Bifunctional iron complexes: Efficient catalysts for C=O and C=N reduction in water

Article abstract of DOI:10.1002/cctc.201300325

The application of bifunctional iron complexes for the hydrogenation of polarized C=X bonds is reported. Two modified Knoelker complexes bearing ionic fragments were synthesized and fully characterized. With these well-defined complexes, the reaction proceeded under mild conditions in pure water, and various alcohols and amines were isolated in good yields. Compared with hydrogenation in organic solvents, better reactivities were observed. Is this the iron age? Well-defined iron-complex-catalyzed reduction of aldehydes, ketones, and imines using molecular hydrogen in water is presented. Under mild conditions, good yields for a broad range of substrates are achieved.

Full text of DOI:10.1002/cctc.201300325

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D. S. Mꢀrel, M. Elie, J.-F. Lohier, S. Gaillard,
J.-L. Renaud*
&& – &&
Bifunctional Iron Complexes: Efficient
Catalysts for C=O and C=N Reduction
in Water
Is this the iron age? Well-defined iron-
complex-catalyzed reduction of
ed. Under mild conditions, good yields
for a broad range of substrates are
achieved.
aldehydes, ketones, and imines using
molecular hydrogen in water is present-
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DOI: 10.1002/cctc.201300325
Bifunctional Iron Complexes: Efficient Catalysts for C=O
and C=N Reduction in Water
Delphine S. Mꢁrel, Margaux Elie, Jean-FranÅois Lohier, Sylvain Gaillard, and
Jean-Luc Renaud*[a]
The application of bifunctional iron complexes for the hydro-
genation of polarized C=X bonds is reported. Two modified
Knçlker complexes bearing ionic fragments were synthesized
and fully characterized. With these well-defined complexes, the
reaction proceeded under mild conditions in pure water, and
various alcohols and amines were isolated in good yields. Com-
pared with hydrogenation in organic solvents, better reactivi-
ties were observed.
Introduction
Hydrogenation of C=C and C=O bonds plays a pivotal role in
fine chemistry. Thus, the reduction of carbonyl compounds to
alcohols is an industrially relevant reaction for the preparation
of fine chemicals, perfumes, agrochemicals, and pharmaceuti-
cals.[1] Taking into account the ecological point of view, among
the known reducing agents, the most promising one is molec-
ular hydrogen. Homogeneous hydrogenation has been an at-
tractive and intensive research area during the last decades,
because organometallic catalysts can be easily tuned and che-
moselectively reduce unsaturated double bonds. However, the
catalysts rely on precious metals such as rhodium, ruthenium,
iridium, or palladium.[2] Owing to their potential toxicity and
their limited availability, their replacement by environmentally
benign and cheaper catalysts is one of the great and exciting
challenges of this decade.
ble, nonexplosive, nontoxic and noncarcinogenic, and, further-
more, it is nonexpensive and easily accessible.
Iron-catalyzed reduction is a recent intensive research area.[6]
Although impressive progresses have been made in iron-cata-
lyzed transfer hydrogenation,[7] little is still known in hydroge-
nation of alkenes, alkynes,[8,9] and C=X bonds (X=O and NR).[10]
We have recently reported on iron-catalyzed reductive amina-
tion of aliphatic carbonyl derivatives using molecular hydro-
gen, under smooth reaction conditions in the presence of
a well-defined catalyst.[11] To the best of our knowledge, there
is no report on iron-catalyzed hydrogenation of aldehydes, ke-
tones, and imines under smooth reaction conditions with
a well-defined catalyst in pure water.[12] In our ongoing work
on iron-complexes-catalyzed reductions, we report our contri-
bution on the reduction of various polarized double bonds in
pure water under low hydrogen pressure and mild reaction
conditions.
Moreover, environmental concerns in chemistry also increase
the demand for more selective chemical processes with a mini-
mum of waste. For that reason, water-soluble organometallic
complexes attract continuously growing interest for applica-
tions in catalysis, because of their environmentally friendly
processing, simple product separation, and pH-dependent se-
lectivity in aqueous media.[3] Therefore, metal-catalyzed synthe-
ses in two-phase or even purely aqueous media has gained in-
creasing attention during the last decades with remarkable in-
dustrial success in C=C and C=O bond hydrogenation, alkene
hydroformylation, and carbonylation reactions.[4] However, cat-
alysis in water remains a challenging field of modern chemistry
and green synthesis.[5] As a solvent for organic reactions, water
possesses a number of attractive properties: it is nonflamma-
Results and Discussion
Synthesis of complexes
To solve the challenge iron-complex-catalyzed reductions and
to anticipate some problems with solubility, we prepared two
new ionic complexes (7 and 14). Their catalytic activities were
evaluated and compared with the activity of the well-known
Knçlker complex (15)[13] in the reduction of polarized double
bonds in water (Figure 1).
