Iron-catalysed tandem isomerisation/hydrosilylation reaction of allylic alcohols with amines

Article abstract of DOI:10.1039/c4ra04037f

An iron(0)-catalysed cascade synthesis of N-alkylated anilines from allylic or homoallylic alcohols and primary and secondary anilines under hydrosilylation conditions has been developed. Notably, a simple Fe(cod)(CO)₃ complex (cod = cycloocta-

Full text of DOI:10.1039/c4ra04037f

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Iron-catalysed tandem isomerisation/
hydrosilylation reaction of allylic alcohols with
Cite this: RSC Adv., 2014, 4, 25892
amines†
Received 21st February 2014
Accepted 29th May 2014
*
Haoquan Li, Mathieu Achard, Christian Bruneau, Jean-Baptiste Sortais
*
and Christophe Darcel
DOI: 10.1039/c4ra04037f
www.rsc.org/advances
An iron(0)-catalysed cascade synthesis of N-alkylated anilines from methodologies providing powerful alternatives to classical
allylic or homoallylic alcohols and primary and secondary anilines multistep sequences. Furthermore, one efficient and atom
under hydrosilylation conditions has been developed. Notably, a economical way to generate in situ saturated carbonyl deriva-
simple Fe(cod)(CO)3 complex (cod ¼ cycloocta-1,5-diene) was used as tives is the redox isomerization of allylic alcohols usually
a precatalyst under visible light activation in ethanol in the presence of promoted by transition metal such as Ru, Rh and Ir.9 Iron
the inexpensive and non-toxic PMHS (polymethylhydrosiloxane) as the Fe(CO)x type complexes have also been described as useful
hydrosilane source. This methodology is based on a three step-one catalysts for allylic alcohol isomerization under irradiation
sequence process involving isomerisation of allylic alcohols/conden- conditions,10,11 and few examples of tandem isomerization–
sation with anilines/reduction reactions.
aldolization reactions were reported.12
In the present contribution, in the continuation of our
research interest on iron-catalysed reduction,13–15 the iron(0)-
catalysed synthesis of N-alkylated anilines from both allylic and
homoallylic alcohols and primary and secondary anilines under
The production of alkylated anilines is an important economic
area in agrochemical, dye, and pharmaceutical industries.1
Classically, their syntheses are based on N-alkylation of primary
aniline derivatives with alkyl halides and transition metal cat-
alysed cross coupling reactions,2 hydroamination of unsatu-
rated C–C bonds,3 or condensation of anilines with carbonyl
derivatives such as aldehydes and ketones followed by reduc-
tion with borohydride type reducing reagents.4 In the latter
eld, catalytic reductive amination reaction of carbonyl deriv-
atives via transition metal catalysed hydrogenation, hydrogen
transfer or hydrosilylation of the in situ formed imines is rising
as a major way in molecular synthesis to obtain amines.5 As
inexpensive, abundant and environmentally benign metal, iron
is a particularly valuable alternative to precious transition
metals, thus intense research on its catalytic potential has
emerged during the past decade.6 Up to now, iron catalysts were
only used scarcely to perform reductive amination.7,8
Table 1 Optimization of the parameters for the reaction of (E)-hex-2-
en-1-ol with N-ethylaniline and PMHSa
Entry
Cat. (mol%)
1a/2a
Temp. (ꢀC)
Conv.b (%)
1
2
3
4
5
6
7
8
Fe(CO)5 (5)
Fe2(CO)9 (2.5)
1.5
1.5
1.5
1.5
1.5
2
1.5
1.5
2
50
50
50
50
50
40
50
40
40
50
50
50
68
31
83
83
0
Fe3(CO)12 (1.7)
Fe(PBO)(CO)3 (5)
Fe(cot)(CO)3 (5)
Fe(IMes)(CO)4 (5)
Fe(cod)(CO)3 (5)
Fe(cod)(CO)3 (5)
Fe(cod)(CO)3 (5)
Fe(cod)(CO)3 (5)
Fe(cod)(CO)3 (2.5)
Fe(cod)(CO)3 (5)
On another hand, in the quest for eco-friendly and atom
economic protocols, cascade and tandem procedures have
attracted signicant attention as efficient synthetic
0
95
88
88
76
78
64
9
10
11
12c
1.1
1.5
1.5
´
UMR 6226 CNRS – Universite de Rennes 1 “Institut des Sciences Chimiques de
Rennes”, Equipe “Organometallics: Materials and Catalysis”, “Centre for Catalysis
and Green Chemistry”, Campus de Beaulieu, 35042 Rennes, France. E-mail:
jean-baptiste.sortais@univ-rennes1.fr; christophe.darcel@univ-rennes1.fr
a
Aniline 2a (0.25 mmol), allylic alcohol 1a (1.1–2 equiv.), PMHS (45 mL,
0.75 mmol, 3 equiv.), iron complex (1.7–5 mol%), EtOH (1 mL), at 50 ꢀC
for 20 h in the presence of visible light activation (24 watt compact
† Electronic supplementary information (ESI) available: Experimental section,
spectral data and 1H and 13C NMR spectra of all compounds are provided. See
DOI: 10.1039/c4ra04037f
b
c
uorescent lamp). Conversion determined by GC. Without light
activation.
