Cationic zinc organyls as precatalysts for hydroamination reactions

Article abstract of DOI:10.1002/chem.201405662

The cationic zinc triple-decker complex [Zn₂Cp?₃]⁺ [BAr^F ₄]^- (BAr^F ₄=B(3,5-(CF₃)₂C₆H₃)₄) exhibits catalytic acti

Full text of DOI:10.1002/chem.201405662

DOI: 10.1002/chem.201405662
Full Paper
&
Zinc Catalysis
Cationic Zinc Organyls as Precatalysts for Hydroamination
Reactions
Maren A. Chilleck,[a] Larissa Hartenstein,[b] Thomas Braun,*[a] Peter W. Roesky,*[b] and
Beatrice Braun[a]
Dedicated to Professor Konrad Seppelt on the occasion of his 70th birthday
Abstract: The cationic zinc triple-decker complex [Zn2Cp*3]+
[BArF ]À (BArF =B(3,5-(CF3)2C6H3)4) exhibits catalytic activity
formed via a similar reaction pathway. Additionally, several
other structurally well-defined cationic zinc organyls have
been examined as precatalysts for intermolecular hydroami-
nation reactions without the addition of a cocatalyst. These
studies reveal that the highest activity is achieved in the ab-
4
4
in intra- and intermolecular hydroamination reactions in the
absence of a cocatalyst. These hydroaminations presumably
proceed through the activation of the CÀC multiple bond of
the alkene or alkyne by a highly electrophilic zinc species,
which is formed upon elimination of the Cp* ligands. The re-
sence of any donor ligands. The neutral complex [ZnCp2S
]
2
(Cp2S =C5Me4(CH2)2SMe) shows a remarkably high catalytic
action of [Zn2Cp*3]+[BArF ]À with phenylacetylene gives the
activity in the presence of a Brønsted acid.
4
hydrocarbonation product (Cp*)(Ph)CCH2, which might be
Introduction
limited by the low solubility of zinc triflate in organic solvents.
Considerable progress has been made by the use of zinc com-
plexes bearing chelating N,N donor ligands. Thus, zinc alkyl
complexes with aminotroponiminato, b-diketiminato, and
phenalenyl-based N,N ligands (Scheme 1) have successfully
been applied as catalysts.[4–6] However, to achieve considerable
catalytic activities, a Brønsted acid has to be added as a cocata-
lyst. The role of the cocatalyst, which does not show significant
catalytic activity, is probably to protonate the alkyl ligand of
the zinc precatalyst, which leads to the formation of a cationic
species.[4c,d,5a,6] It has been assumed that the N,N donor ligand
Hydroamination reactions can be applied to the synthesis of
a broad range of nitrogen-containing molecules.[1] A variety of
different metal-based catalysts have been employed for hydro-
amination catalysis,[2] including complexes of main group
metals and the lanthanides, as well as early and late transition
metals. Various different reaction mechanisms have been dis-
cussed. In principle, hydroamination reactions can be classified
according to whether they proceed through the activation of
the NÀH bond or through an olefin or alkyne activation path-
way.[1b]
In recent years, zinc complexes have attracted increasing in-
terest as catalysts for hydroamination reactions.[3–8] Zinc has
proven to exhibit many favorable properties, and it is a cheap
and nontoxic metal. In hydroamination catalysis, zinc com-
plexes often show a high catalytic activity in combination with
a high tolerance towards polar functional groups. Zinc triflate
was one of the first zinc compounds to be used as catalyst in
hydroamination reactions.[3] However, the catalytic activity is
[a] Dr. M. A. Chilleck, Prof. Dr. T. Braun, Dr. B. Braun
Department of Chemistry
Humboldt-Universitꢀt zu Berlin
Brook-Taylor-Straße 2, 12489 Berlin (Germany)
E-mail: thomas.braun@chemie.hu-berlin.de
[b] L. Hartenstein, Prof. Dr. P. W. Roesky
Institute of Inorganic Chemistry
Karlsruhe Institute of Technology (KIT)
Engesserstraße 15, 76131 Karlsruhe (Germany)
E-mail: roesky@kit.edu
Scheme 1. Precatalysts for zinc-catalyzed hydroamination reactions: a) ami-
notroponiminato zinc complex; b) b-diketiminato zinc complex; c) zinc com-
plex with phenalenyl-based N,N donor ligand; d) zincocene [ZnCp*2] (1);
e) dizincocene [Zn2Cp*2].[4b,5a,6a,7a,b]
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405662.
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acts as a spectator ligand during hydroamination catalysis.
Mandal et al. conducted experimental and theoretical studies
on the intramolecular hydroamination of aminoalkenes cata-
lyzed by zinc complexes with phenalenyl-based N,N donor li-
gands, which suggested that an olefin activation pathway was
favored over the activation of the NÀH bond.[6] In situ NMR ex-
periments and DFT calculations performed by this group sup-
ported the formation of a catalytically active zinc cation.[6a] Re-
cently, zinc complexes which exclusively feature ligands that
are coordinated through zinc–carbon bonds have been applied
as hydroamination catalysts. The precatalysts [ZnCp*2] (1),
[Zn2Cp*2] (Scheme 1), and ZnEt2 led to a high catalytic activity
for the intermolecular hydroamination of phenylacetylenes
with amines in the presence of [PhNMe2H]+[B(C6F5)4]À as coca-
talyst.[7] Again, the formation of catalytically active cationic spe-
cies was suggested.
Table 1. Intramolecular hydroaminations catalyzed by [Zn2Cp*3]+[BArF ]À
4
(2).[a]
Substrate
Product
Conversion
(reaction time)[b]
98% (5 h)
93% (63 h)[c]
99% (6 h)
98% (16 h)
–
Apart from hydroamination catalysis, cationic zinc organyls
have been proposed to act as catalysts or precatalysts in vari-
ous catalytic transformations.[9] However, only a few examples
of structurally characterized cationic zinc complexes that ex-
hibit zinc–carbon bonds have been reported so far.[9–11] In par-
ticular, cationic zinc cyclopentadienyl (Cp) complexes are
rare.[11] In a recent study, the synthesis and structural character-
no reaction
[a] Reagents and conditions: 2 (2 mol%), substrate (0.54 mmol), [FeCp2]
as internal standard (10 mol%), 1,2-difluorobenzene (0.4 mL), 1208C.
