Bulky guanidinate calcium and zinc complexes as catalysts for the intramolecular hydroamination

Article abstract of DOI:10.1016/j.jorganchem.2019.02.005

Mixed guanidinato-amido supported complexes of calcium, LCaN(SiMe₃)₂?2THF (1) [L = {ArNC(NⁱPr₂)NAr} (Ar = 2,6-Me₂-C₆H₃)] and zinc, LZnN(SiMe₃)₂ (2) & L

Full text of DOI:10.1016/j.jorganchem.2019.02.005

Accepted Manuscript
Bulky guanidinate calcium and zinc complexes as catalysts for the intramolecular
hydroamination
Milan Kr Barman, Ashim Baishya, Sharanappa Nembenna
PII:
S0022-328X(18)31031-3
DOI:
https://doi.org/10.1016/j.jorganchem.2019.02.005
JOM 20707
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 20 December 2018
Revised Date: 31 January 2019
Accepted Date: 5 February 2019
Please cite this article as: M.K. Barman, A. Baishya, S. Nembenna, Bulky guanidinate calcium and zinc
complexes as catalysts for the intramolecular hydroamination, Journal of Organometallic Chemistry
(2019), doi: https://doi.org/10.1016/j.jorganchem.2019.02.005.
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ACCEPTED MANUSCRIPT
Graphical Abstract
H
N
R
R
Ar
NH2
iPr
iPr
R
N
N
(THF)n
R
M
N
R'
N(SiMe3)2
N
R
R
H
N
R'
Ar
M = Ca, n=2, Zn, n=0
R
R
2-10 mol% catalyst
r.t -120 oC, C6D6
Conv. upto 99%
14 examples

ACCEPTED MANUSCRIPT
Bulky guanidinate calcium and zinc complexes as catalysts for the
intramolecular hydroamination
Milan Kr. Barman, Ashim Baishya and Sharanappa Nembennaa*
aSchool of Chemical Sciences, National Institute of Science Education and Research (NISER), HBNI, Bhubaneswar,
India 752 050
ABSTRACT
Mixed guanidinato-amido supported complexes of calcium, LCaN(SiMe3)2·2THF (1) [L =
{ArNC(NiPr2)NAr} (Ar = 2,6-Me2-C6H3)] and zinc, LZnN(SiMe3)2 (2) & L1ZnN(SiMe3)2 (3) [L1
= {Ar′NC(NiPr2)NAr′} (Ar′ = 2,6- iPr2-C6H3)] have been synthesized by salt metathesis method.
Reaction between LH or L1H and two equivalents of KN(SiMe3)2 in THF and the resultant
reaction mixture upon treatment with CaI2 or ZnCl2 led to the formation of heteroleptic calcium
or zinc complexes. All three compounds (1-3) were characterized by multinuclear (1H, 13C, 29Si)
magnetic resonance spectroscopy, elemental analysis and single crystal X-ray diffraction
methods. Solid state structures reveal that all are in monomeric in form. Furthermore, we have
shown the catalytic application of these complexes for intramolecular hydroamination of amino
alkenes. Interestingly, compound 1 exhibits as an excellent pre-catalyst for intramolecular
hydroamination of various primary amino alkenes in the absence of any additional activator (co-
catalyst). However, both compounds 1 and 2 show very good catalytic activity of
hydroamination of various secondary amines in the presence of co- catalyst.
*Corresponding author. School of Chemical Sciences, National Institute of Science Education and Research
(NISER), HBNI, Bhubaneswar 751005, India
Tel.: +91-674-2494174; fax: +91-674-2302436
E-mail address: snembenna@niser.ac.in (S. Nembenna)
Keywords: Calcium/Catalysis/Guanidine/Neutral ligand/Zinc
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Introduction
The hydroamination of alkenes or alkynes is an attractive atom-efficient synthetic method
for the synthesis of nitrogen-containing molecules from cheaper precursors [1]. The
hydroamination reaction can be defined as the formal addition of a N-H bond over C-C double or
triple bond, which can either proceed through inter or intra molecular fashion [2]. However, to
overcome the high kinetic barrier for this addition reaction, catalytic pathways are required.
The intramolecular hydroamination of amino alkenes has been rigorously described [3] with
several main group, transition and lanthanide based catalysts. Marks et al., have developed some
highly active lanthanide based catalysts for intramolecular hydroamination reactions [4].
Livinghouse and co-workers have reported homoleptic yttrium and lanthanide
bis(trimethysilyl)amides, [Ln{N(SiMe3)2}3] as catalysts for the intramolecular hydroamination of
amino olefins [5]. As far as main group, in particular, calcium based catalysts are concerned, Hill
and co-workers reported the heteroleptic β-diketiminate supported calcium amide catalysed
intramolecular hydroamination of amino alkenes for the first time [6]. This well-defined calcium
amide complex was originally reported by Chisholm as a lactide polymerization catalyst [7].
Intramolecular hydroamination of amino olefins is extended by the research groups of Hill [8],
Harder [9], Roesky [10], Tamm [11] and many others [12]. Recently, Thiel and co-workers have
isolated and structurally characterized the zinc metal alkyl complex, which is the intermediate in
the β-diketiminate supported heteroleptic zinc amide catalysed intramolecular hydroamination
reaction of an amino olefin in the absence of activator [13].
Traditionally, second- and third- row transition metals have been broadly exploited in
catalytic applications [14]. The recent trend of homogeneous catalysis is the use of main group
metal or elements [9a], [15].
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This is due to their low-cost, low toxicity and the large abundance of main group
elements in comparison to transition or lanthanide metals. In addition, molecular compound
catalyzed organic transformations are in advantageous to understand the reaction mechanism by
isolating reactive intermediates. This is possible because actual chemistry occurs at the
coordination sphere of the metal center.
In this regard, we have previously reported complexes of Mg, Ca and Zn bearing
guanidinate ligand and their catalytic activity in the Tishchenko reaction [16] and heteroleptic
magnesium complexes as pre-catalysts for the hydroboration of esters [17]. Although there are
some reports on synthesis and characterization of guanidinate supported calcium [18] and zinc
[19] complexes, which are utilized as catalysts for ring opening polymerization of lactide,
styrene polymerization, cyclotrimerization of isocyanate, dimerization of aldehyde to
corresponding ester i.e. Tischenko reaction etc. However, to the best of our knowledge there
have been no reports on heteroleptic guanidinate calcium and zinc amide catalysed
hydroamination of amino alkenes till date.
Therefore, herein, we report three new examples of structurally characterized heteroleptic
calcium and zinc amide complexes (1-3). Furthermore, we have investigated the catalytic
application of these complexes for the intramolecular hydroamination reaction of both primary
and secondary amino olefins.
Result and discussion
Two bulky tetra-substituted guanidines such as LH [L = {ArNC(NiPr2)NAr}; (Ar = 2,6-Me2-
i
C6H3)] and L1H [L1 = {Ar′NC(NiPr2)NAr′}; (Ar′ = 2,6- Pr2-C6H3)] have been utilized to
synthesize heteroleptic calcium and zinc amide complexes. The synthesis of heteroleptic bulky
3

