A discrete N,O,N-supported Gallium amido complex for the intermolecular hydroamination of terminal alkynes

Article abstract of DOI:10.1021/om2012584

The diamino-ether ligand {(C₅H₉)NH-C ₆H₄}₂O (1) was found to readily react with 0.5 equiv of Ga₂(NMe₂)₆ via an amine elimination route to afford the N,O,N-supported Ga

Full text of DOI:10.1021/om2012584

Article
pubs.acs.org/Organometallics
A Discrete N,O,N-Supported Gallium Amido Complex for the
Intermolecular Hydroamination of Terminal Alkynes
Frederic Hild and Samuel Dagorne*
́
́
Laboratoire DECOMET, Institut de Chimie de Strasbourg (CNRS-Universite
67000 Strasbourg, France
́
de Strasbourg), 1 rue Blaise Pascal,
S
* Supporting Information
ABSTRACT: The diamino-ether ligand {(C5H9)NH-C6H4}2O
(1) was found to readily react with 0.5 equiv of Ga2(NMe2)6 via
an amine elimination route to afford the N,O,N-supported Ga
amido species {η3-N,O,N-((C5H9)N-C6H4)2O}GaNMe2 (2) in a
reasonable yield (51%). As determined by X-ray crystallography,
the four-coordinate Ga center in 2 adopts an unusual trigonal-
monopyramidal geometry. Compound 2 effectively catalyzes the
hydroamination of terminal alkynes (such as 1-hexyne and
phenylacetylene) in the presence of primary amines (aniline
and butylamine). Kinetic studies on the latter catalytic re-
actions suggest that these proceed with a first-order rate dependence on alkyne and on species 2. In preliminary studies aiming
at the isolation of intermediates relevant to the present catalysis, the dimeric Ga complex [{η2-N,N-((C5H9)N-C6H4)2O}Ga(μ-NHPh)]2
(3) was synthesized by an aminolysis reaction between compound 2 and aniline; its identity was confirmed by X-ray crystallographic
analysis.
active and controlled ring-opening polymerization of cyclic
carbonates.6 The increased reactivity of such group 13 com-
plexes arises from a destabilizing ligand-defined geometry
distortion, resulting in a more Lewis acidic Al metal center.5c
In comparison to “classical” low-coordinate (two- and three-
coordinate) group 13 species, distorted (LX2)AlX′-type com-
pounds may be readily accessible (in a mononuclear form) and
provide a superior steric protection of the Lewis acidic metal
center and, hence, an increased stability. Such entities may thus
well represent a reasonable reactivity/stability balance, render-
ing them suitable for catalysis and, possibly, for novel reactivity
in group 13 chemistry.
On this basis and aiming at the development of novel group
13 mediated reactions, we have become interested in probing
the potential usefulness of distorted (NON)MX′-type species
(M = group 13 metal) as catalysts for the intermolecular hydro-
amination of alkynes, a widely studied atom-efficient process
for the formation of imines and enamines from alkynes and
primary amines.7 However, while various and numerous
metal-based catalysts have been shown to mediate the latter
transformation,7 no group 13 catalyst had been reported when
the present work was initiated.8 Studies in this area have been
thus far restricted to a couple of reports on aluminum-mediated
intramolecular hydroamination of alkenes, found to proceed
under rather harsh conditions.9
INTRODUCTION
■
Well-defined group 13 organo compounds supported by vari-
ous N- and/or O-based multidentate chelating ligands, such as,
for instance, salen- and salan-derived ligands, have been widely
studied and have found numerous applications in homogeneous
catalysis ranging from their use in the mediation of various
Lewis acid assisted organic reactions to that in polymerization
catalysis of polar monomers (cyclic esters, epoxides).1−3
Efficient discrete group 13 based catalysts developed thus far
mainly consist of five-coordinate or higher species of the type
(L2X2)MX′ or (L2X2)M(X′)(L′).1 Indeed, in such complexes,
the high coordination of the metal center and the formation of
robust (L2X2)M chelates typically ensure an excellent stability
and disfavor the formation of undesirable aggregates. Despite
their being usually quite reactive and thus of interest in catalysis,
well-defined low-coordinate (<4) group 13 species have thus
far found very few applications in the latter area, which is
certainly related to their limited stability (in polar and/or
protic medium, for instance).4 In addition, the well-known
propensity of such electrophilic species toward aggregate
formation (through diverse binding/bridging modes) often
complicates their coordination chemistry and their isolation
in a pure form.
