Comparison of decamethyldizincocene [(η5-Cp*) 2Zn2] versus decamethylzincocene [Cp*2Zn] and diethylzinc Et2Zn as precatalysts for the intermolecular hydroamination reaction

Article abstract of DOI:10.1021/om300649q

A comparison of the Zn-Zn bonded species [(η⁵-Cp*) ₂Zn₂] versus the related organometallic zinc compound [Cp*₂Zn] and ZnEt₂ for the intermolecular hydroamination reaction in the presence of equimolar

Full text of DOI:10.1021/om300649q

Article
pubs.acs.org/Organometallics
Comparison of Decamethyldizincocene [(η5‑Cp*)2Zn2] versus
Decamethylzincocene [Cp*2Zn] and Diethylzinc Et2Zn As Precatalysts
for the Intermolecular Hydroamination Reaction
Anja Luhl,† Larissa Hartenstein,† Siegfried Blechert,*,‡ and Peter W. Roesky*,†
̈
†Institute of Inorganic Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstraße 15, 76131 Karlsruhe, Germany
‡Institut fur Chemie, Technische Universitat Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany
̈
̈
S
* Supporting Information
ABSTRACT: A comparison of the Zn−Zn bonded species [(η5-Cp*)2Zn2]
versus the related organometallic zinc compound [Cp*2Zn] and ZnEt2 for
the intermolecular hydroamination reaction in the presence of equimolar
amounts of [PhNMe2H][B(C6F5)4] is reported. All compounds show high
reaction rates under mild conditions and a good functional group tolerance
for the addition of aniline derivatives to primary alkynes. Within this series
the metallocene [Cp*2Zn] is the most active one, whereas the zinc−zinc
bonded species [(η5-Cp*)2Zn2] shows the best selectivity. Most remarkable
is the unexpected excellent catalytic performance of the zinc−zinc bonded
species [(η5-Cp*)2Zn2].
reaction. Many functional groups were tolerated, and high
INTRODUCTION
■
reaction rates under mild conditions were observed.59
The hydroamination reaction, which is the direct addition of
amine N−H bonds to C−C multiple bonds, has been intensely
studied in the last decades.1−22 Catalytic hydroamination was
investigated for a wide range of complexes of various metals,
such as main group metals,2,5,15,23−37 early transition
metals,4,7,9,22 late transition metals,4,7,9,12,14,15,38−43and f-
elements.4,7−9,11−13,15−17 Early transition metal and late
transition metal catalysts show different forms of substrate
activation. Generally complexes of the early transition metals,
the lanthanides, and electron-poor main group metals activate
the amine function.20 These systems are highly efficient
catalysts for hydroamination, but synthetic application is
limited due to their high sensitivity toward moisture and air
and their low tolerance to polar functional groups. In contrast,
compounds of the late transition metals activate the C−C
multiple bond. These kinds of catalysts show normally high
polar functional group tolerance, but are usually based on
relatively expensive metals. Moreover they show low reaction
rates and moderate selectivity for the hydroamination of
nonactivated substrates compared to early transition metals. As
an alternative we and others introduced zinc compounds as a
precatalyst for the hydroamination of alkenes and al-
kynes.21,35,44−58 Zinc catalysts show, besides good catalytic
activity and selectivity, high tolerance toward polar functional
groups. Some of the catalysts show a high stability toward air
and moisture, but it seems that a high stability is not beneficial
for a high activity. Most of the published zinc catalysts were
investigated for the intramolecular hydroamination reaction.
