Novel yttrium and zirconium catalysts featuring reduced Ar-BIANH2 ligands for olefin hydroamination (Ar-BIANH2 = bis-arylaminoacenaphthylene)

Article abstract of DOI:10.1039/c6nj02199a

The novel class of bis-arylaminoacenaphthylenes (Ar-BIANH₂) was employed for the preparation of zirconium and yttrium complexes to be used as catalysts for cyclohydroamination of a number of primary and secondary aminoalkenes. The complex [(2,6

Full text of DOI:10.1039/c6nj02199a

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Novel Yttrium and Zirconium Catalysts Featuring Reduced Ar-
BIANH2 Ligands for Olefin Hydroamination (Ar-BIANH2 = bis-
arylaminoacenaphthylene).
Received 00th January 20xx,
Accepted 00th January 20xx
Alessandro Cimino,a Filippo Moscatelli,a Francesco Ferretti,a Fabio Ragaini,a* Stéphane Germain,b
Jérôme Hannedouche,b Emmanuelle Schulz,b* Lapo Luconi,c Andrea Rossin,c Giuliano
DOI: 10.1039/x0xx00000x
Giambastianic,d
*
www.rsc.org/
The novel class of bis-arylaminoacenaphthylenes (Ar-BIANH2) was employed for the preparation of zirconium and yttrium
complexes to be used as catalysts for the cyclohydroamination of a number of primary and secondary aminoalkenes. The
complex [(2,6-iPr2C6H3-BIAN)Zr(NMe2)2(η1-NHMe2)] was isolated and completely characterized, including X-ray diffraction
analysis. Despite its easy and almost quantitative isolation, it showed only moderate catalytic performance in the
intramolecular hydroamination, irrespective of the cyclization precursor used. On the other hand, in situ generated YIII
complexes obtained with the same class of ligands were found to be very active, leading to the hydroamination of
substrates including those normally reluctant at undergoing cyclization such as those featuring an internal non-activated
C=C double bond. Electron donating substituents and especially steric hindrance on the ligand improves the performance
of the catalysts, allowing in the latter case to decrease the catalyst loading to 1 mol %.
enantioselectivity of the direct amine addition.3 Recently,
some formal hydroamination procedures have been elegantly
reported as alternative strategies to circumvent some of these
Introduction
The direct addition of amines to non-activated carbon-carbon
double bonds, the so-called olefin hydroamination, is one of
the most straightforward atom-economical processes for the
synthesis of valuable nitrogen-containing compounds from
relatively low-cost and ubiquitous starting materials.1 Over the
years, time and efforts have led to the development of a
plethora of metal and metal-free catalysts to promote this
highly desirable transformation inter- and intramolecularly.
Despite some tremendous achievements in terms of catalytic
efficiency, (stereo)selectivities and scope of applications, some
challenges still need to be addressed.2 One of the main
unsolved ones is undoubtedly the relatively poor reactivity of
1,2-dialkyl-substituted olefins compared to terminal olefins
concomitantly with the problematic control of the regio- and
issues.4,5 Nevertheless and despite their brilliant results, these
surrogates are far from ideal from an atom-economical point
of view; there is so still a need for the development of novel
catalysts to directly tackle some of the remaining issues of the
classical hydroamination reaction. Herein, we report our latest
work into the preparation of new zirconium- and yttrium
based complexes and their evaluations as catalysts in the
cyclohydroamination of classical benchmark substrates and
challenging ones.
Some of us have recently reported on the properties of a class
of bis-arylaminoacenaphthylene compounds (Ar-BIANH2)
derived from the reduction of easily available bis-
aryliminoacenaphthenes (Ar-BIAN, Scheme 1. Numbering
refers to compounds prepared and employed in the present
work).6
H
N
H
R
R
N
N
N
R
R
reductant
O2
Ar-BIANH2
Ar-BIAN
1
Ph ( )
4-MeOC6H4 (2)
3,5-(CF3)2C6H3 ( )
Ar =
3
4
2,4,6-Me3C6H2 ( )
2,6-iPr2C6H3 (5)
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the literature, whereas 2,6-iPr2C6H3-BIANH2 (5) had been
Scheme 1. General reactivity of Ar-BIANH2.
isolated by slow diffusion of water vapors into a THF solution
Complexes of the formal dianion of Ar-BIANH2 are well-known
in the literature,7 but their preparation usually requires the
reaction of the parent diimine with a strongly reducing metal
(e.g. Na, Li, Ca), possibly followed by transmetalation of the
alkaline or alkaline-earth intermediate to the targeted metal.
However, alkaline and alkaline-earth complexes are extremely
air- and moisture-sensitive; thus, their practical handling for
the synthesis of new compounds as well as their long-term
storage is troublesome. On the other hand, Ar-BIANH2
compounds are only moderately air-sensitive in the solid state
and can be stored indefinitely under a dinitrogen atmosphere.
Moreover, the use of isolated Ar-BIANH2 ligands holds
important synthetic advantages that widen the ligands
applicability range and facilitate the use of the related
complexes in catalysis. Indeed, new complexes can be
prepared by ligand deprotonation or transamination instead of
the classical reduction/transmetalation protocol. Finally, no
alkaline or alkaline earth-based salts are generated as reaction
by-products throughout the complexation procedure, so that
the obtained complexes may be generated in situ and
employed without any interference from any ionic product
cogenerated during the synthesis.
