Preparation and catalytic performance of novel dimeric tetranuclear zinc complexes in hydroamination of alkenes at room temperature

Article abstract of DOI:10.1055/s-0028-1087418

The synthesis of two novel dimeric tetranuclear zinc complexes has been described. The application of these complexes in hydroamination of unactivated alkenes showed at room temperature high catalytic activity. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1087418

3106
LETTER
Preparation and Catalytic Performance of Novel Dimeric Tetranuclear Zinc
Complexes in Hydroamination of Alkenes at Room Temperature
Preparation of
Novel
D
u
imeric Tetra
s
nuclear
Z
i
nc Coma
plexes fa Biyikal, Karolin Löhnwitz, Peter W. Roesky,*a Siegfried Blechert*b
a
Institut für Anorganische Chemie, Universität Karlsruhe, Engesserstr. 15, 76128 Karlsruhe, Germany
Fax +49(721)6084854; E-mail: roesky@chemie.uni-karlsruhe.de
b
Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany
Fax +49(30)31423619; E-mail: blechert@chem.tu-berlin.de
Received 10 August 2008
reactions at high temperatures only. Therefore, we were
motivated to develop effective zinc catalysts for hydroam-
ination reactions at room temperature.
Abstract: The synthesis of two novel dimeric tetranuclear zinc
complexes has been described. The application of these complexes
in hydroamination of unactivated alkenes showed at room tempera-
ture high catalytic activity.
Herein we report the intramolecular hydroamination of
Key words: alkenes, heterocycles, hydroamination, tetranuclear, nonactivated alkenes at room temperature by using dimer-
zinc
ic tetranuclear zinc complexes, derived from a novel type
of phenolic ligand. It is believed that the unique catalytic
properties of bimetallic complexes originate from double
activation by the two metal centers. The cooperative inter-
action of the metal centers with the substrate molecule al-
lows enhancing the catalytic activity, which cannot be
achieved with monometallic catalysts. In general, the ap-
plication of bimetallic catalysts in organic synthesis is in
contrast to monometallic catalysts rare.2e,2m–2p,2u–2w,3 In re-
sponse to this trend we have established two different
strategies to synthesize novel well-designed phenolic
ligands suitable for two metal centers. We first envisaged
the synthesis of ortho-aminated salicylaldimine ligand 3
from tropolone via a rearrangement reaction. The mecha-
nistically pathway of ortho-aminated salicylaldimines
from brominated tropolones with amines is specified in
the literature and has attracted our attention for our objec-
tives.4 Therefore, we adopted this procedure and synthe-
The catalytic addition of an organic amine N–H to C=C
bonds giving nitrogen-containing molecules is of great in-
terest for academic and industrial research, whereas the
hydroamination of alkenes at room temperature remains a
significant challenge. Numerous single-side metal cata-
lysts for this transformation have been developed, but
their sensitivity towards air and functional groups, their
toxicity and their high price limit their use in organic syn-
thesis.1 Zinc is noted for its mild and unique reactivity in
organic synthesis and tolerates numerous functional
groups. Nowadays zinc is applied as catalyst or reagent in
numerous reactions2 and also in hydroamination reactions
of olefins as ATI- and AT-zinc (ATI = aminotropon-
iminate; AT = aminotroponate) complexes that we recent-
ly reported.1a,e,h,i,p These zinc complexes catalyze the
Zn
O
OMe
Br
Zn
N
OH
N
O
H
N
a
d
N
Br
Br
Br
Br
Br
Br
2
3
1
4
O
HN
b
c
Br
Br
Br
2
Scheme 1 Synthesis of the dimeric tetranuclear zinc complex 4. Reagents and conditions: a) i-PrNH2, hexane, –78 °C to r.t., 20 h, 62%;
b) i-PrNH2, CH2Cl2, –40 °C to 0 °C, 3 h, 93%; c) i-PrNH2, hexane, –78 °C to r.t., 20 h, 62%; d) ZnEt2, hexane, r.t., overnight, 98%.
SYNLETT 2008, No. 20, pp 3106–3110
1
6
.1
2
.2
0
0
8
Advanced online publication: 27.11.2008
DOI: 10.1055/s-0028-1087418; Art ID: G24908ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Preparation of Novel Dimeric Tetranuclear Zinc Complexes
3107
sized the brominated ligand 3 from 1 in one step aldehyde 7 with isopropylamine then afforded the phenol-
(Scheme 1).
ic ligand 8 in 96% yield. The dimeric tetranuclear zinc
complex 9 (Figure 2) was obtained in 84% yield under the
same conditions as for the zinc complex 4.