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Table 2 Scope of the iron-catalysed reaction of allylic and homoallylic alcohols with secondary aminesa
Entry
Alcohol
Product
Conv.b (%)
Yieldc (%)
86
1
2
95
>95
80d
3
4
>98
>95
95
82
5
6
7
>95
83
95
75
89
>98
8
Phytol
75
65
9
>95
>95
81
10
86d
11
12
13
14
>95
>95
78
80
81
70
>95
50
95
31
15
a
Amine (0.25 mmol), allylic alcohol (0.375 mmol, 1.5 equiv.), PMHS (45 mL, 0.75 mmol, 3 equiv.), Fe(cod)(CO)3 (0.0125 mmol, 5 mol%), EtOH (1
mL), at 50 ꢀC for 20 h in the presence of visible light activation (24 watt compact uorescent lamp). b Conversion determined by GC. c Isolated yield.
d
1
Yield determined by H-NMR with 1,3,5-trimethoxybenzene as an internal standard.
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hydrosilylation conditions has been developed. Notably, a
Notably, this transformation works well also with homo-
simple Fe(cod)(CO)3 complex (cod ¼ cycloocta-1,5-diene) was allylic alcohols leading to the corresponding tertiary anilines
used as the precatalyst under visible light activation in ethanol with 80–86% yields (entries 9–11). Furthermore, starting from
using the inexpensive and non-toxic PMHS (poly- allylic alcohols such as (E)-hex-2-en-1-ol and cinnamyl alcohol,
methylhydrosiloxane) as the hydrosilane source (eqn (1)).
several secondary anilines such as indoline and 1,2,3,4-tetra-
hydroquinoline could be coupled successfully leading to the
corresponding tertiary amino compounds with good to excel-
lent isolated yields (70–95%, entries 12–14). In contrast, the
reaction of cinnamyl alcohol with N-benzylaniline led to only
50% conversion and 31% yield (entry 15).
(1)
We started our investigation by the model reaction of (E)-hex-
2-en-1-ol 1a with N-ethylaniline using 3 equiv. of PMHS as the
hydrosilane in ethanol under visible light irradiation (Table 1).‡
With the commercially available Fe(CO)5 (5 mol%), Fe2(CO)9
(2.5 mol%) and Fe3(CO)12 (1.7 mol%) complexes, at 50 ꢀC for 20
h, using 1.5 equiv. of the allylic alcohol 1a, moderate to good
conversions were obtained (31–83%), and the tertiary aniline 3a
was obtained selectively (Table 1, entries 1–3). These results
The substrate scope of amine was then extended to primary
aniline derivatives 4. The optimization of the reaction condi-
tions are summarized in the Table 3. In the presence of 1.5
equiv. of (E)-hex-2-en-1-ol 1a and 3 equiv. of PMHS in ethanol at
70 ꢀC for 20 h, the aniline 4a was fully converted and a mixture
90 : 10 of the monoalkylated and dialkylated anilines was
obtained (entry 1). The decrease of the temperature to 50 ꢀC,
even if it permitted to increase the selectivity to 95 : 5, has a
deleterious effect on the conversion (67%, entry 2). By contrast,
ꢀ
show that under the mild reaction conditions (50 C), the iso-
ꢀ
merisation of the allylic C]C bond took place leading to the
corresponding aldehyde which reacted faster with the
secondary aniline 2a than with PMHS leading to the corre-
sponding imines which can be then reduced. Using Fe(P-
BO)(CO)3 (5 mol%) (PBO ¼ 4-phenylbut-3-en-2-one) which was
the catalyst of choice for the selective hydrosilylation of
carboxylic acids to aldehydes,15b under the same conditions,
similar conversion was obtained (83%, entry 4). With
Fe(cot)(CO)3 (cot ¼ cyclooctatetraene) and (IMes)Fe(CO)4 (IMes
working at 70 C for 20 h but decreasing the quantity of allylic
alcohol 1a used to 1.25 equiv. permitted to obtained an excellent
selectivity towards the monoalkylation (5a/6a ¼ 97 : 3) at high
conversion (91%, entry 4).
