Copper(II) triflate catalyzed allylic arylation of allylic alcohols: Direct and selective access to C-allylanilines

Article abstract of DOI:10.1002/cctc.201300408

Copper(II) triflate was used as a simple and commercially available catalyst in the direct amination of allylic alcohols with anilines to provide C-allylanilines. A wide range of functional groups were tolerated under the reaction conditions, and the products were obtained regioselectivity without the need of an activator. A detailed mechanistic investigation was undertaken. The efficacy of this protocol was demonstrated in the conversion of the 2-allylanilines into substituted quinolines through an oxidative cycloaddition reaction. All(yl) is good: The direct and selective synthesis of C-allylanilines through Cu(OTf)₂-catalyzed (OTf=trifluoromethanesulfonyl) allylic amination involving allylic alcohols and anilines is reported. The reaction can be performed under mild conditions and shows wide functional group tolerance and regioselectivity; the products are obtained in good to high yields. Detailed mechanistic investigations are also discussed. Copyright

Full text of DOI:10.1002/cctc.201300408

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DOI: 10.1002/cctc.201300408
Copper(II) Triflate Catalyzed Allylic Arylation of Allylic
Alcohols: Direct and Selective Access to C-Allylanilines
Ke Chen,[a, b] Houguang Jeremy Chen,[a] Jonathan Wong,[a] Jinglei Yang,[b] and
Sumod A. Pullarkat*[a]
Copper(II) triflate was used as a simple and commercially avail-
able catalyst in the direct amination of allylic alcohols with ani-
lines to provide C-allylanilines. A wide range of functional
groups were tolerated under the reaction conditions, and the
products were obtained regioselectivity without the need of
an activator. A detailed mechanistic investigation was under-
taken. The efficacy of this protocol was demonstrated in the
conversion of the 2-allylanilines into substituted quinolines
through an oxidative cycloaddition reaction.
Introduction
2-Allylanilines are simple building blocks that occupy an im-
portant place in organic synthesis, as they can provide signifi-
cant amplification of molecular complexity.[1] 2-Allylanilines are
traditionally prepared from N-allylanilines through aza-Cope re-
arrangement.[2] Development of protocols that circumvent the
isolation of N-allylanilines and that allow efficient direct access
to C-allylanilines is therefore attractive, as the products ob-
tained are starting materials for the preparation of various bi-
cyclic nitrogen-containing heterocycles through cyclization of
the nitrogen atom onto the double bond.[1] However, the re-
ported syntheses of C-allylanilines by thermal, protic acid pro-
moted, and charge-accelerated rearrangements all have draw-
backs, including harsh reaction conditions, low yields, and lim-
ited substrate scope, and are much less developed (unlike syn-
theses leading to their N-allylaniline analogues).[2c–g] Recent re-
ports of microwave-assisted and heteropoly acid (HPA)
catalyzed processes have also appeared, but these methods
also suffer from similar drawbacks.[2a,b]
Figure 1. Nucleophiles used in reactions involving CÀC bond formation by
allylic substitution.
tively poor reactivity of their alcohols analogues. However,
direct catalytic substitution involving allylic alcohols avoids the
formation of stoichiometric amounts of unwanted chemical
waste during the synthesis of activated substrates and is there-
fore more environmentally benign.[4c,5] Furthermore, the proce-
dure produces water as the sole byproduct and hence is
highly attractive, especially for large-scale synthesis. However,
as a result of the aforementioned poor leaving ability of the
hydroxy moiety, the direct substitution of allylic alcohols typi-
cally requires high temperatures,[6] neat conditions,[7] or consid-
erable amounts of an activator.[8,9]
In reactions involving CÀC bond formation by allylic substi-
tution, malonate,[3] indole,[3,4] thiophene,[4a] pyrrole,[3,4c] benzo-
furan,[4a] furan,[4a,e] and benzothiophene[4a] have all been report-
ed as common nucleophiles (Figure 1). Activated allylic alcohol
derivatives such as allylic halides, carboxylates, and carbonates
are common substrates in these scenarios owing to the rela-
Allylic amination by employing anilines is an attractive route
towards the formation of allylaniline derivatives through ArÀC
or NÀC bond formation. A survey of the literature revealed sev-
eral reports on the transition-metal-catalyzed allylic amination
of allylic alcohol derivatives such as allyl halides, carboxylates,
and carbonates (and to a lesser extent alcohols themselves)
with arylamines (and to a lesser extent anilines because of
a buffering effect, which renders them ineffective for acid-cata-
lyzed nucleophilic substitution reactions). However, these
methods inevitably led to the predominant generation of
N-allylanilines (Scheme 1).
