Copper(I)-Catalyzed Chemoselective Coupling of Cyclopropanols with Diazoesters: Ring-Opening C?C Bond Formations

Article abstract of DOI:10.1002/anie.201612138

Reported herein is an exceptional chemoselective ring-opening/C(sp³)?C(sp³) bond formation in the copper(I)-catalyzed reaction of cyclopropanols with diazo esters. The conventional O?H insertion product is essentially suppressed by j

Full text of DOI:10.1002/anie.201612138

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Communications
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International Edition: DOI: 10.1002/anie.201612138
German Edition: DOI: 10.1002/ange.201612138
À
C C Coupling Reactions
Copper(I)-Catalyzed Chemoselective Coupling of Cyclopropanols with
À
Diazoesters: Ring-Opening C C Bond Formations
Hang Zhang+, Guojiao Wu+, Heng Yi, Tong Sun, Bo Wang, Yan Zhang, Guangbin Dong, and
Jianbo Wang*
Abstract: Reported herein is an exceptional chemoselective
3
3
À
ring-opening/C(sp ) C(sp ) bond formation in the copper(I)-
catalyzed reaction of cyclopropanols with diazo esters. The
À
conventional O H insertion product is essentially suppressed
by judicious choice of reaction conditions. DFT calculations
provide insights into the reaction mechanism and the rationale
for this unusual chemoselectivity.
T
ransition-metal-catalyzed carbene-transfer reactions have
emerged as powerful tools for preparing various functional-
ized organic compounds. In particular, generated from diazo
substrates and late-transition-metal catalysts (e.g. RhII or
CuI), the electrophilic metal-carbene species react with
a
range of nucleophiles, especially heteroatom-based
reagents. These transformations provide efficient methods
for constructing carbon–heteroatom bonds.[1]
In contrast, despite the recent development of elegant
Scheme 1. Reaction of nucleophiles with metal carbenes derived from
diazo compounds.
À
catalytic C H functionalization methods by metal-carbene
insertion, few coupling reactions between a metal carbene
species and a carbon nucleophile have been reported, and
À
offer an important means to form C C single or double
Although significant progress has been made, the scope
bonds. The most commonly explored carbon nucleophiles are with respect to the carbon nucleophiles which can be
electron-rich aromatic and heterocyclic compounds (Sche- employed to couple with diazo compounds remains to be
me 1a). In this type of transformation, benzyl metal species explored. In connection to our interests in both carbene
are generated, and subsequent protonation[2a–f] or nucleo- chemistry and cyclopropanes,[5] we have chosen to explore
philic substitution[2g,h] leads to the construction of C(sp )
cyclopropanols as carbon nucleophiles in transition-metal-
2
À
C(sp3) bonds. The diazo compound itself is also an efficient catalyzed carbene-transfer reactions, considering the fact that
nucleophile (Scheme 1b). These formal dimerizations, which ring-strain relief should provide an additional driving force
have long been considered as useless side reactions, have been for the transformation.[6] While cyclopropanols have been
explored for the synthesis of alkenes[3a–c] and fused rings[3d] previously employed as carbon nucleophiles in reactions with
recently. In addition, alkenes and alkynes have also been protonic acids,[7] hypervalent halogen compounds,[8] or metal
considered nucleophilic reagents in some cases (Scheme 1c). salts,[9] an obvious reaction between cyclopropanols and diazo
The generated intermediates can undergo [1+2] reactions[4a] compounds is nevertheless the metal-catalyzed O H inser-
À
or [3+2] reactions.[4b]
tion with the carbene generated (Scheme 1d).[10] To address
this unmet challenge, herein we describe our development of
a copper(I)-catalyzed coupling between cyclopropanols and
diazo compounds (Scheme 1e).[11,12] The unusual chemose-
lectivity observed in this transformation is rationalized by the
DFT calculations.