We recently communicated on the Zn−Zn bonded compound
decamethyldizincocene ([(η5-Cp*)2Zn2]; Cp* = C5Me5) as
catalyst for the inter- and intramolecular hydroamination
The discovery of [(η5-Cp*)2Zn2] by Carmona et al.60,61 was
the beginning of a broad chemistry dealing with the synthesis
and reactivity of low-valent metal−metal bonded organozinc
compounds.62−73 It was shown by Schulz et al. that [(η5-
Cp*)2Zn2] reacts with the strong Lewis base 4-
(dimethylamino)pyridine (dmap), giving the first Lewis acid−
base adduct of dizincocene [(η5-Cp*)Zn-Zn(dmap)2(η1-
Cp*)].74 Further treatment with [H(OEt2)2][Al{OC(CF3)3}4]
resulted via a protonation of Cp* in the base-stabilized [Zn2]2+
cation [Zn2(dmap)6][Al{OC(CF3)3}4]2.75 A similar protona-
tion reaction of [(η5-Cp*)2Zn2] with various nitrogen-based
ligands (L)71,73 resulted in the Zn−Zn bonded complexes
[(L)2Zn2] upon elimination of Cp*H.69 Carmona et al.
revealed the synthesis of [(η5-Cp*)(OR)(L)xZn2] (R = 2,6-
(2,4,6-Me3C6H2)2C6H3 ; x = 2 and Cp*; x = 1; L = 4-
pyrrolidinopyridine).76,77
Recently, the Zn−Zn bonded compound [(η5-Cp*)2Zn2]
was investigated by us as a catalyst for the inter- and
intramolecular hydroamination reaction.59,78 High reaction
rates under mild conditions for the addition of aniline and
some derivatives to arylethynes were observed. This was the
first application of a Zn−Zn bonded compound as catalyst.
Now we are interested in comparing the catalytic activity of
[(η5-Cp*)2Zn2] (1) with decamethylzincocene [Cp*2Zn] (2)
and diethylzinc ZnEt2 (3) (Scheme 1).
Received: July 11, 2012
Published: October 4, 2012
© 2012 American Chemical Society
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Scheme 1. [(η5-Cp*)2Zn2] (1), [Cp*2Zn] (2), and ZnEt2 (3)
128.71; 131.32; 131.72; 137.80; 146.59; 163.55. See also Supporting
Information.
4-[1-((4-Chlorophenyl)imino)ethyl]-N,N-dimethylaniline. 1H
NMR (C6D6, 300 MHz): δ (ppm) 1.89 (s, 3 H, H-3); 2.48 (s, 6 H,
3
H-6); 6.50 (m, 4 H, H-5, H-2); 7.09 (d, JH−H = 9.0 Hz, 2 H, H-1);
7.99 (d, 3JH−H = 9.0 Hz, 2 H, H-4). 13C{1H} NMR (C6D6, 100 MHz):
δ (ppm) 16.20; 39.34; 111.12; 121.39; 128.84; 129.24; 133.17; 151.41;
151.87; 164.16. See also Supporting Information.
N-[1-(4-(Dimethylamino)phenyl)ethylidene]-2,4,6-trimethy-
1
laniline. H NMR (C6D6, 300 MHz): δ (ppm) 1.87 (s, 3 H, H-4);
2.07 (s, 6 H, H-3); 2.24 (s, 3 H, H-1); 2.51 (s, 6 H, H-7); 6.54 (d,
3JH−H = 9.0 Hz, 2 H, H-6); 6.87 (s, 2 H, H-2); 8.08 (d, 3JH−H = 9.0 Hz,
2 H, H-5). 13C{1H} NMR (C6D6, 100 MHz): δ (ppm) 16.33; 17.97;
20.63; 39.46; 111.28; 125.65; 128.60; 130.77; 133.16; 147.74; 151.81;
163.42. See also Supporting Information.
EXPERIMENTAL SECTION78
■
General Considerations. NMR spectra were recorded on a
Bruker Avance 400 MHz or Avance II NMR 300 MHz spectrometer.
Chemical shifts are referenced to internal solvent resonances and are
reported relative to tetramethylsilane. Deuterated solvents were
obtained from Chemotrade or Euriso-Top GmbH (99 atom % D).
Et2Zn (3) was purchased from Aldrich. [(η5-Cp*)2Zn2] (1) and
[Cp*2Zn] (2) were prepared according to literature procedures.61
Hydroamination Reactions. The substrates were purchased from
Aldrich, AlfaAesar, and Acros.