Scheme 2. Preparation of (2,4,6-Me3C6H2)BIANH2 (4) and (2,6-iPr2-C6H3)BIANH2 (5).
of (2,6-iPr2C6H3-BIAN)Mg(THF)3,10 which is not, in our view, a
synthetically convenient approach. Compound
5
was
alternatively prepared in situ by the same group by treating a
solution of the aforementioned magnesium complex with
methanol.11 Reduction by metallic sodium in place of
magnesium was also mentioned,10 but the procedure was not
described. Taking advantage of these scattered data, we now
obtained pure
4 and 5 by reducing the corresponding Ar-BIAN
compounds in THF with a slight excess of metallic sodium,
followed by the addition of methanol to the so obtained
sodium complexes and without the need to isolate the latter
(Scheme 2).
Hill and coworkers have recently reported on the use of
“dearomatized” Ar-BIAN complexes of alkaline earth metals as
highly active catalysts for the intramolecular hydroamination
of aminoalkenes.3a, 3b Given our interest in the development of
new groups 3 and 4 metals as hydroamination catalysts, we
thought about exploiting our potentially bis-amido ligands for
the preparation of new yttrium and zirconium derivatives. On
this regard, the ability of the isolated Ar-BIANH2 at undergoing
ligand deprotonation in the presence of metal-alkyl or metal-
amido precursors has paved the way to the preparation of new
highly electrophilic compounds.8
Synthesis and characterization of (2,6-iPr2C6H3-BIAN)Zr(NMe2)2(η1-
HNMe2) complex (5a)
The bulky ligand 5 was preliminarily scrutinized for the
isolation of a ZrIV complex. The transamination reaction
between the metal precursor Zr(NMe2)4 and the bis-amino
ligand 2,6-iPr2C6H3-BIANH2 (5) proceeded smoothly already at
room temperature in benzene, with complete conversion after
3h and the evolution of only one dimethylamine equivalent
(Scheme 3). Solvent evaporation provided green crystals of 5a
as an air- and moisture-sensitive microcrystalline solid in about
95% yield. The isolated complex is highly soluble in aromatic
and aliphatic hydrocarbons and its spectroscopic and
diffractometric characterization (vide infra) are consistent with
a five-coordinated metal ion. As a distinctive spectral feature,
Results and Discussion
Ligands synthesis
1
the H NMR at room temperature (298 K) shows two singlets
Among the Ar-BIANH2 ligands employed, Ph-BIANH2 (
1) and 4-
at 2.74 and 1.70 ppm, ascribed to the methyl protons of the
two residual dimethylamido groups and the Me2NH η1-
coordinated to the metal center, respectively. The X-ray
diffraction analysis of 5a confirmed the coordination sphere at
the metal ion. A perspective view of the complex structure is
given in Figure 1 along with a list of selected bond lengths and
angles.
MeOC6H4-BIANH2 ( ) have already been prepared by some of
2
us.6 They had been obtained by reduction of the parent Ar-
BIAN ligands by NaBH4 in methanol. In the original report, the
reaction was carried out at reflux for 3 hours. However, on
repetition of the same reaction with NaBH4 batches obtained
by different suppliers, yields and purities were not always
reproducible. We now developed an improved methodology
by working at room temperature, which afforded the products
reproducibly independent of the origin of the reducing agent.
This novel procedure was also employed to prepare 3,5-
(CF3)2C6H3-BIANH2 (3
).9
The protocol based on NaBH4 was tested on the sterically
hindered 2,4,6-Me3C6H2-BIAN and 2,6-iPr2C6H3-BIAN. However,
in neither case the reduction proceeded to an appreciable
extent even at reflux temperature. The reduced 2,4,6-
Me3C6H2-C6H2-BIANH2 (4) has never been reported before in
Scheme 3. Synthesis of the zirconium complex 5a
.
2 | J. Name., 2012, 00, 1-3
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The less sterically hindered ligands 1 and 3 have also been
5a crystallizes in the P 21 21 21 orthorhombic space group, with scrutinized for the isolation of the corresponding ZrIV amido
four molecules per unit cell. The Zr ion is five-coordinated, in a complexes. Unfortunately, both ligands gave mixtures of
distorted square pyramidal coordination geometry (τ = 0.25). inseparable compounds, which have no longer been processed
The equatorial plane is defined by the two ligand N atoms, one or employed in catalysis. Overall, it can be inferred that the
–NMe2 and the –NHMe2 group, while the last dimethylamido higher the ligand bulkiness, the highest the purity and unicity
substituent occupies the pyramidal apex. The Zr−N distances of the resulting complex. Indeed, the steric hindrance
and the N−Zr−N angles fall in the same range as those found in generated by the aryl substituents is expected to facilitate the
similar five-coordinated Zr amido complexes, with the Zr−N(3) generation of complexes with a one to one metal to ligand
or Zr−N(4) being shorter than the Zr−N(5) bond, as expected ratio, thus avoiding the generation of over-coordinated metal
when passing from an anionic to a neutral donor. The metal ions or complex mixtures.
ion lies slightly above the N(1)-N(2)-N(3)-N(5) plane
[d(Zr−plane) = 0.60 Å]. Tables S1-S4 in the ESI collect all the Catalytic performance of 5a in the intramolecular hydroamination
main structural parameters and refinement details of 5a
.
of aminoalkenes.
