Synthesis of New Chiral Bidentate Isonitrile-Acyclic Diaminocarbene Palladium(II) Compounds and Their Catalytic Activity

Article abstract of DOI:10.1021/acs.organomet.5b00516

Five new chiral bidentate isonitrile-diaminocarbene palladium(II) complexes (5a-5e) as well as the bis-diaminocarbene complex 6 were prepared by metal-mediated nucleophilic addition of amines to the corresponding bisisonitrile-palladium(II) compound 4. The new chelates were fully characterized by 1D and 2D NMR experiments, IR, and ESI-MS. In addition, the X-ray structure of 5a, 5c, and 6 could be analyzed, showing different sterical properties. We furthermore investigated the relationship between their structure and their catalytic activity. The mixed ligand complexes 5a and 5c showed high activity both in Suzuki-Miyaura cross-coupling and in intermolecular asymmetric allylic alkylation (AAA) reactions as well as moderate chiral induction in the AAA.

Full text of DOI:10.1021/acs.organomet.5b00516

Article
pubs.acs.org/Organometallics
Synthesis of New Chiral Bidentate Isonitrile−Acyclic Diaminocarbene
Palladium(II) Compounds and Their Catalytic Activity
Matthias Knorn, Eugen Lutsker, and Oliver Reiser*
Institut fur Organische Chemie, Universitat Regensburg, Universitatsstraße 31, 93053 Regensburg, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: Five new chiral bidentate isonitrile−diamino-
carbene palladium(II) complexes (5a−5e) as well as the bis-
diaminocarbene complex 6 were prepared by metal-mediated
nucleophilic addition of amines to the corresponding
bisisonitrile−palladium(II) compound 4. The new chelates
were fully characterized by 1D and 2D NMR experiments, IR,
and ESI-MS. In addition, the X-ray structure of 5a, 5c, and 6
could be analyzed, showing different sterical properties. We
furthermore investigated the relationship between their
structure and their catalytic activity. The mixed ligand complexes 5a and 5c showed high activity both in Suzuki−Miyaura
cross-coupling and in intermolecular asymmetric allylic alkylation (AAA) reactions as well as moderate chiral induction in the
AAA.
to electron-rich metals4o,5 or by lithium−halogen exchange
INTRODUCTION
■
followed by transmetalation.7 Alternatively, metal−isonitrile
complexes can be directly transformed to their corresponding
acyclic carbene complexes in a highly atom-economic
procedure by the addition of amines (Scheme 1). Especially
While metal complexes of N-heterocyclic carbenes (NHCs)
have attracted much attention in the field of organometallic
chemistry and homogeneous catalysis,1 the corresponding
complexes of acyclic diaminocarbenes (ADCs) are less explored
in their long history (Figure 1). The first ADC−metal complex
Scheme 1. Preparation of a Metal−ADC Complex via Metal-
Mediated Nucleophilic Addition of an Amine to an Isonitrile
Complex
Figure 1. Metal complexes of an N-heterocyclic carbene (NHC) and
an acyclic diaminocarbene (ADC).
