Role of hemilabile diamine ligands in the amine-directed C-H borylation of arenes

Article abstract of DOI:10.1021/om5007837

A study of the role played by the bidentate ligand used in amine-directed C-H borylation is described. Both reaction conversion and selectivity were significantly impacted when steric congestion and electronic perturbations of the bidentate diamine ligand were made, but a more significant influence was imparted by reducing the bite angle of the ligand. N-Benzylaminopyridine was identified as a general ligand that improves both selectivity and yield for most problematic substrates previously reported with picolylamine as ligand.

Full text of DOI:10.1021/om5007837

Article
pubs.acs.org/Organometallics
Role of Hemilabile Diamine Ligands in the Amine-Directed C−H
Borylation of Arenes
Lillian V. A. Hale, Kathryn A. McGarry, Marissa A. Ringgold, and Timothy B. Clark*
Department of Chemistry and Biochemistry, University of San Diego, 5998 Alcala Park, San Diego, California 92110, United States
S
* Supporting Information
ABSTRACT: A study of the role played by the bidentate ligand used in amine-directed C−H borylation is described. Both
reaction conversion and selectivity were significantly impacted when steric congestion and electronic perturbations of the
bidentate diamine ligand were made, but a more significant influence was imparted by reducing the bite angle of the ligand. N-
Benzylaminopyridine was identified as a general ligand that improves both selectivity and yield for most problematic substrates
previously reported with picolylamine as ligand.
INTRODUCTION
■
C−H functionalization reactions have received significant
attention over the past two decades, resulting in a number of
key advances that allow the conversion of otherwise unreactive
C−H bonds into synthetically valuable functional groups.1−5
One method shown to be valuable to the synthetic community
is the C−H borylation reaction.6−14 The distinct advantage of
nature of the diamine ligand was probed for steric congestion,
electronic effects, and the bite angle of the bidentate ligand; the
diamine bite angle was found to have a significant effect on
this reaction lies both in the versatility of the resulting C−B
bonds and the inherent selectivity of the catalysts, which are
highly sensitive to steric congestion.12,15 In the case of arenes,
both product yield and selectivity. We herein report an
improved protocol for amine-directed C−H borylation and
insights into the role of the diamine in the catalytic
borylation ortho to a substituent is kinetically disfavored,
resulting in selective functionalization of appropriately sub-
stituted arenes (such as 1,3-disubstituted arenes) or substrates
with a particularly reactive C−H bond present. Alkanes, on the
transformation.
other hand, are selective for primary C−H bonds,16−18 with
The initial report of amine-directed C−H borylation focused
on picolylamine (3) as the optimal ligand, in conjunction with
only select examples of the functionalization of secondary alkyl
C−H bonds reported.19,20
[Ir(μ-OMe)(COD)]2 (COD = cyclooctadiene), to mediate
ortho-selective C−H borylation. The proposed mechanism
In recent years, substrate-directed C−H borylation has
(Scheme 1)32 is consistent with that suggested by Fernan
́
dez
resulted in the selective formation of ortho-substituted aryl
boronate esters,15,21−31 reversing the inherent selectivity of the
catalyst. Several examples of Lewis base-directed reactions have
been reported for halides,23 ethers,23 esters,23,24 phosphines,31
and nitrogen-containing Lewis bases.26−30 Our work in this area
has largely focused on amine directing groups, which utilize
hemilabile diamine ligands to provide an open coordination site
on the active iridium catalyst upon dissociation of one arm of
the bidentate ligand.29 In this transformation picolylamine (3)
was identified as an optimal ligand to promote the catalytic
transformation (eq 1). Several substrates, however, provided
moderate yields or poor selectivity (for the mono-ortho-
borylated product over the bis-ortho-borylated product). To
improve the yields and selectivity of this transformation, and to
gain insight into the role of the diamine ligand prior to a full
mechanistic interrogation, a ligand study was initiated. The
and Lassaletta for the directed borylation of hydrazones27 and
involves partial dissociation of one arm of the bidentate
diamine ligand to open a coordination site for the required C−
H activation step. Notably, picolylamine (3, blue in Scheme 1)
is less rigid than the traditionally used di-tert-butylbipyridine
(dtbpy) ligand, which facilitates dissociation of one ligand
nitrogen but maintains the second nitrogen (shown in Scheme
1 as the pyridine nitrogen), which is thought to be a critical
component of the catalytic reactivity.
