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Functionalization, which has been published in final form at https://onlinelibrary.wiley.com/doi/abs/10.1002/ange.201800375 . This article may be used
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Designing Homogeneous Bromine Redox Catalysis for Selective
Aliphatic C-H Bond Functionalization
Peter Becker,[a,+] Thomas Duhamel,[a,b,+] Claudio Martínez,[a] Kilian Muñiz*[a,c]
Abstract: The potential of homogeneous oxidation catalysis
employing bromine has remained largely unexplored. The
combination of tetraalkylammonium bromide and 3-chloroperbenzoic
acid offers a unique catalyst system for the convenient and selective
3
oxidation of saturated Csp -H bonds upon photochemical initiation
with visible light. It enables remote intramolecular, position-selective
C-H amination as demonstrated for 20 different examples. For the
first time, a N-halogenated intermediate could be isolated as the
active catalyst state in a catalytic Hofmann-Löffler reaction. In
addition, a novel expeditious one-pot synthesis of Nsulfonyloxaziridines from N-sulfonamides was developed and
exemplified for 15 transformations. These pioneering examples
provide a change in paradigm for molecular catalysis with bromine.

The growing demand for environmentally benign oxidation
conditions has placed catalysis with redox-active halide reagents
into the focus of method development. While homogeneous
catalysis based on the iodine(I) platform is continuously
maturing into a viable synthetic tool,[1] related bromine catalysis
has virtually remained in its infancy. We became interested in
devising the as yet unprecedented bromine catalysis for aliphatic
hydrocarbon functionalization reactions. In a seminal study,
Sharpless had developed conditions for bromine-catalyzed
aziridination of alkenes with chloramine-T under homogeneous
conditions.[2] Later, a bromine catalyzed intramolecular alkene
diamination was reported.[3] In amination chemistry, these
reactions represent the presently small number of homogeneous
bromine-catalyzed alkene oxidations,[4] and the possibility of
employing bromine as redox active homogeneous oxidation
catalyst has been considered strongly limited. The usually low
turn-over rates of homogeneous bromine catalysts have been
suggested to be consequential of rapid catalyst deactivation
(disproportionation and/or over-oxidation to a sink of unreactive
BrV species).[2a] Here, we present the synthesis of pyrrolidines
through a remote hydrocarbon activation of benzylic C-H bonds
mediated by a homogeneous bromide catalyst. The carefully
elaborated conditions for such bromine catalysis represent a

[a]

[b]

[c]
[+]

Dr. P. Becker, T. Duhamel, Prof. Dr. K. Muñiz
Institute of Chemical Research of Catalonia (ICIQ),
The Barcelona Institute of Science and Technology
Av. Països Catalans 16, 43007 Tarragona (Spain)
E-mail: kmuniz@iciq.es
T. Duhamel
Facultad de Química, Universidad de Oviedo
C/Julián Clavería, 33006 Oviedo (Spain)
Prof. Dr. K. Muñiz
ICREA, Pg. Lluís Companys 23, 08010 Barcelona (Spain)
These authors contributed equally.
Supporting information for this article is given via a link at the end of
the document.

uniform approach that is further applicable to a novel oxaziridine
formation from sulfonamides. This work thus demonstrates the
applicability of bromine catalysis in homogeneous oxidation
chemistry without the loss of catalytic activity throughout the
catalysis. Pyrrolidines and related saturated nitrogen-containing
heterocycles are ubiquitous alkaloid scaffolds with outstanding
biological and pharmaceutical activity,[5] for which the direct
amination of an aliphatic C-H bond has been identified as a
logical and economic strategy.[6,7] Our recent development of
related iodine catalyzed Hofmann-Löffler reactions (Figure 1)[8]
has triggered our interest to develop related Br-based protocols.
In contrast to the existing paradigm,[2a] such an approach should
be feasible taking into account historic parent transformations
utilizing stoichiometric amounts of hypobromite as oxidizing
reagent.[9,10] Therefore, it would be particularly interesting as a
proof of principle for bromine oxidation catalysis outside the few
existing examples of alkene functionalization.[2-4,11]
H
R´

