Silver-Catalyzed [2+1] Cyclopropenation of Alkynes with Unstable Diazoalkanes: N-Nosylhydrazones as Room-Temperature Decomposable Diazo Surrogates

Article abstract of DOI:10.1002/chem.201605335

The [2+1] cycloaddition of alkynes with diazo compounds represents one of the most powerful and reliable methods for the construction of cyclopropenes. However, it remains a formidable challenge to accomplish the cyclopropenation of alkynes with non-stabilized diazoalkanes, owing to the fact that such compounds are unstable and prone to detonation. Herein, we report a general silver-catalyzed cyclopropenation reaction of alkynes with unstable diazoalkanes, by for the first time the discovery and application of N-nosylhydrazones as room-temperature decomposiable diazo surrogates. This method allows for the efficient assembly a wide variety of cyclopropene derivatives that are otherwise difficult to access by conventional methods.

Full text of DOI:10.1002/chem.201605335

A Journal of
Accepted Article
Title: Silver-Catalyzed [2+1] Cyclopropenation of Alkynes with Unstable
Diazoalkanes: N-Nosylhydrazones as Room-Temperature
Decomposiable Diazo Surrogates
Authors: Xihe Bi, Zhaohong Liu, Qiangqiang Li, and Peiqiu Liao
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To be cited as: Chem. Eur. J. 10.1002/chem.201605335
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DOI: 10.1002/chem.200((will be filled in by the editorial staff))
Silver-Catalyzed [2+1] Cyclopropenation of Alkynes with Unstable Diazoalkanes:
N-Nosylhydrazones as Room-Temperature Decomposable Diazo Surrogates
Zhaohong Liu,[a] Qiangqiang Li,[a] Peiqiu Liao[a] and Xihe Bi*[a][b]
surrogates applied in [2+1] cyclopropenation of alkynes are
highly limited,[8]-[11] because the dissociation conditions of most
Abstract: The [2+1] cycloaddition of alkynes with diazo
diazo surrogates are hardly compatible with the following
compounds represents one of the most powerful and reliable
cyclopropenation process. As a result, only a few methods are
methods for the construction of cyclopropenes. However, it
known in the literature, including Warkentin’s photolysis of 2-
remains
a
formidable challenge to accomplish the
alkoxy-3,4-oxadiazolines,[8] Gevorgyan’s rhodium-catalyzed ring-
opening of pyridotriazoles,[9] Carreira’s in situ conversion of
trifluoroethylamine into trifluoromethyldiazomethane in aqueous
media,[10] and Valdés’ base-promoted decomposition of 2,2,2-
trifluoroacetophenone tosylhydrazones[11] (Figure 1). Limitation
to electron-withdrawing CF3 group or specific diazo precursors
remains as major disadvantages in these methods. To establish a
general [2+1] cyclopropenation reaction of alkynes with unstable
diazoalkanes, two issues must be overcome simultaneously, (i)
discovery of a diazo surrogate capable of dissociating under mild
conditions, (ii) finding a catalyst able to activate both diazo
species and alkynes. Silver catalysis has recently attracted much
attention in organic synthesis, especially its unique catalytic
performance often observed in many reactions.[12] While the
silver-catalyzed carbene transfer reactions have been less
developed to date, the pioneering reports by Davies and co-works
described silver-catalyzed cyclopropanation of alkenes[13] and
cyclopropenation of internal alkynes[14] with electron-withdrawing
group (EWG)-stabilized diazo compounds. As our continued
studies on silver-catalyzed reactions of alkynes,[15] we here report
our studies of silver-catalyzed cyclopropenation of alkynes by
cyclopropenation of alkynes with non-stabilized diazoalkanes,
owing to the fact that such compounds are unstable and prone
to detonation. Here, we report a general silver-catalyzed
general cyclopropenation reaction of alkynes with unstable
diazoalkanes, by for the first time discovery and application of
N-nosylhydrazones as room-temperature decomposiable diazo
surrogates. This method allows for efficiently assembling a
wide variety of cyclopropene derivatives that are otherwise
difficult to access by conventional methods.
