Understanding the Activation of Air-Stable Ir(COD)(Phen)Cl Precatalyst for C-H Borylation of Aromatics and Heteroaromatics

Article abstract of DOI:10.1021/acs.orglett.0c04210

A newly developed robust catalyst [Ir(COD)(Phen)Cl] (A) was used for the C-H borylation of three dozen aromatics and heteroaromatics with excellent yield and selectivity. Activation of the catalyst was identified by the use of catalytic amounts of water, alcohols, etc., when B2pin2 was used in noncoordinating solvents, while for THF catalytic use of HBpin was required. The results were on par with the in situ based expensive system [Ir(OMe)(COD)]2/dtbbpy or Me4Phen.

Full text of DOI:10.1021/acs.orglett.0c04210

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Letter
Understanding the Activation of Air-Stable Ir(COD)(Phen)Cl
Precatalyst for C−H Borylation of Aromatics and Heteroaromatics
Eric D. Slack* and Thomas J. Colacot*
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ABSTRACT: A newly developed robust catalyst [Ir(COD)(Phen)Cl] (A) was used for the C−H borylation of three dozen
aromatics and heteroaromatics with excellent yield and selectivity. Activation of the catalyst was identified by the use of catalytic
amounts of water, alcohols, etc., when B2pin2 was used in noncoordinating solvents, while for THF catalytic use of HBpin was
required. The results were on par with the in situ based expensive system [Ir(OMe)(COD)]2/dtbbpy or Me4Phen.
Introduction. Pioneering reports by Hartwig/Ishiyama1 and
Maleczka/Smith III2 in the early 2000s propelled the area of
iridium-catalyzed borylation to be a viable alternative route to
aryl and heteroaryl boronates via C−H borylation for a variety of
highly desirable intermediates and products.3 While the earlier
works of Maleczka/Smith III using phosphine based systems2a−c
have not gained popularity due to lower conversions, both
groups eventually identified [Ir(OMe)(COD)]2 as the preferred
precatalyst in conjunction with either 4,4′-di-tert-butyl-2,2′-
bipyridyl (dtbbpy) or 3,4,7,8-tetramethyl-1,10-phenanthroline
(Me4Phen) as a ligand for improved activity.1,3,4 [Ir(OMe)-
(COD)]2 being employed due to the formation of the
innocuous MeOBpin byproduct versus the use of [Ir(Cl)-
(COD)]2 which generates ClBpin and subsequently reacting
with the nitrogen/phosphine based ligands, thereby preventing
ligand complexation with Ir.5−7 Metal-catalyzed borylation
technology has allowed for the total synthesis of various natural
products such as ( )-thysanone,8 (+)-complanadine,9 and drug
targets which would be cumbersome to prepare by classical
methods.10,11 Extensive use of the catalyst systems [Ir(MeO)-
(COD)]2/Me4Phen or dtbbpy led to the notion that this
combination creates the most efficient Ir-based catalyst systems
for C−H borylations with the broadest substrate scope.3,10,11
Although high quality [Ir(OMe)(COD)]2 was originally
commercialized by our group on multigram quantities, multi-
kilogram implementation had presented challenges due to shelf-
life concerns and batch-to-batch variations, as documented by
both academia and industry.12−14 While our attempts to
synthesize a stable [Ir(OMe)(COD)(Phen)] were not
successful, we focused our efforts toward identifying method-
ologies in accessing stable complexes from [Ir(Cl)(COD)]2.
From scattered reports on the use of other electronically diverse
diamine15,16 and phosphine ligands17 for borylations, we saw
this as a possible platform to create highly active and selective Ir
precatalysts. Recently, the importance of these ligands was
exemplified by Baran et al. for the synthesis of the biologically
active Verruculogen and Fumitremorgin A where the role of the
ligand was crucial for regioselective C−H borylation.18 Based on
reports from other laboratories19 and from our own lab20 in
developing effective palladium based precatalysts, we hypothe-
sized that a more stable preformed iridium chloride complex like
A21 (Scheme 1) with the overlooked ligands, such as
phenanthrene (Phen), may improve the catalytic outcome by
preventing interactions of the ligand and proposed “off-cycle”
formation of ClBpin.5−7 Recently we were able to briefly
Scheme 1. Divergent Synthesis of Catalytically Active, A, and
21
Inactive, B, from 1,10-Phenanthroline and [Ir(Cl)(COD)]2
Received: December 20, 2020
Published: February 5, 2021
https://dx.doi.org/10.1021/acs.orglett.0c04210
© 2021 American Chemical Society
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demonstrate the merits of A as precatalyst versus the respective
in situ formed catalyst for the borylation of 1 as a model system
(Scheme 2).21 We found that the formation of the competent
Table 1. Activation Studies of A for the Borylation of 1
Scheme 2. Activity Comparison of Complexe A and B21
additive
(0.5 mol %)
time
(h)
a
no.