reported was synthesized by Tschugajeff et al. in 1915, but
spuriously perceived as a dimeric hydrazine-bridged complex.2
Its correct structure was identified much later by Badley and co-
workers.3 However, only recently have ADCs started to be
recognized as attractive ligands in metal catalysis.4 Being strong
electron donors, ADC ligands increase the electron density at
the metal center in their corresponding complexes. Such donor
properties facilitate the oxidative addition of transition metal
complexes that have been utilized in many cross-coupling
reactions. Compared to N-heterocyclic carbenes, ADC ligands
possess a wider N−C−N carbene angle, which increases the
stability of their complexes and provides better steric control as
well as favors the reductive elimination in many metal-catalyzed
reactions.4a,5
late transition metals such as Pt(II), Pd(II), or Au(I) provide
sufficient electrophilic activation, which is required for the
addition of amine nucleophiles to the isonitriles carbon.4a−m,8
In addition, a facile addition of the nucleophile allows the
employment of only stoichiometric amounts of the latter and
alleviates the workup of the reaction mixture. Moreover, this
route is versatile for the generation of libraries or fine-tuning of
electronic and steric properties of the target complexes. In
recent years, this methodology was especially applied to
palladium(II) compounds, being arguably the most prominent
carbene complexes.4a−k,8 A combination of both their
straightforward synthesis from easily accessible isonitrile
precursors and their key properties makes palladium ADC
compounds an attractive alternative to the well-studied N-
ADC−metal complexes can be synthesized by the direct
complexation of deprotonated carbene precursors, for example
N,N,N′,N′-tetraalkylformamidinium salts,6 or starting from C-
chloro iminium and -formamidinium salts via oxidative addition
Received: June 15, 2015
© XXXX American Chemical Society
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heterocyclic carbene complexes. Relevant to our study, mixed
isonitrile/ADC palladium(II) compounds were synthesized and
employed in Heck and Suzuki−Miyaura cross-coupling
reactions.4a−k
Scheme 3. Preparation of the Palladium(II)−Isonitrile/ADC
Complexes 5a−e and the Bidentate Palladium(II)−Bis-ADC
Complex 6 from the Chiral Palladium(II)−Bisisonitrile
Complex 4
We questioned if the steric prerequisites of bidentate
isonitrile complexes, being so far unexplored in this context,
allow the synthesis of metal−ADC complexes as well. While the
preparation of monodentate chiral acyclic aminocarbene
complexes by the metal-mediated coupling of amino acid
esters and metal−isonitriles was shown recently,8c we were
interested in the effect of introducing asymmetry at the carbene
center in the target molecule imposed by α-chiral isonitrile
precursors. We report here the synthesis of five new chiral
bidentate mixed isonitrile/ADC as well as one chiral bis-ADC-
palladium(II) complex prepared from the corresponding chiral
bisisonitrile complex. All complexes were fully characterized by
1D and 2D NMR experiments, ESI-MS, and IR. In addition, X-
ray structures of the new complexes 5a, 5c, and 6 were
obtained, which allows the analysis of geometry changes
imposed by the nucleophile addition in detail. Finally, we
employed the new complexes as catalysts in the Suzuki−
Miyaura coupling and in the asymmetric allylic alkylation
(AAA) to study both the correlation between their structure
and their reactivity and their ability to transfer chiral
information to the substrate molecules.
formation of the carbene was validated by the typical peak in
the 13C NMR at around 180 ppm as compared to the shifts of
the isonitrile carbons in 4, which are found around 120 ppm.
The formation of the bidentate ADC complex 6 was achieved
using two equivalents of pyrrolidine, evidenced by the vanishing
of the NC isonitrile stretching frequency in the infrared
spectrum at 2237 cm−1. As a consequence of the non-C2-
symmetrical geometry of 6, two different signals at 187.3 and
186.6 ppm could be observed in the 13C resonance spectrum
for the carbene carbons. With the exception of the
bispyrrolidine complex 6, all other attempts to synthesize
biscarbene complexes led to incomplete transformations, most
probably being a consequence of severe steric crowding, as can
be judged from the X-ray structure of the monocarbene
complexes 5a and 5c (vide infra). Single crystals, suitable for X-
ray diffraction analysis,11 could be obtained by vapor diffusion
for the carbene complexes 5a, 5c, and 6 and compared to the
structure of 49a (Table 1).
SYNTHESIS AND CHARACTERIZATION
■
We recently introduced bidentate isonitrile ligands bridged by a
phenylphosphonyl linker and metal complexes derived thereof
for various applications in catalysis.9 As a starting point for this
study, the bisisonitrile ligand 3 and its corresponding palladium
complex 4 were readily prepared from the chiral oxazoline 1 as
previously reported by us9a (Scheme 2).
a
Scheme 2. Synthesis of the Bisisonitrile Ligand 3 and Its
Corresponding PdCl2 Complex
The realization of a square-planar environment around
palladium is the dominating factor in all structures, which
causes a substantial reorganization of the ligand backbone, as
can be seen from the dihedral torsion angle between the (N−
C)carbene−Pd−C (5a (86°), 5c (98°), 6 (63° and 64°)). The
bond distances between the palladium and the isonitrile carbon
(1.909−1.943 Å) were shorter compared to those with the
carbene carbon centers (1.994−2.018 Å), suggesting the strong
susceptibility of an isonitrile ligand for metal back-bonding to
its π*-orbitals, which is also reflected in the deviation of the
linear coordination of the isonitriles to the palladium center
(174−177°).