Amongst small ring molecules, cyclopropenes are arguably the
most strained single ring organic molecules, and occupy a unique
position in hydrocarbon chemistry.[1] The high intrinsic energy of
cyclopropene molecules renders them to be versatile synthetic
imtermediates, but in turn limits their synthesis to a few
practically useful cyclopropenation methodologies.[2] Among
these, the [2+1]-cycloaddition of alkynes with diazo compounds
is a classical, yet reliable, powerful and therefore widely used
route.[2][3] However, the diazo compounds directly used in such
reactions were all limited to those stabilized with electron-
withdrawing[4] or trimethylsilyl groups.[5] The general [2+1]-
cycloaddition of alkynes with unstable diazoalkanes remains
elusive, because of their inherent characteristics such as being
prone to explode leading to the difficulty in preparation and
handling.[6] To overcome this drawback, various diazo surrogates
have been synthetically explored and significantly expanded the
repertoire of diazo-participating reactions.[7] However, the diazo
discovering
N-nosylhydrazones
as
room-temperature
decomposiable diazo surrogates.
[]
Z. Liu, Q. Li, Dr. P. Liao, Prof. X. Bi
Jilin Province Key Laboratory of Organic Functional
Molecular Design & Synthesis
Department of Chemistry
Northeast Normal University
Changchun 130024, China
E-mail: bixh507@nenu.edu.cn
Homepage: http://www.bigroup.com.cn/
Prof. X. Bi
Figure 1. [2+1] Cylopropenation of alkynes with diazo surrogates.
State Key Laboratory of Elemento-Organic Chemistry
Nankai University, Tianjin 300071, China
[] Supporting information for this article is available on the
WWW under
N-tosylhydrazones represent the most synthetically exploited
diazo surrogates in the past decade, as demonstrated by the
rapidly expanding repertoire of their transformations.[16] However,
http://dx.doi.org/10.1002/anie.201xxxxxx.((Please delete if
not appropriate))
1
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the high temperature (> 70 °C) required for their dissociation into
diazo species severely hinders the synthetic extension of N-
tosylhydrazones, especially to the synthetic areas disfavoring high
temperature. This hindrance is evident from the different
outcomes observed in the reactions of diazo compounds with
boronic acids as reported by Ley[17a] and Barluenga,[17b] where the
former achieved boron insertion products based on flow-
generated diazo compounds at 25 °C, whereas the later obtained
deboronated products because of the high dissociation
temperature of N-tosylhydrazones. N-nosylhydrazones, as one
class of N-sulfonylhydrazones, have been considering simply as a
faster decomposable alternative to N-tosylhydrazones, rather than
low-temperature diazo surrogates.[18] We[19] and others[20] have
previously reported the rapid decomposition of o-nosylhydrazides
at room temperature. Based on this observation, we initiated the
study on the dissociation of N-nosylhydrazone 1a and N-
tosylhydrazone 1a’, respectively, at 25 °C in the presence of NaH.
As shown in Figure 2a, the former smoothly released 4-
chlorophenyl diazomethane as a function of time, whereas the
latter remained intact within 24 h. To the best of our knowledge,
this is the first report unraveling N-nosylhydrazones dissociable
at room temperature. Besides, slightly increasing the temperature
of cycloprpenation step to 40 ºC remarkably raised the yield of
product 3a to 85%. Furthermore, the silver salt such as AgOTf
was identified as the most effective catalyst for the
cyclopropenation of alkynes with N-nosylhydrazones. In contrast,
other tested carbene transfer catalysts either gave low yields (e.g.,
Rh2(Oct)4, 42%) or were totally non-effective (e.g., Zn(OTf)2,
Cu(OTf)2, Pd(OAc)2) (Figure 2b) (For details, see supporting
information).
(66%−85%), irrespective of the electronic effect and/or the
position of the substituents on the phenyl ring. The functional
group tolerance was noteworthy, as the formyl, nitro, cyano, and
halo groups were all well tolerated. In addition, this protocol
could be applied to the fused aryl such as fluorine-substituted
substrate 1j, albeit with a slightly decreased yield (3j, 52%). N-
nosylhydradrazones having a heteroaryl group such as indole,
pyridine, thiophene, and furan also proved to be effective partners
(3k−3n, 54%−68%). The structure of cyclopropenes was
unambiguously confirmed by the single-crystal X-ray analysis of
product 3k. Furthermore, N-nosylhydrazones derived from α,β-
unsaturated aldehydes containing a double bond or a triple bond
were effectively used, providing the corresponding cyclopropenes
(3o−3r) in good-to-excellent yields (59%−90%). It is worth
mentioning that the alkynyl N-tosylhydradrazones treated by base
were prone to form pyrazoles by intramolecular cyclization,[21]
while this problem was conquered using N-nosylhydradrazones.
