From isonitriles to carbenes: Synthesis of new NAC - And NHC-palladium(II) compounds and their catalytic activity

Article abstract of DOI:10.1021/om200141s

A series of NAC- and their structurally related NHC-palladium(II) isonitrile complexes, including examples of dinuclear complexes, were prepared. In a second series, NHC-palladium complexes bearing an aryl group on one nitrogen of the NHC ring and different cycloalkyl groups ranging from six- to 15-membered rings have been synthesized. The good yields document the value of the very short route to new palladium(II) NHC complexes. The stuctures have been investigated by crystal structure analyses of some representatives; in addition conformational studies by NMR were conducted. The reactivity in Suzuki cross-coupling reactions was investigated, showing turnover frequencies of up to 18-050 h^-1 and a clear correlation of the size of the cycloalkyl moiety with the turnover number achieved for the coupling of chlorobenzene and 2,5-dimethylphenylboronic acid.

Full text of DOI:10.1021/om200141s

ARTICLE
pubs.acs.org/Organometallics
From Isonitriles to Carbenes: Synthesis of New NACÀ and
NHCÀPalladium(II) Compounds and Their Catalytic Activity
A. Stephen K. Hashmi,* Christian Lothsch€utz, Constantin B€ohling, and Frank Rominger
Organisch-Chemisches Institut, Ruprecht-Karls-Universit€at Heidelberg, Im Neuenheimer Feld 270, 69221 Heidelberg, Germany
S Supporting Information
b
ABSTRACT:
A series of NACÀ and their structurally related NHCÀpalladium(II) isonitrile complexes, including examples of dinuclear
complexes, were prepared. In a second series, NHCÀpalladium complexes bearing an aryl group on one nitrogen of the NHC ring
and different cycloalkyl groups ranging from six- to 15-membered rings have been synthesized. The good yields document the value
of the very short route to new palladium(II) NHC complexes. The stuctures have been investigated by crystal structure analyses of
some representatives; in addition conformational studies by NMR were conducted. The reactivity in Suzuki cross-coupling reactions
was investigated, showing turnover frequencies of up to 18 050 hÀ1 and a clear correlation of the size of the cycloalkyl moiety with
the turnover number achieved for the coupling of chlorobenzene and 2,5-dimethylphenylboronic acid.
’ INTRODUCTION
Scheme 1. Structural Comparison of NHCs and NACs
During the last decades N-heterocyclic carbenes (NHCs)
have revolutionized modern organometallic chemistry.1 Initially
considered as a curiosity, NHCÀmetal complexes found broad
application in a number of different reactions.2 NHCs form
stable complexes with a large number of elements, resulting in a
diverse field of applications.3 Nonetheless, NHCÀPd(II) com-
pounds can be considered as one of the most prominent classes
of NHCÀmetal complexes. In this field the pioneering contribu-
tions from the groups of Herrmann,4 Nolan,5 Organ,6 and
Glorius7 should be mentioned. Structurally related but less
apparent in the literature, another group of carbene ligands
exists, the so-called nitrogen-acyclic carbenes (NACs).8 These
compounds stand out by the absence of the backbone, resulting
in a number of interesting characteristics (Scheme 1).
Depending on the number and type of substituents, NACs can
form diastereoisomers with different NÀCÀN bond angles and
therefore differing electron-donating abilities.9 The first
NACÀPt(II) complex was synthesized by Chugaev and co-
workers in 1915, but was inadvertently formulated as a dimeric
hydrazine-bridged complex.10 Later Shaw and co-workers were
able to assign the correct structure.11 The groups of Richards and
Belluco were next to investigate NACÀPd(II) complexes in the
late 1960s.12 The synthesis of these complexes can be easily
accomplished by the reaction of a nucleophile with a metal-
coordinated isonitrile, commonly resultinginthe acyclic carbenein
high yields. In addition to the use of nitrogen nucleophiles, oxygen
nucleophiles also found broad application in this concept.13 In
addition to this method, there exist other procedures that for example
start from formamidinium or chloramidinium salts.14 In a recent
publication, the catalytic activity in SuzukiÀMiyaura cross-coupling
reactions of special NACÀPd(II) compounds, resulting from the
addition of hydrazones to bis(isonitrile)palladium(II) complexes, was
described.15 Preceding reports dealt with the use of NACÀPd(II)
complexes in Sonogashira, Heck, and SuzukiÀMiyaura reactions.16
In our latest work we addressed the probably simplest synth-
esis of unsymmetrical substituted NHC metal complexes and
their catalytic activity.17 In this article we expand our initial
findings and for the first time compare structurally related
NHCÀPd(II) and NACÀPd(II) complexes with respect to their
catalytic activity and structural parameters.
