Phenalenyl-based organozinc catalysts for intramolecular hydroamination reactions: A combined catalytic, kinetic, and mechanistic investigation of the catalytic cycle

Article abstract of DOI:10.1002/chem.201200868

Herein, we report the synthesis and characterization of two organozinc complexes that contain symmetrical phenalenyl (PLY)-based N,N-ligands. The reactions of phenalenyl-based ligands with ZnMe₂ led to the formation of organozinc complexes [N(M

Full text of DOI:10.1002/chem.201200868

FULL PAPER
DOI: 10.1002/chem.201200868
Phenalenyl-Based Organozinc Catalysts for Intramolecular Hydroamination
Reactions: A Combined Catalytic, Kinetic, and Mechanistic Investigation of
the Catalytic Cycle
Arup Mukherjee,[a] Tamal K. Sen,[a] Pradip Kr. Ghorai,[a] Prinson P. Samuel,[b]
Carola Schulzke,[c] and Swadhin K. Mandal*[a]
Dedicated to Prof. B. M. Deb on the occasion of his 70th birthday
Chem. Eur. J. 2012, 00, 0 – 0
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Abstract: Herein, we report the synthe-
sis and characterization of two organo-
zinc complexes that contain symmetri-
cal phenalenyl (PLY)-based N,N-lig-
actions. The progress of the catalysis
for primary and secondary aminoal-
kene substrates with catalyst 2 was in-
vestigated by detailed kinetic studies,
including kinetic isotope effect meas-
urements. These results suggested
pseudo-first-order kinetics for both pri-
mary and secondary aminoalkene acti-
vation processes. Eyring and Arrhenius
analyses for the cyclization of a model
secondary aminoalkene substrate af-
forded DH° =11.3 kcalmolÀ1, DS° =
supported the formation of a catalyti-
cally active zinc cation and the DFT
calculations showed that more active
catalyst 2 generated a more stable
cation. The stability of the catalytically
active zinc cation was further support-
ed by an in situ recycling procedure,
thereby confirming the retention of
catalytic activity of compound 2 for
successive catalytic cycles. The DFT
calculations showed that the preferred
pathway for the zinc-catalyzed hydro-
ACHTUNGTRENNUNGands. The reactions of phenalenyl-
based ligands with ZnMe2 led to the
formation of organozinc complexes
[N(Me),N(Me)-PLY]ZnMe (1) and [N-
ACHTUNGTRENNUNG(iPr),NACHTUNGTRENNUNG(iPr)-PLY]ZnMe (2) under the
evolution of methane. Both complexes
(1 and 2) were characterized by NMR
spectroscopy and elemental analysis.
The solid-state structures of complexes
1 and 2 were determined by single-
crystal X-ray crystallography. Com-
plexes 1 and 2 were used as catalysts
for the intramolecular hydroamination
of unactivated primary and secondary
aminoalkenes. A combined approach
of NMR spectroscopy and DFT calcu-
lations was utilized to obtain better in-
sight into the mechanistic features of
the zinc-catalyzed hydroamination re-
À35.75 calKÀ1 molÀ1,
and
Ea =
ACHTUNGTRENUNaNG mination reactions is alkene activation
11.68 kcalmolÀ1. Complex 2 exhibited
much-higher catalytic activity than
complex 1 under identical reaction con-
ditions. The in situ NMR experiments
rather than the alternative amine-acti-
vation pathway. A detailed investiga-
tion with DFT methods emphasized
that the remarkably higher catalytic ef-
ficiency of catalyst 2 originated from
its superior stability and the facile for-
mation of its cation compared to that
derived from catalyst 1.
Keywords: density functional calcu-
lations · homogeneous catalysis ·
hydroamination
AHCTUNGTRENNaGUN nds · zinc
· phenalenyl lig-
Introduction
been an enormous effort to develop an efficient catalyst for
this demanding transformation. Several catalyst systems,
which are composed of main-group elements,[2] rare-earth
metals,[3] and transition metals,[4] have been used successfully
to carry out such transformations.[1] In this context, there
has been a growing trend to use low-cost, non-toxic metals,
such as zinc, in the hydroamination reaction. Zinc is an es-
sential, non-toxic, and environmentally friendly metal, as
shown by the presence of about 2 g of zinc ions in the body
of an adult human.[5] In 2005, Roesky, Blechert, and co-
workers first reported the use of an organozinc complex,
[N-isopropyl-2-(isopropylamino)troponiminato]methylzinc
[{(iPr)2ATI}-ZnMe] (ATI=aminotroponiminato), as a cata-
lyst for the hydroamination of aminoalkenes and aminoal-
The hydroamination reaction, which involves the addition of
an amine across a carbon–carbon multiple bond, leads to
the formation of nitrogen-containing heterocyclic mole-
cules.[1] The synthesis of these nitrogen-containing molecules
is of particular interest in academia, as well as in industry,
because these nitrogen-containing molecules are important
as natural products, pharmacological agents, fine chemicals,
and dyes. At present, most of these molecules are synthe-
sized by using multistep procedures. However, hydroamina-
tion offers an alternative strategy to synthesize these nitro-
gen-containing molecules in a more environmentally friend-
ly and atom-economical way. Over the last decade, there has
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
activity of the organozinc catalyst. The activator abstracts
the alkyl group that is attached to zinc, thereby generating
the active cationic zinc intermediate.[6b] Following this study,
several efforts have been made in recent years that use zinc-
based catalysts for intramolecular hydroamination reac-
tions.[6] However, the exact mechanistic pathway for zinc-
catalyzed hydroamination reactions has not yet been eluci-
dated in detail. In principle, there are two possible mecha-
nisms for the zinc-catalyzed hydroamination reaction: 1) ac-
[a] 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)
Fax : (+91)33-48092033
E-mail: swadhin.mandal@iiserkol.ac.in
[b] Dr. P. P. Samuel
Institut fꢁr Anorganische Chemie
Georg-August Universitꢂt Gçttingen
Tammannstrasse 4, 37077 (Germany)
À
tivation of the C C multiple bond followed by nucleophilic
[c] Dr. C. Schulzke
attack by the amine moiety or 2) activation of the amine,
which results in a metal–amide complex, followed by an
olefin-insertion reaction (Scheme 1).[1f,6b] Several studies
have reported that hydroamination reactions with lantha-
School of Chemistry, Trinity College Dublin
Dublin 2 (Ireland)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200868.
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Organozinc Catalysts for Intramolecular Hydroaminations
FULL PAPER
as detailed kinetic studies by NMR spectroscopy, to under-
stand the mechanistic pathway for the zinc-catalyzed hydro-
amination reaction. Herein, we report the synthesis and
characterization
[N(Me),N(Me)-PLY]ZnMe
of
two
organozinc
and
complexes,
(iPr),N(iPr)-
(1)
[N
N
ACHTUNGTRENNUNG
PLY]ZnMe (2), that contain symmetrical phenalenyl (PLY)-
based N,N-ligands. The catalytic activity of complex 2 was
remarkably higher than that of complex 1. DFT calculations
disclosed that the formation and stability of cations from
these organozinc complexes play a key role in their relative
catalytic activity. To better understand the catalytic cycle,
more-detailed DFT calculations were carried out on the key
intermediates in the reaction, as well as on various transition
states (TSs). These calculations reveal that such transforma-
tions proceed through an olefin-activation mechanism. Ki-
netic studies, including kinetic isotope effects, were carried
out on the cyclization of primary and secondary aminoal-
kenes by using catalyst 2, which showed pseudo-first-order
kinetics in the cyclization process.