The role of the ligand in governing the product yield and the
selectivity of the mono-ortho-borylation over the bis-ortho-
borylation products was expected to be governed by two key
Received: July 30, 2014
© XXXX American Chemical Society
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Scheme 1. Proposed Mechanism of Amine-Directed C−H
Scheme 2. Effect of Steric Congestion on Conversion and
Selectivity of C−H Borylation
a
Borylation29
a
Percent conversion and selectivity were determined by 1H NMR
spectroscopy; 1.2 equiv of arene to B2pin2; all conversions are based
on the arene substrate.
steps: the dissociation of one arm of the diamine ligand (step 2:
B to C) and dissociation of the borylated arene (1) to
regenerate the active catalyst (step 5: E to A). In step 2, the
nature of the dissociating nitrogen would have a significant
effect on the rate of the formation of C and likely the overall
reactivity of the catalyst. Alternatively, step 5 will be highly
influenced by the ability of the dissociated nitrogen to
recoordinate to the iridium center and promote dissociation
of 1. Steric congestion around the associated nitrogen will also
have a significant influence on the loss of 1 by inducing steric
strain between the ligand and the increased steric demands of
the borylated arene. A detailed ligand evaluation was expected
to provide insights into the proposed catalytic cycle and provide
a more focused approach to a full mechanistic study.
Figure 1. Electronic effects on conversion and selectivity to 1. Percent
conversion and selectivity were determined by 1H NMR spectroscopy;
1.2 equiv of arene to B2pin2; all conversions are based on the arene
substrate.
RESULTS AND DISCUSSION
■
The ligand study was initiated by examining the influence that
steric congestion around both ligand nitrogens played in
reaction conversion and selectivity for the mono-ortho
borylation product, using the parent N,N-dimethylbenzylamine
(Scheme 2).33 N-Methylpicolylamine (4) and N,N-dimethylpi-
colylamine (5)29 were examined to determine the effect of the
ligand-based amine substituents on the reaction conversion and
selectivity, providing significant loss in selectivity in both cases
while providing an improved conversion with 4. Placement of a
substituent on the alkyl backbone of the ligand (ligand 6)
provided increased selectivity with a decreased conversion.
Substitution on the arene portion of the ligand (ligands 7 and
8) resulted in the most significant improvements; a methyl
adjacent to the pyridine nitrogen (7) provided an increase in
conversion without the loss of selectivity.
The electronic effects of the ligand were examined by varying
the Lewis basicity of both nitrogens (Figure 1). The electron
density of the pendant amine was examined initially.
Converting the pendant amine to a weaker Lewis base (ligands
9 and 10) resulted in improved reactivity of the catalyst over
picolylamine (3), albeit with a significant loss in selectivity.
Electron-deficient pyridine 11 resulted in a slight increase in
selectivity compared to picolylamine (3). Replacement of the
pyridine with a pyrrole or furan (ligands 12−14) resulted in
low conversion, suggesting an important electronic effect of the
pyridine nitrogen in the catalytic cycle. These results are
consistent with dissociation of the amine nitrogen (as shown in
Scheme 1) and suggest that weakly coordinating pendant Lewis
bases increase the catalytic activity.
The final major category that was examined for increased
catalytic activity and selectivity was the bite angle34−36 of the
bidentate ligand. Ligands 15 and 16 (Figure 2) were compared
to picolylamine (3) and found to provide increased conversion
in the case of 15 and exceptional selectivity in the case of 16.
Importantly, the high selectivity of 16 was coupled with good
conversion. Additional aminopyridine ligands were examined
based on the insights gained from the steric and electronic
effects discussed above. Substitution of the ligand-based amine
nitrogen with a benzyl substituent (ligand 17) resulted in a
slight loss of selectivity but with slightly improved conversion.
Addition of a methyl substituent adjacent to the pyridine
nitrogen (ligand 18) did not provide improvement, but an
increased conversion was observed with an acylated amine
nitrogen (ligand 19). Finally, an electron-withdrawing −CF3
substituent on the arene (20) also improved the conversion but
resulted in a significant loss in selectivity.
B
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each case (eq 2, Table 1). Although 3-methoxy- and 3-chloro-
substituted substrates did not improve as much as the 3-methyl-
Table 1. C−H Borylation of N,N-Dimethyl-3-substituted
a
Benzylamines with Ligand 17
Figure 2. Effect of bite angle on conversion and selectivity to 1.