The first step was a bromination and a subsequent methy-
lation of tropolone to give 1.5 Different strategies were es-
tablished for the methylation step of tribromotropolone,
whereas 1 was obtained in moderate yields. The extensive
studies of aryl ethers directed us to the Mitsunobu reac-
tions and we were able to synthesize 1 with diisopropyl-
azodicarboxylate and triphenylphosphine in an excellent
yield of 90%.5b Compound 1 was then converted in one
step into the phenolic ligand 3 in a reasonable yield of
62%.
To specify the transformation we prepared the ami-
notropone 2 from 1 via a basic substitution reaction with
isopropylamine in a very high yield of 93%. The subse-
quent treatment of 2 with an excess of isopropylamine led
to the phenolic ligand 3 via the rearrangement reaction in
a yield of 62%. From 3, we obtained the dimeric tetra-
nuclear zinc complex 4 (Figure 1) by treatment with di-
ethylzinc in excellent yields of 98% after recrystallization
from hexane–toluene.
Figure 1 Perspective solid-state structure of 4 showing the atom-
labeling scheme, omitting hydrogen atoms
In order to examine the influence of bromine on catalytic
activity of the dimeric tetranuclear zinc complex 4 we de-
cided to generate the bromine-free dimeric tetranuclear
zinc complex 9.
These new metal complexes 4 and 9 were characterized
by standard analytical/spectroscopic techniques, and the
solid-state structures of some selected compounds were
established by single crystal X-ray diffraction.6 Both com-
pounds are dimeric in the solid state. Four-membered
metallacycles (Zn1–N1–Zn3–N3) are formed in which
the two anilido nitrogen atoms bridge the zinc atoms. The
Zn–N bond distances are for compound 4 Zn1–N1
2.078(5) Å, Zn1–N3 2.136(4) Å, Zn3–N1 2.118(4) Å, and
Zn3–N3 2.081(4) Å and for compound 9 Zn1–N1
2.016(5), Zn1–N3 2.191(5), Zn3–N1 2.221(5), and
Zn3–N3 2.013(5). As a result of the distorted tetrahedal
arrangment of the Zn and the N atoms the four-membered
metallacycles have a butterfly structure. As a result of the
dimerization the two benzene rings are arranged almost
parallel to each other. In both structures the remaining two
zinc atoms (Zn2 and Zn4) are located at the opposite side
of the dimer. In each part of the dimer two zinc ethyl frag-
ments are coordinated to the ligand framework. The zinc
O
O
OH
O
O
O
OH
H
Br
N
Br
b–d
a
5
6
7
Zn
Zn
O
N
OH
N
H
N
N
e
f
2
9
8
Scheme 2 Synthesis of the dimeric tetranuclear zinc complex 9.
Reagents and conditions: a) NaH, chloromethylethylether, THF, 0 °C
to r.t., 3 h, quant.; b) i-PrNH2, MS 4 Å, toluene, r.t., 1 h; c) KOt-Bu,
BINAP, Pd(OAc)2, 50 °C, overnight; d) HCl [6 M], 5 h, r.t., 20%; e)
i-PrNH2, MS 4 Å, CH2Cl2, r.t., 0.5 h, 96%; f) ZnEt2, hexane, r.t., over-
night, 84%.
The synthesis of 9 started from commercially available al-
dehyde 5, which was protected with chloromethylethyl-
ether in quantitative yield without the need of purification
(Scheme 2). For the next step, it was necessary to con-
dense isopropylamine with the protected salicylaldehyde
6 in the presence of molecular sieves. Afterwards it was
possible to perform the Buchwald–Hartwig reaction. Due
to the low boiling point of isopropylamine the reaction
was performed at 50 °C in a sealed tube. The crude prod-
uct was then deprotected with HCl–MeOH and 7 was ob-
tained in three steps in 20% yield. The condensation of the
Figure 2 Perspective solid-state structure of 9 showing the atom-
labeling scheme, omitting hydrogen atoms
Synlett 2008, No. 20, 3106–3110 © Thieme Stuttgart · New York