[*] Dr. H. Zhang,[+] Dr. G. Wu,[+] H. Yi, T. Sun, B. Wang, Prof. Y. Zhang,
Prof. J. Wang
Beijing National Laboratory of Molecular Sciences (BNLMS) and Key
Laboratory of Bioorganic Chemistry and Molecular Engineering of
Ministry of Education, College of Chemistry, Peking University
Beijing 100871 (China)
Initially, phenylethyl cyclopropanol (1a) and the diazo
ester 2a were employed as the model substrates with [Rh2-
(OAc)4] as the catalyst (Table 1, entry 1). As expected, only
E-mail: wangjb@pku.edu.cn
À
the O H insertion product 3a was obtained in 64% yield.
Prof. G. Dong
After testing a series of other metal catalysts, including RhI,
FeII, AgI, and CuII salts, no improvement was observed
(entries 2–5). Surprisingly, the use of CuI as the catalyst gave
18% yield of the ring-opening/coupling product (entry 7) 4a,
whereas CuBr was not effective (entry 6). Upon examining
the effect of solvents, MeCN was shown to significantly
Department of Chemistry, University of Chicago
Chicago, IL 60637 (USA)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201612138.
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Table 1: Optimization of reaction conditions.[a]
Entry Cat. [M]
Solvent
PhMe
T [8C] 3a, % 4a, %
1
Rh2(OAc)4
60
60
60
60
60
60
60
60
60
70
70
70
64
<5
<5
18
27
18
24
<5
21
6
<5
<5
<5
<5
<5
<5
18
38
11
47
54
2
[Rh(COD)OH] PhMe
3
4
5
6
Fe(OAc)2
AgNO3
CuBr2
CuBr
CuI
PhMe
PhMe
PhMe
PhMe
7
PhMe
8
CuI
MeCN
1,4-dioxane
MeCN
MeCN
MeCN
9
CuI
10
11[c]
12[d]
13[c]
14[c]
CuI
CuI
CuI
CuI
<5
8
57
MeCN/PhMe (1:10) 70
MeCN/PhMe (1:10) 80
6
79
CuI
5
92
[a] Reaction conditions are as follows unless otherwise noted: 1a
(0.10 mmol), 2a (0.10 mmol), and the catalyst (0.01 mmol) were stirred
in 0.5 mL solvent at the indicated temperature for 2 h. [b] The yield was
1
determined by H NMR spectroscopy using MeNO2 as the internal
standard. [c] 1.5 equiv of 2a was used. [d] 2.5 equiv of 2a was used.
À
suppress the O H insertion, and consequently afforded 4a in
38% yield (entries 8 and 9). Considering the high conversion
observed with toluene as the solvent, gratifyingly, use of the
mixed solvent system of MeCN/PhMe (1:10) markedly
enhanced the reaction efficiency (entry 13). Finally, raising
the reaction temperature to 808C improved the yield to 92%
(entry 14).
Scheme 2. Reaction scope. Reaction conditions are as follows if not
otherwise noted: 1a–n (0.20 mmol), 2a–q (0.30 mmol) and CuI
(10 mol%) were stirred in 0.5 mL PhMe/MeCN (10:1) at 808C for 2 h.
All yields are those of isolated products. [a] The reaction was carried
out with 1a (0.20 mmol) and 2l–o (0.40 mmol). TBS=tert-butyldime-
thylsilyl, Ts=4-toluenesulfonyl.
Subsequently, we proceeded to study the scope of this
ring-opening/coupling reaction (Scheme 2). A series of cyclo-
propanols (1a–n), readily prepared by a Kulinkovich reac-
tion, was first examined. The alkyl-substituted cyclopropanols
(1a–d) reacted smoothly with the diazo ester 2a, thus
affording the corresponding products 4a–d in good to
excellent yields. Notably, a range of functional groups, such
as TBS and phenyl ethers, esters, and sulfonates, are compat-
ible with the reaction conditions (4e–h). Moreover, alkenyl-
substituted cyclopropanols (1i and 1j) are also competent
substrates. Next, a series of aryl-substituted cyclopropanols
1ka–kc were subjected to the reaction, and the desired
products 4ka–kc were obtained in good to excellent yields. It
is noteworthy that when the 1,2-disubstituted cyclopropanol
1l was used as the substrate, the reaction resulted in two
constitutional isomers, 4la and 4lb, thus indicating that both
compounds with different ester moieties led to similar good
yields of products (4p–t). Finally, the reactions with the alkyl-
substituted diazoacetates 2l–o proceeded smoothly to afford
the corresponding products 4u–x in good yields.