Crystals of 5a can be conveniently stored for months under
inert atmosphere (N2) and at low temperature (-30 °C) without
any apparent alteration. On the other hand, a progressive
complex decomposition with formation of intractable side-
products takes place at room temperature in aromatic
hydrocarbons (benzene or toluene) as well as in chlorinated
ones (CH2Cl2). Thus, the original bright green solution turns to
dark brown just upon keeping the complex solution at room
temperature in the NMR tube for a few hours. Unfortunately,
all our attempts to get more robust alkyl derivatives of 5a
failed. The reaction of 5a with an excess of Me3Al (10 eq.)
under controlled experimental conditions gave only a complex
mixture of unidentified intermediates along with a relatively
high amount of the free ligand. At odds with what observed
with Zr(NMe2)4, attempts to react the ligand with Zr(Bn)4 led to
the complete recovery of the reagents even after heating the
reaction mixture to reflux for hours. Attempts were also made
to prepare the ZrIV amido complex starting from other Ar-
The freshly prepared bis-amido complex 5a was exploited as
homogeneous
catalyst
for
the
intramolecular
hydroamination/cyclization of model primary and secondary
amines tethered to monosubstituted alkenes (Table 1). Two
different reaction temperatures were used as to differentiate
the catalyst performance and improve the reaction
conversion.
As shown in Table 1, catalyst 5a is active for the targeted
reaction on both primary and secondary aminoalkenes.
However, irrespective of the cyclization precursor used and
the adopted reaction conditions (Table 1, entries 3-8 vs. 1-2),
the chemical conversion was lower compared to that obtained
with the plain Zr(NMe2)4 precursor.
Neither Thorpe–Ingold effects due to the substitution pattern
of the cyclization precursor nor any other stereo-electronic
effect can be reasonably invoked to justify the observed
moderate conversions. On the other hand, these outcomes are
in line with the moderate stability observed for 5a in solution.
Indeed, a progressive decomposition of the catalyst along with
the formation of intractable solid by-products takes place in a
few hours.
BIANH2 ligands. As for ligand 4, NMR evidence suggests that 4a
is generated as a single complex. However, all our attempts to
isolate it as a pure compound failed and its use as in situ
prepared catalyst for the intramolecular hydroamination gave
only very modest results.
Table 1. Intramolecular hydroamination of primary and secondary aminoalkenes
catalyzed by zirconium complexes.a
R2
R2
R2
R2
cat.
R1 = H
R1 = H
R1 = H
R2 = Ph
R2 = -(CH2)5-
R2 = CH3
I
N
NH
R1
ꢀ
, C7D8
II
R1
III
IV
Ia-IVa
Ib-IVb
R1 = Me R2 = Ph
Entry
Catalyst
Substrate
T (°C)
t (h)
2
Conv. (%)b
1
2
3
4
5
6
7
8
Zr(NMe2)4
Ia
Ia
60
60
55
77
17
26
47
20
16
25
Zr(NMe2)4
10
2
5a
5a
5a
5a
5a
5a
Ia
60
Ia
60
10
10
10
10
20
Ia
100
60
IIa
IIIa
IVa
Figure 1. ORTEP drawing of the crystal structure of 5a. Thermal ellipsoids are
drawn at the 50% probability level. Hydrogen atoms on the ligands (apart from
the –NHMe2 group) and the iPr substituents on the phenyl rings attached to N
are omitted for clarity. Selected bond distances (Å) and angles (°): Zr(1)-N(1)
2.133(4), Zr(1)-N(2) 2.207(4), Zr(1)-N(3) 2.065(5), Zr(1)-N(4) 2.029(4), Zr(1)-N(5)
2.406(4), N(5)-H(100) 0.90(7); N(1)-Zr(1)-N(2) 78.15(18), N(1)-Zr(1)-N(4)
101.69(18), N(1)-Zr(1)-N(3) 97.1(2), N(1)-Zr(1)-N(5) 153.5(2).
60
100
a Reaction conditions: toluene solvent (2.5 mL), substrate = 0.21 mmol, catalyst =
5 mol %. b Conversions were determined by GC.
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In situ generation of yttrium complexes
However, the in situ generated catalyst was most often not a
single pure complex. Scheme 4 reports the main species
present in solution, whose identification was supported by
literature precedents (vide supra). However, only in the case of
Given the instability problems generally encountered in the
synthesis and isolation of the ZrIV complexes based on the Ar-
BIANH2 ligands, the YIII derivatives were prepared and
employed in situ in the homogeneous hydroamination
catalysis. This procedure was also dictated by the typically
higher moisture and oxygen sensitivity of organolanthanides if
compared with their ZrIV analogues.
5c and by working with a 1:1 mol ratio yttrium precursor/5 was
1H NMR spectrum obtained that is strongly indicative of a
a
single species with the composition shown (see the ESI). In
other cases, minor components are also observable by NMR
that makes a clear assignment of all signals not possible. All
attempts to isolate and precisely characterize the thus
obtained complexes remained unsuccessful.