Results and Discussion
Synthesis of organozinc complexes: The syntheses of
Scheme 1. Plausible mechanistic pathways for the zinc-catalyzed intramo-
lecular hydroamination reactions.
[N(Me),N(Me)-PLY]ZnMe
PLY]ZnMe (2) were accomplished by reacting 9-N-methyl-
amino-1-N’-methyliminophenalene [HN(Me),N(Me)-PLY]
and 9-N-isopropylamino-1-N’-isoproyliminophenalene [HN-
(iPr),N(iPr)-PLY] with ZnMe2 at 258C and 1108C, respec-
tively (Scheme 2). A solution of [HN(Me),N(Me)-PLY] or
[HN(iPr),N(iPr)-PLY] in toluene was added drop-by-drop to
(1)
and
[NACTHNGUTRENN(UG iPr),NACHTUNGTRENNUNG(iPr)-
AHCTUNGTRENNUNG
nides, early transition metals, and Group 2-metal-based cata-
A
ACHTUNGTRENNUNG
[1g,2
À
lysts proceed through the activation of the N H bond.
d,4a,d] On the other hand, late-transition-metal-based catalysts
U
ACHTUNGTRENNUNG
À
promote C C multiple bond activation with concomitant
a solution of ZnMe2 in toluene in a 1:1.2 stoichiometric
ratio at À788C, and the mixture was stirred at 258C for 3 h
(for 1) or heated at 1108C for 24 h (for 2) to yield complex
1 or 2. The H NMR spectra of both mixtures reveal clean
and almost-quantitative conversion of the reactants into the
products, which can be concluded from the absence of any
attack by the nitrogen nucleophile.[4k,o] However, it has not
yet been established exactly which of the two mechanistic
pathways is operative in the case of the zinc-catalyzed hy-
droamination reaction.[6b,o]
1
As a part of our ongoing interest in the development of
hydroamination catalysts, we recently reported that a hetero-
bimetallic complex, based on Group 2 and Group 4 metals,
can successfully cyclize both unactivated primary and secon-
dary aminoalkenes in a dual fashion.[7] Herein, we report
the use of phenalenyl-based ligands for developing organo-
zinc hydroamination catalysts. Phenalenyl is a well-known
odd-alternant hydrocarbon with high symmetry (D3h) that
has been used as a building block for preparing intriguing
materials as new conjugated electronic systems, such as mul-
tifunctional electronic and magnetic materials, that exhibit
simultaneous bistability in multiple physical channels and
the highest room-temperature conductivity of any neutral
organic solids.[8] Recent articles by Morita, Takui, and co-
workers, and by Hicks, document the state-of-the-art status
of phenalenyl-based materials chemistry.[9] Recently, we ini-
tiated a program with phenalenyl-based ligands for design-
ing molecular catalysts[10] because phenalenyl systems may
have similarities to the well-studied aminotroponimate
ligand system in terms of their metal-binding ability.[11] We
utilized density functional theory (DFT) calculations, as well
À
characteristic N H resonance at d=13.1 or 13.7 ppm in
C6D6, respectively. Complexes 1 and 2 were characterized
by 1H and 13C NMR spectroscopy and elemental analysis.
The solid-state structures of complexes 1 and 2 were deter-
1
mined by single-crystal X-ray crystallography. The H NMR
spectra of complexes 1 and 2 in C6D6 exhibited singlets at
Scheme 2. Synthesis of organozinc complexes 1 and 2 with phenalenyl lig-
AHCTUNGTERGaNNUN nds.
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S. K. Mandal et al.
d=À0.20 and 0.04 ppm, respectively, which are due to the
methylzinc (1.96 ꢃ).[6a] The N-Zn-C bond angles in com-
plexes 1 (average 132.28) and 2 (average 133.38) are smaller
than those in [N-isopropyl-2-(isopropylamino)troponimi-
proton resonances that arise from the methyl group that is
À
bound to the zinc center. The Zn Me resonance is shifted
upfield in both complexes 1 and 2 when compared to that in
ZnMe2 (d=0.51 ppm).[6k] The 13C NMR spectra of com-
plexes 1 and 2 reveal resonances at d=À4.3 and À5.9 ppm,
respectively, which are due to the methyl carbon atom that
is bound to the zinc center; this resonance is also shifted fur-
ther upfield than that in ZnMe2 (d=À4.2 ppm).[6k]
nato]methylzinc (N-Zn-C average 139.058),[6a] thereby
ACHTUNGTRENNUNG
making the N1-Zn1-N2 bond angles (95.48(8)8 in 1 and
93.34(9)8 in 2) wider than that in [N-isopropyl-2-
(isopropylamino)tropACHTNUTGRNEUNG
oniminato]methylzinc (81.948).[6a] The
À
Zn C distances in complexes
(1.955(3) ꢃ) are within the expected range that was ob-
1 (1.954(3) ꢃ) and 2
served in previously characterized methylzinc complexes.[6a,o]
À
X-ray crystal structures of complexes 1 and 2: Crystals of
complexes 1 and 2 were obtained from solutions in toluene
at 08C and 258C, respectively. The molecular structures
were determined by single-crystal X-ray crystallography
(Figure 1). Complexes 1 and 2 crystallized in the triclinic
The C N distances in complexes 1 (average 1.333 ꢃ) and 2
(average 1.334 ꢃ) are in between the C=N double-bond
(1.270 ꢃ)[12] and C N single-bond lengths (1.485 ꢃ).
In
[12]
À
the solid state, the molecules of complexes 1 and 2 are
packed into layers, owing to their planar geometry, with the
phenalenyl moiety maximizing the intermolecular interac-
tions (see the Supporting Information).
¯
space group P1 with one and two molecules in the asymmet-
ric unit, respectively. The X-ray structures of complexes
1 and 2 reveal a trigonal-planar geometry around the zinc
center, with two nitrogen atoms of the phenalenyl ligand
and one carbon atom of the methyl group coordinated to
Catalytic intramolecular hydroamination reactions: The in-
creasing demand for efficient catalysts for the hydroamina-
tion reaction prompted us to test the catalytic activity of
complexes 1 and 2 for intramolecular hydroamination reac-
tions. Previous studies have demonstrated that organozinc-
based complexes can catalyze the intramolecular hydroami-
nation of aminoalkenes.[6a–c,l,o] Recent developments have es-
tablished that organozinc complexes, such as [N-isopropyl-2-
À
À
zinc. The Zn N distances in complexes
1
2
(Zn1 N1
À
À
1.929(2) ꢃ, Zn1 N2 1.9353(19) ꢃ) and
(Zn1 N1
À
1.954(2) ꢃ, Zn1 N2 1.954(2) ꢃ) are comparable to the dis-
tances in [N-isopropyl-2-(isopropylamino)troponiminato]-
(isopropylamino)troponiminato]methylzinc
[{(iPr)2ATI}-
ZnMe] (ATI=aminotroponiminato) and [2-(isopropylami-
no)troponate]methylzinc [(iPrAT)Zn-Me]2 (AT=amino-
troponate), can catalyze the hydroamination of primary ami-
noalkenes.[6a,n] The successful synthesis of complexes 1 and
2, which contain a methylzinc moiety, gave us the opportuni-
ty to test their efficacy for the intramolecular hydroamina-
tion of aminoalkenes.
Intramolecular hydroamination of primary aminoalkenes:
We began our study with unactivated primary aminoalkenes.
