Perhalogenated carba-closo-dodecaborate anions as ligand substituents: Applications in gold catalysis

Article abstract of DOI:10.1002/anie.201209107

Bigger and better! The single-component zwitterionic gold catalyst formed by the coordination of a phosphine ligand bearing an inert and noncoordinating carborane substituent, CB₁₁Cl₁₁^-, to an Au ^I ion exhibited

Full text of DOI:10.1002/anie.201209107

.
Angewandte
Communications
DOI: 10.1002/anie.201209107
Carborane Ligands
Perhalogenated Carba-closo-dodecaborate Anions as Ligand
Substituents: Applications in Gold Catalysis**
Vincent Lavallo,* James H. Wright II, Fook S. Tham, and Sean Quinlivan
+[6]
+ [7]
The success of modern transition-metal catalysis is largely due
to the availability of a diverse range of ligand frameworks.
One of the most universal aspects of ligand design is the
strategic attachment of bulky substituents to influence the
activity of catalysts. Sterically demanding substituents kineti-
cally protect the active metal center and, at the same time,
promote substrate exchange and low coordination, two
processes necessary for turnover. Bulky ligand substituents
also influence the selectivity of the metal center for product
formation and substrate consumption. For these purposes, the
most common groups appended to ancillary ligands are alkyl
and aryl groups, such as adamantyl or 2,6-diisopropylphenyl.
Far less common is the use of dicarba-closo-dodecaborane
(C2B10) clusters[1] as surrogates for alkyl or aryl groups.[2] The
three-dimensional aromatic nature of these species and their
icosahedral shape lend them properties akin to those of both
alkyl and aryl groups. However, owing to the hydridic nature
oxidants, such as C60
and CH3 . Thus, these clusters can
+
be far more inert than even simple hydrocarbons: CH3
abstracts hydrides from n-alkanes with loss of methane at
room temperature. It is these properties that have generated
increasing interest[8] in the use of perhalogenated carborane
anions in silylium catalysis[8i,l] and related processes.[8f] The
anion of choice for most applications is the HCB11Cl11À anion
(1), since it is readily accessible[8g] and arguably the most inert
carborane anion (Scheme 1).
À
of the B H vertices, ligands bearing these substituents tend to
À
undergo undesirable B H activation reactions, such as cyclo-
metalation.[3] It is this tendency for such intramolecular
reactions to occur that has limited the synthetic utility of
dicarba-closo-dodecaborane-bearing ligands in the area of
catalysis.
Scheme 1. Synthesis of phosphine 2.
Interestingly, complexes that contain ligands functional-
À
[4]
ized with related carba-closo-dodecaborate anions (CB11
)
We are interested in using the HCB11Cl11À cluster (1) and
related systems not only as weakly coordinating anions, but
also as super-bulky, inert, charged ligand substituents. The van
der Waals volume (VvdW) of 1 is approximately 350 ꢀ3,[9]
which is more than twice as large as that of an adamantyl
group (VvdW = 136 ꢀ3);[1] thus, 1 is an exceptionally large
substituent. Appending ligands with such molecular architec-
tures may lead to catalytic systems that display superior
activity and stability, particularly when positively charged
intermediates are involved. Herein, we report the synthesis of
a ligand containing the CB11Cl11À cluster as a substituent and
demonstrate its utility by the preparation of unique single-
component AuI catalysts with unprecedented activity for the
hydroamination of alkynes with primary amines.
directly bound to the coordinating atom through the carbor-
ane cage carbon atom have not been reported.[5] Although
similar in size to their neutral “dicarba” cousins (C2B10),
isoelectronic (CB11À) clusters have significantly different
properties. The negative charge is delocalized over all of the
12 cage atoms, and as a result, the anion is very weakly
coordinating. This weak coordination ability can be enhanced
À
by the substitution of some or all of the B H vertices for alkyl
or halo groups. In the case of alkyl substitution, the cluster
becomes more reactive towards substitution and oxidation.
