Zirconium catalysed intermolecular hydroamination reactions of secondary amines with alkynes

Article abstract of DOI:10.1039/c5cc01780g

An in situ generated cationic zirconium complex stabilized by an n-butylamine-bridged bis(phenolato) ligand has been developed to catalyse hydroamination reactions of secondary amines, which is the first example of group 4 metal based catalysts capable of

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Zirconium catalysed intermolecular hydroamination reactions of
secondary amines with alkynes
Qiu Sun,† Yaorong Wang,† Dan Yuan,†* Yingming Yao,†* and Qi Shen†
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
An in situ generated cationic zirconium complex stabilized by
an n-butylamine-bridged bis(phenolato) ligand has been
developed to catalyse hydroamination reactions of secondary
amines, which is the first example of group 4 metal based
catalyst capable of mediating intermolecular hydroamination
reactions of N-aryl/alkyl amines.
As
one
of
the
100%
atomꢀeconomic
catalytic
hydrofunctionalization reactions, hydroamination reactions are a
powerful tool to construct CꢀN bonds and generate important Nꢀ
containing fine chemicals and synthetic intermediates.1A wide
variety of catalytic systems, including complexes of late
transition metals,2 lanthanides,1e,3 and main group elements,1e, 4
have been developed to mediate this transformation. Group 4
metal complexes have shown great promise in this field1e,5,6,7due
to their general reactivity, low toxicity and easy availability. To
date, a variety of structurally diverse titanium and zirconium
complexes have been developed, which exhibited good activities
in catalysing both intraꢀ6 and intermolecular7 hydroamination
reactions. However, the substrate scope of intermolecular
hydroamination reactions has been mainly limited to primary
amines. The only example of intermolecular hydroamination of
Scheme 1 Synthesis of zirconium complexes
Zirconium benzyl complex
2
stabilized by the same
tetradentate bis(phenolato) ligand L1 as in 1 was prepared
according to a reported procedure.11 The ease to modify bridged
bis(phenolato) ligands allows the preparation of structurally
diverse complexes, and also makes a systematic study on the
relationship between structures and catalytic activities possible.
Using a similar protonolysis methodology, complex 3 bearing a
tridentate bis(phenolato) ligand L2 was easily prepared on a
multigram scale in a good yield of 90%. Characterizations of
secondary
amines
employs
a
tethered
zirconium
1
complex 3 include H, 13C NMR, Xꢀray diffraction analysis and
bis(ureate)complex reported by Schafer et al.7w The substrates,
however, are restricted to aliphatic secondary amines.
1
elemental analysis. In its H NMR spectrum, two singlets at 3.02
and 1.98 ppm are assigned to benzylic protons. Two doublets
resonating at 3.32 and 2.97 ppm with the coupling constant of 16
Hz correspond to the diastereotopic protons of the two NCH2Ph
groups. tBu groups give rise to two singlets at 1.80 and 1.36 ppm,
respectively, and signals corresponding to the nBu group are
found at 2.08, 1.01, and 0.32 ppm as multiplets. The solid state
structure of 3 (depicted in Figure 1) obtained from Xꢀray
diffraction analyses on single crystals supports that it has a
monomeric structure. The zirconium ion is fiveꢀcoordinated by
two oxygen atoms, one nitrogen atom, and two carbon atoms
from two benzyl groups. The coordination geometry around the
metal center can be best described as a slightly distorted trigonal
bipyramid, in which C(35) and C(42) occupy equatorial positions,
and O(1) and O(2) occupy axial positions.
