Alkyne and reversible nitrile activation: N, N'-diamidocarbene-facilitated synthesis of cyclopropenes, cyclopropenones, and azirines

Article abstract of DOI:10.1021/ja3014105

We report the synthesis of a variety of diamidocyclopropenes by combining an isolable and readily accessible N,N'-diamidocarbene (DAC) with a range of alkynes (nine examples, 68-97% yield). Subsequent hydrolysis of selected cyclopropenes afforded the corresponding cyclopropenones or α,β- unsaturated acids, depending on the reaction conditions. In addition, the combination of a DAC with alkyl or aryl nitriles was found to form 2H-azirines in a reversible manner (four examples, K_eq = 4-72 M^-1 at 30 °C in toluene).

Full text of DOI:10.1021/ja3014105

Communication
pubs.acs.org/JACS
Alkyne and Reversible Nitrile Activation: N,N′-Diamidocarbene-
Facilitated Synthesis of Cyclopropenes, Cyclopropenones, and
Azirines
Jonathan P. Moerdyk and Christopher W. Bielawski*
Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712, United States
S
* Supporting Information
ABSTRACT: We report the synthesis of a variety of
diamidocyclopropenes by combining an isolable and
readily accessible N,N′-diamidocarbene (DAC) with a
range of alkynes (nine examples, 68−97% yield).
Subsequent hydrolysis of selected cyclopropenes afforded
the corresponding cyclopropenones or α,β-unsaturated
acids, depending on the reaction conditions. In addition,
the combination of a DAC with alkyl or aryl nitriles was
found to form 2H-azirines in a reversible manner (four
examples, Keq = 4−72 M−1 at 30 °C in toluene).
Figure 1. (top) Exposure of SIMes to acetylene affords the formal
C−H insertion product shown. Conditions: 1.1 equiv of acetylene,
THF, −196 to 23 °C, 16 h (93% yield).7 (bottom) Exposure of DAC
s the smallest unsaturated carbo- and heterocycles,
cyclopropenes and azirines have attracted considerable
1 to acetylene affords cyclopropene 2a. Conditions: 1 atm acetylene,
A
C6H6, 23 °C, 8 h (81% yield). Mes = 2,4,6-trimethylphenyl.
interest for their unique structural characteristics.1 Indeed, the
high degree of ring strain possessed by these species has
enabled them to serve many roles in the synthesis of substituted
alkynes, allenes, heterocycles, and other important substrates.2,3
Cyclopropenes are typically prepared via the metal-mediated
addition of electrophilic carbenes to alkynes or the 1,2-
elimination of halocyclopropanes.1,2b,c,3 In contrast, azirines are
often synthesized in situ using the Neber rearrangement of
activated oximes or via the pyrolysis/photolysis of vinyl
azides.2a,4 A practical and atom-economical alternative to the
aforementioned methods is the [2 + 1] cycloaddition of isolable
carbenes with alkynes or nitriles. Unfortunately, such synthetic
approaches are relatively unexplored, largely because of a lack
of suitable carbenes. For example, Arduengo has shown that
exposure of N,N′-dimesityl-4,5-dihydroimidazol-2-ylidene
(SIMes), a well-known, stable, and basic5 N-heterocyclic
carbene (NHC),6 to acetylene (Figure 1 top) or acetonitrile
results in formal C−H insertion rather than cycloaddition.7
Indeed, Bertrand’s phosphinosilyl carbene8 is the only stable
carbene known to undergo chelotropic reactions with
compounds possessing triple bonds and may be limited to
phosphaalkynes (R−CP) and aryl nitriles (Ar−CN).9
products were successfully hydrolyzed to afford cycloprope-
nones or α,β-unsaturated acids.11 Additionally, treatment of 1
with aryl or alkyl nitriles resulted in the first examples of the
formation of 2H-azirines in a reversible fashion.12 In all cases,
the cycloaddition reactions were found to proceed rapidly to
high conversions under mild conditions.
