Rhodium-Catalyzed Regioselective Hydroxylation of Cage B?H Bonds of o-Carboranes with O2or Air

Article abstract of DOI:10.1002/anie.201605880

A rhodium-catalyzed hydroxylation of a cage B4?H bond in o-carboranes with either O₂or air as the oxygen source is described, and serves as a new methodology for the regioselective generation of a series of 4-OH-o-carboranes in a one-pot process. The use of either O₂or air as both the oxidant and the oxygen source makes this protocol very environmentally friendly and practical.

Full text of DOI:10.1002/anie.201605880

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201605880
German Edition: DOI: 10.1002/ange.201605880
À
B H Activation
À
Rhodium-Catalyzed Regioselective Hydroxylation of Cage B H Bonds
of o-Carboranes with O2 or Air
Hairong Lyu, Yangjian Quan, and Zuowei Xie*
Dedicated to Prof. Thomas C. W. Mak on the occasion of his 80th birthday
À
Abstract: A rhodium-catalyzed hydroxylation of a cage B4 H
bond in o-carboranes with either O2 or air as the oxygen source
is described, and serves as a new methodology for the
regioselective generation of a series of 4-OH-o-carboranes in
a one-pot process. The use of either O2 or air as both the
oxidant and the oxygen source makes this protocol very
environmentally friendly and practical.
C
arboranes are a class of boron hydride clusters in which
one or more of the BH vertices are replaced by CH units, and
can be viewed as three-dimensional analogues to benzene.[1]
They are finding many applications in medicine as boron
neutron capture therapy agents,[2] in supramolecular design as
building blocks,[3] and in coordination/organometallic chemis-
try as versatile ligands.[4] As a class of electronically useful
molecules, carboranes have recently been incorporated into
p-conjugated systems for applications in optoelectronic func-
tional materials.[5] As a result, considerable attention has been
directed towards the functionalization of carborane mole-
cules. However, the unique structures of carboranes make the
derivatization difficult, thereby limiting their application
scope. Thus it is eagerly desired to develop new method-
ologies for the selective functionalization of carboranes.
À
Scheme 1. Cage B H hydroxylation of carboranes.
À
Inspired by transition-metal catalyzed C H hydroxylation
of benzenes[13] and recent works on catalytic regioselective
alkenylation,[14] arylation,[15] and alkynylation[16] of cage B H
À
bonds in o-carboranes from our group and others,[17,18] we are
interested in developing catalytic and regioselective hydrox-
À
ylation of cage B H bond in o-carboranes using environ-
mentally benign molecular oxygen as a reagent.
We accidently discovered
a rhodium(III)-catalyzed
B4-hydroxylation reaction by refluxing a toluene solution of
1-COOH-2-CH3-o-C2B10H10 (1a) in the presence of 5 mol%
[{Cp*RhCl2}2] and 2 equivalents of KOAc open to the air. The
significance of the present work is threefold: 1) O2 or air is
employed as a reagent and the sole oxidant, thus making this
method very environmentally benign and practical; 2) to the
best of our knowledge, this is the first example of rhodium-
catalyzed hydroxylation with molecular oxygen, although
À
It has been well-documented that B H bonds in organo-
boranes are generally sensitive toward water,[6] thus leading to
the formation of a large class of boronic/borinic acids which
are finding broad applications in materials science, organic
synthesis, and medicine.[7] In sharp contrast, those in carbor-
anes are much less reactive.[1,8] They can, however, be
activated by transition metals[9] or subjected to electrophilic
substitution reactions,[10] similar to the C H bonds in
rhodium-promoted cage B H
tionalization has been extensively investigated; and 3) this
or organic C H func-
[18a,b,g]
[19]
À
À
À
À
benzenes. For example, perhydroxylation of B H bonds in
B12H122À, CB11H12À, and p-C2B10H12 has been achieved by
refluxing a 30% hydrogen peroxide solution of these clusters
(Scheme 1),[11] though regioselective hydroxylation of cage
also represents the first example of catalytic cage B H
hydroxylation of o-carboranes.
À
In the presence of 5 mol% [{Cp*RhCl2}2] and 2 equiv-
alents of KOAc, a toluene solution of 1-COOH-2-CH3-o-
C2B10H10 (1a) was heated at 958C in the open air for 36 hours
to afford the B4-hydroxylated product 2a in 50% yield
(entry 1, Table 1). The use of O2 in place of air led to the
isolation of 2a in 82% yield (entry 2). Shortening the reaction
time to 24 hours resulted in a decreased yield (entry 3).
Replacement of toluene by chlorobenzene or 1,2-dichloro-
ethane gave 2a in much lower yields (entries 4 and 5).