On the other hand, exhaustive halogenation of the boron
vertices of the cluster introduces a blanket of electron-
withdrawing substituents that enhances the inherent weak
coordination ability of the anion and also confers upon these
molecules exceptional inertness. It has been demonstrated
that perhalogenated carborane counteranions are sufficiently
unreactive that they can form isolable salts with potent
To begin our investigation into the properties of ligands
À
containing the CB11Cl11 moiety, we chose to prepare
a phosphine. Specifically, we targeted iPr2P(CB11Cl11)ÀLi+
(2), since the isopropyl groups of this phosphine should give
distinct resonances in the 1H NMR spectrum that are
straightforward to interpret (Scheme 1). Additionally,
because the CB11C11À fragment is very large, the intermediate
steric bulk of the isopropyl groups should make the phos-
phorus lone pair accessible for coordination.
[*] Prof. Dr. V. Lavallo, J. H. Wright II, Dr. F. S. Tham, S. Quinlivan
Department of Chemistry, University of California Riverside
Riverside, CA 92521 (USA)
E-mail: vincent.lavallo@ucr.edu
Thus, treatment of the trimethylammonium salt of anion
1 with 2 equivalents of nBuLi and subsequent quenching of
the dianionic intermediate with iPr2PCl afforded 2 in 98%
[**] We are grateful to UCR for financial support of this research.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209107.
3172
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 3172 –3176

Angewandte
Chemie
yield (Scheme 1). The anionic phosphine 2 is very soluble in
common polar solvents (CH2Cl2, CHCl3, THF) and is not
sensitive to oxygen in solution or the solid state. A single-
crystal X-ray diffraction study unambiguously confirmed the
structure of 2 and showed a sterically congested environment
around the pyramidalized phosphorus center (sum of CPC
angles: 325.18; Figure 1). The bond between the phosphorus
Figure 2. Top: Synthesis of complexes 3(THT) (left) and 3(LiCl) (right).
Bottom: Solid-state structures of 3(THT) (left) and 3(LiCl) (right; C gray, P
purple, Au gold, Cl green, B brown, S orange). Selected bond lengths
À
À
À
À
[ꢀ] for 3(LiCl): P Au 2.2477(12), P C1 1.943(5), P C2 1.858(6), P C3
À
1.864(5), Au Cl 2.2883(12). Thermal ellipsoids are drawn at the 50%
Figure 1. Solid-state structure of phosphine 2 (C gray, P purple, Cl
green, B brown, Li pink, O red). Hydrogen atoms are omitted for
clarity; thermal ellipsoids are drawn at the 50% probability level.
probability level; hydrogen atoms in both 3(THT) and 3(LiCl) are omitted
for clarity.
zwitterionic Au complex 3(THT) by ligand coordination and
anion metathesis. The driving force for the anion metathesis is
the insolubility of LiCl in FC6H5. A single-crystal X-ray
diffraction study unambiguously confirmed the structure of
À
À
atom and the carborane cage carbon atom (P C1
1.9376(16) ꢀ) is significantly longer than those between the
À
phosphorus atom and the isopropyl groups (P C2 1.8679(19),
P C3 1.8636(18) ꢀ). The lithium countercation of 2 is
coordinated by four THF molecules and does not associate
with the halogen substituents on the negatively charged
carborane (closest Cl···Li distance: 4.283 ꢀ).
