Direct borylation of primary C-H bonds in functionalized molecules by palladium catalysis

Article abstract of DOI:10.1002/anie.201310000

Organoborane compounds are among the most commonly employed intermediates in organic synthesis and serve as crucial precursors to alcohols, amines, and various functionalized molecules. A simple palladium-based system catalyzes the conversion of primary C(sp³)-H bonds in functionalized complex organic molecules into alkyl boronate esters. Amino acids, amino alcohols, alkyl amines, and a series of bioactive molecules can be functionalized with the use of readily available and removable directing groups in the presence of commercially available additives, simple ligands, and oxygen (O₂) as the terminal oxidant. This approach represents an economic and environmentally friendly method that could find broad applications. Crucial additives: A simple palladium-based system catalyzes the conversion of primary C(sp³)-H bonds in complex organic molecules into alkyl boronate esters. Amino acids, amino alcohols, alkyl amines, and a series of bioactive molecules that are modified with readily available directing groups are functionalized in the presence of commercially available additives, simple ligands, and oxygen as the terminal oxidant.

Full text of DOI:10.1002/anie.201310000

Angewandte
Chemie
DOI: 10.1002/anie.201310000
À
C H Borylation
À
Direct Borylation of Primary C H Bonds in Functionalized Molecules
by Palladium Catalysis**
Li-Sheng Zhang, Guihua Chen, Xin Wang, Qing-Yun Guo, Xi-Sha Zhang, Fei Pan, Kang Chen,
and Zhang-Jie Shi*
Abstract: Organoborane compounds are among the most
commonly employed intermediates in organic synthesis and
serve as crucial precursors to alcohols, amines, and various
amounts of transition metals.[4–19] Compared with the selective
3
[4–8]
À
conversion of inert C(sp ) H bonds into carbon–carbon
and carbon–heteroatom bonds,[9–19] the construction of ali-
3
À
À
functionalized molecules. A simple palladium-based system
phatic C B bonds directly from C(sp ) H bonds is of great
interest because organoboranes are commonly employed
intermediates in organic synthesis and crucial precursors to
functionalized molecules.[20,21] Such efficient transformations
3
À
catalyzes the conversion of primary C(sp ) H bonds in
functionalized complex organic molecules into alkyl boronate
esters. Amino acids, amino alcohols, alkyl amines, and a series
of bioactive molecules can be functionalized with the use of
readily available and removable directing groups in the
presence of commercially available additives, simple ligands,
and oxygen (O2) as the terminal oxidant. This approach
represents an economic and environmentally friendly method
that could find broad applications.
could provide a new starting point for different functional-
3
À
ization reactions of specific C(sp ) H bonds and have a great
impact in the fields of medicinal chemistry, chemical biology,
and materials science.
Significant progress has been made on the borylation of
3
À
À
unactivated C(sp ) H bonds, especially for primary C H
bonds of unfunctionalized hydrocarbons, using transition-
T
he direct catalytic functionalization of C H bonds has the
metal catalysis.[22–28] However, the direct borylation of inert
C(sp ) H bonds in functionalized organic molecules is still
challenging. The fundamental problem is to achieve direct
borylation selectively and precisely at a specific position in
a complex molecule. A directing-group-assisted strategy,
which has been shown to be effective for the direct
functionalization of both C(sp ) H and C(sp ) H bonds,
could be employed to control site selectivity. To achieve
borylation of a specific secondary C(sp ) H bond in the
presence of potentially more favored primary and other
À
3
À
potential to revolutionize organic synthesis and to produce
complex natural and man-designed molecules.[1] The imme-
diate challenge in this field is to achieve the catalytic and
highly regioselective functionalization of one specific inert
À
À
C H bond in a complex molecule that contains many C H
bonds and an array of functional groups.[2,3] Compared to the
wide range of well-developed methods for the activation of
2
3
[29]
À
À
3
À
À
aromatic C H bonds, the precise site-selective functionaliza-
À
tion of inert aliphatic C H bonds in complex molecules has
3
À
remained a great challenge. Potential solutions that effec-
tively control chemo-, regio-, and site selectivity could make
secondary C(sp ) H bonds, the directing group must be
powerful enough to achieve the desired selectivity. Further-
more, the catalyst system must be compatible with existing
À
C H bond functionalization more widely applicable for the
À
synthesis of valuable molecules.
functional groups and the C B unit of the product.
