Electronic modification of an aminotroponiminate zinc complex leading to an increased reactivity in the hydroamination of alkenes

Article abstract of DOI:10.1039/b719120k

The electronically modified zinc complex 5-phenylsulfanyl-N-isopropyl-2- (isopropylamino)troponiminate zinc methyl, [{PhS-ATI(iPr)₂}ZnMe], was synthesized. It showed an increased reactivity in the intramolecular hydroamination reaction of non-a

Full text of DOI:10.1039/b719120k

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Electronic modification of an aminotroponiminate zinc complex leading to an
increased reactivity in the hydroamination of alkenes†
Maximilian Dochnahl,a Karolin Lo¨hnwitz,b Jens-Wolfgang Pissarek,a Peter W. Roesky*b and
Siegfried Blechert*a
Received 11th December 2007, Accepted 4th March 2008
First published as an Advance Article on the web 4th April 2008
DOI: 10.1039/b719120k
The electronically modified zinc complex 5-phenylsulfanyl-N-isopropyl-2-
(isopropylamino)troponiminate zinc methyl, [{PhS-ATI(iPr)2}ZnMe], was synthesized. It showed an
increased reactivity in the intramolecular hydroamination reaction of non-activated alkenes compared
to a previously-reported, non-substituted complex.
a catalyst for the intramolecular hydroamination.10 This complex
possesses interesting advantages compared to other systems: (i) it
shows a very high tolerance towards polar functional groups and
(ii) it is remarkably stable towards air and moisture. Zinc is also
Introduction
Organozinc compounds were the first organometallic compounds
having a metal to carbon r-bond.1 Nevertheless, for a long
time organolithium and Grignard reagents were the dominating
one of the least expensive and non-toxic metals which makes its
organometallic reagents in organic synthesis because they show a
use in catalysis highly desirable.
higher nucleophilic reactivity compared to zinc reagents.2 About
30 years ago it was realized that the low nucleophilic reactivity
Results
of organozinc compounds can be used to prepare functionalized
organozinc reagents. Today, numerous catalytic and stoichiometric
zinc mediated organic reactions such as the Negishi cross coupling,
the Simmons–Smith cyclopropanation, the Reformatsky reaction,
the CO2 epoxide copolymerisation and a number of nucleophlic
addition and substitution reactions are known.3 Recently we in-
troduced organozinc complexes as catalysts for the intramolecular
hydroamination reaction, which is the direct addition of N–H
bonds to C–C multiple bonds (Scheme 1).4
In our ongoing research on zinc-catalyzed hydroamination, we
first investigated the influence of steric modifications of the alkyl
groups at the aminotroponimine ligand.11 We were able to show
the major influence of the steric environment around the zinc atom
on both reactivity and stability of the corresponding complexes.
In a second project we aimed to increase the activity of aminotro-
poniminate zinc complexes by changing the electronic properties
of the ligand. Thus, we synthesized a series of aminotroponimines
with electron donating and withdrawing groups. A series of test
reactions revealed that catalyst 2, bearing a thioether moiety in
the 4-position, was superior to all other investigated catalysts.
The corresponding ligand 4 was prepared in a two-step procedure
from the known ligand 3 (Scheme 2).12 Following the literature
procedure, bromination occurred selectively in the 5-position
giving the bromoarene in high yield.13 Nucleophilic displacement
Scheme 1 Intramolecular hydroamination of aminoolefins.