Initially, we performed the intramolecular hydroACTHNUTRGNEUGaN mination
reaction of (1-allylcyclohexyl)methylamine (3) with com-
plexes 1 and 2 in C6D6 whilst maintaining the bath tempera-
ture at 1208C without adding any activator. After heating
for 24 h, we found <5% of the hydroamination product
with complex 2 and no detectable resonances of the hydro-
1
N
1. To increase the catalytic activity of complexes 1 and 2 to-
wards primary aminoalkenes, the reaction was carried out
with an equimolar amount (with respect to the catalyst) of
Figure 1. Molecular structures of organozinc complexes a) 1 and b) 2;
thermal ellipsoids are set at 50% probability level and hydrogen atoms
are omitted for the sake of clarity. Selected distances [ꢃ] and angles [8]
the activator [PhNMe2H][BACHTUNGRTNEUNG(C6F5)4]; we found that, after
8.5 h at 1208C in C6D6, catalyst 2 afforded 91% conversion.
This increase in activity may be attributed to the in situ gen-
eration of the coordinatively unsaturated cationic zinc spe-
cies, in which the activator acts as a methyl-abstracting
agent. Earlier studies on the zinc-catalyzed hydroamination
reaction have established the role of an externally added ac-
tivator, indicating the formation of catalytically active cat-
ionic species.[6a–c,o] The reactions of catalysts 1 and 2 with
dry and degassed primary aminoalkenes proceeded regio-
À
À
À
in complex 1: Zn1 N1 1.929(2), Zn1 N2 1.9353(19), Zn1 C1 1.954(3),
À
À
À
À
N1 C2 1.472(3), N2 C3 1.468(3), N1 C4 1.335(3), N2 C14 1.331(3); N1-
Zn1-C1 131.54(10), N2-Zn1-C1 132.90(10), N1-Zn1-N2 95.48(8), C2-N1-
Zn1 114.90(15), C3-N2-Zn1 114.84(15), C4-N1-Zn1 126.06(15), C14-N2-
À
Zn1 126.26(15). Selected distances [ꢃ] and angles [8] in complex 2: Zn1
À
À
À
À
N1 1.954(2), Zn1 N2 1.954(2), Zn1 C20 1.955(3), N1 C1 1.333(3), N1
À
À
C17 1.478(4), N2 C11 1.335(3), N2 C12 1.481(3); N1-Zn1-C20
133.70(12), N2-Zn1-C20 132.89(12), N1-Zn1-N2 93.34(9), C1-N1-Zn1
120.42(18), C11-N2-Zn1 122.67(19), C17-N1-Zn1 116.93(17), C12-N2-Zn1
115.24(17).
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Organozinc Catalysts for Intramolecular Hydroaminations
FULL PAPER
specifically. All of the substrates were converted into their
corresponding cyclic products by using 5 mol% loading of
the catalyst and activator at 1208C in C6D6.
The catalytic activities of complexes 1 and 2 with different
aminoalkenes are summarized in Table 1. Examination of
the data emphasizes that catalyst 2 has a much higher activi-
Table 1. Intramolecular hydroamination of unactivated primary amino-
ACHTUNGTRENNUNG
alkenes.[a]
Substrate
Product
Cat.
t
Conv.
[%][b]
[h]
1
1
12
7.5
2
3
2
1
98(90)[c]
15
8.5
4
2
91
5
6
7
1
2
1
2
97
12
24
96
8
9
2
1
98
14
110
10
2
97
[a] Reaction conditions: amine (20 mL), catalyst (5 mol%), and activator
[PhNMe2H][B(C6F5)4] (5 mol%) in C6D6 (0.6 mL) at 1208C. [b] Deter-
mined by H NMR spectroscopy against an internal standard. [c] Yield of
ACHTUNGTRENNUNG
Figure 2. Stacked 1H NMR spectra (C6D6) for the cyclization of (1-allyl-
cyclohexyl)methylamine (3) with catalyst 2 after certain time intervals.
1
isolated product after purification. Cat.=catalyst; Conv.=conversion.
ty than catalyst 1 under identical reaction conditions. In ad-
dition, substrates that contain bulky substituents at the b po-
sition with respect to the amine moiety (Thorpe–Ingold
effect) are transformed into their corresponding heterocyclic
products more swiftly (Table 1, entries 2 and 4). 2,2-Diphen-
The higher catalytic activity of complex 2 compared to com-
plex 1 may be attributed to the electronic stability of the in-
situ-generated zinc cation (see later). Complex 2 showed
comparable or higher activity than that of a previously re-
ported organozinc complex in the presence of an acti-
ylpent-4-en-1-amine (Table 1, entry 2) was the most reactive
vator.[6a,n]
ACHTUNGTRENNUNG
substrate, thereby forming its corresponding pyrrolidine
within 7.5 h; the yield of isolated product after column chro-
matography was 90% with catalyst 2, which was in good
Intramolecular hydroamination of secondary aminoalkenes:
Based on the promising reactivity displayed by complex 2
towards primary aminoalkenes, we postulated that it would
also be effective for the intramolecular hydroamination of
unactivated secondary aminoalkenes. To test the catalytic
activities of compounds 1 and 2 towards secondary aminoal-
kenes, we performed the catalytic reactions with a number
of secondary aminoalkenes in C6D6 at 808C with 5 mol%
loading of the catalyst and activator (Table 2). 4-(Bromo-
benzyl)(2,2-diphenyl-4-pentenyl)amine (Table 2, entries 1
and 2) cyclized efficiently with 99% conversion within 2.5 h
by using catalyst 2 and an equimolar amount of the activa-
tor, whilst the conversion with catalyst 1 under identical
conditions was only 5%. The yield of isolated product after
column chromatography (Table 2, entry 2) was 92% with
catalyst 2, which is in good agreement with the conversion
1
agreement with that observed by H NMR spectroscopy. In
the case of (1-allylcyclohexyl)methylamine (3; Table 1,
entry 4), 8.5 h were necessary to achieve 91% conversion.
Substitution with a methyl group at the internal position of
the olefin moiety had a significant negative impact on the
reaction rate (Table 1, entries 7–10) and difficulties in cycli-
zation, for example, resulting in the conversion of 4-methyl-
2,2-diphenylpent-4-en-1-amine into 2,2-dimethyl-4,4-diphen-
ACHTUNGTRENNUNGylpyrrolidine taking 96 h at 1208C (Table 1, entry 8). In situ
NMR studies of the cyclization of compound 3 with catalyst
2 in C6D6 at 1208C revealed a clean conversion of the sub-
strate to afford heterocyclic hydroamination product 4.
Figure 2 shows that the resonances of the olefinic protons of
the aminoalkene moiety disappear with time, whilst the pro-
tons that correspond to the hydroamination product grow.
1
that was observed by H NMR spectroscopy of the reaction
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S. K. Mandal et al.
Table 2. Intramolecular hydroamination of unactivated secondary amino-
ACHTUNGTRENNUNG
alkenes.[a]
either a halogen group or a methoxy group at the para posi-
tion of the phenyl ring, survived under the reaction condi-
tions (Table 2, entries 2 and 6). Again, the catalytic activity
of catalyst 2 was remarkably higher than that of catalyst 1;
a similar observation was also noted in the case of the acti-
Substrate
Product
Cat.
t
Conv.