À
3(THT); however, disorder in the P iPr groups precludes an
accurate discussion of the structural parameters of 3(THT). This
complex is reminiscent of the single-component zwitterionic
Au–tht catalyst supported by an anionic N-heterocyclic
carbene[12] that was recently reported by Tamm and co-
workers.[13] However, in our case, the charged group is bound
directly to the coordinating atom and is therefore much closer
to the Au center. Interestingly, when the reaction was
performed in THF, a solvent in which LiCl is soluble, THT
was liberated, and an unusual anionic complex 3(LiCl) was
isolated (3(LiCl) can also be prepared by the treatment of
isolated 3(THT) with LiCl in THF). In contrast to 3(THT), we
were able to grow single crystals of 3(LiCl) without significant
disorder and thus examined the solid-state structural aspects
of this molecule. The Tolman cone angle[14] for the phosphine
ligand is 2048, which indicates that this ligand is much more
bulky than a standard sterically demanding phosphine ligand,
There is growing interest in single-component[10] gold
catalysts,[11] which can offer a number of advantages over
traditional two-component systems that utilize Ag+ or strong
Brønsted acid additives. These additives not only increase the
cost of the systems, but can also promote side reactions or
different reactivity.[10] Given that our phosphine 2 contains
a pendant weakly coordinating anion, we were intrigued by
the idea of combining 2 with a AuI cation. The typical linear
geometry of AuI is also important, since the CB11Cl11 group of
2 will be fixed in a position close to the metal but removed
from the trans coordination site (with respect to the phos-
phine), where substrates usually bind during catalysis.
When the anionic phosphine 2 was treated with chloro-
(tetrahydrothiophene)gold(I) (ClAu(tht)) in monofluoroben-
zene (FC6H5) at room temperature, a large amount of a white
precipitate formed within 5 min (Figure 2). Analysis of the
precipitate by 31P NMR spectroscopy showed a phosphorus
resonance at d =+ 90 ppm, which is significantly shifted
downfield from that of the starting material 2 (+ 77 ppm)
and in line with the formation of a gold–phosphine complex.
The 1H NMR spectrum clearly showed resonances corre-
sponding to the phosphine ligand as well as coordinated
tetrahydrothiophene and thus suggested the formation of the
À
such as PtBu3 (cone angle: 1828). The P Au bond length is
2.2477(12) ꢀ, which is close to that reported for a neutral
ortho-dicarba-closo-decaborane-substituted phosphine–AuCl
complex (2.232(3) ꢀ).[15] The anionic CB11Cl11 substituent
displays one chloride interaction with the Au ion (closest
À
Au···Cl B distance: 3.118 ꢀ). This distance is greater than the
sum of the covalent Au/Cl radii but in the range of a van der
Waals contact (sum of Au/Cl vdW radii: 3.41 ꢀ). For
comparison, the chloride ligand (trans to the phosphine) is
at a distance of 2.2883(12) ꢀ from the metal center. Hence,
Angew. Chem. Int. Ed. 2013, 52, 3172 –3176
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3173

.
Angewandte
Communications
the carborane substituent retains a significant amount of
weakly coordinating character, even though it is forced into
a position close to the metal center.
Because such high catalytic activity is extremely unusual
for gold-catalyzed reactions, we examined the hydroamina-
tion of phenylacetylene with several different aryl amines at
an ultralow loading (0.001%, 10 ppm). Mesitylamine reacted
effectively with phenylacetylene to afford the corresponding
imine in 67% yield after 24 h (TON = 67000; Table 1,
entry 5). With bulkier 2,6-diisopropylphenylamine, the reac-
tion was even more efficient and afforded the analogous
imine in 85% yield (TON = 85000; Table 1, entry 6). For the
latter reaction, the catalyst was also extremely fast, with an
initial turnover frequency of 29000 in the first hour. Even
higher turnover numbers were observed for the analogous
reactions with 4-fluorophenylacety-
We next turned our attention to the catalytic properties of
complexes 3, which are air- and light-stable, in the hydro-
amination[16] of primary amines with alkynes.[17] As a model
reaction, we chose the addition of aniline to phenylacetylene.
