Ln[N(SiMe3)2]3-Catalyzed Cross-Diinsertion of C≡N/C≡C into an N-H Bond: Facile Synthesis of 1,2,4-Trisubstituted Imidazoles from Propargylamines and Nitriles

Article abstract of DOI:10.1002/chem.201402701

A lanthanide-catalyzed sequential insertion of C≡N and C≡C into an N-H bond is presented. The convenient reaction, which proceeds under mild conditions, is an efficient method for preparing 1,2,4-trisubstituted imidazoles directly from readily available propargylamines and nitriles.

Full text of DOI:10.1002/chem.201402701

DOI: 10.1002/chem.201402701
Communication
&
Insertion Reactions
Ln[N(SiMe3)2]3-Catalyzed Cross-Diinsertion of CꢀN/CꢀC into an
NÀH Bond: Facile Synthesis of 1,2,4-Trisubstituted Imidazoles
from Propargylamines and Nitriles
Longcheng Hong,[a] Yinlin Shao,[a] Lixin Zhang,[a] and Xigeng Zhou*[a, b]
pected to have synthetic potential. The major reason for this
Abstract: A lanthanide-catalyzed sequential insertion of
CꢀN and CꢀC into an NÀH bond is presented. The con-
venient reaction, which proceeds under mild conditions, is
an efficient method for preparing 1,2,4-trisubstituted imid-
azoles directly from readily available propargylamines and
nitriles.
paucity can be attributed to the difficulties encountered in
controlling required selectivity and to the incompatibility of
cross insertion of polar functional groups with non-polar
groups.[6] Heteroatoms often coordinate more strongly to rare
earth metals than alkenes (alkynes), thus inhibiting catalytic
turnover.
Substituted imidazoles are important structural components
of a vast array of naturally occurring and pharmacologically
active molecules.[7] They can also serve as intermediates for the
synthesis of many important drugs,[8] heterocyclic ligands,[9] or
precursors for materials with interesting properties.[10] In this
connection, the limitations of traditional synthetic methodolo-
gy for preparing heterocycles have stimulated considerable in-
terest in developing new, efficient homogeneous catalytic
methods for the synthesis of such compounds.[11] Among
many synthetic strategies developed, the preparation of imid-
azoles directly from alkynes and nitriles is especially attractive
because a large number of these starting materials are com-
mercially available or readily prepared. Recently, Xie and Shen
found that titanacarborane amide can catalyze the cycloaddi-
tion of nitriles with propargylamines to form imidazoles.[12]
However, for terminal alkynes and secondary amines, the pro-
cedure generally suffers from low yields and poor reactivity,
probably owing to competing alkynylation and steric effects,
respectively. Given the fact that larger rare earth metals often
coordinate more strongly to the amido ligand than titanium,
but bind more weakly to alkyl/alkynyl groups than titanium,
we were interested in establishing whether the replacement of
titanium with rare earth metals would inhibit the terminal
alkyne from quenching the MÀC/N bond and be compatible
with sterically demanding disubstituted amines. Herein, we de-
scribe a new catalytic system with wide reactant scope for the
tandem amidination/hydroamination/cyclization reaction of
propargylamines and nitriles.
NÀH bonds are common in natural and synthetic organic com-
pounds. The addition of NÀH bonds across unsaturated CÀC
and carbon–heteroatom functional groups represents an effi-
cient, atom-economical, and desirable route to nitrogen-con-
taining organic compounds, which are important for a variety
of applications.[1] Lanthanide catalysis has played an important
role in the development of many of these processes owing to
the high levels of activity that are observed in the absence of
base or accelerating ligand and their ability to generate unpre-
cedented types of reactivity.[1b,2,3] In addition to the develop-
ment of efficient and selective catalysts for this highly valuable
but challenging transformation, considerable recent interest
has focused on the design of related tandem or sequential re-
actions for the efficient construction of complex molecules.