In addition, some experiments have been carried out and revealed
N-nosylhydradrazones derived from enolizable aldehydes or
ketones were not suitable for this transformation. For example, N-
nosylhydrazides derived from 3-phenylpropanal or acetophenone
decomposed to give a mixture of corresponding alkene and azine
products, but no cyclopropene products could be detected.
Scheme 1. Substrate scope of N-nosylhydrazones. Standard conditions: N-
nosylhydrazone (0.3 mmol), NaH (0.45 mmol) and CH2Cl2 (3.0 mL) were stirred at
rt for 1 h, then alkyne (0.6 mmol) and AgOTf (0.06 mmol) were added, and stirred at
40 oC for 18 h. 3n: t-BuOLi was used as base..
We next sought to investigate the scope of alkynes in this
silver-catalyzed cycloaddition reaction (Scheme 2). The substrate
scope was broad and all the tested internal alkynes underwent the
cyclopropenation with N-nosylhydrazones to afford the
corresponding cyclopropenes (4a−4n) with useful efficiencies
(43%−91% yields). A remarkable steric hindrance effect of
alkyne substrates on the cyclopropenation reaction was observed.
For example, the linear alkynes generally gave high yields (4a−4c,
86%−91%), whereas those with branched chains or a bulky
phenyl ring led to gradually decreased product yields (4d−4h,
43%−56%). Notably, the most bulky 1,2-diphenylethyne also
proved to be reactive, albeit with AgOTFA as catalyst. Diverse
functionalities, including alkynyl, halogen, ester, and amino
Figure 2. (a) Base-promoted dissociation of N-nosylhydrazone 1a and N-
tosylhydrazone 1a’ at 25 °C. (b) Optimization of the reaction conditions.
With the optimal conditions in hand, we next explored the
substrate scope with regard to N-nosylhydrazones. As depicted in
Scheme 1, a range of structurally varied N-nosylhydrazones were
applicable in this silver-catalyzed [2+1] cylopropenation of
alkynes. Both aromatic and aliphatic (alkenyl, alkynyl) N-
nosylhydrazones were readily accommodated in this protocol. For
example, a variety of substituted aryl N-nosylhydrazones (1a−1i)
were able to react with 4-octyne, affording the corresponding
cyclopropene products (3a−3i) in good-to-excellent yields
2
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10.1002/chem.201605335
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groups, were well tolerant, thus affording
a
range of
diluted solution (0.03 M) (Figure 2). By contrast, the reaction of
substrate 6b, which has one carbon atom less in the alkyl chain,
did not cease in the step of forming fused cyclopropene 7b, while
gave a pharmacologically relevant benzo[b]azepine 8 in 85%
yield by a regioselective ring opening reaction.[27] The structure of
8 was confirmed by X-ray crystallographic analysis.
functionalized cyclopropenes (4i−4n, 60%−82%). The tolerance
of bromo group in the presence of a halophilic silver catalyst was
especially noteworthy.[22] Furthermore, under slightly modified
conditions (Ag2CO3 as catalyst, highly diluted reaction solution),
terminal alkynes also proved to be suitable reaction partners in
this silver-catalyzed protocol, which afforded a group of difficult
to be synthesized 1-alkyl-3-aryl cyclopropenes in good yields
(5a−5f, 48%−77%). Notably, Wang and coworkers reported the
reaction of terminal alkynes with N-tosylhydrazones by copper
catalysis afforded allenes,[23] clearly demonstrating the different
catalytic activity of silver catalyst.
Scheme 3. Intramolecular reaction..
The chemoselectivity of enyne systems, i.e., cyclopropanation
vs cyclopropenation, was studied (Scheme 4). We observed that
the reaction of enyne 9a with N-nosylhydrazone 1b by silver
catalysis chemospecifically produced cyclopropene 10a as a sole
product. Interestingly, the same reaction could be switched to
cyclopropanation simply by replacing the silver catalyst with a
rhodium catalyst, which afforded alkynyl cyclopropane 11 with a
cis:trans ratio of 2:1. The non-conjugated enyne 9b displayed the
same chemoselectivity as the conjugated enyne to afford
cyclopropene 10b. Notably, the silver-based catalysis does not
always favor the cyclopropenation selectivity of enynes, as the
Davies group previously observed a complete cyclopropanation in
a silver-catalyzed reaction of enyne 9a with α-diazocarbonyl
compounds.[14] A comparison of our results with those of Davies’
clearly demonstrated the different reactivity of non-stabilized
diazoalkanes and α-diazocarbonyl compounds.
