Use of Cyclopropane as C1 Synthetic Unit by Directed Retro-Cyclopropanation with Ethylene Release

Article abstract of DOI:10.1021/jacs.8b09297

Cyclopropanation of alkenes is a well-established textbook reaction for the synthesis of cyclopropanes, where a "high-energy" carbene species is exploited to drive the reaction forward. However, little attention has been focused toward molecular transformations involving the reverse reaction, retro-cyclopropanation (RC). This is because of difficulties associated with both cleaving the two geminal C-C single bonds and exploiting the generated carbenes for further transformations in an efficient and selective manner. Here, we report that a molybdenum-based catalytic system overcomes the above challenges and effects the RC of cyclopropanes bearing a pyridyl group with the release of ethylene (alkene) and the subsequent intramolecular cyclization leading to pyrido[2,1-a]isoindoles. The reaction allows for the uncommon use of cyclopropanes as C1 synthetic units in contrast to most conventional reactions in which cyclopropanes are used as C3 synthetic units. We anticipate that this new strategy will pave the way for C1 cyclopropane chemistry.

Full text of DOI:10.1021/jacs.8b09297

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Article
Use of Cyclopropane as C1 Synthetic Unit by Directed
Retro-Cyclopropanation with Ethylene Release
Sobi Asako, Takaaki Kobashi, and Kazuhiko Takai
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09297 • Publication Date (Web): 22 Oct 2018
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Use of Cyclopropane as C1 Synthetic Unit by Directed Retro-
Cyclopropanation with Ethylene Release
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Sobi Asako*, Takaaki Kobashi, and Kazuhiko Takai*
Division of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University,
3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530
ABSTRACT: Cyclopropanation of alkenes is a well-established textbook reaction for the synthesis of cyclopropanes,
where “high-energy” carbene species is exploited for driving the reaction forward. However, little attention has been
paid to molecular transformations involving the reverse reaction, retro-cyclopropanation (RC). This is because of
difficulties associated with both cleaving the two geminal C–C single bonds and exploiting the generated carbenes for
further transformations in an efficient and selective manner. Here we report that a molybdenum-based catalytic system
overcomes the above challenges and effects the RC of cyclopropanes bearing a pyridyl group with the release of
ethylene (alkene) and the subsequent intramolecular cyclization leading to pyrido[2,1-a]isoindoles. The reaction allows
for the uncommon use of cyclopropanes as C1 synthetic units in contrast to most conventional reactions in which
cyclopropanes are used as C3 synthetic units. We anticipate that this new strategy will pave the way for C1
cyclopropane chemistry.
(Fig. 1a, bottom). The overall sequence is retro-
INTRODUCTION
cyclopropanation (RC), i.e., fragmentation of
a
Cyclopropanes are three-membered carbocycles that
display unique properties and reactivities because of
their unusual bonding and ring strain. Ubiquitous in
naturally occurring compounds and rationally designed
drugs, cyclopropane-containing molecules themselves
cyclopropane into an ethylene and a carbene species.
This is a highly uphill and difficult process unless
appropriate stabilization of the generated metal carbene
is exerted. Indeed, the generation of metal carbene
largely relies on the cleavage of the weak bonds in
“high-energy” compounds such as diazo compounds,
frequently used but potentially explosive carbene
precursors, and the metal carbene generation by
breaking the two C–C bonds of stable cyclopropanes
are important, 1,2,3 4,5,6,7,8 but they also serve as versatile
synthetic
intermediates
in
modern
organic
synthesis.8,9,10,11,12,13,14,15 Certain transition metals are
capable of cleaving the C–C bond via oxidative addition
to form metallacyclobutane intermediates with strain
release (ca. 27 kcal/mol) as a driving force (Fig. 1a,
top).16,17,18,19 Synthetic chemists have successfully
remains challenging.23 Because of the lack of efficient
and selective catalytic systems that can achieve both RC
and subsequent transformation of metal carbenes, such
reactions that use cyclopropanes as C1 synthetic units
have scarcely been reported. In this context, the
biosynthesis of ethylene, a natural plant hormone, by
oxidative fragmentation of 1-aminocyclopropane-1-
carboxylic acid with concomitant release of HCN (C1
unit) and CO2 may be relevant but lacks the
utilized the key intermediates, mostly as C3 fragments
for various transformations such as addition,
cycloaddition, and cycloisomerization reactions, wherein
activated cyclopropanes such as donor–acceptor
cyclopropanes,20
vinylcyclopropanes,21
and
intermediacy of a carbene species.24,25
alkylidenecyclopropanes22 are often employed to
facilitate the C–C bond cleavage step.
