Insertion of Nitriles into Zirconocene 1-aza-1,3-diene Complexes: Chemoselective Synthesis of N-H and N-Substituted Pyrroles

Article abstract of DOI:10.1002/anie.201407221

The direct insertion of nitriles into zirconocene-1-aza-1,3-diene complexes provides an efficient, chemoselective, and controllable synthesis of N-H and N-substituted pyrroles upon acidic aqueous work-up. The outcome of the reaction (that is, the formatio

Full text of DOI:10.1002/anie.201407221

Angewandte
Chemie
DOI: 10.1002/anie.201407221
Pyrrole Synthesis
Hot Paper
Insertion of Nitriles into Zirconocene-1-aza-1,3-diene Complexes:
Chemoselective Synthesis of N-H and N-Substituted Pyrroles**
Shasha Yu, Meijun Xiong, Xin Xie, and Yuanhong Liu*
Dedicated to Professor Tamotsu Takahashi on the occasion of his 60th birthday
Abstract: The direct insertion of nitriles into zirconocene-1-
aza-1,3-diene complexes provides an efficient, chemoselective,
and controllable synthesis of N-H and N-substituted pyrroles
upon acidic aqueous work-up. The outcome of the reaction
(that is, the formation of N-H or N-substituted pyrroles) results
from the different cyclization patterns, which depend on the
relative stability and reactivity of the enamine–imine tautomers
formed by hydrolysis of the diazazirconacycles.
of novel methodologies, it is somewhat surprising that unlike
the zirconocene–butadiene complexes,[5] so far only the
reactions with tert-butylisocyanide,[2a] ketones,[2b,d,i] special
imine,[6] and PhBCl2
have been achieved. During our
[2h]
ongoing research on the chemistry of early-transition-metal
complexes of conjugated organic substrates,[7] we found that
the cross-coupling reactions of 1-azabuta-1,3-dienes with
nitriles could be efficiently promoted by low-valent zircono-
cene species. Herein, we report the direct insertion of
nitriles[8] into zirconocene-1-aza-1,3-diene complexes, which
provided an efficient, chemoselective, and controllable syn-
thesis of N-H and N-substituted pyrroles in a one-pot
procedure. Interestingly, the chemoselectivity of these reac-
tions depended on the substitution patterns of the azadiene
substrates (Scheme 1).
Z
irconocene p-complexes and/or zirconacycles are among
À
the most important organometallic reagents in promoting C
C bond formation reactions in organic synthesis, because of
their unique structural features and their high reactivities
toward a wide range of electrophiles.[1] In this regard,
zirconocene complexes of 1-aza-1,3-dienes,[2,3] first reported
by Whitby et al. in 1991,[2a] are attractive synthetic precursors
for selective transformations. Based on X-ray crystal analysis,
these complexes are described as s2,p-azazirconacyclopen-
tenes (A) with a folded five-membered-ring moiety rather
than h4-azadiene complexes (B).[2b,i] In some cases, a rapid
ring-flipping process between the two conformers has been
verified by variable-temperature NMR experiments and DFT
calculations (Figure 1).[2a,i] Because of the existence of highly
À
À
polarized Zr C and Zr N bonds, they can be viewed as N,C
dianion or homoenolate equivalents,[4] which engage in
a series of addition reactions. In spite of the potential
synthetic utility of these zirconacycles in the development
Scheme 1. Zirconium-mediated synthesis of N-H or N-substituted
pyrroles.