To gain insight into the reaction mechanism, several
control experiments were carried out (Scheme 3). First, 3a
was treated under standard reaction conditions (Scheme 3a).
À
proximal C C bonds can be cleaved. However, both products
show poor diastereoselectivity.
The scope with respect to the diazo coupling partners was
then examined (Scheme 2). A series of aryl-substituted
diazoacetates (2b–e) was investigated. Various substituents
on the aromatic ring were tolerated, and the reaction was
found to be hardly affected by the electronic properties of the
aryl substituents. The reaction with naphthyldiazoacetate (2 f)
also gave product 4o in 92% yield. Moreover, using the diazo
Scheme 3. Experiments for mechanistic understanding.
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After stirring for 2 hours, no corresponding product 4a was
computed at the same level to check whether the optimized
observed, thus indicating that the formation of 3a and 4a
follows different reaction paths. Subsequently, 1a was treated
with 10 mol% CuI (Scheme 3b). After stirring for 2 hours,
98% of 1a was recovered and only 2% of the ketone 5 (ring-
opening product of 1a) was observed. Increasing the amount
of CuI to 100 mol% resulted in 5% of 5. These results
indicate that CuI alone is not effective to induce the
cyclopropanol ring-opening to generate the alkyl copper
intermediate 6 (Scheme 3c).[9] Thus, it is reasonable to
hypothesize that the copper(I) catalyst first reacts with the
diazo substrate to generate an active intermediate, which
further reacts with the cyclopropanol substrate.
structure is an energy minimum or a transition state, and to
evaluate its zero-point vibrational energy (ZPVE) and
thermal corrections at 353 K. All the transition states have
been verified by IRC (intrinsic reaction coordinate) calcu-
lations. Quasi-harmonic corrections were applied for the
evaluation of the entropic corrections to the Gibbs free
energies by setting the frequencies less than 100 cmÀ1 to
100 cmÀ1.[18] And the single-point energies were calculated at
the M06/6-311 + G(d,p) [Lanl2dz, for Cu] level of theory
using the SMD model[19] with toluene as the solvent based on
the B3LYP/6-31G(d,p)[Lan2lzd, for Cu] optimized geome-
tries.[20] The calculated Gibbs free energies are determined by
adding the thermal correction to Gibbs free energies, based
on the B3LYP level of theory, and single-point energy with
solvation energy correction based on the M06 level of theory.
Gibbs free energies for all species were corrected by a factor
of RTln(29.0) from 1 atmosphere of gas to 1m solution for
standard-state corrections, corresponding to 2.4 kcalmolÀ1.
Figure 1 illustrates the free energy profiles of two reaction
To gain further understanding of the reaction mechanism
À
and in particular the chemoselectivity of O H insertion
versus ring-opening, we performed DFT calculations.
Although the chemoselectivity of the ring-opening is obvi-
ously improved by MeCN as a cosolvent, as a result of the
unclear formation of the [Cu(MeCN)n]+ complex we have
used bipy as the ligand for computational study. Notably, the
reaction with either bipy or (4R,4’R)-4,4’-dibenzyl-4,4’,5,5’-
tetrahydro-2,2’-bioxazole as a ligand in toluene gave similar
chemoselective ring-opening.[13] The calculations were per-
formed on the reaction of the cyclopropanol 1k and CuI
carbene A using bipy as the ligand. All the DFT calculations
were performed using Gaussian09 (RevisionA.02).[14] Geom-
etry optimizations were carried out with B3LYP level of
theory.[15] Lanl2dz basis set[16] was used for copper and
6-31G(d,p) basis set[17] for other atoms (Keyword 5D was
used for calculations). The vibrational frequencies were
À
À
pathways: O H insertion for C O bond formation and ring-
À
opening for C C bond formation. Figure 2 gives key inter-
mediates and transition-state structures, which were prepared
with CYLview.[21]
À
The mechanisms of O H insertion of diazo compounds
with either rhodium or copper as catalysts have been
previously studied in detail with DFT calculations.[22] For
À
comparison, DFT studies with O H insertion have also been
investigated for the current reaction system. In Figure 1,
À
calculations show that in the O H insertion process, nucle-
3
3
À
À
Figure 1. DFT-computed free-energy surface for the copper-catalyzed carbene O H insertion and ring-opening/C(sp ) C(sp ) bond formation.