Over the past several decades, much effort has been
Although palladium catalysis is widely used for many
À
directed towards the direct functionalization of aliphatic
transformations, including the direct borylation of aryl C H
3
C(sp ) H bonds in the presence of stoichiometric or catalytic
bonds,[30] the palladium-catalyzed borylation of unactivated
À
3
À
C(sp ) H bonds with a directing-group strategy has not been
À
achieved because of the relatively high reactivity of C B
[*] L.-S. Zhang, G. Chen, X. Wang, Q.-Y. Guo, X.-S. Zhang, F. Pan,
K. Chen, Prof. Dr. Z.-J. Shi
bonds in the presence of a palladium catalyst, other functional
groups, and many oxidants as well as the steric hindrance of
Beijing National Laboratory of Molecule Science (BNLMS) and Key
Laboratory of Bioorganic Chemistry and Molecular Engineering of
the Ministry of Education
College of Chemistry and Green Chemistry Center
Peking University
the target bonds.[29] Herein, a selective oxidative borylation of
3
À
unactivated primary C(sp ) H bonds in complex structures
that is based on a Pd0/PdII catalytic system and features an N-
heterocycle as the directing group and mild reaction con-
Beijing, 100871 (China)
E-mail: zshi@pku.edu.cn
ditions is reported. This powerful method represents a com-
3
À
plementary approach to the reductive borylation of C(sp ) H
Prof. Dr. Z.-J. Shi
bonds, which may be achieved by iridium or rhodium
State Key Laboratory of Organometallic Chemistry
Chinese Academy of Science
Shanghai, 200032 (China)
catalysis.[20]
We initially chose amino acid derivatives as substrates
because amino acids are among the most abundant and
significant molecules and constitute the fundamental building
blocks of peptides, proteins, and a number of biologically
active molecules. With this choice, the potential compatibility
of versatile functional groups with the developed procedure
[**] Support of this work through the “973” Project by the MOST of
China (2009CB825300) and the NSFC (20925207 and 21002001) is
gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310000.
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À
could be highlighted, to confirm the suitability of this method
for the modification of important natural motifs. We intended
through C N bond formation must be avoided to obtain the
desired products.[31]
to directly and selectively functionalize various unactivated
We chose valine as the standard substrate and B2pin2
(pin = pinacolato) as the borylation reagent in the presence
of palladium acetate as the catalyst under an oxygen (O2)
atmosphere (Table 1). We were pleased to observe a trace
amount of the desired borylation product by GC-MS when
MeCN was used as the solvent (entry 1). When NaOAc
(3.0 equiv) was added to the system, the desired product was
obtained in 16% yield (determined by GC; entry 2), but the
catalyst decomposed and precipitated as palladium black.
Oxidants that are commonly used for palladium-catalyzed
transformations, such as K2S2O8, PhI(OAc)2, benzoquinone
(BQ), silver salts, copper salts, di-tert-butyl peroxide (TBP),
N-fluorobenzenesulfonimide (NFSI), oxone, or Ce(SO4)2,
were screened to improve the yield. Unfortunately, most of
these oxidants inhibited the reaction or destroyed the desired
borylation product (for details, see the Supporting Informa-
tion). Therefore, we started to consider whether an atmos-
phere of O2 itself might act as a mild oxidant for the
regeneration of the palladium catalyst. As a result, we
returned to the initial conditions and tested different ligands
that could stabilize the Pd species and promote the re-
oxidation of the Pd0 species.[32] After screening several
ligands, we found that DMF, NMP, phenanthroline (phen),
and bipyridine inhibited the transformation under O2 atmos-
phere, albeit with no generation of palladium black (entry 3–
5). A series of phosphine ligands were tested, but the yield did
not improve. To our delight, when iPr2S (5.0 equiv) was
added, the yield substantially increased to 48% (determined
by GC; entry 6), confirming the regeneration of the PdII
species to complete the catalytic cycle. Based on preliminary
kinetic studies, the ligand iPr2S only stabilizes the Pd species
to prevent its precipitation and is not obviously involved in
the initial reaction rate (Supporting Information, Scheme S1).