[a] D. S. Mꢀrel, M. Elie, J.-F. Lohier, Dr. S. Gaillard, Prof. J.-L. Renaud
Laboratoire de Chimie Molꢀculaire et Thioorganique
Normandie University, Universitꢀ de Caen Basse Normandie
CNRS, UMR 6507, INC3M, FR 3038
5 Boulevard du Marꢀchal Juin, 14050 Caen (France)
Fax: (+33)0-231452877
E-mail: jean-luc.renaud@ensicaen.fr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300325.
Figure 1. Bifunctional complexes for the catalytic hydrogenation in water.
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ble for X-ray analysis. This analy-
sis confirmed the atom connec-
tion in 13 (Figure 2). Then,
a treatment with trifluoracetic
acid (TFA), followed by the addi-
tion of methyl iodide, provided
the expected iron complex 14 in
67% yield for the last two steps
1
(Scheme 2). H NMR analysis con-
Scheme 1. Synthesis of the ammonium complex 7. Reaction conditions: a) nBuLi, TMSCl, aqueous H2SO4, THF,
À788C then 08C; b) PPh3, Br2, pyridine; CH2Cl2, 08C then RT; c) 3, DIPEA, CH3CN, 708C; d) Fe2(CO)9, toluene, 1108C;
e) CH3I, CH3CN.
firmed that the free amine in 14
was methylated, and that neither
The first bifunctional complex 7 bearing ionic framework
was obtained in three steps. The dialkylation of benzylamine
with the trimethylsilylpropargyl bromide 3, followed by
a [2+2+1] cycloaddition in the presence of Fe2(CO)9 in toluene
at 1108C, led to the iron carbonyl complex 6 (Scheme 1). Sur-
prisingly, the ammonium synthesis by addition of methyl
iodide provided the dimethylammonium complex 7. This result
might be explained as followed: First, a benzyl methyl ammo-
nium iodide forms. Then, a subsequent cleavage of the benzyl
fragment by the iodide atom furnishes the N-methylamine
complex. Complex 7 was obtained after reaction with a second
equivalent of methyl iodide and isolated in 76% overall yield
(two steps from 5).
For the second complex, we thought to introduce a tether
between the iron center and the ionic part to evaluate the
impact on catalytic activity. After Boc-monoprotection of ethyl-
ene diamine and dialkylation of the free amine, the diyne 10
was isolated in 45% overall yield. The Boc group was removed
by acidic treatment and the diyne 11 was obtained in 69%
yield. Unfortunately, the [2+2+1] cycloaddition with Fe2(CO)9
in toluene at 1108C did not furnish any adduct (Scheme 2).
However, we synthesized the ammonium complex by modi-
fying the synthetic approach. Then, the [2+2+1] cycloaddition
of 10 in the presence of Fe2(CO)9 furnished the complex 13 in
58% yield. Single crystals of 13 were obtained and were suita-
Figure 2. Ball-and-stick representation of complex 13. H atoms are omitted
for clarity. Selected bond lengths [ꢂ] and angles [8] for 13: Fe1ÀC1
2.3705(10), Fe1ÀC2 2.1427(10), Fe1ÀC3 2.0651(10), Fe1ÀC4 2.0716(10), Fe1À
C5 2.1434(11), C1ÀO1 1.2314(12); O1-C1-Fe1 135.31(8), O4-C21-Fe1
178.99(16), O5-C22-Fe1 178.81(14), O6-C23-Fe1 179.08(21).
the cyclic amine was alkylated, nor the diammonium salt was
obtained (see the Supporting Information).
Iron-catalyzed hydrogenation of carbonyl derivatives
With the complexes 7, 14 and the Knçlker complex 15 in
hands (Figure 1), we defined the optimal reaction conditions
for the reduction of p-methoxyacetophenone by screening var-
ious parameters such as hydrogen pressure, temperature, con-
centration, and catalyst loading. As no reduction occurred
without activation of the tricarbonyl complexes,[11,14] we also
evaluated the loading of Me3NO. Fortunately, the hydrogena-
tion of p-methoxyacetophenone could be performed in aque-
ous media, and the most representative results are presented
in Table 1.
