Organozinc catalyst on a phenalenyl scaffold for intramolecular hydroamination of aminoalkenes

Article abstract of DOI:10.1021/om400906u

The syntheses and characterization of two organozinc compounds were accomplished by reacting phenalenyl (PLY)-based ligands with ZnMe₂. Reactions of [HN(Cy),O-PLY] and [HN(Cy),N(Cy)-PLY] ligands with ZnMe₂ led to the formation of the

Full text of DOI:10.1021/om400906u

Article
pubs.acs.org/Organometallics
Organozinc Catalyst on a Phenalenyl Scaffold for Intramolecular
Hydroamination of Aminoalkenes
Arup Mukherjee, Tamal K. Sen, Pradip Kr. Ghorai, and Swadhin K. Mandal*
Department of Chemical Sciences, Indian Institute of Science Education and Research-Kolkata, Mohanpur 741252, Nadia,
West Bengal, India
S
* Supporting Information
ABSTRACT: The syntheses and characterization of two organo-
zinc compounds were accomplished by reacting phenalenyl (PLY)-
based ligands with ZnMe2. Reactions of [HN(Cy),O-PLY] and
[HN(Cy),N(Cy)-PLY] ligands with ZnMe2 led to the forma-
tion of the dimeric orange-colored organozinc compound
[N(Cy),O-PLY-ZnMe]2 (1) and red-colored monomeric
organozinc compound [N(Cy),N(Cy)-PLY-ZnMe] (2) under
evolution of methane. Both 1 and 2 were characterized by NMR
spectroscopy, elemental analysis, and single-crystal X-ray
diffraction study. The organozinc compound 2 was tested as a
catalyst for intramolecular hydroamination of both unactivated
primary and secondary aminoalkenes in the presence of an
externally added activator, which generated the zinc-based cation in situ. The catalytic result obtained from the present catalyst 2 was
compared with the catalysts having similar structure from previous studies. The DFT calculation indicates that the stability of the
in situ generated cation plays a significant role in the catalytic activity in the hydroamination reaction.
recently isolated and crystallographically demonstrated for-
mation of a metal alkyl intermediate in the zinc-mediated
intramolecular hydroamination reaction of an aminoalkene.10
As part of our ongoing search for hydroamination catalysts,11
we started using a new class of ligands based on the phenalenyl
moiety for carrying out hydroamination catalysis.12 Phenalenyl
is a well-known odd alternant hydrocarbon with high symmetry
(D3h), having the ability to exist in three redox states, namely,
closed shell cation, open shell neutral radical, and closed shell
anion. Haddon and co-workers have earlier used the open shell
phenalenyl unit as a building block to prepare organic magnetic
semiconductors exhibiting simultaneous bistability in multiple
physical channels and the highest room-temperature con-
ductivity among any neutral organic solids.13 Previous articles
by Morita, Takui, and Hicks document the status of phenalenyl-
based open shell chemistry.14 Recently, we started developing
the organometallic chemistry using phenalenyl (PLY) ligands
leading to the closed shell cationic phenalenyl moiety.12,15 Very
recently, we reported that an organozinc phenalenyl compound
bearing the closed shell cationic phenalenyl unit can act as a
building block for organometallic spintronics materials.16 Apart
from the organometallic spintronics application of phenalenyl-
based compounds, in our recent studies, we have reported
the synthesis of a series of ethylzinc and methylaluminum
catalysts based on phenalenyl ligands for ring-opening
polymerization of ε-caprolactone and rac-lactide.15b,c Herein,
INTRODUCTION
■
Nitrogen-containing heterocycles, such as amines, enamines,
and imines, have attracted significant interest among synthetic
and medicinal chemists over the years, since they are the key
synthetic intermediates of natural products.1 Among the several
synthetic routes, hydroamination, the catalytic addition of
the N−H bond across a C−C multiple bond, is of particular
importance.2 The reaction offers an atom economical pathway
for the synthesis of nitrogen-containing molecules in a more
environmental friendly way than the conventional reaction
pathway. Over the last decades, there have been an enormous
number of efforts to develop an efficient catalyst for this
demanding transformation including those based on main
group elements,3 rare earth metals,4 and transition metals.5,6 In
this regard, the most recent trend is the use of low-cost, non-
toxic metal based catalysts for carrying out hydroamination
catalysis. The hydroamination reaction has been routinely used
in the production of many pharmaceutically important and
biologically relevant drug molecules and natural products.7 It is
important that the product of the hydroamination catalysis is
not contaminated with any toxic metal residue. As a result, the
development of hydroamination catalysts based on nontoxic
metals such as calcium,3a−f magnesium,3b,f and zinc8,9 has been
a growing topic in the past few years. In particular, the use of
zinc-based catalysts and their performance in the hydro-
amination reaction have been promising in recent years, which
started with the report by Roesky, Blechert, and co-workers on
the development of organozinc catalysts based on amino-
troponiminate ligands.8,9a−d,g,h Thiel and co-workers have
Received: September 11, 2013
Published: November 11, 2013
© 2013 American Chemical Society
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we report the syntheses and characterization of two organozinc
complexes, [N(Cy),O-PLY-ZnMe]2 (1) and [N(Cy),N(Cy)-
PLY-ZnMe] (2), containing a phenalenyl ligand backbone. The
catalyst [N(Cy),N(Cy)-PLY-ZnMe] (2) was used for hydro-
amination of unactivated primary and secondary aminoalkenes
in the presence of an externally added activator. NMR spec-
troscopy, kinetic studies, and DFT calculations were employed
to understand the activity of this class of catalyst. Although
early studies8,9,12 considered that the coordinatively unsaturated
zinc cation acts as catalyst, no study has considered the correla-
tion between the stability of the zinc cation and the outcome of
the hydroamination catalysis. In the present study, we have
demonstrated that the stability of the in situ generated cationic
species is responsible for the different rates of hydroamination
reaction observed for different catalysts. The catalytic activity
can be correlated with the HOMO−LOMO energy gap of the
in situ generated cationic species, which is an indicator of their
stability.