[b] The conversion was determined by the integration of the olefin sig-
nals of the substrates in the 1H NMR spectra vs. the internal standard.
[c] Reaction at 1408C.
ization of the cationic triple-decker complex [Zn2Cp*3]+[BArF ]À
4
(2; BArF =B(3,5-(CF3)2C6H3)4; Scheme 2), which can serve as
4
a source of the {ZnCp*}+ cation, was described.[11b]
formation of the corresponding cyclic amines or imines with
high regioselectivities (Table 1), with the exception of (4E)-2,2-
dimethyl-5-phenylpent-4-enylamine, which showed no reaction
under the conditions specified. As described below, the triple-
decker complex 2 is not the catalytically active species of the
hydroamination reaction but a precatalyst.
In addition, the triple-decker complex 2 was used as a preca-
talyst in intermolecular hydroamination reactions of phenylace-
tylenes with aniline derivatives (see below, Table 2). The hydro-
amination of phenylacetylene with 2,4,6-trimethylaniline was
chosen as a test reaction because this reaction also proceeds
at room temperature using the zinc organyls [ZnCp*2] (1),
[Zn2Cp*2], or ZnEt2 as precatalysts.[7a,b] On applying 2.5 mol%
of complex 2 and 40 equivalents each of 2,4,6-trimethylaniline
and phenylacetylene, a quantitative conversion of phenylacety-
lene was achieved after a reaction time of two days in an
NMR-scale experiment at room temperature. The reaction led
to the selective formation of the imine N-(1-phenylethylidene)-
2,4,6-trimethylaniline (3) with Markovnikov regioselectivity
(Scheme 3). The identity of 3 was confirmed by NMR spectros-
Scheme 2. Formation of the triple-decker complex [Zn2Cp*3]+[BArF ]À (2).[11b]
4
Herein, we report on the triple-decker complex 2 and other
cationic zinc organyl compounds as precatalysts for intra- and
intermolecular hydroamination reactions in the absence of any
cocatalyst. Additionally, the use of neutral zinc organyls as cat-
alyst precursors in the presence of a cocatalyst is described. In-
vestigations into the mechanism of the zinc-catalyzed hydro-
amination reaction show the importance of a highly electro-
philic zinc center to achieve a high catalytic activity.
Results and Discussion
The triple-decker complex [Zn2Cp*3]+[BArF ]À (2) was tested as
4
a precatalyst for the intramolecular hydroamination of amino-
alkenes and aminoalkynes. As compound 2 is already a cationic
complex, no additional cocatalyst for the generation of cationic
species was added. All catalytic reactions involving complex 2
were performed in 1,2-difluorobenzene, which is a polar and
non-coordinating solvent. The reactions led to the selective
Scheme 3. Hydroamination of phenylacetylene with 2,4,6-trimethylaniline
catalyzed by 2.
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copy and GC-MS analysis. Although phenylacetylene
was consumed quantitatively after two days, small
amounts of residual 2,4,6-trimethylaniline were still
present in the reaction mixture. The ratio of 2,4,6-tri-
methylaniline to the imine 3 was approximately 1:10.
The incomplete consumption of 2,4,6-trimethylaniline
was also observed using other precatalysts (see
below, Table 3). Note that in recent studies with zinc
organyls as precatalysts, a 2:1 ratio of phenylacety-
lene to 2,4,6-trimethylaniline was used, which led to
an increase in the reaction rates.[7a,b] However, since
we aimed for a full conversion of both substrates, an
equimolar amount of the reagents was applied. The
reaction rate could be increased by first mixing com-
plex 2 and 2,4,6-trimethylaniline and adding phenyl-
acetylene after a period of 1 h. Thus, the time
needed for full conversion of phenylacetylene could
be reduced to approximately one day. The hydroami-
nation with 2.5 mol% of 2 was also repeated on
a larger scale using 2.86 mmol of each substrate.
After a reaction time of 24 h at room temperature,
the yield of isolated 3 was 91%.
Moreover, the long-term catalytic activity using
complex 2 as precatalyst was investigated. 2.5 mol%
of 2 was reacted with 2.86 mmol (40 equiv) each of
2,4,6-trimethylaniline and phenylacetylene. After
complete consumption of the substrates (typically
after 1–2 days), a fresh batch of substrates was
added. Thus, within 12 days 17.2 mmol of both sub-
strates were converted. The yield of the imine 3 iso-
lated was 87%, which corresponds to a turnover
number (TON) of 208. This shows the excellent long-
term catalytic activity obtained by using 2 as precatalyst.
Table 2. Intermolecular hydroaminations catalyzed by [Zn2Cp*3]+[BArF ]À (2).[a]
4
85% (1 h)
32% (7 h)[b]
81% (24 h)[b]
91% (49.5 h)[b,c]
54% (5.5 h)
85% (24 h)
38% (5.5 h)
75% (23.5 h)
62% (4.5 h) 65% (4 h)
79% (23 h)[c] 89% (24.5 h)[c]
78% (5 h)
92% (23 h)[c]
39% (4.5 h)
84% (21.5 h) 81% (25 h)
44% (6 h)
71% (4.5 h) 42% (3.5 h)
100% (23 h)
81% (20 h)
50% (5.5 h)
89% (25.5 h) 71% (24 h)
21% (4 h)
74% (4 h)
91% (24 h)
77% (7 h)
95% (24 h)
100% (5.5 h)
92% (1 h)
100% (5.5 h)
77% (22.5 h)[b]
45% (5.5 h)
81% (24 h)
19% (3.5 h)
85% (29 h)
55% (2.5 h) 60% (3.5 h)
84% (19 h) 96% (22 h)
[a] Conversions (reaction time) of intermolecular hydroamination reactions with 2 as
precatalyst. Reagents and conditions: 2 (2.5 mol%), aniline (0.29 mmol), phenylacety-
lene (0.29 mmol), 1,2-difluorobenzene (0.5 mL), 808C; conversion determined by
1H NMR spectroscopy.[12] [b] Reaction at room temperature. [c] The phenylacetylene
was totally consumed; conversion thus refers to the aniline.
which enhances the nucleophilicity of the amino group. Apart
To investigate the substrate scope of the intermolecular hy-
droamination reaction catalyzed by 2, anilines and phenylace-
tylenes bearing different functional groups were reacted to-
gether, employing 2.5 mol% of 2. All combinations of the ani-
line derivatives 2,4,6-trimethylaniline, aniline, p-methoxyaniline,
p-chloroaniline, and p-fluoroaniline with the phenylacetylene
derivatives phenylacetylene, p-methoxyphenylacetylene, p-
chlorophenylacetylene, and p-fluorophenylacetylene led to the
formation of the corresponding imines with Markovnikov se-
lectivity. The results are summarized in Table 2. Except for the
combination of 2,4,6-trimethylaniline with phenylacetylene or
p-fluorophenylacetylene, the reactions were slow at room tem-
perature and were therefore run at 808C. The reaction prog-
ress was examined by NMR spectroscopy, and the conversions
were determined by integration of the signals of the substrates
from that, no clear trends concerning the influence of the
functional groups at the substrates on the reaction rates are
apparent.
