Lewis acid double metal cyanide catalysts for hydroamination of phenylacetylene

Article abstract of DOI:10.1039/c0cc05335j

Double metal cyanides (DMCs) are highly active recyclable heterogeneous catalysts for hydroamination of phenylacetylene with 4-isopropylaniline. The best hydroamination yields are obtained with Zn-Co DMCs, especially if the particle size is decreased by a

Full text of DOI:10.1039/c0cc05335j

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COMMUNICATION
Lewis acid double metal cyanide catalysts for hydroamination
of phenylacetylenew
Annelies Peeters, Pieterjan Valvekens, Frederik Vermoortele, Rob Ameloot,
Christine Kirschhock and Dirk De Vos*
Received 2nd December 2010, Accepted 17th February 2011
DOI: 10.1039/c0cc05335j
Double metal cyanides (DMCs) are highly active recyclable
heterogeneous catalysts for hydroamination of phenylacetylene
with 4-isopropylaniline. The best hydroamination yields are
obtained with Zn–Co DMCs, especially if the particle size is
decreased by a reverse emulsion synthesis technique.
Hydroamination, the direct addition of an amine to a carbon–
carbon unsaturated bond, is an attractive atom economical
route to amines, imines and enamines.1 The reaction is thermo-
dynamically feasible but high activation barriers exist, even for
the energetically more favourable alkynes.2 Homogeneous
Fig. 1 PXRD of standard Zn–Co DMC (a), nanosized Zn–Co DMC
catalysts have been extensively investigated, but only few
heterogeneous catalysts have been found.1,3–5 Especially with
industrial applications in mind a heterogeneous system offers a
huge advantage in terms of product work-up. Double metal
cyanides (DMCs) are industrially applied solid catalysts for
the ring opening polymerization (ROP) of epoxides producing
polyether–polyols.6–8 They were also reported as catalysts for
copolymerization of epoxides with CO2, for other epoxide
ring opening reactions and for transesterifications.9–15 In the
present work, DMCs are for the first time used as solid Lewis
acid catalysts in hydroamination reactions. Zn–Co based DMCs
were found to be highly active hydroamination catalysts, giving
a 99% yield of the Markovnikov addition product. The reaction
rate could be further improved with a nanosized Zn–Co DMC
prepared by a reverse emulsion technique.
(b) and simulated pattern (c) (left); difference IR spectrum of adsorbed
pyridine on standard Zn–Co DMC (right).
polymerization reactions, the introduction of such CA and
co-CA into the catalyst matrix makes the DMCs more
active.12 In order to assess the effect of particle size, nanosized
DMCs (NDMCs) were prepared by a reverse emulsion synthesis
procedure in which Igepals CA-520 (poly(oxyethylene)5-
(4-isooctylphenyl)ether) is used as an emulsifier to stabilize
the formation of water droplets in the dispersion medium.16
z
The structure of the Zn–Co DMC materials was charac-
terized by powder X-ray diffraction (PXRD) (Fig. 1 (left) and
Fig. S1 in ESIw), which proved the presence of the cubic lattice
structure in all Zn–Co DMCs. The principal reflections of a
simulated pattern, based on the crystal structure of the pure
Zn–Co Prussian blue analogue Zn3[Co(CN)6]2ꢀ12H2O, are all
readily recognized in the experimental patterns (Fig. 1c, left).17
When comparing the PXRD of the standard Zn–Co DMC
and the Zn–Co NDMC, it is clear that even for the nanosized
material, the DMC structure is retained (Fig. 1a and b, left).
Scanning electron microscopy (SEM) readily visualizes the size
and morphology differences for DMCs obtained via the
various procedures.w Typical crystal dimensions decrease from
1–4 mm for the CA-free Zn–Co DMC, over 1–2 mm for the
material prepared in the presence of tert-butanol, down to 200 nm
for the NDMC material (Fig. 2 and Fig. S2–S4, ESIw).