Cationic zirconium complexes have been reported to exhibit
higher activities than their neutral counterparts in catalyzing
intramolecular hydroamination reactions of aminoalkenes and
aminoallenes.8 Recently, our group have reported the zirconium
complex
1 (Scheme 1) stabilized by an amineꢀbridged
tetradentate bis(phenolato) ligand.9 The in situ generated cationic
species from 1 and [Ph3C][B(C6F5)4] showed good activities and
high
regioꢀselectivities
in
catalyzing
intermolecular
hydroamination reactions of primary amines. An insertion
mechanism involving an amido intermediate has been proposed
based on experimental findings. Since the amido intermediate
formation is a prerequisite for secondary amines to undergo
hydroamination reactions,10 it is thus hypothesized that such a
cationic species may be capable of catalyzing hydroamination
reactions of secondary amines. However, no positive results were
obtained from 1. Modification of the zirconium complex was then
carried out through changing the ancillary ligands, which led to
complexes of distinctly different activities.
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reactions of primary amines.9 Cationic species generated in situ
from complex 1ꢀ3 and [Ph3C][B(C6F5)4] (TB) were then tested,
and distinct results were obtained. No conversion was detected in
the presence of either 1 or 2 with TB (entries 4 and 5). In stark
contrast, the reaction between 4a and 5a proceeded in the
presence of 3 and TB, and furnished 100% Markovnikov product
6a in 61% yield (entry 6). Reducing the coordinating sites in the
bis(phenolato) ligand apparently led to complexes with increased
Lewis acidity and larger accessible coordinating sites for the
substrate, which may account for the better catalytic activity of 3.
Optimisation of reaction conditions includes varying substrate
ratios, solvents, cationic reagents, catalyst loadings, and reaction
time. Study on different substrate ratios revealed that 2 equivalent
of 4a gave the best yield of 90% (entries 6ꢀ8). PhCl proved to be
the optimal reaction media (entries 8ꢀ11). It is noteworthy that no
reaction occurred in THF, which may be due to the deactivation
of active species through coordination of THF (entry 11).
Replacement of TB with [PhMe2NH][B(C6F5)4] (AB) lead to
similar results (entry 12), while only little conversion was
detected in the presence of [Et3NH][BPh4] (TEB) (entry 13).
Lowering the catalyst loading to 5 mol% resulted in a lower yield
of 72% even after prolonged reaction time (entry 14). Shortening
the reaction time to 4 h led to the same yield (entry 15). To verify
the role of the zirconium complex, the reaction in the absence of
3 was conducted, and no conversion was detected (entry 16).
After establishing the optimal condition, the scope of the
hydroamination reaction was explored. A variety of terminal
alkynes bearing either aliphatic or aromatic substituents were
studied, and the results are summarized in Table 2. Derivatives of
phenylacetylene bearing either electronꢀdonating or withdrawing
substituents at orthoꢀ, metaꢀ, or paraꢀpositions reacted smoothly
under established conditions, and afforded corresponding amines
6bꢀ6i in good yields of 81ꢀ91%. The disubstituted substrate 1ꢀ
ethynylꢀ3,5ꢀbis(trifluoromethyl)benzene (4j) was converted to the
corresponding amine 6j in 76% yield. The catalytic system
proved also efficient in catalysing the hydroamination reaction of
alkynes bearing heterocycles, such as 2ꢀethynylpyridine (4k) and
2ꢀethynylthiophene (4l), without the deactivation of the
catalytically active species by the coordinating heteroatoms.
Besides aromatic alkynes, aliphatic alkynes, such as 3ꢀphenylꢀ1ꢀ
propyne (4m) and 1ꢀhexyne (4n), were also converted into
corresponding amines in good yields of 92 and 93%, respectively.
In all reactions of terminal alkynes, only Markovnikov products
were detected, showing excellent regioꢀselectivity of this strategy.
However, hydroamination reactions of internal alkynes, such as
diphenylacetylene (4o) and 1ꢀphenylꢀ1ꢀpropyne (4p), became
sluggish and less selective. Products of low yields were isolated,
with the ratio of two isomers amounting to 20:1 in the latter case.
Hydroamination reactions between phenylacetylene and
various secondary amines catalyzed by complex 3 and TB have
also been studied under optimized conditions. The results are
summarized in Table 3. The simplest aromatic secondary amine
Nꢀmethylaniline reacted straightforwardly with phenylacetylene,
and afforded the tertiary amine 7b in 88% yield after reduction.