I

NHSO2R

R´´
unfunctionalized
remote C-H bond

[visible light]
or [blue LED]

Iodine catalysis with an iodine(III)
reagent as terminal oxidant
oxidant
(ArCO2)2IPh

iodine catalyst
ArCO2-I

R´´
R´
N
SO2R
C-H amination products
Iodine catalysis with a photoredoxbased terminal aerobic oxidation
oxidant
iodine catalyst
TPT (1 mol%), air
I-OH

Common N-iodinated intermediate
SO2R
elusive catalyst state
N
(information by
R´
I
R´´
computational chemistry)
H

Figure 1. Catalytic Hofmann-Löffler reactions based on molecular iodine as
catalyst source. Ar = 3-Cl-C6H4.

We started our investigation applying conditions similar to the
iodine-catalyzed Hofmann-Löffler reaction.[8a] When 1a was
converted in the presence of elemental bromine (10 mol%) as
the
catalyst
and
bis(3-chlorobenzoyloxy)iodobenzene
PhI(O2CAr)2 (1.1 equiv.) as the oxidant, 22% of the desired
pyrrolidine product 2a was obtained (Table 1, entry 1). A change
of the hypervalent iodine(III)-oxidant did not significantly improve
the yield of 2a (entry 2). We then turned to a bromide salt, which
can be administered as an ammonium derivative resulting in
better solubility in organic solvents. The combination of
tetrabutylammonium bromide with diacetoxy iodobenzene
(PIDA) and bis(trifluoroacetoxy)iodobenzene (PIFA) increased
the yield to 32 and 40%, respectively (entries 3,4), while the best
result was obtained with PhI(O2CAr)2 (entry 5). Moreover, based
on the particular effect of the 3-chlorobenzoate anion, more

conventional mCPBA (2 equiv.) was identified as an even better
oxidant in combination with 20 mol% of Bu4NBr (entry 6). The
reaction reached full conversion and the cyclized compound 2a
was isolated in 95% yield. The environmentally benign and
economic conditions for mCPBA as the oxidant deserve to be
highlighted. The use of lower concentrations of either Bu4NBr or
mCPBA resulted in lower yields (entries 7-11). However, the
reactions still produced reasonable amounts of product
indicating significant robustness of the homogeneous bromine
catalysis.

entirely. These results demonstrate the first examples of a
Hofmann-Löffler-type reaction under expedious homogeneous
bromine catalysis. They significantly advance the harsh reaction
requirements of the traditional process, which is stoichiometric in
bromine promoter, requires a strong acidic reaction medium and
a basic work-up.[9]

R''

Bu4NBr (20 mol%)
CPBA (2 equiv.)

O O
S
N
R’
H

R

R

R''

N

MeCN, 12 h, RT
2a-r

1a-r

R'
S
O
O

Br catalyst source
TsHN

oxidant,

Ph

solvent, 12 h, 25 ºC
1a
Entry

2a
Conditions

N
Ts

Ph

Conversion (%)[a]

1

Br2 (10 mol%), PhI(O2CAr)2 (1.1 equiv.), DCE
Br2 (10 mol%), PIFA (1.1 equiv.), DCE
Bu4NBr (20 mol%), PIDA (1.1 equiv.), CH3CN

32

4

Bu4NBr (20 mol%), PIFA (1.1 equiv.), CH3CN

40

5

Bu4NBr (20 mol%), PhI(O2CAr)2 (1.1 equiv.), CH3CN 46

6

Bu4NBr (20 mol%), mCPBA (2 equiv.), CH3CN

100 (95%)[b]