Thus, a 1:1 neat mixture of the amine and the alkyne was
added to complex 3(THT) (0.1 mol%), whereupon an exother-
mic reaction occurred. We monitored the reaction by
1H NMR spectroscopy and found that the hydroamination
was more than 95% complete after 1 h (Table 1, entry 1).
lene: a maximum turnover number
Table 1: Hydroamination of alkynes with primary amines in the presence of catalysts 3.
of 92000 was reached with 2,6-
diisopropylphenylamine (Table 1,
entries 7–9). The best results were
obtained with 4-methoxyphenylace-
tylene. With this alkyne, the catalyst
converted all three amines into the
corresponding imines with turnover
numbers of 90000 or greater
(Table 1, entries 10–12).
We postulate that the trend of
increased reactivity with increased
steric bulk of the amine is related to
the decreased binding ability of
bulkier amines and the resulting
imines, which allows for more facile
ligand exchange at the Au center.
Catalyst 3(THT) is also effective for
the hydroamination of aryl amines
with diaryl-substituted alkynes
(Table 1, entries 13–15) and termi-
Entry Cat.
Ar
R1
R2
Catalyst
loading [%]
t [h] T [8C] Yield [%][a]
TON
1
2
3(THT) Ph
3(LiCl) Ph
H
H
H
H
H
H
H
H
H
H
H
H
Ph
Ph
Ph
Ph
Ph
Ph
0.1
0.1
0.01
0.004
0.001
0.001
0.001
0.001
0.001
1
1
25
25
50
50
50
50
50
50
50
50
50
50
80
80
80
80
80
80
>95
>95
>95
88
>950
>950
>9500
22000
67000
85000
54000
75000
92000
90000
94000
>95000
895
3
3(THT) Ph
16
16
24
24
24
24
24
24
24
24
24
24
24
24
24
24
4
3(THT) Ph
5
6
7
3(THT) Mes
3(THT) Dipp
3(THT) Ph
67
85
54
4-FC6H4
4-FC6H4
4-FC6H4
4-MeOC6H4 0.001
4-MeOC6H4 0.001
4-MeOC6H4 0.001
8
9
3(THT) Mes
3(THT) Dipp
3(THT) Ph
3(THT) Mes
3(THT) Dipp
3(THT) Ph
3(THT) Mes
3(THT) Dipp Ph Ph
3(THT) Ph
3(THT) Mes
3(THT) Dipp
75 (60)[b]
92
90
10
11
12
13
14
15
16
17
18
94 (93)[b]
>95 (88)[b]
89.5
Ph Ph
Ph Ph
0.1
0.1
0.1
0.2
0.2
0.2
67
78.5
86.5
86
670
785
435
430
H
H
H
n-C4H9
n-C4H9
n-C4H9
nal
alkyl
alkynes
(Table 1,
>95
>475
entries 15–18). Although the activ-
ity is lower in these cases, the
performance of 3(THT) compares
favorably with that of typical Au
catalysts, which often require cata-
lyst loadings of 1–10 mol%. A low
catalyst loading (0.2 mol%) with an
[a] The yield was determined by NMR spectroscopy by the direct integration of the peak for the alkyne
starting material with respect to the peak for the imine product and is given as the average for two
catalytic reactions. No side reactions were observed. [b] The yield of the isolated product is given in
brackets. Dipp=2,6-diisopropylphenyl, Mes=mesityl (2,4,6-trimethylphenyl).
Identical results were obtained with 3(LiCl) (Table 1, entry 2).
Further catalytic tests were carried out only with 3(THT). When
the catalyst loading was decreased to 0.01% and the mixture
was heated at 508C for 16 h, the yield of the imine was just as
high (Table 1, entry 3). Even at a catalyst loading of 0.004%,
the imine was formed in 88% yield, which corresponds to
a catalyst turnover number (TON) of 22000 (Table 1,
entry 4). The highest reported TON for the gold-catalyzed
hydroamination of an alkyne with a primary amine is around
9000, for a multicomponent acid-activated Au system.[17h]
Control experiments with ClAu(tht) or HCB11Cl11ÀCs+/ClAu-
(tht) at a low catalyst loading (0.1 mol%) afforded the imine
in only trace amounts (< 5%). These results demonstrate the
benefit of the ligand and show that colloidal gold is unlikely to
be responsible for the high activity. A metal-free Brønsted
acid catalyzed pathway can also be ruled out, since iPr2P-
(CB11Cl11)À H3NPh+ does not catalyze the reaction.
internal dialkyl alkyne (3-hexyne) or an alkyl amine
(tBuNH2) as the substrate produced the expected imines
only in trace amounts (< 5%).