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guanidinate supported calcium and zinc amide complexes 1-3, LCaN(SiMe3)2·2THF (1),
LZnN(SiMe3)2 (2) and L1ZnN(SiMe3)2 (3) was achieved by the salt metathesis method. Reaction
of a bulky guanidine ligand, LH with two equivalents of potassium bis(trimethylsilyl) amide,
KN(SiMe3)2 in tetrahydrofuran (THF) and subsequent treatment with one equivalent of CaI2 in
THF afforded the expected product LCaN(SiMe3)2·2THF (1) (Scheme 1). Similarly, the reaction
of a bulky guanidine ligand, either LH or L1H with two equivalents of KN(SiMe3)2 in THF and
subsequent treatment with one equivalent of ZnCl2 led to the formation of LZnN(SiMe3)2 (2) and
L1ZnN(SiMe3)2 (3), respectively (Scheme 1).
Scheme 1. Synthesis of compounds 1-3
Recently, Westerhausen and co-workers reported the structurally characterized heteroleptic
guanidinate
stabilized
calcium
amide,
L1CaN(SiMe3)2·THF·(hexane);
[L1
=
{Ar′NC(NiPr2)NAr′}; (Ar′ = 2,6- iPr2-C6H3)] (4) by deprotonation method [18c]. The compound
4 was synthesized by the reaction of Ca{N(SiMe3)2}2·2THF with L1H in hexane at reflux
temperature for 18 h.
Mixed guanidinato-amido calcium and zinc complexes (1-3) are freely soluble in organic
solvents and air & moisture sensitive. All three compounds were characterized by multinuclear
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(1H, 13C, and 29Si) magnetic resonance spectroscopy and elemental analysis. Furthermore, the
molecular structures of 1, 2 and 3 were confirmed by X-ray single crystal structural analysis. All
three compounds 1-3 exhibit the expected number of signals in the 1H and 13C NMR spectra and
1
are consistent with their composition. The H NMR spectrum of compound 1 exhibits singlet
resonance for the amide moiety, N(SiMe3)2 at 0.1 ppm in C6D6, while both compounds 2 and 3
exhibit singlet resonances for amide moiety, N(SiMe3)2 at 0.2 ppm in C6D6. The 13C NMR
spectra for compounds 1-3 show a characteristic peak for the N3C resonances at 172.5, 169.9
and 170.4 ppm respectively. The 29Si NMR spectra show signals at –1.98, –0.51 and –0.80 ppm
for compound 1, 2 and 3, respectively, and are attributed to the amido, N(SiMe3)2 moiety.
Molecular structures of 1-3
Crystals of the complex LCaN(SiMe3)2·2THF (1) suitable for X-ray diffraction were grown from
o
its n-hexane solution at 0 C for one day. Compound 1 crystallizes in the orthorhombic system
with Pca21 space group. The molecular structure, selected bond distances and bond angles have
depicted in the Fig. 1.
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Fig. 1. Molecular structure of compound 1. All hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and bond angles (o): Ca1–N1 2.411(3), Ca1–N2 2.403(3), Ca1–N4 2.322(3),
C1–N3 1.411(4), Ca1–O1 2.427(2), Ca1–O(2) 2.421(2), Si1–N4 1.690(3); N1–Ca1–N2 56.08(9),
N2–C1–N1 115.22(3), N4–Ca1–N1 108.40(9), N4–Ca1–N2 131.82(10), N1–Ca1–O1 146.67(9),
N1–Ca1–O2 99.52(8), N2–Ca1–O1 92.35(9), N2–Ca1–O2 108.80(10), Si2–N4–Si1 123.30(15).
The molecular structure of 1 reveals that the Ca centre is bonded to the guanidinate ligand in
[N,N’] chelate fashion, and other three sites are occupied by a N atom of the amido ligand and
two oxygen atoms of the THF molecules, resulting in a distorted trigonal bipyramidal geometry.
The Ca1–N4 bond distance 2.322(3) Å in 1 is slightly longer than the reported the Ca-Namido
bond distance in compound 4 (2.284(3) Å). The Ca1–N4 bond distance 2.322(3) Å in 1 is shorter
than the Ca1–N1 and Ca1–N2 bond lengths of 2.411(3) Å and 2.403(3) Å, respectively. The N2–
Ca1–N1 bite angle 56.08(9)o is slightly acute than compound 4 (N2–Ca1–N1 57.05(8)o. The
Ca1–O1 and Ca1–O2 bond distances, 2.427(2) Å and 2.421(2) Å, respectively are longer than
Ca1–O1 bond distance of compound 1 (2.350(2) Å. The molecular structures of 2 and 3, selected
bond distances and bond angles have depicted in the Fig. 2.
Fig. 2. Molecular structures of compounds 2 and 3. All hydrogen atoms are omitted for clarity.
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Selected bond lengths (Å) and bond angles (o) for compound 2 (left): Zn1–N1 1.993(2), Zn1–N2
2.023(3), Zn1–N4 1.858(2), Si2–N4 1.714(3), Si1–N4 1.709(3), N3–C1 1.376(4); N1–Zn1–N2
67.48(11), N4–Zn1–N1 149.26(12), N4–Zn1–N2 142.73(12), N2–C1–N1 110.8(3), Si1–N4–Si2
126.01(15) and 3 (right) Zn1–N1 2.0044(14), Zn1–N2 2.0023(15), Zn1–N4 1.8689(15), Si1–N4
1.7166(15), Si2–N4 1.7151(15), N3–C1 1.366(2); N2–Zn1–N1 67.29(6), N4–Zn1–N2 147.31(6),
N4–Zn1–N1 145.39(6), N2–C1–N1109.92(14), N2–C1–N3125.20(15), Si2–N4–Zn1115.11(8),
Si2–N4–Si1130.64(9).
ꢀ
Compounds 2 and 3 were crystallized in the triclinic P1 and monoclinic P21/c space groups,
respectively. Both compounds 2 and 3 are isostructural. The solid state structures for 2 and 3
display three-coordinate metal centers with distorted trigonal-planar geometries. The molecular
structures reveal that both compounds 2 and 3 are in monomeric in form. The Zn–N(amido) bond
distances in compounds 2 and 3 of 1.858(2) Å and 1.8689(15) Å, respectively, are shorter than
those of [ButZn{µ-N(SiMe3)2] (2.084(2) Å), [C6F5 Zn{µ-N(SiMe3)2}] (2.048(2) Å) [20] and
these distances are longer than that of β
-diketiminato zinc amide comple, DippNacnacZnNMe2 [
Dipp
Nacanac = CH{(CMe)2(2,6-iPr2C6H3N)2}] (Zn-Namido 1.8385(15) Å) [13]. As expected, the
Ca–N (amido) bond length 2.322(3) Å (summation of the covalent radii of calcium and nitrogen
= 2.47 Å) [21] is longer than the Zn–N bond distances (Zn and N = 1.93 Å) in compounds 2 and
3 of 1.858(2) Å and 1.8689(15) Å, respectively. The N1–Zn1–N2 bite angles in compound 2 and
3 67.48(11)o 67.29(6)o, respectively are wider than the bite angle of N2–Ca1–N1 in compound 1.
Calcium and zinc catalysed intramolecular hydroamination reaction
In 1990’s homoleptic calcium and zinc bis(amide) reagents have been reported. [22] Moreover,
these reagents (derivatives) have been widely used in the coordination chemistry by the research
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groups of Bochmann and co-workers [23], Hill and co-workers [15c], Harder and co-workers
[9a], Roesky and co-workers [10a, b], Ruhlandt-Senge and co-workers [24], Westerhausen and
co-workers [25], Carpentier, Sarazin and co-workers [26] and Power and co-workers [27]. In
recent years, a few examples of heteroleptic N,N’-chelated calcium and zinc amide complexes
have been utilized as catalysts for the hydroamination reaction.[15b] In view of this we aimed to
test compounds 1, 2 and 3 for the intramolecular hydroamination/cyclization reaction of both
primary and secondary amino olefins. (see, Tables 1 - 4).
Intramolecular hydroamiation of primary aminoalkenes
Initially, we began our investigation with the addition of 10 mol % of catalyst 1 to the dry &
degassed 2,2-diphenylpent-4-en-1-amine in C6D6 at room temperature. We noticed the complete
conversion of 2,2-diphenylpent-4-en-1-amine to the corresponding five-membered pyrrolidine
derivative within 5 minutes (Table 1, entry 1).
Table 1. Cyclization of 2,2-diphenylpent-4-en-1-aminea
Cat.
(mol%) (min)
Time Temp Conv.
Entry
Precatalyst
(oC)
(%)b
1
2
3
4
10
5
5
25
25
25
25
>99
>99
>99
>99
LCaN(SiMe3)2·2THF (1)
15
30
15
1
2
1
L1CaN(SiMe3)2
·THF (4)
5
5
2
30
25
>99
4
5
120
480
480
80
80
80
>99c
>99c
>99c
6
7
8
LZnN(SiMe3)2 (2)
2.5
2.5
2
L1ZnN(SiMe3)2 (3)
8

