Scandium-Catalyzed para-Selective Alkylation of Aromatic Amines with Alkenes

Article abstract of DOI:10.1021/acs.orglett.9b03451

An efficient para-alkylation of primary and secondary anilines with a variety of sterically encumbered alkenes using a simple β-diketiminato scandium catalyst is reported. This protocol features 100% atom economy, excellent chemo- and regioselectivity, broad substrate scope, and good functional group tolerance. Mechanistic studies disclosed that the reaction probably proceeded via a tandem hydroamination/Hofmann-Martius rearrangement.

Full text of DOI:10.1021/acs.orglett.9b03451

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Scandium-Catalyzed para-Selective Alkylation of Aromatic Amines
with Alkenes
Jianhong Su, Yanping Cai, and Xin Xu*
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science,
Soochow University, Suzhou 215123, P. R. China
S
* Supporting Information
ABSTRACT: An efficient para-alkylation of primary and secondary anilines
with a variety of sterically encumbered alkenes using a simple β-diketiminato
scandium catalyst is reported. This protocol features 100% atom economy,
excellent chemo- and regioselectivity, broad substrate scope, and good
functional group tolerance. Mechanistic studies disclosed that the reaction
probably proceeded via a tandem hydroamination/Hofmann−Martius
rearrangement.
alkenes by using a main group Lewis acid catalyst via Friedel−
he development of efficient synthetic methodologies for
Tthe aromatic amines remains attractive in catalysis Crafts reaction (Scheme 1b).10 However, substrate scope in
this reaction was limited to the sterically bulky diarylamine
because the electrophilic phosphonium cation used could be
deactivated by the Lewis basic substrate, thus greatly restricting
synthetic utility. In our previous study, we designed and
synthesized scandium dialkyl complexes based on unsymmetric
β-diketiminato ligands that were able to mediate regioselective
hydroaminoalkylation of tertiary anilines with a variety of
alkenes in combination with a borate compound.11 In contrast,
when such a catalyst system was applied in the reactions of
primary/secondary anilines with alkenes, it exclusively afforded
para-selective C−H alkylation products in high yields with
good functional group tolerance (Scheme 1c). Mechanistic
studies including isolation of the catalytic intermediate and a
series of control experiments revealed that the reaction
proceeded via a tandem hydroamination/Hofmann−Martius
rearrangement to give a formal para-C(sp2)−H alkylation
product. This development is described herein.
research because this class of compounds plays a central role
in the fine chemical and pharmaceutical industries.1,2 To this
end, functionalization of aniline and its derivatives using
alkenes has emerged and represents an atom- and step-
economical method to a variety of alkyl-substituted aromatic
amines.3−5 In this context, transition-metal-catalyzed hydro-
arylation of alkenes with tertiary anilines has been successfully
achieved with high regioselectivity at either para or ortho
positions.6,7 However, acid- or transition-metal-mediated
Friedel−Crafts-type reactions of alkenes with primary/
secondary anilines containing reactive N−H groups often
result in a mixture of hydroamination (N-alkylation) and
hydroarylation (ortho- and para-C-alkylation) products
(Scheme 1a).8 Selective ortho-alkylation of primary/secondary
anilines has been achieved in a few cases.9 To date, there is
only one precedent for para-alkylation of diphenylamine with
We initiated our study by using β-diketiminato scandium
dialkyl complex 1 together with 1 equiv of [PhNHMe2][B-
(C6F5)4] reagent for the reaction of N-methylaniline (2a) with
α-methylstyrene (3a) under the same conditions typically
applied in our previous case (10 mol % catalyst loading,
amine/alkene molar ratio = 1.5:1, toluene, 120 °C).12
Excitingly, this approach exclusively gave Markovnikov-type
para-alkylation product 4a in 91% isolated yield, with no N-
alkylation or ortho-alkylation products detected. Inspired by
this intriguing result, hydroarylation of a variety of 1,1-
disubstituted alkenes with N-methylaniline 2a was tested, and
the results are summarized in Scheme 2. Reactions of para- or
meta-methyl-substituted α-methylstyrene with 2a selectively
Scheme 1. Selected Examples of Catalytic Hydroarylation of
Alkenes with Primary/Secondary Anilines
Received: September 30, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03451
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Scandium-Catalyzed Hydroarylation of 1,1-
Disubstituted Alkenes with N-Methylaniline
Table 1. Scandium-Catalyzed Hydroarylation of Aromatic
Internal Alkenes with N-Methylaniline
a
a
a
Reaction conditions: N-methylaniline 2a (0.45 mmol), alkenes 3
(0.3 mmol), Sc dialkyl complex 1 (0.03 mmol), [PhNMe2H][B-
(C6F5)4] (0.03 mmol), toluene (2 mL), 120 °C, 36 h, isolated yield.
b
2a (0.90 mmol).
