Assembling zirconium and calcium moieties through an oxygen center for an intramolecular hydroamination reaction: A single system for double activation

Article abstract of DOI:10.1002/anie.201100022

One for two: A zirconium compound combined with a main-group alkaline earth metal through an oxygen bridge has been synthesized. A number of primary and secondary aminoalkenes were successfully converted into cyclic products using this heterobimetallic co

Full text of DOI:10.1002/anie.201100022

Communications
DOI: 10.1002/anie.201100022
Heterobimetallic Catalysis
Assembling Zirconium and Calcium Moieties through an Oxygen
Center for an Intramolecular Hydroamination Reaction: A Single
System for Double Activation**
Arup Mukherjee, Sharanappa Nembenna, Tamal K. Sen, Sankaranarayana Pillai Sarish,
Pradip Kr. Ghorai, Holger Ott, Dietmar Stalke, Swadhin K. Mandal,* and Herbert W. Roesky*
Dedicated to Prof. Dr. Didier Astruc on the occasion of his 65th birthday
[
6]
The catalytic addition of a NÀH bond across a CÀC multiple
bond (hydroamination) offers an efficient and atom-econom-
ical route to produce nitrogen-containing molecules, which
through an oxygen atom. The synthetic strategy takes
advantage of unprecedented syntheses of a number of well-
defined hydroxide precursors of the type LMR(OH) (M = Al,
Ga, or Ge; R = alkyl, aryl, or electron lone pair; L =
CH{N(Ar)(CMe)}2 with Ar= 2,6-iPr C H ), [LSr(m-OH)] ·
[1]
are of great interest to academic and industrial researchers.
Over the last decade, there have been enormous efforts to
develop an efficient catalyst for this demanding transforma-
tion. The most recent trend has been the use of catalysts that
involve Group 2 metals, which show high catalytic conversion
for intramolecular hydroamination reactions under relatively
2
6
3
2
[6–8]
(thf) , and [Cp* (Me)Zr(OH)] (Cp* = C Me ).
thetic development resulted in the access of a new class of
catalysts bearing enhanced Lewis acidic metal centers
This syn-
3
2
5
5
[6]
through oxygen bridging.
[
2]
mild reaction conditions. Moreover, catalysts based on
Group 4 metals have traditionally shown very high activity in
the hydroamination catalysis, as reported by Schafer and
Herein we show for the first time that a Group 4 metal can
be fixed on a Group 2 metal through an oxygen bridge to
carry out intramolecular hydroamination of primary and
secondary aminoalkenes. In this study we have synthesized a
[
3,4]
others.
However, no studies have ever considered the
activation of aminoalkenes by a catalytic system that com-
bines Group 2 and Group 4 metals. Two metal centers with
entirely different chemical properties have attracted chemists
for a long time, not only from the point of synthetic challenge,
Zr-O-Ca-based heterobimetallic compound [Cp* (Me)Zr(m-
2
O)Ca(thf) {N(SiMe ) }] (1). We investigated the catalytic
3
3 2
properties of 1 for intramolecular hydroamination reactions
of primary and secondary aminoalkenes.
[5]
but also from their cooperative activity. In this area, one of
the major challenges has been the development of a system in
which two metal centers will act catalytically in different
manners. Heterobimetallic complexes in general have enor-
mous potential to revolutionize homogeneous catalytic pro-
Synthesis of 1 was accomplished by reacting the mono-
metallic hydroxide precursor [Cp* (Me)Zr(OH)] with [Ca{N-
2
(SiMe ) } ·(thf) ] under elimination of HN(SiMe3)2
3
2
2
2
(Scheme 1). A solution of [Cp* (Me)Zr(OH)] in n-hexane/
2
[
5]
cesses. Recently, we have demonstrated a new synthetic
route by which a plethora of heterometals can be assembled
[*] A. Mukherjee, T. K. Sen, Dr. P. K. Ghorai, Dr. S. K. Mandal
Department of Chemical Sciences
Indian Institute of Science Education and Research-Kolkata
Mohanpur-741252 (India)
Scheme 1. Synthesis of complex 1.
Fax: (+91)-33-25873032
E-mail: swadhin.mandal@iiserkol.ac.in
THF (2:1 ratio) was added drop by drop to the solution of
Dr. S. Nembenna, Dr. S. P. Sarish, Dr. H. Ott, Prof. Dr. D. Stalke,
Prof. Dr. H. W. Roesky
Institut fꢀr Anorganische Chemie, Universitꢁt Gꢂttingen
Tammannstrasse 4, 37077 Gꢂttingen (Germany)
Fax: (+49) 551-39-3373
[
Ca{N(SiMe ) } ·(thf) ] in a 1:1 stoichiometric ratio in n-
3 2 2 2
hexane at 08C and stirred at 258C for 24 h to yield 1.
Compound 1 is insoluble in n-pentane but readily dissolves in
1
13
29
toluene and THF, and it was characterized by H, C, Si
NMR spectroscopy, EI mass spectrometry, and single-crystal
X-ray diffraction studies.
E-mail: hroesky@gwdg.de
[
**] A.M. and T.K.S. are thankful to IISER-Kolkata and CSIR, India,
respectively for research fellowships. S.K.M. thanks DST (grant no.
SR/FT/CS-020/2008), India for financial support. H.W.R. thanks the
Deutsche Forschungsgemeinschaft for support of this work. D.S.
and H.O. are grateful to the DFG-funded SPP 1178, the DNRF-
funded Center for Materials Crystallography (CMC) for support and
the Land Niedersachsen for providing a fellowship in the Catalysis
of Sustainable Synthesis (CaSuS) PhD program.
1
The H NMR spectrum of 1 in C D exhibits three singlets
6
6
at d = À0.36, 0.51, and 1.90 ppm, which are attributed to the
5
proton resonances arising from ZrÀMe, N(SiMe ) , and h -
3
2
C Me groups, respectively. The ZrÀMe proton resonance is
5
5
shifted upfield to d = À0.36 ppm in 1 relative to that observed
[
9a]
for [Cp* (Me)Zr(OH)] (d = À0.2 ppm), whereas the CaÀ
2
N(SiMe ) proton resonance of 1 (d = 0.51 ppm) is shifted
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100022.
3 2
downfield when compared with that observed for the starting
3
968
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 3968 –3972