The two mechanisms generally proposed for gold-cata-
lyzed hydroamination either involve the direct addition of the
amine to a coordinated alkyne or a coordination–insertion
mechanism. In both pathways, charged intermediates undergo
proton-transfer steps that lead to the formation of the
functionalized amine. Although we cannot speculate at this
time on the exact nature of the mechanism operative in the
observed catalysis, it is clear that the anionic CB11Cl11 group
of phosphine 2 is beneficial for the reaction sequence.
We postulate that the extraordinary activity of this system
À
might be due in part to the proximity of the anionic CB11Cl11
group to the Au center during catalysis. This proximity may
lead to electrostatic stabilization of the positively charged
reaction intermediates. Analogously, the charge on the
3174
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 3172 –3176

Angewandte
Chemie
ˇ
39, 2708 – 2716; h) K. Vyakaranam, S. Kçrbe, H. Divisovꢂ, J.
carborane substituent may also act as an electrostatic tether to
prevent phosphine dissociation and subsequent catalyst
decomposition. For large-scale applications of homogenous
gold catalysis or catalysis with systems based on other
precious metals, highly active and efficient catalysts, such as
complexes 3, must be available. Many transition-metal-
catalyzed processes involve cationic intermediates that
Michl, J. Am. Chem. Soc. 2004, 126, 15795 – 15801; i) T. Jelinek, P.
Baldwin, W. R. Scheidt, C. A. Reed, Inorg. Chem. 1993, 32,
1982 – 1990; for an example of an unusual boron-functionalized
cluster compound with a coordinating acetylide functionality,
see: j) A. Himmelspach, M. Finze, S. Raub, Angew. Chem. 2011,
123, 2676 – 2679; Angew. Chem. Int. Ed. 2011, 50, 2628 – 2631.
[6] C. A. Reed, K.-C. Kim, R. D. Bolskar, L. J. Mueller, Science
2000, 289, 101 – 104.
À
might benefit from the use of ligands containing a CB11Cl11
[7] T. Kato, C. A. Reed, Angew. Chem. 2004, 116, 2968 – 2971;
Angew. Chem. Int. Ed. 2004, 43, 2908 – 2911.
or related substituent. We are currently designing other ligand
families that use halogenated carba-closo-dodecaborate sub-
stituents, as well as expanding on the phosphine system
outlined above for use with different transition metals.
[8] For examples of the use of polyhalogenated carboranes as
noncoordinating inert anions, see: a) E. S. Stoyanov, I. V.
Stoyanova, F. S. Tham, C. A. Reed, Angew. Chem. 2012, 124,
9283 – 9285; Angew. Chem. Int. Ed. 2012, 51, 9149 – 9151; b) R.
Ramꢃrez-Contreras, O. V. Ozerov, Dalton Trans. 2012, 41, 7842 –
7844; c) C. Bolli, T. Kçchner, C. Knapp, Z. Anorg. Allg. Chem.
2012, 638, 559 – 564; d) V. Volkis, C. Douvris, J. Michl, J. Am.
Chem. Soc. 2011, 133, 7801 – 7809; e) M. G. Fete, Z. Havlas, J.
