Well-defined four-coordinate iron(ii) complexes for intramolecular hydroamination of primary aliphatic alkenylamines

Article abstract of DOI:10.1002/anie.201402089

Despite the growing interest in iron catalysis and hydroamination reactions, iron-catalyzed hydroamination of unprotected primary aliphatic amines and unactivated alkenes has not been reported to date. Herein, a novel well-defined four-coordinate β-diketi

Full text of DOI:10.1002/anie.201402089

Angewandte
Chemie
DOI: 10.1002/anie.201402089
Iron Catalysis
Well-Defined Four-Coordinate Iron(II) Complexes For Intramolecular
Hydroamination of Primary Aliphatic Alkenylamines**
Elise Bernoud, Pascal Ouliꢀ, Rꢀgis Guillot, Mohamed Mellah, and Jꢀrꢁme Hannedouche*
Abstract: Despite the growing interest in iron catalysis and
hydroamination reactions, iron-catalyzed hydroamination of
unprotected primary aliphatic amines and unactivated alkenes
has not been reported to date. Herein, a novel well-defined
four-coordinate b-diketiminatoiron(II) alkyl complex is shown
to be an excellent precatalyst for the highly selective cyclo-
hydroamination of primary aliphatic alkenylamines at mild
temperatures (70–908C). Both empirical kinetic analyses and
the reactivity of an isolated iron(II) amidoalkene dimer,
ology would be incompatible with the use of primary aliphatic
amines, because these having a greater binding affinity than
electron-deficient amines towards the metal center. Herein,
we report the syntheses of well-defined low-coordinate
iron(II) complexes and their remarkable activities in the
cyclohydroamination of primary aliphatic alkenylamines, as
the first example of iron-catalyzed hydroamination of elec-
tronically unbiased amines.
We hypothesized that well-defined and low-coordinate
iron(II) alkyl complexes stabilized by b-diketiminate ligands
were likely to show a unique reactivity for the selective
hydroamination of primary aliphatic alkenylamines
(Scheme 1).[9] The predilection for electronegative ligands
=
[LFe(NHCH2CPh2CH2CH CH2)]2 favor a stepwise s-inser-
tive mechanism that entails migratory insertion of the pendant
alkene into an iron–amido bond associated with a rate-
determining aminolysis step.
T
he development of more efficient, cost-effective and
environmentally friendly methodologies for the synthesis of
alkylamines is a major goal in modern chemistry. In this
respect, catalytic alkene hydroamination is a key approach
since it offers a waste-free process with 100% atom efficiency
from relatively inexpensive and easily available amines and
olefins.[1] The quest for broader substrate scope and polar
functional-group tolerance has stimulated the development of
hydroamination catalysts based on late-transition metals.[2]
While significant progress has been made in the field, the
reaction of unprotected primary amines, arguably the most
versatile amines to start an “ideal synthesis”,[3] remains
problematic.[4,5] To date, only few research groups have
tacked this challenging issue, but the systems reported so far
are limited in scope and/or based on noble metals of limited
availability, high price and considerable toxicity.[6] Thus, truly
efficient and sustainable hydroamination catalysts for the
preparation of unprotected secondary nitrogen-compounds
are still in demand. As part of our research program, we raise
the challenge to develop such catalysts based on iron as a low-
cost, non-toxic, and abundant metal. To our knowledge,
despite the growing interest in iron catalysis,[7] only iron(III)
chloride has been reported for the hydroamination of
electron-deficient amines.[8] This Lewis acid assisted method-
Scheme 1. Working hypothesis. LnFe-R represents b-diketiminato-
iron(II) alkyl complexes. For details of steps 1–3 see text.
demonstrated by these coordinatively and electronically
unsaturated complexes should drive the preferential forma-
À
À
tion of a Fe N bond over a Fe C bond (Scheme 1, step (1))
and consequently promote catalyst turnover (step (3)).[9b]
À
Even if no direct evidence of alkene insertion into a Fe N
bond (step (2)) has been reported so far,[10] some related alkyl
complexes containing b-hydrogen atoms on the alkyl ligand
may undergo a rapid alkyl isomerization at room temperature
by a reversible b-hydrogen-elimination/alkene reinsertion
process.[11] This process likely proceeds through the formation
of a four-coordinate alkene hydride intermediate complex.
Additionally, the easily electronically and sterically tunability
of b-diketiminate ligands[12] is essential for a fine-tuning of the
metal reactivity and a possible control of the selectivity for
the hydroamination over oxidative amination pathway
(Scheme 1).
