Coinage metal complexes with N-heterocyclic carbene ligands as selective catalysts in diboration reaction

Article abstract of DOI:10.1016/j.tetasy.2006.06.027

We describe the improved catalytic reactivity of terminal alkenes with 1,2-diboranes in the presence of Au(I) and Ag(I) complexes when N-heterocyclic carbene ligands are used. The new catalytic systems are able to diminish the undesired β-H-elimination of the alkylboryl-metal intermediates, which leads to the formation of hydroborated byproducts. The electronic properties and molecular the structure of the precursors of the catalysts could explain the modest asymmetric induction provided.

Full text of DOI:10.1016/j.tetasy.2006.06.027

Tetrahedron: Asymmetry 17 (2006) 1759–1762
Coinage metal complexes with N-heterocyclic carbene ligands
as selective catalysts in diboration reaction
a
b
a
´
´
Rosa Corberan, Jesus Ramırez, Macarena Poyatos,
Eduardo Perisa, and Elena Fernandez
b,
*
*
´
a
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Dpt. Quı´mica Inorganica i Organica, Universitat Jaume I, Av. Vicente Sos Baynat s/n, 12080 Castellon, Spain
Dpt. Quı´mica Fı´sica i Inorganica, Universitat Rovira i Virgili, C/ Marcel.lı Domingo, 43007 Tarragona, Spain
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b
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Received 26 April 2006; accepted 16 June 2006
Available online 25 July 2006
Abstract—We describe the improved catalytic reactivity of terminal alkenes with 1,2-diboranes in the presence of Au(I) and Ag(I) com-
plexes when N-heterocyclic carbene ligands are used. The new catalytic systems are able to diminish the undesired b-H-elimination of the
alkylboryl–metal intermediates, which leads to the formation of hydroborated byproducts. The electronic properties and the molecular
structure of the precursors of the catalysts could explain the modest asymmetric induction provided.
ꢀ 2006 Elsevier Ltd. All rights reserved.
1. Introduction
O
O
O
B
M
Ln
A direct route to 1,2-bifunctional organic compounds can
be carried out by diboration of alkenes through 1,2-
bis(boronate)esters.1 Some of the key advantages of the
metal catalyzed diboration as opposed to noncatalyzed
diboration include the ability to use less reactive but more
stable diboranes, such as tetraalkoxydiboron compounds.2
Thus low valent transition metal catalysts are required to
cleave the B–B bond from the diboron reagent, generally
via oxidative addition, to form the intermediate metal
bis(boryl)complex.3 Subsequent nonsymmetric alkene
coordination and insertion into the M–B bond, eventually
provides regioisomeric intermediates that can follow B–C
reductive elimination generating the desired 1,2-bis(boro-
nate)ester product (Scheme 1).
B
R
B
O
R
B
R
R
R
O
O
R
B
O
O
MLn
O
O
B
B
MLn
O
O
O
B
O
B
O
O
O
O
R
B
B
O
O
O
O
O
B
O
R
Scheme 1.
improvements in ligand design, have allowed this problem
to be circumvented.4 Clean alkene diboration can be
achieved only if b-hydride elimination is inhibited. Progress
was made when Baker, Marder and Wescott5 decided to
use the gold(I) catalytic system [Au(PEt3)Cl] + 1,2-bis(di-
cyclohexylphosphino)ethane on the basis of unknown
mononuclear neutral gold-hydride complexes.6 The desired
1,2-bis(boronate)ester was mainly attained from styrene
and bis(catecholato)diboron, as a model reaction, although
catalyst activity and stability were lower than desired.
Alternatively, base-free Pt systems also contributed to
clean diboration, but they are not appropriate for modifi-
cation with chiral ligands.7
Depending on the catalyst precursor employed, various
amounts of undesired vinylboronate esters can also be ob-
tained by competitive b-hydride elimination processes
(Scheme 1). In particular, when rhodium(I)–phosphine is
involved, as the metal catalytic system, significant amounts
of internal and terminal hydroborated products can be
detected as by-products because of in situ formation of
hydride–rhodium–boryl complexes. However, recent
*
Corresponding authors. Tel.: +34 977 558046; fax: +34 977 559563
(E.F.); e-mail: elenaf@quimica.urv.es
0957-4166/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.06.027

