Iodine(III)-Mediated para-Selective Direct Imidation of Anilides

Article abstract of DOI:10.1002/chem.201501553

The direct, nucleophilic imidation of acetanilide derivatives has been performed under mild, iodine(III)-mediated or -catalyzed conditions, employing lithium triflimide as the nitrogen source. The reaction exhibits exclusive regioselectivity for the para position and shows a good tolerance for varied functional groups at both the ortho or meta positions. Preliminary mechanistic data suggest that the LiNTf₂ reagent plays a key role in the reactivity. The direct, nucleophilic imidation of acetanilide derivatives has been performed under mild, iodine(III)-mediated or -catalyzed conditions, employing lithium triflimide as the nitrogen source. The reaction exhibits exclusive regioselectivity for the para position and shows a good tolerance for varied functional groups at both the ortho or meta positions. Preliminary mechanistic data suggest that the LiNTf₂ reagent plays a key role in the reactivity.

Full text of DOI:10.1002/chem.201501553

DOI: 10.1002/chem.201501553
Communication
&
CÀN Bond Formation
Iodine(III)-Mediated para-Selective Direct Imidation of Anilides
AmØlie Pialat, Julien Berg›s, Axel Sabourin, Robin Vinck, Benoît LiØgault,* and
Marc Taillefer*[a]
Abstract: The direct, nucleophilic imidation of acetanilide
derivatives has been performed under mild, iodine(III)-
mediated or -catalyzed conditions, employing lithium trifli-
mide as the nitrogen source. The reaction exhibits exclu-
sive regioselectivity for the para position and shows
a good tolerance for varied functional groups at both the
ortho or meta positions. Preliminary mechanistic data sug-
gest that the LiNTf2 reagent plays a key role in the reactiv-
ity.
Scheme 1. Direct, intermolecular CÀN bond-forming reactions of simple
Direct CÀN bond forming reactions have recently gained great
arenes.
interest,[1] as the reaction obviates the need for prefunctionali-
zation,[2] which can significantly increase the number of syn-
thetic steps to access a specific target. Since the initial report
on the Pd-catalyzed intramolecular dehydrogenative coupling
of biaryl amides,[3] related intermolecular, ortho-directed, transi-
tion-metal-catalyzed aminations have been the subject of nu-
merous studies (Scheme 1a).[4,5] Essentially developed with Pd,
Cu, and Rh catalysts, and with a wide array of directing
groups, these reactions usually employ an external or internal
oxidant and require high temperatures. However, they offer ex-
cellent regioselectivity for the ortho position. This selectivity
can be partially diverted towards the meta and para positions,
under electrophilic amination conditions, typically conducted
in the presence of a radical initiator or mediated by hyperva-
lent iodine reagents (Scheme 1b). In the former case, reactions
are often performed under relatively harsh conditions and with
superstoichiometric quantities of substrate and/or oxidant.[6] To
address these issues, milder methods have been developed,
for which new reagents or catalyst systems have been specifi-
cally designed.[6g,h] More recently, approaches based on visible-
light-induced photocatalysis have also emerged as promising
alternatives to generate reactive nitrogen-centered radical spe-
cies.[6i–k] In addition, a few reports on iodine(III)-mediated elec-
trophilic aryl amination have appeared in the past few
years.[7–9] As above, excess reagents and high temperatures are
routinely applied in order to obtain the desired reactivity. More
importantly, the regioselectivity, if not innate,[6h,l] is generally
difficult to control, despite the use of additional transition-
metal catalysts,[9c,e] resulting in the isolation of complex mix-
tures of isomers.
Arene functionalization with exclusive selectivity for the para
position has been previously accomplished for CÀC, CÀO, and
CÀX bond formation (X=halide),[10] notably under oxidative
nucleophilic iodine(III)-mediated conditions.[11] In the context of
CÀN bond formation, MuÇiz and co-workers have reported
a single example of para-imidation of a benzylanilide substrate
with non-commercially available PhI(NTs)2.[12] Furthermore,
Zhang and co-workers disclosed a rare example of a para-se-
lective imidation reaction, where 2-alkoxyanilide derivatives
were coupled with N-fluorobenzenesulfonimide (NFSI) in the
presence of a palladium catalyst.[13]
Herein we report our findings on the oxidative direct imida-
tion of anilides, exhibiting exclusive regioselectivity for the
para position (Scheme 1c). The reaction conditions are mild,
employ commercially available reagents in stoichiometric
amounts, and are compatible with varied functional groups. In
addition, an iodine(III)-catalyzed version of this transformation
has been developed and further experiments led us to pro-
pose a mechanism.