[
9b]
material [Ca{N(SiMe ) } (thf) ] (d = 0.38 ppm).
The
C NMR spectrum of 1 reveals a resonance at d = 22.7 ppm,
which is assigned to the zirconium-bound methyl carbon
natocalcium bis(trimethylsilyl)amide [LCa{N(SiMe ) }(thf)]
3 2
3
2
2
2
1
3
(L = CH{N(Ar)(CMe)} , Ar= 2,6-iPr C H ), can activate pri-
2
2
6
3
[
2g]
mary aminoalkenes. The successful synthesis of complex 1
in which a tetravalent Group 4 metal fragment, such as
methyl zirconocene, and a divalent Group 2 metal, such as
calcium, are assembled through an oxygen center gave us a
unique opportunity to check its efficiency towards catalytic
activation of primary and secondary aminoalkenes. Thus our
aim has been to use the single well-defined heterobimetallic
complex 1 for the activation of primary and secondary
aminoalkenes.
2
9
atom. The Si NMR spectrum exhibits a resonance at d =
À13.5 ppm arising from the silicon nucleus of the N(SiMe )
3
2
group.
Analytically pure crystals of 1 were obtained from a cold
toluene/THF solution (3:1 ratio) at 08C, and finally the
structure of 1 was determined by single-crystal X-ray
[10]
crystallography (Figure 1). Compound 1 crystallizes in the
The study started with unactivated primary aminoalkenes.
The reaction of catalyst 1 with dry, degassed aminoalkenes
proceeds regiospecifically, and the primary aminoalkenes
were converted almost quantitatively to the cyclic product
under mild reaction conditions (25–808C). The results of
catalyst 1 with different primary aminoalkenes are summar-
ized in Table 1. The progress of the hydroamination reaction
of the primary aminoalkene (1-allylcyclohexyl)methylamine
(
2; substrate of Table 1, entry 2) to the corresponding cyclic
1
product using catalyst 1 was monitored by in situ H NMR
spectroscopic studies (see the Supporting Information).
The presence of the methyl zirconocene moiety in
compound 1 motivated us to test its efficacy in the activation
of secondary aminoalkenes, as methyl zirconocene has been
found to activate the secondary aminoalkenes in the presence
[4b]
of an externally added activator. At first, we carried out the
Figure 1. View of the molecular structure of 1. Ellipsoids are set at
0% probability; hydrogen atoms have been omitted for clarity.
5
Selected bond distances [ꢃ] and angles [8]: Ca1–O1 2.2068(13), Ca1–
O2 2.4165(14), Ca1–O3 2.4357(13), Ca1–N1 2.3706(16), Zr1–C21
Table 1: Intramolecular hydroamination of unactivated primary amino-
alkenes using catalyst 1.
[
a]
2
1
.326(2), Zr1–O1 1.8879(13); Ca1-O1-Zr1 176.98(7), O1-Ca1-N1
26.27(5). x=centroid of Cp* ring.
[
b]
Entry
Substrate
Product
t [h] Conversion [%]
monoclinic space group P2 /n with one molecule in the
[c]
[d]
1
1
8.6 98, [92]
asymmetric unit. The X-ray structural analysis of 1 reveals
that the calcium atom is bonded through a bridging oxygen
atom to the zirconium center. The calcium atom is surrounded
by four oxygen atoms (one bridging oxygen atom and three
oxygen atoms from THF molecules) and a nitrogen atom of
the amide group. The calcium center adopts a distorted
trigonal bipyramidal geometry, whereas the geometry around
[c]
[d]
2
12
96, [87]
[
e]
f]
2
8
99
80
5
the zirconium center is distorted-tetrahedral, considering h -
[
C Me as single coordination site. The Ca1ÀN1 bond distance
3
4
5
45
5
5
(
2.3706(16) ꢀ) is comparable to that found in [LCa{N-
(
2
SiMe ) }(thf)] (L = CH{N(Ar)(CMe)} , Ar= 2,6-iPr C H ;
3
2
2
2
6
3
[11a]
[g]
[h]
.313 ꢀ).
The bond angle of Ca-O-Zr is nearly linear (ca.
30
20
92
98
1
778) and is wider than the previously observed bond angles
in related heterobimetallic complexes [Cp* (Me)Zr(m-O)Ti-
2
(
(
NMe ) ] (Zr-O-Ti = 169.78) and [Cp* (Me)Zr(m-O)Hf-
2 3 2
[11b]
NMe ) (m-O)Zr(Me)Cp* ] (Zr-O-Hf = 169.48),
reflecting
2
2
2
a higher steric demand in case of compound 1 (see the
Supporting Information).
[a] Reaction conditions: amine (30 mL) in 0.6 mL C D . [b] Determined by
6
6
1
The increasing demand to develop an efficient catalyst for
the hydroamination reaction prompted us to test the catalytic
activity of complex 1 towards intramolecular hydroamination.
A previous study by Scott and co-workers has demonstrated
that the zirconium alkyl cation catalyzes the cyclization of
H NMR spectroscopy against an internal standard. [c] Reaction carried
out with 10 mol% catalyst loading at 258C. [d] Yield of isolated product.
1
[
e] Conversion determined by H NMR spectroscopy using related
oxygen-bridged bimetallic calcium amido catalyst [{(iPrAT)Ca{N-
(
2
SiMe ) }(thf)} ] (iPrAT=2-(isopropylamino)troponate) for substrate
3 2 2
[12]
.
[f] Reaction carried out with 10 mol% catalyst loading at 808C.
[
4a]
secondary aminoalkenes. Recent developments established
[g] Reaction carried out with 20 mol% catalyst loading at 808C.
that divalent Group 2 metal complexes, such as b-diketimi-
[h] Reaction carried out with 13 mol% catalyst loading at 808C.
Angew. Chem. Int. Ed. 2011, 50, 3968 –3972
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 3969