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Figure 2. DFT-computed key intermediates and transition states.
ophilic attack of 1k on A is very facile through transition-
state TS1 with an activation free energy of 14.5 kcalmolÀ1. In
In the previous computational studies by Yu[22a] and
Xie[22b] on copper(I)-catalyzed carbene O H insertion,
À
À
TS1, the distance between the hydrogen atom of O H and the
a water-catalyzed [1,2] proton shift pathway was proposed.
We have also carried out DFT calculations on the similar
cyclopropanol-assisted [1,2] proton shift in our reaction
system (Figure 1, inset). The oxonium ylide O-1 interacts
with 1k through hydrogen bonding, thus generating O-5. The
process is slightly endergonic by 0.5 kcalmolÀ1. The inter-
mediate O-5 is further converted into the enol-copper
intermediate O-6, which undergoes a [1,2] proton shift via
=
oxygen atom of C O is 1.66 ꢀ, thus indicating a hydrogen-
bonding interaction. The formation of the oxonium ylide
O-1 is endergonic by 11.8 kcalmolÀ1. For O-1, the formed
À
À
C O bond has a bond length of 1.59 ꢀ, while the C O bond
=
(1.41 ꢀ) in 1k and C Cu (1.89 ꢀ) bond in A are elongated to
1.49 ꢀ and 2.01 ꢀ, respectively. The previous studies by Yu
À
and co-workers have suggested that the formation of O H
À
insertion product, by a H-shift process of the metal-dissoci-
ated oxonium ylide, has a higher energy barrier because of the
difficulty of an intramolecular [1,2] proton shift.[22a,23] There-
fore, the H-shift processes with metal-associated oxonium
ylide are also anticipated in the current reaction. Here we
have found that O-1 can convert into the metal-associated
a five-membered ring transition-state (TS5) to give the O H
insertion product 4ka. In this pathway, the overall energy
barrier is 37.6 kcalmolÀ1, which is higher than that of
[1,3] proton pathway described above (29.4 kcalmolÀ1).
À
In contrast to O H insertion, 1k can also react with A via
the transition-state TS6, in which 1k, as the carbon nucleo-
phile reacts with copper(I) carbene by ring-opening. The
energy barrier for this pathway is 25.0 kcalmolÀ1. In TS6, the
enol intermediate O-2 quickly via TS2 with an activation free
À1
À
energy of nearly 0 kcalmol . In TS2, the formed C O bond is
=
À
shortened to 1.54 ꢀ and the elongation of C Cu bond is only
formed C C bond has a bond length of 2.43 ꢀ, which is much
À
0.02 ꢀ with the respect of O-1, thus indicating that a low
activation energy barrier is attributed to the minor geo-
metrical variation. Then formation of the copper dissociated
O-3 from O-2 is endergonic by 11.4 kcalmolÀ1. Subsequent
[1,3] proton shift via TS3, assisted by 1k, has an energy barrier
of 33.2 kcalmolÀ1. Thus, the overall energy barrier of this
pathway is as high as 44.4 kcalmolÀ1, indicating that the
metal-free [1,3] proton shift pathway is unlikely, and is
consistent with the study by Yu and co-workers.[22] Alter-
natively, O-2 can be transferred into O-4 with the oxygen
atom coordinated to the copper, and is endergonic by only
longer than a single C C bond (1.54 ꢀ), thus suggesting an
À
early transition state. The distance between O H hydrogen
atom and the carbonyl oxygen atom is 2.08 ꢀ, thus indicating
À
the formation of a hydrogen bond. The elongation of the C C
bond of the three-membered ring is 0.25 ꢀ, while the
=
elongation of C Cu is only 0.04 ꢀ. The high activation
À
energy is attributed to the broken C C bond of the cyclo-
propane moiety. However, this step, leading to the formation
of the enol intermediate C-1, is dramatically exothermic by
38.0 kcalmolÀ1 because of the release of ring-strain. This data
suggests that this step is irreversible. C-1 can transfer into the
intermediate C-2 via the transition-state TS7 with an activa-
tion energy of 22.2 kcalmolÀ1. C-2 is quickly converted into
5.3 kcalmolÀ1.