[a] Dr. K. Chen, H. J. Chen, J. Wong, Dr. S. A. Pullarkat
Division of Chemistry and Biological Chemistry
School of Physical & Mathematical Sciences
Nanyang Technological University
Singapore 637371 (Singapore)
Fax: (+65)63166984
E-mail: sumod@ntu.edu.sg
[b] Dr. K. Chen, Dr. J. Yang
Division of Aerospace Engineering
School of Mechanical and Aerospace Engineering
Nanyang Technological University
Singapore 639798 (Singapore)
For instance, metal-catalyzed allylic aminations involving ani-
lines and alcohols (or activated alcohol derivatives) include,
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300408.
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Results and Discussion
Recent literature reports have shown that copper catalysts can
be efficient in various amination reaction scenarios.[18] We
therefore initiated our study by screening various copper com-
pounds for the aforementioned reaction scenario and found
that Cu(OTf)2 was the most effective. From the preliminary
evaluation of reaction conditions with 1,3-diphenyl-2-propen-
1-ol and p-toluidine as representative substrates, we found
that the best results (90% yield) were obtained with 1,2-di-
chloroethane as the solvent at 708C over 3 h. The same reac-
tion with the use of the palladacycle catalyst required 12 h for
completion and provided the product in 85% yield under the
same conditions. With the optimum conditions thus estab-
lished, we proceeded to screen various substituted anilines. As
shown in Table 1, the reaction showed a marked preference for
the ArÀC products, namely, 2-allylanilines, over the NÀC prod-
ucts, N-allylanilines, if the para position of the aniline was
blocked by a substituent.
Scheme 1. Common allylic substitution reactions that lead to N-allylanilines.
amongst other metals, the use of palladium[6,9,10] and plati-
num[5b,7d,11] and to a lesser extent iron,[12] cobalt,[13] and molyb-
denum.[3] Notably, in all of these aforementioned protocols, the
N-allylaniline is the predominant product formed irrespective
of the catalyst employed. Although there has been reports on
the [Cu(CH3CN)4]PF6 and CuCl2·2H2O–Cu powder catalyzed al-
lylic amination of olefins with aryl hydroxylamines[14] and nitro-
soarenes,[15] catalytic allylic amination of alcohol substrates
with anilines as a route towards allylanilines has, however, not
been explored hitherto.
Recently, we reported the palladacycle-catalyzed direct syn-
thesis of 2-allylanilines from allylic alcohols (Scheme 2).[16] In
line with this report on the development of a direct and
“green” synthesis of C-allylanilines from allylic alcohols and in
A previous report in which proton- and metal-exchanged
montmorillonites were used as catalysts stated that if an elec-
tron-donating substituent such as a methoxy group was pres-
ent at the para position of the aniline, the C-allylaniline was
formed as the major product (93% yield).[19] However,
that reaction required 1508C and over 24 h to reach
completion, and upon extending the substrate scope
to include electron-withdrawing groups (e.g., 4-
chloro-substituted anilines), the N-allylaniline was ob-
tained as the major product, thus exhibiting lack of
product selectivity (in our case, 3af was isolated in
90% yield). For substrates such as 4-hydroxyaniline
bearing both NH2 and OH functionalities, the reaction
preferred to occur selectively at the ortho position of
the NH2 group (3ad, Table 1). This result is consistent
with the relative donor ability of the NH2 and OH
groups and the directing effect they consequently
impart during such allylic arylation scenarios. Similar
products were obtained upon screening of anilines with func-
tional groups such as MeO and PhO. These donating groups
are known to enhance the reactivity of the aniline towards allyl
arylation. As expected, their allylic arylation generated the CÀC
bond products selectively in good to high yields.