In the series, complex 7 appeared to be less active than
complexes 14 and 15 (entries 8, 10, and 11, Table 1). Notably,
high substrate concentrations and an excess of Me3NO, rela-
tively to the metal, did not impede the reduction (entries 6
and 8, 7 and 8, respectively). Finally, the best reaction condi-
Scheme 2. Synthesis of 14. Reaction conditions: a) Boc2O (0.1 equiv.), CH2Cl2,
08C then RT; b) 3, DIPEA, CH3CN, 708C; c) CF3CO2H, CH2Cl2, 08C; d) Fe2(CO)9,
toluene, 1108C; f) CF3CO2H, CH2Cl2, 08C; g) CH3I, CH2Cl2.
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Table 1. Hydrogenation of 4-methoxyacetophenone 16 under various
conditions.[a]
Table 2. (Continued)
Entry Substrate
Product
Catalyst Yield[b]
[%]
Entry
[Fe]
([mol%])
Me3NO
[mol%]
T
[8C]
16
[M]
H2
[bar]
Conv.[b]
[%]
4
5
14
15
31[c]
35[c]
1
2
3
4
5
6
7
8
14 (5.0)
14 (5.0)
14 (5.0)
14 (5.0)
14 (5.0)
14 (2.5)
14 (2.5)
14 (2.5)
14 (1.0)
7 (2.5)
7.5
7.5
7.5
7.5
7.5
3.75
7.5
3.75
1.5
3.75
3.75
85
85
85
60
40
85
85
85
85
85
85
0.2
0.2
0.2
0.2
0.2
0.2
1.0
1.0
1.0
1.0
1.0
5
10
20
20
20
10
10
10
10
10
10
18
80
91
13
0
79
93
93 (86)[c]
85
16
81
6
14
83
7
8
14
15
96
48
9
10
11
9
10
14
15
80
93
15 (2.5)
[a] General conditions: p-methoxyacetophenone (0.5 mmol), [Fe], Me3NO,
in water under hydrogen pressure for 14 h. [b] Determined by 1H NMR
spectroscopy on the crude product. [c] Isolated yield.
11
14
48[c]
tions (good conversions in a reasonable reaction time, 14 h)
were the following ones: a molar solution of carbonyl com-
pound in the presence of 2.5 mol% of iron complex,
3.75 mol% of Me3NO at 858C under 10 bar of hydrogen
(entry 8, Table 1). These conditions were chosen to explore the
scope of the hydrogenation. Nevertheless, the use of 1 mol%
catalyst loading gave full conversion if the reaction time was
extended (entry 9, Table 1).
12
13
14
14
14
14
97
80
71
Having delineated the optimal reaction conditions, we ex-
amined the scope of this reduction in pure water. As polarized
double bonds, we chose first aldehydes and ketones in the
presence of complexes 14 or 15. These results are summarized
in Table 2.
15
14
86
16
17
14
15
96
95
In the optimized reaction conditions, apart from the 3-ace-
tylpyridine and 4-acetylbenzonitrile, which are known to coor-
dinate iron complexes,[15] all ketones and aldehydes were fully
converted. An increase of the temperature to 1008C improved
the yield for these substrates (entries 4 and 11, Table 2). Never-
theless, with 2-acetylpyridine, high yields were recovered
(entry 12). Several functional groups, such as halogen atom
(entries 2–3), ether (entries 1 and 20), nitro (entry 21), nitrile
(entries 4, 5, 23), and trifluoromethyl groups (entry 6), were
91
(56:44)
18
19
14
15
89
(29:71)
20
21
14
14
94
95
Table 2. Iron-catalyzed hydrogenation of various ketones and aldehydes
in pure water.[a]
Entry Substrate
Product
Catalyst Yield[b]
[%]
22
23
14
14
97
1
2
3
14
14
14
90
93
95
94[c]
[a] General conditions: carbonyl derivative (1 mmol), [Fe] (2.5 mol%),
Me3NO (3.75 mol%), in water [molar concentration (C)=1m] under hydro-
gen (10 bar) for 14 h. [b] Isolated yield. [c] Reaction conducted at 1008C.
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well tolerated in this hydrogenation. The reaction could also
be performed with both alkyl and aryl ketones. With alde-
hydes, high isolated yields were obtained (entries 20–23). With
4-cyanobenzaldehyde, the expected hydrogenated product
was obtained in 94% yield at 1008C, demonstrating a higher
reactivity of aldehydes than that of ketones (entry 23 versus
entries 4, 5).