Michl, J. Am. Chem. Soc. 2011, 133, 4123 – 4131; f) O. Allemann,
S. Duttwyler, P. Romanato, K. K. Baldridge, J. S. Siegel, Science
2011, 332, 574 – 577; g) W. Gu, B. J. McCulloch, J. H. Reibens-
pies, O. V. Ozerov, Chem. Commun. 2010, 46, 2820 – 2822; h) S.
Duttwyler, C. Douvris, N. L. P. Fackler, F. S. Tham, C. A. Reed,
K. K. Baldridge, J. S. Siegel, Angew. Chem. 2010, 122, 7681 –
7684; Angew. Chem. Int. Ed. 2010, 49, 7519 – 7522; i) C. Douvris,
C. M. Nagaraja, C.-H. Chen, B. M. Foxman, O. V. Ozerov, J. Am.
Chem. Soc. 2010, 132, 4946 – 4953; j) C. A. Reed, Acc. Chem.
Res. 2010, 43, 121 – 128; k) S. Duttwyler, Y. Zhang, A. Linden,
C. A. Reed, K. K. Baldridge, J. S. Siegel, Angew. Chem. 2009,
121, 3845 – 3848; Angew. Chem. Int. Ed. 2009, 48, 3787 – 3790;
l) C. Douvris, O. V. Ozerov, Science 2008, 321, 1188 – 1190; m) E.
Molinos, S. K. Brayshaw, G. Kociok-Kçhn, A. S. Weller, Orga-
nometallics 2007, 26, 2370 – 2382; n) E. Molinos, S. K. Brayshaw,
G. Kociok-Kçhn, A. S. Weller, Dalton Trans. 2007, 4829 – 4844;
o) T. Kꢄppers, E. Bernhardt, R. Eujen, H. Willner, C. W.
Lehmann, Angew. Chem. 2007, 119, 6462 – 6465; Angew. Chem.
Int. Ed. 2007, 46, 6346 – 6349; p) M. J. Ingleson, G. Kociok-Kçhn,
A. S. Weller, Inorg. Chim. Acta 2005, 358, 1571 – 1580; q) C.-W.
Tsang, Q. Yang, T. C. W. Mak, Z. Xie, Appl. Organomet. Chem.
2003, 17, 449 – 452; r) S. H. Strauss, Spec. Publ. R. Soc. Chem.
2000, 253, 44 – 48; s) C.-W. Tsang, Q. Yang, E. T.-P. Sze, T. C. W.
Mak, D. T. W. Chan, Z. Xie, Inorg. Chem. 2000, 39, 5851 – 5858.
[9] The Vvdw value of HCB11Cl11À was approximated from crystallo-
graphic data by assuming that the cluster is a perfect sphere.
[10] For a recent review on single-component gold catalysts, see:
a) H. Schmidbaur, A. Schier, Z. Naturforsch. B 2011, 66, 329 –
350; for a discussion regarding the effects of Ag+ additives on
multicomponent systems, see: b) D. Wang, R. Cai, S. Sharma, J.
Jirak, S. K. Thummanapelli, N. G. Akhmedov, H. Zhang, X. Liu,
J. L. Petersen, X. Shi, J. Am. Chem. Soc. 2012, 134, 9012 – 9019.
[11] For recent reviews on gold catalysis, see: a) Modern Gold
Catalyzed Synthesis (Eds.: A. S. K. Hashmi, F. D. Toste), Wiley-
VCH, Weinheim, 2012; b) A. Gꢅmez-Suꢂrez, S. P. Nolan,
Angew. Chem. 2012, 124, 8278 – 8281; Angew. Chem. Int. Ed.
2012, 51, 8156 – 8159; c) C. C. J. Loh, D. Enders, Chem. Eur. J.
2012, 18, 10212 – 10225; d) D. Garayalde, C. Nevado, ACS Catal.
2012, 2, 1462 – 1479; e) B.-L. Lu, L. Dai, M. Shi, Chem. Soc. Rev.
2012, 41, 3318 – 3339; f) L.-P. Liu, G. B. Hammond, Chem. Soc.