3-[1-(Phenylimino)ethyl]phenol. 1H NMR (C6D6, 300 MHz): δ
(ppm) 1.82 (s, 3 H, H-5); 6.45 (d, 3JH−H = 6.0 Hz, 2 H, H-6); 6.77 (d,
3
3JH−H = 6.0 Hz, 1 H, H-1); 7.07 (t, JH−H = 6.0 Hz, 2 H, H-2, H-8,);
7.15−7.25 (m, 2 H, H-7); 7.32 (d, 3JH−H = 6.0 Hz, 1 H, H-3,); 7.01 (s
br, 1 H, OH); 7.56 (s, 1 H, H-4). 13C{1H} NMR (C6D6, 100 MHz): δ
(ppm) 17.94; 116.44; 120.05; 123.42; 124.17; 129.21; 129.63; 140.31;
149.94; 156.23; 169.49. See also Supporting Information.
The 1H NMR spectra of N-(methylbenzylidene)aniline,79
methylphenyl(1-phenylvinyl)amine,80 N-(1-phenylethylidene)-4-
chloroaniline,81 N-(1-phenylethylidene)-2,4,6-trimethylaniline,82 N-
[1-(4-bromophenyl)ethylidene)]benzamine,83 N,N-dimethyl-4-[1-
(phenylimino)]ethylaniline,84 N-[1-(1-cyclohexene-1-yl)ethylidene]-
benzamine,85 N-(1-methylheptylidene)benzamine,86 4-chlor-N-(1-
methylheptylidene)benzamine,87 and N-(1-phenylethylidene)-
benzmethanamine88 conform to the literature.
NMR Scale. The catalyst was weighed under argon in an NMR tube.
C6D6 (∼0.5 mL) was condensed into the NMR tube, and the mixture
was frozen at −196 °C. The reactant was injected onto the solid
mixture, and the whole sample was melted and mixed just before
insertion into the core of the NMR machine (t0). The ratio between
the reactant and the product was calculated by comparison of the
integrals of the corresponding signals. Ferrocene was used as an
internal standard for kinetic measurements.
Preparative Scale. A 0.23 mL (215 mg, 2.11 mmol) amount of
phenylethyne, 0.3 mL (285 mg, 2.11 mmol) of mesidine, 21 mg (0.53
mmol) of 1, and 42 mg (0.53 mmol) of [PhNMe2H][B(C6F5)4] were
dissolved in 5 mL of toluene. The subsequent mixture was stirred at
room temperature. The reaction progress was monitored by TLC.
When the reaction was judged to be complete, the mixture was dried
in vacuo and purified by column chromatography on alumina as the
stationary phase. Hexane/ethyl acetate (4:1) was used as eluent.
Finally the solution was concentrated in vacuo to give N-(1-
phenylethylidene)-2,4,6-trimethylaniline as an oil. Yield: 496 mg
(99%).
3-[1-((4-Chlorophenyl)imino)ethyl]phenol. 1H NMR (C6D6,
300 MHz): δ (ppm) 1.69 (s, 3 H, H-5); 6.07 (d, 3JH−H = 9.0 Hz, 1 H,
H-1); 6.46 (d, 3JH−H = 9.0 Hz, 2 H, H-6); 6.93 (d, 3JH−H = 9.0 Hz, 1 H,
3
H-3); 7.01 (t, JH−H = 9.0 Hz, 1 H, H-2); 7.02 (s br, 1 H, OH); 7.07
3
(d, JH−H = 9.0 Hz, 1 H, H-7); 7.45 (s, 1 H, H-4). 13C{1H} NMR
(C6D6, 100 MHz): δ (ppm) 17.72; 114.36; 117.13; 119.53; 121.41;
129.04; 129.23; 129.65; 129.73; 148.78; 156.58; 169.19. See also
Supporting Information.
3-(1-(Mesitylimino)ethyl)phenol. 1H NMR (C6D6, 300 MHz): δ
(ppm) 1.65 (s, 3 H, H-5); 1.95 (s, 6 H, H-6); 2.16 (s, 3 H, H-8); 6.78
(s, 2 H, H-7); 6.20−6.90 (m, 2 H, H-1, OH); 7.00 (t, 3JH−H = 9.0 Hz,
3
1 H, H-2); 7.33 (d, JH−H = 6.0 Hz, 1 H, H-3); 7.44 (s, 1 H, H-4).
13C{1H} NMR (C6D6, 100 MHz): δ (ppm) 17.63; 17.79; 20.50;
114.50; 118.48; 119.04; 126.30; 129.00; 129.46; 129.72; 132.85;
145.10; 156.79; 169.62. See also Supporting Information.