A multitude of metals and catalysts has been employed for this
transformation, especially the lanthanides,4b,c,5 group 4 metals,5,6
the platinum metals7 and also calcium8 and very recently gold.4a,9
However, most of these catalysts have disadvantages like high
prices, toxicity and/or little tolerance towards polar functional
groups. As alternative we recently introduced the zinc complex,
N-isopropyl-2-(isopropylamino)troponiminate zinc methyl (1), as
aTechnische Universita¨t Berlin, Institut fu¨r Chemie, Straße des 17. Juni
135, 10623, Berlin, Germany. E-mail: blechert@chem.tu-berlin.de; Fax: +49
(0)30 3142 3619; Tel: +49 (0)30 3142 2355
bFreie Universita¨t Berlin, Institut fu¨r Chemie und Biochemie, Fabeck-
straße 34-36, 14195, Berlin, Germany. E-mail: roesky@chemie.fu-berlin.de;
Fax: +49 (0)30 8385 2440; Tel: +49 (0)30 8385 4004
† CCDC reference numbers 653787. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/b719120k Electronic sup-
plementary information (ESI) available: Complete spectral data of the
complex and its precursors, complete spectral data of the obtained
products and experimental procedure of the hydroamination. See DOI:
10.1039/b719120k
Scheme 2 Reagents and conditions: (a) Br2, CH2Cl2, 0 ◦C to RT, 1 h,
90%; (b) PhSH, K2CO3, DMF, 70 ◦C, 16 h, 99%; (c) ZnMe2 (1.7 eq.),
toluene, RT, 3 h, 88%.
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proceeded smoothly and delivered the desired ligand 4 in an overall
yield of 89% from 3. The synthesis of the zinc complex 2 was
equally high yielding when the ligand was reacted with ZnMe2
in toluene. Compound 2 was also characterized by single crystal
X-ray diffraction analysis (Fig. 1). It is a monomer in the solid
state with two molecules in the unit cell, and hence, the zinc atom
displays a trigonal planar geometry. Bond lengths and angles are
in the expected range. Comparison with 1 shows that there are no
major structural differences.
of the ligand acts as a donor substituent and therefore increases
the electron density at the nitrogen and the zinc atoms. This effect
seems to increase the stability of the chelate and also renders the
zinc atom more reactive. The new catalyst 2 is easily prepared
in high yields from a well-known aminotroponimine and showed
higher reactivity and stability in solution. These findings make
the new catalyst 2 superior to the first generation catalyst 1 and
will potentially enhance the use of zinc complexes for the catalytic
hydroamination.
Experimental
I
General
1H and 13C-NMR spectra were recorded on a Bruker DRX 500
spectrometer at 353 or 298 K using the deuterated solvent as a
lock and residual solvent peak as the internal reference. MS and
HRMS were recorded on a Finnigan MAT 95 SQ spectrometer.
IR spectra were measured on a Nicolet FT-IR 750 spectrometer.
Elemental analyses were carried out with an Elementar vario EL
or EL III. All reactions were, unless otherwise stated, carried
out using standard Schlenk and glovebox techniques under an
atmosphere of nitrogen. [PhNMe2H][B(C6F5)4] was purchased
from Strem. ZnMe2 was obtained from Aldrich. Benzene-d6 was
Fig. 1 Perspective view of the molecular structure of 2. Hydrogen atoms
are omitted for clarity. Selected distances [pm] and angles [◦]: Zn–N1
198.79(14), Zn–N2 199.04(14), Zn–C14 194.8(2); Zn–N1–C1 113.69(10),
Zn–N1–C8 124.97(10), Zn–N2–C7 114.42(11), Zn–N2–C11 124.42(11),
N2–Zn–C14 139.64(8), N1–Zn1–C14 137.18(8), N1–Zn–N2 81.52(5).
˚
dried over 4 A molecular sieves. Aminoalkenes were prepared
using modified literature procedures from commercially available
starting materials from Aldrich, Acros Organics and Fluka.
Prior to use, all substrates were purified either by distillation or
recrystallization.
The new catalyst 2 was then compared with the first generation
catalyst 1 in a series of test reactions in the hydroamination of non-
activated olefins bearing different functional groups (Table 1).
The reactions were carried out at 80 ◦C in benzene-
d6 with a catalyst loading of 2.5 mol% and 2.5 mol% of
[PhNMe2H][B(C6F5)4].14 The isolated yields were around 90% or
more in all the investigated cases. Entries 1–3 show the tremendous
effect of incorporating donating groups in the substrates. The
cyclization of 5a was accomplished within 40 min instead of 95 min
when the new catalyst 2 was used. Oxygenated furan derivative 6a
took 4 h to reach complete conversion with complex 2 and 6 h
with catalyst 1. Finally, the reaction time for the tosylated pyrrole
7a was reduced from 4 d to 24 h by applying the new catalyst 2.
