Cobalt(III)-Catalyzed C-H Bond Amidation with Isocyanates

Article abstract of DOI:10.1021/acs.orglett.5b00910

The first examples of cobalt(III)-catalyzed C-H bond addition to isocyanates are described, providing a convergent strategy for arene and heteroarene amidation. Using a robust air- and moisture-stable catalyst, this transformation demonstrates a broad iso

Full text of DOI:10.1021/acs.orglett.5b00910

Letter
pubs.acs.org/OrgLett
Cobalt(III)-Catalyzed C−H Bond Amidation with Isocyanates
Joshua R. Hummel and Jonathan A. Ellman*
Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520-8107, United States
S
* Supporting Information
ABSTRACT: The first examples of cobalt(III)-catalyzed C−
H bond addition to isocyanates are described, providing a
convergent strategy for arene and heteroarene amidation.
Using a robust air- and moisture-stable catalyst, this trans-
formation demonstrates a broad isocyanate scope and good
functional-group compatibility and has been performed on gram scale.
n recent years, many elegant strategies employing transition-
metal-catalyzed C−H bond functionalization have emerged
product 3a in 74% yield (Table 1, entry 1). Given that solvent
effects have been observed to play a key role in obtaining
I
for the synthesis of amines and nitrogen heterocycles.1 Among
the rich array of metal complexes that mediate C−H
functionalization, Rh(III)-catalysts have proven to be excep-
tionally versatile due to their unique reactivity and high
functional-group compatibility,2 with additions of C(sp2)−H
bonds to polarized π-bonds providing for convergent
introduction of heteroatom functionality.3−9 In this regard,
we reported direct C(sp2)−H bond addition to isocyanates as a
particularly step- and atom-economic strategy for the
preparation of aromatic, heterocyclic, and alkenyl amides.6f
Direct C(sp2)−H bond additions to isocyanates have also been
accomplished with Re10 and Ru11 catalysts.12 In contrast,
catalytic C−H bond functionalization with earth-abundant first-
row transition metals has emerged only recently,13 and to our
knowledge, additions to isocyanates have not been described.
Herein we report the first examples of cobalt-catalyzed C−H
bond amidation with isocyanates.14,15 This convenient bench-
top procedure is effective for multiple heterocycle directing
groups, shows good functional group compatibility and a broad
scope for aromatic and alkyl isocyanates, and is readily scalable.
For initial evaluation of Co(III)-catalyzed C−H bond
additions to isocyanates, we chose 1-phenyl-1H-pyrazole (1a)
and phenyl isocyanate (2a) as the coupling partners. First
developed by Kanai, Matsunaga, and co-workers for additions
to sulfonyl imines14p we anticipated that the cationic preformed
catalyst [Cp*Co(C6H6)][PF6]2 (4a) might also facilitate C−H
bond amidation with isocyanates (Figure 1). Indeed, the
desired reactivity was achieved when catalyst 4a was utilized in
the presence of catalytic potassium acetate at 80 °C, providing
Table 1. Optimization of Reaction Conditions for Co(III)-
a
Catalyzed Amidation with Phenyl Isocyanate
Co(III)-catalyst
(mol %)
KOAc
(mol %)
temp
(°C)
b
entry
solvent
yield (%)
1
2
3
4
5
4a (10)
4a (10)
4a (10)
4a (10)
4a (10)
4a (2.5)
4a (1)
20
20
20
20
20
5
80
1,4-dioxane
THF
74
62
66
8
80
80
DCE
80
toluene
cd
,
120
120
120
120
120
100
120
100
120
120
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
84 (84)
c
d
6
80 (79)
7
2
66
84
84
60
5
e
8
f
4a (10)
4a (10)
4a (2.5)
4b (5)
4c (2.5)
4a (10)
−
20
20
5
9
10
11
12
13
14
20
5
57
7
−
20
−
a
Conditions: 2a (1.0 equiv), 1a (2.0 equiv) in 1,4-dioxane (2.0 M) for
b
20 h. Determined by GC analysis relative to tetradecane as an
c
external standard. Reaction run on the benchtop under nitrogen.
d
e
Isolated yield at 0.20 mmol scale. Reverse stoichiometry: 2a (2.0
f
equiv), 1a (1.0 equiv). Reaction conducted in 1,4-dioxane (0.5 M).
optimal yield in Ru(II)-11 and Rh(III)-catalyzed C−H
amidations,6 different solvents were evaluated. While the use
of the ethereal solvents 1,4-dioxane and tetrahydrofuran
(entries 1 and 2) as well as 1,2-dichloroethane (entry 3)
provided comparable yields, the nonpolar and noncoordinating
Received: March 29, 2015
Figure 1. Cobalt(III)-catalysts used for C−H functionalization.
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b00910
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Letter
solvent toluene resulted in a low yield (entry 4). Ultimately, the
higher boiling solvent 1,4-dioxane was selected for further
reaction optimization because it allowed reactions to be
conducted at higher temperatures.
Scheme 1. Substrate Scope for Amidation of N-Aryl-1H-
Pyrazoles with Isocyanates
a b
,
Performing the reaction at 120 °C rather than 80 °C
moderately increased the yield (entry 1 vs 5). This reaction is
amenable to a benchtop setup, providing an isolated yield of
84%, identical to that achieved with a glovebox setup (entry 5).
Reducing the catalyst loading from 10 to 2.5 mol % did not
significantly influence the reaction outcome for this substrate
combination (entry 5 vs 6), although at a 1 mol % catalyst