In situ NMR spectroscopic studies reveal that the triple-
decker complex 2 is not the catalytically active species of the
intra- and intermolecular hydroamination reactions. Immedi-
ately after adding the substrates to complex 2, quantitative
formation of HCp* (3 equivalents based on the amount of ap-
plied 2) was observed. Therefore, it can be excluded that the
catalytically active species contains a {ZnCp*} moiety.
The progress of the reaction of phenylacetylene with 2,4,6-
trimethylaniline catalyzed by 2 was monitored by 1H NMR
spectroscopy in intervals of 30 min. For up to 80% of the con-
version, a linear increase of the conversion with the reaction
time was found (see the Supporting Information, Figure S1).
Thus, the reaction rate is apparently independent of the con-
centrations of the substrates, which corresponds to a zero-
order rate law. This observation might indicate that the coordi-
nation of any substrates at the zinc center is not the rate-de-
termining step of the hydroamination catalysis. Note that
a zero-order rate dependence has been reported before for
the hydroamination of phenylacetylene with 2,4,6-trimethylani-
line applying [ZnCp*2] (1), [Zn2Cp*2], or ZnEt2 as precata-
lysts.[7a,b]
1
and the hydroamination products in the H NMR spectra of the
reaction mixtures.[12] With 2.5 mol% of complex 2, conversions
of at least 70% were reached for all substrate combinations
within 24 h. In some cases quantitative conversions of both
substrates were achieved. These results show that the triple-
decker complex 2 exhibits a high tolerance towards different
functional groups at the substrates. The reactions involving
2,4,6-trimethylaniline tend to be faster, which may be ascribed
to the electron donating properties of the methyl groups,
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lyzed hydroamination a reaction
pathway which proceeds on the
activation of the CÀC multiple
bond has already been proposed
in the literature.[3a,b,e,6a,8b] By GC-
MS analysis, very small amounts
of 4 were also found in the reac-
tion mixture of the hydroamina-
tion of phenylacetylene with
2,4,6-trimethylaniline catalyzed
by 2.[14]
The products resulting from
the reaction of the triple-decker
complex 2 with either phenyl-
acetylene or 2,4,6-trimethylani-
Scheme 4. Reactions of 2 with 2,4,6-trimethylaniline or phenylacetylene.
line appear to be highly interest-
ing with regard to a mechanism
To elucidate the mechanism of the hydroamination reaction
with complex 2 as precatalyst, it is essential to determine
which of the substrates is activated by the catalytically active
species. Compound 2 was separately treated either with
4 equivalents of 2,4,6-trimethylaniline or with 4 equivalents of
phenylacetylene, because the occurrence of a reaction may in-
dicate an activation of the substrates by 2. Both reactions led
to the formation of product mixtures (Scheme 4), and the re-
sulting zinc compounds could not be identified. The product
mixtures resulting from both reactions proved to be catalytical-
ly active in the hydroamination of phenylacetylene with 2,4,6-
trimethylaniline when an excess of the substrates was added.
In the reaction of 2 with phenylacetylene, one of the products
in the reaction mixture was identified as 1-(pentamethylcyclo-
penta-2,4-dienyl)-1-phenylethene (4; Scheme 4). By chromato-
graphic purification of the reaction mixture, 4 could be ob-
tained as the major product, which still contained contamina-
tion by small amounts of unidentified byproducts. However,
of the hydroamination reactions. As no zinc-containing prod-
ucts could be identified, attempts were made to synthesize
structurally defined model complexes. For these studies, dieth-
ylzinc was chosen as a starting material. The reaction of ZnEt2
with [H(OEt2)2]+[BArF ]À in the presence of phenylacetylene did
4
not lead to the formation of a well-defined product. In con-
trast, the reaction of a slight excess of ZnEt2 with [H(OEt2)2]+
[BArF ]À in the presence of three equivalents of 2,4,6-trimethyl-
4
aniline or aniline afforded the cationic complexes [ZnEt(2,4,6-
Me3C6H2NH2)3]+[BArF ]À (5) and [ZnEt(C6H5NH2)3]+[BArF ]À (6),
4
4
1
respectively (Scheme 5). The H and 13C NMR spectra of 5 and
1
the identity of 4 could unambiguously be determined by H,
1
1
13C, 13C APT, H/13C HSQC, and H/13C HMBC NMR spectroscopy
(see the Supporting Information, Figures S2 and S3; APT=at-
tached proton test, HSQC=heteronuclear single quantum co-
herence, HMBC=heteronuclear multiple bond correlation) as
well as by GC-MS analysis. The synthesis of 4 via a different re-
action route was reported in the literature and the NMR data
of 4 conform to the literature.[13] Compound 4 is the product
of the formal hydrocarbonation reaction of phenylacetylene
with HCp* and is formed with Markovnikov selectivity. Whether
the nucleophilic species that adds to the triple bond of phenyl-
acetylene is a zinc-coordinated or uncoordinated Cp* anion or
HCp* remains unclear. The reaction of phenylacetylene with
HCp* in the presence of catalytic amounts (2.5 mol%) of 2 also
led to the formation of 4. However, this reaction is not selec-
tive, and a variety of different products are formed. It seems
reasonable that the formation of 4 and the hydroamination re-
action proceed according to similar reaction mechanisms.