Tetracoordinate Al(III) complexes supported by an appro-
priately designed N,N,N-dianionic ligand forcing the metal cen-
ter in a trigonal-monopyramidal coordination geometry (versus
its classically preferred tetrahedral geometry) have been shown
to be effective Lewis acid catalysts for the mediation of several
asymmetric transformations.5 More recently, we reported on
the use of distorted N,O,N-supported Al species for the highly
As shown below, initial studies on alkyne hydroamination
using N,O,N-supported Al amido species were unsuccessful.
Received: December 20, 2011
Published: January 30, 2012
© 2012 American Chemical Society
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As simple Ga(III) precursors such as GaCl3 have been shown,
in some instances, to readily activate alkynes,1,10 we were
prompted to extend our investigations to the Ga analogues.
Here we report on the synthesis and structural character-
ization of a distorted N,O,N-supported Ga amido complex
and its subsequent use in alkyne hydroamination catalysis.
Kinetic studies of the latter catalytic reaction are also
provided and briefly discussed.
As depicted in Figure 1, complex 2 indeed consists of a four-
coordinate Ga species effectively η3-N,O,N-chelated by the bis-
amido-ether {((C5H9)N-C6H4)2O}2− dianionic ligand, with the
Ga metal center adopting an unusual distorted-trigonal-mono-
pyramidal (tmp) geometry. The nitrogens of the three amido
groups (defining the pyramidal base) are thus nearly coplanar
with Ga (sum of N−Ga−N angles 358.72°). The bonding
parameters in 2 are essentially as expected (see Figure 1) with,
in particular, the Ga(1)−N(1) and Ga(1)−N(2) amido bond
distances (1.877(2) and 1.891(2) Å, respectively) lying in the
lower range (1.85−2.05 Å) for terminal Ga−amido bonds.11
Notably, the Ga(1)−N(3) bond distance (1.816(2) Å) is signi-
ficantly shorter, which most likely reflects the ionic contraction
of the Ga(1) and N(3) radii due to the polar character of the
latter bond.12 Importantly, the solid-state data for 2 indicate the
presence of an apical vacant site ideally disposed for co-
ordination to the Lewis acid Ga center. From a structural point
of view, the Ga species 2 represents the first X-ray characterized
compound in which a four-coordinated Ga center adopts a
clear-cut tmp geometry. As for its solution structure, NMR data
for 2 are consistent with a Cs-symmetric structure under the
studied conditions (C6D6, room temperature), in agreement with
solid-state structural data.
RESULTS AND DISCUSSION
N,O,N-Supported Ga Amido Complex (NON)Ga(NMe2) (2).
As illustrated in Scheme 1, the N,O,N-supported Ga amido
■
Scheme 1
As stated in the introduction, the hydroamination of alkynes
was first tested using Al derivatives. More specifically, the
previously reported Al complexes {RN-C6H4}2OAlNMe2 (R =
C5H9, C6H11),6 which are isostructural with the Ga compound 2,
were used and found to be essentially inactive in the inter-
molecular hydroamination of alkynes under the studied
conditions. Thus, using various primary amines and terminal
alkyne substrates, only trace amounts of hydroamination pro-
ducts were observed under harsh reaction conditions (at best
7% conversion to the Markovnikov hydroamination product,
1-hexyne and aniline as substrates, 10 mol % Al species
{(C5H9)N-C6H4}2OAlNMe2, C6D6, 150 °C, 65 h). In contrast,
as summarized in Table 1, the Ga species 2 (using 5 or 10% mol
of 2 versus substrates, C6D6, 100 °C) effectively catalyzes the
species {η3-N,O,N-((C5H9)N-C6H4)2O}GaNMe2 (2) may
be directly prepared in a reasonable yield (51%) via an amine
elimination route by reaction of the diamino-ether ligand
1 with 0.5 equiv of Ga2(NMe2)6. Compound 2 was isolated as
an air- and moisture-sensitive colorless solid, and its molecular
structure was confirmed by X-ray crystallography analysis.