determined unambiguously by single-crystal X-ray diffraction
studies (Figure 1). Complexes 1 and 2 were crystallized in the
monoclinic space group P21/n and C2/c, respectively, with one
molecule in the asymmetric unit cell in both cases. The X-ray
structures of 1 and 2 reveal a distorted tetrahedral and trigonal
planar geometry, respectively, around the zinc center. The mo-
lecular structure of 1 revealed a dimeric structure with oxygen
atoms bridging between two zinc centers, which results in the
formation of a nearly flat Zn1−O1−Zn1i−O1i four-membered
heterocycle. The Zn1−Zn1i distance is 3.25 Å in 1, which can
be compared with the literature data reported earlier for a
similar oxygen-bridged organozinc dimer bearing an amino-
troponimate ligand (3.1283(8) Å)9f and a phenalenyl ligand
(3.1 Å).15c The Zn(1)−N bond distances observed in 2
[Zn(1)−N(1), 1.964(2) Å and Zn(1)−N(2), 1.9536(19) Å]
are comparable with that of the Zn−N bond distance
determined (1.968 Å) in the methylzinc complex of the
N-cyclohexyl-2-(cyclohexylamino)troponiminate ligand.9g The
Zn−N bond length observed in 1 [2.0291(19) Å] is longer when
compared with that of 2 (av 1.958 Å). The Zn−C bond distances
observed in 1 [Zn(1)−C(20), 1.959(2) Å] and 2 [Zn(1)−C(26),
1.966(3) Å] are comparable with that of the Zn−C bond distance
observed (1.944 Å) in a previously characterized organozinc
complex, N-cyclohexyl-2-(cyclohexylamino)troponiminate methyl-
zinc.9g The N−Zn−C bond angles in 2 [N(1)−Zn(1)−C(26),
132.89(10)°; N(2)−Zn(1)−C(26), 133.66(10)°] are smaller
than the bond angles found in the complex [N-isopropyl-2-
(isopropylamino)troponiminato]methylzinc (N−Zn−C,
138.5−139.6°), making the N(1)−Zn(1)−N(2) bond angle
in 2 [93.37(8)°] wider as compared with that observed in the
[N-isopropyl-2-(isopropylamino)troponiminato]methylzinc
complex (N−Zn−N, 81.94°).8a
RESULTS AND DISCUSSION
■
Syntheses of the Organozinc Compounds 1 and 2.
The syntheses of [N(Cy),O-PLY-ZnMe]2 (1) and [N(Cy),N-
(Cy)PLY-ZnMe] (2) were accomplished by reacting 9-N-
cyclohexyl-1-oxophenalene, [HN(Cy),O-PLY], and 9-N-cyclohexyl-
1-N′-cyclohexylphenalene, [HN(Cy),N(Cy)-PLY], with
ZnMe2 at 90 and 110 °C, respectively (Scheme 1). A solution
Scheme 1. Syntheses of Organozinc Compounds 1 and 2
with Phenalenyl Ligands
Catalytic Intramolecular Hydroamination Reactions.
Previous studies have demonstrated that organozinc com-
pounds can catalyze the intramolecular hydroamination reac-
tion of aminoalkenes in the presence of a boron-based Lewis
acceptor.8,9,12 Earlier, we have shown that the organozinc
compounds developed with the phenalenyl ligands exhibited
good to excellent catalytic activity in the case of the intra-
molecular hydroamination reaction of unactivated primary and
secondary aminoalkenes.12 Previously, Roesky, Blechert, and
co-workers have shown that the introduction of the cyclohexyl
substituents on the aminotroponiminate ligand enhances the
catalytic activity of the resulting organozinc complex dramat-
ically in the case of the intramolecular hydroamination reaction
of secondary aminoalkenes.9g In this study, the catalytic activity
of complex 2, having two cyclohexyl substituents on the donor
phenalenyl unit, was tested for the intramolecular hydro-
amination reaction of unactivated primary and secondary
aminoalkenes.
of [HN(Cy),O-PLY] or [HN(Cy),N(Cy)-PLY] in toluene was
added drop-by-drop to a solution of ZnMe2 in 1:1.2 stoi-
chiometric ratio in toluene at −50 °C and heated at 90 °C for
12 h or at 110 °C for 24 h to yield 1 or 2. The 1H NMR spectra
of both reaction mixtures reveal clean and nearly quantitative
conversions of the reactants to products as exhibited by the
absence of any characteristic N−H proton resonance (δ 13.5 ppm
in [HN(Cy),O-PLY] or δ 14.05 ppm in [HN(Cy),N(Cy)-PLY]
in C6D6) of the free phenalenyl ligands.