An acceleration of the reaction rate was also observed in the case
of substrate 8a, bearing a geminal disubstituted double bond. In
this case, the reaction time could be reduced to about one third
with catalyst 2. The strained norbornene derivatives 9a and 10a
selectively reacted at the terminal double bond. In the case of 9a,
cyclisation took place within 2 h with the new catalyst 2, instead of
30 h with catalyst 1. The time for the cyclisation of indole derivative
10a was nearly halved with the new catalyst. Using 2, cyclohexane
derivative 11a completely cyclised within 2 h, whereas the same
compound needed 4.5 h with catalyst 1. The reaction time for
the 1,3-dithiane 12a was reduced from 20 d with complex 1 to
seven days with catalyst 2.
II General procedure for NMR-tube scale intramolecular
aminoalkynes hydroamination
A predried NMR-tube was charged with the aminoalkene
(430 lmol). A solution of 2 (5 mg, 11 lmol, 2.5 mol%) and
[PhNMe2H][B(C6F5)4] (9 mg, 11 lmol, 2.5 mol%) in 0.5 mL
C6D6 was added under a nitrogen athmosphere. The NMR-tube
was flame sealed under vacuum. The reaction mixture was then
heated to 80 ◦C for the stated duration of time. The reaction
1
progress was monitored by H NMR. When the reaction was
judged to be completed, the crude reaction mixture was directly
subjected to column chromatography on silica. All products were
analysed by 1H, 13C, 13C-DEPT, IR, MS, HRMS. The NMR yields
were determined by comparing the integration of a well-resolved
signal for the starting material with a well-resolved signal for the
heterocyclic product.
III Characterisation of new ligands and the catalysts
Conclusions
In conclusion, we have reported the synthesis of a novel, substi-
tuted aminotroponimine ligand 4, its complexation to zinc and
the catalytic activity of the corresponding zinc complex 2. We
have demonstrated that electronic modifications of the ligand can
have a high, positive impact on the catalytic activity of the zinc
complex. We suppose that the thioether moiety at the backbone
(4-Bromo-7-isopropyliminocyclohepta-1,3,5-trienyl)isopropyl-
amine. A 250 mL flask was charged with H-ATI(iPr)2H15 (7.01 g,
34.4 mmol). 50 mL of CH2Cl2 were added and the solution was
cooled to 0 ◦C. A solution of bromine (1.8 mL, 35.1 mmol) in
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Table 1 Comparison of the reactivity of the new catalyst 2 with the first generation catalyst 1a
[{ATI(iPr)2}ZnMe] (1) [{PhS-ATI(iPr)2}ZnMe] (2)
Entry
Substrate
Time
Conversion
Time
Conversion
Product
1
95 min
Quant.b
40 min
Quant.b
93%c
2
3
6 h
Quant.b
4 h
Quant.b
96%c
96 h
95%b
24 h
96%b
87%c
4
5
20 d
30 h
86%b
93%b
7 d
2 h
Quant.%b
90%c
98%b
96%c
6
20 h
Quant.b
12 h
Quant.b
94%c
7
8
4.5 h
20 d
Quant.b
2 h
7 d
Quant.b
92%c
88%
96%b
89%c
a Reagents and conditions: substrate (430 lmol), catalyst (2.5 mol%), [PhNMe2H][B(C6F5)4] (2.5 mol%), C6D6, 80 ◦C. b Determined by 1H NMR. c Isolated
yield
20 mL of CH2Cl2 was added over 15 minutes. In the meantime
the colour changed from bright yellow to brown. The solution
was stirred for another 30 minutes at 0 ◦C and then warmed to rt
and stirred for further 15 minutes. Then 20 mL of a 2 N
NaOH solution were added and the biphasic solution was
transferred into a seperatory funnel. The aqueous phase was
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extracted with CH2Cl2 (2 × 15 mL). The combined organic phases
were then washed with brine (20 mL) and dried over MgSO4. The
crude product could either be purified by recrystallization from
ethanol or by column chromatography on silica (hexanes–TBME
2 : 1). The product was obtained as yellow, fluffy crystals (8.75 g,
30.9 mmol, 90%).