Figure 1. Molecular structure of the Ga amido species 2 (Ortep view). The hydrogens are omitted for clarity: (a) front view; (b) side view. Selected
bond distances (Å): Ga(1)−N(1) = 1.877(2), Ga(1)−N(2) = 1.891(2), Ga(1)−N(3) = 1.816(2), Ga(1)−O(1) = 2.1241(16). Selected bond angles
(deg): N(3)−Ga(1)−N(1) = 119.16(10), N(3)−Ga(1)−N(2) = 117.92(9), N(1)−Ga(1)−N(2) = 121.67(9), N(2)−Ga(1)−O(1) = 82.92(7).
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Table 1. Gallium-Catalyzed Hydroamination of Terminal
a
Alkynes
b
entry
1
alkyne
amine
aniline
t (h)
conversn (%)
1-hexyne
17
24
65
65
24
24
20
7
90
52
c
2
1-hexyne
aniline
d
3
1-hexyne
aniline
15
e
4
1-hexyne
aniline
26
f
5
1-hexyne
butylamine
tBu-NH2
aniline
48
f
6
1-hexyne
<5
22
7
Ph-acetylene
Ph-acetylene
Ph-acetylene
g
8
aniline
100
40
g
9
aniline
1
a
Conditions (unless indicated otherwise): 5 mol % of compound 2,
benzene, 100 °C, [alkyne]0 = [amine]0 = 0.5 M. C6Me6 was used as an
b
1
internal standard. As determined by H NMR after hydrolysis of the
c
d
reaction mixture. [alkyne]0 = [amine]0 = 0.156 M. 2.5 mol % of
Ga2(NMe2)6 was used as a catalyst with [alkyne]0 = [amine]0 = 0.156 M.
e
5 mol % of GaCl3 was used as a catalyst with [alkyne]0 = [amine]0 =
Figure 2. Plot of ln([1-hexyne]0/[1-hexyne]) versus time (h) for the
hydroamination reaction of 1-hexene by aniline catalyzed by the Ga
species 2. Conditions: [1-hexene]0 = [aniline]0 = 0.156 M, 5 mol % of
Ga species 2, C6D6, 100 °C.
f
0.156 M. The reaction was run at 130 °C using 10 mol % of 2.
g
10 mol % of catalyst 2 was used.
hydroamination of terminal alkynes such as 1-hexyne and
phenylacetylene whether an aryl amine (aniline) or an alkyl
amine (butylamine) is used. The highest catalytic activity for
species 2 is observed for the hydroamination of 1-hexyne by
aniline (entries 1 and 2, Table 1): the activities are significantly
lower when either phenylacetylene or butylamine is used as an
alkyne or an amine source, respectively (entry 7 vs 1, entry 5 vs 1,
Table 1). In all cases, as determined by NMR, only the
Markovnikov product is formed (>99%), thus showing that the
present catalysis proceeds with high regioselectivity. Notably,
while the use of a sterically bulky primary amine substrate may
be beneficial to the catalytic activity in some group 4 mediated
hydroamination reactions,7,13 the hydroamination reaction
t
(of 1-hexyne) does not proceed using BuNH2 as an amine
source (entry 6, Table 1), thus indicating that, in the present
case, steric bulk may be detrimental. Finally, as a comparison,
simple Ga(III) precursors such as GaCl3 and Ga2(NMe2)6 were
tested as catalysts and also found to mediate the hydro-
amination of 1-hexene by aniline (entries 3 and 4, Table 1),
albeit with an activity much lower than that of the N,O,
N-supported Ga species 2. Species 2 is less likely than GaCl3
and dimer Ga2(NMe2)6 to form robust and catalytically inactive
aggregates, which may account for its higher catalytic activity in
hydroamination.