[h] [%][b]
1
2
1
2
5
2.5
vation of primary aminoalkenes (Table 1). Complex
2
99(92)[c]
showed comparable activity to that of a previously reported
organozinc complex in the presence of an externally added
activator.[6l,o]
3
1
2
8
4.0
4[d]
96
Kinetic studies of hydroamination reactions: We carried out
detailed kinetic studies to gain further insight into the cycli-
zation of primary and secondary aminoalkenes catalyzed by
complex 2 in C6D6. Kinetic studies of the representative cy-
5
6
1
2
6
2.5
0.5
98
AHCTUNGTRENNUNG
7
8
1
2
3
AHCTUNGTRENNUNG
evolution of the specific resonances of the heterocyclic prod-
99
1
ucts was monitored by H NMR spectroscopy relative to an
internal standard over the course of the first 7 h and 2.5 h
for cyclization of primary and secondary aminoalkenes, re-
spectively.
We carried out detailed kinetic studies for the cyclization
of secondary aminoalkene benzyl(2,2-diphenyl-4-pentenyl)-
9
1
2
5
2.5
3.5
10
99
11
12
1
2
12
97
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
check the order of the reaction for the cyclization of com-
pound 5, we carried out the reaction with 2.69 mm of com-
plex 2 and 53.97 mm of compound 5. A plot of lnACHTUNGTRENNUNG(C/C0)
versus time provided a straight line with a negative slope
(Figure 3a), thus revealing a pseudo-first-order rate for the
cyclization process. To determine the order of the reaction
13
14
1
2
8
1.0
3.2
97
15
16
1
2
9
98
17
18
1
2
8
16
96
19
20
1
2
2
2.5
97
[a] Reaction conditions: amine (20 mL), catalyst (5 mol%), and activator
[PhNMe2H][BACHTUNGTRENNUNG(C6F5)4] (5 mol%) in C6D6 (0.6 mL) at 808C. [b] Determined
by 1H NMR spectroscopy against an internal standard. [c] Yield of isolated
product after purification. [d] Reaction carried out by heating the NMR
probe.
mixture. The secondary aminoalkene that contains a thio-
phene moiety shows a higher reaction rate than the corre-
sponding aminoalkene with a furan moiety (Table 2, en-
tries 8 and 10). This increase may be attributed to a more ef-
fective chelation of the zinc catalyst with the aminofuran
moiety. Secondary aminoalkene substituents, those that have
Figure 3. Kinetic studies on the cyclization of a secondary aminoalkene
monitored by H NMR spectroscopy (C6D6): a) pseudo-first-order plot of
the cyclization of compound 5; b) vanꢄt Hoff plot for the cyclization of
compound 5 catalyzed by complex 2; c) plot of kobs versus [Sub] for the
cyclization of compound 5; and d) H/D KIE for the cyclization of com-
pounds 5 and [D]-5.
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Organozinc Catalysts for Intramolecular Hydroaminations
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with respect to the catalyst, we carried out the cyclization of
compound 5 with various concentrations of complex 2 (1.35
to 5.40 mm) whilst keeping the concentration of substrate 5
fixed (53.97 mm). A plot of kobs versus the concentration of
the catalyst revealed a linear increase in the reaction rate
with catalyst concentration, thereby confirming a first-order
dependency with respect to the catalyst concentration (see
the Supporting Information). This result was further con-
firmed by the vanꢄt Hoff plot (Figure 3b). A plot of lnkobs
Eyring analysis for the cyclization of compound 5 afforded
DH° =11.3 kcalmolÀ1 and DS° =À35.75 calKÀ1 molÀ1 (Fig-
ure 4a), whereas Arrhenius analysis afforded Ea =11.68 kcal
molÀ1 (Figure 4b).
versus lnACHTUNGTRENNUNG[Cat] provided a linear graph (Figure 3b) and the
value of the slope was 1.05 (slope=order of the reaction).[13]
We also examined the order of the reaction with respect to
substrate 5 by keeping the concentration of complex 2 fixed
(2.67 mm) and varying the concentration of substrate 5 from
53.97 mm to 150.63 mm (Figure 3c). We found a decrease in
the reaction rate with increasing initial concentration of
compound 5, thus indicating an inverse-order dependence
on the substrate concentration; a similar observation was
also made in the process of cyclization of the primary ami-
noalkene with catalyst 2 (see the Supporting Information).
Next, the kinetic isotope effect (KIE) for the cyclization
process was investigated to gain further insight into the hy-
droamination reaction. The determination of the KIE is
a very useful mechanistic tool that has already been applied
to organometallic systems.[14] H/D KIE experiments were
carried out under identical reaction conditions, by using
compounds 5 and [D]-5 with 5 mol% of catalyst 2 and
Figure 4. a) Eyring plot for the cyclization of compound 5 catalyzed by
complex 2 in C6D6 over the temperature range 50–808C with the follow-
ing concentrations: [Sub]=53.97 mm, [Cat]=2.67 mm; b) Arrhenius plot
for the cyclization of compound 5 catalyzed by complex 2 in C6D6 over
the temperature range 50–808C with the following concentrations:
[Sub]=53.97 mm, [Cat]=2.67 mm.
In summary, the kinetic analyses of the cyclization of
compounds 3 and 5 catalyzed by complex 2 revealed that
both cyclization processes of primary and secondary amino-
alkenes follow similar kinetics with overall pseudo-first-
order reaction rates. The reaction rates exhibit first-order
dependence on the concentration of the catalyst and in-
verse-order dependence with respect to the concentration of
the substrates. The KIE study on primary and secondary
5 mol% of [PhNMe2H][BACHTNUTRGEN(UGN C6F5)4] (Scheme 3). The first-
order plots of the cyclization reactions of compounds 5 and
[D]-5 in C6D6 at 808C indicate that kobs =0.01092 minÀ1 and
obs =0.00547 minÀ1, respectively, which translate into a KIE
aminoalkenes suggests that the N H bond plays a key role
À
k
of 2.0 (Figure 3d). This observation suggests that hydrogen,
which is derived from the NH group of compound 5, is in-
volved in one of the key steps of the cyclization process,
similar to that observed for the cyclization of the primary
aminoalkene with catalyst 2 (see the Supporting Informa-
tion).
in the cyclization process, that is, it is involved in one of the
rate-limiting steps in the catalytic cycle.
Mechanistic investigation—combined NMR and computa-
tional studies: Previous studies have demonstrated that, in
the presence of [PhNMe2H][B
ACHTNUGTNER(UNNG C6F5)4] as an activator, or-
AHCTUNGTREGgNNUN anozinc complexes exhibit good catalytic activity for pri-
mary and secondary aminoalkenes.[6a–c,n,o] The activator ab-
stracts the methyl group that is attached to the zinc center
to generate a zinc cation in situ, which functions as the cata-
lytically active species. The addition of an activator to the
organozinc complex greatly enhances its catalytic activi-
ty,[6a–c,n,o] which is superior to the catalytic activity of the acti-
vator alone.[15] Herein, the results of the catalytic intramo-
lecular hydroamination of primary and secondary aminoal-
kenes reveal that catalyst 2 has a remarkably higher reactivi-
ty than that of catalyst 1 under identical reaction conditions
(Tables 1 and 2). To understand this notable difference in
catalytic activity and to understand the mechanistic pathway
for the cyclization process, we carried out an in situ NMR
study, as well as a detailed computational study. The in situ
NMR experiments revealed that, after the addition of the
activator in equimolar amounts (Scheme 4), the ZnMe reso-
nance in complex 2 (d=0.04 ppm) vanishes completely and
Scheme 3. Catalytic reaction of complex 2 with benzyl(2,2-diphenyl-4-
pentenyl)amine (5) and benzyl(2,2-diphenyl-4-pentenyl)amine-d ([D]-5)
in C6D6 at 808C.