Table 3. Hydrogenation of imines.[a]
Entry
Substrate
Product
Catalyst
Yield[b]
[%]
1
2
14
15
98[c]
98[c]
For nonfunctionalized substrates, the reactivity of complexes
14 and 15 are comparable. However, with more functionalized
or more challenging derivatives, the ionic complex 14 provid-
ed better results. If an isolated C=C bond was present in the
substrate like in citronellal (entry 22, Table 2), the C=O group
was chemoselectively reduced. With a,b-unsaturated ketones,
the substituents on the ketone moiety and on the C3 position
have a real effect on the chemoselectivity (entries 14–19). Non-
chemoselective reductions were observed with chalcone and
2-cyclohexenone. Both complexes behaved similarly and satu-
rated alcohols were isolated in high yields in both examples
(71–96%, entries 14, 16, 17). With 4-phenyl-but-3-ene-2-one,
a mixture of allylic alcohol and fully hydrogenated product
was isolated. However, complexes 14 and 15 have then a differ-
ent reactivity. Whereas the Knçlker complex 15 led mainly to
the saturated alcohol in 89% yield (allylic alcohol/saturated al-
cohol ratio: 29:71), the ionic complex 14 provided the allylic al-
cohol as a major compound (allylic alcohol/saturated alcohol
ratio: 56:44). Surprisingly, with pseudoionone, chemoselective
reduction of the C=O bond over the C=C double bonds was
observed and yielded the unsaturated alcohol in 86%
(entry 15). Until now, we have no explanation to rationalize
these results. Again, with a challenging substrate such as cyclo-
propylphenyl ketone, higher reactivity was achieved with 14
and the alcohol was isolated in 96% yield (48% with complex
14, entries 7–8). Reaction rates in water are also superior to
those in organic media. Thus, even if a similar yield was ob-
tained for the reduction of the cyclopropylphenyl ketone in
the presence of 15 in pure water (48%) or with the corre-
sponding hydride complex in toluene (46%), the reaction time
was strongly shortened (14 h in water versus 68 h in tolu-
ene).[16] Another relevant improvement of the scope was no-
ticed with 4-acetylbenzonitrile (entries 4–5). Whereas the hy-
dride complex provided less than 1% conversion in toluene, as
reported by Casey and Guan, complex 14 furnished the ex-
pected hydrogenated product in 31% isolated yield within
14 h. Similar observations could be made with benzophenone
(yield>80% in water within 14 h; 55% in toluene within
72 h[16]) or with 4-phenyl-but-3-ene-2-one.
3
4
14
15
61[c]
50[c]
5
6
14
14
96[c]
97
[a] General conditions: imine derivative (1 mmol), [Fe] (2.5 mol%), Me3NO
(3.75 mol%), in water (C=1m) under hydrogen (10 bar) for 14 h. [b] Iso-
lated yield. [c] Reaction conducted at 1008C.
acyclic ones. Then, N-phenyl benzyl amine was synthesized in
97% yield from N-phenyl benzylimine at 858C under 10 bar of
hydrogen in the presence of 2.5 mol% of 14 (entry 6). Again,
the iron-catalyzed hydrogenation in water provided higher
yields and shorter reaction times than the reaction in toluene
(50% yield within 40 h).[16]
Based on previous work, we proposed
a mechanism
(Scheme 3) in which oxidation of one CO ligand with Me3NO
provides the activated 16-electron iron species I. The subse-
quent hydrogen activation leads to the hydride iron complex
II.[11,14–17] Then, as reported in the DFT study by Berkessel, the
C=X bond may be reduced either through a one-step or a step-
wise process.[17] The reduced products are finally released and
the catalytic species I regenerated.
Conclusions
In conclusion, we have demonstrated that bifunctional iron
complexes 14 and 17 were able to efficiently catalyze hydroge-
nation of C=X bonds (X=O and N3) with molecular hydrogen
in pure water under mild reaction conditions. Moreover, we
demonstrated that the use of water as a solvent improved the
reaction rates and that the ionic bifunctional iron gave higher
yields in shorter reaction times with various challenging sub-
strates. This system also tolerated a variety of functional
groups such as halide, ether, nitro, nitrile, and trifluoromethyl
groups.
Iron-catalyzed hydrogenation of carbonyl derivatives
Hydrogenation of C=N bonds in pure water was also investiga-
ted.[10d,16] With stable and water-soluble imines, the corre-
sponding amines were isolated in yields ranging from 61 to
98% in the presence of 14 as the catalyst. Noteworthy, the re-
action had to be performed at 1008C to obtain full conversion
(entries 1–5, Table 3). Here again, catalyst 14 appeared to be
more efficient than 15 and gave higher yields (61 vs. 50%, en-
tries 3, 4). Not only cyclic imines could be reduced but also
Experimental Section
Syntheses
2-Benzyl-4,6-bis(trimethylsilyl)-2,3-dihydrocyclopenta[c]pyrrol-5(1H)-
one iron(0) tricarbonyl (6): In a Schlenk tube under an argon at-
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4.82 (br s, 1H), 3.16 (q, J=5.7 Hz,
2H), 2.79 (t, J=5.7 Hz, 2H), 1.44 (s,
9H), 1.40–1.36 ppm (br s, 2H).
