Catalytic Desymmetric Cycloaddition of Diaziridines with Metalloenolcarbenes: The Role of Donor–Acceptor Cyclopropenes

Article abstract of DOI:10.1002/anie.201906754

A chiral copper(I) complex catalyzes reactions of symmetric diaziridines with enol diazo compounds, which react through N?N bond ring opening in a formal [3+3] cycloaddition to form four chiral centers with high stereocontrol. A broad spectrum of bridged dinitrogen heterocycles were obtained in high yields and excellent diastereo- and enantioselectivities from γ-substituted enol diazoacetates, while their geometrical isomers gave different enantioselectivities. Donor–acceptor cyclopropenes formed from the geometrical isomers of the γ-substituted enol diazoacetates underwent catalytic ring opening to give only the Z isomer of the metalloenolcarbene intermediate, provided excellent yields and selectivities for the 1,5-diazabicyclo[n.3.1]non-2-ene derivatives.

Full text of DOI:10.1002/anie.201906754

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Accepted Article
Title: Catalytic Desymmetric Cycloaddition of Diaziridines with
Metalloenolcarbenes: the Special Role of Donor-Acceptor
Cyclopropenes
Authors: Haifeng Zheng and Michael P. Doyle
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Catalytic Desymmetric Cycloaddition of Diaziridines with
Metalloenolcarbenes: the Role of Donor-Acceptor Cyclopropenes
Haifeng Zheng, Michael P. Doyle*
Dedication to Professor Xiaoming Feng on the occasion of his 55th birthday
Abstract: Chiral copper(I) catalysed reactions of symmetrical
diaziridines with enoldiazo compounds undergo novel N-N bond ring
opening with formal [3+3] cycloaddition to form four chiral centers with
high stereocontrol. A broad spectrum of highly enantioenriched
bridged bis-nitrogen heterocyclic compounds were obtained in high
yield and diastereoselectivity using -substituted enoldiazoacetates,
but their geometrical isomers gave different enantioselectivities.
Donor-acceptor cyclopropenes formed from the geometrical isomers
of -substituted enoldiazoacetates underwent catalytic ring opening to
give only the more selective Z-isomer of the metallo-enolcarbene
intermediate provided optimum yields and selectivities for the 1,5-
diazabicyclo[n.3.1]non-2-ene derivatives.
deactivate an electrophilic catalyst employed to generate the
essential metal carbene intermediate.
We report here the highly stereoselective formal [3+3]
desymmetrization cycloaddition of diaziridines with enoldiazo
compounds that occur with nitrogen ylide formation followed by a
novel N-N bond ring opening. Using a chiral copper(I)/Box
complex catalyst, high enantioselectivities could be obtained with
-substituted enoldiazoacetates, and even higher enantiocontrol
(up to 97% ee) was achieved with their derivative donor-acceptor
cyclopropenes that produced only Z-metalloenolcarbene isomer
(Scheme 1c).
Diazo compounds are versatile reagents that are easily
accessible and have provided an attractive platform for various
transformations.[1] Their metal-catalyzed carbene transfer
cycloadditions,[
2]
strategies
have
enabled
diverse
rearrangements,[3] insertions,
[4]
and ylide transformations.[5]
Except in ylide-forming reactions, basic nitrogen heterocycles,
which can be synthetic building blocks in metal-catalytic carbene
transfer reactions, have rarely been used even though many
compounds produced by these methodologies are biologically
relevant.[6] However, Lacuor and co-workers developed
a
dirhodium(II)-catalyzed 1,2-Stevens-like rearrangement of aryl
diazoacetate with Tröger bases (Scheme 1a, left).[7] In addition,
Schomaker and coworkers found an elegant stereoselective
[
3+1]-ring expansion for the synthesis of highly substituted methyl
azetidines from methylene aziridines (Scheme 1a, right).[8] Each
transformation is initiated by nitrogen ylide formation followed by
C-N bond breakage and migration or rearrangement.
Scheme 1. Background and this work.
Diaziridines are saturated three-membered ring heterocyclic
compounds with two nucleophilic nitrogen atoms. Prior work has
shown that they can be used as the precursors of stabilized
azomethine imines for thermal and Lewis acid catalysed reactions
with unsaturated compounds.[9] Trushkov and co-workers recently
reported the first Lewis acid catalyzed [3+3] annulation of donor-
acceptor cyclopropanes with diaziridines, which resulted in the
formation of perhydropyridazine derivatives in moderate yields
and diastereocontrol (Scheme 1b),[10] indicating the suitability of
diaziridines for nitrogen ylide formation and C-N ring opening.
