Mechanochemical Rhodium(III)-Catalyzed C-H Bond Amidation of Arenes with Dioxazolones under Solventless Conditions in a Ball Mill

Article abstract of DOI:10.1021/acscatal.7b00582

A procedure for the direct mechanochemical rhodium(III)-catalyzed C-H bond amidation of arenes with 1,4,2-dioxazol-5-ones as the nitrogen source has been developed. The transformation proceeds under solventless conditions and does not require additional h

Full text of DOI:10.1021/acscatal.7b00582

Letter
pubs.acs.org/acscatalysis
Mechanochemical Rhodium(III)-Catalyzed C−H Bond Amidation of
Arenes with Dioxazolones under Solventless Conditions in a Ball Mill
Gary N. Hermann and Carsten Bolm*
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen (Germany)
S
* Supporting Information
ABSTRACT: A procedure for the direct mechanochemical
rhodium(III)-catalyzed C−H bond amidation of arenes with
1,4,2-dioxazol-5-ones as the nitrogen source has been developed.
The transformation proceeds under solventless conditions and does
not require additional heating. The corresponding ortho amidated
products are formed in high yields and in shorter reaction times
than in solution.
KEYWORDS: ball mill, C−H amidations, rhodium catalysis, mechanochemistry, solvent-free reactions
itrogen-containing molecules are important for natural
azides served as nitrogen sources (Scheme 1a). Inspired by the
Nproducts, agrochemicals, and pharmaceuticals. Conse- reports on Rh(III)-catalyzed C−H bond amidation processes3
1
quently, in the past decades, the development of efficient C−N
bond formation processes2 has been extensively explored. In
Scheme 1. C−H Bond Amidation Processes under
Mechanochemical Conditions (DG = Directing Group)
recent years, transition-metal-catalyzed C−H bond functional-
ization of traditionally unreactive C−H bonds have been
established as a potent tool to generate a wide range of new
carbon−carbon and carbon−heteroatom bonds. Along these
lines, Chang and others3−5 reported transition-metal-catalyzed
C−H amidation reactions using dioxazolones6,7 as the
amidation reagents where carbon dioxide is formed as the
sole byproduct. For such transformations, [Cp*Rh(III)]
catalysts3 have been proven to be highly efficient. Commonly
significant amounts of solvents are used, and in most of the
cases, relatively harsh reaction conditions are required, still
limiting the overall sustainability of the processes.
In the past decades, ball-milling has been established as a
useful tool for the implementation of a wide range of organic
transformation by means of mechanochemical activation.8 Such
mechanochemically promoted processes often provide advan-
tages compared to common solvent-based methods, including
higher yields, shorter reaction times, lower catalyst loadings,
and mostly the avoidance of organic solvents and elevated
reaction temperatures.9 Also altered reactivity profiles have
been observed.10 In this context, our group reported the first
catalytic mechanochemical C−H bond functionalization.11,12
Under solvent-free conditions, the active Rh(III) species were
formed in situ allowing to couple acetanilides with olefins in an
oxidative Heck-type olefination with dioxygen as the terminal
oxidant. In addition, we applied mechanochemical conditions
to an Ir(III)-catalyzed C−H bond amidation process.13 There,
the mechanochemical activation led to the formation of an
active iridium species, thus enabling an ortho-selective C−N
bond formation process in the absence of solvent. Organic
and encouraged by our previous work,12,13 we wondered
whether dioxazolones could be employed as nitrogen sources in
mechanochemically induced catalytic amidation processes.
Herein, we report on the realization of this concept (Scheme
1b).