998 (w); 962 (w); 949 (w); 891 (w); 847 (w); 813 (m); 738 (m); 690
(m).
MS. (EI, 70 eV): m/z (%) = 312 [M+] (41); 298 (5); 297 (23);
271 (6); 270 (18); 269 (100); 253 (5); 240 (5); 239 (21); 212 (5).
HRMS. C19H24N2S calc.: 312.1660 found: 312.1662
Elemental analysis. calc.: C: 73.03% H: 7.74% N: 8.97%
found: C: 72.84% H: 7.76% N: 9.32%
1H-NMR. (CDCl3, 400 MHz): d (ppm) = 1.23 (d, J = 6.3 Hz,
12 H); 3.77 (sept., J = 6.3 Hz, 2 H); 6.03 (d, J = 12.2 Hz, 2 H);
6.92 (d, J = 12.2 Hz, 2 H); 7.63 (bs, 1 H, NH).
Melting point. 99 ◦C (EtOH)
13C-NMR. (CDCl3, 100 MHz): d (ppm) = 22.8; 46.1; 108.7;
109.7 134.8; 150.5 (Cq).
IR. (ATR): m (cm−1) = 3498 (br w); 3126 (w); 2967 (vs); 2925
(m); 2887 (w); 2865 (m); 2655 (w); 1943 (w); 1902 (w); 1853 (w);
1708 (m); 1581 (s); 1535 (s); 1512 (vs); 1467 (s); 1438 (s); 1383 (s);
1375 (s); 1364 (s); 1353 (s); 1322 (w); 1298 (m); 1260 (m); 1230 (m);
1171 (s); 1133 (m); 1124 (m); 1093 (w); 1075 (m); 1004 (w); 947
(w); 879 (w); 849 (w); 810 (s); 747 (m); 719 (m); 679 (m); 673 (m).
MS. (EI, 70 eV): m/z (%) = 284 [M+; 81Br] (56); 282 [M+; 79Br]
(56); 269 (74); 267 (74); 241 (96); 239 (100); 227 (20); 226 (24); 225
(33); 224 (31); 212 (23); 211 (43); 210 (29); 209 (45); 184 (17); 182
(17); 146 (18); 145 (34); 144 (17); 131 (23); 130 (22); 119 (17); 118
(33); 103 (21); 92 (17); 91 (25); 90 (33); 98 (15); 78 (15); 77 (17); 76
(17); 63 (19).
5-Phenylsulfanyl-N-isopropyl-2-(isopropylamino)troponiminate
zinc methyl (2). A solution of 4 (0.20 g, 0.64 mmol) in 15 mL
of toluene was slowly added to a solution of ZnMe2 (1.2 M in
toluene; 1 mL, 1.2 mmol) in 15 mL of toluene. The solution was
stirred at rt for 3 h. The reaction was completed when the gas
evolution had stopped. All volatiles were removed under reduced
pressure and the yellow residue was washed twice with pentane
(5 mL). Recrystallisation from toluene at −30 ◦C gave the product
as yellow crystals (0.22 g, 0.56 mmol, 88%)
HRMS. C13H19BrN2 calc.: 282.0732 found: 282.0732
Elemental analysis. Calc.: C: 55.13% H: 6.76% N: 9.89% found:
C: 54.89% H: 6.67% N: 9.67%
Melting point. 94 ◦C (EtOH)
1H-NMR. (CDCl3, 500 MHz): d (ppm) = −0.03 (s, 3 H); 1.08
(d, J = 6.3 Hz, 12 H); 3.63 (sept., J = 6.3 Hz, 2 H); 6.34 (d, J =
12.1 Hz, 2 H); 6.89 (t, J = 7.3 Hz, 1 H), 7.01 (m, J = 7.9 Hz, 2 H);
7.32 (d, J = 7.3 Hz, 2 H); 7.42 (d, J = 12.1, 2 H).
13C-NMR. (CDCl3, 125 MHz): d (ppm) = −10.1, 24.0, 48.2,
110.6, 119.0, 125.2, 126.5, 128.2, 129.0, 142.1, 159.7;
MS. (EI, 70 eV): m/z (%) = 390 [M+] (89); 375 (62); 312 (21);
295 (100); 281 (25); 269 (56); 253(44); 239 (26).