7

Bu4NBr (20 mol%), mCPBA (1.5 equiv.), CH3CN

77

8

Bu4NBr (20 mol%), mCPBA (1.2 equiv.), CH3CN

69

9

Bu4NBr (10 mol%), mCPBA (2 equiv.), CH3CN

50

10

Bu4NBr (5 mol%), mCPBA (2 equiv.), CH3CN

11

Bu4NBr (2.5 mol%), mCPBA (2 equiv.), CH3CN

40
s.m

Ph

Ph

N
R

N
O S
O

2f (R = Ns): 71%
2g (R = Ms): 98%

30

3

2a: 95%

X
N
Ts
2b (X = Me): 95%
2c (X = OMe): 94%
2d (X = F): 76%
2e (X = Cl): 68%

2h: 82%

22

2

Ph
N
Ts

Ph
N
O S
O

S

2i: 63%
Ts
N

The reaction conditions show generality for the formation of
pyrrolidines (Scheme 1) and are applicable in the presence of
electron withdrawing and electron donating groups on the arene
affording the products 2b-e with complete selectivity and in good
yields (68-95%). Different sulfonyl protecting groups at the
nitrogen atom are well tolerated providing the respective
nosylated (2f) and mesylated (2g) products in high yields (7198%). Also, a cyclopropylsulfonylamide is readily tolerated
suggesting that potentially competing radical ring opening does
not compete (82%, 2h), while the 2-thiophenylsulfonylamide is
likewise stable (63%, 2i). More congested 2- and 3-tosylgroups
provide comparable yields to the standard 4-tosyl indicating that
increased sterics do not interfere during the cyclisation (87 and
92%, 2j and 2k). Backbone modification is well tolerated (94%,
2l). Acyclic stereocontrol cannot be realized as demonstrated for
compounds 2m/2m’, which produce a 1:1-mixture of
diastereomers due to the radical reaction mechanism. However,
cyclization within ring annulation provides single diastereomers
2n (79%) and 2o (54%). The amination reaction in a-position to
a heteroatom also proceeds in good yields (54 and 98%, 2o and
2p). In addition, bromine catalysis tolerates benzylic tosylamides,
which provide the isoindolines 2q-2t without imine formation,
which is the dominating pathway in iodine catalysis.[8a]
Heteroaromatic substituents do not undergo potentially
competing bromination under these conditions (2r-t), and even
the notoriously oxidation-labile furane core in 2t is tolerated

2j (Ar = 2-Tol): 87%
2k (Ar = 3-Tol): 92%
Ts
Ph
N

Ph
N
Ts
2l: 94%

2n 1:0 dr, 79%

Ph
N
Ts
2m:2m' = 1:1 dr, 74%

N
Ts

2o: 1:0 dr, 54%

2p: 98%

N Ts

N

Ts
N Ts
S
O

S

2q: 77%

Ph
N
Ts +

OMe

O

N Ts

Table 1. C-H Amination under Homogeneous Bromine Catalysis: Optimization.
[a]
1
Estimated from integration in the H NMR spectrum of the crude reaction
[b]
mixture. Isolated yield of 2a after purification. Ar = 3-ClC6H4, PIDA =
PhI(OAc)2, PIFA = PhI(O2CCF3)2.

Ph
N
O S Ar
O

2r: 59%

2s: 48%

2t: 38 (99)%[a]

Scheme 1. C-H Amination under Homogeneous Bromine Catalysis: Scope.
[a]
Addition of mCPBA in four portions. Yield in bracket based on isolated
starting material.

Mechanistically, the reaction could proceed through various
intermediary bromine(I) states, which derive from oxidation of
the bromine source and could be either conventional
hypobromite or 3-chlorobenzoyl hypobromite. Based on control
experiments, we propose a catalysis that is based on 3chlorobenzoyl hypobromite 4 as the active catalyst (Scheme 2).
This hypobromite 4 is directly formed from Bu4NBr and
mCPBA.[12] Independent synthesis of the active catalyst 4 from
the corresponding silver benzoate 3[13] was successful, although
4 is of pronounced instability. Still, addition of 1 equivalent of
standard substrate 1a to the solution of 4 provided a 24%
formation of product 2a (48% normalized yield). In successive
investigation, it was confirmed that hypobromite 4 indeed
initiates catalysis, and a 65% overall formation of 2a was
obtained in the presence of 2 equivalents of mCPBA. The
involvement of free hypobromite was excluded based on Raman
spectroscopy.[14] The involvement of a stabilized anionic species
[Br(O2CAr)2] appears less likely. Such species had been
previously isolated and documented to constitute electrophilic
bromine sources in stoichiometric alkene oxidation reactions.[15]
We have generated such compounds by interaction between
Bu4NBr and PIFA or PhI(mCBA)2, respectively, as they are