Activation parameters for the cyclization of compound 5:
To obtain the activation parameters for the cyclization of
compound 5 by using catalyst 2 in C6D6, a series of reactions
was carried out over a range of temperatures (50–808C). In
each case, a first-order rate constant was obtained from the
resonances at d=0.15 and 2.5 ppm appear that are due to
[16]
plots of ln
N
the generation of CH4
and PhNMe2, respectively, thus
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S. K. Mandal et al.
Scheme 4. Synthesis of the zinc-centered cations 7 and 8.
confirming the role of the activator as a methyl abstractor
and the generation of a zinc cation (Figure 5). Previously,
Roesky and co-workers demonstrated that the use of an
equimolar activator with respect to the methylzinc complex
generated a zinc cation in situ, which, in turn, facilitated the
intramolecular hydroamination reactions of unactivated
aminoalACHTUNGTRENNUNG
kenes.[6b] Earlier, Bochmann and co-workers isolated
a similar type of zinc cation from its corresponding zinc–
Figure 6. Computed HOMOs and LUMOs of the zinc-centered cations of
compounds 7 and 8.
alkyl complex by using a boron-based Lewis acceptor.[17]
puted HOMO–LUMO energy gap of zinc-centered cations
7 and 8 are 64.6 and 70.6 kcalmolÀ1, respectively, thus sup-
porting a higher stability of cation 8 over that of cation 7
(see the Supporting Information).[18]
To further understand the stability of the catalytically
active cation, we investigated the activity of in-situ-generat-
ed cation 8 in successive catalytic cycles. One of the major
problems in homogeneous catalysis is the inability to recycle
the catalysts because they are inseparable from the reaction
mixture. Herein, we monitor the longevity of catalyst 2 by
performing several catalytic runs within the same reaction
vessel to test whether the catalyst remained live over several
catalytic cycles.[19] Four successive catalytic runs were carried
out by using 5 mol% of catalyst 2 and equimolar amounts
of [PhNMe2H][BACHTNUGTRENG(UN C6F5)4] and secondary aminoalkene sub-
Figure 5. 1H NMR spectra of a) [N
b) [N(iPr),N(iPr)-PLY]ZnMe with [PhNMe2H][BACHTGNUTRENNUNG
reveal the absence of the ZnMe resonance and the appearance of CH4
and PhNMe2 resonances.
ACHUTGTNRNEUG(N iPr),NACHTGNUTRENNUNG
strate 5 with respect to an internal standard (hexamethyl
benzene) in C6D6 at 808C by using an in situ recycling pro-
cedure. After complete consumption of the substrate,
a fresh batch of substrate was introduced without introduc-
ing any additional catalyst. After each 4 h interval, the con-
version into the product was checked by recording
G
E
To understand the difference in catalytic activity between
complexes 1 and 2 for the hydroamination reaction, we car-
ried out a computational study on complexes 1 and 2, as
well as on their corresponding cations (7 and 8, Scheme 4),
which were generated after the addition of an equimolar
amount of the activator to solutions of complexes 1 and 2,
respectively. The change in Gibbs free energy (DG) for the
formation of zinc-centered cations 7 and 8 are calculated as
À2.4 and À21.6 kcalmolÀ1, respectively. This result indicates
that the formation of cation 8 is more-favored than that of
cation 7. The frontier orbitals (HOMO and LUMO of cat-
ions 7 and 8), as ascertained by DFT calculations, are shown
in Figure 6. The LUMOs of cations 7 and 8 clearly indicate
that they are primarily zinc-centered. Furthermore, the com-
1
a H NMR spectrum of the reaction mixture. The catalytic
runs were repeated for four successive cycles. We found that
catalyst 2 remained catalytically active in up to four consec-
utive runs (Figure 7); however, a gradual decrease in the ac-
tivity was noted after each 4 h interval by NMR spectrosco-
py. Nevertheless, this result clearly indicates that the catalyt-
ically active species remains live for several consecutive
runs of the catalytic cycles.
Although there have been several reports on hydroamina-
tion reactions with zinc catalysts in recent years,[6] there
have not been any reports on the mechanistic pathways for
zinc-catalyzed hydroamination reactions. The plausible
mechanistic pathways[6b,o] for the zinc-catalyzed hydroamina-
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Organozinc Catalysts for Intramolecular Hydroaminations
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was subjected to computational examination. To corroborate
the kinetic data and to gain further insight into the nature
of the intermediates in the catalytic cycle, we carried out de-
tailed density functional theory (DFT) calculations to inves-
tigate the mechanism of the intramolecular hydroamination
of a model substrate (5). During these calculations, no struc-
tural simplification of any of the key species was imposed
and the data reported herein are focused exclusively on the
most-accessible pathways. DFT calculations have previously
been utilized for unraveling the mechanistic cycle of the hy-
droamination reaction by several groups by using different
metal catalysts.[22]
The calculations were started by examining the possibility
of different ligation modes of the aminoalkene substrate (5)
to coordinatively unsaturated zinc-centered cation 8, which
was generated from complex 2 (Figure 8). In principle, the
substrate can attach to the zinc center in different ways:
through a chelating aminoalkene (AA), a bis-hapto associa-
tion to the zinc center through its olefinic bond (A), or
through its amine functionality (A1). The changes in the
computed Gibbs free energy (DG) for the formation of
structures A and A1 are uphill by 2.1 and 3.8 kcalmolÀ1, re-
spectively, relative to that of AA. This subtle difference in
DG indicates the possibility of these species existing in rapid
equilibrium. Species A and A1 were used as starting points
for the alkene- and amine-activation cycles, respectively, and
the AA species is referred to as the catalyst in its resting
state,[4k] from which A or A1 can be generated.
Figure 7. Catalyst-longevity test for the cyclization of compound 5 with
catalyst 2 in C6D6 at 808C.
tion reaction may involve either activation of the C=C
double bond (also known as alkene activation) or activation
À
of the N H single bond (also known as amine activation,
Scheme 5).[6b] Earlier studies have reported that hydroami-
nation with catalysts of rare-earth metals, early transition
metals, and Group 2 metals proceed through the amine-acti-
vation pathway,[1g,2d,4a,d] whilst late-transition-metal-based
catalysts follow the alkene-activation pathway.[4k,o] Thus, in
this case, zinc catalysts 1 and 2 may follow either the activa-
tion of the C=C bond (alkene activation; Scheme 5, left)[20]
À
or the activation of the N H bond (amine activation;
Scheme 5, right).[21]
Given the lack of spectroscopic evidence of the catalytic
reaction mixture at various temperatures, which would
enable the definite identification of catalytic intermediates
and to gain further insight into the details of the mechanism
of the zinc-catalyzed hydroamination reaction, despite our
several attempts by using NMR spectroscopy, this process
Catalytic hydroamination through activation of the C=C
bond (alkene activation): The catalytic hydroamination of
secondary aminoalkenes by the alkene-activation pathway
includes (Scheme 5, left): 1) coordination of the olefin
moiety to the metal center; 2) nucleophilic attack of the
amine on the olefin moiety that is preactivated by the co-
Scheme 5. Plausible mechanistic pathways for the intramolecular hydroamination of model substrate 5 by using cation 8, which was generated from cata-
lyst 2; counterions are not shown for the sake of clarity.
