Zinc-zinc bonded decamethyldizincocene Zn2(η5- C5Me5)2 as catalyst for the inter- and intramolecular hydroamination reaction

Article abstract of DOI:10.1039/c1cc12461g

The Zn-Zn bonded compound [(η⁵-Cp*)₂Zn ₂] was investigated as catalyst for the inter- and intramolecular hydroamination reaction. High reaction rates under mild conditions were observed. This is the first application of a

Full text of DOI:10.1039/c1cc12461g

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COMMUNICATION
Zinc–zinc bonded decamethyldizincocene Zn2(g5-C5Me5)2 as catalyst for
the inter- and intramolecular hydroamination reactionw
Anja Lu
¨
hl,a Hari Pada Nayek,a Siegfried Blechert*b and Peter W. Roesky*a
Received 27th April 2011, Accepted 5th June 2011
DOI: 10.1039/c1cc12461g
The Zn–Zn bonded compound [(g5-Cp*)2Zn2] was investigated
as catalyst for the inter- and intramolecular hydroamination
reaction. High reaction rates under mild conditions were observed.
This is the first application of a Zn–Zn bonded compound as
catalyst.
knowledge, there have no catalytic applications of any of these
Zn–Zn bonded compounds been reported so far.
We have explored for some time now the catalytic potential
of various zinc alkyl species for the hydroamination reaction.15
Hydroamination is the formal addition of an organic amine
N–H bond to an unsaturated carbon–carbon bond in one step
to give nitrogen containing molecules.16 The catalytic version
of this reaction is of great interest for academic and industrial
research because nowadays most amines are made in multi-step
syntheses. Zinc complexes as catalysts for hydroamination
reaction possess several advantages in comparison to the well
established catalysts: They show a high tolerance towards
polar functional groups and some of them are relatively stable
towards air and moisture. Furthermore, zinc is an inexpensive
and non-toxic metal which makes its use in catalysis attractive.
Herein we report on the application of the Zn–Zn bonded
complex [(Z5-Cp*)2Zn2] as catalyst for the inter- and intra-
molecular hydroamination reactions with different functional
groups on the substrates (Table 1–2). Although some reactions
were already run at room temperature most of the reactions
were carried out at 80 1C in benzene with a catalyst loading of
2.5 mol%, 2.5 mol% of [PhNMe2H][B(C6F5)4] as a cocatalyst
and ferrocene as internal standard. It was shown earlier by us
that the addition of one equivalent of [PhNMe2H][B(C6F5)4]
has a beneficial effect on the reactivity of the zinc catalyst. We
anticipate the formation of a cationic zinc species which is formed
by the protonolysis of the Cp* moiety because Cp*H could be
detected in the NMR spectrum as byproduct. We also tested both
compounds separately in the reaction of phenylethine with anilines
(Table 2, entry 1). The cocatalyst alone did not catalyze
the reaction at all under the described conditions. By using
[(Z5-Cp*)2Zn2] without cocatalyst only polymers were obtained.
First, we compared the catalytic potential of [(Z5-Cp*)2Zn2]
with published systems in the intermolecular hydroamination
reaction. We used the addition of 2,4,6-trimethylaniline to phenyl-
ethyne as a standard reaction because this reaction has also been
studied with many other reported catalysts (Table 1).17 Although a
titanium catalyst is shown in Table 1 (entry 3), most reported
catalysts for this reaction are based on Group 11 metals.
[(Z5-Cp*)2Zn2] (Table 1, entry 1) is the only catalyst that catalyzes
the transformation at room temperature, whereas most of the
other systems operate in the range of 90–110 1C (Table 1, entries
3–6 and 8–11). With typical catalyst and cocatalyst loading of
2.5 mol% each the isolated yield was 99% for this catalytic
The landmark discovery of the Zn–Zn bonded decamethyl-
dizincocene ([(Z5-Cp*)2Zn2]; Cp* = C5Me5) by Carmona
et al.1 was the beginning of a broad chemistry dealing with
the synthesis and reactivity of low-valent metal-metal bonded
organozinc compounds.2 Besides investigations on the
nature of the Zn–Zn bond3 a number of low-valent dizinc
compounds with the general formula R2Zn2 (R = EtMe4C5,1b
[{(2,6-i-Pr2C6H3)N(Me)C}2CH],4 [2,6-(2,6-i-Pr2C6H3)C6H3],5
[(2,6-i-Pr2C6H3)N(Me)C]2,6 [(2,6-i-Pr2C6H3)NCH]2,6a [Me2Si-
{N(2,6-i-Pr2C6H3)}2],7 [(2,4,6-Me3C6H2)N(Me)C)2CH],8 1,2-bis[(2,6-
diisopropylphenyl)imino]acenaphthene),9 bis(phosphinimino)-
methanides,10 tris(3,5-dimethylpyrazolyl)hydridoborate,11 and
aminotroponiminates)12 have been synthesized. Recently some
groups have started to study the reactivity of [(Z5-Cp*)2Zn2]
with H2O, t-BuOH, and NCXyl resulting in the formation of
elemental zinc and the corresponding Zn(II) complexes through
disproportionation.1 Very recently, Schulz et al. reported on
the reaction of [(Z5-Cp*)2Zn2] with the strong Lewis base
4-(dimethylamino)pyridine (dmap) giving the first Lewis
acid–base adduct of dizincocene [(Z5-Cp*)Zn-Zn(dmap)2-
(Z5-Cp*)].13 Further reaction with [H(OEt2)2][Al{OC(CF3)3}4]
resulted in the base-stabilized [Zn2]2+ cation [Zn2(dmap)6]-
[Al{OC(CF3)3}4]2.13
A similar protonation reaction of
[(Z5-Cp*)2Zn2] with various nitrogen based ligands (L)10,12
resulted in the Zn–Zn-bonded complexes [(L)2Zn2] upon
elimination of Cp*H.8 Carmona et al. revealed the synthesis