On the basis of our previous work on the nucleophilic para-
triflation of anilides with AgOTf, where the latter was found to
play an important role for the observed reactivity,[11g] we start-
ed our investigation on the related CÀN bond forming reaction
by employing analogous silver triflimide as the nitrogen
source.[14] In the presence of PhI(OCOCF3)2 (1.2 equiv) in CH2Cl2
at room temperature, acetanilide 1a was converted into imida-
tion product 2a in 30% NMR yield (Table 1, Entry 1). When the
reaction was performed in 1,2-dichloroethane and with
PhI(OAc)2 as the oxidant, the yield was further increased to
[a] Dr. A. Pialat, J. Berg›s, A. Sabourin, R. Vinck, Dr. B. LiØgault, Dr. M. Taillefer
Institut Charles Gerhardt, UMR-CNRS 5253, AM2N, ENSCM
8 rue de l’Øcole normale, 34296 Montpellier Cedex 5 (France)
E-mail: benoit.liegault@enscm.fr
marc.taillefer@enscm.fr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501553.
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Table 1. Optimization of the reaction conditions for the iodine(III)-medi-
ated para-imidation of acetanilide 1a.[a]
Scheme 2. Iodine(III)-catalyzed para-imidation of acetanilide 1a.
Entry
MNR2
Oxidant
Yield [%][b]
1[c]
2
3
4
5[d]
6[d,e]
7[d]
8
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
LiNTf2
PhI(OCOCF3)2
PhI(OCOCF3)2
PhI(OPiv)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
30
50
44
50
60
54
59(73)[16]
nd[f]
B, respectively). Anilides bearing an iodide, bromide, chloride,
fluoride, methyl, or trifluoromethyl group at the ortho position
were all found to be compatible substrates, providing the cor-
responding products 2b–g in 57 to 81% yield (Conditions A)
or 30 to 66% yield (Conditions B). meta-Substituted halo-aceta-
nilides 1h–k were also imidated in yields ranging from 40 to
50%, under Conditions A. These moderate yields may be due
to repulsive electronic interactions between the numerous
lone pairs present in the two SO2CF3 groups of the nucleophile
and the halide substituent of the substrate. This feature
seemed even more pronounced under catalytic conditions;
whereas 2-fluoro-arylimide 2k could be prepared in 56% yield,
the analogous iodo product 2h was only generated in trace
LiNTf2
AgNPhth
PhI(OAc)2
PhI(OAc)2
[a] Reactions performed on 0.25 mmol scale ([1a]=0.125m, unless other-
wise specified); Tf=SO2CF3, NPhth=Phthalimide. [b] Determined by
19F NMR analysis of the crude reaction mixture, using 4,4’-difluorobenzo-
phenone as an internal standard; isolated yield in parentheses.[16] [c] Reac-
tion performed in CH2Cl2. [d] [1a]=0.06m. [e] AgBF4 (10 mol%) was used
as an additive. [f] Product not detected by GC-MS or 1H NMR analysis of
the crude reaction mixture.
50%, then to 60% in a more dilute environment (Entries 4 and
5). Aware of the cost of AgNTf2 as a stoichiometric reagent, we
envisaged substituting it for the more practical LiNTf2,[15] to-
gether with a catalytic amount of AgBF4, presumably acting as
a mild Lewis acid. This modification afforded para-phenylenedi-
amine (PPD) derivative 2a in 54% NMR yield (Table 1, Entry 6).
Later, in a control experiment, we realized that the metallic
counterion Ag+ was in fact unnecessary for the reaction to
proceed. Ultimately, treating anilide 1a with LiNTf2 (1.2 equiv)
and PhI(OAc)2 (1.2 equiv) in 1,2-dichloroethane (DCE) at 208C
over 1.5 h gave compound 2a in 73% isolated yield (Table 1,
Entry 7).[16] Of note, silver-based reagent AgNPhth (NPhth=
phthalimide)[17] was completely ineffective for this transforma-
tion and only led to the recovery of starting material 1a
(Table 1, Entry 8).[6a,b,e,i,k,9a,c,e]
Table 2. Scope of the reaction.[a]
Since first introduced by Fuchigami and Jujita in 1994 and
explored ten years later by the groups of Kita and Ochiai,[18] io-
dine(III)-catalyzed coupling reactions have gained considerable
attention.[19] Indeed, not only do stoichiometric l3-iodane re-
agents generate equimolar amounts of heavy iodoarenes as
byproducts, but they are usually unsuitable for large-scale re-
actions. Hence, we decided to explore the feasibility of a cata-
lytic version of the para-imidation of anilides. Based on these
initial reports,[18b,c] a rapid survey of various aryl iodides, termi-
nal oxidants, Brønsted and/or Lewis acid additives, and sol-
vents led to the use of iodotoluene (10 mol%), meta-chloro-
peroxybenzoic acid (mCPBA, 2 equiv) and trifluoroacetic acid
(TFA, 1 equiv) in DCE at room temperature (Scheme 2). In
these conditions, PPD derivative 2a could be prepared from
acetanilide 1a and silver or lithium triflimide in 46 or 60% iso-
lated yield, respectively. Remarkably, no other isomers were de-
tected.
The scope of this mild oxidative arene imidation was exam-
ined with diversely substituted acetanilides 1, under both stoi-
chiometric and catalytic conditions (Table 2, Conditions A and
[a] Reactions performed on 0.5 mmol scale, at 208C (unless otherwise
specified); isolated yields. [b] Reaction performed at 508C.