1
Both 1 and 2 were characterized by H and 13C NMR spec-
The study started with the unactivated primary amino-
alkenes. First, the intramolecular hydroamination reaction of
(1-allylcyclohexyl)methylamine (3) with complex 2 was
performed in C6D6 maintaining the bath temperature at
120 °C without adding any activator. After heating for 24 h,
we found <10% of the hydroamination product (4) with
troscopy, elemental analyses, and single-crystal X-ray diffraction
1
studies. The H NMR spectra of 1 in THF-d8 and 2 in C6D6
exhibited a singlet at δ −0.65 and 0.09 ppm, respectively, attrib-
uted to the proton resonance arising from the methyl group
bound to the zinc ion. The 13C NMR spectra of 1 and 2
revealed a resonance at δ −13.1 ppm in THF-d8 and −5.1 ppm
in C6D6, respectively, which are assigned to the Zn-CH3 carbon
resonance.
X-ray Crystal Structures of 1 and 2. Suitable crystals of 1
were obtained from slow cooling of a hot toluene solution at
25 °C, while those of 2 were obtained from a cold toluene
solution at 0 °C. Molecular structures of both 1 and 2 were
1
complex 2, as revealed by the H NMR spectrum of the
catalytic reaction mixture. To increase the catalytic activity of
complex 2 toward cyclization of primary aminoalkenes, the
reaction was carried out with an equimolar amount (with
respect to the catalyst) of the activator [PhNMe2H][B(C6F5)4],
whereupon we found that catalyst 2 afforded 93% conversion
after 7.5 h in C6D6 maintaining the bath temperature at 120 °C.
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Figure 1. Molecular structures of organozinc complexes (a) 1 and (b) 2. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms
are omitted for the sake of clarity. Selected bond distances (Å) and angles (deg) in (a) 1: Zn(1)−C(20) 1.959(2), Zn(1)−O(1) 1.9814(16), Zn(1)−
N(1) 2.0291(19), O(1)−C(9) 1.311(3); C(20)−Zn(1)−O(1) 130.88(9), C(20)−Zn(1)−N(1) 127.84(10), O(1)−Zn(1)−N(1) 89.87(7), O(1)−
Zn(1)−O(1) 77.88(7); and (b) 2: Zn(1)−N(1) 1.964(2), Zn(1)−N(2) 1.9536(19), Zn(1)−C(26) 1.966(3), N(1)−C(1) 1.347(3), N(2)−C(11)
1.340(3), N(1)−C(14) 1.484(3), N(2)−C(20) 1.481(3); N(1)−Zn(1)−C(26) 132.89(10), N(2)−Zn(1)−C(26) 133.66(10), N(1)−Zn(1)−N(2)
93.37(8), C(1)−N(1)−Zn(1) 123.57(16), C(11)−N(2)−Zn(1) 120.51(16).
Next, the catalytic efficacy of catalyst 1 was also tested for the
hydroamination reaction of substrate 3 in the presence of this
activator (Table 1, entry 1). The catalytic result suggests that 1
is not an efficient catalyst (Table 1, entry 1). Thus we
proceeded with catalyst 2 for cyclization of various primary
aminoalkene substrates, and the results were compared with the
activity of two structurally related organozinc-based catalysts,
[N(Me),N(Me)-PLY-ZnMe] (5) and [N(iPr),N(iPr)-PLY-
Kinetic studies of the representative cyclization of 3 to 4 were
carried out with catalyst 2 and [PhNMe2H][B(C6F5)4] in
equimolar amounts by 1H NMR spectroscopic monitoring
experiments. The evolution of the specific resonances of the
heterocyclic products was monitored by 1H NMR spectroscopy
relative to an internal standard over the course of the first 7 h.