Cationic gold(I) can promote the retro-Buchner
reaction, a special case of RC, where a substituted
methylene moiety is removed from 1,3,5-
cycloheptatriene, which is in equilibrium with
If the metallacyclobutane intermediate further
undergoes the second cleavage of a C–C bond, a metal
carbene species is formed with the release of ethylene, a
common step involved in the olefin metathesis reaction
norcaradiene, with the release of benzene (Fig. 1b).26
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The intramolecular RC of bicyclo[1.1.0]butane is
proposed to be involved under transition-metal catalysis,
such as Ni27 and Rh28 (Fig. 1c). These reactions take
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advantage of aromatization and the release of extremely
high ring strain (63–67 kcal/mol) 18,19 as driving force.
However, the substrates are then limited to highly
strained cyclopropanes. The intermolecular RC was
observed in the Rh(I)-catalyzed [2+2+1] cycloaddition
reaction of alkylidenecyclopropanes29 and the Rh(I)-
catalyzed [5+2–2] cycloisomerization-type reaction of
allenylcyclopropanes.30 However, the former reaction
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still relies on the higher ring strain of
alkylidenecyclopropanes (38–41 kcal/mol)18,19 and
requires slow addition of diynes to suppress side
reactions following non-RC paths in some cases, and the
latter reaction suffers from low selectivity between the
cycloisomerizations with and without ethylene release
(Fig. 1d and 1e). The PhWCl3/RAlCl2 binary system was
found to catalyze RC of simple cyclopropanes31 and
cyclopropane–olefin metathesis,32 albeit again with low
efficiency (Fig. 1f).33
Figure 1. Organic transformations involving retro-
cyclopropanation with metal catalysts.
Thus, to date, few efficient catalytic systems are
available for molecular transformations featuring RC
with synthetic utility. Here we describe a molybdenum-
catalyzed directed RC/intramolecular cyclization
sequence of cyclopropanes possessing a pyridine moiety
that plays multiple roles (Fig. 1g). With the prevalence
of the chelation assistance in the activation of
chemically inert bonds, such as C–H and C–C bonds,
together with our own experience in this field,34,35 we
envisioned that the use of a pyridyl group at an
appropriate position for RC would be beneficial for the
following reasons: it would 1) facilitate the first
cleavage of a proximal C–C bond in a regioselective
manner, 2) prohibit the undesired decomposition of the
metallacyclobutane intermediate I through a hydride
elimination process,36 3) adequately stabilize the
carbene species II formed after the elimination of
ethylene from I, and 4) efficiently trap the carbene by
intramolecular cyclization to incorporate itself into the
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value-added product. The directing group approaches for
the regioselective transformations of cyclopropanes have
been reported.37 However, the cyclopropanes are still
ketones. 2-(2-Cyclopropylphenyl)pyridine remained
unchanged, indicating the necessary presence of a
substituent at the 1-position of the cyclopropane. Both
an electron donating and withdrawing substituent on the
2-phenylpyridine moiety slowed down the reaction (2m
and 2n). In addition to a pyridyl group, a pyrimidyl
group also worked as a multifunctional unit and the
corresponding pyrimido[2,1-a]isoindole (2o) was
obtained in good yield, whereas an imino group failed to
effect the reaction. All the 6-arylpyrido[2,1-a]isoindole
derivatives synthesized by this method are previously
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used as C3 synthetic units in these cases, because the C–
C bond cleavage takes place only once.