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a Conditions: primary amino olefin (20 µL), 0.5 mL C6D6 on the NMR scale. b Calculated by 1H NMR spectroscopy
and hexamethylbenzene added as an internal standard. c Co-catalyst [PhNMe2H] [B(C6F5)4] equimolar amount (with
respect to catalyst)
Further, the same reaction was performed by using lower catalyst loadings (5 mol %, and 2 mol
%). A complete conversion of 2,2-diphenylpent-4-en-1-amine into cyclic amine was noticed
within 15- 30 minutes (Table 1, entries 2-3). Moreover, the same reaction was performed by
using a catalyst 4 with catalyst loadings 5 mol % and 2 mol %. We observed that compound 4
exhibits similar catalytic activity as that of the compound 1. Further, we decided to test the
catalytic activity of compounds 2 and 3. First, the intramolecular hydroamination of primary
amino alkene i.e, 2,2-diphenylpent-4-en-1-amine with catalyst 2 (5 mol%) was performed in
o
1
C6D6 at 80 C. After heating for 8 h, there was no cyclized product, as revealed by H NMR
spectrum of the reaction mixture. Next, this reaction was performed with an equimolar amount
(with respect to catalyst) of the activator, [PhNMe2H] [B(C6F5)4]. A quantitative conversion of
amino alkene into cyclized product within 120 minutes. This improvement in the catalytic
activity may be attributed to the in situ generation of the coordinatively unsaturated cationic zinc
species, in which the activator acts as amide –NH(SiMe3)2 abstracting agent. However, the same
o
reaction was performed at 80 C in C6D6 with lower catalysts loadings (2.5 mol %) by using
catalysts 2 and 3. In both the cases, longer reaction time is required for the quantitative
conversion.
With optimized reaction conditions for the intramolecular hydroamination of 2,2-diphenylpent-4-
en-1-amine in hand, the reaction scope was expanded to various other primary amino olefin
substrates by using catalysts 1 and 2 ( see, Table 2).
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As expected, a higher catalyst loading and elevated temperature are required for the preparation
of six-membered piperidine and seven-membered azepane derivatives, when compared to five-
membered pyrrolidine derivatives. It is known that catalytic turnover increases for smaller ring
sizes (5 > 6s > 7) (Baldwin’s guidelines).[28] The intramoleular hydroamination reactions of 2,2-
diphenylhex-5-en-1-amine and (1-(but-3-en-1-yl)cyclohexyl)methanamine with complex 1 or 2
yielded the corresponding six-membered piperidine derivatives of 97 % ( Table 2, entry 11), 95
% ( Table 2, entry 12) and 99 % (Table 2, entry 13), 96 % (Table 2, entry 14), respectively.
Table 2. Intramolecular hydroamination reaction of aminoalkenes catalysed by 1 and 2a
Entry
Substrate
Product
Catalyst
(mol%)
Time
(min)
Temp
(oC)
Conv.
(%)b
1
2
1(2)
2(5)
20
25
>99
120
20
80
25
>99
99
3
4
1(2)
2(5)
120
80
99
5
6
1(2)
2(5)
25
25
80
99
98
120
7
1(2)
2(5)
1(2)
2(5)
25
25
80
25
80
98
97
99
98
8
120
20
9
10
120
c
97
11
12
1(10)
2(5)
120
180
60
80
95
10