The
following
cyclopropanol-assisted
À
[1,3] proton shift via TS4 leads to the O H insertion product
4ka, crossing an overall energy barrier of 29.4 kcalmolÀ1
.
4
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H. M. L. Davies, Angew. Chem. Int. Ed. 2011, 50, 2544; Angew.
the final product by a [1,5] proton shift via TS8, thus crossing
an activation free energy of only 5.8 kcalmolÀ1
Chem. 2011, 123, 2592; b) D. Zhang, G. Xu, D. Ding, C. Zhu, J.
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.
Therefore, the DFT calculations show that the ring-
opening pathway via the rate-determining and irreversible
À1
À
TS6 has an activation barrier of 25.0 kcalmol , while O H
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insertion via the rate-determining TS4 needs 28.7 kcalmolÀ1,
À
thus suggesting the formation of C C cleavage product is
À1
À
kinetically favored by 3.7 kcalmol . Therefore, the C C
bond cleavage product will be predominant as compared to
À
the O H insertion product, which is consistent with the
experimental observations.
In summary, we have developed a copper(I)-catalyzed
coupling of cyclopropanols with diazo esters to provide ring-
3
3
À
opened/C(sp ) C(sp ) coupled products, whereas the conven-
À
tional carbene O H insertion pathway can be mostly sup-
pressed. DFT calculations provide insights into the detailed
reaction mechanism and the rationale for this exceptional
chemoselectivity.
Acknowledgements
This project is supported by 973 program of China (No.
2015CB856600) and NSFC (Grant 21332002). We thank Prof.
Zhixiang Yu and Mr. Yi Wang of Peking University for
discussion.
À
[10] For reviews on carbene O H insertion, see: a) D. J. Miller, C. J.
Moody, Tetrahedron 1995, 51, 10811; b) S. F. Zhu, Q.-L. Zhou,
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Conflict of interest
[11] A ring-opening addition of cyclopropanol derivative with
carbene has been reported previously. However, the reaction
follows a free-radical pathway. See: A. Oku, M. Iwamoto, K.
Sanada, M. Abe, Tetrahedron Lett. 1992, 33, 7169.
The authors declare no conflict of interest.
À
Keywords: carbenes · C C coupling reactions · copper ·
À
[12] Formal carbene insertions into C C bonds of cyclobutanol have
diazo compounds · small ring compounds
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[13] When using bipy as a ligand in toluene and the CuI catalyst, 4a is
obtained in 43% yield, along with less than 5% 3a. (4R,4’R)-
4,4’-Dibenzyl-4,4’,5,5’-tetrahydro-2,2’-bioxazole delivers 4a in
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À
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[3] For selected examples of reactions with diazo compounds, see:
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Angewandte
Communications
Chemie
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Manuscript received: December 14, 2016
Revised: January 25, 2017
Final Article published: && &&, &&&&
6
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Angewandte
Communications
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Communications
À
C C Coupling Reactions
H. Zhang, G. Wu, H. Yi, T. Sun, B. Wang,
Y. Zhang, G. Dong,
J. Wang*
&&&& — &&&&
Copper(I)-Catalyzed Chemoselective
Coupling of Cyclopropanols with
À
Diazoesters: Ring-Opening C C Bond
Formations
Making the transfer: Under copper(I)-
catalyzed reaction conditions, the reac-
tion of cyclopropanols with diazo esters
forming products rather than the con-
À
ventional O H insertion products. A
rationale for this unusual chemoselectiv-
ity is provided by computational studies.
3
3
À
gives ring-opening/C(sp ) C(sp ) bond-
Angew. Chem. Int. Ed. 2017, 56, 1 – 7
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7
These are not the final page numbers!