To check the order of the cyclization process, the reaction
was carried out with 5.92 mM 2 and 118.4 mM 3. A plot of
ln(C/C0) versus time provided a straight line with negative
slope, as shown in Figure 3a, revealing a pseudo-first-order rate
of reaction. To determine the order of the reaction with respect
to the catalyst concentration, the cyclization of 3 was carried
out with varied concentrations of 2 (from 6.16 to 9.89 mM)
keeping the concentration of substrate 3 fixed at 117.5 mM. A
plot of kobs versus different catalyst concentrations reveals a
linear increase of the reaction rate with catalyst concentration,
confirming the first-order dependency of the reaction rate with
respect to the catalyst concentration (Figure 3b). This was
further confirmed by the van’t Hoff plot (Figure 3c).17 A plot of
ln kobs versus ln[Cat] provided a linear graph (Figure 3c), and
the value of the slope was determined to be 1.03 (slope = order
of the reaction).17 The order of the reaction with respect to the
substrate concentration was determined by varying the con-
centrations of 3 (113.8 to 448.6 mM) and keeping the con-
centration of catalyst 2 fixed at 5.89 mM (Figure 3d). A plot of
ZnMe] (6), bearing phenalenyl ligands reported earlier.12
A
careful observation of Table 1 suggests that the catalytic activity
of this newly synthesized organozinc complex 2 is much higher
as compared to the previously reported phenalenyl-based organozinc
complex [N(Me),N(Me)-PLY-ZnMe] (5) and comparable or
slightly better when compared with organozinc compound
[N(iPr),N(iPr)-PLY-ZnMe] (6).12 Compounds 2, 5, and 6
differ from each other in terms of the substituents attached
to the donor N-center. Furthermore, examination of the data in
Table 1 emphasizes that catalyst 2 cyclizes the primary
aminoalkenes at different rates depending upon the substituents
at the β-position of the aminoalkenes with respect to the amine
moiety (Thorpe−Ingold effect). The NMR spectroscopic mea-
surements were used to monitor the progress of the cyclization
process of (1-allylcyclohexyl)methylamine (3) with catalyst 2 in
C6D6 maintaining the bath temperature at 120 °C, which
revealed a clean conversion of the aminoalkene substrate into
the heterocyclic hydroamination product (4, Figure 2). Figure 2
reveals that the characteristic resonances of the olefinic protons
of the aminoalkene moiety disappear with the gradual progress
of the reaction, while those of the hydroamination product evolve
with time.
k
obs versus different substrate concentrations indicated a gradual
decrease of the reaction rate, revealing the inverse order of the
reaction with respect to the substrate concentration. This result
complies with the inverse order of intramolecular hydroamina-
tion reaction with respect to substrate concentration that was
reported previously by Michael and Cochran.18
Furthermore, detailed kinetic studies were performed to gain
insight into the cyclization process catalyzed by 2 using the
primary aminoalkene (1-allylcyclohexyl)methylamine (3) in C6D6.
Moreover, we carried out the H/D kinetic isotope effect
(KIE) experiment using 3 and 3-d2 with 5 mol % of catalyst 2
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a
Table 1. Intramolecular Hydroamination of Unactivated Primary Aminoalkenes
a
Reaction conditions: amine (20 μL), catalyst (5 mol %) and activator, [PhNMe2H][B(C6F5)4] (5 mol %) in 0.6 mL C6D6 by maintaining the
b
c
1
reaction bath temperature at 120 °C. Determined by H NMR spectroscopy against an internal standard. Present work.
and 5 mol % of [PhNMe2H][B(C6F5)4] under the same
reaction conditions (Scheme 2). The first-order plots of the
cyclization of 3 and 3-d2 under the reaction conditions result
in kobs = 0.341 h−1 and kobs = 0.101 h−1, respectively, which
translate into a primary KIE of 3.38 (Figure 4). This observation
indicates that the amino group of 3 is involved in the key step of
the primary aminoalkene activation process, which is reminis-
cent of our earlier observation, where we reported an alkene
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1
Figure 2. Stack plots of H NMR spectrum for the reaction of (1-allylcyclohexyl)methylamine (3) with 2 as catalyst at different time intervals.
activation pathway for cyclization of secondary aminoalkenes
with an organozinc phenalenyl-based catalyst in which the
cleavage of the N−H bond takes part in the slowest step of the
mechanistic cycle.12
previously reported phenalenyl-based organozinc complex
[N(Me),N(Me)-PLY-ZnMe] (5) and comparable or slightly
better when compared with organozinc compound [N(iPr),N-
(iPr)-PLY-ZnMe] (6).12
On the basis of the promising reactivity displayed by catalyst
2 toward cyclization of primary aminoalkenes, the activity of 2
in intramolecular hydroamination of unactivated secondary
aminoalkenes was checked. To evaluate the generality of the
catalytic activity of 2 on secondary aminoalkene substrates,
the reaction was investigated with a number of secondary
aminoalkenes in the presence of 5 mol % of catalyst under
a 5 mol % activator loading at 80 °C in C6D6 (Table 2).
Inspection of Table 2 suggests that catalyst 2 is tolerable to
the different functional groups present in the substrate
molecules, and the catalytic activity of 2 is much higher
than the activator alone.19 The secondary aminoalkene sub-
strate bearing a thiophene moiety shows a higher reaction
rate than the corresponding aminoalkene with a furan moiety
for catalyst 2 (Table 2, entries 11 and 14). This may be
attributed to a more effective chelation of the zinc catalysts by
the aminofuran. In this case also, we found that the catalytic
activity of catalyst 2 is much higher as compared to the
Trends in Catalytic Activity. Tables 1 and 2 list the com-
parative reactivity of the present catalyst with that of previously
reported catalysts [N(Me),N(Me)-PLY-ZnMe] (5) and [N-
(iPr),N(iPr)-PLY-ZnMe] (6).12 A closer look at the catalyst
structure reveals that 2 differs from 5 and 6 only in terms of the
substituents attached to the donor nitrogen center. The catalytic
results presented in Tables 1 and 2 indicate that there is a dramatic
difference between the catalytic activity of 5 and that of 2 and 6,
while that of 2 and 6 is appreciably higher than that of 5. The
catalytic activity of 2 is comparable to that of 6 or slightly better. To
understand this trend in the catalytic activity, we carried out DFT
calculations on the active catalyst generated in situ by addition of
externally added activator [PhNMe2H][B(C6F5)4].
