Asymmetric Intramolecular Buchner Reaction: From High Stereoselectivity to Coexistence of Norcaradiene, Cycloheptatriene, and an Intermediate Form in the Solid State

Article abstract of DOI:10.1021/acs.orglett.0c03774

Bicyclic compounds bearing a quaternary stereogenic center have been obtained using asymmetric intramolecular Buchner reaction with excellent yields and level of enantioselectivity. X-ray crystallography determination of the absolute configuration of one product has led to the serendipitous observation of an unusual behavior within the crystal structure, with equilibrating norcaradiene and cycloheptatriene valence isomers at the solid state, as well as an even more unexpected intermediate form. DFT calculations were performed to support these observations.

Full text of DOI:10.1021/acs.orglett.0c03774

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Letter
Asymmetric Intramolecular Buchner Reaction: From High
Stereoselectivity to Coexistence of Norcaradiene, Cycloheptatriene,
and an Intermediate Form in the Solid State
Benjamin Darses,* Pascale Maldivi, Christian Philouze, Philippe Dauban, and Jean-Franco̧ is Poisson
Cite This: Org. Lett. 2021, 23, 300−304
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ABSTRACT: Bicyclic compounds bearing a quaternary stereogenic
center have been obtained using asymmetric intramolecular Buchner
reaction with excellent yields and level of enantioselectivity. X-ray
crystallography determination of the absolute configuration of one
product has led to the serendipitous observation of an unusual
behavior within the crystal structure, with equilibrating norcaradiene
and cycloheptatriene valence isomers at the solid state, as well as an
even more unexpected intermediate form. DFT calculations were
performed to support these observations.
Scheme 1. Buchner Reaction
olycyclic structures containing seven-membered carbo-
Pcycles are widely found in natural products. Among them,
terpenes (daphnanes, tiglianes, ingenanes...) or alkaloids
(colchicine, aconitine, cortistatins...) possess original architec-
tures (Figure 1). However, the cycloheptane ring is not as
common as the five- and six-membered rings within the
commercially available building blocks.
frameworks, while providing higher selectivities for the
addition of the reactive carbenes, more prone to competitive
C−H insertions in intermolecular processes. While the
generation of carbenes by thermal or photochemical
decomposition of diazo precursors generally leads to mixtures
of products, metal-catalyzed denitrogenation of diazo com-
pounds can master the reactivity of carbenes,4 allowing much
more selective aromatic cyclopropanation and providing
opportunities for synthetic developments. Though copper
complexes were initially used,4a dirhodium(II) complexes
rapidly showed greater catalytic activities,4b,c and still remain
the most widely used catalysts to efficiently perform the
Buchner reaction.
Figure 1. Seven-membered carbocycle-containing natural products.
Hence, seven-membered ring carbocycles must be built
using synthetic methods such as ionic cyclizations or ring-
closing metathesis, and also through elegant cycloadditions
requiring the design of complex precursors.1 Alternatively,
enlargement of broadly accessible smaller rings has also been
considered.1a Within the latter, the Buchner reaction is unique
in its ability to induce a neutral expansive dearomatization,
directly affording valuable functionalized cycloheptatrienes
from widely available aromatic precursors.2 This original
transformation consists in an unusual cyclopropanation of an
aromatic ring by a carbene, delivering a norcaradiene, in
equilibrium through an electrocyclic rearrangement with the
generally more stable cycloheptatriene (Scheme 1).3
Despite a great synthetic potential, very few examples of
efficient asymmetric synthesis of cycloheptatrienes have been
reported using the intramolecular Buchner reaction.5,6
Moderate to poor enantioselectivities are generally reported
Received: November 12, 2020
Published: January 4, 2021
The intramolecular Buchner reaction is particularly efficient
to rapidly access bicyclic seven-membered ring-containing
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© 2021 American Chemical Society
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a
for Buchner reactions involving benzylic α-diazoesters or α-
diazoamides. Moreover, these intramolecular processes often
raise the issue of chemoselectivity that translates to the
formation of mixtures of products and, thus, the isolation of
the expected cycloheptatrienes in only moderate yields.5d Very
recently, the use of a cationic chiral Ru-complex has led to a
great improvement in the enantiocontrol of the reaction for the
generation of tertiary stereogenic centers.7 Inspired by our
recent studies on rhodium-catalyzed asymmetric amination,8
we have decided to investigate the use of chiral dirhodium(II)
complexes in the intramolecular Buchner reaction for the
formation of more challenging quaternary stereogenic centers.