As a starting point, we used similar reaction conditions to
those reported by Chang.3a For this, N-(tert-butyl)benzamide
(1a) and methyl-1,4,2-dioxazol-5-one (2a) were treated with a
combination of [{Cp*RhCl2}2] (2.5 mol %) and AgSbF6 (10
mol %) as the catalyst and AgOAc (10 mol %) as an additive in
a mixer mill. To our delight, after milling the reaction mixture
for only 99 min at 30 Hz, the amidated product 3a was formed
in 76% yield (Table 1, entry 1). Substituting [{Cp*RhCl2}2] by
Received: February 22, 2017
Revised: May 30, 2017
Published: June 5, 2017
© XXXX American Chemical Society
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Letter
Table 1. Optimization of the Rh(III)-Catalyzed C−H
Scheme 2. Mechanochemical Rh(III)-Catalyzed C−H Bond
Amidation of Benzamides
a
Amidation under Mechanochemical Conditions
b
entry
catalyst (mol %)
additive
3a (%)
1
2
3
4
5
6
7
[{Cp*RhCl2}2] (2.5)
[{Cp*IrCl2}2] (2.5)
[{Ru(p-cymen)Cl2}2] (2.5)
[{Cp*RhCl2}2] (2.5)
[{Cp*RhCl2}2] (2.5)
[{Cp*RhCl2}2] (2.5)
[{Cp*RhCl2}2] (2.5)
[{Cp*RhCl2}2] (4.0)
[{Cp*RhCl2}2] (5.0)
AgOAc
AgOAc
AgOAc
-
76
-
-
66
53
30
69
91
97
NaOAc
CsOAc
Cu(OAc)2
AgOAc
AgOAc
c
8
d
9
a
Reaction conditions: 1a (1.7 equiv), 2a (0.5−0.8 mmol),15 catalysts,
and additives (10 mol %) were milled in a mixer-mill at 30 Hz, using a
25 mL ZrO2 milling jar with one ZrO2 ball of 15 mm diameter. The
crude reaction mixture was extracted by adding sea sand (2 × 2 g) to
the milling jar and milling the resulting mixture for 5 min (× 2).
b
Determined by 1H NMR spectroscopy with 1,3,5-trimethoxybenzene
c
as the internal standard. Cp* = pentamethylcyclopentadienyl. Use of
AgSbF6 (16 mol %), AgOAc (16 mol %). Use of AgSbF6 (20 mol %),
d
AgOAc (20 mol %).
[{Cp*IrCl2}2] or [{Ru(p-cymen)Cl2}2] did not lead to the
formation of 3a (Table 1, entries 2 and 3). Next, the influence
of additives was investigated14 (Table 1, entries 4−7). In the
absence of AgOAc, 3a was formed in 66% (Table 1, entry 4). A
negative effect was observed, when AgOAc was replaced by
NaOAc or CsOAc (Table 1, entries 5 and 6). In contrast, the
use of Cu(OAc)2 led to product 3a in 69% yield (Table 1, entry
7). Increasing the amount of [{Cp*RhCl2}2], AgSbF6 and
AgOAc enhanced significantly the yield of the reaction (Table
1, entry 8 and 9). In particular, the use of 5 mol % of the
Rh(III) catalyst afforded the amidated product 3a in 97% yield
(Table 1, entry 9).
a
b
Obtained as 1.1:1 regioisomeric mixture. Use of [{Cp*RhCl2}2] (10
mol %), AgSbF6 (40 mol %), AgOAc (40 mol %).
Next, the substrate scope was investigated (Scheme 2). The
use of unsubstituted benzamide 1a in the coupling with methyl-
1,4,2-dioxazol-5-one (2a) afforded 3a in 90% yield (after
column chromatography).16 The use of benzamide 1a as
limiting reagent (1a/2a: 1.0/1.7) afforded product 3a in a
slightly lower yield (64%).17 To study the influence of
substituents on the benzamide core, various substrates bearing
electron-donating groups in para or meta position of the arene
were applied. Using p-methoxy benzamide 1b in the coupling
with 2a provided 3b in a quantitative yield (>99%). In contrast,
the reaction between m-methoxy-substituted benzamide 1c and
2a yielded 3c in only 45% and as a 1 1.1:1 regioisomeric
mixture.18 An analogous transformation of 1d bearing a chloro
group in meta position of the benzamide afforded the
corresponding product 3d as a single regioisomer in 88%
yield. Treating 2a with benzamides containing halogens in para
position, provided the corresponding products in quantitative
yields (3e: > 99%, 3f: > 99% and 3g: 98%).19 A moderate yield
(45%) was observed when 2a was reacted with p-CF3-
substituted benzamide 1h, which could be due to the strong
electron-withdrawing effect of the CF3 group.20 Replacing the
tert-butyl group on the amide nitrogen atom by an adamantyl,
isopropyl, cyclohexyl, or n-butyl group gave the corresponding
products 3i−3l in very good to moderate yields (3i: 83%, 3j:
93%, 3k: 69%, and 3l: 38%).
Subsequently, the amidating reagent was varied. Replacing
the methyl substituent on 1,4,2-dioxozol-5-one 2a by an ethyl,18
tert-butyl18 or phenyl group gave the corresponding amidated
products 3m−3o in yields ranging from 50% to 58%.
Performing the mechanochemical C−H amidation on a gram
scale using 10 mmol of 1a and 6 mmol of 2a as substrates and a
catalyst/additive combination of 2.5 mol % of [{Cp*RhCl2}2],
20 mol % of AgBF4, and 10 mol % of AgOAc in a mixer mill for
99 min gave 3a in 84% yield.