As a consequence, the chloro−palladium bond lengths trans
to the isonitrile moieties were shorter (2.296−2.319 Å) than
those trans to the carbene carbons (2.352−2.378 Å), which is in
agreement with values reported for monodentate palladium and
platinum isonitrile−carbene complexes.1i,4c,e,8o,10b,12 While in
the bisisonitrile complex 4 a bite angle of 88° to palladium by
the dissymmetric arrangement of its isonitrile arms is realized,9a
in the structures of 5a and 6 a significant widening of the
ligands’ bite angle to 91° and 95° is observed due to the higher
steric demand of the amine moieties. Surprisingly, 5c does not
follow this trend (bite angle C−Pd−C = 88°), which appears to
be a consequence of a significant reorganization of the
somewhat flexible ligand backbone. The amine groups are
a
Conditions: (S)-4-tert-butyl-4,5-dihydrooxazole (1) (1.0 equiv), n-
BuLi (1.6 M in hexane, 1.05 equiv), THF (0.4 M), −78 °C, 1.5 h;
POPhCl2 (0.53 equiv), −78 °C to rt, 2 h.
The hypsochromic shift of the isonitrile IR stretching
frequency of around 100 cm−1 to 2237 cm−1 upon transforming
3 to 4 indicates the complexation to this 12-membered chelate.
In comparison to its iron analogue Fe(3)2Cl2,9b showing an
isonitrile IR frequency at 2165 cm−1, it also became evident that
4 is activated for a nucleophilic attack to the isonitrile carbon,
being in line with literature precedent for monodentate
isonitrile metal complexes.4c−f,8f−o,10 Indeed, the addition of
one equivalent of pyrrolidine or dicyclohexyl amine to 4 led to
a new type of chiral bidentate palladium(II)−isonitrile/acyclic
diaminocarbene complexes 5a−e (Scheme 3). In all cases, the
B
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Table 1. Solid-State Molecular Structures and Selected Bond Angles [deg] and Lengths [Å] of the Palladium(II) Complexes 4,9a
5a, 5c, and 6
oriented opposite the tert-butyl groups, thus conferring the
chiral environment imposed by the tert-butyl groups closer to
the metal centers (gearing effect). At 115−119°, wide N−C−N
angles characteristic for acyclic carbenes were observed, which
are significantly wider than most typical angles for cyclic
carbene ligands (∼100−108°).1e
room temperature in ethanol with KOtBu as base to cope with
more user-friendly conditions.4e,f
The isonitrile/acyclic diaminocarbene complexes 5a and 5c
gave the product in good yields without significant differences
(entries 1 and 2). However, it became evident that the bis-ADC
complex 6 showed only minor activity (entry 3). A possible
explanation could be the steric shielding of the palladium(II)
metal center by the amine moieties, which prevents an effective
oxidative addition of the bromobenzene. Similar to related
monodentate palladium isonitrile−ADC complexes, none of
the complexes was able to activate the more challenging
chlorobenzenes under these conditions.4e Application of
different substrates confirmed the high activity of 5a and 5c
to bromobenzenes (entries 4−7).