Zirconocene-1-aza-1,3-diene complexes 2 can be conven-
iently prepared by a ligand-exchange reaction of azadienes
1 with zirconocene–butene complexes[2a,i] generated from the
Negishi reagent.[9] Thus, treatment of 1,4-diphenyl-substituted
azadiene (1a) with the Negishi reagent [Cp2ZrBu2] led to
azazirconacyclopentene 2a (R1 = Ph), which was rapidly and
cleanly coupled with three equivalents of PhCN at room
temperature to give 2,3-diphenyl-1H-pyrrole (3a) in 72%
yield after hydrolysis with 3m HCl (Table 1, entry 1). The
results indicated that only one equivalent of nitrile was
incorporated into the pyrrole product. When the amount of
PhCN was decreased to two equivalents, the yield of 3a also
decreased to 61%. When the reaction was quenched by
a saturated aqueous solution of NaHCO3, the desired product
was not obtained; according to TLC analysis, a complicated
mixture resulted instead. We thus concluded that pyrrole was
formed under acidic conditions. To determine the electronic
effect of the N substituents of azadienes on the reaction
course, we next synthesized substrates 1b–1j bearing various
R1 groups. A series of aryl groups as R1 were compatible with
Figure 1. Structural description of zirconocene-1-aza-1,3-diene com-
plexes.
[*] S.-S. Yu, M.-J. Xiong, X. Xie, Prof. Y.-H. Liu
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Lu, Shanghai 200032 (P. R. China)
E-mail: yhliu@mail.sioc.ac.cn
[**] We thank the National Natural Science Foundation of China (Grant
Nos. 21125210 and 21121062), the Chinese Academy of Science,
and the Major State Basic Research Development Program (Grant
No. 2011CB808700) for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201407221.
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Table 1: Effect of N substituents of azadienes on zirconium-mediated
Table 2: Synthesis of N-H pyrroles.[a]
cross-coupling reactions with nitriles.
Entry
R1
Yield
[%][a]
Entry
R1
Yield
[%][a]
1
2
3
4
5
Ph (1a)
72
68
28
54
63
6[b]
1-naphthyl (1 f)
nBu (1g)
cyclopropyl (1h)
tBu (1i)
74[c]
40
33
49
34
p-ClC6H4 (1b)
p-CF3C6H4 (1c)
o-FC6H4 (1d)
p-MeOC6H4 (1e)
7[b,d]
8[b,d]
9[b,d]
10[e]
NMe2 (1j)
[a] Yields of isolated products. [b] The coupling reactions with nitriles
were carried out at 508C for 2–3 h. [c] Naphthalen-1-amine was also
isolated in 66% yield. [d] Azazirconacycles 2 were prepared at 508C for
3 h. [e] The coupling reaction with nitrile was carried out at 808C for 6 h.
this reaction, furnishing the same product 3a in yields of 54–
74% under mild conditions (Table 1, entries 2 and 4–6).
However, when N-(p-CF3C6H4)-substituted azadiene 1c was
employed, only 28% of the desired product 3a was obtained
(Table 1, entry 3). N-Alkyl-substituted azadienes could also
be used in this reaction, but the product was obtained in lower
yields ranging from 33% to 49% (Table 1, entries 7–9).
Hydrazone 1j was identified as a suitable substrate for the
formation of azazirconacycle 2j,[2a] but a higher reaction
temperature of 808C was required when it was coupled with
benzonitrile (Table 1, entry 10). The above results also
indicated that one amine molecule, derived from the N-R1
moiety of azadiene 1, was eliminated during the process, as
evidenced by the isolation of naphthalen-1-amine in the case
of 1 f (Table 1, entry 6).
The scope of this zirconium-mediated cross-coupling
reaction toward N-H pyrroles was then investigated with
a variety of azadiene substrates bearing an N-phenyl group.
As shown in Table 2, a wide range of nitrile components were
examined. Both electron-withdrawing (Cl, Br, CF3, 3,4-
difluoro) and electron-donating (Me, tBu, OMe) groups on
the aryl ring of nitriles were well tolerated, producing the
corresponding 2,3-disubstituted N-H pyrroles 3b–3h in 59–
78% yields. The coupling reaction of (2-thienyl)nitrile with
azadiene 1a furnished pyrrole 3i in a high yield of 86%. The
use of 2-phenylacetonitrile afforded 3j in a low yield of 27%,
possibly as a result of the instability of the pyrrole product.
