Rhenium-catalyzed oxidative cyanation of tertiary amines with TMSCN

Article abstract of DOI:10.1002/ejoc.201301140

Oxidative cyanation of sp³ C-H bonds at the α position of tertiary amines by using TMSCN as the cyanide donor and a novel high-valent rhenium(V) complex was developed. The reaction offers the corresponding α-aminonitriles in good yields with tert-butyl hydroperoxide as the oxidant under mild and acid-free reaction conditions. Oxidative cyanation of sp³ C-H bonds at the α position of tertiary amines by using TMSCN as the cyanide donor and novel high-valent rhenium(V) complex catalysts is developed. The reactions offer the corresponding α-aminonitriles in good yields by using tert-butyl hydroperoxide as the oxidant under mild and acid-free reaction conditions at room temperature. Copyright

Full text of DOI:10.1002/ejoc.201301140

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201301140
Rhenium-Catalyzed Oxidative Cyanation of Tertiary Amines with TMSCN
Aijun Lin,[a] Hao Peng,[a] Ablimit Abdukader,[a] and Chengjian Zhu*[a,b]
Keywords: Synthetic methods / Cyanation / C–H activation / Rhenium / Amines
Oxidative cyanation of sp3 C–H bonds at the α position of
tertiary amines by using TMSCN as the cyanide donor and
a novel high-valent rhenium(V) complex was developed. The
reaction offers the corresponding α-aminonitriles in good
yields with tert-butyl hydroperoxide as the oxidant under
mild and acid-free reaction conditions.
cently, Wang developed the first redox-neutral [4+2] annu-
lation of benzamides and alkynes through C–H/N–H func-
tionalization for rapid access to 3,4-dihydroisoquinolinones
also on the basis of low-valent rhenium(I) catalysis.[11] The
low-valent-rhenium-catalyzed regio- and stereoselective ad-
dition of imines and indoles to terminal alkynes through
new C–C bond formation was independently reported by
Fukumoto[12] and Wang.[13] High-valent rhenium catalysis,
such as methyltrioxorhenium, was used as an efficient oxi-
dation transformation catalyst;[14] however, the sparse avail-
ability and expensive nature of this reagent limited its wide
application in organic synthesis. Recent work has indicated
that high-valent rhenium bearing different types of ligands
can be applied in different organic reactions. Toste and
Abu-Omar et al. designed and synthesized a variety of high-
valent rhenium oxo and imido complexes that have shown
efficient activities in catalytic reduction reactions.[15] To the
best of our knowledge, there have been few reports on C–
H activation, especially sp3 C–H bond activation, under the
catalysis of high-valent, low toxicity, and air-stable rhenium
complexes.[16] With our continuous interest in rhenium ca-
talysis[17] and sp3 C–H bond activation,[18] we herein present
our results on the oxidative cyanation of sp3 C–H bonds at
the α position of tertiary amines in the presence of a novel
stable oxorhenium(V) complex catalyst (Re-Bu). This
method affords a facile approach to the synthesis of α-
aminonitriles under solvent-free and acid-free reaction con-
ditions.
