Synthesis of hemilabile cyclic (alkyl)(amino)carbenes (CAACs) and applications in organometallic chemistry

Article abstract of DOI:10.1021/jacs.6b05221

A versatile methodology, involving readily available starting materials, allows for the synthesis of stable hemilabile bidentate cyclic (alkyl)(amino)carbenes (CAACs) featuring alkene, ether, amine, imine, and phosphine functionalities. The stability of the free carbenes has been exploited for the synthesis of copper(I) and gold(I) complexes. It is shown that the pendant imine moiety stabilizes the gold(III) oxidation state and enables the C-C bond oxidative addition of biphenylene to the corresponding cationic gold(I) complex. The latter and the corresponding copper(I) complex show high catalytic activity for the hydroarylation of α-methylstyrene with N,N-dimethylaniline, and the copper(I) complex promotes the anti-Markovnikov hydrohydrazination of phenyl acetylene with high selectivity.

Full text of DOI:10.1021/jacs.6b05221

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Communication
Synthesis of Hemilabile Cyclic (Alkyl)(amino)carbenes
(CAACs) and Applications in Organometallic Chemistry
Jiaxiang Chu, Dominik Munz, Rodolphe Jazzar, Mohand Melaimi, and Guy Bertrand
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 15 Jun 2016
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Synthesis of Hemilabile Cyclic (Alkyl)(amino)carbenes
(CAACs) and Applications in Organometallic Chemistry
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Jiaxiang Chu,‡ Dominik Munz,‡ Rodolphe Jazzar, Mohand Melaimi, and Guy Bertrand*
UCSD/CNRS Joint Research Chemistry Laboratory (UMI 3555), Department of Chemistry and Biochemistry, University of
California San Diego, La Jolla, CA 92093-0358, USA
Supporting Information Placeholder
Herein we report simple and scalable synthetic procedures allow-
ing for the preparation of a variety of hitherto unknown hemila-
able starting materials, allows for the synthesis of stable hemila-
ABSTRACT: A versatile methodology, involving readily avail-
bile bidentate CAACs. As proof of principle for the potential of
this novel type of carbenes in organometallic chemistry, we show
that their coinage metal complexes can promote the oxidative
addition of biphenylene (Au), the catalytic hydroarylation of al-
kenes (Cu and Au) and the anti-Markovnikov hydrohydrazination
of alkynes (Cu).
bile bidentate CAACs featuring alkene, ether, amine, imine and
phosphine functionalities. The stability of the free carbenes has
been exploited for the synthesis of copper(I) and gold(I) complex-
es. It is shown that the pendant imine moiety stabilizes the
gold(III) oxidation state, and enables the C–C bond oxidative
addition of biphenylene to the corresponding cationic gold(I)
complex. The latter and the corresponding copper(I) complex
show high catalytic activity for the hydroarylation of α-methyl
styrene with N,N-dimethylaniline, and the copper(I) complex
promotes the anti-Markovnikov hydrohydrazination of phenyl
acetylene with high selectivity.
The most convenient synthesis of the conjugate acid of CAACs
relies on the condensation of an aniline derivative with 2,2-
dialkyaldehydes, followed by deprotonation with LDA, alkylation
with 3-chloro-2-methylpropene, and cyclization under acidic con-
ditions.1b-d We generalized this reaction sequence to unbranched
n
aldehyde. Deprotonation of 1 with BuLi followed by alkylation
with 3-chloro-2-methylpropene gave imine 2 in quantitative yield
on a 55 g scale. A second deprotonation with nBuLi and alkylation
with an alkyl halide bearing a functional group, followed by cy-
clization of the resulting imine 3 under acidic conditions (HCl)
afforded the desired cyclic iminium salts 4 (Scheme 1, method A).
Alternatively, proton induced cyclization of 2 followed by treat-
ment with aqueous ammonia led to the cyclic enamine 5, which
upon reaction with electrophiles gave rise to salts 4 (Scheme 1,
method B). With method A, cyclic iminium salts 4a-c were syn-
thesized with chloromethoxy ethyl ether, methallyl chloride, and
allyl chloride as alkylating agents in moderate to good overall
yields (43%, 47% and 36% yield, respectively, from the aniline).
Base sensitive functionalities can be introduced using method B,
which does not require deprotonation with strong bases. Cyclic
iminium salts 4d-f were prepared through enamine alkylation with
N-methylene-N,N-dimethylammonium chloride (Böhme’s salt),10
N-diisopropylphenyl pivalimidoyl chloride, and methylene diio-
dide, respectively (4d: 46%; 4e: 46%; 4f: 45% yield from the
aniline). The methylene iodide substituted salt 4f can be further
Cyclic (alky)(amino)carbenes (CAACs) have found widespread
application for the stabilization of reactive intermediates, the acti-
vation of small molecules, and as ligands in organometallic chem-
istry.1,2 This is due to their small HOMO–LUMO gap and their
very strong σ-donor and π-accepting capabilities,3 which consid-
erably exceed those of conventional NHCs (N-heterocyclic car-
benes).4 Additionally, the steric demand of CAAC ligands is very
distinct from that of NHCs.5 The possibility of tailoring NHCs for
specific tasks by attachment of ancillary functional groups has
been pivotal for the success of these ligands in organometallic
chemistry.6 Chelating NHC ligands provide remarkable chemical
stability to high-valent metal centers, and have found widespread
application in oxidation catalysis.7 In addition, they allow for
bifunctional cooperativity including the generation of hemilabile
coordination sites,8 and some of their complexes feature lumines-
cent and phosphorescent properties.9
Scheme 1. Synthesis of functionalized CAAC precursors 4a-g.