The reaction with Nꢀethylaniline yielded the corresponding amine
7c in a lower yield of 51% even after a prolonged reaction time of
12 h. In both cases, 100% Markovnikov products formed
Figure 1 Molecular structure of 3 showing 30% probability
ellipsoids; hydrogen atoms are omitted for clarity.
Table 1 Hydroamination/reduction reactions of phenylacetylene
4a and 1,2,3,4ꢀtetrahydroquinoline 5a catalyzed by complexes 1,
2 and 3a
loading
(mol%)
entry
1
cat.
solvent
PhCl
time (h)
6
yield (%)c
0
4a:5a
1
1:1
10
2
3
4
5
2
PhCl
PhCl
PhCl
PhCl
1:1
1:1
1:1
6
6
6
6
10
10
10
0
0
0
0
3
1/TB
2/TB
1:1
1:1
1:2
10
10
10
6
7
3/TB
3/TB
3/TB
3/TB
3/TB
PhCl
PhCl
6
6
6
6
6
61
63
90
89
17
8
PhCl
PhBr
Tol
2:1
2:1
2:1
10
10
10
9
10
11
12
13
2:1
3/TB
3/AB
10
THF
PhCl
PhCl
12
6
trace
87
2:1
2:1
10
10
3/TEB
12
trace
72
3/TB
3/TB
TB
PhCl
PhCl
PhCl
8
4
14
15
16
2:1
2:1
2:1
5
90
10
10
12
0
aConditions unless otherwise stated: 4a (0.22 mL, 2 mmol), 5a
(0.126 mL,
1
mmol),
3
(0.078 g, 0.1 mmol),
[Ph3C][B(C6F5)4](TB),
[PhMe2NH][B(C6F5)4](AB) or
[Et3NH][B(C6H5)4](TEB) (0.1 mmol) if necessary, solvent (3
mL), 110 °C. bThe initially formed enamine was reduced by
c
LiAlH4 to give corresponding tertiary amines. Yield is based on
5a and determined by GC with nꢀhexadecane as internal standard.
No antiꢀMarkovnikov product was detected by GCꢀMS.
To evaluate and compare the catalytic activities of all three
complexes 1ꢀ3, the hydroamination reaction between
phenylacetylene 4a and 1,2,3,4ꢀtetrahydroquinoline 5a was first
studied. None of the complex catalysed the transformation after 6
h reaction at 110 °C (Table 1, entries 1ꢀ3), which is consistent
with previous observation from Zrꢀcatalyzed hydroamination
exclusively. Reactions with indoline (5d),
a
1,2,3,4ꢀ
2
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tetrahydroquinoline derivative (5f), or indoline derivatives (5gꢀj)
resulted in 85ꢀ91% yields (7d, 7fꢀj). Notably, with the
introduction of a coordinating O atom, the substrate 3,4ꢀdihydroꢀ
2Hꢀbenzo[b][1,4]oxazine still gave 7e in a good yield of 82%.
For substrates 5fꢀh, products 7fꢀh with two stereocenters were
formed. Although their yields were good (85ꢀ86%), their
diastereoselectivity was poor (dr = 5:4 ~ 3:1). A bulky secondary
amine diphenyl amine (5k) did not undergo hydroamination
reaction under established conditions.
amines proved unsuccessful. No product was isolated from
reactions of pyrrolidine, piperidine, 1,2,3,4ꢀ
tetrahydroisoquinoline, or morpholine. In sharp contrast, the
tethered zirconium bis(ureate) complex was reported to catalyze
hydroamination reactions of Nꢀalkyl/alkyl amines.7w The two
strategies thus work in complementary to each other.
To further investigate the catalytic process in detail, a kinetic
study has been carried out on the reaction between mꢀ
tolylacetylene 4c and tenꢀfold of indoline 5d. A plot of
ln([4c]0/[4c]) versus time shows a firstꢀorder disappearance of
alkyne 4c (Figure 2). No induction period was observed.