Considering that the transformation could only occur
under alkaline conditions, we tested different bases. Interest-
ingly, in contrast to sodium and lithium salts, potassium and
cesium salts inhibited the reaction so that we mainly focused
on sodium and lithium bases. We found that both strong bases
(such as tBuONa) and very weak bases (such as NaOTf or
NaOTFA; OTf = trifluoromethanesulfonate, OTFA = tri-
fluoroacetate) are not suitable for this transformation. Finally,
among several bases, such as NaOAc or Na2CO3 (Table S4),
lithium carbonate proved to be the most suitable one, and the
desired product was obtained in 54% yield (by GC; entry 7).
Furthermore, to promote the re-oxidation of the Pd0 species
by O2, we added different acids as a proton source,[32] but
found that the reaction was very sensitive to these additional
acids, such as trifluoroacetic acid (TFA), AcOH, pivalic acid
(PivOH), and benzoic acid (entry 8). In the presence of H2O,
the yield was also diminished to 44% (entry 9). Fortunately,
by increasing the amount of B2pin2 to three equivalents and
diluting the solution, the yield could be increased to 64%
(entry 10). While screening further additives, we observed
that the addition of LiF (3.0 equiv) further improved the yield
to 74% (entry 11), whereas LiCl inhibited the reaction.
Finally, in the presence of four equivalents of B2pin2 and
several other additives (entry 15), the desired borylation
3
À
C(sp ) H bonds in amino acids through the formation of
À
reactive C B bonds. After installing the well-studied picolinyl
directing group at the amine moiety, we performed the
designed borylation reaction through the remote activation of
3
À
a C(sp ) H bond, which proceed via two fused palladacycles
as intermediates (Table 1).[31] To achieve the oxidative
3
À
borylation of unactivated C(sp ) H bonds by palladium
catalysis, Pd0/PdII and PdII/PdIV catalytic cycles may both be
considered in the presence of a proper and efficient oxidant.
The oxidant, which serves to complete the catalytic cycle,
must also be mild enough to avoid oxidation of the aliphatic
À
C B bond in the product. Furthermore, the alkyl boronic
ester may easily decompose through transmetalation to a PdII
species, which would lead to the conversion of the borylation
product into other compounds in most cases. Moreover, to
À
ensure the stability of the C B bond under the reaction
conditions and to complete the catalytic cycle, an alkaline
environment is required. Whereas very weak bases may not
be efficient enough to obtain high conversion, strong bases
may accelerate the interaction of the produced alkyl boronic
esters and a PdII species to afford Suzuki—Miyaura products.
Finally, the previously reported intramolecular amination
À
Table 1: Palladium-catalyzed C H borylation under various reaction
conditions.[a]
Entry
Base
Ligand (equiv)
Additive (equiv)
Yield[b] [%]
1
2
3
4
5
6
7
8
–
–
–
–
–
–
–
–
–
–
<5
16
<5
<5
0
48
54
<5
44
64
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
Li2CO3
Li2CO3
Li2CO3
Li2CO3
Li2CO3
Li2CO3
Li2CO3
Li2CO3
Li2CO3
DMF (5.0)
NMP (5.0)
phen (0.2)
iPr2S (5.0)
iPr2S (5.0)
iPr2S (5.0)
iPr2S (5.0)
iPr2S (5.0)
iPr2S (5.0)
iPr2S (5.0)
iPr2S (5.0)
iPr2S (5.0)
iPr2S (5.0)
AcOH (2.0)
H2O (2.0)
–
LiF (3.0)
NaF (3.0)
LiF (3.0)
LiF (3.0)
LiF (3.0)
9
10[c]
11[c]
12[c]
13[c,d]
14[c,d,e]
15[d,e,f]
74
66
77
81
85 (68)
[a] Reaction conditions: 1a (0.10 mmol), B2pin2 (0.20 mmol), base (3.0
equiv), MeCN (1.0 mL), O2 (balloon), 808C, 12 h. [b] Determined by GC
using n-dodecane as the internal standard. The yield of isolated product
is given in parentheses. [c] B2pin2 (0.30 mmol), MeCN (1.5 mL).
[d] NaHCO3 (0.1 mmol) was added as an additional additive. [e] PhCN
(0.1 mL) was added. [f] B2pin2 (0.40 mmol), MeCN (1.5 mL).