1760
R. Corbera´n et al. / Tetrahedron: Asymmetry 17 (2006) 1759–1762
2. Results and discussion
X
X
1) t-BuLi / THF, -78ºC
2) AuCl(PEt3)
A
R
N
N R'
Inspired by these preliminary results and aimed to achieve
clean alkene diboration, we focussed our efforts to find new
catalytic systems that not only performed as clean diborat-
ing catalysts but also those that were able to induce asym-
metry in the bifunctional products. Asymmetric diboration
has only been achieved in a few recent examples in the
literature through chiral Rh(I) catalytic systems,8 despite
the inherent competitive hydroborated by-product forma-
tion. In view of the promising catalytic activity of gold(I)
complexes in the diboration reaction and because of the
lack of any attempted synthesis of 1,2-bis(boronate)esters
with catalytic chiral systems based on gold(I) complexes,
we now report some results of N-heterocyclic carbene–
Au(I)–X complexes, with an extension to the analogous
silver complexes. NHCs are considered as better electron
donors than phosphines, and hence provide enough elec-
tron richness to the metal centre to guarantee B–B cleavage
of the diborane. Additionally, N-heterocyclic carbenes
(NHC) have emerged as a promising family of ligands that
can be used in the design of efficient homogeneous cata-
lysts,9 because of the great stability acquired by the metal
complexes. Our first attempt to perform the catalytic dib-
oration of alkenes with metal–N-heterocyclic carbene
complexes was recently reported with the thermally and
air-stable compound 1 (Scheme 2) which cleanly catalyzed
the diboration of terminal alkenes with bis(catechol-
ato)diboron at room temperature.10 Despite its high activ-
ity, no asymmetric induction was observed, presumably
because the chiral centre was far from the metal. Based
on this previous result, we are now reporting the prepara-
tion of a series of new M–NHC (M = Ag, Au) complexes
whose catalytic activities were explored. Scheme 2 summa-
rizes the synthetic procedure to the new NHC complexes of
Au but also of the analogous of Ag.
X
X
X
X
Ag2O
CH2Cl2, RT
R
N
N
R'
R
R
N
N
R'
R
AuCl(PEt3)
Au
Ag
R'
N
N
R'
N
N
X
X
X
X
A
A
1 M=Ag, X=H, A=AgCl2, R=Me, R'=1-methyl-3-(+)-methylmenthoxide
2 M=Ag, X=Cl, A=I, R=R'=n-But
3 M=Au, X=H, A=Cl, R=Me, R'=1-methyl-3-(+)-methylmenthoxide
4 M=Au, X=Cl, A=I, R=R'=n-But
1-methyl-3-(+)-methylmenthoxide=
-CH2
O
Scheme 2.
NMR, mass spectroscopy and elemental analysis (except
for 3 that was obtained as an oily product and we failed
to get accurate elemental analysis). The molecular structure
of 1 was reported in a previous communication.10 In prin-
ciple, the idea of obtaining these complexes was to study
the effect of introducing an N-heterocyclic carbene in the
Au and Ag complexes in terms of catalytic activity com-
pared to the related phosphine analogues.5 The introduc-
tion of the chlorinated-NHC ligands would also allow us
to examine the catalytic implications of the reduction of
the electrondonating character of the NHC ligand.
Initially, we examined the catalytic activity provided by
compounds [NHC–Au(I)–NHC]X and [NHC–Ag(I)–
NHC]X. The metal modification with an electrondonating
NHC ligand results in a clear enhancement of the catalytic
performances,11 thus supporting the previously reported
work by Baker et al.5 which suggests that arylphosphine
Au complexes are not sufficiently electron rich to serve as
competent alkene diboration catalysts. We were able to
get 100% selectivity for the diborated product in the cata-
lyzed diboration of styrene (Scheme 3), when using 2 and
4, with 46% and 94% conversions, respectively (Table 1,
entries 1 and 2).
The silver complexes were obtained by the direct reaction
of Ag2O with 1-methyl-3-(+)-methylmenthoxide imidazo-
lium chloride or with 1,3-di-n-butyl-4,5-dichloroimidazo-
lium iodide, to provide complexes 1 and 2, respectively.
The Au(I) complexes were obtained in high yield (ca.
90%) by in situ generation of the free carbenes from the
corresponding imidazolium salts in THF at À78 ꢁC with
t-BuLi, and then addition of Au(PEt3)Cl. Alternatively, 3
and 4 were obtained from transmetallation of the corre-
sponding carbenes from the silver complexes 1 and 2 to
Au(PEt3)Cl, although yields were lower (ca. 50%) than
those obtained by direct coordination of the free carbene
to Au. The complexes were characterized by means of
The different activity of these catalysts may be due to the
lability of the Ag–Ccarbene bond, which could favour an
O
O
B
O
O
OH
M-NHC
O
O
O
B
B
OH
OH
R
R
+
R
R
+
R
NaOH
H2O2
O
O
B
B
O
B2(cat)2
Scheme 3.