Percent conversion and selectivity were determined by 1H NMR
spectroscopy; 1.2 equiv of arene to B2pin2; all conversions are based
on the arene substrate.
The small bite angle of 2-aminopyridine ligands likely has
two key roles in the catalytic cycle: it provides rapid
dissociation of one arm of the bidentate ligand (Scheme 1, B
to C) due to the decreased bite angle and the decreased Lewis
basicity of an aniline-type amine; the smaller bite angle could
also increase the steric congestion near the catalyst by reducing
the ability of the dissociated nitrogen to rotate away from the
iridium center.37 These effects on individual mechanistic steps
are subtle and will require a full mechanistic investigation to
determine if the proposed catalytic cycle is consistent with this
analysis.
The ligand study described above resulted in several
improved ligands that could be applied to the substrates that
provided poor yield or selectivity using picolylamine (3).29 The
most problematic substrates are 3-substituted benzylamines,
which provide two monoborylation products (Scheme 3, a and
a
Perent conversion and selectivity were determined by 1H NMR
spectroscopy; 1.2 equiv of arene to B2pin2; conversions are based on
the arene substrate. Isolated yields are of a mixture of a, b, and c.
substituted substrate, a consistent increase in selectivity was
observed. The increased quantities of 22b and 23b are
consistent with the known steric and electronic effects observed
with electronegative atoms in the nondirected C−H borylation
reactions.38 The chlorine and methoxy substituents provide a
slight activation of this sterically congested C−H bond through
inductive effects. The general improvement of the isolated
yields without an increase in conversion is believed to result
from a decrease in unidentified side-products, resulting in clean
product formation.
Scheme 3. Products and Ligand Screen from C−H
a
Borylation of N,N-Dimethyl-3-methylbenzylamine
Several 4-substituted benzylic amines were also known to
provide low yield or selectivity with picolylamine (3) and were
examined with ligand 17 (eq 3, Table 2). Improved yield and/
a
Percent conversion and selectivity were determined by 1H NMR
spectroscopy; 1.2 equiv of arene to B2pin2; all conversions are based
on the arene substrate.
or selectivity was achieved with 4-bromo-, 4-methyl-, and 4-
ethanoate-substituted benzylamines. The 4-methyl-substituted
substrate demonstrated a significant improvement in selectivity
with ligand 17 without a loss in yield. In the case of 25 and 26,
the conversion and selectivity were similar to those of
picolylamine (3), but the poor yields were improved. Of
particular note is the improved yield of boronate ester 25 (from
37% to 65%), which has resulted in a synthetically viable yield
of an intermediate with both a boronate ester and bromine
present for additional transformations.
b) and bisborylation products (c). For these substrates, 85−
89% of the borylated material is represented by product a.
Several ligands identified above were examined with N,N-
dimethyl-3-methylbenzylamine. Ligand 17 was found to
provide the most significant improvement in selectivity for
boronate ester 21a, with a 95:4:1 selectivity of 21a/21b/21c
while maintaining a 93% conversion.
N-Benzylaminopyridine (ligand 17) was examined with other
3-substituted benzylamines, providing improved selectivity in
C
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Table 2. C−H Borylation of N,N-Dimethyl-4-substituted
Benzylamines with Ligand 17
Figure 3. ORTEP drawing for complex 27 with thermal ellipsoids
shown at 50% probability. Only one of the two complexes in the
asymmetric unit is shown for clarity. The benzylaminopyridine ligand
is coordinated to iridium through the pyridine nitrogen (N1) with a
bond length of 2.093(6) Å for the complex shown.
a
Reaction run for 24 h. Percent conversion and selectivity were
1
borylation and provided improved yield and selectivity for most
3- and 4-substituted benzylamines.
determined by H NMR spectroscopy; 1.2 equiv of arene to B2pin2;
conversions are based on the arene substrate. Isolated yields are of a
mixture of a and c.