Probably, the electron density of the triple bond of phenylace-
tylene is lowered by coordination to the zinc center. Thus, phe-
nylacetylene becomes susceptible to nucleophilic attack by
Cp* or the aniline nitrogen atom. Note that for the zinc-cata-
Scheme 5. Formation of cationic ethyl zinc complexes 5 and 6.
6 exhibit the signals of a zinc-coordinated ethyl group, of the
[BArF ]À anion, and of three equivalents of coordinated 2,4,6-tri-
4
methylaniline or aniline (see the Supporting Information, Figur-
es S4–S11). In addition, 6 was characterized by single-crystal X-
ray diffraction.[15] The solid-state structure of the cation of 6 is
shown in Figure 1. The zinc center is coordinated by the ethyl
ligand and three aniline ligands in a distorted tetrahedral fash-
ion. There are no short interatomic contacts between the
[ZnEt(C6H5NH2)3]+ cations and the [BArF ]À anions. The struc-
4
ture of 6 is similar to those of other known cationic ethyl zinc
complexes which also exhibit three neutral donor groups that
are coordinated to the zinc center.[10d–f] Furthermore, Wehm-
schulte and Wojtas reported on the zinc aniline complex
[Zn(CB11Cl11)(NH2CH2CPh2CH2CHCH2)3], which was obtained in
a hydroamination experiment.[9d] As the {CB11Cl11}2À ligand is di-
anionic, [Zn(CB11Cl11)(NH2CH2CPh2CH2CHCH2)3] is a neutral com-
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talyst [H(OEt2)2]+[BArF ]À, a conversion to the imine 3 of 66%
4
was achieved within a reaction time of 3.5 h (Table 3, entry 4).
After the same reaction time under the same reaction condi-
tions, a conversion of 25% was reached with the triple-decker
complex 2 as precatalyst (Table 3, entry 2). Thus, 2 exhibits
a lower catalytic activity in the test reaction than a combination
of complex 1 and [H(OEt2)2]+[BArF ]À. It has to be noted, how-
4
ever, that 2 can be used without an additional cocatalyst,
whereas 1 is catalytically inactive in the absence of a cocata-
lyst.
In addition, the structurally defined cationic complexes
[ZnCp2N(py)2]+[BArF ]À (7; Cp2N =C5Me4(CH2)2NMe2, py=pyri-
4
dine) and [ZnCp2S(py)2]+[BArF ]À (8; Cp2S =C5Me4(CH2)2SMe;
4
Scheme 6), which were published recently,[11d] were examined
as precatalysts in the test reaction. However, the catalytic activ-
ities were low (Table 3, entries 6 and 7). This may be ascribed
to the presence of the pyridine ligands in 7 and 8, which
proved to be essential to stabilize the electrophilic zinc cen-
ter.[11d] The inhibiting effect of pyridine on the catalytic activity
can also be seen when two equivalents of pyridine are added
to complex 2, which effectively suppresses the catalytic activity
(Table 3, entry 3). Pyridine can inhibit hydroamination catalysis
Figure 1. Structure of the [ZnEt(C6H5NH2)3]+ cation in 6. Thermal ellipsoids
were drawn at the 50% probability level. For clarity, the hydrogen atoms are
omitted and only one position of the disordered ethyl ligand is shown. Se-
lected bond lengths [ꢁ] and angles [8]: Zn1ÀN1 2.1447(19), Zn1ÀN2
2.1564(18), Zn1ÀN3 2.128(2), Zn1ÀC19 1.960(9); N1-Zn1-N2 100.53(7), N1-
Zn1-N3 99.87(8), N2-Zn1-N3 99.62(7), N1-Zn1-C19 118.9(3), N2-Zn1-C19
121.7(3), N3-Zn1-C19 112.5(2).
plex. Although the zinc centers in 5 and 6 are proba-
bly highly electrophilic due to the cationic nature of
the complexes, no deprotonation of the amino
Table 3. Hydroamination of phenylacetylene with 2,4,6-trimethylaniline with different
zinc precatalysts.[a]
groups was observed, which is not supportive of an
amine activation pathway. So far it could not be
ruled out that the zinc-catalyzed hydroamination pro-
ceeds via an initial deprotonation of a coordinated
amino group, which would yield a zinc amido spe-
cies.
Entry Precatalyst
(amount)
Cocatalyst/pyridine
(amount)
Conversion (reac-
tion time)
TON[c] (reaction
time)
1
2 (2.5 mol%,
–
32% (7 h)
13 (7 h)
0.007 mmol)
81% (24 h)
91% (49.5 h)[b]
32 (24 h)
36 (49.5 h)
Complex 5 is catalytically active in the hydroamina-
tion of phenylacetylene with 2,4,6-trimethylaniline,
and the catalytic activity is comparable to the activity
of 2 (Table 3, entry 5). After addition of the substrates
to 5, the characteristic signals of the zinc-bound
2
3
4
2 (5 mol%,
0.022 mmol)
–
25% (3.5 h)
5 (3.5 h)
16 (25 h)
80% (25 h)[b]
2 (2.5 mol%,
0.007 mmol)
pyridine (5 mol%) <3% (24.5 h)
<1 (24.5 h)
1
ethyl ligand disappear in the H and 13C NMR spectra,
1 (5 mol%,
0.022 mmol)
[H(OEt2)2]+[BArF ]À
66% (3.5 h)
84% (7 h)
90% (24.5 h)[b]
13 (3.5 h)
17 (7 h)
18 (24.5 h)
4
whereas a new signal appears, which is assigned to
ethane. Therefore, there is probably no ethyl ligand
coordinated to the central zinc atom in the catalyti-
cally active species.