Some of us had previously described
a straightforward
procedure for the generation of amido alkyl ate complexes by
the in situ combination of the yttrium ate precursor
[Li(THF)4][Y(CH2TMS)4]12 and enantiopure chiral
binaphthyldiamine ligands in toluene.3m,
N
-substituted
Catalytic performance of the YIII complexes
13
The as-prepared
mixture was successfully employed to promote the Structurally diverse aminoalkene cyclization precursors were
enantioselective cyclisation of tethered amines to mono- or engaged in the yttrium catalyzed cyclohydroamination
1,2-disubstituted
alkenes
in
valuable
yields
and reaction. In particular, the efficiency of the in situ prepared
enantioselectivities. A similar in situ approach had been complexes was studied with respect to a number of more or
adopted to get binaphthylamido yttrium complexes starting less challenging substrates towards cyclization (Scheme 5). As
from a homoleptic yttrium source, [Y(CH2TMS)3(THF)2].14 These these reactions are all benchmark transformations, the
neutral complexes were particularly active for the catalysts efficiencies were evaluated by NMR, following
transformation of substrates bearing secondary amines.15 substrate conversion. In all cases, only the expected products
Taking into account these results, the same experimental were obtained (see ESI for details).
procedures with both YIII precursors have been applied in the The catalytic activity of the ate complexes 1b
-5b was
presence of the pure Ar-BIANH2 ligands (Scheme 4). A slight preliminarily scrutinized in combination with non-sterically
excess of ligand was introduced into the reaction mixture and demanding substrates (such as IIa and IIIa), in order to
exchange occurred, with each yttrium species, at room evaluate the influence of the ligand structure and bulkiness on
temperature, accompanied by a significant color change from the complexes reactivity. A complete list of the catalytic
deep red/dark violet for the ligands to green for all complexes, outcomes is given in Table 2. The catalyst performance of the
except for 3b, bluish-violet. Solutions were stirred for 30 yttrium tetra-alkyl precursor in the cyclization of both primary
minutes and then directly engaged with the targeted substrate aminoalkenes is also reported for the sake of comparison. As
to check the catalytic ability. It should be stressed that the in Table 2 shows, IIa was conveniently transformed into the
situ preparation of the catalysts was found to be completely corresponding cyclization product with high conversion and in
reproducible and the same outcome (differences in conversion a relatively short time, using 6 mol % of the tetra-alkyl
values within 2%) was observed for catalytic reactions precursor [Li(THF)4][Y(CH2TMS)4] at room temperature (Table
repeated under the same experimental conditions.
2, entry 1). However, the cyclization of IIIa occurred with more
difficulty under these conditions (Table 2, entry 9), likely due
to a less relevant Thorpe Ingold effect. All the in situ prepared
YIIIAr-BIAN ate complexes (1b
-5b) proved to catalyze these
transformations, albeit with variable efficiency. In the case of
ate complexes bearing non sterically hindered Ar-BIANH2
ligands (1b and 2b), only a moderate catalytic performance
was obtained (Table 2, entries 2-3 and 10-11). A similar trend
was observed with complex 3b, where the presence of
electron withdrawing (EWG) trifluoromethyl substituents on
the
N-phenyl rings (Table 2, entries 4 vs. 2 and 12 vs. 10)
translated into a poorly active system for the targeted
reaction. On the contrary, the use of bulky ligands decorated
with electron donating groups (EDG) clearly led to a strong
acceleration of the cyclization process with almost complete
substrate conversions in less than 1 h (Table 2, entries 5-6 vs. 2
and 13-14 vs. 10). Noteworthy, catalyst 5b offered the highest
performance, leading to complete cyclization of both
aminoalkene precursors to the corresponding pyrrolidines in a
relatively short time (between 1 and 3 h) at room temperature
and with a catalyst loading as low as 1 mol % (Table 2, entries
8 and 16).
Scheme 4. In situ preparation of Ar-BIAN alkyl ate and neutral yttrium complexes
(only the main species formed are shown).
4 | J. Name., 2012, 00, 1-3
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Table 3. Intramolecular hydroamination of Va catalyzed by ate and neutral rare-earth
complexes.a
Entry
Catalyst
t (h)
16
Conv. (%)b
88
1
2
[Li(THF)4][Y(CH2TMS)4]
1b
68
> 95
92
3
2b
0.8
4
4
4b
91
5
5b
0.5
0.5
1
> 95
> 95
92
6
5bc
7
5bd
8
[Y(CH2TMS)3(THF)2]
0.25
0.5
0.5
> 95
92
9
4c
5c
10
92
aReactions were carried out in C7D8, under argon at room temperature, with 6
Scheme 5. Intramolecular hydroamination with Ar-BIAN yttrium complexes.
mol
%
catalyst, unless otherwise stated. bDetermined by in situ 1H NMR
spectroscopy. c 2 mol % catalyst. d 1 mol % catalyst.
Table 2. Intramolecular hydroamination of primary amines catalyzed by ate rare-earth
complexes.a
for the sterically hindered complex 5b, where almost complete
cyclization of Va takes place in very short reaction times, also
at very low catalyst loadings (Table 3, entries 5-7). The neutral
YIII complexes were highly active species, with almost complete
conversion in about half an hour (entries 8-10). As already
demonstrated,15 neutral complexes are particularly active in
the hydroamination/cyclization of secondary amines.
Nevertheless, substrate Va was not sterically demanding
enough to allow for an accurate evaluation of the ligand
effects on the reactivity of the yttrium complexes in the
hydroamination reaction. For this reason, the more
challenging precursors VIa and VIIa were engaged to complete
the study (Scheme 5, Table 4). While the former is the
precursor for less favorable 6-membered heterocycles
(piperidine derivatives), the latter presents a less reactive
internal double bond. To get the cyclization products, all
reactions had to be performed at higher temperatures (50 °C
for VIa and 70 °C for VIIa, respectively) while keeping the
catalyst loading at 6 mol %.