The elegant work of the research groups of Marks and Moland-
er involving lanthanide-catalyzed intramolecular multiple inser-
tion of alkenes or alkynes into an NÀH bond allows facile
access to bi- and tricyclic azacycles.[4] Furthermore, lanthanide-
catalyzed intra- and intermolecular cross-diinsertion reactions
of an alkene and an alkyne into an NÀH bond has also been
studied.[5]
Apart from the above investigations, the lanthanide-cata-
lyzed sequential insertion of alkenes (alkynes) and other polar
unsaturated functional groups into NÀH bonds has remained
unexplored to date, despite such tandem reactions being ex-
Considering that Ln[N(SiMe3)2]3 could serve as readily avail-
able, inexpensive, and highly versatile reagents for catalytic NÀ
H and CÀH bond functionalizations,[3g–i,13,14] we initiated our re-
search on the model reaction of benzonitrile 1a with propar-
gylamine 2a by using Ln[N(SiMe3)2]3 as the precatalyst
(Table 1). By screening reaction conditions, it was found that
the sequential amidination/cyclization reaction of propargyl-
amines with nitriles can be catalyzed by Sm[N(SiMe3)2]3 at 608C
(1a/2a=1:1.2) to give 3aa in 93% yield (Table 1, entry 15). Ele-
vating or decreasing the reaction temperature resulted in
lower yields of product (Table 1, entries 6–9). THF was found to
[a] L. Hong, Y. Shao, L. Zhang, Prof. Dr. X. Zhou
Department of Chemistry
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials
Fudan University, Shanghai 200433 (P. R. China)
E-mail: xgzhou@fudan.edu.cn
[b] Prof. Dr. X. Zhou
State Key Laboratory of Organometallic Chemistry
Shanghai 200032 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402701.
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To our delight, the catalytic system is effective for the trans-
formation of a number of terminal alkynes. For example, reac-
tion of 2a with 1a and 1b provided the desired products 3aa
and 3ba in 93% and 83% yields, respectively. In contrast,
these transformations in the Ti-based catalytic system provided
the products in only 20–21% yield, even with a longer reaction
time and a higher reaction temperature.[12] This difference may
be attributed to the stronger Lewis acidity of Sm compared to
Ti, thus preventing the terminal alkyne from quenching the
metal–carbon/nitrogen bond.
Table 1. Optimization of reactions conditions for the preparation of
imidazoles.[a]
Entry
Ln
Solvent
T [8C]
t [h]
Yield [%][b]
1
2
3
4
5
6
7
8
La
Sm
Nd
Gd
Y
Sm
Sm
Sm
Sm
Sm
Sm
Sm
Sm
Sm
Sm
SmCl3
Sm(OTf)3
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
THF
hexane
DCE
DMF
THF
THF
RT
RT
RT
RT
RT
40
60
80
100
60
60
60
60
60
60
60
60
24
24
24
24
24
24
24
24
24
36
24
24
24
24
24
24
24
9
51
22
4
29
68
70
53
43
79
84
–
–
–
93
–
–
The use of various propargylamines was also examined
(Table 3). Propargylamines with an internal/terminal CꢀC bond
are all compatible with this reaction. Arylalkynes are more re-
active than alkylalkynes, presumably because aryl substituents
facilitate the coordination of the alkyne moiety to the catalyst
through their weak interaction with the metal center,[15] thus
offering higher yields of products (3ak versus 3ah--aj). Surpris-
ingly, when the optimized conditions were applied to the cycli-
zation of N-phenyl propargylamine (2d) in an attempt to pre-
pare N-phenyl imidazole (3ad), the reaction did not go to
completion. When Yb[N(SiMe3)2]3 was used as catalyst and the
reaction temperature raised to 1408C, with xylene as the sol-
vent, 3ad was obtained together with significant amounts of
amidine intermediates (Scheme 1). The sluggishness of this re-
action likely results from steric hindrance of the aryl group and
the increasing acidity of N-aryl propargylamines compared
with N-alkyl propargylamines, thus leading to a preference for
amidinate abstraction over alkyne insertion. Simply switching
the catalyst to Y[N(SiMe3)2]3 provided desired compounds 3 in
decent yields (Table 3, 3ad–af). In addition, this catalytic proto-
9
10
11
12
13
14
15[c]
16
17
THF
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.2 mmol), catalyst
(0.01 mmol), solvent (1 mL), under N2. [b] Yield of isolated product.