Three points of these results are particularly noteworthy:
First, the mechanochemical activation allowed performing the
reaction in the absence of any solvents, yielding the ortho
amidated products in high yields by using similar catalysts
loading as in solvent.3a Second, the desired products were
formed in significant shorter reaction times (99 min) compared
to the solvent-based standard protocol (12 h), and third, no
additional heating was required.
To broaden the scope of the mechanochemical C−H
amidation, the directing moiety was modified (Scheme 3).
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Letter
(with DCE as the solvent at 40 °C),3a,23 both processes appear
to follow similar reaction pathways with the C−H bond
cleavage being turnover-limiting.
Scheme 3. Mechanochemical Rh(III)-Catalyzed C−H Bond
Amidation with Different Directing Groups
Finally, we intended to confirm the competence of a
rhodacyle to undergo amidation under the reported mecha-
nochemical reaction conditions (Scheme 5). Following our
Scheme 5. Formation of Rhodacycle 6 and Its Catalytic
Reactivity
previously reported protocol,12b rhodacycle 6 was prepared by
milling of 4c with [{Cp*RhCl2}2] (1.0 equiv) and NaOAc (6.0
equiv) for 2 × 99 min in a mixer mill. Then, a catalytic quantity
of 6 (10 mol %) was combined with AgBF4 (20 mol %) and
AgOAc (20 mol %) and applied in the coupling between 4c and
2a. After 99 min of milling, product 5c was isolated in 67%
yield, confirming our hypothesis that rhodacycle 6 (in its
cationic form) was a likely intermediate in the catalytic cycle of
the mechanochemically induced C−H bond amidation
process.24
In summary, we developed a mechanochemical rhodium-
(III)-catalyzed C−H bond amidation of arenes with 1,4,2-
dioxazol-5-ones as amidating reagents. Under solventless
conditions, the amidated products are formed in high yields,
without additionally heating and in shorter reactions times than
in solution. Preliminary investigations revealed mechanistic
analogies to the solvent-mediated protocol.
a
Use of [{Cp*RhCl2}2] (10 mol %) AgSbF6 (40 mol %), AgOAc (40
b
mol %). 2 × 99 min at 30 Hz.
Pleasingly, a wide variety of directing groups could be applied.21
For example, the coupling of 2a with ketoxime 4a or
benzo[h]quinoline (4b) worked well, providing the corre-
sponding products in good yields (5a: 78% and 5b: 81%). In
addition, 2-phenylpyridine (4c), 2-phenyl pyrimidine (4d) and
oxazoline 4e were suitable substrates forming the products in
good to moderate yields, albeit under slightly more demanding
conditions (5c: 70%, 5d: 24% and 5e: 34%). For some
substrates (e.g., 5b and 5c), higher catalyst loadings were
required compared with those in solution.3a Most interesting,
isobutyrophenone, a very challenging weakly coordinating
directing group3a was also effective under ball-milling
conditions smoothly, generating product 5f in 49% yield by
using 2a as the amidation reagent. Changing the substituent on
the amidation reagent 2 from a methyl to a phenyl group led to
product 5g in 49% yield together with a small amount (10%) of
diamidation product 5g′.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.7b00582.
■
S
1
Experimental procedures, characterization data, and H
and 13NMR spectra for the synthesized compounds
(PDF)
For gaining insight into the reaction mechanism a kinetic
isotopic effect (KIE)22 was determined by performing
intermolecular competition reactions between [D5]1a and
[H5]1a with 2a as the coupling partner (Scheme 4). Under
mechanochemical conditions, the KIE value was 4.8. As this
number was close to the one (4.4) determined in an analogous
experiment under the previously reported reaction conditions
AUTHOR INFORMATION
Corresponding Author
*E-mail: carsten.bolm@oc.rwth-aachen.de. Homepage: http://
bolm.oc.rwth-aachen.de/. Fax: (+49) 241-8092-391.
■
ORCID
Scheme 4. KIE Experiment of the Mechanochemical
Carsten Bolm: 0000-0001-9415-9917
Notes
Rh(III)-Catalyzed C−H Bond Amidation
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
C.B. thanks RWTH Aachen University for support from the
Distinguished Professorship Program funded by the Excellence
■
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Letter
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(20) In some cases, low yields could be improved by using more
catalysts or/and a longer reaction time as illustrated by the data given
in parentheses in Schemes 2 and 3.
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