afforded para-alkylation products 4b and 4c in 92% and 87%
yields, respectively. Remarkably, a variety of α-methylstyrene
derivatives containing polar functional groups are tolerated in
the reactions. For example, halogen (F, Cl, and Br) substituted
alkene substrates are compatible with the catalyst system,
giving rise to the corresponding alkylation products 4d−4g
with marginally lower isolated yields (79%−85%). α-
Methylstyrene with polar electron-donating groups at the
para-position, such as t-butyldimethylsilyloxy (TBSO), me-
thoxy, methylthio, and N,N-dimethylamino groups, was also
suitable for the reactions. The resulting alkylation products
4h−4k were isolated in excellent yields (90%−92%). Trans-
formation of α-isopropenylnaphthalene to 4l was easily
achieved in 89% yield. Double hydroarylation of para-
disubstituted alkene substrate furnished addition product 4m
when excess N-methylaniline reagent was used. When using
sterically encumbered α-substituted styrenes as substrates, the
reactions still took place, providing access to the para-
alkylation products 4n−4q with construction of quaternary
carbon centers in satisfactory yields (81%−88%). It should be
noted that hydroarylation occurred selectively at the CC
double bond that is conjugated with the aromatic ring, with
retention of the unconjugated one (i.e., the generation of 4o).
In addition, heterocyclic aromatic olefins containing a
thiophene or benzofuran framework are applicable in the
present study to form anticipated addition products 4r−4t.
However, aliphatic alkenes such as 1-hexene, trans-3-hexene,
and cyclohexene are not applicable in this hydroarylation
reaction. Less sterically hindered aromatic alkene, e.g., styrene,
is also precluded for the reaction due to the formation of an
oligomer under this catalyst system.11
a
Reaction conditions: N-methylaniline 2a (0.45 mmol), alkenes 5
(0.3 mmol), Sc dialkyl complex 1 (0.03 mmol), [PhNMe2H][B-
(C6F5)4] (0.03 mmol), toluene (2 mL), 120 °C, isolated yield.
trisubstituted alkenes to generate 6c and 6d with a newly
formed quaternary carbon at the para-position of N-methylani-
line, albeit with lower yields (Table 1, entries 3 and 4).
Notably, a tricyclic compound with a large π-conjugated
system, namely, acenaphthylene, can be facilely hydroarylated
by N-methylaniline to give the corresponding dearomatizative
product 6e in 60% isolated yield within 48 h. Attempts to
extend the substrate scope to tetra-substituted aromatic
alkenes failed under our typical conditions probably because
of the severe steric hindrances in the alkene.
Given that the scandium dialkyl 1/[PhNMe2H][B(C6F5)4]
catalytic system exhibited excellent performance in chemo- and
regioselective para-alkylation of N-methylaniline with terminal
or internal aromatic alkenes, we next screened aniline
substrates under our typical reaction conditions (α-methyl-
styrene as an alkene coupling partner); the results are
summarized in Scheme 3. Substrates with an electron-donating
group (methyl) or electron-withdrawing groups (F, Cl, and Br)
at the ortho-position of N-methylaniline were compatible with
the reactions, exclusively yielding addition products 7a−7d
without noticeably changing the catalytic activity. When
increasing the steric hindrance at the nitrogen atom, even
with some fused rings, such para-alkylation reactions still took
place to generate products 7e−7j in high yields. It is
remarkable that primary anilines are also involved in this
system. For instance, aniline and o-chloro-/methoxy-substi-
tuted aniline reacted with α-methylstyrene to furnish the para-
addition products 7k−7m with a 10 mol % catalyst loading.
The molecular structure of compound 7m was further
confirmed by single-crystal X-ray diffraction (for details, see
Subsequently, reactions of 1,2-disubstituted and 1,1,2-
trisubstituted aromatic alkenes with N-methylaniline 2a were
examined under analogous conditions (Table 1). Treatment of
indene or 1,2-dihydronaphthalene with 2a successfully gave
para-alkylation compounds 6a (82%) and 6b (84%),
respectively, without other regioisomers being observed. The
reactions also proceeded for more sterically crowded
B
DOI: 10.1021/acs.orglett.9b03451
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Organic Letters
Letter
Scheme 3. Scandium-Catalyzed Hydroarylation of α-
Methylstyrene with Primary or Secondary Anilines
Scheme 4. Selected Control Experiments for the
Mechanistic Study of the Scandium-Catalyzed para-
Selective Alkylation Reaction
a
a
Reaction conditions: aniline 2 (0.45 mmol), α-methylstyrene 3a (0.3
mmol), Sc dialkyl complex 1 (0.03 mmol), [PhNMe2H][B(C6F5)4]
(0.03 mmol), toluene (2 mL), 120 °C, 36 h, isolated yield.