R. Corbera´n et al. / Tetrahedron: Asymmetry 17 (2006) 1759–1762
1761
Table 1. Catalytic diboration/oxidation reaction of alkenes with M–NHC complexes and bis(catecholato)diborona
Entry
Catalytic system
Alkene
T (h)
Temperature (ꢁC)
Conv.b (%)
Diol (%)
1
2
3
4
5
6
7
8
2
4
1
3
3
3
5
6
6
6
6
6
7
Styrene
Styrene
Styrene
Styrene
Styrene
Vinylcyclohexane
Styrene
Styrene
Styrene
Vinylcyclohexane
p-Fluorostyrene
p-Methoxystyrene
Styrene
60
60
60
60
20
60
60
60
24
24
24
24
60
25
25
25
25
70
25
25
25
70
70
70
70
25
46
94
76
69
86
40
40
13
42
100
51
13
10
100
100
100
100
100
100
100
91c,d
9
100c,e
100
100
100
93
10
11
12
13
a Standard conditions: alkene/diborane/M–carbene complex = 0.5/0.55/0.025. Solvent: THF.
b Determined by 1H NMR before oxidation.
c Determined by GC with chiral column FS-Cyclodex B-IP, 50 m · 0.25 mm of the ketal derivatives.
d ee = 9%.
e ee = 4%.
equilibrium between mono- and bis-carbene species, in
solution.12 In fact, while we have assigned a bis-NHC
structure for both 1 and 2 (Scheme 2), we believe that the
species present in solution have a mono-NHC structure,
according to data reported in the literature for similar com-
plexes.12b The NMR and crystallographic data do not per-
mit unambiguous determination of electronic differences
between the NHC–Ag moieties as a function of the
NHC, although studies have been made in order to ratio-
nalize the nature of the species present in solution.13 On
the other hand Au–NHC complexes are known to have
stronger Au–Ccarbene bonds,14 thus allowing well defined
molecular arrangements both in solid state and in solution.
This may somehow justify the differences in the catalytic
behaviour upon introduction of the chloro substituents in
imidazolylidene rings of the silver and gold complexes. In
the silver complex the introduction of the electron attract-
ing atoms results in partial deactivation of the catalyst
(compare entries 1 and 3) in the case of gold the effect is
just the opposite (compare entries 2 and 4). We do not have
a satisfactory explanation for this observation, but the
structural versatility of the Ag–NHC complexes may also
play a role, implying that the catalytic Ag(I) and Au(I) spe-
cies have a different structural nature in solution. In any
case, we would expect that diminishing the electrondonat-
ing character of the ligand would imply a reduction of
the catalytic performance, as in the trend that we have
observed for our Ag–NHC complexes and in agreement
to previously reported results.5 For the chiral complexes 1
and 3, again, total chemoselectivity for the diborated prod-
uct was observed, with moderate to high conversions when
the reaction was carried out at 70 ꢁC (Table 1, compare
entries 3–5). Lower conversions were obtained when the
catalytic diboration was carried out with more sterically
hindered substrates, such as vinylcyclohexane (Table 1,
compare entries 4 and 6). However, no asymmetric induc-
tion was observed in the corresponding diols, after the
oxidation procedure.
decided to test a series of chiral NHC–Ag–Cl complexes
(Fig. 1, complexes 5–7) that have previously been described
in the literature.15
Ph
N
N
Ph
N
N
Ph
N
N
Ph
AgCl
AgCl
AgCl
5
7
6
Figure 1.
These compounds were tested in the catalytic diboration/
oxidation of styrene (Table 1, entries 7–9 and 13) and 6
was tested in the diboration of a series of terminal olefins
(entries 10–12). As shown for the catalysts described above
(1–4) the diborated product remains the major product,
although the conversion is lower, probably as a conse-
quence of the more sterically hindered catalytic system in-
volved. Interestingly, 6 was able to induce, modestly, some
enantioselectivity, with ee values between 4% and 9%. We
believe that this result may be due to the higher sterical
crowding of the metal in 6 compared to the rest of the chi-
ral catalysts. We are aware that these ee values are very
low, but they must be compared to the maximum values
(33%) achieved by the sophisticated P,N-Quinap–Rh(I)
complex, so we believe chiral NHC–M(I)–X complexes
may be a useful alternative for the clean asymmetric cata-
lytic diboration of alkenes. Compound 6 was also tested
in the diboration of vinylcyclohexane at 70 ꢁC, showing
quantitative conversion but negligible asymmetric induc-
tion (entry 10). The diboration of p-fluorostyrene and p-
methoxystyrene (entries 11 and 12) allowed us to establish
that the more electron attracting the substituents are, the
higher the conversion to the diborated species.
3. Conclusion
Aiming to study whether we could provide effective dibora-
tion catalysts that could also induce some asymmetry, we
We can conclude that we have described a series of NHC
complexes of Au(I) and Ag(I) that have been tested in the

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R. Corbera´n et al. / Tetrahedron: Asymmetry 17 (2006) 1759–1762
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Acknowledgements
´
Ramırez, J.; Segarra, A. M.; Fernandez, E. Tetrahedron:
The authors want to thank the CICYT of Spain, for the
financial support (CTQ2004-04412/BQU and CTQ2005-
05187/BQU) and Bancaixa (P1.1B2004-07) for financial
support. R.C. thanks the MEC for a fellowship, J.R.
thanks the Generalitat de Catalunya for a fellowship.
Asymmetry 2005, 16, 1289.
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