Elemental analysis. calc.: C : 61.30% H : 6.69% N : 7.15% S :
8.18% found: C : 60.78% H : 7.08% N : 7.05% S: 21%
Isopropyl-(7-isopropylimino-4-phenylsulfanylcyclohepta-1,3,5-
trienyl)amine (4). A 10 mL flask was charged (4-bromo-7-
isopropyliminocyclohepta-1,3,5-trienyl)isopropylamine (282 mg,
1.00 mmol), K2CO3 (701 mg, 5.07 mmol) and 3 mL of
DMF. Thiophenol (115 ll, 1.12 mmol) was added in one portion
X-Ray crystallographic studies of 2†
A suitable crystal was covered in mineral oil (Aldrich) and
mounted on a glass fiber. The crystal was transferred directly to the
−73 ◦C cold stream of STOE IPDS 2T diffractometer. Subsequent
computations were carried out on an Intel Pentium Core2Duo PC.
◦
and the solution was stirred at 70 C overnight. The suspension
was then cooled to rt and poured on a biphasic mixture of water
and TBME (1 : 1, 40 mL). The aqueous phase was extracted with
TBME (2 × 20 mL). The combined organic phases were washed
with water and brine (30 mL). After evaporating the solvents, the
crude product could either be purified by recrystallization from
ethanol or by column chromatography on silica (hexanes–TBME
2 : 1). The product was obtained as yellow needles (315 mg,
998 lmol, 99%).
1H-NMR. (CDCl3, 400 MHz): d (ppm) = 1.25 (d, J = 6.0 Hz,
12 H); 3.82 (sept., J = 6.0 Hz, 2 H); 6.20 (d, J = 12.4 Hz, 2 H);
6.99 (d, J = 12.0 Hz, 2 H); 7.07–7.12 (m, 1 H); 7.15–7.27 (m, 4 H);
7.70–7.95 (br s, 1 H, NH).
Crystal data for 2. C20H26N2SZn, M = 391.86, triclinic, space
group P-1, a = 978.20(8) pm, b = 1028.33(9) pm, c = 1125.87(9)
pm, a = 106.262(7), b = 97.856(7), c = 112.659(7)◦, V =
964.31(14) pm3, T = 200(2) K, Z = 2, l = 1.385 mm−1, 6896
reflections collected, 3364 unique, R1 = 0.0235 (I > 2r(I)),
wR2 = 0.0599 for all 3364 data, 222 parameters, all non hydrogen
atoms calculated anisotropic; the positions of the H atoms were
calculated for idealised positions. The structure was solved and
refined using SHELXS-97 and SHELXL-97.16
13C-NMR. (CDCl3, 100 MHz): d (ppm) = 22.9; 46.1, 109.0,
118.1 (Cq); 125.1; 127.0; 128.8; 139.4; 140.1 (Cq); 151.4 (Cq).
IR. (ATR): m (cm−1) = 3195 (w); 3070 (w); 3056 (w); 2966 (m);
2928 (w); 2867 (w); 1578 (s); 1536 (s); 1511 (s); 1478 (s); 1453 (m);
1437 (s); 1386 (s); 1366 (m); 1351 (m); 1326 (w); 1301 (w); 1263
(m); 1235 (m); 1174 (s); 1121 (w); 1077 (w); 1042 (w); 1024 (m);
Acknowledgements
The authors acknowledge support from the Cluster of Excellence
“Unifying Concepts in Catalysis” coordinated by the Technische
Universitaet Berlin and funded by the Deutsche Forschungsge-
meinschaft.
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14 The addition of one equivalent of [PhNMe2H][B(C6F5)4] was shown
to have a beneficial effect on the reactivity of the zinc catalyst. We
attribute this to the formation of a cationic zinc species which is
formed by the protonolysis of the zinc–methyl moiety. The disap-
1
pearance of the zinc–methyl group has been monitored by H-NMR
spectroscopy. It is most likely that the cocatalyst also accelerates the
protonolysis of the alkylzinc moiety which is formed during the catalytic
cycle.
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