[c] 1.2 equiv of 2a was used.
be the solvent of choice (Table 1, entries 10–14). The use of
other rare earth metal sources such as SmCl3 or Sm(OTf)3 was
ineffective (Table 1, entries 16 and 17). In addition,
the reaction proceeds in a 5-exo-dig fashion; no
product derived from a 6-endo-dig cyclization was
observed, indicating excellent regioselectivity.
Table 2. Nitrile scope.[a]
With optimized conditions established for the cycli-
zation of 1a with 2a, we proceeded to explore differ-
ent substrate combinations, as shown in Table 2. In
general, the presence of strong coordinating atoms
on nitriles led to lower yields because the Lewis basic
sites of such starting materials can coordinate prefer-
entially to the highly Lewis acidic lanthanide ions,
thus precluding h2-coordination of alkynes and the
migratory insertion processes necessary for cycliza-
tion. For example, reaction of 1a with aromatic nitrile
1e, which contains a methoxy group on the aromatic
ring, gave 3ea in moderate yield, while the use of 4-
nitrobenzonitrile (1 f) was ineffective. Similarly, 2-cya-
nothiophene (1i) gave an inferior result for the prep-
aration of 3ia. The trend is quite a contrast to that
previously observation.[6] Notably, the position of
a methyl group on the aromatic ring has an effect.
For example, a benzylnitrile with a methyl at the
ortho position (1n) gave expected product 3na in
a lower yield than the conversion of substrate 1k,
which bears a tert-butyl at the para-position, to give
3ka, probably owing to a steric effect. Nevertheless,
this method is applicable to most aliphatic nitriles.
[a] Reaction conditions: 1 (0.4 mmol), 2a (0.48 mmol), Sm[N(SiMe3)2]3 (0.02 mmol), THF
(2 mL) under N2, yield of isolated product. [b] 3 equiv of 2a was used.
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spectroscopy; equation 3). Moreover, the amidine
intermediate was also observed in the reaction of 1a
with 2d (Scheme 1). These results are in accordance
Table 3. Propargylamine scope.[a]
with an amidinate complex being
intermediate.
a reactive
In order to draw more support for a mechanism,
we carried out the reaction with the N-deuterated
propargylamine [D]-2k. Under the aforementioned
conditions, reaction of 1a with [D]-2k gave the deu-
terated product [D]-3ak, enriched at the benzylic po-
sition with >92% D, in 78% yield (Scheme 2). This
result clearly indicates that the CÀC double bond iso-
merization takes place after protonation (Scheme 3,
route A) rather than before protonation (route B).
Based on the results described above, a reasonable
pathway for the cyclization of propargylamines with
nitriles described herein is shown in Scheme 4. Acti-
vation of the NÀH bond leads to the formation of
a lanthanide-amido complex (A) together with the
liberation of HN(SiMe3)2. Coordination and sequential
insertion of one nitrile into the LnÀN bond of A gives
a key lanthanide-amidinate intermediate (B). Subse-
quently, the intramolecular addition of CꢀC bond to
the LnÀN bond of B leads to cyclization, affording
vinyl complex C. Then, predominant protonation of C
with another propargylamine affords cyclization inter-
mediate D and regenerates active intermediate A. Fi-
nally, double-bond migration of D results in the for-
mation of the imidazole product 3. Examples of inser-
tions of unsaturated functional groups into the lan-
[a] Reaction conditions: 1 (0.4 mmol), 2 (0.48 mmol), Sm[N(SiMe3)2]3 (0.02 mmol), THF
(2 mL), under N2, isolated yield. [b] Reaction was carried out at 1408C under N2, using
xylene as the solvent, Y[N(SiMe3)2]3 as catalyst, and 1.5 equiv of propargylamine was
used.