Figure 3. Plot of kobs versus Ga species 2 concentration. Conditions:
[1-hexyne]0 = [aniline]0 = 0.156 M, C6D6, 100 °C.
alkynes showed a rate dependence on amine concentration.7,14
While being less active than Cp2TiMe2,15 the hydroamination
activity of Ga complex 2 is comparable, for instance, to those
of the Ti species (η5-Cp*)2Ti(η2-Me3SiCCSiMe3) and the
Ta alkyl imido species (Me3CCH2)3TaNtBu.16
To gain insight into the present Ga-catalyzed hydroamina-
tion reaction, kinetic studies were performed using 1-hexyne
and aniline as substrates. As depicted in Figure 2, these data
agree with an apparent first-order reaction in substrate (kobs
≈
0.03 h−1 under the studied conditions). In addition, the reac-
tion was studied at different concentrations of catalyst 2, yield-
ing the observation of a linear correlation between [2] and
With the intent to identify (and possibly isolate) key inter-
mediates relevant to the present hydroamination catalysis, stoi-
chiometric reactions of 1-hexyne or/and aniline with 1 equiv
of Ga species 2 were run and afforded in all but one case
an intractable mixture of products. Thus, the aminolysis reac-
tion between the Ga complex 2 and 1 equiv of aniline (toluene,
100 °C, 1 h) allows the formation of the corresponding Ga-
NHPh complex [{η2-N,N-((C5H9)N-C6H4)2O}Ga(μ-NHPh)]2
kobs (Figure 3). In contrast, the concentration in amine (10, 20,
and 30 equiv) apparently does not affect the reaction rate
(Figure 4). Altogether, these kinetic data agree with the pre-
sent Ga-catalyzed hydroamination being of first-order rate
dependence in alkyne as well as in catalyst 2 while being of
zero-order in amine. As a comparison, it may be noted that
various kinetic studies on group 4 mediated hydroamination of
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N-C6H4)2O}Ga moieties being bonded to one another via two
μ-NHPh amido units (Figure 5). These two amido groups are
disposed in a trans configuration relative to the nearly planar
Ga2N2 core. The centrosymmetric dimer 3 (with an inversion
center located within the Ga2N2 core) features two four-
coordinate Ga centers, each adopting a slightly distorted
tetrahedral geometry. Notably and unlike species 2, the central
N,O,N oxygen of the chelating ligand is not coordinated to the
Ga center in compound 3. All geometrical and bonding param-
eters are overall rather as expected (Figure 5). In solution,
the NMR data (C6D6, room temperature) are consistent with
an effective Ci-symmetric structure and thus with the solid-state
structure being retained in solution.
As deduced from a high-temperature NMR-scale monitoring
experiment, the formation of compound 3 from species 2
and aniline is quantitative at 100 °C in C6D6 within 5 min,
thus suggesting that Ga-NHPh amido species may readily form
under (hydroamination) catalytic conditions. Nevertheless,
monitoring a NMR-scale hydroamination of 1-hexyne by
aniline using either complex 2 or 3 as catalyst (20 mol % of
Ga species 2 or 3 vs substrates, 100 °C, C6D6) showed that
both Ga species are not stable under catalytic conditions and
Figure 4. Plot of kobs versus amine concentration for the hydro-
amination reaction of 1-hexene by aniline catalyzed by the Ga species
2. Conditions: [1-hexyne]0 = 0.156 M, 5 mol % of Ga species 2, C6D6,
100 °C.