4-Chloro-N-[1-(cyclohex-1-en-1-yl)ethylidene]aniline. 1H
NMR (C6D6, 300 MHz): δ (ppm) 1.40−1.61 (m, 4 H, H-3, H-4);
1.61 (s, 3 H, H-6); 1.92−2.09 (m, 2 H, H-5); 2.42−2.55 (m, 2 H, H-
2); 6.14−6.23 (m, 1 H, H-1); 6.43 (d, 3JH−H = 9.0 Hz, 2 H, H-7); 7.10
3
(d, JH−H = 9.0 Hz, 2 H, H-8). 13C{1H} NMR (C6D6, 100 MHz): δ
(ppm) 15.08; 21.27; 22.03; 24.60; 26.04; 120.66; 128.76; 128.85;
133.57; 139.42; 150.97; 166.06. See also Supporting Information.
N-(1-(Cyclohex-1-en-1-yl)ethylidene)-2,4,6-trimethylaniline.
1H NMR (C6D6, 300 MHz): δ (ppm) 1.32−1.53 (m, 4 H, H-3, H-4);
1.57 (s, 3 H, H-6); 1.94 (s, 6 H, H-7); 1.97−2.13 (m, 2 H, H-5); 2.20
(s, 3 H, H-9); 2.61−2.70 (m, 2 H, H-2); 6.12−6.26 (m, 1 H, H-1);
6.80 (s, 2 H, H-8). 13C{1H} NMR (C6D6, 100 MHz): δ (ppm) 15.15;
17.78; 21.26; 22.03; 22.10; 24.79; 25.96; 128.53; 128.87; 130.93;
132.24; 135.68; 147.27; 165.47.
The following NMR spectra were recorded directly from NMR-
scale reactions without further purification
N-[1-(4-Bromophenyl)vinyl]-N-methylaniline. 1H NMR
(C6D6, 300 MHz): δ (ppm) 2.86 (s, 3 H, H-4); 4.60 (s, 1 H, H-5);
4.75 (s, 1 H, H-6); 6.69 (t, 3JH−H = 6.0 Hz, 1 H, H-1); 6.75 (d, 3JH−H
=
RESULTS AND DISCUSSION
■
9.0 Hz, 2 H, H-3); 6.95 (m, 2 H, Har); 7.05−7.15 (m, 4 H, Har).
13C{1H} NMR (C6D6, 100 MHz): δ (ppm) 40.79; 100.05; 121.01;
121.20; 127.39; 128.73; 128.94; 131.27; 133.39; 148.65; 152.53. See
also Supporting Information.
In this contribution, we present a comparison of the reaction
scope and substrate selectivity of [(η5-Cp*)2Zn2] (1) with
decamethylzincocene [Cp*2Zn] (2) and diethylzinc ZnEt2 (3)
(Scheme 1).78 Compound 3 was reported by us and others as
being active as a catalyst for the intramolecular hydroamination
reaction, while compound 2 has not been used as a catalyst so
far.48,54 Generally the reactions we studied were run in benzene
with a catalyst loading of 2.5 mol %, with 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.21,44,52−58 We
anticipate the formation of a cationic zinc species that is formed
by the protonolysis of the Cp* moiety, because Cp*H could be
N-[1-(4-Bromophenyl)ethylidene]-4-chloroaniline. 1H NMR
3
(C6D6, 300 MHz): δ (ppm) 1.61 (s, 3 H, H-3); 6.39 (d, JH−H = 9.0
Hz, 2 H, H-2); 7.09 (d, 3JH−H = 9.0 Hz, 2 H, H-1); 7.28 (d, 3JH−H = 9.0
3
Hz, 2 H, H-5); 7.52 (d, JH−H = 9.0 Hz, 2 H, H-4). 13C{1H} NMR
(C6D6, 100 MHz): δ (ppm) 16.30; 120.72; 125.26; 128.83; 129.00;
131.34; 131.45; 137.79; 149.92; 164.14. See also Supporting
Information.