3108
M. Biyikal et al.
LETTER
atom (Zn1 and Zn3) which are part of the four-membered strates were converted in 99% or quantitatively within the
metallacycles are coordinated onto the phenolate oxygen time stated. Isolated yields are given in Table 1. The re-
atoms (O1 and O2) and the anilido oxygen atoms (N1 and sults in entries 1–3 showed in the case of both zinc com-
N3), whereas the other zinc atoms (Zn2 and Zn4) are also plexes that the ring-closure reactions with sterically
coordinated to the phenolate oxygen atoms and to the imi- demanding gem-substituted substrates occur faster ac-
ne nitrogen atoms (N2 and N4). As a result of the different cording to the Thorpe–Ingold effect.7 By using precatalyst
coordination sphere the coordination number of Zn1 and 9 the reaction took 5–8 hours longer for the conversion of
Zn3 is four, whereas Zn2 and Zn4 are only threefold coor- these substrates than with precatalyst 4 (entries 1–3). It
dinated. In solution a dynamic monomer–dimer equilibri- was also possible to reduce the reaction times by heating.
um is observed for compounds 4 and 9. Whereas in the 13C In the case of benzyl-(2,2-dimethyl-pent-4-enyl)-amine
NMR spectra only one set of signals is observed, broad the reaction time could be reduced from 30 hours at room
peaks, which cannot clearly be assigned, are seen in the 1H temperature to 30 minutes at 80 °C with the precatalyst 4
NMR spectra of compounds 4 and 9. Variable tempera- and to 1 hour with the precatalyst 9 quantitatively (entry
ture experiments at elevated temperatures resulted in a 4). Furan- and thiophene-substituted substrates decreased
sharpening of the lines but still broad signals are seen.
the catalytic activity of both precatalysts at room temper-
ature (entry 5 and 6). However, a tremendous acceleration
of the reaction was observed at 80 °C, whereas both cata-
lysts converted (2,2-diphenyl-pent-4-enyl)-furan-2-yl-
methyl-amine in 20 minutes to 1-furan-2-ylmethyl-2-
methyl-4,4-diphenyl-pyrrolidine (entry 7). The substrates
shown in entries 8 and 9 could be converted within a short
time and in high yield. In both cases the brominated pre-
catalyst 4 catalyzed the reactions 2 hours faster than the
precatalyst 9. The cyclization of norbornene substrates
took place exclusively at the terminal double bond. With
the precatalyst 4 the thiophene-substituted norbornene de-
rivative was cyclized in 38 hours quantitatively at room
temperature. The same reaction with the precatalyst 9
took 30 hours longer for a conversion of 99% under the
same conditions (entry 10). At 80 °C the reaction time
could be reduced to 1.5 hours in both cases (entry 11). The
same effect was observed for the tosylated substrate (en-
tries 12 and 13). In these cases the reaction time could be
reduced from 69 hours at room temperature to 50 minutes
at 80 °C with the precatalyst 4, and to 1 hour with the pre-
catalyst 9.
The precatalysts 4 and 9 were tested in the hydroamina-
tion reactions of nonactivated olefins bearing different
functional groups. The reactions were carried out at room
temperature in C6D6 with a precatalyst loading of 2.5
mol% and 5.0 mol% of commercially available
[PhNMe2H][B(C6F5)4] as cocatalyst. The precatalysts 4
and 9 are catalytic inactive in the case of exclusion of the
cocatalyst. The two equivalents of cocatalyst were used to
remove the ethyl groups from the bimetallic zinc com-
plexes to liberate coordination sites and to form more re-
active cationic zinc complexes.
However, the challenge of the NMR-scaled reactions was
to homogenize the in situ formed catalysts in C6D6 that
generated an insoluble ionic phase at room temperature in
most of the cases. Ultrasonic techniques failed and there-
fore the NMR tubes were kept in horizontal position to en-
large the surface area and to enhance the transfer of the
substrates into the ionic phase. This method allowed us to
convert (1-allyl-cyclohexylmethyl)-benzyl-amine with
the precatalyst 4 in 4.5 hours quantitatively (entry 2). The
same reaction took 6 hours for a conversion of 99% in the
vertically held NMR tube. An explicit result was observed
in the conversion of (2,2-diphenyl-pent-4-enyl)-furan-2-
ylmethyl-amine with the precatalyst 4. The reaction took
only 23 hours for a quantitative conversion with enlarged
surface area of the ionic phase, whereas with a small sur-
face area 56 hours were needed for a conversion of 98%
(entry 6).
