. Preparation of neutral palladium complexes Pd1–13.
The complexes were obtained as pale yellow solids, except
for those containing phosphanes bearing the ferrocenyl group
(Pd8 and Pd12), which were red. The complexes were charac-
terised by IR, chemical microanalysis (or MS) and multinuclear
NMR in solution. As expected,[5,6] the complexes were found to
exist as mixtures of two diastereomeric species in solution, due
to the presence of the chiral ligand and the allyl moiety. Hence,
two singlets in the 31P{1H} NMR spectra, often partially over-
lapped, could be observed. All of the C and H atoms of the
complexes are in principle different in each diastereomer; this
could be clearly seen in the duplication of signals in the part
1
corresponding to the allyl moiety in the H and 13C NMR spec-
1
tra of the complexes. Full details can be found in the Experi-
trum and the H NMR spectrum was broad and uninformative,
1
mental Section. Integration of the 31P and H NMR spectra al-
indicating that no definite species were formed. In the case of
the solid obtained from Pd10, both 31P and 1H NMR spectra
showed that it corresponded to bis(phosphane) complex
Pd10′. The formation of this compound indicates that a symme-
trisation (disproportionation) reaction yielding the bis(phos-
phane) and the bis(solvato) complexes had taken place, as pre-
viously reported for Pd complexes with other monophosphane
ligands.[7,8]
lows the estimation of the diastereomeric ratio in solution. It
was found that this was approximately 1:1 for all complexes,
except for Pd12, for which it was 1:1.2. Interestingly, in com-
plexes bearing a phosphane containing the thianthryl group
(Pd3 and Pd13) the H atoms of the allyl group gave rise to
extremely wide peaks in the 1H NMR spectra. In addition, no
peaks could be detected for the allyl group in the 13C{1H} spec-
1
tra at room temperature. Low-temperature H NMR spectra of
In contrast to the unsuccessful attempts described above,
the coordination of the S atom of the thianthryl group in com-
plexes Pd3 and Pd13 was successfully accomplished, yielding
cationic complexes Pd3′′ and Pd13′′ as pale yellow solids after
recrystallisation (Scheme 5). It should be mentioned that the
NMR of Pd13′′ shows the presence of a small quantity of
bis(phosphane) complex Pd13′.
Pd13 in CD2Cl2 (see the Supporting Information) showed, how-
ever, that the expected allylic hydrogen atoms appeared when
the spectrum was recorded at –80 °C.
Cationic Complexes
The next type of Pd complexes prepared were cationic bis-
The κ2P,S complexation of the ligands was confirmed by the
phosphanes of the type [Pd(η3-2-methylallyl)(P)2]PF6. As de- downfield shift of the 31P signals [Δδ(Pd3′′–Pd3) = 26.2 ppm;
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Scheme 4. Unsuccessful attempts to force κ2P,O- and κ2P,S-coordination in Pd complexes of DBF- and DBT-based ligands.
For complex Pd3′′, single crystals suitable for X-ray crystal-
lography could be obtained. A representation of its molecular
structure is given in Figure 1.
The unit cell of Pd3′′ contains two independent molecules,
corresponding to the two different isomers of the complex.
They can be named as syn and anti with regard to the relative
disposition between the methyl group of the allyl fragment and
the methoxy group of the phosphinite. A particular feature of
the structure is the expected[10] nonplanar, “butterfly“ shape of
the thianthryl substituent of the phosphane, which is folded
along its S–S axis. Interestingly, for both isomers in the crystal
the thianthrene moiety is folded such that it remains parallel to
the C(20)–C(22) bond of the allyl group. The Pd atom is in a
distorted square-planar environment, the interatomic distances
and angles of which are similar in the two isomers present in
the unit cell and also similar to those in other Pd complexes
containing five-membered P,S chelate rings, such as in a com-
plex with a 4-diphenylphosphinophenothiazine ligand de-
scribed recently by Silaghi-Dumitrescu and co-workers.[11] The
distance between the Pd atom and the C atom of the allyl
group trans to the P atom is larger than with the C atom trans
Scheme 5. Successful κ2P,S-coordination in Pd complexes of TA-based ligands.