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amounts. PPD derivatives 2l–n possessing carbonyl-containing
functional groups Ac, OAc, and CO2Me, and 2-ethynyl-imide
2o, were obtained in 30–53 and 40% yield, respectively, using
PhI(OAc)2 as the oxidant. Cbz-protected anilines (Cbz=carbox-
ybenzyl) are also compatible coupling partners, as illustrated
with product 2p, prepared in 42% yield under Conditions A.
This provides the opportunity for complementary deprotection
conditions and further functionalization. Interestingly, when
acetanilide 1q, bearing a tert-butyl group blocking the para
position, was submitted to the reaction conditions, neither
ortho nor meta imidation product could be detected. This sug-
gests that a different mode of action than those generally pro-
posed for electrophilic amination processes[6,9] may be operat-
ing (see below). Finally, strong electron-donating (OMe) or
-withdrawing (NO2) groups were found to be more problemat-
ic, owing to exacerbated or lessened reactivity, leading to
over-oxidized products or recovered starting material, respec-
tively. It is, however, worth mentioning that this iodine(III)-
mediated/catalyzed arene CÀN bond-forming reaction is the
first method to provide a direct access to a varied range of
substrates[20] under mild conditions, and with exclusive regiose-
lectivity, thus avoiding very difficult separations between two
or more isomers.[6e,g,i,k,9a,c,e]
Scheme 3. Proposed mechanism of the iodine(III)-catalyzed para-selective
imidation of anilides (R=CH3 or CF3, Ar=phenyl or tolyl).
mation of species A from l3-iodane ArI(OCOR)2 and LiNTf2 in-
creases the electrophilicity of the iodine(III) centre towards ani-
lide 1a, thus leading to intermediate B. Next, nucleophilic
attack of the proximal triflimide anion occurs in an associative
fashion,[11g,23] with concomitant loss of iodoarene and lithium
(trifluoro)acetate. Aromaticity is restored to yield triflimide 2a.
Under catalytic Conditions B, the ArI(OCOR)2 species is regener-
ated in the presence of mCPBA. It is difficult at this time to ac-
count for the exceptional regioselectivity encountered in these
reactions;[11] nevertheless, it is unlikely that it could solely be
explained by steric effects and/or enhanced electrophilicity at
the para position of intermediate B.
Finally, the products may be selectively deprotected to yield
the corresponding aryl triflamides (ArNHTf),[24] which have
been shown to be of interest for both medicinal chemistry and
agrochemical industry.[25] For example, aryl triflamide 3d was
readily obtained from compound 2d in nearly quantitative
yield in only 10 min under mild basic conditions [Eq. (3)]. In ad-
dition, aniline 4d was prepared in very good yield, though
under stronger conditions [Eq. (4)].
To gain some insight into this mode of reactivity, several ex-
periments were performed. Acetanilide 1d was chosen as the
starting material, because it allows for a better identification of
the different species present during the analyses. Running
a typical experiment under Conditions A, in the presence of
1,1-diphenylethylene or 3,5-di-tert-butylhydroxytoluene (BHT),
had little to no effect on the outcome of the reaction, there-
fore ruling out a mechanism involving radical species. We then
observed that, although prolonged exposure of 1d to
PhI(OAc)2 led to no reaction, the additional presence of LiBF4
induced slow decomposition [Eqs. (1) and (2)]. This is indicative
of the probable role of Li+ as a weak Lewis acid, which may
be responsible for the activation of the hypervalent iodine re-
agent towards the poorly nucleophilic anilide starting material.
Additionally, lithium triflimide, initially poorly soluble in chlori-
nated solvents, could be instantly brought into solution in the
presence of an equimolar amount of PhI(OAc)2. The resulting
1
species A was characterized by H, 19F, and 13C NMR, showing
evidence for weak coordination of the lithium atom to the car-
bonyl functional groups of the acetate moieties (Scheme 3).[17]
Notably, whereas a defined coordination mode around the lith-
ium atom could not be established,[21] displacement of one or
two of the acetate “ligands” to form PhI(OAc)(NTf2) or
PhI(NTf2)2 species,[12] respectively, can reasonably be exclud-
ed.[22]
On the basis of the above observations and previous stud-
ies,[7,11,19] a proposed mechanism is depicted in Scheme 3. For-
In summary, we have developed a practical method for the
direct, nucleophilic imidation of acetanilides, with exclusive re-
gioselectivity for the para position. The reaction conditions
employ either a stoichiometric amount of PhI(OAc)2 or a combi-
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Acknowledgements
The authors thank the CNRS and the ENSCM for their support
and the Minist›re de l’Enseignement SupØrieur et de la Recher-
che for Ph.D. grants (A.P. and J.B.).
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Keywords: CÀN bond formation
· hypervalent iodine ·
oxidative coupling · para-phenylenediamines · umpolung
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Communication
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Received: April 21, 2015
Published online on June 10, 2015
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