’ RESULTS AND DISCUSSION
The synthesis of both NACÀ and NHCÀPd(II) compounds
utilizes the addition of a nitrogen nucleophile to the coordinated
Received: February 14, 2011
Published: March 31, 2011
r
2011 American Chemical Society
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Scheme 2. Synthesis of Bis(isonitrile)palladium(II)
Complexes
Table 1. Synthesis of the NACÀPalladium Complexes 5À17
Scheme 3. Synthesis of 3c from p-Toluidine and
3-Bromocyclohexene
Scheme 4. Formation of NAC and NHC Complexes
isonitrile ligand. On coordination of an isonitrile to a metal such
as palladium(II), platinum(II), or gold(I), the IR stretching
frequency shows a hypsochromic shift in the range of more than
100 cmÀ1, indicating an activation for the reaction with a
nucleophile. This spectroscopic property identifies the isonitrile
ligands as virtually pure donor ligands. Moreover, isonitriles are
isoelectronic to NHCs so that the isonitrileÀmetal bond can be
characterized as a carbene-type bond. The synthesis of the
bis(isonitrile)palladium(II) complexes was simply achieved by
the treatment of (CH3CN)2Cl2Pd(II) with two equivalents of
the corresponding isonitrile in toluene.
Inthecaseof 3 the isonitrile was synthesizedbytwo consecutive
3,3-sigmatropic rearrangements of in situ generated N-(cyclohex-
2-en-1-yl)-4-methylbenzene, forming the corresponding aniline
derivative 3a. This compound can be transformed into the
isonitrile 3c by standard procedures.
On addition of a simple primary or secondary amine to
bis(isonitrile)palladium(II) complexes, the outcome is a
NACÀPd(II) complex (Scheme 4a). When substituted 2-
(chloroethyl)amines are used, the primary attack of the amine
a For the syntheses of the amines see Supporting Information. b In case
of this amine 2.00 equiv of the palladium precursor were used in order to
form the dimeric NACÀPd(II) complex.
causes the formation of a secondary nucleophilic center on the
nitrogen atom of the isonitrile, closing the heterocycle by a
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Scheme 5. Synthesis of 18 and Solid-State Molecular
Structurea
a All the hydrogen atoms and the chloride counterion are omitted for
clarity.
analogues 8 and 16 in good to moderate yields (entries 4, 12). In
two other examples we used aniline derivatives as nucleophiles
(entries 10, 11). Compound 9 was formed by the attack of an
amine bearing a styryl group in the backbone (entry 5). The
compound is perfectly stable and might find application in
functionalized polymers, as the olefin moiety should serve as
monomer in polymerization reactions.
Figure 1. Solid-state molecular structures of compounds 8b and 14.
Most hydrogen atoms are omitted for clarity.
Compounds 10 and 13 were inspired by the impressive
work of the Glorius group, which has shown that IBOX-
carbenes are extremely good ligands for a number of different
reactions, whereby the catalytic activity is characterized by the
dynamic behavior of the adjacent aliphatic ring.8 Two exam-
ples (entries 8 and 13) prove the applicability of this method
to the formation of bimetallic compounds. The structure of
complexe 14 was unambiguously proven by X-ray structure
analysis (Figure 1).
The synthesis of 8 was repeated several times. In one case an
excess of 1-adamantylamine was used, thus leading to the
formation of a small amount of a certain byproduct, 8b.
Fortunately, we could grow single crystals suitable for X-ray
analysis. On treatment of the initially formed complex with
1-adamantylamine, an internal complex was formed with the
amine in trans-position to the carbene. A hydrogen-bonded
chloride in the outer coordination sphere compensates the
positive charge. The differing yields for the carbenes must be
traced back to the workup process because the small pre-
cipitate was not always ideal. The addition of the amine to
the isonitrile was quantitative in nearly all cases, as proven
by the IR stretching frequencies of the reaction mixtures.
The workup was accomplished by recrystallization of the
crude products.
Due to the presence of partial double bonds in the carbene
moiety, we observed the formation of rotamers. Taking the
example of 11, this fact can be exemplified impressively. Having
a closer look at the aromatic region of the 1H NMR spectrum,
one can identify two different signals that must be assigned to
the hydrogen atom on the nitrogen atom adjacent to the
Figure 2. Aromatic region of the 1H NMR spectrum of 11.
nucleophilic substitution (Scheme 4b). For reasons of simplicity
we did not use the free 2-(chloroethyl)amines, but the corre-
sponding ammonium salts in the presence of triethylamine
as base.