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c
99
13
14
1(10)
2(5)
150
60
80
180
96
aConditions: For catalyst 1; amine (20 µl) and catalyst (2 mol %); for catalyst 2 amine (20 µl) catalyst (5
b
1
mol %) and activator (5 mol%) in 0.5 mL C6D6 on NMR scale. Determined by H-NMR spectroscopy
using hexamehyl benzene as an internal standard. c catalyst 1 (10 mol%)
Intramolecular hydroamination of secondary aminoalkenes
After the successful conversion of primary amino olefins into cyclized amines by using catalysts
1 and 2, we aimed to test compounds 1 and 2 catalysed intramolecular hydroamination reaction
of secondary amines. The intramolecular hydroamination reaction of secondary aminoalkene was
carried out by using N-benzyl-2,2-diphenylpent-4-en-1-amine as a model substrate (see, Table
3).
Table 3. Cyclization of N-benzyl-2,2-diphenylpent-4-en-1-aminea
Cat.
(mol%)
Time
(h)
Temp
(oC)
Conv.
(%)b
Entry
1
Precatalyst
5
24
120
95
LCaN(SiMe3)2
·
2THF (1)
THF (4)
L1CaN(SiMe3)2
·
2
5
23
120
93
3
4
5
6
5
2
12
2
80
80
80
80
99
94
98
94
LZnN(SiMe3)2 (2)
2.5
5
2
L1ZnN(SiMe3)2 (3)
2.5
12
3
aConditions: secondary amino olefin (20 µL), equimolar amount (with respect to catalyst) of co-catalyst
[PhNMe2H] [B(C6F5)4], 0.5 mL C6D6 on the NMR scale. bCalculated by 1H NMR spectroscopy and
hexamethylbenzene added as an internal standard.
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We carried out with addition of 5 mol % catalyst 1 and equimolar amount (with respect to
catalyst) of activator [PhNMe2H] [B(C6F5)4] (5 mol %) to a N-benzyl-2,2-diphenylpent-4-en-1-
o
amine in C6D6 in a J Young valve NMR tube. The above reaction mixture was heated to 120 C
for 24 h. We noticed the formation of (R)-1-benzyl-2-methyl-4,4-diphenylpyrrolidine in 95 %,
1
which is confirmed by H NMR spectrum of the reaction mixture. Catalyst 4 shows similar
catalytic activity for the above reaction. However, both catalysts 2 and 3 show better catalytic
activity for the conversion of N-benzyl-2,2-diphenylpent-4-en-1-amine into (R)-1-benzyl-2-
methyl-4,4-diphenylpyrrolidine when compared to catalysts 1 and 4. (Table 3, entries 3-6).
Table 4. Intramolecular hydroamination reaction of secondary aminoalkenes catalysed by 1 and
3a
Entry
Substrate
Product
Catalyst
Time
(h)
Temp
(oC)
Conv.
b
(%)
1
2
1
24
2
120
80
>95
2
99
3
4
1
2
34
4
120
80
95
96
5
6
1
2
36
4
120
80
94
97
7
8
1
2
24
2
120
80
98
99
9
1
2
26
3
120
80
96
98
10
12