DMF=N,N-dimethylformamide, NMP=N-methyl-2-pyrrolidinone,
phen=phenanthroline.
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Table 2: Substrate scope for amino acid, amino alcohol, and amine
derivatives.[a]
product 2a (Table 2) was isolated in 68% yield with a good
diastereoselectivity (d.r.) of 83:17. The diastereoselectivity
was probably induced during the formation of two fused five-
membered palladacycles as key intermediates; their relative
configuration is determined by the sterically hindered carbo-
methoxy group.
With the optimized conditions in hand, different natural
and unnatural amino acids were tested, and the desired
borylated products were produced in good to excellent yields
Entry
1
2
Yield[b] d.r.[c]
[%]
1
2
3
2a
2b
2c
68
70
71
83:17
(Table 2). With isoleucine, the reaction selectively occurred at
3
À
the primary C(sp ) H bond, and the absolute configuration of
the starting material did not influence the reaction efficiency
(l-isoleucine vs. dl-isoleucine, 2b vs. 2c). Notably, for l-
threonine, which bears a tert-butoxy moiety at the b-position
of the amino group, the yield improved (2d). tert-Leucine,
with a quaternary carbon atom, was less reactive, possibly
because of a steric effect (2e). Substituents at the a- and b-
positions of the amino group are necessary for this trans-
formation, and with two methyl substituents at the b-position
and one methyl group at the a-position, the corresponding
product was isolated in 64% (2 f). Furthermore, for a series of
easily accessible valinol substrates, valuable functional groups
could be accommodated within the substrate. For example,
–
–
4
2d
84
–
5
2e
2 f
2g
2h
2i
61
64
72
62
68
41
58
–
–
benzyl (Bn) and benzoyl (Bz) moieties, which contain
2
À
aromatic C(sp ) H bonds, were tested as protecting groups
6
(2g–2h). The absence of products that correspond to
borylation of the C(sp ) H bonds confirmed the key role of
2
À
the picolinamide directing group.[31] As the tetrahydropyranyl
(THP) group is broadly used for protecting hydroxy groups,
we tested this protecting group and found that it is well
tolerated and therefore useful for masking a nearby hydroxy
group (2i). An acetyl moiety could also act as a protecting
group, but the corresponding product was isolated in a lower
yield (2j). As silyl groups serve as valuable protecting groups
in traditional organic synthesis, we tested tert-butyldimethyl-
silyl (TBS) and triisopropylsilyl (TIPS) moieties as alcohol
protecting groups. In both cases, the corresponding products
were obtained in good yield in the absence of the additive LiF
7
87:13
90:10
91:9
8
9
10
11[d]
2j
86:14
>20:1
(2k–2m). Notably, for a phenylalanine substrate, which bears
2
À
unactivated C(sp ) H bonds, borylation at the d-position of
the amino group was successfully achieved and thus provides
a supplementary method for the functionalization of phenyl-
2k
alanine residues in biological structures (2n). Furthermore,
2
12[d]
13[d]
14
2l
60
51
63
82
>20:1
À
targeted unactivated C(sp ) H bonds in the lateral chain of
several important antibiotics, such as Cefalexin, Cefaclor, and
Ampicillin sodium, could also be functionalized in good yields
(2o). By employing well-established synthetic methods,
a series of valuable ortho functionalization reactions could
2m
2n
2o
–
–
–
2
[30]
À
be achieved by the conversion of these C(sp ) B bonds.
Through the preceding transformation of unactivated
3
À
À
C(sp ) H bonds into C B bonds, a series of valuable
functionalization reactions at the target positions were
achieved (Scheme 1a). Under simple oxidative conditions,
a hydroxy group can be installed with high efficiency. In
15
À
a reaction on the gram scale, the initially unactivated C H
[a] Reaction conditions: 1 (0.10 mmol), B2pin2 (0.40 mmol), Pd(OAc)2
(0.02 mmol), iPr2S (0.50 mmol), Li2CO3 (0.30 mmol), LiF (0.30 mmol),
NaHCO3 (0.10 mmol), MeCN/PhCN (1.5:0.1), 808C, O2 (balloon), 12 h.