Thermogravimetric analyses (TGA) under O2-atmosphere of
the Zn–Co DMC catalysts all show a weight loss of approximately
35 wt% between 300 and 400 1C (Fig. S5 and S6, ESIw). This is
attributed to the loss of the cyanide ligands of the material.13
Energy dispersive X-ray spectroscopy (EDX) was used to
DMCs combine two transition metals in their structure
and can be considered as analogues of Prussian blue
(Fe4[Fe(CN)6]3ꢀ14H2O). This framework of FeIII–CN–FeII
bridges is based on a face-centered cubic unit cell.12 In this
study, a standard DMC material was synthesized by addition
of a water-soluble metal cyanide, K3[Co(CN)6,] to an excess
amount of ZnCl2 or another water-soluble metal salt.z As
a complexing agent (CA) tert-butanol was used; poly-
(tetramethylene ether) glycol (PTMEG) was added as the
co-complexing agent (co-CA). The co-CA may act as a
protecting agent and thus affect the particle size;12 in epoxide
Centre for Surface Chemistry and Catalysis, Department of Microbial
and Molecular Systems, K.U. Leuven, Kasteelpark Arenberg 23,
3001 Leuven, Belgium. E-mail: Dirk.DeVos@biw.kuleuven.be
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c0cc05335j
c
4114 Chem. Commun., 2011, 47, 4114–4116
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The unique nature of the zinc site in the Zn–Co DMC
derived from ZnCl2 is further underscored by a comparison
with other Zn catalysts. The Zn source was altered from ZnCl2
to ZnSO4ꢀ7H2O (entry 10) or Zn(CF3SO3)2 (entry 11). Much
lower activities, however, were found with the sulfate anion
and the triflate anion. This is in agreement with results in the
literature for polymerization reactions catalyzed with
DMCs.18 When the catalytic activity of ZnCl2 as such was
tested, no hydroamination product was formed, even after
24 h (entry 12). This illustrates the special character of the
Zn-sites in Zn–Co DMCs: the activity is not due to mere
deposition of Zn salts on a solid support. The Co cyanide salt
K3[Co(CN)6] was tested as well under the same conditions and
showed no hydroamination activity.
In the literature, Zn2+ ion exchanged zeolite H-beta (Zn-beta)
has been proposed as a hydroamination catalyst.26,27 Here, Zn
Lewis acid sites are combined with Brønsted acid sites of the
parent zeolite material. Zn-beta (50% of sites exchanged) was
compared with the DMC for the test reaction. The zeolite
catalyst was activated at 200 1C under vacuum overnight.
After 24 h, a 71% yield of the Markovnikov hydroamination
product was found (entry 13). Even after rigorous water
exclusion, some acetophenone byproduct was formed (14%),
lowering the selectivity significantly. The purely Lewis acid
Zn–Co DMCs are in contrast highly selective and active
towards the hydroamination product; less stringent activation
conditions are needed to avoid acetophenone formation.
The stability of the Zn–Co-DMC-PTMEG catalyst was
investigated with a hot filtration test. A sample was taken
from the stirred reaction mixture after 2 h at 110 1C and
filtered without cooling the reaction mixture. The resulting
filtrate showed no further catalytic activity after 4 h at the
reaction temperature (Fig. S8, ESIw). Zn–Co-DMC-PTMEG
Fig. 2 SEM images of DMC material: Zn–Co-DMC-pure prepared
by the standard method (left) and Zn–Co-NDMC-Igepal prepared by
the reverse emulsion method (right).