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11
12
13
14
1
2
34
4
120
80
93
94
94
95
1
36
4
120
80
N
2
aConditions: For catalysts 1 and 2 amine (20 µl), catalyst (5 mol %) and activator (5 mol %) in 0.5 mL C6D6 on
NMR scale . bDetermined by 1H NMR spectroscopy using hexamthylbenzne as an internal standard.
As summarized in table 4, a variety of secondary aminoalkenes underwent intramolecular
hydroamination reaction in the presence of catalysts 1 and 2 and equimolar amount of activator
(Table 4, entries 1-14) in C6D6. In each case, a quantitative conversion was noticed. Progress of
1
the reaction was monitored by H NMR spectroscopy of the reaction mixture using internal
standard hexamethyl benzene. As is the case in primary amino olefins, a longer reaction time is
required for the formation of six-membered piperidine derivatives (table 4, entries 11-14). For
the conversion of primary amines into cyclized amines calcium amide is superior catalyst when
compared zinc amides. However, in the case of secondary amines, zinc amides are better catalyst
than calcium amides.
Conclusion
Three new heteroleptic calcium and zinc complexes (1-3) bearing bulky guanidinato and amido
ligands have been synthesized and structurally characterized. Furthermore, we have
demonstrated the formation of carbon-nitrogen bonds in both primary and secondary
aminoalkenes by intramolecular cyclization (hydroamination) reaction catalysed by heteroleptic
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calcium and zinc amide complexes. These catalysts show similar catalytic activity with reported
calcium/zinc amide and other main group metal complexes. These complexes (1-3) are important
precursors for isolation of guanidinate supported hydride and hydroxides, alkoxides etc. Such
studies are in progress in our laboratory and we publish in due course.
Experimental section
All manipulations were carried out using standard Schlenk line technique and glove box under an
inert atmosphere of nitrogen. Solvents were collected from MBraun Solvent Purification System
and degassed prior to use. Benzene-d6 was dried over potassium mirror and freeze-thaw was
performed twice prior to use. NMR spectroscopic data were recorded on a Bruker AV-400
spectrometer (1H NMR, 400 MHz; 13C{1H} NMR, 101 MHz, 29Si{1H} NMR, 80 MHz).
Deuterated benzene was used as the solvent, and chemical shift values (
δ
) are reported in parts
128.0 for 13C).
1
per million relative to the residual signals of the solvent (
δ
7.16 for H and
δ
Elemental analyses were performed in a Vario Micro Cube Elementar CHNS /O analyzer. All
the chemicals were purchased from Sigma-Aldrich, Alfa Aesar, Acros organics and used as
received. [PhNMe2H][B(C6F5)4] was purchased from Strem chemicals and used without any
further purification. Precursors such as LH [16], L1H [29] and L1Ca(NSiMe3)2.THF [30] were
prepared according to literature procedures. Aminoalkenes substrates both primary and
secondary such as 2,2-diphenylpent-4-en-1-amine [31], (1-allylcyclohexyl)methylamine [32], (1-
(2-methylallyl)cyclohexyl)methanamine [32], 4-methyl-2,2-diphenylpent-4-en-1-amine [32],
(E)-2,2,5-triphenylpent-4-en-1-amine [33], 2,2-diphenylhex-5-en-1-amine [34], (1-(but-3-en-1-
yl)cyclohexyl)methanamine [33], N-benzyl-2,2-diphenylpent-4-en-1-amine [33], N-(4-
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methylbenzyl)-2,2-diphenylpent-4-en-1-amine [35], N-(4-methoxybenzyl)-2,2-diphenylpent-4-
en-1-amine [12c], 1-(1-allylcyclohexyl)-N-benzylmethanamine [12c], 1-(1-allylcyclohexyl)-N-
(4-bromobenzyl)methanamine, N-benzyl-2,2-diphenylhex-5-en-1-amine [36], N-benzyl-1-(1-
(but-3-en-1-yl)cyclohexyl)methanamine [33] were prepared according to the literature
procedures and dried by distilling. All hydroamination products are known compounds and were
identified by comparing to the reported literature 1H NMR spectroscopic data.
Synthesis of compound 1
To a mixture of bulky guanidine free ligand, LH [L = {ArNC(NiPr2)NAr}; (Ar = 2,6-Me2-C6H3)]
(0.5 g, 1.42 mmol) and KN(SiMe3)2 (0.574 g, 2.87 mmol) was added THF (20 mL) and stirred at
room temperature for 4 h. The resultant reaction mixture was then added drop by drop to a stirred
o
suspension of CaI2 (0.417 g, 1.42 mmol) in THF (5 mL) at –78 C. The reaction mixture was
slowly allowed to warm to room temperature and stirred for 24 h. All the volatiles were removed
under vacuum. The solid residue was extracted with n-hexane (~40 mL) and filtered through
Celite and the volume of the clear solution was reduced approximately to 10 mL and kept it for
o
crystallization at 0 C. Colourless crystals suitable for X-ray diffraction analysis were obtained
after one day. A crystalline compound was isolated. Yield: 0.83 g (84 %); mp = 140 -145
NMR (C6D6, 298 K, 400 MHz) 7.11 (d, J = 7.4 Hz, 4H, Ar-H), 6.93 (t, J = 7.4 Hz, 2H, Ar-H),
°
C. 1H
δ
3.87 (sept, J = 8 Hz, 2H, CH(CH3)2), 3.52 (br, 8H, OCH2CH2), 2.59 (s, 12H, CH3), 1.39 (br, 8H,
OCH2CH2), 0.75 (d, J = 6.9 Hz, 12H, CH(CH3)2), 0.10 (s, 18H, Si(CH3)3); 13C {1H} NMR (C6D6,
298 K, 100 MHz):
(OCH2CH2), 50.2 (N-iPr-CH), 25.7 (OCH2CH2), 24.7 (Ar-CH3), 23.0 (Ar-CH3), 20.5 (iPr-CH3),
20.2 (iPr-CH3), 6.4 (Si-C); 29Si {1H} NMR (C6D6, 298 K, 79 MHz):
δ 172.5 (NCN), 151.1 (Ar-C), 132.6 (Ar-C), 130.1 (Ar-C), 121.4 (Ar-C), 67.9
δ
1.89 (NSi(CH3)3). Despite
several attempts, accurate elemental analysis could not be obtained for compound 1
15