Δδ(Pd13′′–Pd13) = 26.4 ppm] characteristic when a five-mem-
bered ring is formed.[9] Two peaks appeared in the 31P{1H} spec-
1
tra and two sets of signals were present in the H and 13C{1H}
spectra, indicating that complexes Pd3′′ and Pd13′′ exist as
mixtures of the two diastereomers, as was also the case with
precursor complexes Pd3 and Pd13. The ratio between the cati-
onic complexes is roughly 60:40, meaning that the sulfur com-
plexation step occurs with a small degree of diastereoselectiv-
ity. It is worth noting that complexes Pd3′′ and Pd13′′ showed
1
well-defined NMR spectra. In the H and 13C{1H} NMR spectra,
the expected two sets of peaks assignable to the allyl group
appear, in contrast with the cases of Pd3 and Pd13 (vide infra).
This is probably due to the rigid κ2P,S-coordination of the ligand to the S atom, indicating a higher trans influence of the phos-
in the cationic complexes.
phinite group relative to the thioether group. It should be
–
Figure 1. ORTEP representation (thermal ellipsoids drawn at 50 % probability level, H atoms and PF6 anions removed for clarity) of anti-Pd3′′ (left) and syn-
Pd3′′ (right). Interatomic distances [Å] and angles [°] for anti-Pd3′′: Pd(1A)–C(19A), 2.107(11); Pd(1A)–C(20A), 2.212(10); Pd(1A)–C(21A), 2.211(10); P(1A)–Pd(1A)–
C(19A), 98.9(3); C(19A)–Pd(1A)–C(21A), 65.9(4); C(21A)–Pd(1A)–S(1A), 106.7(3); S(1A)–Pd(1A)–P(1A), 88.18(9); C(1A)–S(1A)–C(12A), 100.3(4); C(6A)–S(2A)–C(7A),
100.9(5). For syn-Pd3′′: Pd(1B)–C(19B), 2.238(10); Pd(1B)–C(20B), 2.167(9); Pd(1B)–C(21B), 2.210(11); P(1B)–Pd(1B)–C(21B), 98.8(3); C(21B)–Pd(1B)–C(19B), 66.8(4);
C(19B)–Pd(1B)–S(1B), 105.2(3); S(1B)–Pd(1B)–P(1B), 88.65(8); C(1B)–S(1B)–C(12B), 100.4(5); C(6B)–S(2B)–C(7B), 102.0(4).
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pointed out that the S atom coordinated to Pd is a stereogenic tion, because the same hydrovinylation catalyst tends to isom-
centre and that each of the molecules in the crystal structure erise the initially formed 3-arylbut-2-enes to the more stable 2-
has a different absolute configuration.
arylbut-2-enes. It has been found by us[5,6,8b,14b,15] and by oth-
ers[14a] that [PdCl(η3-allyl)P] (P = monophosphorus ligand) com-
plexes are excellent catalytic precursors of the active species
after halide abstraction, so the potential of complexes Pd1–13
in hydrovinylation was explored (Scheme 6).
The complexation studies described here with the Pd–η3-
methallyl moiety can be compared with analogous recently de-
scribed studies with the Ru–η6-arene moiety.[4] For the Ru sys-
tems it was found that both DBT- and TA-based phosphanes
effectively coordinated to the Ru atom in a κ2P,S-mode but the
DBF-based ligands did not. This shows that with the systems
studied, the DBF-based phosphanes have the weakest tendency
to act as bidentate ligands, whereas the TA-based ones show
the strongest tendency. The softer character of sulfur relative
to oxygen and the greater flexibility of the TA group relative to
DBF and DBT probably account for the differences in coordina-
tion abilities of these ligands.
Scheme 6. Pd-catalysed enantioselective hydrovinylation of styrene.
The activation of the catalysts was carried out with silver
tetrafluoroborate in the presence of styrene. After removal of
silver chloride by filtration, the solution was pressurised with
ethylene at 25 °C. The results obtained are given in Table 1.