For the synthesis of NACÀPd(II) complexes we used a
number of different amines in order to demonstrate the high
scope of this methodology. For this study we used two different
bis(isonitrile)palladium(II) complexes bearing sterically very
demanding groups R. As amines we introduced aliphatic as well
as aromatic representatives. Table 1 summarizes the outcome of
our experiments. Besides the use of less sterically demanding
amines such as diethyl amine, pyrrolidine, or piperidine (entries
1, 2, 3, 7) we also managed to synthesize 1-adamantyl-substituted
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Table 2. Synthesis of the NHCÀPalladium Complexes
19À26
Scheme 6. Formation of Diastereoisomers
Figure 3. Aliphatic region of the 1H NMR spectrum of 22. Optimized
structure of the simplified ligand (B3LYP, cc-PVDZ).
formation of the NHCÀPd(II) complexes was achieved in
THF at room temperature. In this study we prepared a number
of compounds bearing large aliphatic rings (Table 2, entries 2, 3,
4, and 6). The choice of these substituents was made on the basis
of our earlier work. In catalytic studies we found cyclohexyl-
substituted NHCÀPd(II) carbenes to be active catalysts for the
activation of aryl chlorides in SuzukiÀMiyaura cross-coupling
reactions at room temperature in technical grade solvents.
Although the yields were moderate, we were interested in the
role these groups would play in further catalytic reactions. Thus
we prepared a whole series of new NHC complexes of this type,
varying the ring size (Table 2). The synthesis of these organo-
metallic compounds succeeded without any problems, docu-
menting the broad scope of our new method.17 Our protocol
allowed the incorporation of a chiral side group (entry 5) and of
large aliphatic rings. So, we could synthesize a NHC containing a
cyclopentadecyl unit for the first time. Moreover we prepared a
NHC complex starting from 3 showing enormous steric demand
(entry 7). The last entry in Table 2 shows a NHC with an
annulated cyclohexyl ring. In 2008 Hahn and co-workers re-
ported a synthesis delivering comparable Rh(I) complexes in a
four-step synthesis, requiring reagents such as elemental sulfur
and potassium.19 The workup was simply accomplished by
recrystallization or by precipitation from ethereal solution.
Our method also allowed the introduction of a phenyl group
in the backbone of the NHC ligand. Due to the cis-configuration,
these compounds form mixtures of diastereoisomers (Scheme 6).
carbene carbon. In this special case we found an isomeric ratio
of 2.5:1.
In another experiment we tried to synthesize a bidentate
NACÀPd(II) complex.18 In doing so, we obtained complex 18
(Scheme 5). Interestingly, only one isonitrile and one chloro
ligand were replaced, forming a cationic complex.
As outlined above, we were also able to prepare NHCÀPd(II)
compounds by a related procedure (Scheme 4). The synthesis of
the 2-(chloroethyl)ammonium salts was accomplished according
to standard procedures (see Supporting Information). The
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a
Table 3.
Table 4. Suzuki Cross-Coupling of Chlorobenzene and 2,5-
Dimethylphenylboronic Acid by the New NHCÀPalladium
Complexes
a Average of two runs. Reaction conditions: 1.00 mmol of bromoben-
zene, 1.15 mmol of boronic acid, 1.00 mmol of KOtBu, 2.00 mL of
EtOH, 12 h, RT. The yield was determined by GC, using dodecane as
internal standard.
a Average of two runs. Reaction conditions: 1.00 mmol of bromoben-
zene, 1.15 mmol of boronic acid, 1.00 mmol of KOtBu, 2.00 mL of
EtOH, 12 h, RT. The yield was determined by GC, using dodecane as
internal standard.
The phenyl group can be oriented in two different ways, pointing
toward either the isonitrile or the chloro ligand, in which one
direction is preferred. The isomeric ratios are in the range of 2:1
(24) and 6:1 (22).