[*] Dr. E. Bernoud,[+] Dr. P. Ouliꢀ,[+] Dr. R. Guillot, Dr. M. Mellah,
Dr. J. Hannedouche
Institut de Chimie Molꢀculaire et des Matꢀriaux d’Orsay (ICMMO),
UMR CNRS 8182, Univ Paris-Sud
Initially, we synthesized a novel low-coordinate iron(II)
alkyl complex 2a·THF containing the 2,4-bis(2,4,6-trimethyl-
phenylimino)pent-3-yl ligand L by a two-step metathesis
procedure (Scheme 2). Reaction between the lithium salt of
2,4-bis(2,4,6-trimethylphenylimino)pentane HL and anhy-
drous iron(II) chloride leads to the isolation of yellow crystals
of the four-coordinate tetrahedral ate complex [LFe(m-Cl)2Li-
Orsay, 91405 (France)
E-mail: jerome.hannedouche@u-psud.fr
[+] These authors contributed equally to this work.
[**] We thank ANR JCJC DHAMFER, Univ Paris Sud and CNRS for
financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402089.
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Scheme 3. Room temperature stoichiometric reactivity of 2a and 3.
Reaction conditions: 1. toluene, 258C, 3 days, 75%.
Scheme 2. Synthesis of 2a·THF. Reaction conditions: 1. a) nBuLi
(1.01 equiv), THF, À788C to 258C, 2 h, b) FeCl2 (1.01 equiv), 258C,
16 h, 74% (two steps); 2. LiCH2SiMe3 (1 equiv), Et2O, 258C, 16 h,
72%.
addition of a yellow solution of 2a to 3 results in the
immediate formation of a dark-red solution, from which red-
orange crystals can be isolated in 75% yield. The X-ray
structure of a single-crystal reveals a centrosymmetric dimer
(THF)2] (1; Scheme 2). The proposed structure for 1 was
unambiguously determined by X-ray diffraction.[13,14] Subse-
quent metathesis reaction of 1 with LiCH2SiMe3 affords
[LFeCH2SiMe3·THF] (2a·THF) in 72% yield as an orange
air-sensitive crystalline solid (Scheme 2). Complex 2a·THF is
soluble in Et2O, THF, and toluene and slightly soluble in
hexane. It can be stored in crystalline state or in a [D6]benzene
solution at room temperature for weeks without noticeable
decomposition.
=
[LFe(NHCH2CPh2CH2CH CH2)]2 (2b) in the solid state, in
which two iron atoms are bridged by two amido ligands from
two molecules of 3 (Figure 2).[14] Each iron atom has
Solid-state analysis of a single-crystal of 2a·THF[14] reveals
that the iron center is four-coordinate and adopts a trigonal-
pyramidal geometry in which the apical position is occupied
by the THF ligand (Figure 1). No agostic interactions are
Figure 2. ORTEP drawing of 2b. Thermal ellipsoids are set at 30%
probability. Hydrogen atoms omitted for clarity. Selected bond
lengths [ꢁ] and bond angles [8]: N(1)–Fe 2.0459(14), N(2)–Fe
2.0267(13), N(3)(i)–Fe 2.0665(15), N(3)–Fe 2.0698(14), Fe–Fe(i)
2.7828(8); N(2)-Fe-N(1) 92.72(5), N(2)-Fe-N(3)(i) 110.21(6), N(1)-Fe-
N(3)(i) 125.88(5), N(2)-Fe-N(3) 120.81(5), N(1)-Fe-N(3) 114.11(5),
N(3)-Fe-N(3)(i) 95.44(5). Symmetry transformation used to generate
equivalent atoms: (i)=Àx; 1Ày; 2Àz.
Figure 1. ORTEP drawing of four-coordinate b-diketiminatoiron(II)
alkyl complex 2a·THF. Thermal ellipsoids are set at 30% probability.