“Ir” (0.5 mol % Ir)
solvent
conv
c
b
1
A
A
none
H2O
none
iPrOH
NaOtBu
HOtBu
iPrOH
H2O
heptane
heptane
heptane
heptane
heptane
heptane
0−20%
100%
90%
100%
5%
15%
0%
72%
69%
99%
18
8
7
c
b
b
b
b
b
2
c
3
[Ir(OMe)(COD)]2
d
4
A
A
A
A
A
A
A
2
d
precatalyst was highly dependent on the reaction conditions
employed (Scheme 1). We exemplified this by mixing
[Ir(Cl)(COD)]2 with Phen using the solvent of choice
(hexanes) for borylation forming an inactive green cationic
complex B as shown in Scheme 1. While the same combination
of ligand with [Ir(Cl)(COD)]2 in THF led to a purple colored
precatalyst A, which after isolation could be activated, leading to
high conversion of 1 to 1a at low, 0.5 mol %, Ir loading (Scheme
2).21
The formation of B under the in situ conditions had
contributed to the poor performance of the in situ [Ir(Cl)-
(COD)]2/Phen system and hence was overlooked previously.
Although THF was required to make the active precatalyst A,
our recent deliberate efforts to utilize THF as a solvent for
broadening the scope of the reaction using A often gave erratic
or poor results. This prompted us to systematically investigate
the activation of A to create a robust precatalyst system. Herein,
we report our results of this study, which will hopefully promote
the borylation technology widely for both academia and
industry.
5
18
18
1
4
4
d
6
d
e
7
THF
d
e
8
THF
THF
d
e
9
NaOMe
HBpin
d
e
10
THF
4
a
b
c
Determined by GC analysis. 0.33 M. Reaction conditions: “Ir” (0.5
mol %), additive (0.5 mol %), B2pin2 (1.1 equiv), solvent, 80 °C, 1 (1
d
equiv), 80 °C. Reaction conditions: (1) A (0.5 mol %), B2pin2 (1.1
equiv), additive (0.5 mol %), solvent, 80 °C, 1 h. (2) 1 (1 equiv), 80
e
°C. 1.4 M.
Supporting Information, we proposed a similar intermediate D
(Scheme 3), analogous to the reported intermediate C.5 Our
Scheme 3. Plausible Activation of A with Additives
Results and Discussion. Since THF gave erratic results, we
decided to reinvestigate the reaction with heptane as the solvent
(Scheme 2). Surprisingly, erratic results were also obtained from
borylations in heptanes even when using A. We initially
suspected that trace moisture in purchased anhydrous solvent
might be altering the activity of A, which prompted us to
conduct the study by mixing a catalytic amount of A with model
substrate 1 and bis(pinacolato)diboron (B2pin2) using rigor-
ously dried heptanes at 80 °C using our conditions.21
Surprisingly, it gave only trace conversion (ca. 10% by GC) of
starting material, 1 to 1a (Table 1, entry 1). To account for the
possibility that A might be deactivated by one of the reactants in
our reaction mixture, we varied the order of addition of
substrate, catalyst, reagent, and solvent. All perturbations14 led
to the same outcome: no reaction! Aware of Hartwig’s proposed
active Ir borylation catalyst (dtbbpy)Ir(Bpin)3(COE) (C)5 we
sought to determine how complex A might transform to a similar
species under our reaction conditions. However, our efforts to
isolate the phenanthroline analogue of C following methodology
developed for dtbbpy5 and Me4Phen14 was not successful.