The effect of the R4 group at the alkene terminus of the
azadienes was also tested, and aryl groups bearing halide,
methyl, and methoxy substituents were all compatible with
the process, leading to 3k–3n in good yields of 67–82%. An
azadiene bearing one more substituent (R2 group) at the
imino carbon atom on the azadiene was also coupled
smoothly with benzonitrile to give the corresponding 2,3,5-
trisubstituted pyrrole 3o in 46% yield.
[a] Yields of isolated products. [b] The coupling reaction with nitrile was
carried out at 508C for 1 h. [c] The coupling reaction with nitrile was
carried out at room temperature for 1 h, and 508C overnight.
incorporated into the pyrrole products in these cases. As
shown in Table 3, this one-pot process offers a highly flexible
synthesis of pyrroles with various substituents. For example,
the reaction worked well for various aryl nitriles, providing
4a–4c in 78–82% yields. Heteroaryl nitriles, including (2-
thienyl)nitrile and even (2-pyridyl)nitrile, were coupled
efficiently with a 1,3,4-trisubstituted azadiene to afford the
corresponding pyrroles 4d and 4e in 73% and 87% yields,
respectively. In the case of (2-pyridyl)nitrile, the reaction was
quenched by a saturated aqueous solution of NaHCO3, and
the desired pyrrole 4e was formed during the purification
process. 2-Phenylacetonitrile was well accommodated to give
product 4 f in good yield of 68%. Regarding the N substitu-
ents on the azadienes, N-aryl- or N-benzyl-substituted aza-
dienes were compatible with this reaction, and substrates with
an n-hexyl or a phenyl group as R3 were also smoothly
converted to the pyrrole derivatives 4j and 4k in 73% and
65% yields, respectively. The structures of N-substituted
pyrroles were unambiguously confirmed by X-ray crystallo-
graphic analysis of compounds 3k and 4e.[10] Polysubstituted
pyrroles are of significant interest, as they can find a lot of
applications in synthetic,[11] pharmaceutical,[12] and material
science.[13] Our method provides a mild and efficient route to
these compounds.[14]
A proposed reaction mechanism for the formation of N-H
and N-substituted pyrroles is shown in Scheme 2. Insertion of
3
ꢀ
À
the C N bond of the nitrile into the Zr C(sp ) bond of
azazirconacyclopentene 2 affords diazazirconacycle 5. How-
ever, the nitrile attacking from this side may encounter large
steric hindrance. Alternatively, azazirconacycle 2 might be in
equilibrium with alkenylazazirconacyclopropane 6, although
the equilibrium lies far toward azazirconacycle 2. The
Interestingly, when 1,3,4-trisubstituted azadienes were
used as the substrates, N-substituted pyrroles 4 were formed
selectively under similar reaction conditions. Unlike in the
above reactions, the N-R1 group of the azadienes was
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Table 3: Synthesis of N-substituted pyrroles.[a]
tautomerization,[15] and the equilibrium ratio depends very
much on the substitution patterns. In the cases of 1,4- and
1,2,4-substituted azadienes (path a), fast tautomerization to
the more stable tautomer 9 with an imine moiety derived from
azadienes occurs, which undergoes ring closure via the
enamine intermediate to afford the corresponding cyclized
intermediate 11. This process is followed by the elimination of
the amine to give the di- or trisubstituted N-H pyrrole 3. In
the case of 1,3,4-trisubstituted azadienes (path b), the enam-
ine moiety in 8, derived from azadienes, may be stable enough
3
=
because of the presence of the R group on the C C bond to
undergo the subsequent intramolecular nucleophilic addition/
elimination to deliver the N-substituted pyrrole 4.