Selected bond lengths [ꢁ] and bond angles [8]: N(1)–Fe 2.0273(9),
N(2)–Fe 2.0072(8), Fe–O 2.2257(8), C(9)–Fe 2.0492(11); N(2)-Fe-N(1)
93.07(4), N(2)-Fe-C(9) 139.74(4), N(1)-Fe-C(9) 116.58(4), N(2)-Fe-O
95.76(3), N(1)-Fe-O 98.68(3), C(9)-Fe-O 105.20(4).
a distorted tetrahedral geometry and the iron–amido bond
(i)
À
À
lengths (Fe N(3) 2.0665(15) ꢀ, Fe N(3) 2.0698(14) ꢀ) are
longer than those encountered in related monomeric com-
plexes, possibly because of the highly crowded nature of both
metal centers.[16] Dimer 2b which is insoluble in Et2O and
hexane, and sparingly soluble in THF and toluene, can be
stored as a solid at room temperature for weeks without any
evident decomposition. This experiment demonstrates 2a is
sufficiently basic to deprotonate primary aliphatic amines and
that in presence of such amines, the amido complex 2b (or its
monomer in solution) will be favored.
evident as the shortest relevant Fe···H contact distance is over
À
3.6 ꢀ. The two Fe N(diketiminate) bonds are slightly differ-
À
À
ent (Fe N(1) 2.0273(9) ꢀ, Fe N(2) 2.0072(8) ꢀ) but are,
despite the higher coordination number, similar to those
observed in closely related three-coordinate complexes as is
the bite angle of the bidentate ligand.[15] The Fe C bond (Fe
C(9) 2.0492(11) ꢀ) is consistent with that of three- and four-
coordinate iron(II) b-diketiminate containing a sp3-hybrid-
ized hydrocarbyl ligand.[11b,15a]
À
À
To assess the viability of an intramolecular alkene
insertion reaction, 2b was heated in toluene at 908C. GC
analysis of the crude reaction shows the presence of the
starting amine 3, 2-methylpyrrolidine (4), imine 5 and 2,2-
To test the reactivity of 2a on primary aliphatic amines,
the stoichiometric reaction of 2a and 2,2-diphenylpent-4-en-
1-amine (3) was carried out (Scheme 3). Room temperature
2
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(entry 8). A plot of the reaction selectivity[20] (%)
Table 1: Influence of the catalyst, temperature, concentration, solvent and additive
on the efficiency of the cyclohydroamination reaction of 3.[a]
versus the conversion (%) in the cyclization of 3 by
2a·THF (10 mol%) shows that the selectivity
evolves with the degree of conversion.[13] The selec-
tivity remains high (> 93%) until a 66% conversion
then decreases to 83% at full conversion. This
observation suggests that the selectivity might be
dependent on the concentration of primary amine
present in the reaction medium. To maintain a mini-
mum concentration in primary amine, cyclopentyl-
amine (7) was added as noncyclizable primary amine.
The addition of 20 mol% of 7 preserves the high
selectivity, as only traces of 5 and 6 are detected
(Table 1, entry 9). Moreover, only 10 mol% of cyclo-
pentylamine was enough to obtain 4 in 94% GC
yield as an almost single product (entry 10).
Entry Catalyst
(mol %)
Additive
T [8C]
3
4
5
6
(mol %)[b]
[%][c] [%][c] [%][c] [%][c]
1[d]
2
3
4
2b (100)
2b (5)
–
–
–
–
–
–
–
–
90
90
90
50
90
90
70
90
90
90
90
9
2
1
94
1
3
7
19
6
2
16
82
73
6
80
80
87
2
58
12
14
0
10
10
5
5
2
3
1
18
4
13
0
9
6
2
6
3
1
2a·THF (10)
2a·THF (10)
2a·THF (10)
2a·THF (10)
2a·THF (10)
[Fe{N(SiMe3)2}2] (10)
2a·THF (10)
2a·THF (10)
2c (10)
5[e]
6[f]
7[f,g]
8[f]
9[f,h]
10[f,h]
11[f]
7 (20)
7 (10)
–
89
94
94
Our catalytic system is efficient for the selective
formation of five- and six-membered rings from
primary alkenylamines (Table 2). The exo-cycliza-
tion can occur in good yields with a 1,2-disubstituted
alkene bearing a phenyl group, a dimethylsubstituted
3
2
[a] Reaction conditions: [3]=0.56m, toluene, 24 h unless otherwise stated.