In this communication, we report an efficient synthesis of
enantioenriched cycloheptatrienes through catalytic asymmet-
ric intramolecular Buchner reaction using cyano-substituted
diazo derivatives.9,10
Table 1. Screening of Chiral Ligands and Temperature
Our investigations started by reacting benzyl β-cyano α-
diazoacetamide 1a in the presence of 1 mol % of the
commercially available chiral dirhodium(II) complex (tetrakis-
[N-phthaloyl-(S)-tert-leucinate] (Rh2[S-pttl]4), in dichloro-
methane at 0 °C (Table 1, entry 1). We were delighted to
observe the selective formation of the expected Buchner
product 2a in 95% yield and with a promising 87:13
enantiomeric ratio. Lowering the temperature to −30 °C did
not impede the efficiency of the reaction while improving the
enantiomeric ratio to 90.5:9.5 (Table 1, entry 2). A screening
was thus undertaken at this temperature to find the most
efficient chiral catalyst.
Changing the amino acid side chain or introducing fluorine
atoms on the phthaloyl moiety of the ligands proved
deleterious to the asymmetric induction (Table 1, entries 3−
5), while a tert-butyl group in position 4 of the phthaloyl group
slightly improved the enantioselectivity (Table 1, entry 6).11
The best result was obtained with the S-bpttl ligand,12 where
the phenyl is replaced by a 2,3-naphthoyl motif (Table 1, entry
7). However, other isomeric naphthalene derivatives led to
lower enantiomeric ratio (Table 1, entries 8−9). The dosp or
the btpcp ligands offered moderate to poor enantioselectivities
in this transformation (Table 1, entries 10−11).13 Variation of
the solvent also led to lower enantioselectivities (see Table S1,
SI). Carrying the reaction at −50 °C with the S-bpttl ligand
allowed both an improvement of the enantiomeric ratio to 96:4
and the yield to 95% (Table 1, entry 12). Similar results were
obtained using of 0.5 mol % of rhodium catalyst (Table 1,
entry 13).14
The optimized reaction conditions have then been applied
to a variety of substituted aromatic derivatives (Scheme 2).
Substitution of the aromatic ring in para position allowed the
preparation of the corresponding Buchner products in
excellent yields and enantioselectivities. The presence of an
alkyl or a silyl group (2b−c) maintained a similar selectivity,
and halogens induced even greater enantiomeric ratio (2d−f).
Nitrogen-containing functional groups such as an azide or a
phthalimide also afforded the expected products both in high
yields and selectivities (2g−h). The best enantiomeric ratios
were obtained with an aromatic bearing a substituent
extending the π-conjugation such as a phenyl or a phenyl-
acetylene moiety (2i−j). The asymmetric formation of
cycloheptatrienes is therefore very efficient, both in terms of
yield and enantioselectivity, with a moderately electron-
donating or -withdrawing group at the para position of the
aromatic ring. However, with more electron-donating groups,
such as a methoxy (2k), the racemic product was obtained.
a
Reaction conditions: To a solution of 1a (0.300 mmol, 1.00 equiv)
in CH2Cl2 (6 mL) at −30 °C, under an argon atmosphere, was added
Rh2L4 (3.0 μmol, 1 mol %). The mixture was stirred at this
b
temperature for 2 h. Isolated yields after flash chromatography.
c
d
e
Determined by chiral HPLC. Reaction carried at 0 °C. Reaction
f
carried at −50 °C for 24 h. 0.5 mol % of Rh2L4.
This could be due to an equilibrating process involving an
achiral spiro intermediate.15 This phenomenon has previously
been postulated to explain such interconversion, and the
presence of the nitrile substituent is probably favoring the
push−pull racemization process.16 The reaction is not limited
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c
Scheme 2. Reaction Scope
Scheme 3. Gram-Scale Reaction
adiene equilibrium in favor of the latter. It is therefore
important to evaluate the position of the equilibrium between
the two forms: observing the proton and carbon NMR
chemical shifts of C6−H provides useful information.17 Indeed,
they are known to be located at a weighted value reflecting the
ratio between the two fast-equilibrating isomers.2 The
structural isomers 2i and 2o, deriving respectively from a 4-
phenyl or a 3-phenyl-substituted substrate, provide good
comparison data. Whereas the C6−H 1H and 13C NMR
chemical shifts respectively appear at 4.92 and 98.8 ppm for 2i,
showing a vinylic character, those of compound 2o at 3.74 and
62.1 ppm are more consistent with a cyclopropane ring (Figure
2). However, the imprecise position of the equilibrium
a
b
7:1 mixture of regioisomers. For the shown major regioisomer.