tert-Butyl (2-(bis(3-(trimethylsilyl)-
prop-2-yn-1-yl)amino)ethyl)carba-
mate (10): In a Schlenk tube under
an argon atmosphere, the protect-
ed amine 9 (1.2 g, 7.49 mmol) and
propargylic bromide
3
(2.87 g,
15.0 mmol) were introduced in dry
acetonitrile (75 mL). Diisopropyle-
thylamine
(DIPEA)
(3.9 mL,
22.48 mmol) was added and the
reaction mixture was stirred for 1 h
at RT. Then, the reaction mixture
was heated at 708C for 14 h. After
cooling down the reaction mixture
to RT, water (50 mL) was added
and the aqueous layer was extract-
ed with EtOAc (3ꢃ50 mL). Com-
Scheme 3. Postulated mechanism of the catalytic hydrogenation in water.
bined organic layers were dried
over MgSO4. After filtration, the
solvent was removed under vacuum. The resulting crude product
was purified by flash chromatography on silica gel (eluent: pen-
tane/EtOAc: 98:2) to furnish 10 as a yellow oil (1.68 g, 59%).
1H NMR (CDCl3, 400 MHz): d=5.06–4.94 (br s, 1H), 3.40 (s, 4H), 3.23
(q, J=5.2 Hz, 2H), 2.67 (t, J=5.6 Hz, 2H), 1.44 (s, 9H), 0.17 ppm (s,
18H). 13C NMR (CDCl3, 100 MHz): d=156.1 (sꢃ1), 101.0 (sꢃ2), 90.1
(sꢃ2), 79.2 (sꢃ1), 51.9 (tꢃ1), 43.5 (tꢃ2), 37.9 (tꢃ1), 28.6 (qꢃ3),
0.1 ppm (qꢃ6). IR: n˜ =2961, 2166, 1713, 1499, 1248, 1170,
mosphere, the protected amine
5 (424 mg, 1.28 mmol) and
Fe2(CO)9 (457 mg, 1.27 mmol) were introduced in degassed toluene
(13 mL). The reaction mixture was stirred at 1108C for 14 h. After
cooling down at RT, the resulting mixture was purified on neutral
alumina oxide column chromatography surrounded by a pad of
Celite (eluent: pentane/EtOAc 100:0 to 90:0). The pure product 7
was obtained as a yellow-brown powder (542 mg, 85%). 1H NMR
(CDCl3, 400 MHz): d=7.37–7.27 (m, 5H), 4.00 (s, 2H), 3.77 (d, J=
12.4 Hz, 2H), 3.37 (d, J=12.4 Hz, 2H), 0.23 ppm (s, 18H). 13C NMR
(CDCl3, 100 MHz): d=208.7 (sꢃ3), 182.5 (sꢃ1), 138.4 (sꢃ1), 128.7
(dꢃ2), 128.2 (dꢃ2), 127.5 (dꢃ1), 113.5 (sꢃ2), 69.5 (sꢃ2), 59.1 (tꢃ
1), 53.0 (tꢃ1), 51.3 (tꢃ1), À0.7 ppm (qꢃ6). IR: n˜ =3031, 2956,
2788, 2062, 1999, 1986, 1620, 1244, 838 cmÀ1. HRMS (m/z): [M+H]+
calculated for C23H30NO4Si2Fe, 496.1063; found: 496.1059.
.
838 cmÀ1 HRMS (m/z): [M+H]+ calculated for C19H37N2O2Si2,
381.2394; found, 381.2399.