In order to evaluate the capability of complexes 5 as chiral
inductors, we investigated the asymmetric allylic alkylation14 as
a second process. In contrast to the Heck, Suzuki−Miyaura, or
Negishi cross-coupling, the mechanism of this reaction involves
a controlled nucleophilic attack to a metal-coordinated allylic
substrate, to yield optically active products. By the design of
new catalysts for this reaction, a wide bite angle of the ligand,
which creates a chiral cavity for the allyl system, was identified
as a decisive feature for a successful transformation with an
effective chiral induction.14c,g,15 While the most common
ligands for this reaction are bidentate donor ligands that
contain phosphorus, nitrogen, and sulfur or a combination of
these atoms,14e−h,15b the use of NHC ligand systems has been
explored only recently.16 So far, there were no reports on
palladium−isonitrile or acyclic carbene catalysis for this
transformation. Using optimized conditions17 we found 4 to
be a capable catalyst, although 10 mol % was necessary for full
conversion within 60 min (Table 3, entry 1). Application of 5a,
5c, and 6 revealed substantial differences in the catalytic
activity. While complex 5c leads to quantitative yield after 120
min (entry 3), the pyrrolidine-containing complex 5a reaches
87% yield after a prolonged reaction time of 18 h (entry 2).
However, similar to the Suzuki−Miyaura coupling, catalyst 6
led to poor results only (entry 4). Here again, an increase of
steric crowding seems to be responsible for the observed
CATALYTIC EXPERIMENTS
■
Having analyzed the structural characteristics of 5a, 5c, and 6,
we investigated the correlation between their structure and
their catalytic performance. We started with the Suzuki−
Miyaura cross-coupling of 4-methoxybromobenzene with 4-
methylphenylboronic acid as a model system (Table 2). In
terms of practicability, we used technical grade solvent without
prior degassing. In all cases, a loading of 1 mol % of catalyst was
sufficient to perform this reaction. While most protocols using
palladium−ADCs require elevated temperatures or environ-
mentally unfavorable solvents for such transforma-
tions,4c,f,g,6h,8e,10,13 we were able to perform the reaction at
Table 2. Suzuki−Miyaura Cross-Coupling Using Complexes
a
5a, 5c, and 6 as Catalyst
b
entry
R1
R2
catalyst
yield [%]
1
2
3
4
5
6
7
5a
5c
6
76
84
33
75
83
77
81
4-OMe
4-Me
H
4-OMe
4-tBu
5a
5a
5c
5c
4-OMe
4-NO2
H
4-Me
2-OMe
a
Conditions: bromobenzene (1.0 equiv, 1.0 mmol), boronic acid (1.2
equiv, 1.2 mmol), KOtBu (1.0 equiv, 1.2 mmol), 1 mol % catalyst, 2
mL of EtOH, rt, 20 h. Entries 1−3: NMR yield; entries 4−7: isolated
yield.
b
C
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calcd for C26H41N3O5P104Pd [M − 2Cl− + HCOO−]+ 610.1819, found
610.1819.
Table 3. Asymmetric Allylic Alkylation Using Complexes 4,
5a, 5c, and 6 as Catalysts
a
5c: 1H NMR (600 MHz, CDCl3) δ 7.73 (dd, J = 13.6, 7.5 Hz, 2H),
7.62 (t, J = 7.4 Hz, 1H), 7.48 (td, J = 7.5, 4.7 Hz, 2H), 5.74 (t, J = 11.7
Hz, 1H), 5.57 (d, J = 9.8 Hz, 1H), 5.08−5.01 (m, 1H), 4.74−4.62 (m,
2H), 4.22−4.15 (m, 1H), 3.91−3.84 (m, 1H), 3.77 (d, J = 9.9 Hz,
1H), 3.33 (t, J = 11.6 Hz, 1H), 2.30 (d, J = 10.6 Hz, 1H), 1.92−1.46
(m, 10H), 1.39−1.22 (m, 5H), 1.16 (s, 9H), 1.19−1.04 (m, 2H), 1.08
(s, 9H), 0.93−0.79 (m, 2H); 13C NMR (151 MHz, CDCl3) δ 182.9,
133.3, 131.3, 128.8, 125.8, 70.2, 68.4, 66.7, 65.1, 64.4, 57.4, 34.4, 34.1,
31.6, 31.0, 30.6, 29.7, 27.6, 26.3, 25.6, 25.2 (isonitrile carbon not
detected); IR (neat) 2967, 2934, 2860, 2235, 1552, 1467, 1439, 1406,
1373, 1347, 1250, 1131, 1095, 1037, 1019, 995, 825, 752, 735, 695
cm−1; ESI-MS, m/z [M − 2Cl + HCOO−]+ 708.28.