Reactions performed on 0.300 mmol of substrate for 24 h. Isolated
c
Figure 2. H and 13C NMR chemical shifts of C6−H for different
products 2.
1
yields, and e.r. determined by chiral HPLC.
to N-tert-butyl derivatives, as a N-benzyl amide (2l) also
afforded the cycloheptatriene in good yield though in
moderate enantioselectivity. It was also possible to introduce
an additional methylene between the carbene and the aromatic
ring with only a slight decrease of efficiency of the reaction.
The 6,7-bicyclic derivative 2m was thus isolated in 74% yield
and 90.5:9.5 e.r.
An ester in the meta position proved very interesting as it
leads to the isolation of a single regioisomer, notably as the
norcaradiene product (2n) in 73% yield and an excellent
enantioselectivity of 98.5:1.5. This product corresponds to the
addition of the carbene to the most hindered of the two
accessible positions of the aromatic ring. Similarly, a meta
phenyl group afforded the same chemical reactivity, but in this
case, the isolated product 2o is a 7:1 mixture of norcaradiene
regioisomers, in favor of the most hindered one.
We decided to test the reliability of the transformation on a
synthetically relevant scale. Applying the same reaction
conditions on 5.00 mmol (1.28 g) of 1a delivered more than
one gram of the expected product 2a with exactly the same
yield and enantiomeric ratio. On 5.00 mmol scale, the catalyst
loading could also be notably lowered to 0.05 mol % (i.e., 3.6
mg), affording 2a in 93% yield and an unchanged stereo-
selectivity (Scheme 3).
frequently leads to contradictions. For example, ( )-2m was
described as a norcaradiene by Reisman,9a whereas Xu
reattributed the structure of the same product to a cyclo-
heptatriene.9b Observing the chemical shifts corresponding to
1
C6−H, which appear at 4.56 and 81.9 ppm by H and 13C
NMR, it is possible to evaluate that this compounds may better
be represented as a cycloheptatriene rather than a norcar-
adiene.
In order to determine the absolute configuration of the
prepared bicycles, we also decided to use the bromine
anomalous diffusion at Mo Kα wavelength, on compound
2e.18 We first examined the NMR spectral data of this
compound as we did for 2i, 2m, and 2o. In this case, the
chemical shifts of 1H and 13C NMR for C6 (δ = 5.13 and 111.7
ppm respectively; CDCl3, r.t.) clearly account for an even more
pronounced sp2 character and therefore a highly prevalent
cycloheptatriene form in solution.
The absolute configuration of 2e was then determined to be
R by single-crystal X-ray crystallography. Interestingly, the
asymmetrical unit displays two types of molecular forms in
sharp contrast with the NMR observations in solution (Figure
3). The form A appears as a statistical disorder of the two
valence isomers of 2e: the norcaradiene A-NCD and the
cycloheptatriene A-CHT, in a 0.512/0.488 ratio. On the other
hand, the form B is a slightly thermally agitated structure,
which seems to be structurally laying at the intermediacy of the
As aforementioned (Scheme 2), the substitution of the
products can sometimes shift the cycloheptatriene/norcar-
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ing bicyclic skeletons. The asymmetric intramolecular Buchner
reaction allowed the highly stereoselective installation of a
quaternary stereogenic center, delivering a library of diversely
substituted products. The unexpected observation of a
compound present as the cycloheptatriene form in solution,
but as a mixture of cycloheptatriene and norcaradiene in the
solid state prompted us to undergo some DFT calculations to
rationalize this observation. Further investigations on this
striking valence isomerization in solution or in the crystalline
structure are currently under progress, and so is the calculation
of the enantiodetermining transition state with the chiral
catalyst to rationalize the origin of the enantioselectivity.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03774.
Figure 3. X-ray structures of 2e: structures A and B present in the
crystal, and decomposition of A into its valence isomers.