ACCEPTED MANUSCRIPT
Synthesis of compound 2
To a mixture of free ligand LH (0.5 g, 1.422 mmol) and KN(SiMe3)2 (0.57 g, 2.87 mmol) was
added THF (20 mL) and stirred at room temperature for 4 h. The resultant reaction mixture was
then added drop by drop to a stirred suspension of ZnCl2 (0.194 g, 1.422 mmol) in THF (~5 ml)
at –78 oC. The reaction mixture was slowly allowed to warm to room temperature and stirred for
24 h. All the volatiles were removed under vacuum. The solid residue was extracted with n-
hexane (~40 mL) and filtered through Celite. The volume of the clear solution was reduced
approximately to 10 mL and few drops of THF was added and kept it for crystallization at –30
oC. Colourless crystals suitable for X-ray diffraction analysis were obtained after three days. A
o
1
crystalline compound was isolated. Yield: 0.69 g (84 %); mp = 120 – 125 C. H NMR (C6D6,
298 K, 400 MHz) 6.97 (d, J = 7.4 Hz, 4H, Ar-H), 6.91-6.87 (m, 2H, Ar-H), 3.87 (sept, J = 6.6
δ
Hz 2H, CH(CH3)2), 2.40 (s, 3H, Ar-CH3), 2.27 (s, 9H, Ar-CH3), 0.68 (d, J = 7.0 Hz, 12H,
CH(CH3)2), 0.20 (s, 18H, Si(CH3)3); 13C{1H} NMR (C6D6, 298 K, 100 MHz):
δ
169.9 (NCN),
147.0 (Ar-C), 133.5 (Ar-C), 128.4 (Ar-C), 123.0 (Ar-C), 51.0 (N-iPr-CH), 24.1 (Ar-CH3), 18.9
29
(iPr-CH3), 5.1 (Si-C); Si {1H} NMR (C6D6, 298 K, 79 MHz):
δ 0.51 (NSi(CH3)3). Anal Calcd
for C29H50N4Si2Zn; C, 60.44; H, 8.75; N, 9.72. Found C, 60.29; H, 8.31, N, 10.09.
Synthesis of compound 3
The compound was synthesized by using a similar procedure to that employed for the
preparation of 2, but by using L1H [L1 = {Ar NC(NiPr2)NAr = 2,6- iPr2-C6H3)] (0.5 g, 1.08
′ ′} (Ar′
mmol), KN(SiMe3)2 (0.435 g, 2.18 mmol) and ZnCl2 (0.147 g, 1.07 mmol).Yield: 0.65 g (88
%).1H NMR (C6D6, 298 K, 400 MHz)
δ
7.10 (d, J = 1.8 Hz, 2H, Ar-H), 7.05 (t, J = 4 Hz, 4H, Ar-
H), 3.65 (sept, 6H, CH(CH3)2), 1.40 (d, J = 6.8 Hz, 6H, CH(CH3)2), 1.29 (d, J = 6.9 Hz, 3H,
CH(CH3)2), 1.24 (d, J = 6.9 Hz, 24H, CH(CH3)2), 0.75 (d, J = 7.1 Hz, 3H, CH(CH3)2), 0.20 (s,
16