Our initial experiments explored the reaction of 1 with
acetylene. Stirring a solution of 1 in C6H6 ([1]0 = 0.13 M)
under acetylene (1 atm) for 8 h at ambient temperature
resulted in the formation of a cloudy solution that, upon
concentration and washing with diethyl ether, afforded a white
solid in 81% yield.13 The isolated product was identified as
cyclopropene 2a (Figure 1 bottom), in part on the basis of the
appearance of signals associated with a strained olefin observed
1
at 7.23 (2H) and 117.9 ppm (CDCl3) in the compound’s H
and 13C NMR spectra, respectively. The structure of 2a, which
constitutes a rare14 example of a diamidocyclopropene, was
unambiguously elucidated by single-crystal X-ray diffraction
(XRD) analysis (Figure 2).
DAC 1 was found to undergo [2 + 1] cycloadditions with a
variety of terminal, internal, aryl, alkyl, and silyl-protected
alkynes to give the corresponding cyclopropenes in good to
excellent isolated yields (68−97%; Table 1). In general, the
terminal alkynes were reacted at equimolar concentration with
1 ([1]0 = 0.20 M) in C6H6 for 16 h at 23 °C, whereas the
internal alkynes were stirred at 60 °C using 2 equiv of alkyne
under otherwise identical conditions. With the exception of 2h,
Herein we disclose that the N,N′-diamidocarbene (DAC)10
1
is the first isolable carbene to undergo formal [2 + 1]
cycloadditions with alkynes as well as alkyl and aryl nitriles.
DACs are an emerging class of stable carbenes that display
many reactivities often expected from in situ-generated
electrophilic carbenes, such as methylene.10 Simply exposing
1, which can be isolated in one step from the condensation
product of N,N′-dimesitylformamidine and dimethylmalonyl
dichloride,10d to various alkynes was found to afford the
corresponding diamidocyclopropenes in high yields. These
Received: February 12, 2012
Published: March 30, 2012
© 2012 American Chemical Society
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Scheme 1. Cyclopropene Formation Followed by Ring
a
Expansion and Isomerization
Figure 2. ORTEP diagram of 2a, with thermal ellipsoids drawn at 50%
probability and H atoms omitted for clarity. Selected distances (Å) and
angles (deg): C1−C25, 1.464(2); C1−C26, 1.471(2); C25−C26,
1.285(2); C25−C1−C26, 51.90(10); C26−C25−C1, 64.32(11);
C25−C26−C1, 63.78(11).
a
Conditions: (i) DMAD (1 equiv), C6H6, 23 °C, 5 min, 97% yield; (ii)
Table 1. Summary of the [2 + 1] Cycloadditions of DAC 1
with Various Alkynes
a
C6H6, 23 °C, 40 h, 41% yield from 1 (R = CO2Me); (iii) C6H6, 60 °C,
12 h, 80% yield from 1.
Scheme 2. Treatment of Various Alkynes with 1 Followed by
Hydrolysis of the Resultant Cyclopropenes (e.g., 2e and 2g)
under Acidic Conditions Enables Access to the
Corresponding Cyclopropenone or α,β-Unsaturated Acid(s)
a
Depending on the Reaction Temperature and Time
a
Conditions: (i) 1, C6H6, 60 °C, 16 h then 50:50 v/v HCl/AcOH, 80
°C, 2 h (66−70% yield); (ii) 1, C6H6, 60 °C, 16 h then 50:50 v/v
HCl/AcOH, 100 °C, 48 h (32−43% yield).