Previous studies have demonstrated that addition of
[PhNMe2H][B(C6F5)4] as an activator in the solution of the
methylzinc complex generates the coordinatively unsaturated
zinc cation in situ, which functions as a catalytically active
species.8,9,12 The activator abstracts the methyl group attached
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Figure 3. Kinetic studies of primary aminoalkene cyclization process monitored by 1H NMR spectroscopy in C6D6: (a) plot of ln(C/C0) versus time
for the cyclization of 3; (b) plot of kobs versus [Cat] for the cyclization of 3; (c) van’t Hoff plot of ln kobs versus ln[Cat]; (d) plot of kobs versus [Sub]
for the cyclization of 3 using catalyst 2.
Scheme 2. Cyclization of (1-Allylcyclohexylmethyl)amine
(3) and (1-Allylcyclohexylmethyl)amine-d2 (3-d2) Using 2
as Catalyst in C6D6, Maintaining the Bath Temperature at 120 °C
1
to the zinc center to generate the zinc cation in situ. The H
NMR experiment in the present work also supports formation
of a zinc-centered cation on addition of a stoichiometric
amount of activator (Scheme 3). The 1H NMR spectroscopy of
the reaction mixture reveals the complete disappearance of the
zinc methyl resonance of 2 at δ 0.09 ppm and appearance of
Figure 4. H/D KIE study for the formation of 3 and 3-d2 using catalyst
two new resonances at δ 0.15 and 2.5 ppm due to the
2 in C6D6.
generation of CH420 and PhNMe2, respectively (see Supporting
Information Figure S2). This observation confirms the role of
the activator as a methyl abstractor and the generation of a zinc
cation, which is in accordance with earlier reports.8b,9g,12
Furthermore, in order to understand the catalytic activity
trend of catalysts 2, 5, and 6 in the intramolecular hydro-
amination reaction of primary and secondary aminoalkenes
(Tables 1 and 2), we carried out a computational study on the
active catalyst, zinc-centered cation. In particular, the DFT
calculation was utilized to calculate the HUMO−LUMO energy
gap of the zinc cation to assess their stability.21 The computed
frontier orbitals (HOMO and LUMO of zinc-centered cation 7
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a
Table 2. Intramolecular Hydroamination of Unactivated Secondary Aminoalkenes
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Table 2. continued
a
Reaction conditions: amine (20 μL), catalyst (5 mol %), and activator, [PhNMe2H][B(C6F5)4] (5 mol %), in 0.6 mL of C6D6 at 80 °C.
Determined by H NMR spectroscopy against an internal standard. Present work.
b
c
1
Scheme 3. Generation of Zinc-Centered Cation 7
that it is a primarily zinc-centered one. Furthermore, the com-
puted HOMO−LUMO energy gap of the zinc-centered cation 7
is 73.3 kcal/mol. This value is higher as compared to the pre-
viously reported value for zinc-centered cation 8 (64.6 kcal/mol),
generated from [N(Me),N(Me)-PLY-ZnMe] (5), and slightly
higher than the zinc-centered cation 9 (70.6 kcal/mol), generated
from [N(iPr),N(iPr)-PLY-ZnMe] (6).12 This reveals compara-
tively higher stability for the cation 7 over that of 8 and 9.12
Figure 6a displays the HOMO−LUMO energy gap calculated in
the cations 7, 8, and 9, which indicates the formation of the
most stable cation from the present catalyst 2. Furthermore, the
catalytic reactions using substrate (1-allylcyclohexyl)-
methylamine (3) were performed under identical conditions
generated from 2) as ascertained by DFT calculations are
presented in Figure 5. The LUMO of cation 7 clearly indicates
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C6D6 maintaining the bath temperature at 120 and 80 °C,
respectively, by in situ recycling methodology.22 The H NMR
1
integration was used to calculate the conversion with respect to
a known amount of an internal standard (hexamethylbenzene).
After full consumption of the substrates, fresh batches of new
substrates were added without introducing any additional
catalyst. After each 7.5 h (for reaction with primary amino-
alkene substrate) and 2.5 h (for reaction with secondary
aminoalkene substrate) interval, the conversions into the
1
products were checked by recording H NMR spectra of the
reaction mixtures. The catalytic runs were repeated for five
successive cycles. It was found that 2 remained catalytically
active up to five consecutive runs (Figure 7); however, a
gradual decrease in the activity was noted by NMR spectros-
copy. Nevertheless, this result clearly indicates that the catalyt-
ically active species (cation 7 in the present case) remains live
for several consecutive runs of the catalytic cycles, indicating
good cation stability in the reaction medium.
Figure 5. Computed (a) HOMO and (b) LUMO of the zinc-centered
cation 7.
using catalysts [N(Cy),N(Cy)-PLY-ZnMe (2), [N(Me),N-
(Me)-PLY-ZnMe] (5), and [N(iPr),N(iPr)-PLY-ZnMe] (6).