Previous work from Hartwig’s group had shown that similar to
the activity of the [Ir(OMe)(COD)]2 complex, [Ir(OH)-
(COD)]2 was also catalytically competent in the presence of
either Me4Phen or dtbbpy ligand.1b This suggests that a similar
pathway to Hartwig’s proposed mechanism might be occurring
in our system.5 To test whether adventitious water was causing
erratic results in our studies, we deliberately added a catalytic
amount of H2O (1:1/Ir:H2O) to rigorously dried heptanes.
Notably, this led to quantitative conversion of our model
substrate 1 (Table 1, entry 2) to 1a consistently. In addition,
MeOH, iPrOH, and NaOMe also gave good results. From the
reported mechanisms and experiments described in 1d from the
efforts to isolate D, or [Ir(OH)(Phen)(COD)], or the
corresponding −OMe complex were not successful; however,
aldehydes were observed when higher boiling alcohols (i.e. 1-
octanol and 1-hexanol). Although we propose the formation of
[Ir(OH)Phen(COD)] in analogy with Hartwig’s observation,
the corresponding pathway from this catalyst to D has not yet
been studied. Hartwig had shown the effectiveness of the
methoxide addition to [Ir(Cl)(COD)]2/dtbbpy; however, it
produced decreased yields.1b In addition, previous methods
using [Ir(OMe)(COD)]2/Me4Phen or dtbbpy showed im-
proved activity while premixing the ligand with HBpin/B2pin2
prior to the addition of substrate, with the observation of a dark
red solution at rt.14 When we attempted the same process using
A mixed with B2pin2 followed by the addition of water or
isopropanol at room temperature (25−27 °C), a heterogeneous
mixture was observed, which upon heating at 80 °C for 1 h
changed to a homogeneous red solution. Addition of the
substrate to the above solution with continued heating for 2 h
gave complete conversion (Table 1, entry 4), versus 8 h when 1
was added prior to heating (Table 1, entry 2). Similar results
were obtained for NaOMe, MeOH, iPrOH, and H2O
respectively, while negative results were obtained for both
NaOtBu and tBuOH (Table 1, entries 5 and 6). Although
catalyst A was synthesized using THF as a solvent, the
conditions employed above for heptanes gave complications
due to borylation of THF.22 While iPrOH could be used for full
conversion of 1 in heptanes, its use in THF was unsuccessful,
giving only trace conversion (Table 1, entry 7). Interestingly, 1
equiv of HBpin w.r.t. A gave complete conversion (Table 1,
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entry 10) versus the use of water or sodium methoxide (Table 1,
entries 8 and 9).
loadings (Table 2, 15a, 18a). However, this was in very good
agreement with previously disclosed methods using both dtbbpy
and Me4Phen in conjunction with [Ir(OMe)(COD)]2.3,11 The
borylation of (−)-nicotine, 18, was of particular interest due to
its pharmaceutical applications. We were able to reduce the Ir
loading by half using A versus the reported 1 mol %
[Ir(Cl)(COD)]2/Me4Phen in situ system for nearly the same
yield up to a 10 g scale.12 The order of addition and activation of
the catalyst was important with respect to substrate 20. Addition
of substrate 20 prior to the activation of the catalyst gave no yield
of 20a, presumably due to coordination of 20 to Ir. With our
protocol 20a was isolated in 87% yield even at 0.5 mol % catalyst
loading (Table 2).
Ozerov et al. performed a thorough analysis of turnover
numbers (TON) for several substrates and catalyst along with
their recently developed catalyst [(5-methyl-1,3-phenylene)bis-
(oxy))bis(diisopropylphosphane)], (Ir(POCOP)).23 We se-
lected 4 as a model substrate considering its regioselectivity in
lieu of benzene. Under our optimized procedure, we obtained a
TON of 1940 within 24 h for 4a with a yield of 97%, in
comparison their TONs of 1720 (Scheme 4 and Table 2, 4a).5,24
Scheme 4. Comparison of Ir(POCOP) Catalyst and Catalyst
A TON for the Borylation of 1,2-Dichlorobenzene24
While THF is considered to be an inferior solvent, likely due
to byproduct formation,22 its use was required for substrates
insoluble in heptanes and MTBE.24 Efforts to apply the same
methodology used with heptanes gave inferior results in
accordance with the literature results.22,25 Through optimization
using substrate 23, we found that the ideal concentration of
substrate was ca. 1.4 M, which gave similar conversions as those
obtained in heptanes (Table 3, 23a). With optimized conditions
Table 2. Borylation of Aromatic Substrates
Table 3. Optimization of Conditions Using THF as a
a
Solvent
a
Conditions: (1) A (0.5 mol %), “HBpin−“ (0.5 mol %), B2pin2 (1.1
equiv), solvent, 80 °C, 1 h. (2) 1 (1 equiv), 80 °C.