N-[1-(4-Bromophenyl)ethylidene]-2,4,6-trimethylaniline. 1H
NMR (C6D6, 300 MHz): δ (ppm) 1.63 (s, 3 H, H-4); 1.94 (s, 6 H, H-
3
3); 2.22 (s, 3 H, H-1); 6.83 (s, 2 H, H-2); 7.29 (d, JH−H = 9.0 Hz, 2
3
H, H-6); 7.62 (d, JH−H = 9.0 Hz, 2 H, H-5). 13C{1H} NMR (C6D6,
100 MHz): δ (ppm) 16.38; 17.72; 20.58; 124.87; 125.07; 128.70;
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Table 1. Intermolecular Hydroamination of Phenylethyne with 2,4,6-Trimethylaniline59
detected in the NMR spectrum as a byproduct. As mentioned
above, it was shown earlier that [(η5-Cp*)2Zn2] can be
protonated upon elimination of Cp*H with preservation of the
Zn−Zn bond.69,75,76 For [Cp*2Zn] Braun et al. reported on a
similar protonation reaction, which gave the base-stabilized
[(Cp*)Zn(L)]+ cation (L = Me2N(CH2)2PiPr2, Cy2P-
(CH2)2PCy2, Cy2PCH2PCy2).89
In order to prove that a combination of catalyst and
cocatalyst is needed, both zinc catalysts 1 and 2 were separately
used without cocatalyst in the reaction of phenylethine with
anilines. In these reactions only polymers were obtained by
using 1, and no reaction was observed by using 2 without
cocatalyst. Similar results were reported earlier by us using
compound 3 without cocatalyst. The cocatalyst alone or
another proton source such as benzoic acid did not catalyze the
reaction at all under the described conditions. Thus, a proton-
catalyzed conversion can be excluded.54
As test reaction we used the addition of 2,4,6-trimethylaniline
to phenylethyne because this reaction has also been studied
with many other reported catalysts (Table 1).59,90−96 Most
reported catalysts for this reaction shown in Table 1 are based
on group 11 metals, although there are some exceptions such as
a titanium catalyst (Table 1, entry 5). All three zinc catalysts
(1−3) (Table 1, entries 1−3) catalyze the transformation at
room temperature, whereas most of the other systems operate
in the range 90−110 °C (Table 1, entries 4−9 and 11−13).
With typical catalyst and cocatalyst loading of 2.5 mol % each,
the isolated yield was 99% for the test reaction by using
compounds 1−3. Since we could show that the reaction rate of
1 could even be further increased by applying an amine to
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a
Table 2. Intermolecular Hydroamination of Anilines and Alkynes Catalyzed by [Cp*2Zn]
a
Reagents and conditions: catalyst (2.5 mol %), [PhNMe2H][B(C6F5)4] (2.5 mol %), C6D6, 60 °C, conversion determined by 1H NMR, ferrocene
b
c
as internal standard. Amine (0.5 mmol), alkyne (0.75 mmol). Amine (0.5 mmol), alkyne (1 mmol). bypr = byproduct: the corresponding enamine.
d
e
f
g
h
Reaction at 80 °C. Reaction at room temperature. Reaction at 120 °C. No conversion. Unknown byproduct.
alkyne ratio of 1:2, we adapted these conditions also by using
compounds 2 and 3 as catalysts. Among the series of the three
zinc compounds 1−3, compound 2 showed the most rapid
conversion (Table 1, entries 1−3). Compound 3 is slightly
more active than compound 1, but, as seen from the kinetics
(see below), both catalysts show almost the same rate for the
first 90% of conversion. By performing a full screening (see
below) 1 and 3 roughly showed the same activity. Although the
reaction temperature for the zinc compounds 1−3 was
significant lower, the time needed for a full conversion was
comparable or even shorter than the other catalysts shown in
Table 1. Therefore, in the whole series shown in Table 1
compound 2 is the most active system for the test reaction.