Communications
[
4b]
catalytic hydroamination reaction of (1-allylcyclohexylme-
moiety. To check the generality of the catalytic activity of 1
on secondary aminoalkenes, we investigated the reaction with
a number of secondary aminoalkenes in the presence of the
thyl)benzylamine (substrate of Table 2, entry 1) with 1 in
C D , maintaining the bath temperature at 1108C without
6
6
adding any activator. After 20 h of heating we found less than
0% conversion to the hydroamination product. To increase
activator at 1108C in C D6 (Table 2) revealing very high
6
1
catalytic conversion of a range of secondary aminoalkenes to
the corresponding cyclic products. The catalytic activity of 1
compares well with that observed for alkyl zirconium catalyst
the catalytic activity of 1 for secondary aminoalkenes, we
performed the reaction with an equimolar amount of the
activator [PhNMe H][B(C F ) ] (with respect to the catalyst)
[
4a]
for a range of other secondary aminoalkenes.
2
6 5 4
and we found that after 20 h at 1108C in C D , the conversion
We carried out detailed kinetic studies to gain further
insight into the catalytic process of 1 using the primary and
secondary aminoalkenes in C D . A plot of ln(C/C ) for 2
versus time provides a straight line with negative slope
(Figure 2a), revealing an overall first order rate for the
6
6
was 99% (Table 2, entry 1). This fact may be attributed to the
generation of the in situ cationic zirconium species where the
activator acts as a methyl abstracting reagent. The activator,
6
6
0
[
PhNMe H][B(C F ) ], has been used previously to generate
2 6 5 4
a cationic zirconium species through abstraction of a methyl
Table 2: Intramolecular hydroamination of unactivated secondary ami-
[
a]
noalkenes using catalyst 1.
[
b]
Entry
Substrate
Product
t [h] Conversion [%]
1
20 99
2
3
4
5
20 98
50 99
[
[
c]
c]
30 96, [90]
30 99, [93]
Figure 2. a) Plot of ln(C/C ) versus time for the cyclization of 2
0
(&; substrate of Table 1, entry 2) and [D D
]2 (*) catalyzed by 1 in C
2
6
6
2
at 258C. For [D ]2: y=À0.00116xÀ0.13541, R =0.97333; For 2:
2
2
y=À0.00281xÀ0.16261, R =0.98849. b) Plot of lnk versus ln[Cat.]
obs
for the cyclization of 2 by 1 in C D at 258C (van’t Hoff plot).
6
6
2
y=1.04811xÀ1.23556, R =0.98007.
6
48 99
reaction. A detailed kinetic study reveals the first-order rate
dependence on catalyst concentration (see the Supporting
Information). The first-order rate of the reaction with respect
to the catalyst concentration was further confirmed from
vanꢁt Hoff plot. A plot of lnkobs versus the natural logarithm
of the catalyst concentration provides a straight line (Fig-
ure 2b) with a slope of 1.04 (where the slope corresponds to
the reaction rate). To gain a preliminary understanding of the
mechanistic pathway of the cyclization process, we carried out
an H/D kinetic isotope effect (KIE) experiment under the
7
8
9
36 99
50 98
24 99
48 69
reaction conditions using 2 and [D ]2. The experiment
2
resulted in a substantial KIE of 2.42 (Figure 2a, see also the
Supporting Information). This observation suggests that a
hydrogen of the amino group of 2 is involved in the key step of
the primary aminoalkene activation process. Similarly, we
carried out the kinetic study for cyclization of (1-allylcyclo-
hexylmethyl)benzylamine (substrate of Table 2, entry 1)
using catalyst 1. In this case we also found that the overall
1
0
[a] Reaction conditions: amine (30 mL), catalyst (10 mol%), [PhNMe H]
2
[
B(C F ) ] (10 mol%) in 0.6 mL C D at 1108C. [b] Determined by
H NMR spectroscopy against an internal standard. [c] Yield of isolated
6
5
4
6
6
1
product.
3
970 www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 3968 –3972