EXPERIMENTAL SECTION
■
The results described above are a clear indication that the
nature of the ligand has subtle influences on the catalytic cycle
that can determine the catalyst efficiency and selectivity. In an
effort to isolate an active catalyst, several protocols were
examined to incorporate ligand 17 into an iridium complex.8,39
All attempts to form a trisboryl complex from [Ir(μ-
OMe)(COD)]2, 17, and B2pin2 or HBpin by methods
analogous to those reported in the literature provided complex
mixtures. Direct addition of 17 to [Ir(μ-Cl)(COD)]2, however,
provided iridium complex 27 as a yellow solid in quantitative
yield and high purity (eq 4).40 Recrystallization by slow
General Methods. Oxygen- and moisture-sensitive or hygroscopic
materials were handled under purified nitrogen in an inert atmosphere
glovebox. All solvents were dried and degassed by standard procedures
unless used for purification. TLC analysis used 500 μm, 60 Å silica
layer fluorescence UV plates. Flash chromatography was carried out on
hand-packed columns of Brockman I basic aluminum oxide, 50−200
μm, 60 Å. NMR spectra were collected on an NMR spectrometer at
400 or 500 MHz for 1H NMR and 125 MHz for 13C NMR. 1H NMR
spectra are referenced to CDCl3 at 7.26 ppm or to an internal
tetramethylsilane (TMS) standard at 0.00 ppm. All benzylic amines
were synthesized by known procedures.29 All deuterated solvents,
bis(pinacolato)diboron, [Ir(μ-OMe)(COD)]2, [Ir(μ-Cl)(COD)]2,
and ligands were purchased and used without further purification.
Complex 27 was examined by single-crystal Xray diffraction using a
Bruker APEX-II Platform CCD diffractometer using graphite-
monochromated Mo Kα radiation (λ = 0.710 73 A) by ω scans at
100 K. The structure was solved and refined using the
SHELXTLPLUS package. The structure was solved by direct methods.
Representative Procedure for the C−H Borylation of
Benzylic Amines with Various Bidentate Ligands. To a 50 mL
PTFE-valved reaction tube, equipped with a stir bar and charged with
[Ir(μ-OMe)(COD)]2 (0.020 g, 0.030 mmol), were added 6-methyl-2-
(aminomethyl)pyridine (7, 0.0073 g, 0.060 mmol), B2pin2 (0.424 g,
1.67 mmol), N,N-dimethylbenzylamine (0.270 g, 2.00 mmol), and 4.0
mL of methylcyclohexane. After 16 h at 70 °C, the volatiles were
removed in vacuo. The unpurified reaction mixture was analyzed by 1H
NMR spectroscopy, and characteristic peaks for 1, 2,29 and N,N-
dimethylbenzylamine (typically the benzylic methylene hydrogens)
were used to determine the reaction conversion and selectivity.
Representative Procedure for the C−H Borylation of
Substituted Benzylic Amines with N-Benzylaminopyridine
(17). To a 50 mL PTFE-valved reaction tube, equipped with a stir
bar and charged with [Ir(μ-OMe)(COD)]2 (0.020 g, 0.030 mmol),
were added N-benzylaminopyridine (17) (0.011 g, 0.060 mmol),
B2pin2 (0.424 g, 1.67 mmol), 3-chloro-N,N-dimethylbenzylamine
(0.339 g, 2.00 mmol), and 4.0 mL of methylcyclohexane. After 16 h
at 70 °C, the volatiles were removed in vacuo. The unpurified reaction
evaporation from tetrahydrofuran and pentane provided X-ray
quality crystals that were used to assign the structure
unambiguously (Figure 3). Complex 27 is a square-planar,
Ir(I), d8 complex in which the diamine ligand is bound in a
monodentate fashion through the pyridine nitrogen.
All attempts to convert complex 27 to a boron-containing
active catalyst by addition of B2pin2 or HBpin were
unsuccessful. Subjecting complex 27 to the reaction conditions
for C−H borylation, however, demonstrated that the complex
is catalytically active.41 Notably, the catalytic reactivity of 27 is
similar to the standard reaction conditions when [Ir(μ-
Cl)(COD)]2 and 17 are added separately and are significantly
improved over the conditions with [Ir(μ-Cl)(COD)]2 in the
absence of added ligand.41
In summary, a detailed ligand screen has been reported that
demonstrates the effects of steric congestion, electronics, and
the bite angle of the bidentate ligand. The bite angle of the
ligand was found to have the most significant effect on
improving both conversion and selectivity of mono-borylation
products over bis-borylation products. N-Benzylaminopyridine
was identified as the optimal ligand for substrate-directed C−H
1
mixture was analyzed by H NMR spectroscopy, and characteristic
peaks for 22a, 22b, 22c,29 and 3-chloro-N,N-dimethylbenzylamine
(typically the benzylic methylene hydrogens) were used to determine
the reaction conversion and selectivity. Purification by chromatog-
raphy with basic aluminum oxide (98.5:0.5:1 dichloromethane/
methanol/triethylamine) followed by bulb-to-bulb distillation (30
min at 60 °C, 0.5 mmHg), to remove unreacted starting material,
D
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provided 22 (0.453 g, 1.53 mmol, 77%) as a 90:8:2 mixture of 22a/
22b/22c.