Surprisingly, even certain sophisticated gold and silver catalysts
(Table 1, entries 4 and 10−13) could not compete. The good
performance of compounds 1−3 is not limited to 2,4,6-
trimethylaniline to phenylethyne but can also be extended to
the addition other anilines to arylethynes. The addition of
aniline to phenylethyne, which is also shown in Tables 2−4
(entry 1A, each), was evaluated with 32 other published
catalysts. Although very different reaction conditions were
reported, it clearly can be seen from Table S1 that the
performance of compounds 1−3 is excellent for this reaction.
We then investigated catalytic activities of compounds 1−3
in various intermolecular hydroamination reactions by using
substituted primary and secondary amines (mostly anilines)
with different functional groups and different arylethynes and
aliphatic alkynes (Tables 2−4). In this screening we applied an
amine to alkyne ratio of 1:1.5 because for all substrates other
than phenylacetylene the rates were comparable to that of a 1:2
ratio. In general, for all catalysts quantitative Markovnikov
regioselectivity was observed for all reactions (Tables 2−4). For
comparison, in Table 4 the recently communicated results of
our screening with compound 1 as catalyst are given, although
slightly different conditions were used in the earlier studies.59,97
To these data, which were covering only aromatic substrates,
we now add the results of the intermolecular hydroamination
reactions by using different aliphatic alkynes (see below, Table
4).
First we discuss the hydroamination reactions by using
compound 2 as catalyst. As functional groups on the substrates,
halides and even OH were tolerated in the catalysis by using
compound 2 as catalyst. The reactions were run in benzene at
60 °C with the exception of 2,4,6-trimethylaniline (Table 2,
entry D), which already reacted in acceptable rates at room
temperature and 1-octyne (Table 2, entry 6) which mostly was
converted at higher temperatures. Besides anilines and
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a
Table 3. Intermolecular Hydroamination of Anilines and Alkynes Catalyzed by Et2Zn
a
Reagents and conditions: catalyst (2.5 mol %), [PhNMe2H][B(C6F5)4] (2.5 mol %), C6D6, 60 °C, conversion determined by 1H NMR, ferrocene
b
c
as internal standard. Amine (0.5 mmol), alkyne (0.75 mmol). Amine (0.5 mmol), alkyne (1 mmol). bypr = byproduct: the corresponding enamine.
d
e
f
g
Unknown byproduct. Reaction at 80 °C. Reaction at room temperature. No conversion at 80 °C.
a59
,
Table 4. Intermolecular Hydroamination of Anilines and Alkynes Catalyzed by [(η5-Cp*)2Zn2]
a
Reagents and conditions: catalyst (2.5 mol %), [PhNMe2H][B(C6F5)4] (2.5 mol %), C6D6, 60 °C, conversion determined by 1H NMR, ferrocene
b
c
d
as internal standard. Amine (0.5 mmol), alkyne (0.5 mmol). Amine (0.5 mmol), alkyne (0.75 mmol). Isolated yield. bypr = byproduct: the
corresponding enamine. Reaction at room temperature. No conversion.
e
f
arylethynes also benzylamine (Table 2, entry E), 1-
ethynylcyclohexene (Table 2, entry 5), and 1-octyne (Table
2, entry 6) were used as substrates. With the exception of
benzylamine (Table 2, entry E) all reactions of the primary
amines achieved a conversion of 90−100% by using compound
2 as catalyst. Reactions of the secondary amine N-methylaniline
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are less favored. Thus reaction of N-methylaniline with 4-
ethynyldimethylaniline, 3-ethynylphenol, and ethynylcyclohex-
ene showed poor results (Table 2, entries 3B−5B). In contrast
to the amines, aliphatic substituents on the alkyne function did
not have any negative influence on the reactions. The primary
alkynes 1-ethynylcyclohexene (Table 2, entry 5) and 1-octyne
(Table 2, entry 6) were transformed to the corresponding
imines by using compound 2 as catalyst. Thus, the nature of the
amine and its pKa value seem to have a major influence on the
catalysis, whereas the substituents on the terminal alkynes are
less important. The rates and the yields were substrate
dependent. We also tried to introduce an OH group as
functional group onto the aliphatic chain of the alkyne by using
4-pentyne-1-ol as substrate, but only product mixtures could be
obtained.
(98% yield, in 24 h at 120 °C; Figure S1, Table S1).45 It can be
clearly seen from Tables 2−4 (entry 1A, each) that the
organometallic catalysts 1−3 are superior. Compound 2
catalyzes the same reaction in 9 h in quantitative conversion
at only 60 °C reaction temperature.
Kinetic investigations have shown a substrate dependence of
the rate. For the addition of 2,4,6-trimethylaniline to phenyl-
ethyne a zero-order kinetics in substrate was observed for the
initial 80% of conversion (Figure 1). This kinetic data supports
As second catalyst we investigated the commercially available
ZnEt2 (3). The ease of accessibility of compound 3 is an
advantage compared to the metallocene compounds 1 and 2.
For comparing the catalytic performance, a similar range of
substrates and reaction conditions to that for compound 2 were
chosen (Table 3). In general compound 3 is slower than
compound 2 not only in the test reaction (Table 1, entries 2
and 3) but also for all reactions shown in Tables 2 and 3.
Moreover, the amount of the enamine, which is the byproduct
of the mainly formed Schiff base, is higher for most of the
investigated reactions in which the byproduct is formed. As
observed for compound 2, also compound 3 catalyzes most of
the reactions in high conversions. The exceptions are the
secondary amine N-methylaniline (Table 3, entry B) and 3-
ethynylphenol (Table 3, entry 4). The reasons for these
hampered conversions might be the steric hindrance of N-
methylaniline and the phenolic OH group of 3-ethynylphenol,
which might react with the catalysts to form a zinc phenoxide.
In contrast to compound 2, which obviously tolerates the OH
group of 3-ethynylphenol (Table 2, entry 4), compound 3 is
more sensitive. 1-Octyne was completely transformed by using
compound 3 as catalyst, but higher reaction temperatures (80
°C) were needed for this aliphatic alkyne for most of the
reactions (Table 3, entry 6). In the case of the aliphatic alkynes
1-octyne and 1-ethynylcyclohexene compounds 2 and 3 are less
efficient than other easily accessible catalysts. Thus, the
[Ti(NMe2)4]-catalyzed addition of aniline to 1-hexyne
proceeds already in 2 h, but 10 mol % of catalysts and a
reaction temperature of 75 °C were needed.98
We then compared the catalytic activity of compounds 2 and
3 with that of Zn−Zn bonded complex 1 (Table 4). Parts of the
results shown in Table 4 were communicated by us earlier.59
We now added new results for the aliphatic alkynes shown in
Table 4, entries 5 and 6. As observed for compounds 2 and 3,
the aliphatic alkynes were mostly converted in high yield. As
seen from Tables 1−4 compound 1 shows a slower catalytic
rate in most of the investigated reactions than those observed
for compounds 2 and 3. On the other hand, the relative amount
of byproduct (enamine) formed by using compound 1 as
catalyst is significantly lower than that by the other two
catalysts. Thus, Zn−Zn bonded complex 1 shows a better
selectivity. During our catalytic experiments we did not observe
the formation of any elemental zinc. This observation together
with the different selectivity of complex 1 in comparison to
complex 2 indicates that compound 1 does not simply
disproportionate to elemental Zn and 2 during the reaction.