presumably produced under the conditions from Table 1, entries
4 and 5. However, isolated anionic bromine(I) compounds do not
show activity in the cyclization of 1a, that would be comparable
to the active catalyst 4.[14] This is an important mechanistic
difference in behavior when compared to the corresponding
iodine-based catalysts.22 In contrast to previous mechanistic
investigations of iodine catalyzed Hofmann-Löffler reactions,[8] it
was possible to obtain the N-brominated intermediate 5a from a
reaction between 1a, an ammonium bromide and mCPBA. This
synthesis resembles the formation of 5a under the real catalytic
conditions. N-Brominated 5a could be unambiguously identified
by NMR spectroscopy and by HRMS measurements displaying
correct isotope pattern. It displays a remarkably high reactivity
as evidenced from its estimated half-life time of 30 min. The
isolation of 5a constitutes the first detection and structural
characterization of an N-halogenated intermediate in halogencatalyzed Hofmann-Löffler reactions. In comparison to free 1a,
the UV spectrum of N-brominated 5a exemplifies a
bathochromic shift to around 300 nm, which is in agreement with
a photochemical activation under normal environmental
daylight.[14] It also explains that reactions with LED light sources
experience a strong decrease in yield.[14] The required energy for
N-Br bond cleavage is significantly different from related
iodinated species.[8b] This observation demonstrates that the
development of light-initiated bromine oxidation catalysis cannot
be predicted readily from the corresponding iodine catalysis.

OAg
CD2Cl2,
3

Cl

NHTs

Ph

Br-O2CAr (4)

Ph
N
Ts
2a (24%)
2a (65%)

12 h, 25 ºC

1a

with mCPBA (2 equiv)

NHTs

Ph

1a

Me2(C18H37)2NBr (1 equiv)
ArCO3H (1 equiv)
CD3CN, 1 h, 25 ºC

Ts
N

Ph

1a (66%) + 2a (34%)

Br
NTs

B
Y

HBr

Br
A

Bromine(-I/I)
Catalysis
Manifold

hν
(visible light)

R4NOH

H2O

NTsBr

R4NBr

Y
5
ArCO2H
NHTs

4
1

Figure 2. Mechanistic proposal: exploiting Br catalyst stability for a Br(-I/I)
catalysis manifold. Y = aryl, alkoxy; Ar = 3-Cl-C6H4.

4 (52%)

CD2Cl2, CH3CN,

Y

Y

Cl

- AgBr

N
Ts
2

ArCO3H
OBr

Br2

NHTs
Y

ArCO2Br

O

O

fast reaction. This is the first case of direct pyrrolidine formation
from Br-based Hofmann-Löffler reactions as previous methods
always terminate at the C-Br stage requiring subsequent base
administration for C-N bond formation.[9,10,16] The liberated
hydrogen bromide regenerates the catalyst precursor R4NBr
from neutralization with R4NOH. These individual events provide
the molecular basis for selective catalysis within a defined
bromine(-I/I) catalysis manifold. Over-oxidation to an unreactive
bromine catalyst in an oxidation state higher than +I could never
be detected. For example, isolated 4 does not react with excess
mCPBA, and treatment of Bu4NBr with 1-5 equivalents of
mCPBA does not provide bromine over-oxidation, but
decomposition of the oxidant instead.[14]

Br

5a
(day light)

Scheme 2. Potential catalysts, control experiments and isolation of Nbrominated intermediate.