b
entry
catalyst
time
yield [%]
% ee
c
d
1
2
3
4
4
60 min
18 h
100 (89)
8 (R)
6 (S)
5a
5c
6
87
100
5
e
120 min
18 h
45 (S)
5d: 1H NMR (600 MHz, CDCl3) δ 7.94−7.77 (m, 2H), 7.68−7.43
(m, 5H), 7.37−7.12 (m, 3H), 5.46−3.15 (m, 9H), 1.09−0.78 (m,
18H); IR (neat) 3220, 2960, 2870, 2363, 2233, 1573, 1402, 1339,
1331, 1245, 1185, 1133, 1096, 1021, 995, 813, 749, 693 cm−1; ESI-
HRMS, m/z calcd for C28H39N3O5P106Pd [M − 2Cl− + HCOO−]+
634.1662, found 634.1674.
n.d.
a
Conditions: rac-(E)-1,3-diphenyl allylacetate (1.0 equiv, 0.5 mmol,
126 mg), dimethyl malonate (3.0 equiv, 1.5 mmol, 172 μL), BSA (3.0
equiv, 1.5 mmol, 370 μL), KOAc (20 mol %, 0.1 mmol, 10 mg), 10
b
mol % catalyst, 3 mL of THF, 60 °C. Enantiomeric excess
determined by chiral HPLC (Chiralcel OJ-H), absolute configuration
5e: 1H NMR (600 MHz, CDCl3) δ 8.36−7.40 (m, 7H), 7.19−6.68
(m, 2H), 5.81 (d, J = 10.4 Hz, 1H), 5.08−3.29 (m, 9H), 1.19−0.68
(m, 18H); IR (neat) 3284, 3206, 2963, 2874, 2233, 1566, 1510, 1461,
1372, 1301, 1245, 1129, 1100, 1025, 995, 828, 749, 693 cm−1; ESI-
HRMS, m/z calcd for C28H39N3O6P104Pd [M − 2Cl− + HCOO−]+
648.1611, found 648.1616.
c
d
e
in parentheses. Isolated yield in parentheses. [α]2D0 +5.0. [α]D20
−10.2.
differences. In agreement with this reasoning, only 5c showed
moderate enantioselection in the reaction.
1
6: H NMR (600 MHz, CDCl3) δ 7.66−7.60 (m, 3H), 7.54−7.49
(m, 2H), 5.50 (d, J = 10.2 Hz, 1H), 5.22 (d, J = 9.6 Hz, 1H), 5.14−
5.10 (m, 1H), 5.01−4.96 (m, 1H), 4.91−4.86 (m, 1H), 4.67−4.61 (m,
1H), 4.42−4.37 (m, 1H), 4.30 (dd, J = 22.2, 11.1 Hz, 1H), 4.19−4.12
(m, 2H), 4.09−3.98 (m, 2H), 3.96−3.91 (m, 1H), 3.74−3.68 (m, 1H),
3.44−3.36 (m, 2H), 3.26 (dd, J = 15.8, 8.1 Hz, 1H), 2.19−2.00 (m,
5H), 1.95−1.76 (m, 3H), 1.11 (s, 9H), 1.06 (s, 9H); 13C NMR (151
MHz, CDCl3) δ 187.3, 186.6, 133.4, 130.9, 129.2, 125.2, 66.4, 66.4,
65.3, 63.1, 57.3, 56.9, 47.4, 46.8, 34.5, 33.9, 29.9, 27.4, 27.2, 25.4, 25.0,
24.9; IR (neat) 2955, 2879, 1554, 1439, 1375, 1316, 1241, 1129, 1089,
1050, 1025, 989, 912, 841, 805, 749, 694, 662 cm−1; ESI-MS, m/z [M
− 2Cl − H]+ 623.23.