Entry
1
Catalyst
Substrate
IIa
t (h)
2
Conv. (%)b
83
[Li(THF)4][Y(CH2TMS)4]
2
1b
IIa
17
75
3
2b
IIa
17
75
4
3b
IIa
16
63
5
4b
IIa
0.75
0.25
0.4
1
> 95
> 95
> 95
> 95
< 10
46
6
5b
IIa
7
5bc
IIa
8
5bd
IIa
9
[Li(THF)4][Y(CH2TMS)4]
IIIa
IIIa
IIIa
IIIa
IIIa
IIIa
IIIa
IIIa
18
10
11
12
13
14
15
16
1b
2b
88
88
50
3b
45
75
4b
1.5
0.25
0.5
3
> 95
> 95
> 95
> 95
5b
5bc
5bd
aReactions were carried out in C7D8, under argon at room temperature, with 6
mol
%
catalyst, unless otherwise stated. bDetermined by in situ 1H NMR
spectroscopy. c 2 mol % catalyst. d 1 mol % catalyst.
Table 4. Intramolecular hydroamination of demanding amines catalyzed by ate and
neutral rare-earth complexes.a
Overall, it can be inferred that the use of sterically hindered
diamine ligands, preferably bearing EDG as aryl substituents,
favors the generation of the targeted amido alkyl ate
complexes and avoids the formation of tetra-amido species,
generally recognized to be less catalytically active for
promoting the hydroamination reaction.16
Entry
1
Catalyst
Substrate
VIa
t (h)
24
21
20
17
24
18
18
24
24
24
24
24
24
24
24
Conv.(%)b
35
[Li(THF)4][Y(CH2TMS)4]
2
2b
VIa
10
3
4b
VIa
80
4
5b
VIa
> 95
< 10
10
The catalytic performance of the most representative
complexes from this series was then evaluated in the
cyclization of the model secondary aminoalkene Va (2,2-
dimethyl-pent-4-enyl)-methyl-amine (Scheme 5, Table 3).
Again, all the in situ prepared ate and neutral complexes were
active towards the cyclization of this secondary amine.
Interestingly, and as a general comment, in the presence of ate
complexes the cyclization occurred more rapidly compared to
the transformation of the primary aminoalkene analogue IIIa
(Table 3, entries 1-2 vs. Table 2, entries 9-10). As observed
with substrates IIa and IIIa, complexes possessing EDG as aryl-
ligand substituents caused an increase in the reaction rate
(Table 3, entries 3-5). This effect was even more pronounced
5
[Y(CH2TMS)3(THF)2]
VIa
6
4c
VIa
7
5c
VIa
34
8
[Li(THF)4][Y(CH2TMS)4]
VIIa
VIIa
VIIa
VIIa
VIIa
VIIa
VIIa
VIIa
15
9
2b
20
10
11
12
13
14
15
4b
> 95
> 95
43
5b
5bc
[Y(CH2TMS)3(THF)2]
42
4c
5c
35
88
a Reactions were carried out in C7D8, under argon at 50 °C for the transformation
b
of VIa and 70 °C for VIIa, with 6 mol % catalyst, unless otherwise stated.
Determined by in situ 1H NMR spectroscopy. c 2 mol % catalyst.
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As Table 4 shows, all catalysts promoted the cyclization bidentate systems in combination with late transition metals;
reactions of VIa and VIIa, albeit with a reduced reactivity if studies in this direction are currently ongoing in our labs.
compared with the previously reported cyclization precursors.
Once again, complexes bearing more sterically hindered
Experimental Section
ligands offered the best performance with both aminoalkenes.
Complete conversion of both substrates was obtained with the
4b and 5b ate complexes (Table 4, entries 4, 10-11). Cyclization
of VIIa up to 88% was also obtained with the neutral complex
5c (Table 4, entry 15). Neutral complexes bearing less hindered
ligands (i.e. 4c) or free of any ligand did not seem to resist the
high temperatures required to promote the cyclization process
(Table 4, entries 13-14). Overall, the amido alkyl ate complex
5b and (to a lesser extent) the neutral yttrium complex 5c are
rare examples of active species that can promote
hydroamination in case of challenging substrates like those
General Procedure
Unless otherwise stated, all the manipulations concerning the
ligands synthesis were performed under
a
nitrogen
atmosphere by using standard Schlenk techniques. As for the
investigation of zirconium compounds reactivity, all air- and/or
moisture-sensitive reactions were performed under inert
atmosphere in flame-dried flasks using standard Schlenk-type
techniques or in a nitrogen filled dry-box. Catalytic reactions
were performed under inert atmosphere (N2) in a 10 mL round
bottom flask and the reaction course was monitored by GC-MS
analysis. As for the investigation of yttrium compounds, all
manipulations were carried out under an argon atmosphere by
using standard Schlenk or glove box techniques.
possessing internal non-activated double bonds.
A non-
negligible activity was maintained even at low catalyst loadings
(Table 4, entry 12).3a, 3m, 13, 16-17
Ar-BIANs were synthesized as previously reported (see ESI for
full procedure and NMR characterization).20 Amino alkenes
Conclusions
Ia 21
,
IIa 16
,
IIIa 22
,
IVa 23
,
Va 24
,
VIa 25
,
VIIa 16
,
In this paper, we have investigated the reactivity of ZrIV and YIII
complexes prepared from different Ar-BIANH2 compounds in
the intramolecular hydroamination of primary and secondary
aminoalkenes. A new and convenient synthetic methodology
was described for the preparation and isolation of the bis-
anilido ligands from the respective bis-imino precursors (Ar-
BIAN). The effectiveness of the proposed methodology for the
generation of the Ar-BIANH2 ligand class has allowed for the
14
[Li(THF)4][Y(CH2SiMe3)4]12 and Y(CH2SiMe3)3(THF)2
were
prepared according to reported procedures. When employed
in yttrium catalyzed reactions, aminoalkenes were dried over
calcium hydride, transferred under vacuum and further dried
for at least 1h over 3Å molecular sieves with a few drops of
toluene-d8 prior to use. Methanol was freshly distilled under
nitrogen over Mg(OMe)2. Distilled water was degassed by
irradiating with ultrasounds under nitrogen in a cleaning bath.