Scheme 1. Tandem cyclization of N-phenyl propargylamine.
col also allow the efficient cyclization of substrates bearing
a silyl and cyclopropyl groups at another alkyne terminus.
Compound 3ak was further confirmed by single-crystal X-ray
analyses as shown in Figure 1.
To shed light on the mechanism of the reaction, the follow-
ing experiments were conducted. Firstly, propargylamido yttri-
um complex 5, generated in situ from [(TMS)2N]2YCl and prop-
argyl sodium, was used to catalyze the cyclization of 1a with
2k; the yield of 3ak was 67%, as determined using GC
[Eq. (1)]. This result suggests that initial protonation of the cat-
alyst with 2k to form a propargylamido-yttrium intermediate
might be involved. In another experiment, N-methyl-N-(prop-
argyl)benzimidamide (6) was synthesized by the literature
method (equation 2).[16] Subsequent cyclization was performed
under the aforementioned conditions, giving the desired prod-
uct in almost quantitative yield (determined by using NMR
thanide–amidinate linkage are rare.[17] To our knowledge, the
present work represents the first example of alkyne insertion
into a lanthanide-amidinate linkage.
In summary, we have demonstrated a Ln[N(SiMe3)2]3-cata-
lyzed cyclization of a range of nitriles with propargylamines,
thus significantly expanding the scope of such transformations.
The reactions proceeded smoothly with excellent regioselectiv-
ity, providing convenient access to 1,2,4-trisubstituted imid-
azoles, which are difficult to synthesize by other means. This
lanthanide-mediated tandem insertion of CꢀN and CꢀC bonds
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Experimental Section
General procedure
In a glovebox, Sm[N(SiMe3)2]3 (12.6 mg, 0.02 mmol) and THF (2 mL)
were added to a Schlenk tube equipped with a magnetic stirring
bar and a Teflon cap. The sealed reaction tube was taken out of
the glovebox, and the progargylamine (0.48 mmol) and nitrile
(0.4 mmol) were added under a N2 atmosphere. Then, the reaction
mixture within the sealed Schlenk tube was stirred in an oil bath at
608C. After completion of the reaction, as indicated by GC-MS
analysis, the mixture was quenched with water (2 mL) and the re-
sulting solution was extracted with ethyl acetate (3ꢁ5 mL). The or-
ganic layers were combined and dried over sodium sulfate. The
pure 1,2,4-trisubstituted imidazole product was obtained by flash
column chromatography on silica gel with ethyl acetate/hexane
(1:1 to 3:1).
Figure 1. ORTEP drawing of 3ak. Selected bond length [ꢃ]: C1ÀN1 1.354(3),
C1ÀN2 1.323(3), C2ÀN1 1.377(3), C2ÀC3 1.347(4), C3ÀC11 1.506(4), C3ÀN2
1.374(3).
Acknowledgements
We thank the National Natural Science Foundation of China,
973 program (2011CB825304) and The Research Fund for the
Doctoral Program of Higher Education of China.
Scheme 2. Deuterium-labeling experiment.
Keywords: CÀC coupling · imidazoles · nitriles ·
lanthanides · propargylamines
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COMMUNICATION
&
Insertion Reactions
L. Hong, Y. Shao, L. Zhang, X. Zhou*
&& – &&
Ln[N(SiMe3)2]3-Catalyzed Cross-
Diinsertion of CꢀN/CꢀC into an NÀH
Bond: Facile Synthesis of 1,2,4-
Trisubstituted Imidazoles from
Propargylamines and Nitriles
Cross-diinsertion: Sequential insertion
of CꢀN and CꢀC bonds into an NÀH
bond can be effected using a lanthanide
catalyst (see scheme). The method effi-
cient gives direct access to 1,2,4-trisub-
stituted imidazoles from readily avail-
able nitriles and propargylamines.
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