ACCEPTED MANUSCRIPT
18H, Si(CH3)3); 13C{1H} NMR (100 MHz, C6D6, 25 °C):
δ 170.4 (NCN), 143.4(Ar-C), 143.0
(Ar-C), 133.7 (Ar-C), 125.5 (Ar-C), 124.0 (Ar-C), 123.7 (Ar-C), 51.0 (N-iPr-CH), 29.5 (iPr-
CH3), 28.2 (iPr-CH3), 25.4 (iPr-CH3), 23.9 (iPr-CH3), 23.4 (iPr-CH3), 22.9 (iPr-CH3), 5.6 (Si-C);
29Si{1H} NMR (80 MHz, C6D6, 25 °C):
δ 0.80 (NSi(CH3)3). Anal Calcd for C37H66N4Si2Zn: C,
64.55; H, 9.66; N, 8.14. Found C, 64.39; H, 9.82; 7.75.
General procedure for the intramolecular hydroamination of primary aminoalkenes.
Compound
1 catalysed hydroamination reactions
Primary aminoalkene (20 µL), catalyst (2-10 mol %), hexamethylbenzene (known amount used
as internal standard) and C6D6 (0.5 mL) were charged in a J. Young valve NMR tube inside the
1
glove box. The progress of the reaction was monitored by H NMR spectroscopy and NMR
1
yields were calculated by comparing the integration of well resolved H NMR signal of cyclic
amine with that of internal standard (hexamethyl benzene).
Compounds
2 and 3 catalysed hydroamination reactions
Primary aminoalkene (20 µL), catalyst (2.5–5 mol %), equimolar amount of co-catalyst
[PhNMe2H][B(C6F5)4] (2.5–5 mol %) hexamethylbenzene (known amount used as internal
standard) and C6D6 (0.5 mL) were charged in a J. Young valve NMR tube inside the glove box
Sealed NMR tube was taken out from the glovebox and heated at 80 oC on a pre-heated oil bath.
The progress of the reaction was monitored by 1H NMR spectroscopy.
General procedure for the intramolecular hydroamination of secondary aminoalkenes.
Secondary aminoalkene (20 µL), catalyst (5 mol %), co-catalyst [PhNMe2H][B(C6F5)4] (5 mol
%) and internal standard hexamethyl benzene (known amount) C6D6. (0.5 mL) were charged in a
17