Benzene and toluene were purified by distillation from
sodium/triglyme benzophenone ketyl and stored over
activated 4Å molecular sieves. Benzene-d6 was dried over
sodium/benzophenone ketyl and condensed in vacuo over
activated 4Å molecular sieves prior to use. Toluene-d8 was
dried with sodium benzophenone ketyl, transferred under
vacuum and stored over activated 3Å molecular sieves. All
other reagents and solvents were used as purchased from
commercial suppliers without further purification (unless
otherwise stated). 1D (1H and 13C) and 2D (COSY H,H, HETCOR
H,C) NMR spectra were recorded either on a Bruker Avance
300 MHz instrument (300.13 and 75.47 MHz for 1H and 13C,
respectively) or on Bruker AM250, Bruker AV300 and AV360
and DRX400 NMR spectrometers, operating at 250, 300, 360
and 400 MHz respectively. Chemical shifts are reported in ppm
isolation and characterization of
a bis-amido zirconium
complex (5a) by means of a transamination reaction. The same
bis-anilido ligands have also been successfully employed for
the in situ synthesis of YIII complexes, to be tested for the
intramolecular hydroamination reaction. While 5a has shown
only moderate catalyst performance, the in situ prepared
organoyttrium complexes have been found to be excellent
catalyst candidates for the hydroamination with a number of
substrates, including those normally reluctant at undergoing
cyclization. Complexes prepared from the sterically hindered
ligand
5 (containing two ortho isopropyl groups on each aryl
ligand bound to N) showed the highest activity, both in
combination with zirconium and yttrium. This is in line with
what observed when Ar-BIAN compounds are used as ligands
in the palladium- or nickel catalyzed polymerization of
olefins,18 but contrasts with what observed when the same
ligands are employed to generate catalysts for several
reactions like Pd-catalyzed olefin/CO copolymerization, Ru-
catalyzed allylic aminations by nitroarenes and others, steric
hindrance inhibiting the reaction in these cases.19 Such a
behavior is indicative of a ligand active role in preventing
catalyst deactivation through aggregation or over-coordination
and highlights the importance of steric effects in the
hydroamination reactions here described. Overall, the
successful approach to the synthesis of early transition metal
and rare-earth complexes based on Ar-BIANH2 ligands paves
the way to the future exploitation of these bis-amino
(δ) relative to TMS, referenced to the chemical shifts of
residual solvent resonances (1H and 13C). The multiplicity of the
13C
13C
{
{
1H
1H
}
}
NMR spectra was determined on the basis of the
JMOD sequence and quoted as: CH3, CH2, CH and C for
primary, secondary, tertiary and quaternary carbon atoms,
respectively. The C, H, N elemental analyses were made on a
Thermo FlashEA 1112 Series CHNS-O elemental analyzer. The
GC-MS analyses were performed on a Shimadzu QP2010S
apparatus equipped with
a SUPELCO SPB-1 fused silica
capillary column (30 m length, 0.53 mm i.d., 15 µm film
thickness).
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6NJ02199A
Journal Name
ARTICLE
General synthesis of Ar-BIANH2 with NaBH4 as reducing agent
MeOH (20 mL), collected by filtration on a glass frit, washed
with water (3×10 mL) and dried under vacuum.
The synthesis is a modification of that previously reported by
some of us.6 Ar-BIAN (1 mmol) and freshly distilled methanol
(7 mL) were added to a Schlenk flask under a nitrogen
atmosphere. Solid NaBH4 (3 mmol) was then added. The color
of the solution changed from yellow/orange to red/purple. The
reaction was maintained under stirring overnight. The mixture
was then concentrated to half volume under vacuum. The
precipitate was collected by filtration on a glass frit, washed
with degassed water (3×10 mL), and dried under vacuum.
Satisfactory elemental analysis values could not be obtained
due to the air sensitivity of the Ar-BIANH2 derivatives, as also
previously observed.6
2,4,6-Me3C6H2-BIANH2 (4). Dark-purple solid, 56% yield. 1H
NMR (400 MHz, C6D6)
8.2 Hz and 7.0 Hz, 2H, H4), 6.82 (s, 4H, H10), 6.71 (d,
2H, H3), 4.69 (s, 2H, NH), 2.21 (s, 12H, ortho-CH3), 2.19 ppm (s,
6H, para-CH3). 13C NMR (100 MHz, C6D6) 138.8 (C8), 137.0
(C2), 133.5 (C11), 133.0 (C9), 129.8 (C10), 127.8 (C4), 126.7 (C1),
125.6 (C7), 125.40 (C5), 119.3 (C3), 21.0 (para
H3), 18.7 ppm
ortho
H3). The signal corresponding to C6 overlaps with C6D6.