Screening for base additives proved that 2 equivalents of
KOAc was the optimal choice (see Table S1 in the Supporting
Information). Higher or lower reaction temperatures did not
enhance the yield of 2a (entries 6 and 7). Control experiments
showed that no reaction was observed in the absence of either
À
B H bonds of these icosahedron boron clusters still remains
elusive.[12] In contrast, transition-metal-catalyzed direct cage
À
B H hydroxylation of carboranes has not been reported thus
far.[1,8]
[*] H. Lyu, Dr. Y. Quan, Prof. Dr. Z. Xie
Department of Chemistry and State Key Laboratory of Synthetic
Chemistry, The Chinese University of Hong Kong
Shatin, N.T., Hong Kong (China)
E-mail: zxie@cuhk.edu.hk
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201605880.
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Table 1: Optimization of reaction conditions.[a]
Table 2: Synthesis of cage B4-hydroxylated-o-carboranes.[a]
Entry Catalyst (mol%)
1[c] [{Cp*RhCl2}2] (5)
Base (equiv) Solvent T [8C] Yield [%][b]
KOAc (2)
KOAc (2)
KOAc (2)
KOAc (2)
KOAc (2)
KOAc (2)
KOAc (2)
toluene
toluene
toluene
PhCl
DCE
toluene
95
95
95
95
95
85
50
82
76
46
12
70
71
80
2
[{Cp*RhCl2}2] (5)
3[d] [{Cp*RhCl2}2] (5)
4
5
6
7
8
[{Cp*RhCl2}2] (5)
[{Cp*RhCl2}2] (5)
[{Cp*RhCl2}2] (5)
[{Cp*RhCl2}2] (5)
toluene 105
[{Cp*RhCl2}2] (2.5) KOAc (2)
toluene
toluene
toluene
toluene
toluene
95
9[e] [{Cp*RhCl2}2] (2.5) KOAc (2)
95 N.R.
95 N.R.
10
11
12
–
KOAc (2)
KOAc (2)
KOAc (2)
[{Cp*IrCl2}2] (5)
Pd(OAc)2 (10)
95
95
28
trace
[a] Reactions were conducted on 0.05 mmol scale in 2 mL of solvents in
a closed flask. [b] Yield of isolated product. [c] The reaction was carried
out in the open air. [d] Reaction time was 24 h. [e] 1a was replaced with
1-methyl-o-carborane. Cp*=C5Me5.
[a] Reactions were conducted on 0.2 mmol scale in 5 mL of toluene at
958C in a closed flask for 36 h (yield is that of isolated product). [b] Air
instead of O2. [c] 5 mol% [{Cp*RhCl2}2] was loaded. [d] TMS was
removed after work up.
the directing group COOH or rhodium catalyst (entries 9 and
10). Lowering the catalyst loading to 2.5 mol% just slightly
affected the yield of 2a (entry 8). Other transition metals
offered poor results (entries 11 and 12). Thus, the parameters
in entry 8 in Table 1 were chosen as the optimal reaction
conditions.
Under such optimized reaction conditions, the scope of
this reaction was investigated. Alkyl substituents at the cage
C2 gave the B4-hydroxylated products 2 in 70–80% yields
upon isolation, whereas benzyl groups at the same position
afforded only 40–45% yields of 2 (2a–e and 2q–s vs. 2g–i;
Table 2). Electron-donating groups on the phenyl ring of
styryl groups generally offered slightly higher yields of 2 than
those with electron-withdrawing substituents (2k–n vs. 2o,p).
This reaction was tolerant of chloro, fluoro, and methoxy
groups, which could be utilized for further synthetic elabo-
ration (2n–p). Oxygen-containing substituents, 1n and 1s,
reacted smoothly to the corresponding give 2n and 2s in 72
and 73% yields, respectively. For R = TMS, the desilylation
product 2 f was isolated in 30% yield. The cage boron
substituted substrate 1-COOH-2-Me-9,12-Me2-o-C2B10H8
(1t) was also examined, thus resulting in the formation of
2-Me-4-OH-9,12-Me2-o-C2B10H8 (2t) in 49% yield (see page
S15).
Scheme 2. Transformations of 2a: a) Tf2O, pyridine, DCM, RT, 1 h;
b) Ac2O, DMAP, toluene, RT, 12 h; c) PhCH2Br, K2CO3, CH3CN, 908C,
12 h; d) TMSCl, Et3N, DCM, RT, 12 h. DCM=dichloromethane,
DMAP=4-dimethylaminopyridine, TMS=trimethylsilyl.