However, the strong coordinating ability of diaziridines could also
Enoldiazoacetate 1a and diaziridine 2a were selected as
model substrates to evaluate the title reaction. Commercially
available dirhodium(II) complexes were initially employed as
catalysts, but no cross-coupled products were obtained (Table 1,
entries 1 and 2). 1H NMR spectral analysis showed only the
formation of the corresponding donor-acceptor cyclopropene that
did not undergo further reaction over the extended reaction time.
Strong coordination between the diaziridine and dirhodium(II)
carboxylate was evident by colorimetric assay, indicating
deactivation of the catalyst for ring opening of the donor-acceptor
cyclopropene.[11] Gold complexes (Table 1, entries 3 and 4),
which were efficient catalysts in previous transformations of
vinyldiazo compounds,[12] also failed to afford any coupling
product, and a similar lack of reactivity was found with catalytic
[
a]
Dr. H. F. Zheng, Prof. Dr. M. P. Doyel
Department of Chemistry, The University of Texas at San Antonio
One UTSA Circle, San Antonio, TX 78249 (USA)
E-mail: michael.doyle@utsa.edu
6
AgSbF (Table 1, entry 5). However, when the reaction was
performed in the presence of copper(I) complexes, formal [3+3]
cycloaddition occurred, affording 1,5-diazabicyclo[3.3.1]non-2-
ene product 3a in good yields (Table 1, entries 6-8). This outcome
Supporting information for this article is given via a link at the end of
the document.
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involved cleavage of the N-N bond of the diaziridine rather than
the expected C-N bond cleavage.[10] The most effective catalyst
was found to be [Cu(CH CN) ][BF ], producing the desired
3 4 4
product 3a in 81% yield at room temperature (Table 1, entry 8).
Attempts to optimize the yield of 3a by changing the solvent were
unsuccessful (Table 1, entries 9-11).
Table 1. Optimization of reaction conditions for [3+3] annulation of 1a and 2a.[a]
Entry
ML
n
x (mol %)
Solvent
Yield 3a (%)[b]
1
2
3
4
5
6
7
8
9
Rh
Rh
IPrAu(CH
2
(OAc)
(S-DOSP)
4
2
2
5
5
5
5
5
5
5
5
5
CH
CH
CH
CH
CH
CH
CH
CH
2
2
2
2
2
2
2
2
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
2
2
2
2
2
2
2
2
-
-
-
-
2
4
3
CN)BF
4
JohnPhosAuCl
AgSbF
[Cu(OTf)]
Cu(CH
Cu(CH
Cu(CH
Cu(CH
Cu(CH
6
-
Scheme 2. Scope of the copper(I)-promoted [3+3] annulation reaction
2
·C
6
H
6
40
73
81
20
45
24
3
3
3
3
3
CN)
CN)
CN)
CN)
CN)
4
PF
4
BF
4
BF
4
BF
4
BF
6
4
4
4
4
CHCl
CH ClCH
toluene
3
1a reacted with 2a to produce 3a in only 55% yield with 30% ee
1
1
0
1
2
2
Cl
(Scheme 3a). Expecting that -substituted enoldiazoacetates
could enhance enantiocontrol, enoldiazoacetate 1f and diaziridine
[
(
a] Reactions were performed with 1a (0.24 mmol) and 2a (0.2 mmol) in solvent
4 mL). [b] Yields of isolated product.
2a were selected as model substrates to determine optimum
With optimized conditions in hand, we examined the scope of
conditions (for details, see SI). A systematic inspection of
tridentate (pybox), bidentate (bis-oxazoline), and double-side arm
bisoxazoline (sabox) ligands (see Supplementary Information)
revealed that the optimum results were obtained with
reactions with enoldiazo compounds 1 (1d-1h were mixtures of
geometrical isomers) with diaziridine 2 (Scheme 2). The tert-
butyldimethylsilyl (TBS) protected enoldiazoacetate (1a) gave the
corresponding 1,5-diazabicyclo[3.3.1]non-2-ene product in higher
yield than did the triisopropylsilyl (TIPS) protected enoldiazo
compound (1b). In addition, enoldiazosulfone 1c[13d] and
enoldiazoacetamide 1d[13a] gave the corresponding [3+3]-
cycloaddition products, albeit in lower yields. Furthermore, the
terminal alkyl-substituted enoldiazo compounds (1e-1g, Z/E =
at -30 oC. The desired bridged
Cu(CH
nitrogen heterocyclic derivative 3f was isolated in 91% yield with
20:1 dr and 85% ee (Scheme 3a). Variation of the silicon
protecting group (1i) and substituents of diaziridines (2b, 2c, and
h) generally provided 1,5-diazabicyclo[3.3.1]non-2-ene
3 4 4 2 2
CN) BF /L10 in CH Cl
>
2
derivatives (3q-3w) in high yields with good diastereocontrol, but
with only modest enantioselectivities (Scheme 3b).