Experimental procedures, characterization data, compu-
tational details, and crystallographic data (PDF)
Accession Codes
two extreme valence isomers. The C1−C6 distance of 1.853(9)
Å is indeed between those of A-NCD and A-CHT (1.56(4) Å
and 2.475 Å respectively). Similarly, the C6−C7−C1 angle has
a value of 77.2(5)° for B, far from the 109.5° expected for a
perfect tetrahedron or the 111(2)° angle observed for A-CHT,
but slightly closer to the 62(2)° angle observed for A-NCD.
Consequently, B could be seen as an artificially observable
transition state between the two valence isomers A-NCD and
A-CHT, stabilized within the crystalline matrix. To the best of
our knowledge, this behavior has never been reported for this
type of compound. Moreover, in the solid state, the undeniable
presence of a large amount of norcaradiene seems to indicate a
network stabilization of this isomer.
The observation of both forms of 2e in the solid state,
including an intermediate structure, as well as its different
behavior in solution, revealed by NMR, indicate an equivalent
or close stability of the two isomers and a fast exchange
between both. Although the equilibrium is a known
phenomenon, the easy balance between the two isomers, by
physical state modification, may lead to contradictory product
descriptions, as well as an unexpected reaction of the
misidentified isomer upon condition modifications. We thus
decided to model the thermodynamics profile of the isomers of
2e at 298 K using Density Functional Theory (DFT)
calculations to evaluate the Gibbs energies of the norcaradiene
and cycloheptatriene isomers (see SI for details). The
optimized geometries thus showed a weak stabilization of
−3.6 kcal mol−1 in favor of the cycloheptatriene form (in
accordance with the NMR observation). Moreover, the
activation free energy for the interconversion, computed at
6.7 kcal mol−1, is low and consistent with a possible fast
exchange between the two valence isomers.
CCDC 1996828 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Benjamin Darses − Univ. Grenoble Alpes, CNRS, DCM,
38000 Grenoble, France; Institut de Chimie des Substances
Naturelles, Université Paris-Sud, Université Paris-Saclay,
91198 Gif-sur-Yvette, France; orcid.org/0000-0002-
2284-5931; Email: benjamin.darses@univ-grenoble-
alpes.fr
Authors
Pascale Maldivi − Univ. Grenoble Alpes, CEA, CNRS, IRIG,
SyMMES, 38000 Grenoble, France; orcid.org/0000-
0003-2008-1090
Christian Philouze − Univ. Grenoble Alpes, CNRS, DCM,
38000 Grenoble, France
Philippe Dauban − Institut de Chimie des Substances
Naturelles, Université Paris-Sud, Université Paris-Saclay,
91198 Gif-sur-Yvette, France; orcid.org/0000-0002-
1048-5529
Jean-Franco̧ is Poisson − Univ. Grenoble Alpes, CNRS, DCM,
38000 Grenoble, France; orcid.org/0000-0002-4982-
7098
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03774
It is worth noting that optimized geometrical parameters are
also in very good agreement with the experimental crystal data
(see Table S2, SI). In particular, the calculated transition state
(TS) structure is close to the observed form B found in the
crystal of 2e (Figure 3): a C1−C6 distance of 1.91 Å and a C6−
C7−C1 angle of 79.5°; the stabilization of this unconventional
form B within the crystalline matrix is quite puzzling, and to
the best of our knowledge unprecedented.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We wish to thank the Labex ARCANE and CBH-EUR-GS
(ANR-17-EURE-0003) for their financial support. The Nano-
■
́
In conclusion, we have reported an efficient strategy to
prepare enantioenriched seven-membered carbocycle-contain-
Bio-ICMG platforms (FR 2607), the Departement de Chimie
́
Moleculaire, and the Institut de Chimie des Subtances
303
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Angew. Chem. 2019, 131, 8276−8280. (b) Nasrallah, A.; Lazib, Y.;
Boquet, V.; Darses, B.; Dauban, P. Catalytic intermolecular C(sp3)−H
amination with sulfamates for the asymmetric synthesis of amines.
Org. Process Res. Dev. 2020, 24, 724−728.
(9) (a) Nani, R. R.; Reisman, S. E. α-Diazo-β-ketonitriles: Uniquely
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(10) For asymmetric synthesis of norcaradienes using cyano-
substituted α-diazoesters, see: Smith, K. L.; Padgett, C. L.; Mackay,
W. D.; Johnson, J. S. Catalytic, Asymmetric Dearomative Synthesis of
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Naturelles are acknowledged for their technical and/or
financial support.
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