of [(Z5-Cp*)(OR)(L)xZn2] (R = 2,6-(2,4,6-Me3C6H2)C6H3
and Cp*; L = 4-pyrrolidinopyridine).14 To the best of our
a Institut fur Anorganische Chemie, Karlsruher Institut fur
¨
¨
Technologie (KIT), Engesserstr. 15, 76131 Karlsruhe, Germany.
E-mail: roesky@kit.edu; Fax: (+49) 721-608-44854;
Tel: (+49) 721-608-46117
b Technische Universitat Berlin, Institut fur Chemie, Straße des 17.
¨
Juni 135, 10623 Berlin, Germany.
¨
E-mail: blechert@chem.tu-berlin.de; Fax: +49 (0)30-31 42 36 19;
Tel: +49 (0)30-31 42 23 55
w Electronic supplementary information (ESI) available: Table SI1,
Kinetic Studies and NMR spectra. See DOI: 10.1039/c1cc12461g
c
8280 Chem. Commun., 2011, 47, 8280–8282
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reaction (Table 2, entry 1E). Although the reaction temperature
for [(Z5-Cp*)2Zn2] was significant lower, the time needed for a
full conversion is comparable to the other catalysts. Therefore,
[(Z5-Cp*)2Zn2] is the most active system in the series shown in
Table 1 for the standard reaction. Surprisingly, even some
sophisticated gold and silver catalysts (Table 1, entries 2 and
8–11) could not compete. The reaction rate of [(Z5-Cp*)2Zn2]
could even be further increased by applying an amine to alkyne
ratio of 1 : 2.
Table 1 Intermolecular hydroamination of phenylethyne with 2,4,6-
trimethylaniline
Ratio
Mol% amine/
Entry/
Ref. Cat.
T
Yield
[1C] t [h] [%]
cat.
alkyne
Quant.
[(g5-Cp*)2Zn2]
23 11.25
23 5.5
2.5
2.5
1 : 1
1 : 2
Recently the hydroamanination of phenylethyne with ani-
line catalyzed by Zn(OTf)2 followed by a reduction was
reported.18 Although our yield is a bit lower (89% vs. 98%
for Zn(OTf)2) [(Z5-Cp*)2Zn2] catalyzes this reaction in shorter
time (15h vs. 24h) at lower reaction temperature (80 1C vs.
120 1C) with half of the catalyst loading (Fig. S1w).
(NMR)
Quant.
(NMR)
1
217a
40 16
81 (NMR)
5
1 : 1
As shown in Table 2 we then investigated various inter-
molecular hydroamination reactions by using substituted
primary and secondary anilines with different functional groups
and different arylethynes. In this screening we used an amine to
alkyne ratio of 1 : 1, since we aimed to have full conversion
without any by-product or starting material. Quantitative
Markovnikov regio-selectivity was observed for all reactions
shown in Table 2. As functional groups halides, methoxy, and
even OH were tolerated. The reactions were run in benzene at
80 1C with the exception of 2,4,6-trimethylaniline (Table 2,
entry E) and some reactions of 3-methoxyaniline (Table 2,
entry 3D), which already reacted in acceptable rates at room
temperature. With a few exceptions all reactions run with a
conversion of 90%–100%. In general the reactions of the
secondary aniline diphenylamine did not proceed whereas the
reaction of N-methylaniline gave lower yields with 4-ethynyl-
dimethylaniline, 3-ethynylphenol, and 1-ethynyl-4-methoxy-
benzene (Table 2, entries 3B–5B). Kinetic investigations have
shown a substrate dependence of the rate. For the standard
reaction (addition of 2,4,6-trimethylaniline to phenylethyne) a
zero order kinetic was observed for the first 90% of conversion
(Fig. S2w). The steric demand of the aniline has no influence on the
reaction rate. In contrast, the addition of 3-methoxyaniline to
4-ethynyl-dimethylaniline (Table 2, entry 3D) is more complicated
and no clear rate law could be assigned (Fig. S3w). Nevertheless,
both kinetic measurements show neither any induction period nor
an unstable conversion. Obviously a stable catalytic species is
formed in situ but in the present stage we do not know the exact
nature and the oxidation state of the catalytic active species.
Finally, we investigated the catalytic activity of [(Z5-Cp*)2Zn2]
in the intramolecular hydroamination reaction. Surprisingly the
incorporation of functional groups (Table SI1w, entries 1–2) did
not have a significant influence on the conversion. All intra-
molecular cyclizations of secondary amino olefins were completed
in 10–40 min in quantitative yields. In contrast, the cyclization of a
primary amino alkyne went much slower (Table SI1w, entry 4).
Beside the imine, the corresponding enamine was obtained as a
byproduct.
317b
105 24
80 (isolated) 5
1 : 1
CuSTAa
110 4
8
110 6
75 (GC)
95 (GC)
37
2 : 1
2 : 1
2 : 1
417c
517d
617e
CuA1SBA-15
Cu-K-10b
10 wt
(%)
110 20
20
95 (GC)
2 : 1
91 (isolated)
(Ph3P)AuCH3 +
H3PW12O40
717f
70
2
93 (NMR) 0.2 : 1 1.1 : 1
R = CH2CotBu,
R0 = CH2Ph (I)
R = CH2CONHtBu,
R0 = CH2Ph (II)
90
90
7
0
817f
12
2
1 : 1.5
R = CH2CotBu,
R0 = CH2CotBu (III)
R = C6H10OH,
R0 = CH2Ph (IV)
90
90
16
1
R = CH2COtBu,
R0 = CH2Ph (V)
R = CH2CONHtBu,
R0 = CH2Ph (VI)
917f
90
90
84
75
12
2
1 : 1.5
R = CH2COtBu,
R0 = CH2COtBu (VII)
R = C6H10OH,
R0 = CH2Ph (VIII)
90
90
80
58
1017f
90 12
16
46
2
2
1 : 1.5
1 : 1.5
In conclusion, we have shown the first application of a Zn–Zn
bonded compound as a catalyst. [(Z5-Cp*)2Zn2] can be applied
as a homogenous catalyst for the intra- and intermolecular
hydroamination reaction in the presence of equimolar amounts
of [PhNMe2H][B(C6F5)4]. Many functional groups are tolerated.
The addition of 2,4,6-trimethylaniline to phenylethyne catalyzed
1117f
90 12
a
b
Copper salt of silicotungstic acid; montmorillonite K-10.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 8280–8282 8281