(5 mol%)
5
6
7
8
9
5 (2.5 mol%,
0.007 mmol)
–
–
–
34% (6 h)
79% (27 h)
14 (6 h)
32 (27 h)
Moreover, the catalytic activities of several cationic
and neutral zinc Cp complexes were compared in the
test reaction of phenylacetylene with 2,4,6-trimethy-
laniline (Table 3). The reactions were performed in
1,2-difluorobenzene at room temperature applying
a 1:1 molar ratio of 2,4,6-trimethylaniline and phenyl-
acetylene. Note that for the comparison of catalytic
activities, each triple-decker cation in 2 was consid-
ered to be the source of only one catalytically active
cationic zinc center. For all precatalysts, the formation
of the respective protonated cyclopentadienes was
observed during catalysis. [ZnCp*2] (1) has been re-
ported to be the zinc complex with the highest cata-
lytic activity in the hydroamination of phenylacety-
lene with 2,4,6-trimethylaniline to date.[7a] Using
5 mol% of 1 in the presence of 5 mol% of the coca-
7 (5 mol%,
0.016 mmol)
<3% (26 h)
<1 (26 h)
8 (5 mol%,
0.016 mmol)
19% (27 h)
4 (27 h)
9 (5 mol%,
0.022 mmol)
[H(OEt2)2]+[BArF ]À
10% (3.5 h)
34% (24.5 h)
2 (3.5 h)
7 (24.5 h)
4
(5 mol%)
10 (5 mol%,
0.022 mmol)
[H(OEt2)2]+[BArF ]À
61% (3.5 h)
73% (7 h)
85% (25 h)[b]
12 (3.5 h)
15 (7 h)
17 (25 h)
4
(5 mol%)
[a] Reagents and conditions: molar ratio 2,4,6-trimethylaniline/phenylacetylene=1:1,
1,2-difluorobenzene (0.5 mL), RT; conversion determined by 1H NMR spectroscopy.
[b] Phenylacetylene was totally consumed; conversion thus refers to 2,4,6-trimethyla-
niline. [c] TON=turnover number. The TON values for 2 are based on the assumption
that each molecule is the source of only one catalytically active cationic zinc center.
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mosphere of argon. 1,2-Difluorobenzene was purchased from
ABCR and dried over CaH2 before use. ZnEt2 was obtained from
Sigma-Aldrich as a 1m solution in hexane. The reagents for the in-
termolecular hydroamination experiments (2,4,6-trimethylaniline,
aniline, p-methoxyaniline, p-chloroaniline, p-fluoroaniline, phenyl-
acetylene, p-methoxyphenylacetylene, p-chlorophenylacetylene, p-
fluorophenylacetylene) were purchased from Sigma–Aldrich, ABCR,
or Acros Organics and stored in an argon-filled glovebox. 2,4,6-Tri-
methylaniline, aniline, and p-fluoroaniline were dried over NaH
prior to use. [ZnCp*2] (1), [Zn2Cp*3]+[BArF ]À (2), [ZnCp2N(py)2]+
4
[BArF ]À (7), [ZnCp2S(py)2]+[BArF ]À (8), [ZnCp2N2] (9), [ZnCp2S2] (10),
4
4
and [H(OEt2)2]+[BArF ]À were prepared according to literature pro-
4
cedures.[17,11b,d,16,18] The NMR spectra were recorded on a Bruker
Avance 400, a Bruker DPX 300, a Bruker Avance III 300, a Bruker
Avance III 500, or a Bruker Avance II 300 spectrometer. For NMR ex-
periments in 1,2-difluorobenzene, the NMR tubes were equipped
1
with a glass capillary filled with C6D6. H NMR chemical shifts were
Scheme 6. Zinc precatalysts 7-10.[11d,16]
referenced to the residual proton signals of the deuterated sol-
vents (C6D5H: d=7.15 ppm; CHCl3: d=7.26 ppm). 13C{1H} NMR
shifts were referenced to the 13C NMR signal of the solvent (C6D6:
d=128.06 ppm; CDCl3: d=77.16 ppm). 19F NMR shifts were refer-
either by occupying coordination sites at the zinc center of the
catalytically active species or by acting as a Brønsted base.
Due to the adverse effect of pyridine on the catalytic activity,
1
enced to external C6F6 (d=À162.9 ppm). The H and 13C{1H} NMR
spectra of compounds 4–6 are depicted in the Supporting Infor-
mation. GC-MS (EI) spectra were recorded on an Agilent Technolo-
gies 6890/5973N GC-MS system. Liquid injection field desorption
ionization (LIFDI) mass spectra were measured at a Micromass Q-
TOF 2 mass spectrometer equipped with a LIFDI 700 (Linden CMS)
ion source. Microanalyses were measured at a HEKAtech Euro EA
3000 elemental analyzer. The NMR and GC-MS analytical data of
the following imines are given in the Supporting Information: N-(1-
(p-methoxyphenyl)ethylidene)-2,4,6-trimethylaniline, N-(1-(p-chloro-
phenyl)ethylidene)-2,4,6-trimethylaniline, N-(1-(p-fluorophenyl)ethy-
lidene)-2,4,6-trimethylaniline, N-(1-phenylethylidene)aniline, N-(1-(p-
methoxyphenyl)ethylidene)aniline, N-(1-(p-chlorophenyl)ethylide-
ne)aniline, N-(1-(p-fluorophenyl)ethylidene)aniline, N-(1-phenylethy-
lidene)-p-methoxyaniline, N-(1-(p-methoxyphenyl)ethylidene)-p-me-
thoxyaniline, N-(1-(p-chlorophenyl)ethylidene)-p-methoxyaniline, N-
(1-(p-fluorophenyl)ethylidene)-p-methoxyaniline, N-(1-phenylethyli-
dene)-p-chloroaniline, N-(1-(p-methoxyphenyl)ethylidene)-p-chloro-
aniline, N-(1-(p-chlorophenyl)ethylidene)-p-chloroaniline, N-(1-(p-flu-
orophenyl)ethylidene)-p-chloroaniline, N-(1-phenylethylidene)-p-flu-
oroaniline, N-(1-(p-methoxyphenyl)ethylidene)-p-fluoroaniline, N-(1-
(p-chlorophenyl)ethylidene)-p-fluoroaniline, and N-(1-(p-fluorophe-
nyl)ethylidene)-p-fluoroaniline.
the neutral complexes [ZnCp2N
] ] (10;
(9) and [ZnCp2S
2
2
Scheme 6) were employed as precatalysts in the presence of
[H(OEt2)2]+[BArF ]À to generate a catalytically active cationic
4
species in situ. Complex 9 is a relatively poor catalyst (Table 3,
entry 8). Although upon addition of the cocatalyst and the
substrates the quantitative formation of HCp2N was observed,
the presence of the potentially coordinating and basic amino
functions of HCp2N in solution may inhibit catalytic activity. In
contrast, 10 exhibits a high catalytic activity (Table 3, entry 9),
which is comparable to that of 1. In addition, 1 and 10
showed similar activities in the hydroamination of p-fluorophe-
nylacetylene with p-methoxyaniline. Therefore, both 1 and 10
are excellent precatalysts for the intermolecular hydroamina-
tion of phenylacetylenes with anilines.