reaction rate was first-order and the reaction rate depends
directly on the catalyst concentration (see the Supporting
Information).
the intramolecular hydroamination of primary and secondary
aminoalkenes through a single catalytic system.
Preliminary investigations into the cyclization process of
primary aminoalkenes indicate that the activation of primary
aminoalkenes occurs by the amine activation pathway. The
Received: January 3, 2011
Revised: January 27, 2011
Published online: March 23, 2011
1
H NMR spectrum of the reaction mixture exhibits the
Keywords: bimetallic compounds · calcium ·
formation of free HN(SiMe ) (at d = 0.1 ppm), indicating
.
3
2
homogeneous catalysis · hydroamination · zirconium
involvement of the calcium center during the catalytic cycle.
This fact is also supported by the substantial kinetic isotope
effect (KIE of 2.42), which suggests that amine activation is
one of the most important steps during the catalytic cycle. The
amine activation pathway has been established previously
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[
2d,g]
using a calcium-based catalyst for primary aminoalkenes.
Moreover, the proton resonances arising from the ZrÀMe
group and from the zirconium bound Cp* groups do not
undergo any significant changes during the activation of the
primary aminoalkenes. This result indicates that the zirco-
nium center does not participate in the activation process.
However, in the case of secondary aminoalkenes, we found
that after the addition of an equimolar amount of the
activator (with respect to the catalyst), the ZrÀMe resonance
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2
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À1
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À1
and 12.5 kcalmol for generation of a cation at the zirconium
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3 2
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3 2 2
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(
thf) ] at room temperature under elimination of HN(SiMe ) .
2 3 2
We have demonstrated that the two different catalytically
active metal centers present in compound 1 can be used to
activate the primary and secondary aminoalkenes. A number
of primary and secondary aminoalkenes was successfully
converted to cyclic products using the catalyst 1. Preliminary
investigation indicates that the calcium center activates the
primary aminoalkenes and the zirconium center activates the
secondary aminoalkenes. This work represents a major step
forward in the development of heterobimetallic catalysis for
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3971

Communications
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[
10] Crystal data for 1: C H CaNO Si Zr, M = 809.48, 0.4 ꢄ 0.26 ꢄ
39 75 4 2 r
3
0
9
4
0
2
.2 mm , monoclinic, space group P2 /n, a = 14.4608(12), b =
1
.4611(16),
c = 16.7106(15) ꢀ,
b = 109.4660(10)8,
V=
3
À3
.4339(7) nm , Z = 4, 1calc = 1.213 Mgm
,
m
(MoKa) =
À1
.453 mm , T= 100(2) K, q range for data collection = 3.02–
[12] S. Datta, P. W. Roesky, S. Blechert, Organometallics 2007, 26,
4392 – 4394.
6.038, 37016 reflections measured, 8592 independent (Rint
=
3
972
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 3968 –3972