1
readily undergo protonolysis: only H NMR signals for as yet
unidentified Ga derivatives and for the free ligand {(C5H9)-
NH-C6H4}2O are observed as the catalysis proceeds. Of
relevance to the subject, it should be noted that the thermolysis
of related Ga amido derivatives such as R′2Ga(μ-NHR)2GaR′2
complexes (R, R′ = alkyl, aryl) has been reported to afford
various polynuclear Ga amido imido/imido species.17 Although
further studies are clearly required, the possible formation of
(3; Scheme 1), which was isolated as a colorless powder in a
moderate yield. As deduced from X-ray crystallographic studies,
complex 3 crystallizes as a centrosymmetric dimer and its
molecular structure may be described as two {η2-N,N-((C5H9)
Figure 5. Molecular structure of the dimeric Ga amido species 3 (Ortep view). The hydrogens are omitted for clarity (except the NH groups).
The C5H9 groups are also partially omitted for clarity: only the nitrogen-bound carbons are represented. Selected bond distances (Å): Ga(1)−N(1) =
1.900(2), Ga(1)−N(2) = 1.887(2), Ga(1)−N(3) = 2.027(2), Ga(1)−N(3)′ = 2.035(1). Selected bond angles (deg): N(2)−Ga(1)−N(1) = 113.92(9),
N(2)−Ga(1)−N(3) = 120.84(9), N(1)−Ga(1)−N(3) = 114.47(9).
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1
solid that was further dried under vacuum (40 mg, 20% yield). H
such Ga amido imido/imido species may be involved in these
Ga-mediated hydroamination reactions.
3
4
NMR (300 MHz, C6D6): δ 7.26 (dd, JHH = 8.1 Hz, JHH = 1.5 Hz,
4H), 7.05 (dt, 3JHH = 7.8 Hz, 4JHH = 1.5 Hz, 4H), 6.71 (m, 10H), 6.68
In summary, the diamino-ether ligand {(C5H9)NH-C6H4}2O
may readily react with Ga2(NMe2)6 to afford the corresponding
monomeric Ga species (NON)GaNMe2 (2), in which the four-
coordinate Ga center adopts an unusual tmp geometry. Unlike
its Al analogues (NON)AlNMe2, species 2 effectively catalyzes
the intermolecular hydroamination of terminal akynes by pri-
mary amines. Thus, while discrete Ga derivatives are typically
less reactive than their Al analogues for the mediation of
numerous organic reactions (such as various Lewis acid assisted
reactions), the present results further illustrate that Ga derivatives
may be a profitable alternative (in catalysis) where Al analogues
fail.
(dd, 3JHH = 8.1 Hz, 4JHH = 1.5 Hz, 4H), 6.43 (dt, 3JHH = 7.8 Hz, 4JHH
=
1.5 Hz, 4H), 3.75 (q, 3JHH = 7.2 Hz, 4H), 3.40 (s, NH, 2H), 1.20−2.10
(m, 32H). 13C NMR (300 MHz, C6D6): δ 150.4 (Cipso), 147.0 (Cipso),
145.0 (Cipso) 129.6 (Ar), 128.3 (Ar), 127.7 (Ar), 119.2 (Ar), 117.4
(Ar), 113.9 (Ar), 113.2 (Ar), 57.5 (CH-C5H9), 35.3 (C5H9), 34.9
(C5H9), 24.6 (C5H9), 24.5 (C5H9).
General Procedure for the Hydroamination Catalysis. In a
nitrogen-filled glovebox, the appropriate amounts of alkyne and amine
were added by syringe and dissolved in C6D6 in a vial equipped with a
Teflon-tight screw cap. The Ga complex 2 as well as an internal
standard (C6Me6, 0.2 equiv vs substrates) were then added to yield a
colorless solution. The vial was tightly closed, placed in a temperature-
controlled oil bath (100 °C), and heated for the appropriate time. The
mixture was subsequently cooled to room temperature and hydrolyzed
with aqueous HCl (1 M). Conversion of the reagents was monitored
Further studies in this area will focus on the characterization
of key intermediates relevant to these hydroamination reactions
and on widening the scope of application of this catalysis.
