Transition Metal Catalyzed Direct Amination of the Cage B(4)-H Bond in o-Carboranes: Synthesis of Tertiary, Secondary, and Primary o-Carboranyl Amines

Article abstract of DOI:10.1021/jacs.6b07086

Transition metal catalyzed regioselective amination of the cage B(4)-H bond in o-carboranes has been achieved for the first time using O-benzoyl hydroxylamines or organic azides as the amination reagents, leading to the preparation of a series of tertiary and secondary carboranyl amines. Both amination reactions proceeded under mild conditions without the addition of any external oxidants. Hydrogenolysis of the resultant product 4-N(CH₂Ph)₂-o-carborane afforded the primary carboranyl amine, 4-amino-o-carborane, in quantitative yield.

Full text of DOI:10.1021/jacs.6b07086

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Article
Transition Metal Catalyzed Direct Amination of
Cage B(4)-H Bond in o-Carboranes: Synthesis of
Tertiary, Secondary and Primary o-Carboranyl Amines
Hairong Lyu, Yangjian Quan, and Zuowei Xie
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b07086 • Publication Date (Web): 14 Sep 2016
Downloaded from http://pubs.acs.org on September 15, 2016
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Transition Metal Catalyzed Direct Amination of Cage B(4)−H Bond in o-
Carboranes: Synthesis of Tertiary, Secondary and Primary o-Carboranyl
Amines
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Hairong Lyu, Yangjian Quan,* and Zuowei Xie*
Department of Chemistry and State Key Laboratory of Synthetic Chemistry, The Chinese University of Hong Kong,
Shatin, N. T., Hong Kong, China
Supporting Information Placeholder
noalkyl-o-carboranes have been extensively employed as
ligands for transition metal complexes.12 Despite the recent
advances in carborane chemistry, straightforward and gen-
eral synthesis of cage B-aminated-o-carboranes is rather
limited.3b,4,13,14 For instance, 3-NH2-o-C2B10H11 is prepared by
the reduction of o-carborane with Na in liquid NH3, fol-
lowed by oxidation with KMnO4. Such reaction has the risk
of fire and explosion.15 Recently, the synthesis of B(3)/B(9)-
aminated-o-carboranes has been reported through a Pd-
catalyzed Buchwald-Hartwig amination of B(3)/B(9)-iodo-
o-carboranes (Scheme 1) or B(3)-bromo-o-carborane.16
However, transition metal catalyzed direct cage B−H ami-
nation remains elusive.4,13
ABSTRACT: Transition metal catalyzed regioselective
amination of cage B(4)-H bond in o-carboranes has been
achieved for the first time using O-benzoyl hydroxylamines
or organic azides as the amination reagents, leading to the
preparation of a series of tertiary and secondary carboranyl
amines. Both amination reactions proceeded under mild
conditions without the addition of any external oxidants.
Hydrogenolysis of the resultant product 4-N(CH2Ph)2-o-
carborane afforded primary carboranyl amine, 4-amino-o-
carborane, in quantitative yield.
Carboranes, a class of three-dimensional relatives of
benzene, have proved as useful basic units in boron
neutron capture therapy agents,1 in supramolecular
design/materials,2 and in coordination/organometallic
chemistry.3 Thus, functionalization of carboranes has
received a growing interest. A number of methods have
been developed for the functionalization of cage CH and
Scheme 1. Synthetic Routes to Cage B-Aminated-o-
Carboranes
BH
vertices.4
In
general,
cage
C−H
activation/functionalization is relatively easier to be
achieved than that of B−H ones as the cage CH proton is
acidic (pKa ~ 23).5 We and other groups have taken up the
challenge to develop transition metal catalyzed direct cage
B−H functionalization.6-8 With the help of a carboxylic acid
directing group, catalytic cage B(4)-alkenylation,6c B(4,5)-
dialkenylation,6e B(4,5)-diarylation,6f B(4)-alkynylation,6g
and B(4)-hydroxylation6i have been achieved. These results
indicate that the weakly coordinating directing group
−COOH not only plays a key role in regioselectivity and
mono-/di-selectivity of the reactions, but also is removable
after the reaction. We also note that a key intermediate
bearing five-membered metallacycle MBCCO is involved in
the aforementioned catalytic cycles. Such intermediate may
react with amination reagents to realize the direct
amination of cage B−H bond.
Inspired by the recent achievements in catalytic C-H
amination17 and the unique role of weakly coordinating
carboxylic ligand in regioselective activation of cage B-H
bond,6c,e,f,g,i we have developed two efficient methods for
direct and regioselective amination of o-carboranes using
O-benzoyl hydroxylamines and organic azides as the amino
Carborane derivatives containing organic nitrogen
groups have received much attention due to their potential
applications in medicinal chemistry9 and catalysis.10 For
example, carborane-amino acid or -nucleoside combina-
tions serve as excellent candidates for cancer treatment in
boron neutron capture therapy (BNCT).9,11 Moreover, ami-
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sources, respectively. These new findings are reported in
this communication (Scheme 1).