RESULTS AND DISCUSSION
9
To test this hypothesis, we commenced our
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investigation
using
2-[2-(1-
phenylcyclopropyl)phenyl]pyridine (1a) as a model
substrate (Fig. 2). Close similarity between the proposed
mechanism in our strategy (Fig. 1g) and that of the
olefin metathesis, significant advances of which have
unknown compounds, and exhibit strong fluorescence.42
been made with molybdenum catalysts38,39 besides our
recent interest in molybdenum catalysis40,41 prompted us
to use this metal as a catalyst of choice; hence we first
explored various combinations of molybdenum
complexes and ligands. After extensive experimentation,
we found that a catalytic amount of Mo(benzene)2
combined
with
9,10-phenanthrenequinone
(L1)
effectively catalyzed the RC/intramolecular cyclization
reactions to afford 6-phenyl[2,1-a]pyridoisoindole (2a)
in quantitative yield. The reaction reached completion
with 20 mol % catalyst after 24 h at 160 °C in
mesitylene, whereas at lower temperatures the reaction
rate was significantly decreased (Table S1), partly
because the thermodynamic driving force of releasing
ethylene decreases as the temperature decreases (G = –
3.7, –2.3, and +1.1 kcal/mol, at 160, 120, and 25 °C,
respectively). The reaction still proceeded with 10 mol
% catalyst loading, but with slower reaction rates (Table
S1).
Screening of various ligands revealed that o-quinones
were uniquely effective in the reaction; L1 and L2
performed best (Fig. 2a). While catechol showed similar
catalytic activity (L3), common bidentate nitrogen- or
phosphine-based ligands completely shut off the reaction
(L4–L7). Mo(benzene)2 alone could effect the reaction
to some extent, but the reaction in the absence of
molybdenum did not proceed at all. Other Mo(0)
complexes such as Mo(CO)6 and Mo(CO)3(MeCN)3 also
worked, but less efficiently (Table S2).
The scope of the directed RC reaction is illustrated in
Fig. 2b. Cyclopropanes possessing an electron-rich and
electron-deficient aryl group at the 1-position of the
cyclopropane both reacted smoothly (2b–2g).
Cyclopropanes with a bulky 2-tolyl or alkyl substituents
were less reactive but they still underwent the RC
reactions at higher temperature and after longer reaction
time (2h–2k). Due to the moderate instability of 6-
alkylpyrido[2,1-a]isoindoles (2i–2k) under atmospheric
conditions, these products were oxidized and then
isolated as the corresponding alkyl 2-(2-pyridyl)phenyl
Figure 2. Molybdenum-catalyzed synthesis of
pyrido[2,1-a]isoindoles via retro-cyclopropanation. a,
Gibbs free energies and enthalpies of the reaction
calculated at the M06/6-31G(d,p)mesitylene(SMD) level of
1
theory and the effects of ligands. H NMR yields are
shown. b, Scope of the molybdenum-catalyzed RC. The
reactions were performed on a 0.2 mmol scale with
Mo(benezene)2/L1 (20 mol%) in mesitylene (0.25 M).
a
b
Isolated yields are shown. 160 °C, 24 h. 160 °C, 18 h.
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d
e
c180 °C, 24 h. 180 °C, 72 h. Isolated as ketone after
oxidation. Ph, phenyl; tBu, tert-butyl; Me, methyl.
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The importance of a pyridyl directing group for the
molybdenum-catalyzed RC was illustrated by the
reaction of 2-(1-phenylcyclopropyl)-1,1’-biphenyl. It did
not afford any products with ethylene loss or ring-
opened products both in the presence and absence of
added pyridine (Fig. 3a). Monitoring the reaction in p-
xylene-d10 in an NMR tube confirmed the evolution of
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1
ethylene, observed in 26% H NMR yield after 24 h at
160 °C and in 31% yield after leaving the resulting
mixture at ambient temperature for another day (Fig.
3b). Results of the RC with 2-substituted cyclopropanes
also confirmed the liberation of alkene. Thus, a
cyclopropane possessing a cis-decyl substituent at the 2-
position (3cis) was cleanly converted to 2a in 87% yield
with dodecenes detected in 87% total yield (Fig. 3c).