To support the mechanism, zirconium-containing inter-
mediate 5e, derived from the coupling reaction of the
azadiene (1p) and 2-pyridinecarbonitrile, was successfully
isolated (74% yield) and characterized by X-ray single-crystal
diffraction (Figure 2). The analysis of 5e showed a seven-
[a] Yields of isolated products. [b] The reaction was quenched by
a saturated aqueous solution of NaHCO3. [c] Azazirconacycle was
prepared at 508C for 3 h, and the coupling reaction with nitrile was
carried out at 508C for 1 h. [d] The coupling reaction with nitrile was
carried out at room temperature for 1 h, and 508C overnight.
Figure 2. Molecular structure of complex 5e. Hydrogen atoms are
omitted for clarity. Selected bond lengths [ꢀ] and angles [deg]: Zr1–N1
2.024(2), Zr1–N2 2.199(2), N1–C6 1.251(3), N2–C9 1.414(3), C6–C7
1.545(4), C7–C8 1.503(4), C8–C9 1.346(4); N1-Zr1-N2 93.52(9), C9-
N2-Zr1 115.03(16), C17-N2-Zr1 128.42(17), C17-N2-C9 112.9(2), C9-
C8-C7 128.7(2).
membered azazirconacycle. The results indicated that pyr-
roles were formed after hydrolysis of the zirconacycles. The
Zr1–N1 distance of 2.024(2) ꢀ indicates a s bond between Zr
and the sp2-hybridized imino N atom.[8h] The Zr1–N2 distance
of 2.199(2) ꢀ is in the range of a single bond, and the N2 atom
is only slightly pyramidalized (summation of bonding angles
at N2 356.358). The N1–C6 distance (1.251(3) ꢀ) corresponds
to an imine moiety.[8h] The large distances of Zr1–C9 and Zr1–
C8 (exceeds 3.0 ꢀ) do not indicate a p interaction between Zr
Scheme 2. Possible reaction mechanism.
=
and the C C bond.
addition of a CN group to the allyl zirconium moiety through
a syn-SE2’[7d] process leads to 5, which upon hydrolysis gives
the enamine–imine intermediate 8. Intermediate 8 equili-
brates with its tautomers, such as 9,[2d] through enamine–imine
In summary, we have disclosed a new reactivity of
zirconocene-1-aza-1,3-diene complexes in insertion reactions
with nitriles, providing an efficient and chemoselective route
to N-H and N-substituted pyrroles upon acidic aqueous work-
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=
À
[6] The insertion of a C N bond into the Zr C bond of an
azazirconacyclopentene was described in one case. It was
reported that the reduction of [ZrCl4(THF)2] with 2 equiv of
up. These two types of pyrroles are obtained selectively by
choosing azadiene substrates with different substituents. The
outcome of the reaction results from the different cyclization
patterns, which depend on the relative stability and reactivity
of the corresponding enamine–imine tautomers formed by
hydrolysis of the diazazirconacycles. Further investigations to
explore the synthetic utility of zirconocene-1-aza-1,3-diene
complexes are currently underway in our group.
=
=
Mg in the presence of 2 equiv of (cyclo-C6H11)N CH-C(Me)
CH(Ph) afforded a bicyclic azazirconacycle, which might be
formed through the insertion of the C N bond into the Zr C
bond of the azazirconacyclopentene. See reference [2f].
=
À
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Received: July 15, 2014
Published online: && &&, &&&&
Keywords: azazirconacycles · cyclization · insertion · nitriles ·
.
pyrroles
ꢀ
À
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Pyrrole Synthesis
S.-S. Yu, M.-J. Xiong, X. Xie,
Y.-H. Liu*
&&&& — &&&&
Insertion of Nitriles into Zirconocene-1-
aza-1,3-diene Complexes:
Chemoselective Synthesis of N-H and N-
Substituted Pyrroles
Exercise control: N-H and N-substituted
pyrroles can be prepared through the
direct insertion of nitriles into zircono-
cene-1-aza-1,3-diene complexes and their
subsequent acidic aqueous work-up. The
outcome of the reaction depends on the
relative stability and reactivity of the
corresponding enamine–imine tauto-
mers formed through the hydrolysis of
the diazazirconacycles.
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