(entry 6, Table 1). Substrate scope of other O-benzoyl hy-
1
2
3
4
5
6
7
8
droxylamines was also evaluated. To our delight, a variety
of alkylamine moieties were readily introduced to the cage
B(4) position by simply treatment of 1a with various O-
benzoyl hydroxylamines, giving 3o-3v in moderate to high
isolated yields (Table 2).
The optimization of reaction conditions was summarized
in Table S1 of the Supporting Information. Our initial reac-
tion of 1-COOH-2-CH3-C2B10H10 (1a) with O-benzoyl hy-
droxylmorpholine (2a) in the presence of 10 mol %
Pd(OAc)2 and 2 equiv of AgOAc in toluene at 110 oC for 12 h
gave no desired amination product (entry 1, Table S1). The
addition of 2 equiv of K2HPO4 offered the B(4)-aminated-o-
carborane 3a in 49% GC yield, whereas 2 equiv of KOAc
only led to a trace amount of 3a (entries 2-3, Table S1).
Lowering the loading of K2HPO4 to 1 equiv and the reaction
temperature to 100 oC afforded higher yields of 3a (entries
4-5, Table S1). It was later found that the amination reac-
tion proceeded well in the absence of AgOAc, giving 3a in
93% GC yield (entries 6-7, Table S1). Lowering the catalyst
loading to 5% led to a decreased yield of 3a (entry 9, Table
S1). In view of the yield of 3a, entry 7 in Table S1 was chosen
as the optimal reaction conditions.
Table 2. Scope of O-Benzoyl Hydroxylamine Sub-
stratesa,b
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Table 1. Synthesis of B(4)-Morpholinated-o-
Carboranesa
a
Reactions were conducted on 0.2 mmol scale in 5 mL of
toluene in a closed flask. b Yield of isolated products.
The aforementioned direct catalytic B-H amination gave
tertiary carboranyl amines. We wondered if primary amido
groups could be introduced to the cage B(4) position using
organic azides as the amination reagents. The results
showed that replacement of O-benzoyl hydroxylamine with
tosyl azide in the aforementioned optimal reaction condi-
tions did not give any target product (entry 1, Table S2 in
the SI). Inspired by Rh- or Ru-promoted cage B-H activa-
tion6h,6j,7c,7f and Rh- or Ru-catalyzed C-N bond forming
reactions using organic azides as reagents,17b we then
screened these transition metals as the catalysts.
[Cp*RhCl2]2 only afforded the aminated product 4-TsNH-2-
CH3-C2B10H10 (5a) in 12% yield (entry 2, Table S2). To our
delight, in the presence of 2.5 mol % [Ru(p-cymene)Cl2]2
and 2 equiv of NaOAc, reaction of 1a with TsN3 in toluene
entry
1
R1
Me
yield (%)b
79 (3a)
72 (3b)
71 (3c)
2
Et
nBu
3
4
iPr
52 (3d)
53 (3e)c
46 (3e)
68 (3g)
74 (3h)
73 (3i)
5
TMS
6
H
7
benzyl
8
4-CH3-benzyl
4-Cl-benzyl
4-F-benzyl
4-OMe-benzyl
3-CH3-benzyl
styryl
o
at 100 C for 12 h gave the desired amination product 5a in
9
95% GC yield (entry 10, Table S2). Further screening of ba-
ses and reaction temperatures did not offer better results.
Thus, entry 10 in Table S2 was chosen as the optimal reac-
tion conditions.
10
11
71 (3j)
78 (3k)
70 (3l)
12
13
14
Under the above optimized reaction conditions, the
scope of such amination was investigated. A variety of alkyl
and benzyl substituents at the cage C(2) position gave the
amination products 5 in very good to excellent isolated
yields, and no obvious electronic effects were observed
(entries 1-3 and 5-9, Table 3). Compound 1e (R1 = TMS) af-
forded the corresponding product 5d in a good yield of 67%
(entry 4, Table 3). In addition, the scope of sulfonyl azides
was also examined (Table 4). Arylsulfonyl azides reacted
smoothly regardless of the substituents on the phenyl ring,
giving 5j-5o in > 80% isolated yields. Benzylsulfonyl azide
worked equally well, whereas butylsulfonyl azide resulted
in relatively lower isolated yield of 72% (5p and 5q in Table
4). These results show that Ru-catalyzed direct cage B-H
amination is generally more efficient than Pd-catalyzed one
48 (3m)
58 (3n)
4-CH3-styryl
a
Reactions were conducted on 0.2 mmol scale in 5 mL of
toluene in a closed flask. b Yield of isolated products. c TMS
was removed after work up.