Under the catalytic conditions, the reaction between 4 and the
substrate 1 provides the N-brominated tosylamide 5, which
undergoes light-induced homolysis to the N- centered radical
intermediate A (Figure 2). The selectivity of the C-H
functionalization event arises from the involvement of a
kinetically preferred 1,5-H abstraction at this stage. A kinetic
isotope effect of 3 was determined for this pathway as the ratedetermining step. The intermediary alkyl bromide B forms
through either a radical chain mechanism involving 5, 3chlorobenzoyl hypobromite or from radical recombination. Given
the occurrence of free 1a in the stoichiometric experiment with
5a from Figure 4, the former appears more reasonable.
Subsequent nucleophilic cyclization requires the activated
benzylic position or an a-alkoxylation to provide a sufficiently

In contrast to the successful formation of pyrrolidines, the
corresponding 1,3- or 1,4-C-H abstractions to aziridines or
azetidines, respectively, do not constitute viable reaction
pathways. For these derivatives, however, exposure to the
Br/mCPBA system afforded an unexpected result. When
exploring the reaction with 3-phenylpropanyl-1-amine 6a as
substrate, a single product was obtained in 86% yield, which
was identified as the N-sulfonyloxaziridine 7a (Scheme 3). This
overall sequence thus represents a new one-pot approach to the
important class of N-sulfonyloxaziridines, which depict unique
oxidants within the synthetic arsenal of modern organic
chemistry.[17] Moreover, this synthesis represents a rare case in
halogen catalysis, in which oxidants are generated within a
catalytic oxidative transformation.[18] The reaction was
rationalized to consist of two individual consecutive oxidation
events. In the absence of available hydrogen atoms for a
kinetically competent C-H abstraction the initially formed Nbromo species 5 undergoes an a-elimination to the aldimine C.
The oxaziridine formation is then accomplished by oxidation of
the intermediary aldimine with the terminal oxidant mCPBA.[19]
Importantly, attempts to conduct the same transformation with
catalytic amounts of iodine or Bu4NI did not succeed
demonstrating a superior performance of bromine over iodine.
The involvement of the N-Br intermediate 5 and the absence
of the alternative benzylic bromination product is important as it
confirms that the reactions from Figure 2 do proceed through a
Hofmann-Löffler pathway and do not derive from the potential

direct benzylic C-H bromination under conventional radical
conditions. The oxaziridine formation from linear sulfonamides 6
is a general reaction as displayed in Scheme 3 with 15
examples. Standard N-tosyl phenylpropylamide gives the
corresponding oxaziridine 7b in 60% yield as a single oxidation
product. Variation of the arene substituent provides the products
7c-d (59-70%), while the sulfonyl variation includes mesyl (7e,
44%), nosyl (7f, 65%) and 2-thiophenylsulfonyl (7g, 64%). A
phenyl ether derivative 7h could also be accessed conveniently
(68%).

R

Bu4NBr (20 mol%)
CPBA (2 equiv.)

O O
S
N
R’
n H

O
R
N
n
S R’
O
7a-n O

MeCN, 12 h, RT

6a-n (n = 0,1,2)
BrO2CAr
R

- HO2CAr

HO3CAr

O O
S
N
R’
n
Br
5
N
O

C

n
7a: (n = 1) 86%
7b: (n = 2) 60%

7c: (X = OMe) 59%
7d: (X = Cl) 70%
Ts
S

N
O

O
7h: 68%

O
S
N
O

O
S
N
O

Financial support was provided by the Spanish Ministry for
Economy and Competitiveness and FEDER (CTQ2017-88496R
grant to K. M., and Severo Ochoa Excellence Accreditation
2014-2018 to ICIQ, SEV-2013-0319). The authors are grateful to
the CERCA Program of the Government of Catalonia and to
COST Action CA15106 “C–H Activation in Organic
Synthesis (CHAOS)”.
Keywords: Amination • Bromine • C-H Functionalization •
Catalysis • Hofmann-Löffler Reaction
[1]
[2]

O
N
7n: 85%

[3]