a
Unless otherwise noted, all reactions were performed in C6H6 at 23
°C for 16 h using equimolar concentrations of 1 ([1]0 = 0.2 M) and
t
isolation of the product (41% yield). Additionally, heating
the alkyne shown. Abbreviations: Bu = tert-butyl; Ph = phenyl; Cy =
b
either 2i or 3 at 60 °C for 12 h resulted in the formation of a
cyclohexyl; Bu = butyl; Pr = propyl; Et = ethyl. Isolated yields of the
c
1
corresponding cyclopropene products. 1 atm acetylene, [1]0 = 0.13
third product that displayed downfield H NMR shifts [6.54
d
e
(2H) and 6.62 (2H) ppm] as well as a lack of olefinic 13C NMR
signals. In agreement with these data, single-crystal XRD (see
the SI) of the isolated solid (80% yield based on 1) revealed
this compound to be the conjugated alkylideneamide 4. We
believe that the overall transformation of 2i to 4 was driven by
the ring-opening rearrangement of the cyclopropene to a
norcaradiene derivative that underwent electrocyclic ring
expansion to the corresponding cycloheptatriene 3,15 and
ultimately isomerized to its more stable isomer 4. This process
represents a novel, metal-free C−C bond-forming reaction that
involves the migration of a sterically hindered aryl group.
Considering that the carbene center in 1 is in the same
oxidation state as the carbon atom in carbon monoxide, a gas
reluctant to undergo [2 + 1] cycloaddition reactions,16 we
reasoned that hydrolysis of the aforementioned N,N-
diamidocyclopropenes would provide a new synthetic
route11,17 to cyclopropenones, substrates desired for their
utility in preparing heterocycles, cyclopropenylium ions, and
other unsaturated carbonyl species.17,18 To test this hypothesis,
2g was heated to 80 °C in a 1:1 v/v mixture of glacial acetic
acid and concentrated HCl for 2 h. Following purification over
silica gel, 2,3-diphenylcyclopropenone was isolated in 66% yield
(Scheme 2, route i). Although no ring-opened products were
observed under these conditions, heating 2g at 100 °C for 48 h
M, 8 h. 60 °C. [alkyne]0 = 0.4 M, 60 °C.
all of the synthesized cyclopropenes were found to be high-
melting-point solids (mp >140 °C) and stable to the ambient
atmosphere. Regardless, in all cases, no competitive alkyne C−
H insertion processes were observed, and the cyclopropenes
formed constitute the first examples of their kind arising from
alkynes and an isolable carbene.
While the aforementioned reactions afforded stable cyclo-
propenes as the sole products, the outcome of the reaction of 1
with an electron-deficient alkyne, dimethyl acetylenedicarbox-
ylate (DMAD), was found to be time-dependent (Scheme 1).
Stirring 1 with a stoichiometric quantity of DMAD in C6D6 at
23 °C for 5 min afforded the cyclopropene product 2i (97%
isolated yield), as evidenced in part by a diagnostic signal
observed at 125.8 ppm in the 13C NMR spectrum of this
compound. However, when the same reaction was allowed to
proceed for >30 min at 23 °C, a second product that exhibited
1
upfield H and 13C NMR shifts (1H, 5.98 and 6.14 ppm; 13C,
119.58 and 121.05 ppm) gradually formed. The new
spectroscopic signals were consistent with a loss of aromaticity
and the formation of the fused cycloheptatriene 3. The
structural assignment was subsequently confirmed by single-
crystal XRD [see the Supporting Information (SI)] following
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Communication
in the same acid mixture followed by extraction afforded trans-
α-phenylcinnamic acid, albeit in modest yield (32%; Scheme 2,
route ii). Similarly, hydrolysis of 2e afforded either 2-methyl-3-
propylcyclopropenone (70%) or a 53:47 mixture of the
isomeric ring-opened carboxylic acids trans-2-methylhex-2-
enoic acid and trans-2-ethylidenepentanoic acid (43% overall
yield), depending on the reaction conditions. As there are
relatively few methods17b,c for the efficient preparation of
dialkylcyclopropenones, especially in comparison with their
diaryl analogues, the synthesis of 2-methyl-3-propylcyclopro-
penone in a single vessel and good yield is noteworthy and
underscores the ability of DACs to serve as an equivalent to
CO in useful cycloaddition reactions.
Table 2. Thermodynamic Data for the Reversible [2 + 1]
Cycloadditions of 1 with Various Nitriles
a
a
b
Conditions: 1 equiv of RCN, C7D8, unless otherwise noted. 10
Having successfully demonstrated that the DAC 1 is a
suitable [2 + 1] cycloaddition partner for alkynes, subsequent
attention shifted toward studying the reaction of 1 with nitriles,
another class of substates that contain triple bonds.19 The
addition of excess benzonitrile (2 equiv) to 1 in C6D6 ([1]0 =
0.09 M) at 23 °C for 2 h afforded a product that displayed
NMR signals consistent with the formation of 2H-azirine 5a.
For example, 13C NMR analysis of the above-mentioned
reaction mixture revealed a signal at 167.6 ppm (C6D6)
diagnostic9,20 of a 1:1 cycloadduct of the nitrile and the DAC as
opposed to an ylide.21 To our surprise, attempts to isolate the
azirine product by removing the residual solvent under vacuum
followed by washing with pentane returned the starting
material, 1. Similar results were obtained with other
functionalized derivatives of benzonitrile as well as acetonitrile.
Collectively, these observations suggested to us that the
interaction between the DAC and the aforementioned nitriles
is reversible (see below). Nevertheless, the formation of a 2H-
azirine product was later unequivocally determined by single-
crystal XRD analysis of 5c (see Figure 3 and the SI for details).
The equilibrium constants (Keq) for the reversible reactions
of 1 with various nitriles were measured in C7D8 by variable-
temperature NMR spectroscopy and used to construct van’t
Hoff plots (see the SI). Linear fits of these data enabled the
determination of ΔH and ΔS for each reaction investigated.
These studies revealed that the [2 + 1] cycloaddition of 1 was
more favored for electron-deficient nitriles, as evidenced by the
larger Keq and more negative ΔH values measured (Table 2).
Treatment of 1 with acetonitrile was also found to afford the
corresponding azirine 5d (diagnostic 13C NMR signal in C6D6
equiv of CH3CN.
at 169.9 ppm) in a reversible manner (Keq = 4 M−1 at 30 °C).22
Although the Keq for the reaction with the alkyl nitrile was
lower than those measured for the aryl analogues, this result
represents the first example of an isolable carbene undergoing a
[2 + 1] cycloaddition with an alkyl nitrile as opposed to
affording the formal C−H insertion product that is typically
observed.7,23
In summary, we report a series of [2 + 1] cycloaddition
reactions through the combination of a stable, isolable
diamidocarbene (DAC) with various alkynes and nitriles to
afford cyclopropenes and 2H-azirines, respectively. In all cases,
the reactions proceeded under mild conditions (23−60 °C)
and reached high conversions. The azirination reactions were
found to be reversible at ambient temperature, which is more
typical of given transition metals and a first for an organic
compound,24 and may complement known25 methods for
activating nitriles. Aside from being among the first examples of
their kind, the aforementioned diamidocyclopropenes were
hydrolyzed to afford either the corresponding cyclopropenones
or ring-opened acids, depending on the conditions employed.
Additionally, exposure of the DAC to electron-deficient
substrates facilitated the discovery of new C−C bond-forming
reactions. Collectively, these transformations underscore the
ability of DACs to function as useful synthetic reagents in
applications that extend beyond facilitating the formation of
three-membered rings. We expect these results to broaden the
utility of isolable carbenes in synthesis, structural analysis, and
dynamic covalent chemistry.26
ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed experimental procedures; X-ray crystallographic data
for 2a−c, 3, 4, 5c, and 6; and NMR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
bielawski@cm.utexas.edu
Notes
The authors declare no competing financial interest.
Figure 3. ORTEP diagram of 5c, with thermal ellipsoids drawn at 50%
probability and H atoms omitted for clarity. Selected distances (Å) and
angles (deg): C1−C25, 1.436(2); C1−N3, 1.5184(18); N3−C25,
1.2753(19); N3−C25−C1, 67.81(10); C25−N3−C1, 61.15(10);
C25−C1−N3, 51.05(9).
ACKNOWLEDGMENTS
■
We are grateful to the National Science Foundation (CHE-
0645563), the Robert A. Welch Foundation (F-1621), and the
Alfred P. Sloan Foundation for their support.
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1493.
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