Introduction
The invention of efficient and selective chemical methods
for C–H bond functionalization is currently an important
field in reaction design.[1] The direct utilization of C–H
bonds offers a number of advantages from both environ-
mental and economical viewpoints as prefunctionalization
of the substrates is not required and the synthetic pro-
cedures are generally shorter. Transition-metal-catalyzed C–
H bond activation, especially that of unactivated sp3 C–H
bonds, has emerged as an important method, but it remains
a challenge in organic synthesis.[2,3] Recently, the direct oxi-
dative cyanation of C–H bonds adjacent to nitrogen atoms
has attracted much interest, as bifunctional organic com-
pounds with adjacent functional groups are highly useful
and versatile intermediates in organic synthesis; moreover,
these compounds have been widely used in the construction
of biologically active natural products, such as alkaloids
and vicinal diamines.[4] Several examples of metal-based
catalysts, such as Ru,[5] Fe,[6] V,[7] and Mo,[8] in the presence
of oxidants for the direct oxidative cyanation of tertiary
amines have been reported. We have also documented the
gold(III)-catalyzed oxidative α-cyanation of sp3 C–H bonds
of tertiary amines with trimethylsilyl cyanide (TMSCN) as
the cyanide donor, and the corresponding α-aminonitriles
were obtained in good to excellent yields.[9]
Homogeneous rhenium catalysis owing to its particular
reactivity, chemoselectivity, functional group compatibility,
and stability has attracted the attention of chemists in the
past few years. Kuninobu and Takai reported low-valent
rhenium(I)-based complexes for direct sp2 C–H bond acti-
vation and subsequent C–C bond formation.[10] Very re-
Results and Discussion
The strategy for the preparation of high-valent rhenium
complexes Re-Pr and Re-Bu is shown in Scheme 1. Reac-
tion of readily available oxazoline ligands 1[19] with ReV-oxo
dimethyl sulfide complex 2 in EtOH under reflux condi-
tions resulted in a bright green solid that could be separated
by filtration and thoroughly washed with cold ether and
ethanol.[20] Slow evaporation of a solution of the Re-Bu
complex offered X-ray-quality acicular crystals in high yield
[a] State Key Laboratory of Coordination Chemistry, School of
Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, P. R. China
E-mail: cjzhu@nju.edu.cn
http://hysz.nju.edu.cn/cjzhu/
[b] State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry,
345 Lingling Road, Shanghai 200032, P. R. China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301140.
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Rhenium-Catalyzed Oxidative Cyanation of Tertiary Amines
at room temperature.[21] The ORTEP diagram of the Re-Bu the most suitable oxidant (Table 1, entry 2 vs. entries 10–
complex is shown in Figure 1. The Re-Bu complex pos- 13). Relative to K3[Fe(CN)6], CH2(CN)2, and CH3CN,
sesses distorted octahedral geometry, in which the terminal TMSCN was the best cyanide source (Table 1, entry 2 vs.
double-bonded oxygen atom is trans to the oxygen atom of entries 14–16). CH2(CN)2 afford 4a in 42% yield, which can
one oxazoline ligand, and the chloride atom is trans to the be attributed to oxidative degradation of malononitrile
oxygen atom of the another oxazoline ligand. The length of through cleavage of the C–CN bond by the rhenium com-
the Re=O bond is 1.718(6) Å, which is within the normal plex.[23] Further studies indicated that reducing the catalyst
range of double-bonded rhenium oxo complexes.[22]
loading to 3 and 1 mol-% clearly affected the catalytic effi-
ciency (Table 1, entries 17 and 18).
Table 1. Optimization of the reaction conditions.[a]
Entry Cat.
Oxidant
“CN–”
Yield[b]
[%]
Scheme 1. Synthesis of oxorhenium oxazoline complexes Re-Pr and
Re-Bu.
1
2
3
4
5
6
7
8
Re-Pr
Re-Bu
KReO4
NH4ReO4
[nBu4N][ReOCl4]
ReOCl2(OPPh3)(SMe2)
ReOI2(OEt)(PPh3)2
ReO2I(PPh3)2
Re-Bu
Re-Bu
Re-Bu
Re-Bu
Re-Bu
Re-Bu
Re-Bu
Re-Bu
Re-Bu
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TMSCN
TMSCN
TMSCN
TMSCN
TMSCN
TMSCN
TMSCN
TMSCN
TMSCN
78
86
8
7
38
43
29
56
45[c]
Ͻ5
n.r.
Ͻ5
n.r.
9
10
11
12
13
14
15
16
17
18
CH3CO3tBu TMSCN
PhCO3tBu
mCPBA[d]
O2
TBHP
TBHP
TBHP
TBHP
TBHP
TMSCN
TMSCN
TMSCN
K3[Fe(CN)6] n.r.