Conclusion
The cationic triple-decker complex [Zn2Cp*3]+[BArF ]À (2) was
4
successfully applied as a precatalyst in intra- and intermolecu-
lar hydroamination reactions, without the addition of a cocata-
lyst. In addition, the catalytic activities of different cationic and
neutral zinc organyls were compared. The organyl ligands
proved beneficial for a high catalytic activity because the cata-
lytically active species are probably generated by protonation
of the ZnÀC bonds. To preserve the high electrophilicity of the
zinc center, all other reagents in the reaction mixture (e. g., the
anion and the solvent) had to be weakly coordinating.
General procedure for the intramolecular hydroamination
with precatalyst 2
In an NMR tube, [Zn2Cp*3]+[BArF ]À (2; 15 mg, 0.011 mmol,
4
2 mol%) and [FeCp2] (10 mg, 0.054 mmol, 10 mol%) as an internal
standard were dissolved in 1,2-difluorobenzene (0.4 mL). After ad-
dition of the aminoalkene or aminoalkyne (0.535 mmol; see
Table 1), the reaction mixture was warmed to 120 or 1408C and
the reaction progress was monitored by NMR spectroscopy. The
conversion was determined by the integration of the olefin signals
1
of the substrates in the H NMR spectra vs. the internal standard.
Experimental Section
General procedure for the intermolecular hydroamination
with precatalyst 2
Reagents and General Methods
Unless stated otherwise, the experiments were performed with
a Schlenk line under an atmosphere of argon or in an argon-filled
glovebox with oxygen levels below 10 ppm. All solvents were puri-
fied and dried by conventional methods and distilled under an at-
An NMR tube was charged with [Zn2Cp*3]+[BArF ]À (2; 10 mg,
0.007 mmol, 2.5 mol%). A premixed solution of the aniline deriva-
tive (0.29 mmol) and the phenylacetylene derivative (0.29 mmol) in
1,2-difluorobenzene (0.5 mL) was added at room temperature (see
4
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Full Paper
Table 2). The NMR tube was sealed and heated at 808C, if appropri-
1.14 mmol) was added at room temperature. The resulting orange
solution was stirred for 30 min at room temperature. The volatiles
were then removed under reduced pressure. The resulting oily resi-
due was washed with hexane (3ꢂ5 mL) at À708C and dried under
1
ate (see Table 2). The reaction progress was monitored by H, 13C,
and, if suitable, 19F NMR spectroscopies. The conversion was deter-
1
mined by integration of H NMR signals of the substrates and the
corresponding hydroamination product.[12] After termination of the
catalysis, the identity of the resulting imine was confirmed by GC-
MS analysis under non-inert conditions.
vacuum, which gave 306 mg of a beige solid. The H and 13C NMR
1
spectra indicated that a mixture of various products was obtained,
which except for HCp* could not be identified. For a hydroamina-
tion experiment, an NMR tube was charged with the thus-obtained
product mixture (50 mg) and
a premixed solution of 2,4,6-
Synthesis of N-(1-phenylethylidene)-2,4,6-trimethylaniline
trimethylaniline (70 mL, 0.50 mmol) and phenylacetylene (55 mL,
0.50 mmol) in 1,2-difluorobenzene (0.5 mL) was added at room
temperature. After a reaction time of 4 days at room temperature,
the 1H and 13C NMR spectra showed that phenylacetylene was
completely consumed, whereas 3 and 2,4,6-trimethylaniline were
present in a molar ratio of about 1:0.4.
(3)
In a Schlenk tube [Zn2Cp*3]+[BArF ]À (2; 100 mg, 0.071 mmol,
4
2.5 mol%) was dissolved in 1,2-difluorobenzene (2 mL), and a pre-
mixed solution of 2,4,6-trimethylaniline (401 mL, 2.86 mmol) and
phenylacetylene (314 mL, 2.86 mmol) in 1,2-difluorobenzene (3 mL)
was added at room temperature. The resulting yellow solution was
stirred for 24 h at room temperature. The volatiles were then re-
moved under reduced pressure and the resulting oily residue was
purified by column chromatography on silica gel (eluent=petrole-
um ether/ethyl acetate 4:1+2% triethylamine) under non-inert
conditions to give 3 as a yellow oil (0.62 g, 91% yield). 1H NMR
(400.1 MHz, CDCl3, 298 K): d=8.03 (m, 2H, ArH), 7.48 (m, 3H, ArH),
6.88 (s, br, 2H, ArH), 2.29 (s, 3H, CH3), 2.08 (s, 3H, CH3), 2.00 ppm
(s, 6H, CH3); 13C{1H} NMR (75.5 MHz, C6D6, 298 K): d=164.7 (NCCH3),
147.4 (CAr), 139.6 (CAr), 131.8 (CAr), 130.5 (CAr), 129.0 (CAr), 128.5 (CAr),
127.5 (CAr), 125.6 (CAr), 20.9 (CH3), 18.1 (CH3), 16.9 ppm (CH3); GC-
MS (70 eV): m/z: 237 [M+], 222 [M+ÀCH3], 207 [M+À2CH3].
Reaction of 2 with phenylacetylene and subsequent hydro-
amination
[Zn2Cp*3]+[BArF ]À (2; 758 mg, 0.54 mmol) was dissolved in 1,2-di-
4
fluorobenzene (6 mL). Phenylacetylene (238 mL, 2.17 mmol) was
added at room temperature, upon which the color of the solution
immediately changed to dark brown and, within the next minutes,
to orange. The resulting solution was stirred for 1 h, and the vola-
tiles were removed under reduced pressure. The oily residue was
washed with hexane (10 mL, then 2ꢂ5 mL), lyophilized, washed
with hexane (4ꢂ5 mL), and lyophilized again, which gave 318 mg
1
of a brown solid. The H and 13C NMR spectra exhibited the signals
of 4 and a variety of unidentified products. For a hydroamination
experiment, a sample of the product mixture (50 mg) was placed
in an NMR tube and dissolved in 1,2-difluorobenzene (0.6 mL).