[b] 7=cyclopentylamine. [c] Determined by GC analysis. [d] Without 3. [e] THF as
solvent. [f] [3]=0.96m. [g] 68 h (unoptimized). [h] 48 h.
diphenylpentan-1-amine (6) in a 9:16:58:18 ratio respectively
Table 2: Intramolecular hydroamination of primary amines.[a]
(Table 1, entry 1).[17] The formation of the hydroamination
product 4, the oxidative amination product 5, and the reduced
product 6 are reminiscent of an intramolecular migratory
insertion of the carbon–carbon double bond into the iron–
amido bond of 2b (or its monomer, Scheme 1).[18] The
reactivity of 2b is a rare illustration of the migratory insertion
of an alkene into the metal–amido bond of a well-defined and
isolated metal amido complex.[19]
Entry
Alkenylamines
Product
Yield [%][b]
94[c] (85)
1
2
3
4
89[d]
85[d]
Encouraged by these initial stoichiometric results, we
evaluated the ability of dimer 2b to catalytically promote the
reaction of 3. To our delight, only 5 mol% of 2b leads to
almost complete conversion of 3 into 4, 5, and 6 with a 82%
yield in 4 (Table 1, entry 2). In contrast with other late-
transition-metal catalytic systems, no olefin isomerization
product was detected.[6f–h] Furthermore, comparable reactiv-
ity and hydroamination selectivity[20] can be achieved with the
use of the monomeric iron alkyl complex 2a·THF instead of
2b as precatalyst (Table 1, entry 3). Conducting the reaction
at 508C results in red-orange crystals after 12 h (which persist
during the remaining reaction time) and a poor conversion of
6% after 24 h (Table 1, entry 4). X-ray analysis of a single
crystal reveals that the crystals were 2b, demonstrating the
formation of complex 2b from precatalyst 2a·THF under
catalytic conditions. Gratifyingly, reactions could also be run
in the coordinating solvent THF with a positive outcome
(Table 1, entry 5 versus 3), which indicates catalyst tolerance
of Lewis basic functional groups in the starting amines (see
below). A slight improvement in yield of 4 is obtained by
either an about twofold increase in the substrate concen-
tration in toluene (Table 1, entry 6 versus 3) or a decrease of
the reaction temperature to 708C (Table 1, entry 7). It is
worth noting the b-diketiminate ligand plays a crucial role in
the catalyst control of the selectivity of the reaction, as
[Fe{N(SiMe3)2}2][21] affords the olefin isomerization product
as the main product under identical reaction conditions
>95[d]
5
6
91[d] (75)
98[d,e] (78)
[a] Reaction conditions: 2a·THF (10 mol%), cyclopentylamine 7
(10 mol%), [D8]toluene, 908C, 48 h. [b] Yield of isolated product in
parenthesis. [c] Determined by GC analysis [d] Determined by in situ
1H NMR spectroscopy using ferrocene (0.4 equiv) as internal standard.
[e] d.r=1:1.
allene, or even a coordinating methoxy group without
noticeable catalyst deactivation (Table 2, entries 4–6). How-
ever, the cyclohydroamination does not proceed without
a geminal disubstitution on the tether or with 1,2-dialkylsub-
stituted alkenes.[22]
To gain a better insight into the reaction mechanism,
kinetic measurements[13] were conducted. Monitoring the
concentration of 3 over the course of the reaction reveals
apparent first-order kinetics up to at least 90% conversion
and over a threefold initial concentration range ([3] = 0.56–
1.65m).[23] Nevertheless, the observed first-order rate constant
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decreases with increasing initial substrate concentration,
which is consistent with an inverse order dependence of the
reaction rate on 3 concentration.[2b,6d,h,24] Over the course of
2–3 half-lives, the hydroamination of 3 is first order with
respect to 2a·THF concentration over a fourfold range
([2a·THF] = 0.048–0.192m). These data suggest the involve-
ment of a monomeric iron species in the rate-determining step
of the catalytic cycle. As expected, the transformation also
exhibits an inverse rate dependence on the concentration in
cyclopentylamine 7.[25] Additionally, kinetic analysis of the
reaction of N-proteo-amine 3 versus N-deutero-amine [D2]-3
yields a kinetic isotope effect (KIE) of kH/kD = 3 (908C).[6d,j,26]
This is highly reminiscent of a primary isotope effect, which
In summary, we have demonstrated that novel well-
defined four-coordinated b-diketiminatoiron(II) alkyl com-
plex 2a·THF is an excellent precatalyst for the highly selective
cyclohydroamination of primary aliphatic alkenylamines at
mild temperatures (70–908C). Reactivity study of isolated
iron(II) amidoalkene dimer complex 2b has indicated that
alkene migratory insertion into the iron–amido bond occurs
without the assistance of an additional amine. This study,
supported by preliminary kinetic measurements, features
a stepwise s-insertive mechanism with aminolysis as the
turnover-limiting step of the catalytic cycle. To our knowl-
edge, this work establishes for the first time that well-defined
iron complexes are viable catalysts for the selective hydro-
amination of unprotected primary amines with great binding
affinity and polar functional groups. Further studies are
currently ongoing to attain a deeper understanding of the
mechanism.