Similarly, a cyclopropane with a trans-decyl substituent
(3trans) afforded 2a in 57% yield with dodecenes
detected in 54% total yield.
The proposed mechanism commences with
coordination of
a pyridyl group to the active
molybdenum species generated in situ from Mo(0) and
o-quinone (Fig. 3d). The regioselective cleavage of the
proximal C–C bond of the cyclopropane ring affords the
molybdenacyclobutane
intermediate
(I),
which
undergoes a second cleavage of the C–C bond with
release of ethylene to form the molybdenum carbene
species (II). The C–N bond forming intramolecular
cyclization finally yields the pyridoisoindole product
and regenerates the active species.
Figure
3.
Molybdenum-catalyzed
retro-
cyclopropanation with release of alkene and the
proposed mechanism. Yield of ethylene determined after
leaving the resulting reaction mixture in an NMR tube at
ambient temperature for another day. Ph, phenyl.
*
The proposed mechanism was further corroborated by
density functional theory (DFT) calculations using
Mo/L1/C2H4
and
2-[2-(1-
methylcyclopropyl)phenyl]pyridine as a model metal
complex and substrate, respectively (Fig. 4).
Calculations with other possible conformations and
model systems are provided in the Supporting
Information. Gibbs free energies and enthalpies
calculated at the UM06/SDD:6-31G(d,p) level of theory
in mesitylene at 160 °C are shown here. The proximal
selective cleavage of the C–C single bond proceeds from
complex A with triplet ground state to afford
molybdenacyclobutane B in its singlet state, during
which spin crossover takes place. After this point, the
singlet potential energy surface of the productive
reaction path mostly lies lower than the triplet energy
surface. The [2+2] cycloreversion from B affords
molybdenum carbene C with coordinated ethylene.
Dissociation of a pyridyl group from the molybdenum
center (C to Dsinglet) and its intramolecular nucleophilic
attack to the carbene affords the cyclized product
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described here adds to the growing repertoire of non-
(Dsinglet to E).43 The transition state Gibbs energies of
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diazo approaches to carbene species.23 Investigations to
expand the scope of cyclopropanes for the RC reactions
by fine tuning of quinone ligands, are ongoing to further
explore the applications of cyclopropanes as diazo
surrogates.
each step relative to A were calculated to be 14.7, 5.9,
and 14.3 kcal/mol, respectively.
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ASSOCIATED CONTENT
Supporting Information. Experimental procedures, DFT
studies, physical properties of the compounds, and NMR
spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*asako@cc.okayama-u.ac.jp
*ktakai@cc.okayama-u.ac.jp
Figure 4. DFT-examined paths for the retro-
cyclopropanation. Gibbs free energies and enthalpies
(italicized) at 160 °C are given in kcal/mol, calculated at
the UM06/SDD:6-31G(d,p)mesitylene(SMD) level of
theory. *Ref. 43.
ORCID
Sobi Asako: 0000-0003-4525-5317
Kazuhiko Takai: 0000-0002-2572-0851
Notes
The authors declare no competing financial interest.
CONCLUSION
We have demonstrated that cyclopropanes can be used
as uncommon C1 carbene units by releasing ethylene
with a simple molybdenum/quinone catalyst. The
strategic use of a pyridyl group with the catalyst enabled
the efficient RC of cyclopropanes without extra strain,
and the convenient synthesis of pyridoisoindole
derivatives, compounds of interest for materials science.
Although diazo compounds have played a leading role
as sources of carbene species in organic synthesis, their
inherent explosive and toxic nature continues to demand
stable and safe surrogates for the generation of carbene
species. The C1-fragment cyclopropane chemistry
ACKNOWLEDGMENTS
We thank MEXT for financial support (KAKENHI Grant-
in-Aid for Scientific Research (A) No. 26248030 to K.T.
and Challenging Research (Exploratory) No. 17K19122 to
S.A.). The generous amount of computational time from
the Research Center for Computational Science, Okazaki,
is gratefully acknowledged.
REFERENCES
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Molybdenum carbene complex D without coordination of the pyridyl group could not be located at triplet spin state. Instead,
molybdenacyclobutane complex D′ without pyridyl coordination was found.
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