Most of the ligands produced active systems in the reaction
Pd-Catalysed Hydrovinylation
The hydrovinylation reaction is a catalysed heterocodimerisa-
tion between ethylene and a conjugated diene.[1b,1d] This reac-
tion is interesting because it creates a C–C bond by using ethyl- but with very different activities depending on the substituents
ene, which is an inexpensive feedstock, and because the double at the phosphorus atom. On close inspection, certain trends
bond incorporated into the molecule can subsequently be ma- can be identified. In general the order of decreasing activity
nipulated in a multitude of ways. In addition, because a stereo- depending on the heterocycle is DBF >> DBT > DBTO2 >> TA;
genic centre is created, the reaction can be carried out enan- indeed, the two TA-based systems are completely inactive even
tioselectively.[1a,1c,1d] Despite its interest, activity and selectivity at 6 h reaction time (Entries 3 and 13). This order correlates
issues hamper its full development. The most typical catalytic quite nicely with the coordination ability of the heteroatom in
systems involve a NiII or PdII precursor stabilised with a mono- the heterocycle so it is not surprising that the two TA-based
phosphorus ligand because for these metals bidentate ligands phosphanes give completely inactive systems given the fact
inhibit the reaction.[2c] The presence of groups capable of es- that they act as true bidentate ligands, which are known to
tablishing secondary coordination interactions with the metal inhibit the hydrovinylation reaction.[2c] Within the DBF- and
can be beneficial for the reaction, as shown by RajanBabu and DBT-based families of ligands the order depending on the R
co-workers[2b,12] and by Leiner and co-workers[3a,13] in Ni-based substituent is tBu > iPr > Fc >> OMe ≈ Me, which means that,
systems. The catalytically active species is thought to be a metal roughly, the bulkier the ligand the more active the system be-
hydride. In general, nickel systems are more commonly used comes. This trend contrasts with previous results obtained with
because they are very active and can be highly enantioselective diarylphosphanes.[15c] It can be observed that except for meth-
but with the penalty of requiring (usually)[3b] very low tempera- oxy- and methylphosphanes (Entries 1–5, 10) the change of a
tures. In contrast, palladium-based systems work well at room polycyclic aryl group[5,6,15c,16] for a heterocyclic substituent
temperature.[6,14] The challenge, apart from improving the en- makes the system much more active but less selective in the
antioselectivity, is the control of the regioselectivity of the reac- hydrovinylation reaction. With regard to the enantioselectivities,
Table 1. Results of styrene hydrovinylation with Pd1–13 complexes.
Entry[a]
Precursor
Time [h]
Conversion[b] [%]
Codimers[c] [%]
Selectivity[d] [%]
TOF[e] [h–1
]
ee[f] [%]
1
2
3
4
5
6
7
8
Pd1
Pd2
Pd3
Pd4
Pd5
Pd6
Pd7
Pd8
Pd9
Pd10
Pd11
Pd12
Pd13
2
4
6
4
13.3
33.2
<5
13.3
32.9
–
>99
92.6
–
66
81
–
50
6 (S)
<5
–
<5
<5
6 (R)
13 (R)
20 (S)
<5
10 (S)
18 (S)
14 (R)
–
18.8
55.6
58.6
91.4
67.3
48.0
19.9
84.5
67.0
<5
18.5
55.6
57.6
91.4
67.3
48.0
19.4
83.9
66.9
–
> 99
73.2
92.6
74.9
80.4
91.4
94.5
85
6
91
0.5
0.5
1
1
2
1
1
6
1129
1839
667
476
95
823
659
–
9
10
11
12
13
85.8
–
[a] Catalytic conditions: Pd complex (0.02 mmol), styrene (20 mmol), AgBF4 (0.022 mmol) at 25 °C and P ≈ 15 bar of initial pressure of ethylene in 15 mL of
dichloromethane. [b] Conversion of starting styrene. [c] Total amount of codimers. [d] Percentage of 3-phenylbut-1-ene/codimers. [e] TOF values calculated
from the total amount of codimers formed. [f] Enantiomeric excess of 3-phenylbut-1-ene at the stated time.
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