Scheme 2. Palladacycle-catalyzed synthesis of C-allylanilines.
the context of our general interest in the development of effi-
cient CÀP, CÀC, and CÀN bond-formation protocols,[17] we con-
tinued our search for a significantly cheaper, more efficient,
and easily accessible catalyst for this useful protocol.
We therefore explored the application of copper catalysts in
the direct synthesis of 2-allylanilines from allylic alcohols and
anilines without the requirement of any additional additive or
activating agent. In comparison to the palladacycle-catalyzed
scenario, cheaper and more active copper(II) trifluoromethane-
sulfonate (triflate, ÀOTf) promoted the allylic arylation reaction
in a much shorter reaction time (3 versus 12 h) with
a wider range of substrates in a more efficient
manner (Scheme 3). Key mechanistic insights into the
reaction mechanism were also obtained by control
reactions and variable-temperature NMR (VT-NMR)
spectroscopy. The potential for further transforma-
tions of the formed products is also demonstrated.
We then proceeded to extend the substrate scope to ani-
lines devoid of any para substituents. This class of substrate
was not studied in our previous report in which a palladacycle
catalyst was used and has also never been reported in other
studies involving the formation of C-allylanilines. Employing
Scheme 3. Cu(OTf)2-catalyzed allylic arylation of allylic alcohols.
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hols and anilines also has a pro-
found influence on the product
regioselectivity. The less-hin-
dered position on the allylic al-
cohol substrate is preferred by
the incoming aniline for forma-
tion of the CÀC bond. Further-
more, in the case of anilines in
which the para position is
blocked by substituents, the re-
Table 1. Cu-catalyzed allylic arylation of allylic alcohols with various anilines.[a]
action
showed
significantly
better regioselectivity with re-
spect to the allylic alcohol owing
to the steric hindrance offered
by the incoming aniline nucleo-
phile. Consequently, in the case
of anilines in which the para po-
sition is vacant, this regioselec-
tivity preference was not promi-
nent (Table 3, Entries 1 and 3).
To obtain mechanistic insight,
a set of reactions including one
that was closely monitored by
VT-NMR spectroscopy were sub-
sequently performed. A stepwise
reaction was performed between
allylic alcohol 1a/1b and 2,4-di-
[a] All reactions were performed at 708C for 3 h with the allylic alcohol (0.5 mmol) and the amine (0.75 mmol).
Yield of the isolated product is given in parentheses.
the new protocol, C-allylaniline products, that is, 4-allylanilines,
were selectively generated (Table 2). If the para position and
one of the two ortho positions were simultaneously available,
4-allylanilines were still preferentially formed instead of 2-allyl-
Table 2. Cu-catalyzed allylic arylation of allylic alcohols with various
anilines.[a]
anilines (6aa–al, Table 2) irrespective of the electronic nature
of the ortho substituent. Notably, anilines with nitro, chloro,
and methyl substituents at the ortho position were previously
employed as substrates in the direct amination of allylic alco-
hols by using Pt(cod)Cl2–diphosphine[5b,11c] and MoO2(acac)2
catalytic systems (cod=1,5-cyclooctadiene, acac=acetylaceto-
nate).[3] However, in such instances they gave the correspond-
ing N-allylanilines as the main products.
Reactions involving anilines bearing electron-withdrawing
functional groups such as CF3 and CN (present at either the
para or ortho position) also selectively gave the CÀC products,
that is, 2-allylanilines or 4-allylanilines, respectively (3ah and
3ai, Table 1; 6af and 6ag, Table 2). This study thus extends the
direct arylation protocol to include ortho-substituted anilines,
which were conspicuously absent from all previous reports in-
volving the synthesis of C-allylanilines.