Meanwhile, changing the loading of amine (1, 2 and 10 equiv.)
did not influence the reaction rate, revealing a zeroꢀorder
Table
2
Hydroamination/reduction reactions of 1,2,3,4ꢀ
tetrahydroquinoline 5a and various alkynes catalyzed by 3/TBa,b
dependence on amines. Overall, the rate law is deduced as
[alkyne]1[amine]0.
ν ~
1.4
y = 2.88958*10-4
x
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
1000
2000
3000
4000
5000
time (s)
Figure 2 Plot of ln[mꢀtolylacetylene]0/[mꢀtolylacetylene] versus
time (s) for the hydroamination reaction of mꢀtolylacetylene 4c
and indoline 5d catalyzed by complex 3 and TB. Conditions:
[4c]0 = 0.3636 mol·L−1, [5d]0 =3.6374 mol·L−1; 10 mol% of
complex 3 and TB, PhBrꢀd5, 110 °C.
aConditions: 4a (0.126 mL, 1 mmol), 5 (2 mmol), 3 (0.078 g, 0.1
mmol), [Ph3C][B(C6F5)4] (TB) (92 mg, 0.1 mmol) were stirred in
b
c
PhCl (3 mL) for 4 h at 110 °C. Isolated yield. Reduction was
not performed. d24 h reaction time.
Table 3 Hydroamination/reduction reactions of phenylacetylene
and various secondary aminesa,b
a Conditions: 3 (1 mmol), 4a (2mmol, 0.23 mL), 1 (0.079 g, 0.1
mmol), [Ph3C][B(C6F5)4] (TB) (92 mg, 0.1 mmol) were stirred in
PhCl (3 mL) for 4h, 110 °C. bIsolated yield. c12 h reaction time.
Scheme 2 Plausible mechanism for hydroamination reactions
catalysed by 3 and TB
Further extending the substrate scope to secondary aliphatic
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Based on all findings described above, an amido intermediate
B derived from the cationic species A has been proposed
(Scheme 2). Coordination of alkynes takes place, giving rise to
the intermediate C. Increased Lewis acidity and larger accessible
coordination sites of the metal center in 3 apparently facilitates
the formation of C, which may account for its enhanced activity
in comparison to 1 and 2.7y An σꢀbond insertion into the ZrꢀN
bond occurs to form D. The regioselectivity of hydroamination
reactions may arise from the repulsion between the bulky ligand
L2 and the R group in C, which favors the intermediate D leading
to Markovnikov products. Aminolysis of D affords E, which
subsequently liberates the enamine product along with the
regeneration of B.
In summary, an in situ generated cationic zirconium complex
stabilized by an nꢀbutylamineꢀbridged bis(phenolato) ligand has
been developed to catalyze hydroamination reactions of
secondary amines. This is the first example of group 4 metal
based catalyst capable of catalysing intermolecular
hydroamination reactions of Nꢀaryl/alkyl amines. Study on
development of group 4 metal complexes to further broaden the
scope of hydroamination reactions is onꢀgoing in our laboratory.
Financial support from the National Natural Science
Foundation of China (Grants 21132002, 21372172 and
21402135), the Major Research Project of the Natural Science of
the Jiangsu Higher Education Institutions (14KJA150007),
PAPD, and the Qing Lan Project are gratefully acknowledged.
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Notes and references
Key Laboratory of Organic Synthesis of Jiangsu Province, College of
Chemistry, Chemical Engineering and Materials Science, Dushu Lake
Campus, Soochow University, Suzhou 215123, People’s Republic of
China; Eꢀmail: yuandan@suda.edu.cn (D. Y.); yaoym@suda.edu.cn (Y.
Y.); Fax: 86 512 65880305; Tel: 86 512 65882806
† Electronic Supplementary Information (ESI) available: experimental
details, characterization data, kinetic study and crystallographic data
(CCDC of complex 3: 1051622). See DOI: 10.1039/c000000x/
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