Subsequently,
a
variety of B(4)-morpholinated-o-
carborane derivatives were synthesized under the optimal
reaction conditions. Effects of cage carbon substituents R1
on the reaction results were examined. Both liner alkyl and
benzyl substituents gave the B(4)-aminated products 3 in
68-79% isolated yields (entries 1-3 and 7-12, Table 1). No
obvious electronic effects were observed (entries 7-12, Table
1). However, branch alkyl, TMS and styryl groups led to the
decreased yields of 3 (entries 4-5 and 13-14, Table 1). If R1 =
H, the corresponding product 3e was isolated in 46% yield
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in view of the isolated yields of amination products 3 and 5
(Tables 1 and 2 vs Tables 3 and 4).
To gain mechanistic insights into these B-H amination
1
2
3
4
5
6
7
8
reactions, several control experiments were carried out.
Under the optimal reaction conditions, 1a, 4k and nor-
bornene were mixed in 1:1:1 molar ratio, giving an azacyclo-
propane 7 and 5k in 43% and 51% yields, respectively
(Scheme S1a in the SI), whereas only a trace amount of aza-
cyclopropane 7 was detected by GC-MS in the absence of
the Ru catalyst (Scheme S1b). These results suggest that a
Ru-nitrene intermediate may be involved in the Ru-
catalyzed amination.17b On the other hand, comparison of
the reaction rates of 1a-d6 (1-COOH-2-Me-3,4,5,6,7,11-D6-o-
C2B10H4) and 1a under the optimal reaction conditions gave
very small KIE values of kH/kD = 1.15 for Pd system and 0.96
for Ru one (Schemes S2 and 3), which indicates that the
cyclometalation (B-H activation) step may not be involved
in the rate-determining step. However, many attempts to
isolate the five-membered cyclometalated intermediates
failed. Based on the aforementioned experimental results
and literature reports,6,17 two plausible reaction mecha-
nisms for Pd- and Ru-catalyzed direct cage B−H amination
are proposed in Schemes S4 and 5, respectively (see the SI
for detail).
Table 3. Synthesis of B(4)-Aminated-o-Carboranes
Using Tosyl Azidea
9
entry
R1
yield (%)b
90 (5a)
88 (5b)
85 (5c)
67 (5d)c
91 (5e)
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1
Me
2
3
4
5
6
7
8
9
Et
iPr
TMS
benzyl
4-CH3-benzyl
4-Cl-benzyl
4-OMe-benzyl
84 (5f)
94 (5g)
93 (5h)
92 (5i)
In conclusion, we have developed two catalytic systems
for regioselective and efficient direct amination of cage
B(4)−H bond in o-carboranes, for the first time, using O-
benzoyl hydroxylamines or organic azides as aminating
agents, where –COOH acts as a traceless directing group.
This work builds a toolbox for the preparation of previously
inaccessible tertiary, secondary and primary o-carboranyl
amines directly from o-carboranes, which might find appli-
cations in medicine, catalysis and materials.1-3 These amina-
tion reactions have a broad substrate scope and are tolerant
of functional groups, which offers useful references for se-
lective C−H amination in organic compounds and B-H
amination in other boron clusters.
a Reactions were conducted on 0.2 mmol scale in 5 mL of
toluene in a closed flask. b Yield of isolated products. c TMS
was removed after work up.
Table 4. Scope of Sulfonyl Azide Substratesa,b
ASSOCIATED CONTENT
Supporting Information
Detailed experimental procedures, complete characteriza-
tion data, NMR spectra, and X-ray data in CIF format for
3e, 3o, 3p, 5a and 5k. This material is available free of
charge via the Internet at http://pubs.acs.org.
a Reactions were conducted on 0.2 mmol scale in 5 mL of
toluene in a closed flask. b Yield of isolated products.
AUTHOR INFORMATION
Corresponding Author
It was noteworthy that the benzyl groups in 3u was easily
removed through hydrogenolysis in the presence of 10
mol % Pd/C to afford quantitatively 4-amino-o-carborane
6u, which may serve as a new precursor for cage B(4) func-
tionalization of o-carboranes (Scheme 2).18
* yjquan@cuhk.edu.hk
* zxie@cuhk.edu.hk
1
Notes
Compounds 3, 5 and 6u were fully characterized by H,
13C, and 11B NMR spectroscopy as well as high-resolution
mass spectrometry (see the SI for detail). The molecular
structures of 3e, 3o, 3p, 5a and 5k were further confirmed
by single-crystal X-ray analyses (Figure S1-S5).
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by grants from the Research
Grants Council of HKSAR (Project No. 14304115),
NSFC/RGC Joint Research Scheme (N_CUHK442/14) and
Vice-Chancellor's (CUHK) Discretionary Fund.
Scheme 2. Synthesis of 4-Amino-o-Carborane
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