In conclusion, we have reported different strategies for the
synthesis of two novel multidentate phenolic ligands and
their corresponding zinc complexes and showed their high
catalytic activity in intramolecular hydroamination reac-
tions of unactivated olefins at room temperature and at
80 °C. The catalytic activity of our novel dimeric tetra-
nuclear zinc complexes was compared to our previously
reported ATI–zinc complexes and showed a significant
higher catalytic activity.1a,e,i Especially the brominated
zinc complex 4 showed a higher catalytic activity at room
temperature than the zinc complex 9. This is ascribed to
the electron-poor arene ring system caused by the bromine
substitution, which increases the Lewis acidity of the zinc
complex. These results prompted us to focus on mecha-
nistic investigations and the enhancement of the reactivity
and selectivity of new brominated multimetallic zinc
complexes.
The cocatalyst [PhNMe2H][B(C6F5)4] itself showed in hy-
droamination reactions a very minor activity only. In the
case of benzyl-(2,2-dimethyl-pent-4-enyl)-amine with a
cocatalyst loading of 5.0 mol% only traces of product
were observed after 30 hours at room temperature. At
80 °C only 14% conversion of the substrate was achieved
during 1 hour. The addition of precatalyst 4 accelerated
the reaction resulting in complete conversion in 30 min-
utes at 80 °C (entry 4).
In all hydroamination reactions the precatalyst 4 showed
a higher catalytic activity than the precatalyst 9. All sub-
Synlett 2008, No. 20, 3106–3110 © Thieme Stuttgart · New York

LETTER
Preparation of Novel Dimeric Tetranuclear Zinc Complexes
3109
Table 1 Results of the Hydroamination Reactions Catalyzed with the Precatalysts 4 and 9a
Entry
Substrate
4
9
Product
Time (h)
3
Yield (%)
Time (h)
8
Yield (%)
Ph
Ph
1
2
99
93
99
91
H
N
H
N
NTs
4.5
11
H
N
N
3
4
30
80
88
38
1b
77
83
0.5b
H
N
N
S
S
Ph
Ph
Ph
Ph
5
6
96
6
93
H
N
Ph
Ph
N
O
6
7
23
94
97
77
93
98
O
0.33b
0.33b
Ph
Ph
H
N
N
O
8
9
9
7
88
92
11
9
82
93
O
N
H
N
H
N
N
S
10
11
38
99
91
68
94
89
1.5b
1.5b
N
N
S
H
N
Ts
N
12
13
97
96
94
96
69
79
1b
0.83b
H
N
NTs
a Reagents and conditions: substrate (0.43 mmol), catalyst (2.5 mol%), [PhNMe2H][B(C6F5)4] (5.0 mol%), C6D6, r.t.; NMR tubes were kept in
horizontal position; conversions are determined by 1H NMR; all conversions are 99–100%; isolated yield of >95% purity.
b The reaction was performed at 80 °C.
K. C.; Gil, A.; Branchadell, V. Eur. J. Org. Chem. 2007,
3311. (c) Datta, S.; Roesky, P. W.; Blechert, S.
Supporting Information for this article is available online at
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Acknowledgment
The authors acknowledge support from the Cluster of Excellence
‘Unifying Concepts in Catalysis’ coordinated by the Technische
Universität Berlin and funded by the Deutsche Forschungsgemein-
schaft.
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Synlett 2008, No. 20, 3106–3110 © Thieme Stuttgart · New York

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M. Biyikal et al.
LETTER
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b = 17.2017 (7) Å, c = 22.2739 (9) Å, b = 126.790 (2)° at
200 K, Z = 8, V = 8931.0(6) Å3, r = 1.749 g cm–3,
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reflections (Rint = 0.0860). The structure was solved by
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refined by full-matrix least-square techniques with I > 2s(I)
to R1 = 0.0483 and wR2 = 0.1190.
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Single-Crystal X-ray Diffraction Data of Compound 9
C34H56N4O2Zn4 (814.31): Oxford Diffraction Xcalibur S
Sapphire, space group P61 (No. 169) with a = 11.21070 (10)
Å, c = 50.8375 (12) Å, at 150 K, Z = 6, V = 5533.25 (15) Å3,
r = 1.466 g cm–3, 2qmax = 50°, 41505 reflections collected,
6515 unique reflections (Rint = 0.0898). The structure was
solved by Patterson methods (SHELXS-97 and SHELXL-
97) and refined by full-matrix least-square techniques with
I > 2s(I) to R1 = 0.0605 and wR2 = 0.0812.
Positional parameters, hydrogen-atom parameters, thermal
parameters, bond lengths and angles have been deposited as
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