N,N-Bis(3-(trimethylsilyl)prop-2-yn-1-yl)ethane-1,2-diamine (11): In
a two-necked round-bottom flask under an argon atmosphere, the
protected amine 10 (2.27 g, 5.96 mmol) and TFA (23.54 mL,
149 mmol) were introduced in dry dichloromethane (35 mL). The
resulting mixture was stirred 1 h at 08C, then at RT for 18 h. TFA
was neutralized by a saturated solution of sodium bicarbonate (pH
ꢀ7–8). The aqueous layer was washed with a sodium hydroxide
solution (3m) (3ꢃ30 mL), and then extracted with dichlorome-
thane (70 mL). The organic layer was washed with brine, and then
dried over MgSO4. After filtration, the organic solvent was removed
2,2-Dimethyl-5-oxo-4,6-bis(trimethylsilyl)-1,2,3,5-tetrahydrocyclo-
penta[c]pyrrol-2-ium iodide iron(0) tricarbonyl (7): In a Schlenk
tube, to a solution of 6 (496 mg, 1.0 mmol) in acetonitrile (9 mL)
methyl iodide (0.37 mL, 6.0 mmol) was added. The reaction mixture
was stirred at 808C for 18 h, and then the solution was filtered
through a pad of Celite. The solvent was partially removed and
a 1:1 mixture of pentane/Et2O was added. The resulting precipitate
was collected on a frit to afford complex 7 as a light-brown
1
under vacuum to afford 11 as a yellow oil (1.15 g, 69%). H NMR
(CDCl3, 400 MHz): d=3.42 (s, 4H), 2.78 (t, J=5.4 Hz, 2H), 2.60 (t,
J=5.4 Hz, 2H), 0.16 ppm (s, 18H). (NH2 not observed). 13C NMR
(CDCl3, 100 MHz): d=101.2 (sꢃ2), 89.6 (sꢃ2), 55.7 (tꢃ1), 43.5 (tꢃ
2), 39.4 (tꢃ1), 0.0 ppm (qꢃ6). IR: n˜ =2958, 2356, 2165, 1583, 1452,
1322, 1249, 982, 836, 758 cmÀ1. HRMS (m/z): [M+H]+ calculated for
C14H29N2Si2, 281,1869; found, 281,1867.
1
powder (499 mg, 89%). H NMR ([D6]DMSO, 400 MHz): d=4.88 (d,
J=15.6 Hz, 2H), 4.67 (d, J=15.6 Hz, 2H), 3.51 (s, 3H), 3.42 (s, 3H)
0.25 ppm (s, 18H). 13C NMR ([D6]DMSO, 100 MHz): d=207.1 (sꢃ3),
180.4 (sꢃ1), 107.5 (sꢃ2), 66.6 (sꢃ2), 65.9 (tꢃ2), 56.6 (qꢃ1), 55.8
(qꢃ1), À1.20 ppm (qꢃ6). IR: n˜ =2954, 2066, 2016, 1996, 1622,
1413, 1247, 838 cmÀ1
C18H28FeNO4Si2, 434.0906; found: 434.0916.
.
HRMS (m/z): [MÀI]+ calculated for
(2-(2-Aminoethyl)-4,6-bis(trimethylsilyl)-2,3-dihydrocyclopenta[c]pyr-
rol-5(1H)-one) iron tricarbonyl (12): In a two-necked round-bottom
flask under an argon atmosphere, the protected amine 13 (1.62 g,
3 mmol) and TFA (5.80 mL, 75 mmol) were introduced in dry di-
chloromethane (15 mL) and the resulting mixture was stirred for
1 h at 08C, and at RT for 14 h. The TFA was neutralized by a saturat-
ed solution of sodium bicarbonate (pHꢀ7–8). The aqueous layer
was extracted with dichloromethane and washed with a potassium
hydroxide solution (3m) (3ꢃ30 mL), then concentrated under
vacuum. The viscous orange oil was dissolved in a pentane/Et2O
50:50 mixture, filtered through a pad of Celite, and the solvent
tert-Butyl (2-aminoethyl)carbamate (9):[18] In a two necked round
bottom flask under an argon atmosphere, a solution of ethylene
diamine 8 (33,8 mL, 500 mmol) in dry dichloromethane (500 mL)
was added dropwise to a solution of Boc2O (10,91 g, 50 mmol) in
dry dichloromethane (250 mL) at 08C. The reaction mixture was
stirred 3 h at 08C then overnight at RT. Water (80 mL) was added
and the organic layers were washed with water (2ꢃ80 mL) and
dried over Na2SO4. After filtration, the solvent was removed under
vacuum to furnish the tert-butyl (2-aminoethyl)carbamate
9
1
was removed to furnish 12 as an orange oil (1.06 g, 79%). H NMR
1
(8.01 g, 76%) as a colorless oil. H NMR (CDCl3, 400 MHz): d=4.92–
(CDCl3, 400 MHz): d=3.82 (d, J=12.4 Hz, 2H), 3.36 (d, J=12.4 Hz,
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2H), 2.90–2.85 (br s, 4H), 1.57–1.50 (br s, 2H), 0.25 ppm (s, 18H).
(NH2 not observed). 13C NMR (CDCl3, 100 MHz): d=208.7 (sꢃ3),
113.2 (sꢃ2), 69.5 (sꢃ2), 58.2 (tꢃ1), 53.5 (tꢃ2), 40.6 (tꢃ1),
À0.7 ppm (qꢃ6) (the carbon signal of the CO of the ligand was
and the European Union (FEDER funding). ERDF fundings (AI-
Chem & INTERREG IVa program) are gratefully acknowledged for
financial support. Dr. Rꢀmi Legay is thanked for NMR analysis;
Dr. Solenne Moulin and Mꢀlinda Ipouck are warmly thanked for
their help during the preparation of the manuscript.
not observed). IR: n˜ =2953, 2059, 1980, 1615, 1245, 835 cmÀ1
.