The scope of the selective preparation of secondary aniline
derivatives starting from the corresponding primary anilines
was then evaluated and the results are summarized in Table 4.
By reaction with p-methoxyaniline, (E)-hex-2-en-1-ol 1a led to
the N-(n-hexyl)-p-methoxyaniline with 72% yield (entry 2).
Similarly, good results were obtained with cinnamyl alcohol
affording the alkylated aniline derivatives in 66–91% isolated
yields (entries 3–5 and 7–11). Notably there is no strong inu-
ence of the electronic or steric parameters of the para-substit-
uent of the aniline partner on the performance of the catalytic
system. Furthermore, it is noticeable that when the p-iodoani-
line was used, even if the conversion reached 40%, no desired
alkylated aniline derivative can be identied (entry 6). The
reaction of a-substituted allylic alcohols such as but-3-en-2-ol
with p-toluidine under similar conditions led to the N-isobutyl-
p-toluidine with 91% isolated yield (entry 12).
¼
1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene),15a the
starting materials were recovered quantitatively (entries 5 and
6). Interestingly, using Fe(cod)(CO)3 (5 mol%), under similar
conditions led to 95% conversion and the N-ethyl-N-hexylani-
line 3a was isolated with 86% yield (entry 7). When the
temperature was lowered to 40 ꢀC, even with 2 equiv. of the
allylic alcohol 1a, the conversion decreased (88%, entries 8 and
9). Similarly, when decreasing the amount of alcohol 1a to 1.1
equiv., or the loading of the Fe(cod)(CO)3 complex (2.5 mol%)
slightly lower conversions were observed (76 and 78%, respec-
tively, entries 10 and 11). Notably the visible light activation
seems important as in its absence a drastic decrease of the
conversion was observed (64% versus 95%, entries 12 and 7).
Hence, we selected the Fe(cod)(CO)3 complex as the pre-
catalyst for further investigation of the scope of the reaction
(Table 2). Using N-ethylaniline (1 equiv.) as the amine partner,
the condensation with several allylic alcohols (1.5 equiv.) were
investigated. (E)- and (Z)-hex-2-en-1-ols similarly led to the
N-ethyl-N-hexylaniline with 86 and 80% yields, respectively
(entries 1 and 2). With allylic alcohols bearing a geminated or a
tri-substituted olenic moiety, the tandem reaction proceeded
also nicely as the corresponding tertiary anilines were obtained
with good isolated yields (entries 4 and 5). Starting from cin-
namyl alcohol, 75% of the isolated aniline could be obtained
(entry 6). Interestingly, starting from (Z)-but-2-ene-1,4-diol, the
4-(N-ethyl-N-phenyl-amino)butan-1-ol was isolated with 89%
yield (entry 7). By reaction with phytol, a linear diterpene allylic
alcohol, the corresponding N-ethyl-N-phytylaniline was
obtained in 65% yield for a conversion of 75%.