Scheme 2. Substrate scope of alkynes. Modified conditions: N-nosylhydrazone (0.5
mmol), NaH (0.75 mmol) and CH2Cl2 (10.0 mL) were stirred at rt for 1 h, then
terminal alkynes (5.0 mmol) and Ag2CO3 (0.15 mmol) were added, and stirred at
40 °C for 18 h. 4d: AgOTFA (20 mol%) was used.
To demonstrate the scalability of this protocol, we conducted
a reaction on large scale, and observed that the gram-scale
synthesis with 2.44 g of N-nosylhydrazone 1b proceeded well
under the standard conditions. Further, the synthetic utility of
cyclopropenes was demonstrated by the rearrangement of 3b into
naphthol 3ba[24] and the peracid oxidation leading to α-
alkylchalcone 3bb,[25] respectively.
The high strain energy of ring-fused cyclopropenes makes
their synthesis and isolation to be a difficult task.[26] We attempted
to apply our method to attain this aim. After several trials, we
eventually isolated an eight-member-fused cyclopropene 7a in a
moderate yield (42%) starting from N-nosylhydrazone 6a, in a
Scheme 4. Chemoselectivity of enynes.
3
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Based on the experimental results and related precedents,[28]
we suggest a presumed mechanistic description for the silver-
catalyzed cyclopropenation reaction of alkynes with non-stabili
zed diazoalkanes (Scheme 5). Base-promoted dissociation of N-
nosylhydrazones 1 first occurs to generate unstable diazo
compound A, which readily undergoes dinitrogen extrusion by a
silver catalyst to give a transient silver carbenoid B.[28a]
Subsequently, coordination to the carbon-carbon triple bond of
alkynes takes place affording the coordinated complex C.
Following the migratory insertion, an organosilver intermediate D
is generated. Finally, the cyclopropene products are produced,
with the regeneration of Ag(I) catalyst for the next catalytic cycle.
Apparently, the function of the silver salts as a robust catalyst
probably could be ascribed to the superior alkynophilicity of in
situ formed silver carbenoid.[12]
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In summary, we have developed the first silver-catalyzed
general [2+1] cyclopropenation reaction of alkynes with unstable
diazoalkanes, by for the first time discovering the capability of N-
nosylhydrazones acting as room-temperature diazo surrogates.
The newly developed method features in broad substrate scope,
high reaction efficiency, good functional group tolerance, and
unexpected cylopropenation selectivity in enyne systems, thus
demonstrating its remarkable synthetic potency. Furthermore, a
variety of cyclopropene derivatives that are otherwise difficult to
access by conventional methods were obtained in generally good
yields. Further expansion of this N-nosylhydrazone strategy
combined with silver catalysis to asymmetric synthesis and other
synthetic areas is currently under way and will be reported in due
course.
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Acknowledgements
This work was supported by the NSFC (21522202, 21502017, 21372038), the
Ministry of Education of the People’s Republic of China (NCET-13-0714), the Jilin
Provincial Research Foundation for the Basic Research (20140519008JH),
Fundamental Research Funds for Central Universities (2412015BJ005,
2412015KJ013, 2412016KJ040), and the Jilin Province Key Laboratory of Organic
Functional Molecular Design & Synthesis (No. 130028658).
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Keywords: N-nosylhydrazones • alkynes • cylopropenation
• silver-catalyzed • diazo surrogates
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Chemistry - A European Journal
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Cyclopropenation 
Zhaohong Liu, Qiangqiang Li,
Peiqiu Liao, Xihe Bi*
…..........…...… Page – Page
Silver-Catalyzed [2+1]
The silver-catalyzed general [2+1]
cyclopropenation reaction of alkynes synthetic method toward a variety of
with unstable diazoalkanes has been cyclopropene derivatives that are
developed, by for the first time
discovering the potential of N-
nosylhydrazones as room-
temperature decomposiable diazo
surrogates.
This report displayed a powerful
Cyclopropenation of Alkynes
with Unstable Diazoalkanes: N-
Nosylhydrazones as Room-
Temperature Decomposable
Diazo Surrogates
otherwise difficult to access by
conventional methods.
6
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