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S. K. Mandal et al.
AHCTUNGTRENNUNG
Intermediate B carries the hydrogen atom on the quater-
nized ammonium center, which undergoes a proton transfer
from the ammonium unit to the carbon atom that is at-
Figure 8. Various forms of the catalytically competent species (computed
Gibbs free energy changes in kcalmolÀ1 relative to AA are given in pa-
rentheses); counterions are not shown for the sake of clarity.
À
tached to the zinc center. Cleavage of the Zn C bond leads
to the formation of intermediate C. In the most-accessible
pathway that commences from intermediate B, the ammoni-
Figure 9. Free-energy profile for the intramolecular hydroamination of compound 5 by catalyst 8 through an alkene-activation pathway. Only the most-
accessible steps are shown; counterions are not shown for the sake of clarity.
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À
um unit is suitably oriented toward the zinc center, such
that its deprotonation does not cause major structural reor-
ganization (TS [B-C], Figure 9). Transformation from B into
C proceeds through TS [B-C], which involves an energy bar-
rier of 22.8 kcalmolÀ1. Thereafter, the incoming substrate
displaces the desired pyrrolidine product (6) from inter-
mediate C and regenerates the catalytically active species
(A), which again participates in the catalytic cycle. Transfor-
mation of intermediate C into catalytically active species A
involves a transition state that constitutes the zinc complex,
substrate (5), and cyclic pyrrolidine 6 (TS [C+5A+6]). The
transformation from C into A involves an energy barrier of
10.9 kcalmolÀ1. Transformation from B into C involves the
highest uphill energy (22.8 kcalmolÀ1) of the cycle, thus indi-
cating that it is the slowest step in the catalytic process. This
Catalytic hydroamination through activation of the N H
bond (amine activation): An alternative pathway for the cat-
alytic hydroamination of secondary aminoalkenes by amine
activation includes (Scheme 5, right): 1) formation of the
À
Zn N bond, which leads to the zinc–amide complex; 2) in-
sertion of the olefinic unit into the Zn N bond; and 3) dis-
À
placement of the cyclic pyrrolidine by a new substrate mole-
cule to regenerate the active catalyst (A1). The Gibbꢄs free
energy profile for the most-accessible reaction pathway for
the intramolecular hydroamination reaction is shown in
Figure 11. For the amine-activation process, the first step in-
volves the generation of intermediate A1 (Figure 8), which
can be in rapid equilibrium with catalyst-resting state AA
(Figure 11) because A1 is only 3.8 kcalmolÀ1 higher in
À
energy than AA. The formation of the Zn N bond, thereby
À
transformation, which involves cleavage of the N H bond,
leading to the initial zinc–amide complex, was postulated to
proceed through transition state TS [A1-B1] (Figure 11). It
is evident from the calculations that the formation of the
is the rate-determining step of the catalytic cycle, which is in
agreement with the data that were obtained from the KIE
studies. Figure 10 shows the energy-optimized structures of
the key intermediates and the transition states in the catalyt-
ic hydroamination of compound 5 catalyzed by cation 8.
À
Zn N bond is almost insurmountable, with an energy barri-
er of 44.8 kcalmolÀ1, thereby making the amine-activation
pathway much-less favorable than the alternative alkene-ac-
tivation pathway; the highest energy barrier is only 22.8 kcal
molÀ1 in the case of the alternative alkene-activation path-
way. These values are comparable to those that were calcu-
lated recently for the iridium-catalyzed hydroamination re-
action by Tobisch, Stradiotto, and co-workers.[4k] Thus, fur-
ther optimization of other intermediates was not performed
for the amine-activation process because, from the first step
of the mechanistic cycle, it is evident that the amine-activa-
tion pathway involves a much-higher energy barrier than
any steps in the alkene-activation pathway, thus indicating
that the alkene-activation pathway is preferred. This result
may be correlated to the earlier-established fact[23] that the
À
low kinetic basicity of the Zn C bond, as a consequence of
À
the high covalency with respect to the Zn N bond, leads to
À
the formation of Zn C bond being thermodynamically
À
more-preferred over the Zn N bond. Furthermore, Boch-
mann and co-workers have earlier isolated and crystallo-
graphically characterized an h2-bound toluene–ZnII com-
plex.[24] Figure 12 shows the energy-optimized structures of
the key intermediates and transition state of the catalytic
hydroamination of compound 5 catalyzed by compound 8
for the amine-activation pathway.
Once the alkene-activation pathway was established as
the preferred pathway, we further performed DFT calcula-
tions on the catalytic cycle of the hydroamination reaction
catalyzed by cation 7, which was generated from complex 1,
to understand the observed remarkable difference in catalyt-
ic efficiency between catalysts 1 and 2 (Table 1 and Table 2).
The Gibbꢄs free energy profile, which considers the most-ac-
cessible reaction pathway for the intramolecular hydroami-
nation reaction is shown in Figure 13. This energy profile in-
dicates that, in the case of catalyst 1, the catalytic pathway
from structures A’ to C’ involves slightly higher energy bar-
riers than that using catalyst 2, which supports the lower cat-
alytic activity of complex 1 versus complex 2. However, this
slight difference in the Gibbꢄs free energy profile does not
Figure 10. Optimized structures of the key intermediates and transition
states for the alkene-activation pathway catalyzed by cation 8, which was
generated from complex 2; hydrogen atoms (other than B and TS [B-C])
and counterions are not shown for the sake of clarity.
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S. K. Mandal et al.
electrophilic zinc center, there-
by forming intermediate A. In
turn, intermediate A can exist
in rapid equilibrium with the
catalyst-resting
having the chelating mode of
aminoalkene coordination;
state
(AA)
3) the activated alkene moiety
in intermediate A is then at-
tacked by the nucleophilic ni-
trogen atom of the amino
group, thus leading to the for-
mation of intermediate B;
4) proton transfer from the am-
monium unit in structure
B
onto the carbon atom that is at-
tached to the Zn center leads to
À
the cleavage of the Zn C bond
and generates intermediate C,
which proceeds through TS [B-
C]; and 5) displacement of the
cycloamine by the incoming
substrate occurs and regener-
ates the catalytically active spe-
cies (A).
Conclusion
We have prepared and charac-
terized two organozinc com-
plexes that contain symmetrical
phenalenyl (PLY)-based N,N-
ligACTHNUTRGNEUGaN nds. These organozinc com-
plexes were used as catalysts
for hydroamination reactions.