CH2(CN)2
CH3CN
42
n.r.
71[e]
47[f]
TMSCN
TMSCN
Re-Bu
[a] All reactions were performed with 3 (0.5 mmol), cyanide
(1.2 equiv.), and oxidant (2.5 equiv.). [b] Yield of isolated product;
n.r.: no reaction. [c] The temperature was 60 °C. [d] mCPBA =
meta-chloroperoxybenzoic acid. [e] 3 mol-% catalyst was used.
[f] 1 mol-% catalyst was used.
Figure 1. ORTEP molecular structure representation of Re-Bu.
Initial studies on the catalytic activities of the high-valent
With the optimal conditions for the highly selective oxi-
oxorhenium(V) oxazoline complexes Re-Pr and Re-Bu were dative cyanation of tertiary amines in hand, the scope of the
performed to explore their potential in the oxidative reaction was investigated by using TMSCN as the cyanide
cyanation reactions of tertiary amines. N,N-Dimethylaniline source, TBHP as the oxidant, and Re-Bu (5 mol-%) as the
3 was chosen as the model substrate to optimize the reac- catalyst at room temperature. As shown in Table 2, substi-
tion conditions with various rhenium complex catalysts, tuted N,N-dimethylanilines with electron-donating and elec-
oxidants, and cyanide sources at room temperature tron-withdrawing groups were selectively and efficiently
(Table 1). In the presence of Re-Pr (5 mol-%), correspond- converted into the corresponding α-aminonitriles in good
ing aminonitrile 4a was obtained in 78% yield with trimeth- yields (Table 2, entries 2–7). N,N-Dimethyl-o-toluidine of-
ylsilyl cyanide (TMSCN) as the cyanide source and tert- fered a slightly lower yield (70% yield) than both N,N-di-
butyl hydroperoxide (TBHP, 5–6 m in decane) as the oxi- methyl-m-toluidine (78% yield) and N,N-dimethyl-p-
dant. To our delight, upon testing Re-Bu, the yield of 4a toluidine (82% yield) owing to steric hindrance. Upon using
improved to 86% (Table 1, entry 2). Perrhennate salts N-methyl-N-ethylaniline as the substrate, the N-methyl
(KReO4 and NH4ReO4) offered 4a in low yield (Table 1, group was oxidized chemoselectively to offer the corre-
entries 3 and 4). Other monooxorhenium or dioxorhenium sponding N-ethyl-N-phenylaminoacetonitrile in 72% yield
compounds offered 4a in moderate yield (Table 1, entries 5– (Table 2, entry 8). This system was also applied efficiently
8). A high temperature was detrimental to the oxidative to cyclic amines: piperidine, pyrrolidine, and tetrahydroiso-
cyanation reaction owing to the generation of more oxidat- quinoline derivatives were all converted into the corre-
ive byproducts such as N-methyl-N-phenylformamide sponding α-cyanoamines in good yields (Table 2, entries 9–
(Table 1, entry 9). Moreover, upon testing different oxidants 11). Unluckily, products 4i–k were isolated without any
with the use of Re-Bu as the catalyst, TBHP proved to be enantioselectivities. Primary and secondary amines such as
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A. Lin, H. Peng, A. Abdukader, C. Zhu
SHORT COMMUNICATION
benzylamine and dibenzylamine were tested under the opti- cleophilic attack C gives the corresponding α-cyanated
mal reaction conditions, but the corresponding products product and regenerates the oxorhenium(V) complex to
were not obtained. To demonstrate the practical utility of complete the catalytic cycle.
this method, scale-up experiments were performed with
10.0 mmol of N,N-dimethylaniline, and 4a was obtained in
83% yield (1.21 g).
Table 2. Re-Bu complex catalyzed oxidative cyanation of N,N-di-
alkylanilines.[a]
Scheme 2. The proposed reaction mechanism for the high-valent
rhenium complex catalyzed oxidative cyanation of tertiary amines.