study the chemical composition of the surface of the Zn–Co
DMC. The analysis shows a Zn : Co ratio of 1.85, slightly
more than the stoichiometric ratio of 1.5. An excess amount of
Zn is incorporated in the structure, which in the case of
polymerization reactions was found to be beneficial for the
catalytic activity of the DMCs.18 Infrared (IR) measurements
on a Zn–Co DMC showed the n(CN) peak at 2194 cmꢁ1
(Fig. S7, ESIw). Pyridine was adsorbed onto the standard
Zn–Co DMC at 50 1C and excess pyridine was removed at
175 1C, followed by IR measurement. The difference of the IR
spectra before and after adsorption of pyridine is shown in
Fig. 1 (right). The IR bands at 1609 and 1450 cmꢁ1 indicate
the presence of strong Lewis acid sites, while Brønsted acid
sites (IR bands at 1639 and 1545 cmꢁ1) were absent, proving
that the DMCs are purely Lewis acid catalysts.15,19 The zinc
sites are presumed to be the catalytically active sites.15
As a test reaction, the addition of 4-isopropylaniline to
phenylacetylene was selected. Batch experiments were performed
with DMC catalysts at 110 1C in toluene.w All reactions only
yielded Markovnikov-type addition products, as proven by
NMR. The enamine (4-isopropylphenyl)(1-phenylvinyl) amine
is formed, and isomerizes to the imine (4-isopropylphenyl)-
(1-phenylethylidene)amine.
Table 1 Hydroamination yields (%) of Markovnikov addition product
of 4-isopropylaniline with phenylacetylenea
Catalyst
Yield—3 hb Yield—24 hb
1
2
3
4
5
6
7
8
9
Zn–Fe(II)-DMC-PTMEGc
Zn–Fe(III)-DMC-PTMEGc
Cu–Fe(II)-DMC-PTMEGc
Cu–Co-DMC-PTMEGc
Zn–Co-DMC-PTMEGc
Zn–Co-DMC-PEGc
0%
0%
0%
0%
33%
0%
8%
21%
13%
0%
1%
0%
5%
0%
92%
18%
67%
79%
40%
8%
28%
0%
71%
99%k
96%
99%
Different metal combinations were tested (Table 1, entries 1–5).
Zn–Co DMC was found to have a very high activity in
comparison to the other metal combinations, for which the
hydroamination yields were negligible (entries 1–5). In epoxide
polymerizations this combination was also found to be the
most active, although Zn–FeIII proved to be an efficient
catalyst in that case as well;20 by contrast, in transesterification
catalysis with DMCs the Zn–FeII combination was the most
active one.13,14 DMCs with different co-CAs were prepared
(entries 5–7). The PTMEG based catalyst had the best activity
for the present reaction, with a 92% yield after 24 h. A very
high selectivity for the hydroamination product was observed
(>99%). When the co-CA (entry 8) was omitted in the
synthesis, or both the CA and co-CA (entry 9), the yields
were not as high, but surprisingly, the pure Zn–Co DMC,
without any complexing agents, still had a considerable activity.
In comparison, for the polymerization of epoxides, this pure
Prussian blue analogue material shows no activity.12,21–25
Zn–Co-DMC-P123c
Zn–Co-DMC-/d
Zn–Co-DMC-puree
10 Zn–(SO4)-Co-DMC-PTMEGc,f
11 Zn–(CF3SO3)2-Co-DMC-PTMEGc,g 7%
h
12 ZnCl2
0%
7%
48%
46%
57%
13 Zn-betai
14 Zn–Co-DMC-PTMEGc,j
15 Zn–Co-NDMC-Igepalc
16 Zn–Co-NDMC-Igepalc,j
a
Reaction conditions: 0.5 mmol of alkyne, 1.0 mmol of amine, 50 mg
b
c
of catalyst in toluene (1 ml) at 110 1C. Determined by GC. PTMEG
poly(tetramethylene ether) glycol, PEG polyethylene glycol, P123 Pluronics
co-block polymer, Igepals CA-520. No co-CA used. No CA nor
d
e
f
co-CA. ZnSO4.7H2O instead of ZnCl2. Zn(CF3SO3)2 instead of
g
h
i
ZnCl2. 5 mol% ZnCl2. 50% of the cation exchange capacity (CEC)
of H-beta zeolite exchanged with Zn(CH3CO2)2, activated overnight
j
under vacuum at 200 1C. 1 mmol of alkyne and 0.5 mmol of amine.
Isolated yield: 88%.
k
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4114–4116 4115

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(conditions of entry 5) was also found to be a recyclable catalyst.