1) nBuLi
2) E-X
Method A
Dipp
1) nBuLi
N
H+
2)
Dipp
Dipp
Dipp-NH2
+
N
N
Dipp
Cl
MgSO4
rt
N
3
5
E
1) H+
2) NH4OH
2
O
1
Dipp
4
E
N
quantitative
Dipp
quantitative
Method B
E-X
Method B
Method A
tBu
N
N
N
N
N
N
N
Dipp
Dipp
BF4
Dipp
Dipp
Dipp
Dipp
N
I
OEt
NMe2
PPh2
4g: 69%a
Dipp
PF6
4a: 43%
BF4
BF4
4c: 36%
BF4
4d: 46%
I
I
4b: 47%
4e: 46%
4f: 45%
Dipp: 2,6-diisopropylphenyl. aYield from 4f.
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derivatized as shown by the substitution reaction with HPPh2 in
the presence of Hünig’s base, which afforded 4g in 69%.
Scheme 3. Oxidative addition of biphenylene to (6e)AuCl.
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2
3
4
5
6
7
Solid-state structure of (6e)AuCl (bottom left) and of 7
(bottom right). X-ray crystal structure with thermal ellip-
soids (30% probability); solvent molecules, hydrogen and
anion were omitted for clarity. Selected bond lengths (Å):
(6e)AuCl (left): Au-Cc 1.984(8), Au-Ni 3.391(7); 7 (right):
Au-Cc 2.077(3), Au- Ni 2.246(3), Au-Ca 2.051(3), Au-Cb
2.091(3).
To our delight the deprotonation of all cyclic iminium salts 4a-g
afforded cleanly the corresponding free carbenes 6a-g (Scheme
2). Carbene 6f was found to be the least stable at room tempera-
ture decomposing after only a few hours, whereas the methylene
ethoxy substituted carbene 6a was found to be indefinitely stable
in the solid state and was therefore isolated.
A series of the corresponding copper (6)CuCl and gold (6)AuCl
complexes were easily synthesized in good yields by addition of a
solution of carbene to copper chloride and (tetrahydrothio-
phene)gold chloride, respectively (Scheme 2). The connectivity of
the coinage metal complexes (6d)CuCl, (6f)CuCl and (6e)AuCl
(Scheme 3) was confirmed by single crystal X-ray crystallog-
raphy. They are mononuclear complexes that do not show any
interaction of the tethered functional groups with the metal center
(See Supporting Information).
8
9
tBu
B(C6F5)4
Dipp
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N
N
N
Dipp
Dipp
N
1.) KB(C6F5)4
2.) biphenylene
Dipp
III
Au
tBu
CH2Cl2
r.t., 12 h
I
Au
Cl (6e)AuCl
7
70%
Scheme 2. Synthesis of free carbenes 6a-g and of their cop-
per and gold complexes.
(6a)CuCl: 70%
(6b)CuCl: 59%
Dipp
M
Dipp
Dipp
(6d)CuCl: 77%
(6e)CuCl: 74%
(6f)CuCl: 75%
(6g)CuCl: 58%
(6d)AuCl: 83%
(6e)AuCl: 79%
CuCl or (THT)AuCl
rt
N
KHMDS
rt
N
N
4a-g
6a-g
E
E
E
Cl
To demonstrate the potential of these novel carbenes in organo-
metallic chemistry, we selected the gold complex (6e)AuCl,
which features the imine functionality. Whereas CAAC ligands
support low oxidation states, their propensity to stabilize high-
valent complexes has rarely been explored.11 Although it is very
well-known that oxidative addition on gold(I) is kinetically chal-
lenging,12 it was very recently shown by Bourissou13 and Toste14
that the metal center of some cationic gold(I) complexes could
undergo oxidative addition of biphenylene. We reasoned that the
imine moiety of (6e)AuCl, which does not coordinate the soft
gold(I) (Au-Ni > 3.39 Å), should be able to help stabilize the gold
center in the +III oxidation state.15 Indeed, monitoring the room
temperature reaction of (6e)AuCl with biphenylene in presence of
KB(C6F5)4 by 13C NMR spectroscopy revealed a shift of the car-
bene signal from 231.7 to 251.0 ppm and of the imine carbon
from 177.3 to 196.1 ppm. An X-ray diffraction study confirmed
the activation of a C–C bond and the formation of the cationic
gold(III) complex 7 in which the imine ligand coordinates to the
gold center (Au-Ni = 2.246(3) Å) (Scheme 3). Note that with
(CAAC)AuCl complexes without functional groups on the side-
chain, the oxidative addition does not occur even upon heating to
80 °C in the presence of KB(C6F5)4. These results demonstrate
that the pendant imine strongly stabilizes the gold(III) center, and
suggest that such hemilabile bidentate ligands should be suitable
for catalysis involving high oxidation states.