2,4,6-trimethylaniline (28 mL, 0.20 mmol) and phenylacetylene
(22 mL, 0.20 mmol) were added to the solution at room tempera-
ture. After 3 days at room temperature, the 1H and 13C NMR spectra
showed that both substrates were consumed and that the imine 3
had quantitatively been formed.
Investigation of the long-term catalytic activity of 2
In a Schlenk tube, [Zn2Cp*3]+[BArF ]À (2; 100 mg, 0.071 mmol,
4
2.5 mol%) was dissolved in 1,2-difluorobenzene (3 mL), and a pre-
mixed solution of 2,4,6-trimethylaniline (401 mL, 2.86 mmol) and
phenylacetylene (314 mL, 2.86 mmol) in 1,2-difluorobenzene (4 mL)
was added at room temperature. The resulting solution was stirred
at room temperature, and the progress of the reaction was exam-
ined by GC-MS analysis of small samples. After consumption of the
substrates, which typically took 1–2 days, a fresh batch of 2,4,6-tri-
methylaniline and phenylacetylene (each 2.86 mmol) was added. In
total, 17.2 mmol each of 2,4,6-trimethylaniline and phenylacetylene
reacted within 12 days. Purification by column chromatography
under non-inert conditions gave 3 as a yellow oil (3.53 g, 87%
yield; TON=208).
Synthesis of (Cp*)(Ph)CCH2 (4)
[Zn2Cp*3]+[BArF ]À (2; 300 mg, 0.21 mmol) was dissolved in 1,2-di-
4
fluorobenzene (3 mL), and phenylacetylene (94 mL, 0.86 mmol) was
added at room temperature, upon which the solution turned dark
brown. Over the following minutes, the color changed to orange.
The resulting solution was stirred for 2 h at room temperature. The
volatiles were then removed under reduced pressure and the oily
residue was purified by column chromatography on silica gel
(eluent=petroleum ether+1% ethyl acetate) under non-inert con-
ditions to give a colorless oil. According to the 1H and 13C NMR
spectra, the product contained 4 as the main product and minor
amounts of various unidentified byproducts. 1H NMR (400.1 MHz,
C6D6, 298 K): d=7.10–7.01 (m, 5H, ArH), 5.26 (m, 2H, CHaHb), 1.72
(s, 6H, CH3), 1.60 (s, 6H, CH3), 1.18 ppm (s, 3H, CH3); 13C{1H} NMR
(100.6 MHz, C6D6, 298 K): d=152.6 (C), 142.9 (C), 140.8 (C), 135.9
(C), 127.6 (CH), 127.2 (CH), 127.1 (CH), 115.6 (CH2), 62.1 (C), 20.6
(CH3), 11.2 (CH3), 10.5 ppm (CH3); GC-MS (70 eV): m/z: 238 [M+],
223 [M+ÀCH3], 208 [M+À2CH3], 193 [M+À3CH3], 103 [M+ÀCp*].
In situ NMR spectroscopic investigation of the hydroamina-
tion of phenylacetylene with 2,4,6-trimethylaniline catalyzed
by 2
[Zn2Cp*3]+[BArF ]À (2; 20 mg, 0.014 mmol, 2.5 mol%) was dissolved
4
in 1,2-difluorobenzene (0.5 mL), and 2,4,6-trimethylaniline (80 mL,
0.57 mmol) was added at room temperature. The resulting solution
was stirred for 1 h at room temperature, after which phenylacety-
lene (63 mL, 0.57 mmol) was added. The progress of the reaction
1
was monitored by recording H NMR spectra at intervals of 30 min.
The conversion was determined by integration of a signal of the
imine 3 (d=7.92 ppm; m, 2H, ArH), using the signal of the ortho
protons of the [BArF ]À anion (d=8.12 ppm; m, br, 8H) as an inter-
4
nal standard.
Synthesis of [ZnEt(2,4,6-Me3C6H2NH2)3]+[BArF ]À (5)
4
ZnEt2 (1m solution in hexane, 3.0 mL, 3.0 mmol) was added to a so-
lution of 2,4,6-trimethylaniline (832 mL, 5.93 mmol) in 1,2-difluoro-
Reaction of 2 with 2,4,6-trimethylaniline and subsequent
hydroamination
benzene (10 mL) at room temperature. Solid [H(OEt2)2]+[BArF ]À
4
[Zn2Cp*3]+[BArF ]À (2; 400 mg, 0.29 mmol) was dissolved in 1,2-di-
(2.00 g, 1.98 mmol) was then added, which led to the evolution of
gas. The resulting solution was stirred for 15 min at room tempera-
4
fluorobenzene (4 mL), and 2,4,6-trimethylaniline (160 mL,
Chem. Eur. J. 2015, 21, 2594 – 2602
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
ture, and the volatiles were removed under reduced pressure. The
residue was washed with hexane (4ꢂ10 mL). Upon drying under
vacuum, 5 was obtained as a colorless solid (2.49 g, 92% yield).
1H NMR (400.1 MHz, 1,2-F2C6H4, C6D6 capillary, 298 K): d=8.10 (m,
premixed solution of p-methoxyaniline (54 mg, 0.44 mmol) and p-
fluorophenylacetylene (53 mg, 0.44 mmol) in 1,2-difluorobenzene
(0.5 mL) was added. The NMR tube was sealed and heated at 808C.
1
The reaction progress was monitored by H, 13C, and 19F NMR spec-
br, 8H, o-H, BArF À), 7.45 (m, br, 4H, p-H, BArF4À), 6.63 (s, 6H, 2,4,6-
troscopy, and the conversion was determined by integration of
a
1H NMR signal of p-methoxyaniline (d=3.46 ppm; s, 3H, OMe)
4
Me3C6H2NH2), 3.76 (s, br, 6H, NH2), 2.00 (s, 9H, p-CH3, 2,4,6-
Me3C6H2NH2), 1.82 (s, 18H, o-CH3, 2,4,6-Me3C6H2NH2), 0.92 (t,
3J(H,H)=8.1 Hz, 3H, CH2CH3), 0.11 ppm (q, 3J(H,H)=8.1 Hz, 2H,
CH2CH3); 13C{1H} NMR (100.6 MHz, 1,2-F2C6H4, C6D6 capillary, 298 K):
and the imine N-(1-(p-fluorophenyl)ethylidene)-p-methoxyaniline
(d=3.53 ppm; s, 3H, OMe). Conversions with 1 as precatalyst: 56%
(3.5 h), 83% (23.5 h; p-fluorophenylacetylene was totally con-
sumed). Conversions with 10 as precatalyst: 56% (3.5 h), 78%
(23.5 h; p-fluorophenylacetylene was totally consumed).