HRMS (m/z): [M+H]+ calculated C18H29FeN2O4Si2, 449,1015; found,
449.1083.
Keywords: aldehydes
hydrogenation · iron · water
·
homogeneous catalysis
·
(2-(2-Boc-aminoethyl)-4,6-bis(trimethylsilyl)-2,3-dihydrocyclopen-
ta[c]pyrrol-5(1H)-one) iron tricarbonyl (13): In a Schlenk tube under
an argon atmosphere, amine 5 (427 mg, 1.26 mmol) and Fe2(CO)9
(460 mg, 1.26 mmol) were introduced in degassed toluene (12 mL).
The reaction mixture was stirred at 1108C for 14 h. After cooling
down at RT, the resulting mixture was purified on neutral alumina
oxide column chromatography surrounded by a pad of celite
(eluent: pentane/EtOAc 100:0 to 95:5). Pure 13 was obtained as
[1] For recent reviews, see: a) F. D. Klingler, Acc. Chem. Res. 2007, 40, 1367;
b) W. P. Hems, M. Groarke, A. Zanotti-Gerosa, G. A. Grasa, Acc. Chem. Res.
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Fuente, S. Castillon, C. Claver, ChemCatChem 2010, 2, 1346; e) J.-H. Xie,
S.-F. Zhu, Q.-L. Zhou, Chem. Rev. 2011, 111, 1713.
1
a yellow-brown powder (368 mg, 58%). H NMR (CDCl3, 400 MHz):
d=4.83–4.78 (br s, 1H), 3.82 (d, J=12.4 Hz, 2H), 3.39 (d, J=
12.4 Hz, 2H), 3.31 (q, J=6.4 Hz, 2H), 2.93 (t, J=6.4 Hz, 2H), 1.44 (s,
9H), 0.25 ppm (s, 18H). 13C NMR (CDCl3, 100 MHz): d=208.6 (sꢃ3),
182.3 (sꢃ1), 113.0 (sꢃ2), 156.2 (sꢃ1), 79.6 (sꢃ1), 69.5 (sꢃ2), 54.4
(tꢃ1), 53.3 (tꢃ2), 39.2 (tꢃ1), 28.5 (qꢃ3), À0.7 ppm (qꢃ6). IR: n˜ =
2955, 2061, 1987, 1609, 1247, 840 cmÀ1. HRMS (m/z): [M+H]+ cal-
culated for C23H37FeN2O6Si2, 549,1540; found, 549.1543.
[3] a) B. Cornils, W. A. Hermann, Aqueous-Phase Organometallic Catalysis,
2nd ed., Wiley-VCH, Weinheim, 2002; b) U. M. Lindstrçm, Chem. Rev.
2002, 102, 2751.
(N,N,N-Trimethyl-2-(5-oxo-4,6-bis(trimethylsilyl)cyclopenta[c]pyrrol-
2(1H,3H,5H)-yl)ethanaminium) iron tricarbonyl (14): In
a two
[4] D. Sinou, Adv. Synth. Catal. 2002, 344, 221.
necked round bottom flask under an argon atmosphere, the iron
complex 13 (1.0 g, 2.39 mmol) and potassium carbonate (991 mg,
7.17 mmol) were introduced in dry dichloromethane (20 mL).
Methyl iodide (0.89 mL, 14.34 mmol) was added and the resulting
mixture was stirred overnight at RT. The solution was filtered
through a pad of Celite. The solvent was partially removed and
a mixture of pentane/Et2O 50:50 was added to precipitate the
complex. Filtration afforded 14 as a light-brown powder (1.26 g,
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b) U. M. Lindstrçm, F. Andersson, Angew. Chem. 2006, 118, 562; Angew.
Chem. Int. Ed. 2006, 45, 548; c) F. Joꢄ, Acc. Chem. Res. 2002, 35, 738;
d) T. Dwars, G. Oehme, Adv. Synth. Catal. 2002, 344, 239; e) M. C. Pirrung,
Chem. Eur. J. 2006, 12, 1312; f) S. Kobayashi, K. Manabe, Acc. Chem. Res.
2002, 35, 209; g) C. Romain, S. Gaillard, M. K. Elmkaddem, L. Toupet, C.