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Table 2 Intermolecular hydroamination of anilines and alkynes catalyzed by [(Z5-Cp*)2Zn2]a
Entry Substrates
5 h, 87%.
(22% bypr)c
18 h, 89% (6% bypr)c
1
2
3
9 h, 95%
9 h, 96%
5 h, Quant.
3.5 h, 82%
15 h, 99%
1 h, 20%
11.25 h, Quant.d 99%b,d
36 h, 93%. (7% bypr)c
4 h, 88%;
10 h, Quant.
18 h, 75% 48 h, 93%
19.66 h, 93%d 21.66 h, 95%d
d
5 h, Quant.
34 h, 99%
25 h, 93%d 48 h, 99%d 19.75 h, 93% 21.75 h, 96%d
e
—
d
4
6.5 h, 82% 30 h, 97%
9 h, 41% 5 h, 38%
9 h, 97%
41.75 h, 69% 43.75 h, 70%d
5 h, 92%
(8% bypr)c
e
—
5
8 h, Quant. (4% bypr)c 3 h, 71%
a
Reagents and conditions: substrate (0.5 mmol), catalyst (2.5 mol%), [PhNMe2H][B(C6F5)4] (2.5 mol%), C6D6, 80 1C, conversion determined by
1H NMR; isolated yield; bypr = byproduct: the corresponding enamine; reaction at room temperature; no conversion.
b
c
d
e
by [(Z5-Cp*)2Zn2] proceeds already at room temperature. Thus,
for this reaction [(Z5-Cp*)2Zn2] is more active than any other
catalyst.
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This work was supported by the Cluster of Excellence
‘‘Unifying Concepts in Catalysis’’ coordinated by the TU Berlin,
by the Deutsche Forschungsgemeinschaft, and the Fonds der
Chemischen Industrie.
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This journal is The Royal Society of Chemistry 2011