Supporting Information). In contrast, when using an aniline
substrate with a protecting group (e.g., methyl group) at the
para-position under our standard conditions, hydroarylation
still occurred but selectively gave ortho-alkylated product 8 in
high yield.
exclusive formation of para-alkylated primary aniline 7k within
5 h through a rearrangement reaction (Scheme 4d).
Apparently, the combination of scandium complex 1 with
the borate reagent we employed can serve as an active catalyst
for both hydroamination and Hofmann−Martius rearrange-
ment reactions.
It is well-known that rare-earth metal alkyl complexes are
prone to protonolysis with primary or secondary amines to
form rare-earth amide complexes, which serve as active species
for subsequent hydroamination reactions with alkenes to afford
N-alkylation products.3,13 It is also well-documented that metal
Lewis acids can mediate Hofmann−Martius rearrangement
reactions of N-alkylated anilines to give the corresponding
ortho- or para-alkylated anilines.14 Thus, the above scandium-
catalyzed para-alkylation reaction of primary/secondary ani-
lines might proceed via a tandem hydroamination/Hofmann−
Martius rearrangement reaction. Alternatively, classical Frie-
del−Crafts reaction through electrophilic aromatic substitution
could also lead to the same results.8 To elucidate which
pathway is preferred, several control experiments were
conducted (Scheme 4). First, the stoichiometric reaction of
in situ generated cationic scandium alkyl complex11b with N-
methylaniline was investigated. The reaction clearly gave the
expected cationic scandium amide complex 9 in 80% isolated
yield (Scheme 4a) via protonolysis of the highly reactive Sc−C
bond by the N−H group. We found that the coordinated N-
methylaniline molecule in the complex stabilized the scandium
center. When directly applying 10 mol % of complex 9 as a
catalyst in the reaction of N-methylaniline with α-methylstyr-
ene under standard conditions, it produced para-alkylation
product 4a in 79% yield (Scheme 4b), which suggests that
scandium amide complex 9 behaved as a catalytic intermediate
in the hydroarylation reaction.
Third, a deuterium labeling experiment was also performed
to provide more information about the reaction mechanism.
The reaction of deuterated N-methylaniline (2a-d) with α-
methylstyrene (3a) catalyzed by 1/[PhNHMe2][B(C6F5)4]
gave the para-alkylation product 4a-d (Scheme 4e), which
showed ca. 65% deuterium incorporation at the newly formed
methyl unit as expected. In addition, we also observed 30%
deuterium incorporation at the ortho position of 4a-d. This is
because substrate 2a-d could be partially converted into ortho-
deuterated N-methylaniline under our catalytic system (for
details, see Supporting Information). All the above observa-
tions indicate that such scandium-catalyzed hydroarylation
reaction proceed with a tandem hydroamination/Hofmann−
Martius rearrangement mechanism, rather than a classical
Friedel−Crafts reaction mechanism. We also propose that
highly para-selective rearrangement may be caused by both the
sterically hindered alkene substrate and the bulky scandium
catalyst.
In summary, para-selective alkylation of aromatic primary
and secondary amine using various sterically demanding
alkenes has been achieved by utilization of an easily accessible
β-diketiminato scandium dialkyl complex in combination with
a borate reagent. The corresponding C(sp2)−C(sp3) bond-
forming reaction occurred with a 100% atom economy and
good functional group tolerance. Remarkably, this protocol
showed excellent chemo- and regioselectivity in comparison
with widely investigated acid- or transition-metal-catalyzed
Friedel−Crafts alkylation reactions.8 Mechanistic studies
disclosed that the resulting para-alkylated anilines were formed
by an intermolecular hydroamination reaction followed by
Hofmann−Martius rearrangement. This study may provide
new insights into N−H and C−H functionalization reactions
Second, the reaction of 2,4,6-trimethylaniline with α-
methylstyrene was carried out in the presence of 1/
[PhNMe2H][B(C6F5)4], which successfully led to the isolation
of N-alkylation product 10 (Scheme 4c), albeit requiring an
increased amount of catalyst loading probably because of
severe steric hindrance at the aniline substrate. In addition, we
prepared a secondary aniline compound 1115 and then treated
it under our catalyst system at 120 °C, which resulted in the
C
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Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b03451.
Experimental details, compound characterizations, and
X-ray crystallographic data for compound 7m (PDF)
Accession Codes
CCDC 1956634 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: xinxu@suda.edu.cn.
ORCID
(7) Song, G.; Luo, G.; Oyamada, J.; Luo, Y.; Hou, Z. ortho-Selective
C-H addition of N,N-dimethyl anilines to alkenes by a yttrium
catalyst. Chem. Sci. 2016, 7, 5265.
Xin Xu: 0000-0003-2426-9816
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21871204), 1000-Youth Talents Plan,
PAPD, and the project of scientific and technologic infra-
structure of Suzhou (SZS201708).
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