Rev. 2012, 41, 3129 – 3139; g) M. Rudolph, A. S. K. Hashmi,
Chem. Soc. Rev. 2012, 41, 2448 – 2462; h) C. D. Pina, E. Falletta,
M. Rossi, Chem. Soc. Rev. 2012, 41, 350 – 369; i) M. Rudolph,
A. S. K. Hashmi, Chem. Commun. 2011, 47, 6536 – 6544; j) M.
Bandini, Chem. Soc. Rev. 2011, 40, 1358 – 1367; k) M. Malacria,
L. Fensterbank, V. Gandon, Top. Curr. Chem. 2011, 302, 157 –
182; l) A. S. K. Hashmi, Angew. Chem. 2010, 122, 5360 – 5369;
Angew. Chem. Int. Ed. 2010, 49, 5232 – 5241; m) D. J. Gorin,
Experimental Section
For complete experimental details, see the Supporting Information.
CCDC 909516 (2), 909517 (3(THT)), and 909518 (3(LiCl)) contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Received: November 13, 2012
Revised: January 18, 2013
Published online: February 13, 2013
Keywords: carborane anions · gold · homogeneous catalysis ·
.
hydroamination · phosphanes
[1] For a recent review on dicarba-closo-dodecaborane clusters, see:
M. Scholz, E. Hey-Hawkins, Chem. Rev. 2011, 111, 7035 – 7062.
[2] For selected recent examples of ligands containing the C2B10
cluster, see: a) N. Fey, M. F. Haddow, R. Mistry, N. C. Norman,
A. G. Orpen, T. J. Reynolds, P. G. Pringle, Organometallics 2012,
31, 2907 – 2913; b) A. M. Spokoyny, C. W. Machan, D. J. Cling-
erman, M. S. Rosen, M. J. Wiester, R. D. Kennedy, C. L. Stern,
A. A. Sarjeant, C. A. Mirkin, Nat. Chem. 2011, 3, 590 – 596;
c) J. I. van der Vlugt, Angew. Chem. 2010, 122, 260 – 263; Angew.
Chem. Int. Ed. 2010, 49, 252 – 255; for selected examples of the
synthesis and reactivity of transition-metal systems containing
anionic nido boron clusters, see: d) F. Teixidor, R. Benakki, C.
ViÇas, R. Kivekꢁs, R. Sillanpꢁꢁ, Inorg. Chem. 1999, 38, 5916 –
5919; e) J. D. Hewes, C. W. Kreimendahl, T. B. Marder, M. F.
Hawthorne, J. Am. Chem. Soc. 1984, 106, 5757 – 5759.
[3] For a comprehensive list of C2B10 cyclometalation reactions, see
Ref. [2a] and references therein.
[4] For a comprehensive review on the chemistry of the carba-closo-
dodecaborate anion, see: S. Kçrbe, P. J. Schreiber, J. Michl,
Chem. Rev. 2006, 106, 5208 – 5249.
[5] For examples of the use of the carba-closo-dodecaborate anion
as an X-type ligand with metals directly bound to the carborane
cage carbon atom, see: a) R. J. Wehmschulte, L. Wojtas, Inorg.
Chem. 2011, 50, 11300 – 11302; b) A. Himmelspach, M. Zꢁhres,
M. Finze, Inorg. Chem. 2011, 50, 3186 – 3188; c) A. Himmels-
pach, J. A. P. Sprenger, J. Warneke, M. Zꢁhres, M. Finze,
Organometallics 2012, 31, 1566 – 1577; d) M. Finze, J. A. P.