Conclusions
In summary, we have described a novel high-valent
rhenium complex catalyzed oxidative cyanation reaction of
tertiary amines with trimethylsilyl cyanide and TBHP under
acid-free conditions at room temperature for the first time.
The reaction proceeds with high efficiency to give the corre-
sponding α-cyanated amines, which are extremely useful
synthetic intermediates in the construction of biologically
important compounds. Further studies concerning the
mechanistic details and asymmetric catalysis are now in
progress in our laboratory.
Experimental Section
Typical Procedure for the Synthesis of Oxazoline Ligands: An
amino alcohol (5 mmol) was added to a solution of 2-hydroxy-
benzonitrile (0.69 g, 5 mmol) and triphenylphosphine (4.8 g,
18.3 mmol) in CH3CN (40 mL) under an atmosphere of argon at
room temperature. To the resulting white suspension was added
triethylamine (3.1 mL, 44.2 mmol) with stirring, and a clear color-
less solution was obtained. CCl4 (9.9 mL, 100.0 mmol) was added
dropwise to the reaction mixture over 4 h. During the course of the
addition of the CCl4, a precipitate formed and the reaction mixture
changed color to dark red. The reaction was stirred for a further
[a] All reactions were performed with the tertiary amine
(0.5 mmol), TMSCN (1.2 equiv.), and TBHP (2.5 equiv.). [b] Yield
of isolated product. [c] Tertiary amine (10.0 mmol).
48 h, which resulted in a dark red suspension. The solution was
then filtered, and the colorless residue was washed with diethyl
ether (2ϫ30 mL). The filtrate and washing were combined, the re-
sulting precipitate was removed by filtration, and the process was
repeated until no more solids precipitated. The filtrate was removed
under reduced pressure to leave a sticky, dark red-brown residue,
which was extracted into hexane (200 mL). The solvent was evapo-
rated under reduced pressure to give a viscous, colorless residue.
The residue was purified by column chromatography (SiO2; hexane/
ethyl acetate, 50:1). The corresponding oxazoline was obtained as
a colorless oil.[19]
A probable mechanism for the oxidative cyanation of the
sp3 C–H bonds of tertiary amines catalyzed by the Re-Bu
complex is showed in Scheme 2. With an excess amount of
TBHP as the oxidant, the oxorhenium(V) complex readily
affords dioxorhenium(VII) product A, which further com-
bines with N,N-dimethylaniline to form intermediate com-
plex B. Intermediate B produces iminium ion transition
state C through electron transfer from the α-carbon-cen-
1
1a: H NMR (300 MHz, CDCl3): δ = 13.0 (s, 1 H), 7.65–7.62 (m,
tered radical and subsequent hydrogen transfer. Then, nu- 1 H), 7.38–7.34 (m, 1 H), 7.02–6.98 (m, 1 H), 6.87–6.84 (m,1 H),
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Rhenium-Catalyzed Oxidative Cyanation of Tertiary Amines
4.42–4.39 (m, 1 H), 4.13–4.09 (m, 1 H), 4.08–4.05 (m, 1 H), 1.80–
1.75 (m, 1 H), 1.03–1.00 (m, 3 H), 0.98–0.90 (m, 3 H) ppm.
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1
1b: H NMR (300 MHz, CDCl3): δ = 12.4 (s, 1 H), 7.67–7.61 (m,
1 H), 7.40–7.37 (m, 1 H), 7.02–6.99 (m, 1 H), 6.87–6.85 (m, 1 H),
4.47–4.45 (m, 2 H), 3.95- 3.92 (m, 1 H), 1.92–1.89 (m, 1 H), 1.67–
1.65 (m, 1 H), 1.42–1.40 (m, 2 H), 1.02–0.99 (m, 3 H), 0.97–0.95
(m, 3 H) ppm.