δ 7.26 (d, J J =
= 8.2 Hz, 2H, H5), 6.98 (dd,
J
= 6.9 Hz,
δ
-C
(
-C
1
2,6-iPr2C6H3-BIANH2 (5). Dark-purple solid, 69% yield. H NMR
(400 MHz, C6D6)
7.23 – 7.11 (m, 8H, H5, H10 and H11), 6.92
(dd,
= 8.0, 7.3 Hz, 2H, H4), 6.51 (d, = 7.0 Hz, 2H, H3), 4.99 (s,
2H, N ), 3.54 (sept, = 6.9 Hz, 4H, C (CH3)2), 1.19 (d, = 6.9
Hz, 12H, CH3), 1.09 ppm (d,
= 6.8 Hz, 12H, CH3). 13C NMR (100
MHz, C6D6)
144.8 (C9), 138.4 (C8), 136.1 (C2), 127.4 (C6), 127.2
(C4), 126.0 (C11), 125.3 (C5), 123.8 (C10), 119.9 (C3), 28.3
H(CH3)2), 24.1 H3), 23.4 ppm H3). Three signals
δ
J
J
H
J
H
J
J
δ
Ph-BIANH2 (1). Purple solid, 70% yield. 1H NMR (300 MHz,
(
C
(C
(C
δ 7.43 (d, J J
= 8.1 Hz, 2H, H5), 7.22 (d, = 6.9 Hz, 2H, H3),
corresponding to quaternary carbons were not detected or
overlap with C6D6.
C6D6)
7.17–7.11 (m,
(m, = 8.4 and
J
= 8.1 Hz, 2H, H4 overlapping with C6D6), 7.05
= 7.5 Hz, 4H, H10), 6.77 (m, 2H, H11 overlapping
J
J
with H9), 6.74 (d,
J
= 8.4 Hz, 4H, H9), 5.01 ppm (s, 2H, N
H
). 13C
Synthesis of 5a.
144.9 (C8), 136.3 (C2), 129.4 (C10), 128.5
To a solution of
5
(0.100 g, 0.20 mmol) in dry and degassed
mL), solution of
NMR (75 MHz, C6D6)
δ
(C6), 127.7 (C4), 126.5 (C5), 126.2 (C7), 121.4 (C3), 120.2 (C11),
116.7 ppm (C9). The signal corresponding to C1 was not
detected or overlaps with C6D6.
benzene (3
a
tetrakis(dimethylamido)zirconium (0.053 g, 0.20 mmol) in dry
and degassed benzene (2 mL) was added. The reaction mixture
was maintained at room temperature, under stirring, for 3 h
and then concentrated in vacuo to give the crude mixture as a
dark-green solid. The crude sample was washed with pentane
and filtered to afford analytically pure green crystals of 5a in
95% isolated yield. Crystals suitable for X-ray diffraction
analysis were grown from a concentrated toluene solution at -
30 °C. Crystals of 5a are indefinitely stable under nitrogen
atmosphere at -30 °C. On the other hand, a progressive
complex decomposition and formation of intractable side-
products takes place in aromatic hydrocarbons already at
room temperature. Such an undesired side effect occurs
irrespective of the concentration of 5a in solution.
1
4-CH3OC6H4-BIANH2 (2). Dark-purple solid, 75% yield. H NMR
(400 MHz, C6D6)
H4 and H3), 6.75 (d,
H10), 4.92 (s, 2H, N
MHz, C6D6)
154.8 (C11), 138.4 (C8), 136.7 (C2), 127.7 (C4),
H3).
δ
7.41 (d,
= 9.1 Hz, 4H, H9), 6.71 (d,
), 3.31 ppm (s, 6H, OCH3). 13C NMR (100
J
= 7.9 Hz, 2H, H5), 7.26–7.00 (m, 4H,
J
J = 9.1 Hz, 4H,
H
δ
126.1 (C5), 120.9 (C3), 118.9 (C9), 115.0 (C10), 55.2 ppm (O
C
The signals corresponding to C1, C6 and C7 were not detected
or overlap with C6D6.
1
3,5-(CF3)2C6H3-BIANH2 (3). Red solid, 71%. H NMR (400 MHz,
C6D6)
Hz, 2H, H4), 7.20 (s, 2H, H11), 7.09 (d,
4H, H9), 5.20 ppm (s, 2H; N ). 13C NMR (100 MHz, C6D6)
143.2 (C8), 135.5 (C2), 132.2 (q, 2JC,F = 33.0 Hz, CF3), 128.4 (C6),
128.0 (C4), 127.5 (C5), 125.1 (C7), 123.8 (q, 1JC,F = 273 Hz, C
F3),
123.5 (C1), 120.5 (C3), 116.0 (C9), 113.0 ppm (C11). 19F{1H} NMR
(282 MHz; C6D6) -63.3 ppm.
δ
7.51 (d,
J
= 8.2 Hz, 2H, H5), 7.23 (pst,
J
= 8.1 Hz and 6.9
J
= 6.9 Hz, 2H, H3), 6.66 (s,
H
δ
C
C
δ
General synthesis of sterically crowded Ar-BIANH2 with Na as
reducing agent
Metallic sodium cut into small pieces (85 mg, 3.7 mmol) was
placed in a Schlenk flask under a nitrogen atmosphere, washed
with THF twice (2×3 mL) and then suspended in THF (25 mL).