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Renaud, ChemSusChem 2008, 1, 505; c) K. Junge, K. Schrçder, M. Beller,
Chem. Commun. 2011, 47, 4849; d) S. Chakraborty, H. Guan, Dalton
Trans. 2010, 39, 7427; e) M. Darwish, M. Wills, Catal. Sci. Technol. 2012,
2, 243; f) B. A. F. Le Bailly, S. P. Thomas, RSC Adv. 2011, 1, 1435; g) V. K. K.
Praneeth, M. R. Ringenberg, T. R. Ward, Angew. Chem. 2012, 124, 10374;
Angew. Chem. Int. Ed. 2012, 51, 10228.
[7] For recent publications, see: a) S. Zhou, S. Fleischer, K. Junge, S. Das, D.
Addis, M. Beller, Angew. Chem. 2010, 122, 8298; Angew. Chem. Int. Ed.
2010, 49, 8121; b) P. O. Lagaditis, A. J. Lough, R. H. Morris, J. Am. Chem.
Soc. 2011, 133, 9662; c) C. Sui-Seng, F. Freutel, A. J. Lough, R. H. Morris,
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1394; e) N. Meyer, A. J. Lough, R. H. Morris, Chem. Eur. J. 2009, 15, 5605;
f) A. A. Mikhailine, M. I. Maishan, R. H. Morris, Org. Lett. 2012, 14, 4638;
g) D. E. Prokopchuk, J. F. Sonnenberg, N. Meyer, M. Zimmer-De Iuliis,
A. J. Lough, R. H. Morris, Organometallics 2012, 31, 3056; h) J. F. Sonnen-
berg, N. Coombs, P. A. Dube, R. H. Morris, J. Am. Chem. Soc. 2012, 134,
5893; i) P. E. Sues, A. J. Lough, R. H. Morris, Organometallics 2011, 30,
4418; j) P. O. Lagaditis, A. J. Lough, R. H. Morris, J. Am. Chem. Soc. 2011,
133, 9662; k) A. A. Mikhailine, R. H. Morris, Inorg. Chem. 2010, 49, 11039;
l) P. O. Lagaditis, A. J. Lough, R. H. Morris, Inorg. Chem. 2010, 49, 10057.
[8] a) S. C. Bart, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc. 2004, 126,
13794; b) A. M. Archer, M. W. Bouwkamp, M.-P. Cortez, E. Lobkovsky, P. J.
Chirik, Organometallics 2006, 25, 4269; c) R. J. Trovitch, E. Lobkovsky, E.
Bill, P. J. Chirik, Organometallics 2008, 27, 1470; d) E. J. Daida, J. C.
Peters, Inorg. Chem. 2004, 43, 7474.
1
85%). H NMR (CD3CN, 400 MHz): d=4.00 (d, J=12.7 Hz, 2H), 3.59
(t, J=5.3 Hz, 2H), 3.47 (d, J=12.7 Hz, 2H), 3.31–3.27 (m, 2H), 3.20
(s, 9H), 0.2 ppm (s, 18H). 13C NMR (CD3CN, 125 MHz): d=209.8 (sꢃ
3), 183.3 (sꢃ1), 114.1 (sꢃ2), 69.8 (sꢃ2), 64.2 (tꢃ1), 54.4 (qꢃ3), 53.7
(tꢃ2), 50.1 (tꢃ1), À0.8 ppm (qꢃ6). IR: n˜ =2954, 2066, 2016, 1996,
1622, 1413, 1247, 838 cmÀ1. HRMS (m/z): [MÀI]+ calculated for
C21H35FeN2O4Si2, 491,1479; found, 491.0529.
General procedure for iron-catalyzed hydrogenation of C=X
bond in water
A 25 mL dried autoclave containing a stirring bar was charged
with the substrate (1 mmol), the iron complex (2.5 mol%), and
Me3NO (3.75 mol%) and degassed water (1 mL) under an argon at-
mosphere. The autoclave was pressured to 10 bar of hydrogen and
stirred at 858C or 1008C for 14 h. The reactor was cooled down to
RT, and the remaining hydrogen was carefully vented. The resulting
aqueous layer was extracted with CH2Cl2 (3ꢃ5 mL). Combined or-
ganic layers were dried over MgSO4. After filtration, the solvent
was removed under vacuum. The resulting crude product was puri-
fied by distillation or by chromatography on flash silica gel.
Acknowledgements
[9] For recent publication on alkene reduction with borohydride reagent,
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We gratefully acknowledge financial support from the “Ministꢁre
de la Recherche et des Nouvelles Technologies” for PhD grant for
D.M., CNRS (Centre National de la Recherche Scientifique), the
“Rꢀgion Basse-Normandie”, the “CRUNCH ” interregional network
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Received: April 30, 2013
Published online on && &&, 0000
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers!