A simple access to transition metal cyclopropenylidene complexes

Article abstract of DOI:10.1039/c4cc10375k

We report the first example of BAC-Cu complex (BAC = bis(diisopropylamino)cyclopropenylidene) and its use as a carbene-transfer reagent, allowing access to Au-, Pd-, Ir- and Rh-BAC compounds. Catalytic experiments show the high activity of the [CuCl(BAC)]

Full text of DOI:10.1039/c4cc10375k

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A Simple Access to Transition Metal
Cyclopropenylidene Complexes
Cite this: DOI: 10.1039/x0xx00000x
a
a
b
a
Yannick D. Bidal Mathieu Lesieur , Mohand Melaimi , David B. Cordes ,
,
a
b
a
Alexandra M. Z. Slawin , Guy Bertrand* and Catherine S. J. Cazin*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report the first example of BACꢀCu complex (BAC = complex [CuCl(BAC)] 2 was obtained in 92% yield after 2 hours at
8
0°C in acetonitrile (Scheme 2). It is worth mentioning that 2 is
bis(diisopropylamino)cyclopropenylidene) and its use as a
carbeneꢀtransfer reagent, allowing access to Auꢀ, Pdꢀ, Irꢀ and
RhꢀBAC compounds. Catalytic experiments show the high
activity of the [CuCl(BAC)] complex in Click chemistry.
perfectly air and moisture stable, which allows for a convenient
workꢀup procedure.
Cyclopropenylidenes, the simplest aromatic ring system displaying a
carbene centre, are amongst the most discussed compounds within
1
1a
the carbocyclic carbene family. Since the early 1970’s, various
transition metal complexes bearing cyclopropenylidene ligands have
been prepared. Their synthesis has been achieved via oxidative
2
dꢀf,3
addition of dihalocyclopropenes,
cyclopropenium salts or lithium adducts,
deoxygenation, desulfurization and deselenization routes.
However these methodologies require harsh conditions that are not
by the reaction of
2
dꢀf,4
or using
2e,5
6
Scheme 1. Synthesis of the cyclopropenylidene 1
1
a
conducive to the generation of libraries of complexes. The isolation
6
of the cyclopropenylidene 1 in 2006 (Scheme 1) allowed to broaden
1
a,7
the number of complexes available, but Group 11 remains to date
scarcely studied, with no example of gold and copper compounds,
8
and only one example of a silver derivative. As a consequence, the
catalytic activity of cyclopropenylidene metal complexes has
9
remained rather unexplored compared to that of the Nꢀheterocyclic
1
0
carbene (NHC) counterparts.
Scheme 2. Synthesis of the CuꢀBAC complex 2
1
As part of our ongoing efforts to develop straightforward synthetic
accesses to NHC transition metal complexes, we have shown that
CuꢀNHC compounds could be obtained by simple reaction of the
The H NMR spectrum of 2 at room temperature contains three
i
broad signals corresponding to the Pr groups. Upon cooling the
solution to 240 K, the signals significantly sharpen to the expected
doublets and multiplet (assigned to CH and CH, respectively). Such
a dynamic behaviour has already been observed for the free carbene
and its corresponding salt 1·HBF . The free activation energy for
site exchange of the Pr groups was found to be 53 kJ/mol for the
free carbene and 75 kJ/mol for the cyclopropenium salt. In the case
of complex 2, a ꢁG of 63.5 kJ/mol was experimentally determined
and is likely to be related to a similar dynamic process (see ESI for
1
1
conjugate acid of NHCs with Cu O, and subsequently used as
2
3
1
2
NHCꢀtransfer reagents. Such a methodology provides a versatile
synthetic access to a library of NHC complexes, without the
6a
1
4
13
i
formation of the highly reactive free carbene at any stage. Herein,
we report that this synthetic strategy can also be applied to the
cyclopropenylidene series, allowing access to Au, Pd, Ir and Rh
complexes. In addition, we show that a BACꢀCu complex has a high
catalytic activity in an important catalytic transformation namely the
≠
details). The structure of 2 was unambiguously confirmed by a
15
[3+2] cycloaddition of azides and alkynes.
single crystal Xꢀray diffraction study (Fig. 1). Complex 2 has a
pseudoꢀlinear geometry with a Cl1ꢀCu1ꢀC1 angle of 175.6(2)°. The
C1ꢀCu1 bond distance [1.880(6) Å] is strikingly shorter than
previously reported CꢀCu bond lengths for IPr (IPr = (N,N’ꢀbisꢀ
2,6ꢀ diisopropylphenyl]imidazolꢀ2ꢀylidene) [1.953(8) Å] and 1,2,3ꢀ
The first step of the synthetic strategy consists of the formation of a
I
BACꢀCu complex by reaction of the cyclopropenium chloride
1
4
16a
1
·HCl with Cu O under microwave heating. Indeed such a reaction
2
benefits from short reaction times, and in this manner the desired
[
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1
7
triazolꢀ4ꢀylidene CuCl complexes [1.9577(16) Å]. This is in to previously reported procedures for palladium, that for example
1
b,18,19
agreement with the strong σꢀdonor ability of BAC.
Its steric have made use of palladium black and the BAC·HCl salt (44%
2
0
1a,2d
hindrance was assessed by calculating its %VBur at 1.9 Å and found yield).
The present methodology affords the desired complex
to be 31.6%. This value is slightly larger than that of ICy [30.5%, with a 90% yield over the two synthetic steps. The NMR spectra of
ICy = (N,N’ꢀbisꢀ[dicyclohexyl]imidazolꢀ2ꢀylidene] and significantly complexes 3ꢀ6 also show the presence of a dynamic phenomenon on
≠
smaller than that of two common NHCs, IMes [38.0%, IMes= (N,N’ꢀ the NMR timescale with ꢁG for this process in the 50.0ꢀ64.2 kJ/mol
bisꢀ[2,4,6ꢀtrimethylphenyl]imidazolꢀ2ꢀylidene]
and
IPr range (see ESI for details). All complexes were obtained in microꢀ
analytical purity and their structure was unambiguously confirmed
16,21,22
(49.6%).
15
by single crystal Xꢀray diffraction. The fact that [CuCl(BAC)] 2
and [AuCl(BAC)] 4 can be isolated in such a straightforward and
pure manner is also a highlight of the approach. Indeed, all attempts
to prepare 2 from the free BAC 1 failed. In the case of gold,
2
3
similarly to the behaviour of the smallest CAAC carbenes, BAC
reacts with [AuCl(THT)] quantitatively affording the cationic
+
[
(
Au(BAC) ] complex, regardless of the reagent stoichiometry used
2
see ESI for XRD data).
The key copper synthon 2 appeared a logical choice to initially
evaluate the catalytic performance of transition metal BACꢀ
containing complexes. We chose to test its catalytic activity in the
24
[
3+2] cycloaddition of azides and alkynes. The reaction of heptyl
Figure 1. Molecular structure of 2. Hydrogen atoms are omitted for
azide with phenyl acetylene, under mild conditions (room
temperature, solventꢀfree), using a low catalyst loading was
investigated. Comparison with [Cu(Cl)(SIMes)] [SIMes = (N,N’ꢀbisꢀ
clarity. Selected bond lengths (Å) and angles (º): CuꢀC, 1.880(6);
1
5
CuꢀCl, 2.107(2); CꢀCuꢀCl, 175.6(2).
[
2,4,6ꢀ(trimethyl)phenyl]imidazolidinꢀ2ꢀylidene], which represents
With complex 2 in hand, its ability to transfer the carbocyclic
carbene was examined. Results are summarised in Scheme 3. Four
metals (Rh, Ir, Pd, Au) belonging to Groups 9, 10 and 11 were
selected to test the viability and versatility of the synthetic approach.
In all cases, the transfer of the cyclopropenylidene occurs
quantitatively and selectively. Spectacularly, for Au, Ir and Rh, the
carbene transfer reaches completion within one minute at room
temperature! In the case of palladium, while the transfer occurs
quantitatively to form the targeted dimeric species, 4 hours and an
operating temperature of 40°C are required to reach completion.
This represents an outstandingly straightforward method compared
25
the stateꢀofꢀtheꢀart
for such a reaction, showed that the
cyclopropenylidene complex 2 has a higher catalytic activity than its
NHC analogue. Relative kinetic profiling curves, under identical
conditions, are presented in the ESI. Various substrates with a broad
range of functionalities (ꢀNO , ꢀCN, ꢀOH, ꢀC=O) were tested. All
desired compounds were obtained with an excellent isolated yield
2
using 0.5 mol% of 2 under mild condition (solventꢀfree, 25°C)
(Scheme 4).
N
N
N
N
Cl
1/2
Pd
Cl Ir
Cl
2
3, 98%
5, 98%
N
N
Cu
Cl
N
N
N
N
2
CO
Au
Cl
Rh
Cl
CO
6, 81%
4, 98%
Scheme 3. Transfer of cyclopropenylidene from copper to palladium, gold, iridium and rhodium. ORTEP representations of 3ꢀ5. Hydrogen
atoms are omitted for clarity. Selected bond lengths (Å) and angles (º): for 3: PdꢀC1, 1.926(4); PdꢀCl1, 2.2892(12); PdꢀCl2, 2.3311(12); Pdꢀ
Cl21, 2.4215(11); for 4: AuꢀC1, 1.971(2); AuꢀCl, 2.2843(7); C1ꢀAuꢀCl, 179.22(6); for 5: IrꢀC1, 2.011(2); IrꢀCl, 2.3596(7); C1ꢀIrꢀCl,
15
9
1.75(6).
2
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COMMUN AT75ION
IC10
N
R
1
University of California, San Diego, La Jolla, CA 92093ꢀ0343, USA.
N
N
2
(0.5 mol%)
R
1
N
3
+
R
2
Electronic Supplementary Information (ESI) available: Synthesis and
characterisation of ligand precursor, complexes, azide substrates and
solvent-free, rt
R2
catalysis products, general procedure for catalysis, VTꢀNMR
N
N
N
N
≠
Hept
Bn
Ph
Ph
experiments,
∆
G and %VBur determination, NMR spectra of complexes
N
N
N
N
N
N
N
N
ꢀ
and catalysis products, and crystal data for 2ꢀ5 and [Au(BAC)
2
]+OTf See
Ph
6%, 5h20
Ph
Ph
96%, 4h30
tBu
96%, 4h
DOI: 10.1039/c000000x/
9
97%, 2h
N
N
1
2
a) K. Öfele, E. Tosh, C. Taubmann, W. A. Herrmann, Chem. Rev.
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,
N
97%, 5h
N
N
N
N
2
Ph
Ph
O
2
N
NC
96%, 2h30
Bn
,
5
1
304ꢀ5313; d) S. J. Connon, Angew. Chem. Int. Ed., 2014, 53, 1203ꢀ
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N
N
Bn
N
N
N
N
N
N
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Ph
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OH
85%, 7h
97%, 2h
96%, 1h
Scheme 4. Scope of the [3+2] cycloaddition. Reaction conditions: 2
0.5 mol%), azide (1.00 mmol), alkyne (1.10 mmol), 25°C, solventꢀ
(
free.
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In conclusion, the synthesis of the first cyclopropenylidene
copper(I) complex and its use as an efficient carbene transfer
reagent have been achieved. The approach results in the
straightforward generation of Pd and Rh complexes, as well as
the first isolation of Au and Ir complexes bearing a BAC
ligand. Preliminary catalytic studies of the parent CuꢀBAC
complex are extremely promising and show that this ligand
family may in certain instances surpass the NHC congeners. As
one of the most important criteria for the development of
catalysts is a versatile synthetic access to a library of
complexes, we hope that the present work will encourage the
study and development of highly active systems bearing cyclic
carbenes other than the classical 5ꢀmembered heterocyclic
standards.
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The authors gratefully acknowledge the Royal Society
2
(
University Research Fellowship to CSJC) and the DOE (DEꢀ
D. Holschumacher, C. G. Hrib, P. G. Jones, M. Tamm, Chem.
Commun., 2007, 3661ꢀ3663.
FG02ꢀ13ER16370) for financial support. We also thank the
EPSRC National Spectrometry Service Centre in Swansea for
HRMS analyses and Melanja Smith and Dr Tomas Lebl for
assistance with variableꢀtemperature NMR experiments.
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4
a
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1
4 This reaction can also be carried out using conventional heating. See
ESI for details.
5 CCDC 1012888 (
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1
5
.
+
ꢀ
[
2
Au(BAC) ] OTf complex. These data can be obtained free of charge
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