were comparable (entry 11) or lower. However, we found the
reactions to be extremely fast. For this reason we were interested
in the kinetics of these transformations. In order to study the
catalyses concerning the reaction time, we found ReactIR to be
the best solution. However, we had to choose another combina-
tion of aryl bromide and boronic acid because our standard
system was not suitable for this methodology. The combination
of p-bromobenzaldehyde and 2-methoxyphenylboronic acid was
appropriate for this task. Figure 4 shows the kinetics of the
experiments using compounds 10 and 13 as well as 6 and 11 as
catalysts. NAC 13 showed a TOF of 18 050 hÀ1 and a yield of
93%. Structurally related 10 showed a slightly smaller TOF
(17 820 hÀ1) and a comparable yield of 96%. Delivering compar-
able yields (6, 83%; 11, 84%), the differences concerning the
TOF of the reactions are quite remarkable (14 050 hÀ1, 11; 3700
1
Moreover, interesting electronic effects manifest in the H
NMR spectra of these compounds. The phenyl groups in the
backbone are in short contact with the aliphatic groups of the
aromatic moieties of the NHCÀPd(II) complexes. This orienta-
tion causes a shielding of these groups, and the corresponding
signals are shifted toward higher field (Figure 3).
Some of the complexes synthesized were tested in Suzu-
kiÀMiyaura cross-coupling reactions using bromobenzene or
chlorobenzene and 2,5-dimethylphenylboronic acid as coupling
partner. The outcome of these experiments is listed in Tables 3
and 4. In this study we were interested only in structure
Àperformance relationships of the catalysts. For this reason,
we did not vary the reaction parameters. In order to have robust
and user-friendly conditions, all reactions were performed at
room temperature in technical grade solvent without prior
degassing. In the case of NACÀPd(II) complexes the com-
pounds 11 and 12 delivered the highest yields (TON = 600 and
610, respectively). On the contrary, catalyst 9 showed only little
reactivity (TON = 230). The cationic complex 18 was comple-
tely inactive. In comparison with the structurally related
NHCÀPd(II) complexes (Table 3, entries 9, 10, 11) the yields
h
À1, 6).
In contrast to the NACÀPd(II) complexes the NHCÀPd(II)
analogues were also able to activate phenyl chloride at room
temperature under these difficult conditions. Our experiments
showed a clear dependence of the reaction outcome on the ring
size of the adjacent aliphatic ring and the backbone substitu-
tion (Figure 6, Table 4). The catalysts bearing the six- and
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yield. At the moment we have no explanation for this finding.
However, this comparison clarifies that the catalytic activity is
highly sensitive with regard to the geometric properties of the
ligands and that parameters such as the percent buried volume do
not always provide the right explanation.20 In order to test the
robustness of our catalysts, we performed a catalysis in rum (54
vol %) as solvent. Using 1 mol % of catalyst 22 we found 20% of
coupling product starting from chlorobenzene.
’ CONCLUSION
We have synthesized new, highly stable NACÀPd(II) and
NHCÀPd(II) complexes by a very short and efficient route. The
catalytic activity of the NHC compounds is much higher than
that of the NAC analogues. This might originate from an
improved stability and a longer catalyst lifetime. Nonetheless,
we found high TOFs for the NACÀPd(II) complexes of 18.050
Figure 4. Time-course of the reactions and overlay of observed
absorption bands of starting material and product.
h
À1 and more.21 Furthermore, we dicovered a dependence of the
catalyst performance and the ring size of adjacent aliphatic rings.
Paired with a phenyl-substituted backbone, large rings on the
NHC lead to excellent results with high TONs for phenyl
chloride at room temperature.22 In our opinion this dependence
is partially due to a hydrophobic effect caused by the unpolar
aliphatic ring. Moreover, the phenyl group in the backbone exerts
a steric effect on the isopropyl groups of the aromatic substituent
of the NHC as proven by 1H NMR spectroscopy (Figure 3). This
might lead to enhanced rigidity of the aromatic side group,
improving reductive elimination. The increasing ring size
raises the degree of conformational freedom of this side
group, thus facilitating oxidative addition. At the moment we
are further exploring this ligand concept for other reactions and
metals.
’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic information
b
Figure 5. Solid-state molecular structure of NHCÀPd complex
files (CIF) of compounds 4, 8b, 14, and 18, as well as
characterization data of all products. This material is available
free of charge via the Internet at http://pubs.acs.org.
(Table 3, entry 10).
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: hashmi@hashmi.de.
’ ACKNOWLEDGMENT
The authors want to express special thanks to the Umicore AG
for donations of the metal salts. Christian Lothsch€utz is grateful
for a Ph.D. grant from the Studienstiftung des dt. Volkes e.V.
Special thanks is addressed to the analytics department of the
Ruprecht-Karls-Universit€at Heidelberg.
Figure 6. Relationship between ring size, backbone substitution, and
TON (Table 4). The number in parentheses declares the ring size of the
aliphatic ring. The abbreviation Ph indicates the presence of a phenyl
group in the backbone of the catalyst.
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