Subsequent to screening anilines, we also evaluated the ap-
plicability of this method to various allylic alcohol substrates
(Table 3). Better reactivity and higher yields were obtained for
allylic alcohols in which the R’ group was aryl or tBu (Table 3,
Entries 7, 9, 10, and 12). However, reactions in which relatively
less activated alkyl-substituted allylic alcohols (Table 3, En-
tries 1–6 and 8) were used required higher reaction tempera-
tures or longer reaction times to obtain better yields of the
products. These results are consistent with the fact that alkyl-
substituted allylic alcohols are known to be more prone to
side reactions. The steric hindrance offered by the allylic alco-
[a] All reactions were performed at 708C for 3 h with the allylic alcohol
(0.5 mmol) and the amine (0.75 mmol). Yield of the isolated product is
given in parentheses.
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Table 3. Cu-catalyzed allylic arylation of various allylic alcohols with
anilines.[a]
Table 3. (Continued)
Entry
Allylic alcohol
Product, yield[b] [%]
11
1g
4gj, 72
4hj, 82
12
1h
[a] Unless otherwise indicated, the reaction was performed at 708C with
the allylic alcohol (0.5 mmol) and the amine (0.75 mmol). [b] Yield of the
isolated product. [c] Reaction was performed at 908C for 6 h. [d] Ratio of
6ba/7ba=5:1 and the total yield of (6ba/7ba) was 78%. [e] Ratio of
6bj/7bj=2.2:1 and the total yield of (6bj/7bj) was 85%. [f] Reaction was
performed at 908C for 12 h.
Entry
1[c]
Allylic alcohol
Product, yield[b] [%]
1b
1b
1b
1b
1b
6ba, 65[d]
3ba, 79
6bj, 59[e]
3bj, 85
2[c]
3[c]
4[c]
5[c]
Scheme 4. Stepwise reaction conducted to elucidate the reaction pathway.
methylaniline (2j) in the presence of Cu(OTf)2 (5 mol%), as
shown in Scheme 4. The results revealed that initially formed
kinetically favored product 8aj/8bj subsequently gave rise to
thermodynamically favored product 3aj/3bj upon heating.
To confirm the above hypothesis, the reaction between al-
lylic alcohol 1a and 2,4-dimethylaniline (2j) was monitored by
VT-NMR spectroscopy (Figure 2, see also the Supporting Infor-
6bb, 73
1
mation). In the H NMR spectra of allylic alcohol 1a, N-allylani-
6[c]
1b
1c
3bb, 74
4cj, 75
line 8aj, and C-allylaniline 3aj in [D4]-1,2-dichloroethane, the
resonance for the a-CH proton is distinct and appears at d=
5.39, 5.14, and 4.85 ppm, respectively, at room temperature
and at d=5.40, 5.18, and 4.92 ppm, respectively, at 808C.
These signals provide a good spectroscopic handle to monitor
the progress of the reaction. Only the resonance signal due to
a-CH of allylic alcohol 1a was observed immediately after the
NMR sample tube containing the reaction mixture was intro-
duced into the magnet maintained at 808C (Figure 2, 0 min).
As expected, the signal of the a-CH proton of CÀN product
8aj was observed 5 min after the start of the VT-NMR spec-
troscopy experiment. This indicated that the kinetically fa-
vored
7
8[f]
1d
1e
1 f
3dj, 60
4ej, 85
4 fj, 71
9
10
CÀN product was initially formed. After 10 min, the character-
istic resonance for the a-CH proton of CÀN product 8aj and
that of CÀC product 3aj were observed simultaneously. Sub-
sequently, the concentration of thermodynamically favored
CÀC product 3aj increased as the reaction progressed. After
50 min, allylic alcohol 1a and CÀN product 8aj were com-
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8aj into thermodynamically fa-
vored product 3aj rather than
an aza-Cope rearrangement or
Hofmann–Martius sequence.
Another control experiment
with 8bj was conducted under
the optimum conditions, and
3bj was obtained as the major
product
instead
of
4bj
(Scheme 7). This result further
confirmed that the transition of
8bj into 3bj does not occur
through an aza-Cope rearrange-
ment, as 4bj should be the sole
product if the reaction proceed-
ed though this route.