Recently the hydroamination of phenylethyne with aniline
catalyzed by Zn(OTf)2 followed by a reduction was reported
Figure 1. Conversions vs time diagram of the catalytic hydroamination
of 2,4,6-trimethylaniline to phenylethyne in a 1:2 ratio using
compounds 1−3 as catalysts and [PhNMe2H][B(C6F5)4] as
cocatalyst; [aniline] = 1 mol·L−1. The rate constants are k = 0.065
× 10−3·mol·L−1·s−1 (1), 0.172 × 10−3·mol·L−1·s−1 (2), and 0.086 ×
10−3·mol·L−1·s−1 (3) for the first 80% of the reaction (concentration 1
mol·L−1).
the results given in Table 1. Compound 2 catalyzes the
hydroamination most rapidly, whereas compounds 1 and 3
show similar rates in this reaction. The kinetic data of other
transformations such as the addition of 2,4,6-trimethylaniline to
1-octyne (Figure S2; Tables 2 and 3, entry 6D each) and 1-
bromo-4-ethynylbenzene (Figure S3; Tables 2 and 3, entry 2D
each) also show that compound 2 is the fastest catalyst, but a
clear zero-order kinetics cannot be extracted from the first
reaction. Nevertheless, both kinetic measurements show neither
any induction period nor an unstable conversion. Obviously a
stable catalytic species is formed in situ for all three catalysts,
but at the present stage we do not know the exact nature and
the oxidation state of the catalytically active species. Based on
the results of other research groups, we propose a cationic
complex as the active species.75
CONCLUSIONS
■
In conclusion, we have shown that the three organometallic
zinc compounds 1−3 are remarkably good catalysts for the
intermolecular hydroamination reaction in the presence of
equimolar amounts of [PhNMe2H][B(C6F5)4]. Many func-
tional groups are tolerated.
The addition of 2,4,6-trimethylaniline to phenylethyne
catalyzed by all three compounds proceeds at room temper-
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Article
(21) Jenter, J.; Luhl, A.; Roesky, P. W.; Blechert, S. J. Organomet.
Chem. 2011, 696, 406−418.
ature. Also the addition of other aniline compounds and their
derivatives to arylethynes proceeds smoothly. For some of
these reactions, compounds 1−3 are more active than any other
catalyst. On the other hand terminal aliphatic alkynes are better
converted by other catalysts. Within the series of compounds
1−3 the metallocene 2 is the most active one, whereas the
zinc−zinc bonded species 1 shows the best selectivity. Since
diethylzinc (3) is commercially available, it is the catalyst that is
the most easily accessible. Thus, each of the three investigated
compounds has its advantages. Most remarkable is the broad
catalytic application of the zinc−zinc bonded species 1.
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ASSOCIATED CONTENT
■
S
* Supporting Information
(29) Crimmin, M. R.; Arrowsmith, M.; Barrett, A. G. M.; Casely, I. J.;
Hill, M. S.; Procopiou, P. A. J. Am. Chem. Soc. 2009, 131, 9670−9685.
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Experimental details, NMR spectra, kinetic plots, and Table S1
are available free of charge via the Internet at http://pubs.acs.
org.
(31) Datta, S.; Gamer, M. T.; Roesky, P. W. Organometallics 2008,
27, 1207−1213.
AUTHOR INFORMATION
■
(32) Datta, S.; Gamer, M. T.; Roesky, P. W. Dalton Trans. 2008,
2839−2843.
Corresponding Author
*(S.B.) Fax: +49 (0)30-31 42 36 19. Tel: +49 (0)30-31 42 23
55. E-mail: blechert@chem.tu-berlin.de. (P.W.R.) Fax: (+49)
721-608-44854. Tel: (+49) 721-608-46117. E-mail: roesky@kit.
edu.
(33) Datta, S.; Roesky, P. W.; Blechert, S. Organometallics 2007, 26,
4392−4394.
(34) Dunne, J. F.; Fulton, D. B.; Ellern, A.; Sadow, A. D. J. Am. Chem.
Soc. 2010, 132, 17680−17683.
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Notes
The authors declare no competing financial interest.
(36) Neal, S. R.; Ellern, A.; Sadow, A. D. J. Organomet. Chem. 2011,
696, 228−234.
ACKNOWLEDGMENTS
■
(37) Zhang, X.; Emge, T. J.; Hultzsch, K. C. Organometallics 2010,
29, 5871−5877.
This work was supported by the Cluster of Excellence
“Unifying Concepts in Catalysis” coordinated by the TU
Berlin, by the Deutsche Forschungsgemeinschaft, and by the
Fonds der Chemischen Industrie.
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