Sprenger, Chem. Eur. J. 2009, 15, 9918 – 9927; e) S. V. Ivanov,
J. J. Rockwell, O. G. Polyakov, C. M. Gaudinski, O. P. Anderson,
K. A. Solntsev, S. H. Strauss, J. Am. Chem. Soc. 1998, 120, 4224 –
4225; for examples of the functionalization of the cage carbon
atom with potentially coordinating atoms (P,N,O; no transition-
metal complexes of these molecules have been reported), see:
f) M. J. Nava, C. A. Reed, Inorg. Chem. 2010, 49, 4726 – 4728;
g) M. Finze, J. A. P. Sprenger, B. B. Schaack, Dalton Trans. 2010,
Angew. Chem. Int. Ed. 2013, 52, 3172 –3176
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3175

.
Angewandte
Communications
B. D. Sherry, F. D. Toste, Chem. Rev. 2008, 108, 3351 – 3378; and
Ref. [10b].
[12] T. Kçsterke, J. Kçsters, E.-U. Wꢄrthwein, C. Mꢄck-Lichtenfeld,
C. Schulte to Brinke, F. Lahoz, F. E. Hahn, Chem. Eur. J. 2012,
18, 14594 – 14598.
[13] S. Kronig, E. Theuergarten, C. G. Daniliuc, P. G. Jones, M.
Tamm, Angew. Chem. 2012, 124, 3294 – 3298; Angew. Chem. Int.
Ed. 2012, 51, 3240 – 3244.
[14] C. A. Tolman, Chem. Rev. 1977, 77, 313 – 348.
[15] M. A. McWhannell, G. M. Rosair, A. J. Welch, Acta Crystallogr.
Sect. C 1998, 54, 13 – 15.
[16] For selected reviews on hydroamination reactions, see: a) J. L.
Klinkenberg, J. F. Hartwig, Angew. Chem. 2011, 123, 88 – 98;
Angew. Chem. Int. Ed. 2011, 50, 86 – 95; b) I. Krossing, Angew.
Chem. 2011, 123, 11781 – 11783; Angew. Chem. Int. Ed. 2011, 50,
11576 – 11578; c) T. E. Mꢄller, K. C. Hultzsch, M. Yus, F.
Foubelo, M. Tada, Chem. Rev. 2008, 108, 3795 – 3892; d) R. A.
Widenhoefer, X. Han, Eur. J. Org. Chem. 2006, 4555 – 4563.
[17] For examples of the gold-catalyzed hydroamination of alkynes,
see: a) S. Fleischer, S. Werkmeister, S. Zhou, K. Junge, M. Beller,
Chem. Eur. J. 2012, 18, 9005 – 9010; b) E. Alvarado, A. C. Badaj,
T. G. Larocque, G. G. Lavoie, Chem. Eur. J. 2012, 18, 12112 –
12121; c) R. Kinjo, B. Donnadieu, G. Bertrand, Angew. Chem.
2011, 123, 5674 – 5677; Angew. Chem. Int. Ed. 2011, 50, 5560 –
5563; d) K. L. Butler, M. Tragni, R. A. Widenhoefer, Angew.
Chem. 2012, 124, 5265 – 5268; Angew. Chem. Int. Ed. 2012, 51,
5175 – 5178; e) K. D. Hesp, M. Stradiotto, J. Am. Chem. Soc.
2010, 132, 18026 – 18029; f) A. Leyva-Pꢆrez, J. R. Cabrero-
Antonino, A. Cantꢃn, A. Corma, J. Org. Chem. 2010, 75, 7769 –
7780; g) V. Lavallo, G. D. Frey, B. Donnadieu, M. Soleilhavoup,
G. Bertrand, Angew. Chem. 2008, 120, 5302 – 5306; Angew.
Chem. Int. Ed. 2008, 47, 5224 – 5228; h) E. Mizushima, T.
Hayashi, M. Tanaka, Org. Lett. 2003, 5, 3349 – 3352; for an
example of a system with a low catalyst loading for the hydration
of alkynes with water (a related process), see: i) N. Marion, R. S.
Ramꢅn, S. P. Nolan, J. Am. Chem. Soc. 2009, 131, 448 – 449.
3176
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 3172 –3176