loading the yield dropped to 66% (entry 7). Using the reverse
stoichiometry with 1a as the limiting reagent provided an
identical yield to that achieved under standard conditions
(entry 5 vs 8), although isolation of pure product was more
challenging due to byproduct formation. Conducting the
reaction at a concentration of 0.5 M did not affect the reaction
yield (entries 5 vs 9); however, the higher concentration of 2.0
M was selected to provide conditions that minimize solvent
waste. Lowering the reaction temperature to 100 °C to operate
below the boiling point of 1,4-dioxane resulted in a moderate
drop in yield to 60% when 2.5 mol % of 4a was employed
(entries 6 vs 10). The noncationic dimeric complex
[Cp*CoCl2]2 (4b) provided only a 5% yield even at a higher
catalyst loading and an elevated temperature (entry 11). When
operating below the solvent boiling point, a comparable yield
was observed with preformed cationic catalyst 4c relative to 4a
(entry 10 vs 12). This result indicates that, for Co(III)-
catalyzed C−H bond additions to isocyanates, a completely
noncoordinating counterion provides no rate enhancement
relative to PF6.16 Removal of potassium acetate dramatically
reduced the yield of desired product (entry 13). Moreover, no
product was observed when catalyst 4a was excluded (entry
14), demonstrating that a Co(III)-catalyst is required for this
C−H functionalization. Due to the lower cost of the PF6
counterion, cobalt catalyst 4a was selected for evaluating the
substrate scope. A catalyst loading of 10 mol % was used to
achieve higher conversion for less reactive substrates (see
Scheme 1). Finally, given that C−H bond amidation with
isocyanates has been reported for Re, Ru, and Rh, ICP-MS
analysis of Co(III)-catalyst 4a was performed to detect the
levels of several second and third row metals known to mediate
C−H functionalization. Complex 4a showed less than 1 ppm of
Re, Ru, Rh, Ir, and Pt, and Pd at 1 ppm (see Supporting
Information for details).
a
Conditions: isocyanate 2 (0.20 mmol), N-aryl-1H-pyrazole 1 (0.40
b
mmol) in 1,4-dioxane (2.0 M) at 120 °C for 20 h. Isolated yields after
chromatography. Reaction conducted using 2.5 mol % of catalyst 4a
and 5 mol % of KOAc.
The substrate scope was first evaluated with a series of
functionalized isocyanates 2 and N-aryl-1H-pyrazoles 1
(Scheme 1). For aromatic isocyanates, electron-rich (3b),
electron-neutral (3c, 3e), and electron-deficient (3d, 3f−h)
derivatives all coupled in high yield. Additionally, high yields
were maintained when substituents were placed at the para
(3b−d), meta (3e−g), and ortho (3h) positions. Linear alkyl
isocyanates coupled to provide the N-benzyl amide 3i and N-
hexyl amide 3j in good yields, while a branched alkyl isocyanate
gave the N-cyclohexyl amide 3k in moderate yield. A higher 10
mol % catalyst loading was necessary for some of the
isocyanates. For example, while a good yield was obtained
when employing 2.5 mol % of catalyst 4a in the synthesis of 3a,
a dramatically lower 19% yield of 3j was observed at this lower
loading for hexyl isocyanate.
c
electron-deficient (3o) substituents. For meta-substituted
derivatives, C−H functionalization occurred exclusively at the
less hindered position to give amides 3p and 3q, thereby
demonstrating that regioselectivity can be controlled by steric
effects.17 Notably, incorporation of both bromo (3n) and
methyl ketone (3o) groups proceeded in good yield,
highlighting the functional-group compatibility of Cp*Co(III)
catalysis, which consequently enables subsequent transforma-
tions for late stage introduction of molecular complexity.
The directing group scope of this reaction was also
investigated (Scheme 2). In addition to the amidation of N-
aryl-1H-pyrazoles, the 2-pyridyl (3r) and 2-pyrimidinyl (3s)
directing groups also provided amide products albeit in
modestly diminished yields. The 2-pyrimidyl directing group
A series of substituted N-aryl-1H-pyrazoles 1 were then
evaluated and provided amide products 3 in good yields for
electron-rich (3l, 3p), electron-neutral (3m−n, 3q), and
B
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Letter
showcase the ability of earth-abundant Cp*Co(III) catalysis to
mediate practical synthetic transformations.
Scheme 2. Directing Group Scope for C−H Bond
a b
,
Amidation
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, gram scale synthesis, and characterization
data. The Supporting Information is available free of charge on
the ACS Publications website at DOI: 10.1021/acs.or-
glett.5b00910.
AUTHOR INFORMATION
Corresponding Author
■
* E-mail: jonathan.ellman@yale.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Institutes of Health
(R01-GM069559). We thank the reviewer who suggested that
we perform ICP-MS analysis of Co(III)-catalyst 4a.
a
Conditions: 2a (0.20 mmol), directing group 1 (0.40 mmol) in 1,4-
b
dioxane (2.0 M) at 120 °C for 20 h. Isolated yields after
chromatography.
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D
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