The catalytic result reveals that the catalyst 2 is the most
efficient one. Under identical conditions, catalyst 5 led to only
8% conversion, while the catalysts 2 and 6 led to nearly 87%
and 80% conversion, respectively. This catalytic trend can
grossly be correlated with the HOMO−LUMO energy gap of
the corresponding in situ generated cations as reflected from
Figure 6b. The cation 7 with highest HOMO−LUMO energy
gap results in the best activity, while the cation 8 with the
lowest HOMO−LUMO energy gap results in the worst
catalytic activity.
To further examine the stability of the catalytically active
zinc-centered cation and its longevity in the catalytic cycle, we
have investigated the catalytic activity of 2 in the successive
catalytic cycles continuously. The cation 7 generated in situ
from 2 by addition of a stoichiometric amount of activator was
used to carry out this experiment. In this case, the longevity of
catalyst 2 was monitored by performing several catalytic runs
within the same reaction vessel to test whether the catalyst
remained live for several catalytic cycles.22 Five successive
catalytic runs were carried out in two different reactions by
using 5 mol % of catalyst 2 and an equimolar amount of
[PhNMe2H][B(C6F5)4] with primary aminoalkene substrate 3
in one reaction and with secondary aminoalkene substrate
benzyl(2,2-diphenylpentenyl)amine (10) in another reaction in
SUMMARY AND CONCLUSION
■
The present study reports the development of a nontoxic
metal-based hydroamination catalyst containing phenalenyl
ligands. Reactions of phenalenyl ligands with ZnMe2 led to
the formation of organozinc complexes [N(Cy),O-PLY-
ZnMe]2 (1) and [N(Cy),N(Cy)-PLY-ZnMe] (2) under evolu-
tion of methane. The solid-state structures of both complexes 1
and 2 were determined by single-crystal X-ray diffraction study.
Complex 2 was used as a catalyst for intramolecular hydro-
amination of unactivated primary and secondary aminoalkenes
in the presence of an externally added activator. The activator
in situ generates the catalytically active cation, and the DFT
calculation was employed to understand the stability of the
active cation. The performance of the catalyst 2 was further
compared with the activity of earlier reported organozinc
catalysts bearing phenalenyl-based ligands. The DFT calcu-
lation indicated that the stability of the catalytically active zinc-
centered cations is primarily responsible for the observed
catalytic activity. The more stable cation leads to better catalytic
activity in the intramolecular hydroamination reaction of both
primary and secondary aminoalkene substrates.
EXPERIMENTAL SECTION
All manipulations were performed under a dry and oxygen-free
atmosphere (N2) using standard Schlenk techniques or inside a
■
Figure 6. (a) Plot of the computed HOMO and LUMO energy gap of the zinc-centered cations 7, 8, and 9 generated from the methylzinc
complexes 2, 5, and 6, respectively. (b) Plot of the % of conversion of 3 to the corresponding cyclic product 4 after 6 h with the HOMO−LUMO
energy gap of the zinc-centered cations 7, 8, and 9 (for the catalytic conditions see Table 1).
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Figure 7. Catalyst longevity test for the cyclization of (a) (1-allylcyclohexyl)methylamine (3) with catalyst 2 in C6D6 maintaining the reaction bath at
120 °C and (b) benzyl(2,2-diphenyl-4-pentenyl)amine (10) with catalyst 2 in C6D6 at 80 °C.
nitrogen-filled MBraun glovebox maintained at below 0.1 ppm of O2
and H2O level, utilizing glassware that was oven-dried (130 °C) and
evacuated while hot prior to use. All solvents were distilled from
Na/benzophenone prior to use. Other chemicals were purchased
commercially and used as received. Deuterated benzene was purchased
from Cambridge Isotope Laboratories, dried by sodium potassium
alloy, and stored over 4 Å molecular sieves prior to use. Dimethyl zinc
solution and [PhNMe2H][B(C6F5)4] were purchased from Acros
added dropwise to the ZnMe2 solution (1.2 M in toluene; 1.0 mL,
1.2 mmol) in toluene (20 mL) at −50 °C. The reaction mixture was
slowly warmed to ambient temperature and heated at 110 °C for 24 h.
The resulting deep red solution was then passed through an activated
Celite pad, concentrated to approximately 10 mL under reduced
pressure, and kept at 0 °C. After 1 day, suitable red-colored crystals of
the title compound were developed from the reaction mixture. Yield:
0.280 g (64%). 1H NMR (C6D6, 400 MHz, 298 K): δ 7.59 (2H, d, J =
7.3 Hz, Ar−H), 7.46 (2H, d, J = 9.1 Hz, Ar−H), 7.26 (2H, d, J = 9.8
Hz, Ar−H), 7.14 (1H, t, J = 7.3 Hz, Ar−H), 3.69−3.71 (2H, m, N−
CH−(CH2)2), 1.59−1.92 (12H, m, Cy−H), 1.24−1.36 (8H, m, Cy−
H), 0.09 (3H, s, Zn−Me) ppm. 13C NMR (C6D6, 100 MHz, 298 K): δ
160.5 (Ar), 135.1 (Ar), 130.1 (Ar), 129.3 (Ar), 125.5 (Ar), 120.2 (Ar),
119.0 (Ar), 108.9 (Ar), 60.3 (NCH−(CH2)2), 35.8 (CH2), 26.3
(CH2), 26.2 (CH2), −5.1 (Zn−Me) ppm. Melting point: 143−144 °C.