(14) Preshlock, S. M.; Ghaffari, B.; Maligres, P. E.; Krska, S. W.;
Maleczka, R. E., Jr.; Smith, M. R., III. J. Am. Chem. Soc. 2013, 135,
7572.
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2005, 127, 10539.
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Webster, C. E.; Hall, M. B. J. Am. Chem. Soc. 2005, 127, 2538.
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Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 13684.
(2-Benzylaminopyridine)chloro(1,5-cyclooctadiene)iridium-
(I) (27). To a round-bottom flask containing [Ir(μ-Cl)(COD)]2
(0.372 mmol, 0.250 g) and 17 (0.744 mmol, 0.137 g) was added
THF (10.0 mL). The reaction was stirred at room temperature for 7 h,
then concentrated to give a yellow, amorphous solid (0.3879 g, 0.746
mmol, 100%). Suitable crystals for X-ray analysis were grown from a
solution of 27 (20.2 mg) in tetrahydrofuran/pentane (2:1, 0.75 mL)
by slow evaporation. Mp: 153.3−158.0 °C. 1H NMR (500 MHz,
CDCl3): δ 8.07 (d, J = 5.9 Hz, 1H), 7.43 (d, J = 7.6 Hz, 2H), 7.39−
7.28 (m, 5H), 6.61 (t, J = 6.9 Hz, 1H), 6.41 (d, J = 8.6 Hz, 1H), 4.53
(d, J = 6.0 Hz, 2H), 4.46 (br s, 2H), 3.25 (br s, 2H), 2.23 (br s, 4H),
1.66−1.63 (m, 2H), 1.53−1.49 (m, 2H). 13C NMR (125 MHz,
CDCl3): δ 157.1, 147.3, 137.7, 137.3, 128.8, 127.6, 126.9, 113.7, 107.9,
71.0 (br), 46.9, 32.2 (br), 31.1 (br). IR (thin film, CDCl3): 3280 (br),
2936 (m), 2911 (m), 2880 (m), 2833 (m), 1616 (s), 1570 (s), 1521
(s), 1448 (s) cm−1. HRMS (EI): calcd for C20H24ClIrN2 [M + Na]+
m/z 541.1132, found [M + Na]+ 541.1128. The structure was
confirmed by X-ray crystallography.41
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ASSOCIATED CONTENT
■
S
* Supporting Information
1
(26) Kawamorita, S.; Miyazaki, T.; Ohmiya, H.; Iwai, T.; Sawamura,
Representative H NMR spectral data of crude and purified
M. J. Am. Chem. Soc. 2011, 133, 19310.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: clarkt@sandiego.edu.
■
(29) Roering, A. J.; Hale, L. V. A.; Squier, P. A.; Ringgold, M. A.;
Wiederspan, E. R.; Clark, T. B. Org. Lett. 2012, 14, 3558.
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The authors declare no competing financial interest.
(31) Crawford, K. M.; Ramseyer, T. R.; Daley, C. J. A.; Clark, T. B.
Angew. Chem., Int. Ed. 2014, 53, 7589.
(32) Boller, T. M.; Murphy, J. M.; Hapke, M.; Ishiyama, T.; Miyaura,
ACKNOWLEDGMENTS
■
N.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 14263.
We acknowledge financial support from the National Science
Foundation (CHE-1151092), the University of San Diego, and
Research Corporation for Science Advancement. The National
Science Foundation is thanked for NMR (0417731) and X-ray
(CHE-1126585) facilities at USD. We also thank Dr. John
Greaves (University of California, Irvine) for mass spectrom-
etry and Dr. Victor G. Young, Jr. (University of Minnesota) and
Dr. Christopher J. A. Daley (University of San Diego) for
assistance with X-ray data analysis.
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