A combined approach that in-
volved NMR experiments and
DFT calculations was adopted
to unravel the mechanistic
pathway for the zinc-catalyzed
hydroamination reaction. The
Figure 11. Free-energy profile for the intramolecular hydroamination of compound 5 catalyzed by complex 8
through an amine-activation pathway. Only the most-accessible step is shown; counterions are not shown for
the sake of clarity.
adequately explain the origin of the remarkable difference
in catalytic activity. It seems more likely that the dramatic
difference originates from the higher stability and more-
facile formation of cation 8 from complex 2 than that of
cation 7 from complex 1. The energy-optimized structures
of the key intermediates and transition states in the catalytic
hydroamination of compound 5 catalyzed by compound 7
can be found in the Supporting Information.
reactions of phenalenyl-based ligands with ZnMe2 led to the
formation of organozinc complexes [N(Me),N(Me)-PLY]-
AHCTUNGTRENGUZNN nMe (1) and [NACHTUNGTREG(NNUN iPr),NACHTNUGERTN(NUGN iPr)-PLY]ZnMe (2) under the evo-
lution of methane. The solid-state structures of complexes
1 and 2 were determined by single-crystal X-ray crystallog-
raphy. Complexes 1 and 2 were used as catalysts for the in-
tramolecular hydroamination of unactivated primary and
secondary aminoalkenes. Complex 2 exhibited a catalytic ac-
tivity that was remarkably higher than that of complex
1 under identical reaction conditions. Detailed NMR spec-
troscopic studies and DFT calculations were utilized to un-
derstand the mechanistic cycle of the zinc-catalyzed hydroa-
mination reaction. In situ NMR experiments supported the
formation of a catalytically active zinc cation and the DFT
calculations showed that the more-active catalyst (2) gener-
Mechanistic conclusions: Thus, from this investigation, we
arrive at the following mechanistic conclusions (Scheme 6):
1) the zinc-catalyzed hydroamination reaction preferably
follows the alkene-activation pathway rather than the alter-
native amine-activation pathway; 2) the first step of alkene
activation involves coordination of the alkene moiety to the
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Organozinc Catalysts for Intramolecular Hydroaminations
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ated a stable cation. The stability of the catalytically active
cation was further supported by the in situ recycling proce-
dure, which confirmed the persistent nature of the cation for
successive hydroamination cycles. Kinetic studies of the cyc-
lization of both primary and secondary aminoalkenes cata-
lyzed by complex 2 revealed that both cyclization processes
followed similar reaction kinetics with pseudo-first-order re-
action rates. KIE studies on primary and secondary aminoal-
À
kenes suggested that the cleavage of the N H bond in both
primary and secondary aminoalkenes plays a key role in the
cyclization process. The DFT calculations showed, for the
first time, that the preferred pathway for the zinc-catalyzed
hydroamination reaction involves alkene activation rather
than amine activation. A detailed investigation with DFT
calculations supported the conclusion that the remarkably
higher catalytic efficiency originates from the higher stabili-
ty and more-facile formation of the zinc-centered cation
from complex 2 than from complex 1.
Figure 12. Optimized structures of the key intermediates and transition
state for the amine-activation pathway catalyzed by compound 8; hydro-
gen atoms (except in B1) and counterions are not shown for the sake of
clarity.
Experimental Section
All manipulations were performed under a dry nitrogen atmosphere by
using standard Schlenk techniques or inside an MBraun glovebox that
was maintained at or below a level of 0.1 ppm O2 and H2O with oven-
Figure 13. Free-energy profile for the intramolecular hydroamination of compound 5 catalyzed by compound 7 through an alkene-activation pathway.
Only the most accessible steps are shown; counterions are not shown for the sake of clarity.
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S. K. Mandal et al.
Synthesis
PLY]ZnMe (1):
[HN(Me),N(Me)-PLY]
of
[N(Me),N(Me)-
solution of
(0.222 g,
A
1.0 mmol) in toluene (25 mL) was
added dropwise to solution of
a
ZnMe2 (1.0 mL, 1.2 mmol, 1.2m in tol-
uene) in toluene (20 mL) at À788C.
The reaction mixture was slowly
warmed to ambient temperature and
stirred at 258C for 3 h. The resulting
red solution was then concentrated to
approximately 30 mL under reduced
pressure and kept at 08C. After 1 day,
deep-red crystals of the title com-
pound developed in the reaction mix-
ture. Yield: 0.250 g (82%); m.p. 200–
2028C; 1H NMR (C6D6, 400 MHz,
298 K): d=7.64 (d, J=7.3 Hz, 2H;
ArH), 7.54 (d, J=9.7 Hz, 2H; ArH),
7.24 (t, J=7.3 Hz, 1H; ArH), 7.10 (d,
J=9.1 Hz, 2H; ArH), 3.15 (s, 6H;
NMe2), À0.20 ppm (s, 3H; ZnMe);
13C NMR (C6D6, 100 MHz, 298 K): d=
162.1, 135.2, 130.7, 129.8, 125.6, 120.5,
117.8, 109.1, 40.0, À4.3 ppm; elemental
analysis calcd (%) for C16H16N2Zn:
C 63.70,
C 63.53, H 5.73, N 9.32.
Synthesis of [N(iPr),N
ZnMe (2): A solution of [HN
(iPr)-PLY] (0.278 g, 1.0 mmol) in tolu-
ene (25 mL) was added dropwise to
solution of ZnMe2 (1.0 mL,
1.2 mmol, 1.2m in toluene) in toluene
(20 mL) at À788C. The reaction mix-
ture was slowly warmed to ambient
temperature and then heated at 1108C
H 5.35,
N 9.29;
found:
G
ACHTUNGTRENNUNG
E
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
a
Scheme 6. Proposed mechanism for the intramolecular hydroamination of aminoalkenes catalyzed by cation 8,
which was generated from complex 2; counterions are not shown for the sake of clarity.
dried glassware (1308C) that was evacuated whilst hot prior to use. All
solvents were distilled from Na/benzophenone prior to use. Other chemi-
cals were purchased commercially and used as received. Deuterated ben-
zene was purchased from Cambridge Isotope Laboratories, dried with
a sodium/potassium alloy, stored over 4 ꢃ molecular sieves prior to use.
1H and 13C NMR spectra were recorded on a JEOL-ECS 400 MHz spec-
trometer with C6D6 as the solvent. The following abbreviations are used
to describe the peak patterns, when appropriate: s (singlet), d (doublet),
t (triplet), m (multiplet), and br (broad). Elemental analysis was per-
formed by the Analytisches Labor des Instituts fꢁr Anorganische Chemie
der Universitꢂt Gçttingen. Melting points were measured in sealed glass
tubes on a Bꢁchi melting point B540 instrument and are uncorrected.
for 24 h. The resulting deep-red solution was then concentrated to ap-
proximately 10 mL under reduced pressure and kept at 258C inside a glo-
vebox. After a few days, red crystals of the title compound developed in
the reaction mixture by using the slow-solvent-evaporation technique.
Yield: 0.280 g (78%); m.p. 143–1458C; 1H NMR (C6D6, 400 MHz,
298 K): d=7.59 (d, J=7.3 Hz, 2H; ArH), 7.48 (d, J=9.1 Hz, 2H; ArH),
7.22 (t, J=7.3 Hz, 1H; ArH), 7.16 (d, partially obscured by C6D6, J=
9.7 Hz, 2H; ArH), 3.99–3.96 (m, 2H; NCHMe2), 1.34 (d, J=6.7 Hz,
12H; CHMe2), 0.04 ppm (s, 3H; ZnMe); 13C NMR (C6D6, 100 MHz,
298 K): d=160.4, 135.2, 130.2, 129.3, 125.6, 120.3, 118.7, 108.7, 51.1, 25.0,
À5.9 ppm; elemental analysis calcd (%) for C20H24N2Zn: C 67.23, H 6.72,
N 7.84; found: C 66.84, H 6.31, N 7.55.