Anal. Calcd for C26H32N2Zn: C, 71.30; H, 7.36; N, 6.40. Found: C,
71.86; H, 7.41; N, 6.37.
General Procedure for the Intramolecular Hydroamination
of Primary Aminoalkenes. All NMR tube scale reactions were
performed in a N2-filled glovebox. A predried NMR tube was charged
with a solution of the catalyst (5 mol %), [PhNMe2H][B(C6F5)4]
(5 mol %), and hexamethylbenzene (measured amount used as
internal standard) in 0.6 mL of C6D6 under a nitrogen atmosphere.
Thereafter, the 1H NMR of the reaction mixture was checked to make
sure the abstraction of the Zn−Me protons was complete. Amino-
alkene (20 μL) was added to the solution, the NMR tube was sealed,
and the reaction mixture was heated in a preheated oil-bath maintained
at 120 °C for the stated duration of time. The progress of the reaction
was monitored by 1H NMR spectroscopy. The NMR yields were
determined by comparing the integration of the internal standard with
a well-resolved signal of the heterocyclic product.
General Procedure for the Intramolecular Hydroamination
of Secondary Aminoalkenes. All NMR tube scale reactions were
performed in a N2-filled glovebox. A predried NMR tube was charged
with a solution of the catalyst (5 mol %), [PhNMe2H][B(C6F5)4]
(5 mol %), and hexamethylbenzene (measured amount used as
internal standard) in 0.6 mL of C6D6 under a nitrogen atmosphere.
Thereafter, the 1H NMR of the reaction mixture was checked to make
sure the abstraction of the Zn−Me protons was complete. Amino-
alkene (20 μL) was added to the solution, the NMR tube was sealed,
and the reaction mixture was heated at 80 °C in a preheated oil-bath
for the stated duration of time. The progress of the reaction was
monitored by 1H NMR spectroscopy. The NMR yields were
determined by comparing the integration of the internal standard
with a well-resolved signal of the heterocyclic product.
General Procedure for Kinetic Experiments. All manipulations
were performed in a N2-filled glovebox. Kinetic experiments were
performed using NMR techniques on a JEOL-ECS 400 MHz and
Bruker Avance 500 MHz spectrometers. A standard solution of catalyst
was made by weighing the complex 2 into a vial and adding deuterated
solvent. The detailed kinetic experiments were carried out on the
1
Organics. H and 13C NMR spectra were recorded with a JEOL-ECS
400 MHz and Bruker Avance 500 MHz spectrometers using C6D6 as
solvent at 298 K unless otherwise stated. Elemental analyses were
performed using a Perkin-Elmer 2400 CHN analyzer. Melting points
were measured in sealed glass tubes with a Buchi melting point B 540
̈
instrument. 9-N-Cyclohexyl-1-oxophenalene13b and 9-N-cyclohexyl-1-
N′-cyclohexylphenalene15c were synthesized according to the literature
procedures. The aminoalkene substrates 2,2-diphenylpent-4-en-1-amine,6e
(1-allylcyclohexyl)methylamine,23 2,2-dimethylhex-5-enylamine,4e
4-methyl-2,2-diphenylpent-4-en-1-amine,24 4-methyl-(1-allylcyclohexyl)-
methylamine,24 benzyl(2,2-diphenyl-4-pentenyl)amine,6e (4-bro-
mobenzyl)-(2,2-diphenyl-4-pentenyl)amine,6e (4-methoxybenzyl)-(2,
2-diphenyl-4-pentenyl)amine,25 (2,2-diphenylpent-4-enyl)furan-2-ylme-
thylamine,8b (2,2-diphenylpent-4-enyl)thiophen-2-ylmethylamine,8b
(1-allylcyclohexylmethyl)benzylamine,6e (1-allylcyclohexylmethyl)-
(4-bromobenzyl)amine,26 2-Furan-2-ylmethyl-3-methyl-2-aza-spiro[4.5]-
decanamine,9g (1-allylcyclohexylmethyl)(4-methoxybenzyl)amine,26 and
benzyl(2,2-dimethyl-4-pentenyl)amine6e were prepared as reported pre-
viously and dried by distilling twice from CaH2. The hydroamination
products are known compounds and were identified by comparison to the
literature NMR spectroscopic data.4e,8b,9g,6e,24−26
Synthesis of [N(Cy),O-PLY-ZnMe]2 (1). A solution of [HN-
(Cy),O-PLY] (0.277 g, 1 mmol) in toluene (20 mL) was added
dropwise to a ZnMe2 solution (1.2 M in toluene; 1.0 mL, 1.2 mmol) in
toluene (20 mL) at −50 °C. The reaction mixture was slowly warmed
to ambient temperature and heated at 90 °C for 12 h. The solution
was filtered through a Celite pad under hot conditions and kept at
25 °C. Suitable orange-colored crystals of the title compound were
developed from the reaction mixture within 2 h. Yield: 0.220 g (62%).