[b] Yield of isolated product. [c] d.r. determined by H NMR spectrosco-
py. [d] Without LiF.
bond was converted into methanesulfonate (OMs) and OTBS
groups in moderate yields over three steps (3a–3b). By
transforming the hydroxy group 4 into an excellent leaving
1
À
group and a subsequent SN2 reaction, a targeted C H bond
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could be readily converted into an azide or a thioether (5, 6);
these transformations are difficult to achieve from the same
starting materials using traditional methods. Furthermore, the
hydroxy group could be further oxidized to an aldehyde
moiety to yield compound 7, which could be a new starting
point for a series of further transformations, including
Horner–Wadsworth–Emmons reactions to yield structures
such as 8 and aldol reactions. To further demonstrate the
utility of this method for organic synthesis and to confirm its
efficiency in the modification of complex molecules, we
explored strategies to functionalize natural product deriva-
functionalized complex molecules. Mild reaction conditions,
which feature a weakly alkaline environment and oxygen as
the terminal oxidant, are crucial for achieving excellent
compatibility with an array of functional groups; therefore,
the utility of this method for organic synthesis was confirmed.
These studies are not only important for understanding the
reactivity of bonds that were traditionally considered as inert,
but also provide an efficient synthetic method for the
construction of valuable chemicals from readily available
and inexpensive natural products. Hopefully, these studies
will inspire the development of new functionalization reac-
tions for peptides, proteins, and other bioactive molecules for
the postsynthetic derivatization of biomolecules.
tives (Scheme 1b). We were pleased to discover that the inert
3
À
primary C(sp ) H bond of the angular methyl group in
starting material 9, a derivative of the steroid estrone, could
be successfully functionalized, and that the final product 10
was obtained as a single diastereomer.
Experimental Section
General procedure for the palladium(II)-catalyzed borylation of
amino acid, amino alcohol, and amine derivatives: B2pin2 (101.6 mg,
0.40 mmol; white crystals after recrystallization), Li2CO3 (22.2 mg,
0.30 mmol; white powder), NaHCO3 (8.4 mg, 0.10 mmol), and
Pd(OAc)2 (4.5 mg, 0.02 mmol; Acros) were added to a dried 50 mL
tube under air. Then, LiF (7.8 mg; no LiF was added for 1k–1m) was
added to the tube in the glove box. After degassing under vacuum and
refilling with O2 (three times), the tube was then sealed well. A
mixture of 1b (25.0 mg, 0.10 mmol; colorless oil), iPr2S (75 mL,
0.50 mmol; Alfa Aesar), MeCN (1.5 mL; purified by distillation), and
PhCN (0.1 mL) was quickly added to the tube under stirring at room
temperature. Then, the tube was immediately moved to a Wattecs
Parallel Reactor at 808C. The solution was stirred at 808C for 12 h.
After the completion of the reaction, the solution was directly
purified by preparative thin-layer chromatography (PTLC; petro-
leum ether/ethyl acetate or petroleum ether/CH2Cl2/ethyl acetate or
toluene/ethyl acetate as the eluent; without concentrating the
solution) to afford 2b (26.3 mg, 70% yield) as a colorless oil.
In conclusion, the direct transformation of unactivated
3
À
À
C(sp ) H bonds into reactive C B bonds was achieved by
palladium catalysis. This method offers a new strategy to
À
selectively construct C B bonds from unactivated primary
3
À
C(sp ) H bonds and may become a powerful strategy for the
synthesis of new derivatives of amino acids and other
Received: November 18, 2013
Revised: January 21, 2014
Published online: March 5, 2014
À
Keywords: amino acids · borylation · C H functionalization ·
directing groups · palladium
.
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Scheme 1. Applications of the C H borylation reaction. [a] B2pin2,
Pd(OAc)2, iPr2S, Li2CO3, LiF, NaHCO3, MeCN/PhCN, 808C, O2 balloon,
16 h. [b] NaBO3·4H2O, THF/H2O, 308C, 1 h. [c] MsCl, NEt3, CH2Cl2,
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808C, 1 h. [f] NaSMe, KI, DMF, 808C, 1 h. [g] DMP, CH2Cl2, RT.
[h] triethyl phosphonoacetate, EtONa, THF, 0 8C–RT. DMP=Dess–
Martin periodinane.
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