Table 3 Optimization of the parameters for the reaction of (E)-hex-2-
en-1-ol with aniline and PMHSa
Entry
1a/4
Temp. (ꢀC)
Conv.b (%)
5a : 6ab
1
2
3
4
1.5
1.5
1.5
1.25
70
50
rt
>98
67
50
90 : 10
95 : 5
90 : 10
97 : 3
70
91
a
Aniline 4a (0.25 mmol), allylic alcohol 1a (1.25–1.5 equiv.), PMHS
(45 mL, 0.75 mmol, 3 equiv.), Fe(cod)(CO)3 (5 mol%), EtOH (1 mL), at
50–70 ꢀC for 20 h in the presence of visible light activation (24 watt
compact uorescent lamp). b Determined by GC.
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Table 4 Scope of the iron-catalysed reaction of allyl alcohols with primary aminesa
Entry
Alcohol
Product
Conv.b (%)
Yieldc (%)
1
2
95
88
72
>95
3
95
81
68
76
—
91
66
74
71
76
4
95
5
90
6
40d
>95e
82e
80
7
8
9
10
11
>95
82
12
>95f
91
13
14
15
16
>95
>95
>95
78
71
72
92
61
17
80
64
a
Amine (0.25 mmol), allylic alcohol (0.31 mmol, 1.25 equiv.), PMHS (45 mL, 0.75 mmol, 3 equiv.), Fe(cod)(CO)3 (0.0125 mmol, 5 mol%), EtOH
b
c
(1 mL), at 70 ꢀC for 20 h in the presence of visible light activation (24 watt compact uorescent lamp). Determined by GC. Isolated yield.
d
e
Conversion determined by 1H-NMR with 1,3,5-trimethoxybenzene as an internal standard. Less than 10% of dialkylated aniline derivative
detected in the crude mixture. f Reaction at 100 ꢀC for 20 h instead of 70 ꢀC for 20 h.
Interestingly anilines featuring a hydroxylated N-substituent now well established with iron carbonyl based catalysts,11 light
can be prepared with moderate yields (61–92%) by reaction of activation helping to generate the Fe(CO)3 active species by
(Z)-but-2-ene-1,4-diol with various substituted primary aniline decoordination of the cod ligand from the starting Fe(cod)(CO)3
compounds (entries 13–17).
pre-catalyst.16 Aer which the condensation of aldehydes with
The rst step in this tandem reaction is the isomerisation of anilines led to the formation of imines and iminium as the
the (homo)allylic alcohol to the corresponding aldehyde. This is intermediates which can be reduced under hydrosilylation
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conditions with PMHS. The iron catalysed reduction of imines7
and more particularly with PMHS was already reported
recently.8
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In summary, Fe(cod)(CO)3 complex was efficiently used as a
precatalyst for the selective synthesis of tertiary and secondary
anilines derivatives starting from allylic and homoallylic alcohols
and secondary and primary anilines, respectively, under hydro-
silylationconditions usingcheap andabundantPMHSreagentas
the hydrosilane reagent. It must also be pointed out that this
reaction was performed in ethanol under mild conditions (50–
70 ꢀC under visible light activation). This process correspond to a
formal reductive amination of (homo)allyllic alcohols via a
tandem isomerisation/condensation/hydrosilylation reaction.
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Acknowledgements
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We are grateful to the University of Rennes 1, CNRS, Rennes
´
Metropole and the Britanny council for nancial support. H.L.
thanks the foundation Rennes 1 for a Grant.
Notes and references
˜
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‡ Representative experimental conditions: A 20 mL oven dried Schlenk tube
containing a stirring bar, was charged with Fe(cod)(CO)3 (3.1 mg, 0.0125 mmol)
and then purged with argon/vacuum three times. Ethanol (1 mL), amine (0.25
mmol, 1 equiv.), (homo)allylic alcohol (0.313 mmol, 1.25 equiv.), PMHS (45 mL, 3
equiv.) were added under argon. The reaction mixture was stirred in a preheated
oil bath at 50–70 ꢀC for 20 h under light irradiation (using 24 watt compact
uorescent lamp). Then the reaction mixture was condensed under reduced
pressure. The residue was then puried by silica gel column chromatography
using a mixture of diethylether/petroleum ether as the eluent to afford the desired
product.
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´
´
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´
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´
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´
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´
´
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