After 3 runs, the high yield of hydroamination product was
intact: a 92% yield of the imine was found in the third run. Thus,
Zn–Co-DMC-PTMEG is a stable, efficient and recyclable hydro-
amination catalyst. The amine/alkyne ratio was altered in a
reaction with Zn–Co-DMC-PTMEG. The highest yield was
obtained with 0.5 mmol amine and 1 mmol alkyne. After 3 hours
48% of the hydroamination product was formed, and after
24 hours, a 99% yield based on amine was obtained (entry 14).
Nitrogen adsorption measurements were performed on
Zn–Co-DMC-PTMEG. A typical Type 1 adsorption isotherm
was recorded, characteristic of microporous materials
(Fig. S9, ESIw). The micropore radius of Prussian blue analogues
is small.17,24 We estimated the pore size by the Horvath–
Kawazoe model as o4.6 A. It can therefore be safely assumed
that most of the catalysis takes place at the outer surface of the
DMC material. In that case, one expects an activity increase as
the outer surface of the material is increased. Therefore,
nanosized DMC particles were prepared by a reverse emulsion
synthesis, using various neutral or ionic emulsifying agents.
Best results were obtained with Igepals CA-520. At a 2 : 1
amine : alkyne ratio, the initial rate with the NDMC was more
than 40% higher than with the standard DMC (entries 5 and 15,
after 3 h). When an excess alkyne was used, the hydroamination
yield amounted to 57% already after 3 h (entries 14 and 16).
The emulsifier also acts as a co-CA in this synthesis, since it is
incorporated into the structure in the same way as PTMEG;16
an oxygen atom of the emulsifier is coordinated to a Zn atom
in the DMC structure. A nitrogen adsorption isotherm of the
Zn–Co NDMC (Fig. S9, ESIw) was also recorded. A large
increase in the outer surface area can be seen between the
standard and reverse emulsion prepared DMC (21 m2 gꢁ1 and
170 m2 gꢁ1), confirming a smaller particle size. Thus the rate of
hydroamination can be improved by lowering the size of the
catalyst particles by the reverse emulsion method, indicating
that the outer surface area is indeed important for the catalytic
activity.
poly(tetramethylene ether)glycol (PTMEG, 1000 g molꢁ1), polyethylene
glycol (PEG, 2000 g molꢁ1) or Pluronics P123 (P123, 5750 g molꢁ1).
This solution was vigorously stirred at 80 1C. Next, a solution
of
1 mmol metal cyanide salt (K3[Co(CN)6], K3[Fe(CN)6] or
K4[Fe(CN)6]ꢀ3H2O) in deionized water (10 ml) was added dropwise.
The stirring was continued for 30 min. 25 ml of the complexing agent
tert-butanol (tBuOH) was added and the resulting mixture was stirred
for an additional 3 hours at 80 1C. The solids were centrifuged and
washed with a 1 : 1 mixture of tBuOH and deionized water. The
resulting catalyst was then dried at 60 1C overnight, followed by
drying at 80 1C under vacuum for 8 hours. Synthesis of nanosized
Zn–Co DMC (reverse emulsion method):16 10 g of Igepals CA-520,
7.7 ml of tBuOH and 109 ml of cyclohexane were mixed with a
mechanical stirrer (300 rpm) at room temperature. A solution of
10 mmol ZnCl2 in 2 ml of deionized water was added dropwise. Next,
a solution of 2 mmol K3[Co(CN)6] in 2 ml of water was added
dropwise as well. The resulting mixture was stirred for 1 hour at room
temperature. The formed catalyst particles were centrifuged and
washed with tBuOH. The particles were dried overnight at 60 1C
and afterwards dried under vacuum at 80 1C.
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Notes and references
z Synthesis of standard DMCs:12 10 mmol of the metal salt (ZnCl2,
ZnSO4ꢀ7H2O, Zn(CF3SO3)2 or CuCl2ꢀH2O), were dissolved in 100 ml
of deionized water, together with 1 mmol of the co-complexing agent,
c
4116 Chem. Commun., 2011, 47, 4114–4116
This journal is The Royal Society of Chemistry 2011