Scheme 4. Gold and copper complexes with imine func-
tionality in the side chain of the CAAC ligand lead to high
catalytic activity for the hydroarylation reaction.
cat. (1 mol%),
KB(C6F5)4 (2 mol%)
NMe2
Ph
Ph
+
Me2N
C6D6, 120 °C
cat. = (6e)AuCl: 1 h, 45% (12 h, 97%)
cat. = (6e)CuCl: 1 h, 47% (12 h, 97%)
Et
Et
CAAC:
N
cat. = (CAAC)AuCl: 1 h, 33% (12 h, 89%)
cat. = (CAAC)CuCl: 1 h, traces (12 h, traces)
Dipp
the same experimental conditions, the monodentate CAAC gold
complex appeared to be also efficient (89 % conversion) but the
corresponding copper complex gave only traces of the hydroaryla-
tion product. Note that the copper catalyzed intermolecular17 hy-
droarylation of alkenes had not yet been reported.
The intermolecular hydroamination reaction of alkynes and al-
kenes involves the transfer of a proton as well. The synthesis of
the anti-Markovnikov product remains a challenge.18 Recently,
we reported that ancillary ligand-free copper catalysts afford sole-
ly the Markovnikov addition product in the reaction of phenyla-
cetylene with dimethylhydrazine.19 Monnier et al. showed that
copper(I) with a cyanide anion leads to 90% selectivity for the
anti-Markovnikov product with dinpropylamine as the reaction
partner.20 We found that the tert-butoxide analogue of the copper
We reasoned that a basic moiety on the side-chain of the CAAC
ligands should also assist reactions involving a formal proton
transfer. We therefore evaluated the potential of the gold imine
complex (6e)AuCl for the hydroarylation of α-methyl styrene
with N,N-dimethyl aniline. We had previously reported that anti-
Bredt NHC gold complexes promote this reaction with excellent
chloride complex, namely (6e)CuOtBu, promotes at 120 C the
o
addition of dimethylhydrazine to phenyl acetylene. A 95% con-
version was observed after 24 h and the anti-Markovnikov prod-
uct was obtained with 96% selectivity (Scheme 5). It is important
to note that under the same experimental conditions, the
(CAAC)CuOtBu complex is much less selective, demonstrating
again the prominent role played by the imine functionality.
o
conversion but upon heating at 135 C for 24 hours.16 We found
that the reaction proceeds much faster with (6e)AuCl, a 97% con-
version was observed after only 12 hours at 120 °C (Scheme 4).
Encouraged by the high catalytic activity of this gold complex, we
wondered whether the corresponding copper complex (6e)CuCl
could also promote the hydroarylation reaction. We were pleased
to also observe a 97% conversion after 12 h. Interestingly, under
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Scheme 5. (6e)CuOtBu catalyzes the anti-Markovnikov
addition of dimethylhydrazine to phenyl acetylene.
1
2
3
4
5
6
7
8
cat. (1mol%)
Me2NNH2
NMe2
N
N
Ph
Ph
H
NMe2
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120 °C, 24 h
95%
H
Ph
Me
cat. = (6e)CuOtBu:
cat. = (CAAC)CuOtBu:
4
:
:
96
77
23
Et
Et
CAAC:
N
Dipp
9
The versatile methodology discussed in this paper should allow
for the preparation of a variety of hemilabile bidentate CAACs,
which will expand the number of applications of this class of car-
benes.
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ASSOCIATED CONTENT
Supporting Information
Synthetic procedures, catalytic experiments, NMR spectra, solid-
state structures. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
(10) Böhme, H.; Mundlos, E.; Otto-Erich Herboth, O.-E. Chem. Ber.
1957, 90, 2003.
guybertrand@ucsd.edu
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De Buck Becker, J.; Rudolph, M.; Scholz, C.; Rominger, F. Adv. Synth.
Catal. 2012, 354, 133. (b) Livendahl, M.; Goehry, C.; Maseras, F.;
Echavarren, A. M. Chem. Commun. 2014, 50, 1533.
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D. Angew. Chem. Int. Ed. 2015, 54, 5236. (b) Joost, M.; Amgoune, A.;
Bourissou, D. Angew. Chem. Int. Ed. 2015, 54, 15022.
Author Contributions
‡ These authors contributed equally.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
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This work was supported by the DOE (DE-FG02-13ER16370)
and the NSF (CHE-1359809). Thanks are due to the SIOC and
Prof. Yaofeng Chen (JC), and the German Academic Exchange
Service (DM) for postdoctoral fellowships. Dr. Eder Tomás-
Mendivil is thanked for helpful discussions. A. L. Rheingold, M.
Gembicky and C. E. Moore are greatly acknowledged for their
help with X-ray diffraction studies.
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