7l: 71%

7k: 44%
F

7e (R = Ms): 44%
7f: (R = Ns): 65%
Br
Ts
N
O
7i: 55%

Acknowledgements

O O
S
N
O

O

7j: 72%
O
S
N
O
7m: 60%

O
N
R

Ts

X

O

O

n

O O
S
R’

N

O
N

Ts

O
N
S
7g: 64% O O

R

- HBr

- HO2CAr

formation have been designed. The reactions proceed within
defined homogeneous catalytic cycles and rely on an
engineered bromine(I) catalyst that is directly accessed from
oxidation with the terminal oxidant mCPBA. For the first time in
halide catalysis, the crucial N-halogenated intermediate as the
initiator of the radical C-H functionalisation could be isolated and
characterized. These accomplishments will be an instrumental
guide for the development of general bromine catalysts and
complimentary radical C-H transformations.

[4]

CN
N
O

Ts

Ts

7o: 97%

Scheme 3. Oxaziridine synthesis from linear N-sulfonamides: scope.

With respect to the variation of the N-tosyl phenylethylamide
motif 6a, its 2-brominated arene derivative 7i was accessed in
55% yield and structurally characterized by solid state X-ray
analysis.[20] Variation of the sulfonyl group includes the
benzylsulfonyl derivative (7j, 72%) and the cyclopropylsulfonyl
derivative (7k, 44%). Although potentially labile under reaction
conditions involving free radicals, the two transformations
proceed with chemoselectivity in favor of the oxaziridine
formation. The reaction scope further includes a 2naphthylsulfonyl (7l, 71%), a 4-fluorobenzenesulfonyl (7m, 60%)
and an aliphatic neopentyl substitution (7n, 85%) in the final
product. Usually, oxaziridines were never formed under any
conditions for the examples of pyrrolidine formation from
Scheme 1. A single exception was encountered for the nitrilesubstituted derivative 6o.[21] This outcome is due to the
increased acidity of the a-hydrogens, which prevents hydrogen
atom transfer and thus leads to imine and ultimately oxaziridine
formation.
In summary, protocols for unprecedented bromine(-I/I)catalysis in C-H aminative functionalization and oxaziridine

[5]
[6]

[7]

[8]

[9]
[10]
[11]

[12]
[13]
[14]
[15]
[16]
[17]

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3, 2375.
a) K. Muñiz, Pure Appl. Chem. 2013, 85, 755 (2013); b) For non-related
Bromine(III) chemistry: M. Ochiai, K. Miyamoto, T. Kaneaki, S. Hayashi,
W. Nakanishi, Science 2011, 332, 448.
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Ber. Dtsch. Chem. Ges. 1910, 43, 2035.
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1971, 1.
It is interesting to compare this context in main group catalysis with the
recent case of non-related C-H functionalization using conventional
transition metal Rh and emerging Co catalysis. For reviews on Rh: a) P.
B. Arockiam, C. Bruneau, P. H. Dixneuf, Chem. Rev. 2012, 112, 5879;
Co: b) M. Moselage, J. Li, L. Ackermann, ACS Catal. 2016, 6, 498.
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[18]
[19]
[20]

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X-ray crystallographic data for compound 7i has been deposited with
the
Cambridge
Crystallographic
Data
Centre
database
(http://www.ccdc.cam.ac.uk/) under code CCDC 1583024.

[21]

For mechanistically non-related oxidative a-oxidation of carbonyl
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Entry for the Table of Contents (Please choose one layout)
Layout 1:

COMMUNICATION
Br enters catalysis! New
homogeneous bromine catalysis by
combining an ammonium bromide
with mCPBA as oxidant enables the
first catalytic variant of the classic
bromine-mediated Hofmann-Löffler
reaction. The same conditions
transform sulfonamides into
oxaziridine derivatives within a single
operation.

P. Becker, T. Duhamel, C. Martínez, K.
Muñiz*
Page No. – Page No.
Designing Homogeneous Bromine
Redox Catalysis for Selective
Aliphatic C-H Bond Functionalization