Typical Procedure for the Synthesis of Oxorhenium Oxazoline Com-
plexes: Ligand 1a or 1b (1 mmol) was dissolved in ethanol (50 mL),
followed by 2,6-lutidene (0.29 mL, 1 mmol). ReOCl2(OPPh3)-
(SMe2) (299 mg, 0.46 mmol) was added to the flask within 5 min.
The solution was heated at reflux under an atmosphere of argon
for 4 h, cooled to room temperature, and filtered to yield a green
solid, which was washed with cold ether (3ϫ10 mL) and then
dried.[20]
1
Re-Pr: Dark green powder (223 mg, 75%); m.p. 251 °C. H NMR
[2]
[3]
(300 MHz, CDCl3): δ = 7.91 (d, J = 7.5 Hz, 1 H), 7.73 (d, J =
7.5 Hz, 1 H), 7.46–7.40 (m, 1 H), 7.26–7.21 (m, 1 H), 6.97–6.92 (m,
1 H), 6.85–6.72 (m, 3 H), 5.17–5.14 (m, 1 H), 4.91–4.87 (m, 2 H),
4.69–4.67 (m, 1 H), 4.60–4.57 (m, 1 H), 4.45–4.39 (m, 1 H), 2.98–
2.86 (m, 2 H), 1.10–1.05 (m, 12 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 178.8, 171.4, 168.4, 164.5, 136.5, 130.8, 121.7, 119.5,
118.3, 110.1, 109.1, 76.3, 69.6, 67.9, 29.0, 19.3, 18.5, 15.1,
14.1 ppm. HRMS (ESI): calcd. for [C24H28N2O5Re]+ 609.1526;
found 609.1523.
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Re-Bu: Bright green powder (254 mg, 82%); m.p. 265 °C. 1H NMR
(300 MHz, CDCl3): δ = 7.82 (s, 1 H), 7.65 (d, J = 1.5 Hz, 1 H),
7.27–7.22 (m, 1 H), 7.07–7.04 (m, 1 H), 6.89–6.86 (m, 1 H), 6.68–
6.58 (m, 3 H), 5.51–5.49 (m, 1 H), 4.95–4.91 (m, 2 H), 4.80–4.78
(m, 1 H), 4.64–4.59 (m, 1 H), 4.48–4.46 (m, 1 H), 2.63–2.54 (m, 2
H), 1.84–1.40 (m, 2 H) 1.10–0.92 (m, 8 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 178.1, 171.4, 167.9, 164.5, 136.0, 130.9,
130.2, 122.3, 121.8, 118.7, 117.4, 109.9, 109.4, 73.8, 69.4, 68.1, 43.8,
42.7, 41.8, 25.6, 23.9, 21.9, 21.5, 21.2 ppm. HRMS (ESI): calcd. for
[C26H32N2O5Re]+, 637.1846; found 637.1835.
Typical Procedure for the Oxidative Cyanation of Tertiary Amines
Catalyzed by Re-Bu: A mixture of the amine (0.5 mmol), trimethyl-
silyl cyanide (0.6 mmol), Re-Bu (5 mol-%), and TBHP (5–6 m in
decane, 1.25 mmol) was stirred at room temperature for 5 h. At the
end of the reaction, as monitored by TLC, the reaction was
quenched by the addition of a saturated solution of NaHCO3
(2 mL), and the mixture was extracted with ethyl acetate (3–5 mL).
The combined organic layer was washed with brine, dried with an-
hydrous Na2SO4, and concentrated under reduced pressure to give
the crude product, which was purified by column chromatography
on silica gel. The fraction was collected and concentrated to give
the desired product.
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The authors gratefully acknowledge the National Natural Science
Foundation of China (NSFC) (grant numbers 21172106,
21074054), the National Basic Research Program of China (grant
number 2010CB923303), and the Research Fund for the Doctoral
Program of Higher Education of China (grant number
20120091110010) for their financial support.
Eur. J. Org. Chem. 2013, 7286–7290
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SHORT COMMUNICATION
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Received: July 30, 2013
Published Online: October 9, 2013
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