1
.5:1-5:1) showed excellent reactivity, from which products 3e-3g
were obtained in 91-94% yields with diastereoselectivities that
were good to excellent (9:1 to >15:1 dr). The ester substituent had
little impact on the reaction outcome.
The substrate scope of diaziridines 2 was also explored
(
Scheme 2). Both electron-withdrawing and electron-donating
substituents on the 4-position of phenyl ring of diaziridine
produced the corresponding products (3i-3k) in good yields, and
substituents at different positions of the phenyl ring had little effect
on the reaction (3l and 3m). Diaziridines bearing electron-
donating substituents provided higher yields than those with
electron-withdrawing groups (EWG) (3i and 3j versus 3k), and 1-
naphthyl (2f) and 2-thienyl (2g) substituted diaziridines were
suitable substrates (70% and 50% product yields). The [6,3]-
bicyclic diaziridine substrate 2i also underwent [3+3]-
cycloaddition with 1e in good yield and diastereoselectivity.
Recent progress with enantioselective cycloaddition
reactions of enoldiazo compounds[13] prompted us to examine
enantiocontrol in [3+3]-cycloaddition with diaziridines catalysed
by copper(I) complexes with chiral ligands. Our objective was to
prepare [n.3.1]-bridged nitrogen heterocyclic compounds in good
yields with both high diastereo- and enantio-selectivities.
However, the terminal hydrogen substituted enoldiazo compound
Scheme 3. Catalytic enantioselective [3+3] cycloaddition reaction.
Although product yields were good and diastereocontrol was
high, we were disappointed with enantioselectivity. Our extensive
survey of catalysts and ligands had left us with
3 4 4
Cu(CH CN) BF /L10 as optimum, but different E/Z-ratios 1f
showed inconsistencies in product %ee values. Considering that
the Z/E-mixture may influence enantioselectivity, we prepared the
Z-isomer of 1i (Z/E > 20:1) and 1i whose isomeric
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yields and excellent stereoselectivities (3q, 3e and 3z). The more
sterically encumbered isopropyl-substituted cyclopropene 4d
afforded 3g in only 25% ee, but the n-octyl substituent did not
diminish stereocontrol from selectivities found with methyl and
ethyl substituents. For reactions with para-substituted phenyl
substituents of diaziridines 2, both electron-withdrawing and
electron-donating groups provided good product yields and
excellent dr with er greater than 96:4 (3r-3t). Ortho- and meta-
methyl substituuents of the diaziridine influence reaction
stereoselectivities (meta-3u was obtained in 89% yield, 7:1 dr,
and 89% ee, but ortho-3v was obtained in 87% yield, >20:1 dr,
and 55% ee). 1-Naphthyl- and 2-thienyl-substituted diaziridines
Scheme 4. Control experiments.
(
2g) gave the corresponding bridged nitrogen heterocyclic
derivatives 3w and 3x with lower diastereocontrol. The [6,3]-
bicyclic diaziridine substrate (2h) also underwent cycloaddition to
ratio was predominantly the E-isomer (E/Z = 1.3:1). In addition,
the donor–acceptor-substituted cyclopropene 4b, that had proven
to be efficient metallo-vinylcarbene precursors,[14c] was used in
place of the diazo compound for the asymmetric [3+3]
cycloaddition reaction. For reactions forming 3q (Scheme 4),
enantioselectivity was significantly higher using Z-1i or 4b than
with the E/Z-mixtures of 1i. Stereoselectivities obtained with Z-1i
matched those with 4b, and 3q was obtained with >20:1 dr and
the corresponding chiral
1,5-diazabicyclo[4.3.1]non-2-ene
product 3y in 59% yield with 12:1 dr and 95% ee. In addition, the
chiral product 3f was further transformed to products 5f and 6f in
good yields without loss of stereoselectivities (Scheme 6b). The
absolute configuration of 3t was determined to be (1R,4S,5S,9R)
by X-ray diffraction of the related derivative 5t (Scheme 6c).[15]
95% ee (Scheme 4b and 4c). Monitoring the reaction of diaziridine
2a with Z/E-1f showed that (a) both Z and E isomer underwent
cycloaddition, but the Z-isomer exhibited higher reactivity, and (b)
cyclopropene 4a was not observed under the reaction conditions,
which indicated that the reaction rate for [3+3]-cycloaddition with
the metallo-enolcarbene was faster than that for the formation of
the donor-acceptor cyclopropene. As described in Scheme 5,
these experiments suggested that the Z-metallo-enolcarbene 7
showed high reactivity and enantioselectivity for cycloaddition,
and that cyclopropenes underwent catalytic ring opening to give
only the Z-isomer of the metallo-enolcarbene intermediate that
provided excellent results. As a consequence, the donor-acceptor
cyclopropene, conveniently formed either thermally or
[
14]
catalytically from its parent enoldiazo-E/Z-mixture, is preferred
over the enoldiazo-E/Z-mixture as the reactant for [3+3]-
cycloaddition with diaziridines.