in THF for 23, we explored the scope for an array of substrates,
including pyridines and diazole-based substrates. Pyridines
substituted ortho to the heteroatom gave good conversion at 0.5
mol % Ir loading (Table 4, 25a, 28a, 29a, 30a, and 31a).
Table 4. Borylation of Heteroaromatic Substrates
a
b
A, B2pin2, iPrOH,substrate, and heptanes. 80 °C. (1) A, B2pin2,and
iPrOH were loaded in heptanes and heated at 80 °C for 1 h. (2)
c
d
e
Substrate. Run at 5 and 10 g scale. Run at 1 mmol. Substrate added
before induction/activation.
a
Reaction conditions: (1) A, B2pin2, and HBpin were loaded, diluted
b
with THF, and heated at 75 °C for 1 h. (2) Substrate added. NMR
yield of all isomers. Mixture of of isomers (1:4 ratio) at 80 °C.
c
Using the same methodology we saw similar trends for other
substrates, giving complete conversion (high yields) with low Ir
loadings (Table 2, entries 4a−11a), except for substrate 8,
where the loading had to be increased to 2 mol % (Table 2, 8a).
Mildly coordinating carbonyl groups and tertiary amines also
showed good reactivity at 0.5 mol % loading of Ir (Table 2,
entries 12a, 16a, and 17a). Substrates with stronger
coordinating groups such as nitriles and pyridines, in heptanes,
were more challenging and hence required higher catalyst
Counter to expectations, optimized conditions in THF
improved the borylation yield of 15a to 87% from 30% in
heptanes (Table 2 and 4, 15a), demonstrating the importance of
the conditions. Curiously when this methodology was applied to
produce 30a and 30b, we obtained a 1:4.5 isomer ratio with 84%
conversion by 1HNMR, a result similar to what was previously
disclosed with the [Ir(OMe)(COD)]2/dtbbpy system at rt.1b
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Transition to MTBE, often used to avoid difficulties with THF,
at 80 °C as well gave the same isomer ratio. However, activating
A (3 mol %), followed by cooling to 25−26 °C, followed by the
addition of 2,5-dichloropyridine (30) while stirring for 18 h gave
an improved 1:8 isomer ratio of borylated products with 86%
conversion (Table 5). We applied this methodology (Table 6)
AUTHOR INFORMATION
Corresponding Authors
■
Eric D. Slack − Johnson Matthey, West Deptford, New Jersey
08066, United States; Email: eric.slack@jmusa.com
Thomas J. Colacot − Johnson Matthey, West Deptford, New
Jersey 08066, United States; orcid.org/0000-0001-9976-
1376; Email: thomas.colacot@milliporesigma.com,
tcolacot@yahoo.com
Table 5. Selectivity Improvement of A for the Borylation of
2,5-Dichloropyridine24a
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c04210
Notes
The authors declare the following competing financial
interest(s): Johnson Matthey has a financial interest in
Compound A.
a
ACKNOWLEDGMENTS
Reaction conditions: Ir source, B2pin2, solvent, 1 h at temperature.
■
b
1
Then substrate was added. Determined by H NMR. From ref 27.
This paper is dedicated to the loving memory of Prof. Victor
Snieckus (1937−2020), for his pioneering work in the area of
C−H functionalization, with emphasis on directed ortho
metalation. We thank Dr. Peter Gildner (FMC Corporation,
Newark, Delaware) for the preliminary work at the Colacot
group. We also thank Dr. Maria Luisa Palacios-Alcolado for her
full support to finish this project successfully.
Table 6. Ortho Borylation Using Traceless Directing Phenols
Applying Catalyst A
a
REFERENCES
a
■
Reaction conditions: (1) A, HBpin, B2pin2, THF 75 °C, 1 h. (2)
b
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c04210.
Experimental details and spectral data of all compounds
(PDF)
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