1H NMR (THF-d8, 500 MHz, 298 K): δ 7.79 (4H, d, J = 8.5 Hz, Ar−
H), 7.69−7.67 (4H, m, Ar−H), 7.39 (2H, d, J = 10.0 Hz, Ar−H), 7.28
(2H, t, J = 7.5 Hz, Ar−H), 7.04 (2H, d, J = 9.0 Hz, Ar−H), 3.99−3.95
(2H, m, N−CH−(CH2)2), 1.92−1.72 (14H, m, Cy−H), 1.53−1.45
(4H, m, Cy−H), 1.31−1.25 (2H, m, Cy−H), −0.65 (6H, s, Zn−Me)
ppm. 13C NMR (THF-d8, 125 MHz, 298 K): δ 177.2 (Ar), 163.1 (Ar),
138.0 (Ar), 135.9 (Ar), 131.6 (Ar), 130.7 (Ar), 130.2 (Ar), 129.3 (Ar),
126.0 (Ar), 125.9 (Ar), 121.2 (Ar), 119.1 (Ar), 111.3 (Ar), 59.8
(NCH−(CH2)2), 35.3 (CH2), 26.4 (CH2), 25.4 (CH2), −13.0 (Zn−
Me) ppm. Melting point: 310−312 °C. Anal. Calcd for
C40H42N2O2Zn2: C, 67.28; H, 5.89; N, 3.92. Found: C, 67.18; H,
5.79; N, 3.87.
Synthesis of [N(Cy),N(Cy)-PLY-ZnMe] (2). A solution of
[HN(Cy),N(Cy)-PLY] (0.358 g, 1.0 mmol) in toluene (25 mL) was
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cyclization of 3. Stock solutions containing substrate 3, an internal
standard (hexamethylbenzene), and activator were prepared by
dissolving in deuterated solvent in a similar fashion as described
above. The activator, internal standard, and catalyst were added to a
predried screw cap NMR tube. Thereafter, the 1H NMR of the
reaction mixture was checked to ensure the complete abstraction of
the Zn−Me protons. A measured amount of aminoalkene substrate
was added to this solution, the NMR tube was sealed and placed at the
appropriate temperature, and the progress of the catalysis was
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1
monitored by H NMR spectroscopy after described time intervals.
The concentrations of substrate and product were determined by
relative integration to a known concentration of hexamethylbenzene
dissolved in deuterated solvent.
Computational Details. Geometry optimizations and vibrational
frequency analyses were carried out without any symmetry constraints
at the level of density functional theory (DFT) based methods as
implemented in the electronic structure program Gaussian 03.27 We
used Beck’s three-parameter hybrid exchange functional28 combined
with the Lee−Yang−Parr nonlocal correlation function29 abbreviated
as B3LYP. The split-valence basis set with diffuse functions, namely, 6-
311++G, has been employed for all atoms. Vibrational frequencies
were calculated for optimized molecular structures to verify that no
negative frequencies were present for minimum energy structures.
X-ray Crystallographic Studies. The data of 1 and 2 were
collected from a shock-cooled crystal at 100(2) K30 on a Bruker
SMART-APEX II diffractometer with a D8 goniometer equipped with
a fine-focus INCOATEC Mo microsource.31 The data sets of the
compounds were integrated with SAINT,32 and an empirical
absorption correction (SADABS) was applied.33 The structures were
solved by direct methods (SHELXS-97) and refined by full-matrix
least-squares methods against F2 (SHELXL-97).34 All non-hydrogen
atoms were refined with anisotropic displacement parameters. The
hydrogen atoms were refined isotropically at calculated positions using
a riding model. CCDC-911744 (1) and CCDC-805581 (2) contain
the supplementary crystallographic 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.
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Organometallics 2013, 32, 1858−1865.
ASSOCIATED CONTENT
* Supporting Information
■
S
Complete ref 27, NMR of the stoichiometric reaction mixture,
NMR characterization data of the complexes, and X-ray
crystallographic details. This information is available free of
charge via the Internet at http://pubs.acs.org.
(7) (a) Jiang, T.; Livinghouse, T. Org. Lett. 2010, 12, 4271−4273.
(b) McGrane, P. L.; Livinghouse, T. J. Am. Chem. Soc. 1993, 115,
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1992, 57, 1323−1324. (d) Patil, N. T.; Pahadi, N. K.; Yamamoto, Y.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: swadhin.mandal@iiserkol.ac.in.
■
Notes
The authors declare no competing financial interest.
(8) (a) Zulys, A.; Dochnahl, M.; Hollmann, D.; Lohnwitz, K.;
̈
Herrmann, J.-S.; Roesky, P. W.; Blechert, S. Angew. Chem. 2005, 117,
7972−7976; Angew. Chem., Int. Ed. 2005, 44, 7794−7798.
ACKNOWLEDGMENTS
■
A.M. and T.K.S. are thankful to IISER-Kolkata and CSIR, India,
respectively for research fellowships. S.K.M. thanks SERB
(DST), New Delhi, India, for financial support.
(b) Dochnahl, M.; Lohnwitz, K.; Pissarek, J.-W.; Biyikal, M.; Schulz,
̈
S. R.; Schon, S.; Meyer, N.; Roesky, P. W.; Blechert, S. Chem.−Eur. J.
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2007, 13, 6654−6666. (c) Dochnahl, M.; Lohnwitz, K.; Luhl, A.;
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