1
by H NMR spectroscopy of the C6D6 organic phase: i.e., the ratios
between the alkyne substrate and the hydrolysis products (i.e. the
corresponding ketones) were calculated from the integral intensities
of the corresponding signals. In all cases, the reagents either did
not react or were converted to the expected hydroamination products,
as deduced from the 1H NMR integration ratio (reagents +
hydroamination product)/internal standard.
EXPERIMENTAL SECTION
■
All experiments were carried out under N2 using standard Schlenk
techniques or in a Mbraun Unilab glovebox. Toluene and pentane
were collected after being passed through drying columns (SPS
apparatus, MBraun) and stored over activated molecular sieves (4 Å)
for 24 h in a glovebox prior to use. C6D6 was purchased from
Eurisotop (CEA, Saclay, France), distilled from CaH2, degassed under
a N2 flow, and stored over activated molecular sieves (4 Å) in a
glovebox prior to use. All other chemicals were purchased from
Aldrich and were used as received, with the exception of the amines
and the alkynes: the latter were all distilled over KOH and
subsequently stored over activated molecular sieves (4 Å) for at
least 24 h in a glovebox prior to use. The NMR spectra were recorded
on Bruker AC 300 and 400 MHz NMR spectrometers in Teflon-
valved J. Young NMR tubes at ambient temperature unless indicated
otherwise. 1H and 13C chemical shifts are reported vs SiMe4 and were
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, figures, and CIF files giving a summary of X-ray
crystallographic data for complexes 2 and 3 and graphs associated
with kinetic studies on compound 2 mediated hydroamination of
1-hexene by aniline. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: dagorne@unistra.fr.
■
determined by reference to the residual H and 13C solvent peaks.
Elemental analyses were performed at the Service de Microanalyse
1
́
of the Universite de Strasbourg (Strasbourg, France). The diamino-ether
ACKNOWLEDGMENTS
ligand 1 and Ga2(NMe2)6 were prepared according to reported literature
■
procedures.18
We thank the CNRS and The University of Strasbourg for
financial support. Lydia Brelot and Corinne Bailly (Service de
Crystallographie, Institut de Chimie de Strasbourg) are gratefully
acknowledged for the X-ray analysis of complexes 2 and 3. F.H.
acknowledges the ADEME and the Reg
fellowship.
{η3-N,O,N-((C5H9)N-C6H4)2O}GaNMe2 (2). In a nitrogen-filled
glovebox, the diamino-ether ligand (C5H9NH-C6H4)2O (1; 770 mg,
2.29 mmol) was charged in a Schlenk flask and a toluene solution
(10 mL) of Ga2(NMe2)6 (460 mg, 1.15 mmol) was added to yield a
colorless solution. The reaction mixture was then heated for 40 h
at 100 °C in an oil bath to yield a pale yellow solution that was
subsequently cooled to room temperature and evaporated to dryness
in vacuo, affording an off-white solid residue. The latter was dissolved
in pentane and cooled to −35 °C over 5 days. The Ga complex 2
precipitated as a white solid that was further dried under vacuum (525
mg, 51% yield). Anal. Calcd for C25H33GaN2O: C, 64.31; H, 7.20; N,
9.37. Found: C, 63.29; H, 6.99; N, 9.25. 1H NMR (400 MHz, C6D6):
δ 7.26 (dd, 3JHH = 8.4 Hz, 4JHH = 1.6 Hz, 2H), 7.05 (dt, 3JHH = 7.6 Hz,
4JHH = 1.2 Hz, 2H), 6.68 (dd, 3JHH = 8.4 Hz, 4JHH = 1.6 Hz, 2H), 6.42
́
ion Alsace for a Ph.D.
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■
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