À
reflects N H(D) bond cleavage during the turnover-limiting
step.
On the basis of these kinetic data and the observed
stoichiometric and catalytic reactivities of isolated dimer 2b,
a stepwise s-insertive mechanism[1,27] for the iron-catalyzed
cyclohydroamination of primary alkenylamines is proposed
(Scheme 4). Initial s-bond metathesis between the coordina-
Received: February 5, 2014
Published online: && &&, &&&&
Keywords: alkenes · amines · homogeneous catalysis ·
.
hydroamination · iron
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Scheme 4. Proposed stepwise s-insertive mechanism for alkenylamine
3 (see text for details).
tively unsaturated 2a·THF and basic amine 3 would lead to
the formation of catalytically active iron amido species A,
which in solution would be in rapid equilibrium with dimer
À
2b. Subsequent rapid alkene insertion into the Fe N bond
would give iron alkyl intermediate B, which might undergo
two related pathways. Turnover-limiting aminolysis of B with
either 3 or 7 would liberate the hydroamination product 4 and
give species A or 2c, respectively. An equilibrium between 2c
and A might account for the regeneration of the active species
A from 2c. To probe this, 2c[28] was independently synthe-
sized[13] and its catalytic behavior was evaluated in the
cyclohydroamination of 3. Complete conversion of 3 occurs
within 24 h using 10 mol% of isolated 2c, affording the
hydroamination product in 94% yield, which is consistent
with an equilibrated process (Table 1, entry 11). The evolu-
tion of B through an undesired but slower b-hydride
elimination process would explain the formation of 5 and 6
as minor side-products.[29]
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(%) + 5 (%) + 6 (%)].
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[23] A first-order kinetic behavior is observed in the absence of
cyclopentylamine.
[24] The catalyst inhibition pathway may involve reversible binding
of an additional free amine.
[25] For cyclohydroamination rate depression upon the addition of
non cyclizable amine: M. R. Gagne, C. L. Stern, T. J. Marks, J.
Am. Chem. Soc. 1992, 114, 275 – 294.
[26] A KIE of similar magnitude is observed in the absence of
cyclopentylamine.
À
[10] For an example of related Fe H insertion into a carbon – carbon
triple bond: Y. Yu, A. R. Sadique, J. M. Smith, T. R. Dugan,
R. E. Cowley, W. W. Brennessel, C. J. Flaschenriem, E. Bill, T. R.
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Commun. 2002, 2886 – 2887; b) J. Vela, S. Vaddadi, T. R.
Cundari, J. M. Smith, E. A. Gregory, R. J. Lachicotte, C. J.
Flaschenriem, P. L. Holland, Organometallics 2004, 23, 5226 –
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[27] For a recent discussion of s-insertive and non-insertive mech-
anistic pathways for catalysts proceeding by amine activation: S.
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[12] Y.-C. Tsai, Coord. Chem. Rev. 2012, 256, 722 – 758.
[13] See Supporting Information for details.
[28] The nuclearity of 2c is unknown.
[29] b-H elimination might liberate an enamine that tautomerizes to
imine 5 generating {Fe-H} species. These might either insert into
[14] CCDC 909964 (1), 909965 (2a·THF) and 909963 (2b) contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
=
the C C bond of 3 to form 6 after aminolysis or act as a Brçnsted
base to give A or 2c. For examples of low-coordinate b-
diketiminatoiron(II) hydride and their insertion into double
bond; see J. M. Smith, R. J. Lachicotte, P. L. Holland, J. Am.
Chem. Soc. 2003, 125, 15752 – 15753; For {Fe-H} acting as a base;
see Reference [11b].
[15] a) T. J. J. Sciarone, A. Meetsma, B. Hessen, Inorg. Chim. Acta
2006, 359, 1815 – 1825; b) A. Panda, M. Stender, R. J. Wright,
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Iron Catalysis
E. Bernoud, P. Ouliꢁ, R. Guillot,
M. Mellah,
J. Hannedouche*
&&&& — &&&&
Well-Defined Four-Coordinate Iron(II)
Complexes For Intramolecular
Hydroamination of Primary Aliphatic
Alkenylamines
Iron horseshoe: A well-defined four-coor-
dinate b-diketiminatoiron(II) alkyl com-
plex is a precatalyst for the highly selec-
tive cyclohydroamination of primary ali-
phatic alkenylamines at mild tempera-
tures. Its mechanism is also elucidated.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!