ACCEPTED MANUSCRIPT
J. Young valve NMR tube inside the glove box. Sealed NMR tube was taken out from the
o
glovebox and heated at 80 - 120 C on a pre-heated oil bath. The progress of the reaction was
1
monitored by H NMR spectroscopy and NMR yields were calculated by comparing the
1
integration of well resolved H NMR signal of cyclic amine with that of internal standard
(hexamethyl benzene).
Crystallographic Details
Crystallographic data for compounds 1, 2 and 3 are summarized in Table 5. Suitable crystals of
compounds 1, 2 and 3 were removed from the Schlenk flask under an inert atmosphere and
immediately coated with silicon oil on a glass slide and mounted on a glass fiber, and then placed
quickly in a stream of liquid nitrogen on an X-ray diffractometer. X-ray diffraction data were
collected using a Bruker APEX-II CCD diffractometer equipped with an Oxford low-
temperature device, operating at T = 100 K. Data collection was monitored with Apex II
software, and preprocessing was done with SADABS [37] integrated with Apex II. Using Olex2
[38] the structures were solved with the ShelXT structure solution program using Direct Methods
and refined with the ShelXL refinement package using Least Squares minimisation [39]. All
non-hydrogen atoms were refined anisotropically. Hydrogen atom positions were calculated
geometrically and refined using the riding model.
18

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Table 5. Crystal data and structure refinement details of compounds 1, 2 and 3
1
2
3
CCDC
1835652
1835654
1835653
Empirical formula
Formula weight
Temperature
Crystal system
Space group
a (Å)
C37H66CaN4O2Si2
695.20
C29H50N4Si2Zn
576.28
C37H66N4Si2Zn
688.49
100(2) K
100(2) K
100(2) K
Monoclinic
P21/c
Orthorhombic
Pca21
Triclinic
ꢀ
1
P
18.302(18)
9.710(4)
17.428(7)
b (Å)
c (Å)
12.818(12)
17.682(15)
90
90
90
11.479(4)
16.187(7)
70.395(2)
84.806(2)
77.247(2)
1657.4(12)
12.746(5)
18.721(7)
90
101.187(2)
90
α
β
γ
(°)
(°)
(°)
V (Å3)
4148(2)
4080(2)
Z, Calculated density
3
3
3
4, 1.121 Mg/m
4, 1.113 Mg/m
2, 1.155 Mg/m
µ
/mm-1
F(000)
0.835
620
0.689
1496
0.243
1520
Theta range for data 2.23 to 25.30
collection/ o
1.34 to 25.75
1.19 to 25.50
Index ranges
–22
–10
–21
≤
≤
≤
h
k
l
≤
≤
≤
14,
5,
20
–11
–13
–19
≤
≤
≤l ≤ 19
h
k ≤
≤
11,
14,
–21
–15
–22
≤
h
≤
21,
15,
22
≤ k ≤
≤ l ≤
Reflections collected
unique
/
21514 / 6619 [R(int) =
0.0599]
48524 / 7598 [R(int) =
0.0434]
17884/6231 [R(int) =
19

ACCEPTED MANUSCRIPT
0.1456]
Completeness to theta
Absorption correction
99.1 %
98.6 %
99.9 %
Empirical
Empirical
Empirical
Max.
transmission
Data
parameters
Goodness-of-fit on F2
and
min. 0.7452 and 0.6422
0.7453 and 0.5967
0.7461 and 0.6652
/
restraints
/
6619 / 1 / 429
1.019
6231 / 0 / 339
0.923
7598 / 0 / 415
1.040
Final
[I>2sigma(I)]
R
indices R1 = 0.0451, wR2 =
0.1014
R1 = 0.0471, wR2 =
0.1059
R1 = 0.0307, wR2 =
0.0745
Acknowledgements
The authors thank National Institute of Science Education and Research (NISER), HBNI,
Bhubaneswar, Department of Atomic Energy (DAE), Govt. of India. A. B. acknowledges
University Grant Commission (UGC) Govt. of India for his fellowship.
Appendix A. Supplementary material
CCDC -1835652(1), 1835654 (2) and 1835653 (3) contain the supplementary crystallographic
data for this paper.
Appendix B. Supplementary material
Copies of 1H, 13C and 29Si NMR for 1, 2 and 3.
20

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Highlights:
•
•
•
•
Intramolecular hydroamination
Guanidinate complexes of calcium and zinc metals
Effective catalysts for hydroamination of amino alkenes
X-ray structures of heteroleptic calcium and zinc complexes
26