Scheme 6. Scope, transformation and single crystal structure
Scheme 5. [3+3]-Cycloaddition of enoldiazo compounds and cyclopropenes.
Based on the absolute configuration of 3t and previous
reports,[14] we propose in Figure 1 the probable mechanism and
stereocontrol model for the catalytic asymmetric formal [3+3]
cycloaddition. The chiral Cu(I)/L10 complex catalyzed dinitrogen
Under the optimized reaction conditions the cycloaddition
reactions between a wide range of donor–acceptor substituted
cyclopropenes 6 and diaziridines 2 were investigated (Scheme
6
2
a). The TIPS protected 4b, methyl-substituted cyclopropenes 4c, (N ) extrusion of enoldiazo compound (Z-1) forms the Z-
and enoldiazo compound Z-1j gave the desired products in good
metalloenolcarbene intermediate; and, alternatively, the chiral
Cu(I)/L10 catalyst reacts with the cyclopropene to also afford only
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the Z-metalloenolcarbene intermediate. Subsequently, the
diaziridine attacks the Z-metallo-enolcarbene from the β-Si face
to form intermediate 8 (Figure 1b, TS-I). Then, the silyl enol ether
induced nucleophilic substitution onto the second nitrogen occurs
with cleavage of the N-N bond to produce the corresponding
product 3t with the (1R,4S,5S,9R,)-configuration. However, for
the E-enoldiazo compound, steric hindrance between R group
and the Cu(I) complex weakens the metal carbene bond, resulting
lower reactivity and enantioselectivity (Figure 1b, TS-II).
Keywords: enoldiazo compounds• diaziridines• metal carbene •
cyclopropenes • asymmetric [3+3] cycloadditions
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acceptor-substituted cyclopropenes as the metalloenolcarbene
precursors in the presence of the same Cu(I)/bisoxazoline
complex catalyst with higher enantioselectivities. These results
demonstrated the intriguing reactivity of saturated nitrogen
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Acknowledgements
We acknowledge the U.S. National Science Foundation
(
CHE−1763168) for funding this research. The NMR
[15] CCDC 1910545 (5t)..
[
16] Displacement of bialkylsulfide in a [3+1]-cycloaddition reaction has been
reported:Y.-M. Deng, L .A. Massey, P. Y. Zavalij, M. P. Doyle, Angew.
Chem. Int. Ed. 2017, 56, 7479; Angew. Chem. 2017, 129, 7587..
spectrometer used in this research was supported by a grant from
the U.S. National Science Foundation (CHE−1625963).
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
H. F. Zheng, M. P. Doyle
Page No. – Page No.
Catalytic Desymmetric Cycloaddition
of Diaziridines with
Metalloenolcarbenes. The Special
Role of Donor-Acceptor
Cyclopropenes
Herein, we introduced symmetrical diaziridines in metalloenolcarbenes chemistry,
and found novel N-N bond ring opening with formal [3+3] desymmetrization
cycloaddition. In addition, chiral copper(I) catalyst could realize the high
stereoselective formal [3+3] cycloaddition of enoldiazo compounds. Futhermore, the
excellent asymmetric version was achieved by using donor–acceptor-substituted
cyclopropenes as the metalloenolcarbene precursors in the presence of same chiral
Cu(I)/bisoxazoline complex catalyst, delivering
a
various of chiral 1,5-
diazabicyclo[n.3.1]non-2-ene derivatives.
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