Starting materials: 9-N-Methylamino-1-N’-methylimino-phenalene[8d] and
9-N-isopropylamino-1-N’-isoproylimino-phenalene[10b] were synthesized
according to literature procedures. Dimethyl zinc (1.2m in toluene) and
General procedure for the intramolecular hydroamination of primary
aminoalkenes: All of the reactions were performed in an NMR tube in
a N2-filled glovebox. A pre-dried NMR tube was charged with the amino-
[PhNMe2H][BACHTUNGTRENNUNG(C6F5)4] were purchased from Acros Organics. Aminoal-
alkene (20 mL) and a solution of the catalyst (5 mol%), [PhNMe2H]
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG[B-
kenes were prepared from commercially available starting materials (Al-
drich, Acros Organics, and Fluka). Aminoalkene substrates 2,2-diphenyl-
pent-4-en-1-amine,[4m] (1-allylcyclohexyl)methylamine,[25a] 2,2-dimethyl-
C6D6 (0.6 mL) was added under a nitrogen atmosphere. The NMR tube
was sealed and the reaction mixture was heated in a preheated oil bath
that was maintained at 1208C for the stated time. The reaction progress
was monitored by 1H NMR spectroscopy. Yields were determined by
comparing the integration of the internal standard with a well-resolved
signal for the heterocyclic product. Purification: 2-methyl-4,4-diphenyl
pyrrolidine (cyclized product of Table 1, entry 2) was purified by column
chromatography on silica gel (CH2Cl2/MeOH, 10:1); yield of isolated
product: 90%.
hex-5-enylamine,[3e]
methyl(1-allylcyclohexyl)methylamine,[25b]
enyl)amine,[4m] (4-bromobenzyl)(2,2-diphenyl-4-pentenyl)amine,[4m] (4-
meth
oxybenzyl)(2,2-diphenyl-4-pentenyl)amine,[25c] (2,2-diphenylpent-4-
enyl)furan-2-ylmethylamine,[6b]
4-methyl-2,2-diphenylpent-4-en-1-amine,[25b]
4-
benzyl(2,2-diphenyl-4-pent-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
(2,2-diphenylpent-4-enyl)thiophen-2-yl-
methylamine,[6b] (1-allylcyclohexylmethyl)benzylamine,[4m] (1-allylcyclo-
hexylmethyl)(4-bromobenzyl)amine,[25d] 2-furan-2-ylmethyl-3-methyl-2-
azaspiroACHTUNGTRENNUNG
[4.5]decanamine,[6o] (1-allylcyclohexylmethyl)(4-methoxybenzy-
l)amine,[25d] and benzyl(2,2-dimethyl-4-pentenyl)amine[4m] were prepared
according to literature procedures and dried by distilling twice from
CaH2. The hydroamination products are known compounds and were
General procedure for the intramolecular hydroamination of secondary
aminoalkenes: All of the reactions were performed in an NMR tube in
a N2-filled glovebox. A pre-dried NMR tube was charged with the amino-
alkene (20 mL) and a solution of the catalyst (5 mol%), [PhNMe2H]
AHCTUNGTRENNUNG
identified by comparison with their literature NMR spectroscopic data.[3-
ACHTUNGTRENNUNG[B-
c,6b,o,25]
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Organozinc Catalysts for Intramolecular Hydroaminations
FULL PAPER
C6D6 (0.6 mL) was added under a nitrogen atmosphere. The NMR tube
was sealed and the reaction mixture was heated at 808C in a pre-heated
oil bath for the stated time. The reaction progress was monitored by
1H NMR spectroscopy. Yields were determined by comparing the inte-
gration of the internal standard with a well-resolved signal for the hetero-
cyclic product. Purification: 1-(4-bromobenzyl)-2-methyl-4,4-diphenylpyr-
rolidine (cyclized product of Table 2, entry 2) was purified by column
chromatography on silica gel (n-hexane/EtOAc, 9:1); yield of isolated
product: 92%.
frequencies were calculated for optimized molecular structures to verify
that no negative frequencies were present for the minimum-energy struc-
tures and one negative frequency were present for the TS structures.
Acknowledgements
Catalyst longevity test for intramolecular hydroamination reaction of
compound 5: The catalyst-longevity experiment was carried out inside
a N2-filled glovebox. A pre-dried NMR tube was charged with the amino-
A.M. and T.K.S. are thankful to the IISER-Kolkata and the CSIR, India,
respectively, for research fellowships. S.K.M. thanks the CSIR (Sanction
Grant No. 01
ACHTUNGTRENNUNG
alkene (20 mmol) and a solution of the catalyst (5 mol%), [PhNMe2H]
ACHTUNGERTN[NUNG B-
thanks the DST, India, for financial support. S.K.M. thanks NMR and ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ray facilities at IISER-Kolkata, and also thanks Prof. H. W. Roesky for
his support and constant encouragement.
1 equiv with respect to the substrate) in C6D6 (0.6 mL) was added under
a nitrogen atmosphere. The NMR tube was sealed and the reaction mix-
ture was heated at 808C for 4 h. The reaction was monitored by H NMR
1
spectroscopy after 4 h. Conversion was determined by comparing the in-
tegration of a well-resolved signal for the heterocyclic product with that
of the internal standard. The reaction time was further prolonged to
make sure that full consumption of the substrate was accomplished
before starting the next catalytic cycle. After completion of the first
cycle, the NMR tube was cooled and a fresh batch of substrate was
added without adding any further catalyst and the NMR tube was heated
again at 808C for 4 h. This process was repeated for four consecutive
cycles.
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General procedure for the kinetic experiments: All manipulations were
performed in a N2-filled glovebox. Kinetic experiments were performed
by using 1H NMR spectroscopy on a JEOL-ECS 400 MHz spectrometer
and each rate constant represents an individual kinetic experiment. A
standard solution of the catalyst was made by weighing complex 2 into
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a similar fashion as that described above. The substrates, internal stan-
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the progress of the catalysis was monitored by 1H NMR spectroscopy
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a known concentration of hexamethyl benzene that was dissolved in deu-
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X-ray crystallography of complexes 1 and 2: A suitable crystal of com-
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IPDS II Stoe image-plate diffractometer (graphite-monochromated MoKa
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at 100 K on a Bruker SMART-APEX II diffractometer with a D8 goni-
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displacement parameters. The hydrogen atoms were refined isotropically
on calculated positions by using a riding model. No restraints were used.
CCDC-870913 (1) and CCDC-870914 (2) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Computational details: Geometry optimizations, transition-state (TS) cal-
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any symmetry constraints at the level of density functional theory
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Published online: && &&, 0000
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These are not the final page numbers!

Organozinc Catalysts for Intramolecular Hydroaminations
FULL PAPER
Hydroamination
A. Mukherjee, T. K. Sen, P. K. Ghorai,
P. P. Samuel, C. Schulzke,
S. K. Mandal* ................. &&&& — &&&&
Phenalenyl-Based Organozinc Cata-
lysts for Intramolecular Hydroamina-
tion Reactions: A Combined Catalytic,
Kinetic, and Mechanistic Investigation
of the Catalytic Cycle
Phenalenyl-based organozinc com-
plexes were used as catalysts for the
intramolecular hydroamination reac-
tions of unactivated primary and sec-
ondary aminoalkenes. DFT calcula-
tions predicted that the organozinc
complex followed an alkene-activation
pathway for the cyclization of secon-
dary aminoalkenes.
Organozinc Complexes
New phenalenyl-based organozinc complexes were synthe-
sized, characterized, and applied in intramolecular hydro-
AHCTUNGTREGaNNNU mination reactions of unactivated primary and secondary
aminoalkenes. The frontispiece illustrates that the zinc-
catalyzed hydroamination of secondary aminoalkene
substrates proceeds through the alkene activation pathway
ascertained by NMR spectroscopy, kinetic studies, and
DFT calculations, which are described in the Full Paper
by S. K. Mandal et al. on page && ff.
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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