d=162.5 (q, 1J(C,11B)=50 Hz, i-C, BArF À), 135.6 (s, CAr, 2,4,6-
4
Me3C6H2NH2), 135.1 (s, o-C, BArF À), 133.1 (s, CAr, 2,4,6-Me3C6H2NH2),
4
130.6 (s, CAr, 2,4,6-Me3C6H2NH2), 129.7 (q, br, 2J(C,F)=32 Hz, m-C,
BArF À), 126.1 (s, CAr, 2,4,6-Me3C6H2NH2), 124.9 (q, 1J(C,F)=272 Hz,
4
CF3, BArF À), 117.6 (m, p-C, BArF4À), 19.5 (s, p-CH3, 2,4,6-
Acknowledgements
4
Me3C6H2NH2), 16.3 (s, o-CH3, 2,4,6-Me3C6H2NH2), 11.4 (s, CH2CH3),
1.5 ppm (s, CH2CH3); LIFDI-TOF-MS (1,2-F2C6H4): m/z: 363
[ZnEt(2,4,6-Me3C6H2NH2)2]+, 228 [ZnEt(2,4,6-Me3C6H2NH2)]+; ele-
mental analysis calcd (%) for C61H56N3BF24Zn: C 53.74, H 4.14, N
3.08; found: C 53.90, H 4.04, N 2.63.
We are grateful to Dipl.-Chem. Anne-Kristin Trꢃtzschler and
Dipl.-Chem. Stefan Scheifler for experimental support.
Keywords: alkynes
hydroamination · zinc organyls
·
homogeneous
catalysis
·
Synthesis of [ZnEt(C6H5NH2)3]+[BArF ]À (6)
4
ZnEt2 (1m solution in hexane, 0.74 mL, 0.74 mmol) was added to
a solution of aniline (135 mL, 1.48 mmol) in 1,2-difluorobenzene
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(5 mL) at room temperature. [H(OEt2)2]+[BArF ]À (500 mg,
4
0.49 mmol) was added as a solid, which led to the formation of
gas. The resulting solution was stirred for 30 min at room tempera-
ture. The volatiles were then removed under reduced pressure,
and the residue was washed with hexane (10 mL+2ꢂ5 mL). After
drying under vacuum, 6 was obtained as a colorless solid (535 mg,
88% yield). Single-crystals that were suitable for X-ray diffraction
were obtained by slow evaporation of a solution of 6 in 1,2-
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1
difluorobenzene at À308C. H NMR (400.1 MHz, 1,2-F2C6H4, C6D6 ca-
pillary, 298 K): d=8.09 (m, br, 8H, o-H, BArF À), 7.45 (m, br, 4H, p-H,
4
BArF À), 7.14 (m, br, 6H, ArH, C6H5NH2), 6.96 (m, br, 3H, ArH,
4
C6H5NH2), 6.62 (m, br, 6H, ArH, C6H5NH2), 4.05 (s, br, 6H, NH2), 1.09
(t, 3J(H,H)=8.2 Hz, 3H, CH2CH3), 0.26 ppm (q, 3J(H,H)=8.2 Hz, 2H,
CH2CH3); 13C{1H} NMR (100.6 MHz, 1,2-F2C6H4, C6D6 capillary, 298 K):
d=162.5 (q, 1J(C,11B)=50 Hz, i-C, BArF À), 138.8 (s, CAr, C6H5NH2),
4
135.1 (s, o-C, BArF À), 130.7 (s, CAr, C6H5NH2), 129.7 (q, br, J(C,F)=
2
4
32 Hz, m-C, BArF4À), 125.7 (s, CAr, C6H5NH2), 124.9 (q, J(C,F)=272 Hz,
1
CF3, BArF À), 119.0 (s, CAr, C6H5NH2), 117.6 (m, p-C, BArF4À), 11.9 (s,
4
CH2CH3), À2.5 ppm (s, CH2CH3).
General procedure for the hydroamination of phenylacety-
lene with 2,4,6-trimethylaniline using different precatalysts
An NMR tube was charged with the zinc precatalyst (see Table 3). If
appropriate, an equimolar amount of [H(OEt2)2]+[BArF ]À was
4
added. A premixed solution of 2,4,6-trimethylaniline and phenyl-
acetylene in a 1:1 molar ratio and, if appropriate, pyridine in 1,2-di-
fluorobenzene (0.5 mL) were added at room temperature (see
Table 3). The NMR tube was sealed, and the progress of the reac-
1
tion was examined by H and 13C NMR spectroscopy. The conver-
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1
sion was determined by integration of a H NMR signal of 2,4,6-tri-
methylaniline (d=2.01 ppm; s, 3H, CH3) and the imine 3 (d=
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Hydroamination of p-fluorophenylacetylene with
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[ZnCp2S2] (10; 10 mg, 0.022 mmol), was placed in an NMR tube,
and solid [H(OEt2)2]+[BArF ]À (22 mg, 0.022 mmol) was added. A
4
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Full Paper
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3
¯
1351.13; crystal size 0.46ꢂ0.36ꢂ0.25 mm ; triclinic; P1; a=12.8998(5),
b=14.7138(6), c=16.5320(6) ꢁ; a=96.875(3), b=107.163(3), g=
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=
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[12] The conversions of the intermolecular hydroamination reactions were
determined by calculating the substrate/product ratios by integration
[19] A. L. Spek, J. Appl. Crystallogr. 2003, 36, 7–13.
1
of the H NMR signals of the substrates and products. For this purpose,
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University of Gçttingen, Gçttingen (Germany), 1997; b) G. M. Sheldrick,
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an appropriate resonance of one of the substrates and the signal of the
methyl group on the imine function of the product were applied. This
procedure is applicable because all intermolecular hydroaminations
lead to the quantitative formation of the corresponding imines. In
cases in which the phenylacetylene derivative was completely con-
sumed, the conversion refers to the aniline derivative.
Received: October 15, 2014
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Published online on December 17, 2014
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