Preliminary mechanistic studies were carried out to shed
some light on this hydroxylation pathway. Under the optimal
reaction conditions, reaction of 1a with 18O2 resulted in the
formation of the 18O-labled product 2a’ in 81% yield with
high fidelity, and proved that the oxygen atom in the hydroxy
group originates from molecular oxygen (Scheme 3a; deter-
mined by HRMS. See the Supporting Information). Mean-
while, the experimental results in the presence of 2 equiv-
alents of H218O ruled out oxygen incorporation from H2O
(Scheme 3b). In contrast, using 1-COOK-2-CH3-o-C2B10H10
(1a’) as the substrate, the yield of 2a significantly dropped to
58% (Scheme 3c). If 1 equivalent of K2CO3 was added to the
reaction shown in Scheme 3d to remove the in situ generated
acid in the reaction system, only trace amounts of 2a were
observed by GC-MS with decarboxylated species 1-CH3-o-
C2B10H11 as the major product (Scheme 3d). These control
experiments suggested the importance of acid in this reaction.
To illustrate the synthetic utility of the resultant B4-
hydroxylated carborane derivatives, several transformations
of 2a are illustrated in Scheme 2. The OH group in 2a was
conveniently converted into OTf for further derivatization. In
contrast, several carborane incorporated esters and ethers
were also easily prepared from 2a.
1
Compounds 2 and 3a–6a were fully characterized by H,
13C, and 11B NMR spectroscopy as well as high-resolution
mass spectrometry. The molecular structures of 2b and 2e
were further confirmed by single-crystal X-ray analyses (see
Figures S1 and S2).
2
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molecular oxygen is employed as a reagent and the sole
oxidant. This reaction serves as a new methodology for the
selective preparation of a series of 4-hydroxy-o-carborane
derivatives in an environmentally benign and simple one-pot
process. This work can also offer useful references for
À
selective hydroxylation of B H bonds with O2 in other
boron clusters.
Experimental Section
o-Carboranyl monocarboxylic acid 1 (0.20 mmol), [{Cp*RhCl2}2]
(2.5 mol%, 0.005 mmol), and KOAc (2 equiv, 0.40 mmol) were
mixed in toluene (5 mL). After the reaction mixture was cooled to
À788C, the reaction tube was evacuated and back-filled with O2
(3 times, balloon). The tube was closed when the temperature was
warmed up to room temperature. The resulting mixture was stirred at
958C for 36 h. After hydrolysis with water (10 mL) and extraction
with diethyl ether (10 mL ꢀ 3), the organic solutions were combined
and concentrated to dryness in vacuo. The residue was subjected to
flash column chromatography on silica gel (230–400 mesh) using
n-hexane and ethyl acetate (10:1) as the eluent to give the product 2.
Scheme 3. Control experiments.
Acknowledgments
This work was supported by grants from the Research Grants
Council of The Hong Kong Special Administration Region
(Project No. 14304115) and the Vice-Chancellorꢁs (CUHK)
Discretionary Fund.
On the basis of the aforementioned experimental results
and our previous work of iridium-catalyzed alkenylation of
o-carboranes,[14a] a possible mechanism is proposed for the
present catalytic reaction (Scheme 4). Acid-base reaction of
1 with KOAc gives the potassium salt 1’.[20] Coordination of
the oxygen atom of 1’ to the rhodium(III) center, followed by
regioselective electrophilic attack at the more electron-rich
cage B4 site generates the intermediate A.[14,10f] It is proposed
À
Keywords: B H activation · carboranes ·
homogeneous catalysis · hydroxylation · rhodium
À
that oxygen insertion into the Rh B bond proceeds to
produce the seven-membered rhodocycle B.[21] Protonation
of B affords the peroxy species C and regenerates the
rhodium(III) catalyst. C undergoes decomposition and decar-
boxylation to give the final product 2. The formation of B was
observed by MALDI-TOF MS (see the Supporting Informa-
tion), and verified by control experiments using 18O2.
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4
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Revised: August 8, 2016
Published online: && &&, &&&&
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À
B H Activation
H. Lyu, Y. Quan, Z. Xie*
&&&& — &&&&
Rhodium-Catalyzed Regioselective
À
Hydroxylation of Cage B H Bonds of o-
Carboranes with O2 or Air
Cage decorations: The title reaction of
carboranes in a one-pot process. The use
of either O2 or air as both the oxidant and
the oxygen source makes this protocol
environmentally friendly and practical.
Cp*=C5Me5.
À
a cage B4 H bond in o-carboranes with
either O2 or air as the oxygen source
serves as a new method for the regiose-
lective generation of a series of 4-OH-o-
6
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