The sterically crowded Ar-BIAN (1.5 mmol) was then added as
a solid and the reaction mixture was stirred for 5 h. The color
of the mixture progressively turned from orange/yellow to red
and finally dark green. MeOH (2 mL) was added and the
mixture stirred until the color turned to purple. The solvent
was evaporated under vacuum. The residue was suspended in
3
1.23 (d, JHH = 6.8 Hz, 12H,
1H NMR (300 MHz, C6D6, 293K):
δ
3
B
CH(CH3ACH3B, H15,18), 1.29 (d, JHH = 6.8 Hz, 12H, CH(CH3ACH3 ,
H16,19), 1.70 (br s, 6H, HN(CH3)2, H21), 2.74 (br s, 12H, N(CH3)2,
H20), 3.86 (sept, 3JHH = 6.8 Hz, 4H, C
H
(CH3)2, H14,17), 6.15 (d, 3JHH
= 6.9 Hz, 2H, H3), 6.89 (dd, JHH = 8.4 Hz and 6.9 HZ, 2H, H4),
3
7.09 (d, JHH = 8.4 Hz, 2H, H5), 7.28 (m, 6H, H10, 11, 12). 13C
{ }
1H
24.6
H(CH3)2, C14,17), 38.6
3
NMR (75 MHz, C6D6, 293 K, selected data):
δ
(CH(CH3ACH3 ), 26.1 (
C
H(C
H3ACH3 ), 28.2 (
C
B
B
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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ARTICLE
Journal Name
(HN(
C
H3), C21), 41.7 (N( H3), C20), 119.2 (C3), 123.7 (C5), 123.9 Single crystal X-Ray data were collected at low temperature
C
(C10,12), 125.2 (C11), 127.1 (C4), 137.1 (C2), 144.7 (C9,13), 145.0 (100 K) on an Oxford Diffraction XcaliburPX diffractometer
(C1), 148.4 (C8). Anal. Calcd (%) for C42H55N5Zr (725.18): C, equipped with a CCD area detector using Cu Kα radiation (λ =
69.56; H, 8.20; N, 9.66. Found: C, 69.62; H, 8.25; N, 9.60.
1.5418 Å). The program used for the data collection was
CrysAlis CCD 1.171. Data reduction was carried out with the
program CrysAlis RED 1.171 and the absorption correction
was applied with the program ABSPACK 1.17. Direct methods
implemented in Sir97 were used to solve the structures and
the refinements were performed by full-matrix least-squares
against F2 implemented in SHELX97. All the non-hydrogen
atoms were refined anisotropically, while the hydrogen atoms
were fixed in calculated positions and refined isotropically with
the thermal factor depending on the one of the atom to which
they are bound (riding model). Disorder on some methyl
groups of the –NMe2 substituents was not explicitly treated
during the refinement, since no significant improvement of the
R factor could be achieved. Molecular plots were produced by
the program ORTEP3.
General procedure for zirconium catalyzed intramolecular
hydroamination reactions of aminoalkenes Ia-IVa
All catalytic tests were set-up in an inert atmosphere in a N2-
filled drybox. The amido precursor 5a was tested as neutral
catalyst in the intramolecular hydroamination reaction in a
two-necked 10 mL round-bottom flask equipped with a
magnetic stirring bar, a glass stopper and a septum. In a typical
procedure, a solution of the catalyst (5 mol %) in dry and
degassed toluene (1 mL) was treated in one portion with a
solution of the aminoalkene (0.21 mmol) in dry and degassed
toluene (1.3 mL) and ferrocene as an internal standard (0.2 mL
of a stock 0.17 M ferrocene solution in toluene). Toluene was
used as reaction solvent and the catalyst content was fixed to
5 mol % for each run. Afterwards, the system was heated at
the final temperature and the reaction course was periodically
monitored by analyzing a samples of the mixture by GC-MS
analysis at fixed times.
CCDC-1431252 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
ccdc.cam.ac.uk/community/requestastructure.
General procedure for in situ preparation of complexes 1b to 5b.
Acknowledgements
In a glovebox, a solution of [Li(THF)4][Y(CH2TMS)4] (0.042
mmol) in C7D8 (1 mL) was stirred for few minutes. The
considered Ar-BIANH2 ligand (0.050 mmol) was solubilized in
C7D8 (1 mL) and the corresponding solution was slowly added
dropwise into the [Li(THF)4][Y(CH2TMS)4] solution under
vigorous stirring. As the ligand was slowly added, the dark
violet to red reaction mixture (according to the ligand
structure) turned to a dark green to blue colored solution. The
homogeneous reaction solution was then allowed to stir 30
min at ambient temperature.
A. C. and F. M. thank the Erasmus Project for a fellowship that
allowed their stay in France. F. F. thanks the University of
Milan for a PostDoc fellowship. G. G., L. L. and A. R. thank the
Fondazione Cariplo (“Crystalline Elastomers” project) for
funding this research activity. S. G., J. H. and E. S. thank
MENSR, CNRS, and the Groupe de Recherche International
(GDRI) “Homogeneous Catalysis for Sustainable Development”
for financial support.
References
General procedure for in situ preparation of complexes 4c and 5c
The same procedure was used starting from a solution of
Y(CH2SiMe3)3(THF)2 (0.042 mmol) and the Ar-BIANH2 ligands
(0.050 mmol).
1. For general and more specialized reviews on
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DOI: 10.1039/C6NJ02199A
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DOI: 10.1039/C6NJ02199A
Graphical abstract:
Even demanding aminoalkenes can be cyclized by yttrium complexes of Ar-BIANH2; a related zirconium complex has been
crystallographically characterized.