CONCLUSION
■
In conclusion, the metal-mediated synthesis of ADCs was
successfully applied to the chiral bisisonitrile complex 4. This
straightforward synthesis led to five novel palladium bidentate
isonitrile−ADC complexes (5a−5e) as well as one bis-ADC
complex (6), which were characterized by NMR, IR, and ESI-
MS. The solid-state structures of 5a, 5c, and 6 could be
compared with the known bisisonitrile complex 4, and finally
the catalytic activity of 5a, 5c, and 6 was investigated. While the
bis-ADC complex 6 showed only weak activity in the Suzuki−
Miyaura cross-coupling and the intermolecular AAA, the mixed
ligand compounds 5a and 5c led to excellent results. In
addition, a moderate enantiomeric excess was obtained using
complex 5c in the AAA. However, a clear correlation between
their structure and their ability for chiral induction could be
observed.
ASSOCIATED CONTENT
* Supporting Information
■
S
. The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00516.
Experimental and catalytic details, compound character-
ization, optimization table for AAA, HPLC data, X-ray
data, and NMR spectra (PDF)
Crystallographic data (CIF)
Crystallographic data (CIF)
EXPERIMENTAL SECTION
■
General Procedure for the Preparation of 5a−5e and 6.
Stoichiometric addition of amine (1.0 or 2.0 equiv) to a stirred
solution of palladium(II)−bisisonitrile complex 4 (0.5 mmol, 1.0
equiv) in 50 mL of DCM furnished the desired complex by
evaporation of the solvent as a yellow solid. Crystals suitable for X-
ray analysis were obtained by vapor diffusion of Et2O in a DCM
solution of the complex.
Crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: oliver.reiser@chemie.uni-regensburg.de.
■
5a: 1H NMR (300 MHz, CD3CN) δ 7.76−7.63 (m, 3H), 7.59−7.49
(m, 2H), 5.46 (d, J = 10.1 Hz, 1H), 4.71 (td, J = 10.5, 3.2 Hz, 1H),
4.64 (dd, J = 22.1, 11.4 Hz, 1H), 4.45−4.38 (m, 1H), 4.36 (dd, J = 9.0,
5.2 Hz, 1H), 4.27−4.11 (m, 2H), 3.92 (dd, J = 10.4, 2.7 Hz, 1H), 3.74
(td, J = 10.5, 3.9 Hz, 1H), 3.20−3.08 (m, 1H), 3.03−2.86 (m, 1H),
2.11−1.96 (m, 4H), 1.13 (s, 9H), 1.02 (s, 9H); 13C NMR (75 MHz,
CD3CN) δ 180.8, 134.1, 133.1, 129.7, 129.5, 69.8, 69.2, 66.2, 64.6,
56.7, 49.5, 34.9, 34.4, 28.1, 26.2, 25.9, 25.6 (isonitrile carbon not
detected); IR (neat) 2963, 2876, 2228, 1567, 1451, 1373, 1242, 1131,
996, 862, 751, 695 cm−1; ESI-MS, m/z [M − 2Cl− + HCOO−]+
598.17.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Support by the Studienstiftung des Deutschen Volkes (Fellow-
ship for E.L.) is gratefully acknowldeged.
■
REFERENCES
■
5b: 1H NMR (600 MHz, CDCl3) δ 7.88−7.68 (m, 2H), 7.68−7.45
(m, 3H), 5.80 (t, J = 10.0 Hz, 1H), 5.08−3.06 (m, 10H), 2.11−1.49
(m, 6H), 1.14−0.85 (m, 18H); IR (neat) 2948, 2866, 2229, 1566,
1439, 1372, 1241, 1133, 995, 854, 753, 693 cm−1; ESI-HRMS, m/z
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E
DOI: 10.1021/acs.organomet.5b00516
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F
DOI: 10.1021/acs.organomet.5b00516
Organometallics XXXX, XXX, XXX−XXX