Finally, the synthetic utility of
the obtained 2-allylaniline prod-
ucts was illustrated by a simple
transformation. The 2-allylani-
lines were readily converted
under mild conditions into sub-
stituted quinolines through an
Figure 2. 1H NMR spectra at 808C showing the gradual appearance and subsequent change in the concentrations
of the CÀN and CÀC products with reaction time (see the Supporting Information for more detailed spectra, in-
cluding the initial and final spectra at RT).
pletely converted into CÀC product 3aj, as determined by the
complete disappearance of the signals for 1a and 8aj and the
appearance of the signal for 3aj only (see the Supporting In-
formation for detailed spectra). After cooling of the magnet to
1
room temperature, the H NMR spectrum showed that the re-
action was clean, and only the signals for CÀC product 3aj
Scheme 6. Control experiment that illustrates allylic arylation.
and an excess amount of 2,4-dimethylaniline were observed.
To further understand how the kinetically favored CÀN prod-
uct was converted into the thermodynamically favored CÀC
product, several monitored experiments were conducted. A
control experiment for the Hofmann–Martius rearrangement
step was set up with 9, and rearrangement product 10 was
not obtained under the optimum reaction conditions used for
the catalysis or at elevated temperatures and longer reaction
times (Scheme 5). This result confirmed unambiguously that
Cu(OTf)2 cannot promote the Hofmann–Martius rearrangement
under the optimum conditions employed.
Scheme 7. Control experiment to confirm the allylic arylation sequence.
Additionally, a mixture of 3aj and 3ak was observed upon
treatment of 8aj with aniline 2k (1 equiv.) under the optimum
reaction conditions (Scheme 6). This result indicated that CÀN
cleavage was involved in the conversion of N-allylaniline into
the C-allylaniline. Therefore, the reaction proceeded by further
allylation in the conversion of kinetically favored product
oxidative cycloaddition reaction (Scheme 8). This cycloaddition
therefore provides a new route towards the synthesis of multi-
substituted quinolines of biological importance. Additionally,
the one-pot synthesis of a quinoline derivative from an allylic
alcohol and aniline was also demonstrated, as shown in
Scheme 8. These reaction conditions, however, were not opti-
mized with respect to the cyclization step, and therefore, the
products were obtained in only moderate yields.
The utility of 2-allylanilines in the synthesis of biologically
relevant heterocycles was previously demonstrated by other
groups. Some notable examples include the use of C-allylani-
lines in the efficient Pd-catalyzed synthesis of N-aryl-2-benzylin-
dolines by Wolfe et al.,[1k] as well as in the enantioselective syn-
thesis of 2,5-dihydrobenzo[b]azepine derivatives by an allylic
amination/ring-closing metathesis sequence by You et al.[1a]
significant number of highly efficient reaction protocols per-
A
Scheme 5. Control experiment to study a possible Hofmann–Martius rear-
rangement.
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Conclusions
In summary, we have demonstrated the use of a simple and
commercially available catalyst, copper(II) triflate, for the direct
and selective synthesis of C-allylanilines in good to high yields
by direct allylic amination from relevant allylic alcohols and
anilines. A wide range of functional groups were tolerated
under the reaction conditions, and the products were obtained
regioselectivity without the need of an activator. A detailed
mechanistic investigation was undertaken, which included con-
trol experiments and VT-NMR spectroscopy analysis. The effica-
cy of this protocol in providing subsequent access to substitut-
ed quinolines was also demonstrated. The development of
an asymmetric variant is currently being pursued in our
laboratory.
Keywords: allylic compounds · amination · arylation · copper ·
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Received: May 29, 2013
Published online on && &&, 0000
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FULL PAPERS
K. Chen, H. J. Chen, J. Wong, J. Yang,
S. A. Pullarkat*
&& – &&
Copper(II) Triflate Catalyzed Allylic
Arylation of Allylic Alcohols: Direct
and Selective Access to C-Allylanilines
All(yl) is good: The direct and selective
synthesis of C-allylanilines through
mild conditions and shows wide func-
tional group tolerance and regioselec-
tivity; the products are obtained in
good to high yields. Detailed mechanis-
tic investigations are also discussed.
Cu(OTf)2-catalyzed (OTf=trifluorometh-
anesulfonyl) allylic amination involving
allylic alcohols and anilines is reported.
The reaction can be performed under
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers!