Chemo- and Enantioselective Intramolecular Silver-Catalyzed Aziridinations

Article abstract of DOI:10.1002/anie.201704786

Asymmetric nitrene-transfer reactions are a powerful tool for the preparation of enantioenriched amine building blocks. Reported herein are chemo- and enantioselective silver-catalyzed aminations which transform di- and trisubstituted homoallylic carbamates into [4.1.0]-carbamate-tethered aziridines in good yields and with ee values of up to 92 %. The effects of the substrate, silver counteranion, ligand, solvent, and temperature on both the chemoselectivity and ee value were explored. Stereochemical models were proposed to rationalize the observed absolute stereochemistry of the aziridines, which undergo nucleophilic ring opening to yield enantioenriched amines with no erosion in stereochemical integrity.

Full text of DOI:10.1002/anie.201704786

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Accepted Article
Title: Chemo- and enantioselective intramolecular silver-catalyzed
aziridination
Authors: Minsoo Ju, Cale Weatherly, Ilia Guzei, and Jennifer M.
Schomaker
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Angewandte Chemie International Edition
10.1002/anie.201704786
Chemo- and Enantioselective Intramolecular Silver-catalyzed
Aziridinations
Minsoo Ju, Cale D. Weatherly, Ilia A. Guzei, and Jennifer M. Schomaker*
[
10]
Abstract: Asymmetric nitrene transfer reactions are a powerful tool
for the preparation of enantioenriched amine building blocks. Herein,
we report chemo- and enantioselective silver-catalyzed aminations
that transform di- and trisubstituted homoallylic carbamates into
BOX ligand has also been described (Scheme 1D). Styrenes
gave ee up to 84%, but replacing Ph with alkyl groups lowers the
ee; to our knowledge, no successful examples of asymmetric
aziridinations of trisubstituted alkenes have been reported.
[
9
4.1.0]-carbamate-tethered aziridines in good yields and ee up to
2%. The effects of the substrate, silver counteranion, ligand,
Scheme 1. Approaches to metal-catalyzed chemoselective nitrene transfers.
solvent and temperature on both chemoselectivity and ee were
explored to complement the scope of traditional metal-catalyzed
nitrene transfer reactions. Stereochemical models were proposed to
rationalize the observed absolute stereochemistry of the aziridines,
which undergo nucleophilic ring-opening to yield enantioenriched
amines with no erosion in stereochemical integrity.
Asymmetric, metal-catalyzed nitrogen-atom transfer reactions
offer excellent opportunities to transform simple precursors into
enantioenriched amines, key building blocks for pharmaceuticals,
[
1,2]
agrochemicals, natural products and ligands.
Additions of
metal nitrenes to alkenes are particularly useful, as the strain in
the resulting aziridines enables facile nucleophilic ring cleavage
to furnish enantioenriched amines, amino acids, aminoalcohols
[
3]
and β-lactams. Compared to asymmetric epoxidations, the
corresponding aziridinations are underexplored and display
narrower substrate scope. In this communication, we report an
intramolecular chemo- and enantioselective silver-catalyzed
nitrene transfer that furnishes di- and trisubstituted aziridines in
good yields and ee. Ring-opening with diverse nucleophiles
occurs at the distal aziridine carbon to yield amines with
excellent retention of stereochemical information.
Our group has reported tunable, chemo- and site-selective
nitrene transfers catalyzed by Ag(I) complexes supported by
[
11-16]
diverse N-chelating ligands.
We wondered if the flexibility
exhibited by these catalysts might improve both the scope and
the ee of intramolecular alkene and allene aziridinations. Such
studies would provide corroboration of our hypothesis that lower-
coordination Ag(I) complexes favour aziridination, while higher
Early investigations of asymmetric metal-catalyzed nitrene
transfer were pioneered by Evans and Jacobsen (Scheme 1A-
[
4,5]
B).
Cu supported by bis(oxazoline) (BOX) and diimine
ligands were employed with imidoiodinanes as nitrogen sources.
The biggest drawback of Cu-based catalysis is the limited scope,
which is largely restricted to styrenes, terminal olefins and
[
11]
coordination at the metal prefers C-H bond amination instead.
In light of the need to favor low-coordinate Ag(I) complexes,
preliminary studies focused on bidentate asymmetric ligands
using AgClO as the metal salt (see Supporting Information for
4
full details). Chiral BOX ligands gave the best balance between
chemoselectivity for aziridination and ee. Optimizations using
homoallylic carbamates 1-(Z) and 2-(E) were carried out with
various Ag salts and BOX ligands (Table 1, full details in the SI).
[
6]
conjugate acceptors. Chiral Ru catalysts supported by salen
[
7]
ligands (Scheme 1C) use sulfonyl azides as nitrene precursors
to furnish aziridines in up to 99% ee; however, the reaction
works best with styrenes and terminal alkyl olefins. Dinuclear Rh
catalysts and Zhang's porphyrin-Co complexes have also
[
1a,8]
enjoyed success with certain classes of substrates.
L2 and L5 gave good chemoselectivity and ee with AgClO
entries 1, 5), while other ligands displayed either poor yield or
4
Surprisingly, few examples of metal-catalyzed intramolecular
asymmetric aziridinations have been reported, despite their
potential for synthesizing valuable synthetic building blocks. A
(
ee (entries 2-4). Various Ag salts with L2 and L5 (entries 6-13)
-
showed that coordinating anions, such as OAc (entry 6), were
chiral Rh catalyst, [Rh
styrenes in up to 76% ee in moderate yields. Aziridination of
homoallylic sulfamates using [Cu(MeCN) ]PF supported by a
2 4
(4S-MEOX) ], gave aziridination of
[
9]
not effective. AgOTf (entries 7-8) was superior using L5 vs. L2,
while AgBF
gave lower ee compared to AgClO
3) was on par with AgClO , but due to lower cost, AgClO
4
4
and AgPF
6
(entries 9-12) behaved similarity, but
. The ee with AgSbF (entry
was
4
6
4
6
1
4
[
*] M. Ju, Dr. C. D. Weatherly, Dr. I. A. Guzei, Prof. J. M. Schomaker
chosen for further study. Similar ee was seen using other non-
coordinating anions with 2-(E) (entries 14-18) and L2/L5.
Department of Chemistry
University of Wisconsin-Madison
Further optimization used L2 and L5 with AgClO
the SI for details). CH Cl was the optimal solvent, in contrast to
Cu-catalyzed aziridination, where CH
estingly, addition of an extra 10 mol % of AgClO
yields at -20 °C and increased ee to >90%. This likely results
4
(Table S3 in
1
101 University Avenue, Madison WI 53706 (USA)
2
2
E-mail: schomakerj@chem.wisc.edu
[
10]
3
CN was preferred. Inter-
delivered high
Supporting information for this article is given via a link at the end of
the document.
4
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Angewandte Chemie International Edition
10.1002/anie.201704786
Table 1. Initial optimization of the asymmetric aziridination.
Table 2. Scope of the asymmetric aziridination.
from the role the Ag salt plays in breaking up polymeric PhIO, a
[
17]
process that suffers at low temperatures.
3
Sc(OTf) could be
substituted for additional AgClO with L5 also showing
4
,
increased ee upon cooling to -20 °C (see Table S3).
With optimized conditions in hand, the scope was explored
(Table 2). Dialkylsubstituted alkenes 1-(Z) and 2-(E) (entries 1-2)
gave excellent yields and >90% ee. The remainder of the mass
balance in Table 2 was C-H insertion product, easily separated
from the aziridine by column chromatography or recrystallization.
Changing the length or steric bulk of the alkyl side chain (entries
3
-4) furnished products in high ee. Benzyl and ether-containing
alkyl groups were well-tolerated (entries 5-6); the ee of 5a
increased to >99% after one recrystallization. In contrast to Cu-
catalyzed enantioselective aziridination, styrene 7-(E) initially
gave racemic aziridine 7a. Interestingly, changing the solvent to
6 6
C H , improved the ee to 70%; decreased ee was observed in
toluene and xylene. Other substrates did not show this effect,
[
19]
are underway.
Although allylic carbamate substrates
suggesting that Ag-π interactions may be responsible for the
increased ee.
precludes issues of chemoselectivity, no ee was noted.
[
10,18]
X-ray crystallography (SI for details) of 5a resulting from
reaction of 5-(E) with L2 (Scheme 2) contained (R,R) absolute
stereochemistry. Reaction of 14 with L2 gave (S,R)-14a, while
L5 furnished the antipodes. Interestingly, the observed absolute
stereochemistry of trans-5a was opposite to that seen in Cu-
Gratifyingly, 1,1',2-trisubstituted alkenes 8-(E)-12-(E) gave
good chemoselectivity and ee for 8a-12a (entries 8-12). A 1,2,2'-
substitution pattern in 13-(E) (entry 13) lowered ee to 22%,
although the yield and chemoselectivity remained high. Steric
bulk in the tether between the alkene and carbamate of 14-(Z)
[
10]
catalyzed aziridination of homoallylic sulfamates.
(
entry 14) resulted in lower yield, but good ee after one
recrystallization. However, moving the bulk closer to the alkene
in 15-(Z) (entry 15) had detrimental effect on the ee.
Asymmetric aziridination of allene 16 to methyleneaziridine 16a
entry 14) gave a promising 62% ee; further optimization studies
The results in Table 2 point to several alkene features
important to obtaining high ee with Ag-catalyzed nitrene transfer.
These include at least one substituent on the distal alkene car-
bon, monosubstitution only at the proximal alkene carbon, distal
substituents that are not conjugated to the alkene and a two-
a
(
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Angewandte Chemie International Edition
10.1002/anie.201704786
Scheme 2. Determination of the absolute stereochemistry.
transfer reagent on reaction behavior and point to the
importance of further studies to better understand such effects.
Table 3. Comparison of carbamates and sulfamates in Cu- and Ag-catalyzed
asymmetric nitrene transfer.
carbon tether between the carbamate and the site of
unsaturation. These observations, coupled with our knowledge
of the absolute aziridine stereochemistry (Scheme 2), suggested
the model for stereochemical induction proposed in Figure 1.
1
2
High ee is observed when substitution is present at R , R , or at
1
2
both R and R . The poor ee observed when the proximal alkene
3
carbon
R
is substituted is rationalized by steric clashing
between substrate and catalyst. Furthermore, the dramatic
solvent effect observed with the styrene 7-(E) suggests that
potential π-cation interactions between the Ph group and the
silver complex may erode the ee in non-aromatic solvents. Using
Nucleophilic ring-opening of aziridines introduces valuable
[
3]
functionality into the amine products. To demonstrate that the
stereochemical integrity of the aziridination is maintained during
subsequent manipulations, 2a was opened with diverse
nucleophiles to give enantioenriched amines 18-23 (Scheme
[
20,21]
3
).
These ring-openings display reactivity complementary to
that of the corresponding cyclic sulfamates, which open at the
[
10]
proximal aziridine carbon.
For example, 2a undergoes ring-
opening with halides, azides, carboxylic acids, sulfides and
cuprates at the distal aziridine carbon. No erosion in either dr or
ee of the amine products was noted. These aziridines could also
be transformed into (2-aziridinyl)acetic acids by carbamate
[
22]
cleavage and oxidation of the primary alcohol.
Figure 1. Stereochemical model for asymmetric aziridination.
Scheme 3. Ring-openings of enantioenriched aziridines.
C
6
H
6
as solvent (Table 2, entry 7) may interfere with these
substrate-catalyst interactions, restoring some ee to the reaction.
Other aromatic solvents (C F and C CF ) also restored
some ee, but not as effectively as C
6
H
5
6
H
5
3
6
6
H .
A comparison of Ag(I) vs. Cu(I) catalysts supported by L2
using both carbamates and sulfamates was carried out to better
understand the differences between these two systems (Table
3
1
). Our Ag(I) catalysts required carbamates as precursors (entry
); a sulfamate gave only trace aziridine (entry 3). This was
surprising, as we have previously shown sulfamates to be
[12,13]
effective in other Ag-catalyzed nitrene transfers.
Closer
1
examination by H NMR showed formation of an iminoiodinane
intermediate, which eventually furnished the aziridine over a
period of days (Figure S2 for details). We propose this arises
from either increased Lewis acidity of Ag(I) compared to Cu(I),
or more likely, the increased size of the Ag(I) cation (115-126
pm) vs. a Cu(I) ion (96 pm). The sulfamate oxygen and the Ph of
the iminoiodinane may bind to Ag, slowing down the formation of
the active silver nitrenoid species, even at rt (Figure S2). This is
not an issue in Ag catalysts with fewer open coordination sites
In conclusion, we have described the first general examples of
intramolecular, asymmetric aziridination proceeding via a silver-
catalyzed nitrene transfer pathway. A BOX-supported AgClO
4
complex transforms di- and trisubstituted homoallylic carba-
mates to [4.1.0]-carbamate-tethered aziridines in good yields
and ee ~90%. Nucleophilic ring-opening occurs smoothly at the
distal aziridine carbon to furnish enantioenriched amines with no
erosion of stereochemical information. Future efforts will explore
ligands for asymmetric Ag-catalyzed C-H amination and expand
the scope of asymmetric allene aziridination.
[
12,13]
on the metal.
4 6
Attempts to utilize [Cu(MeCN) ]PF with
carbamates gave no reaction (entry 2), while sulfamates were
[
10]
successful as previously reported (entry 4).
These results
highlight the impact of differences in metal identity and nitrogen
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Angewandte Chemie International Edition
10.1002/anie.201704786
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1
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Acknowledgements
This work was funded by an NSF-CAREER Award 1254397 and
the Wisconsin Alumni Research Foundation to JMS. The NMR
facilities at UW-Madison are funded by the NSF (CHE-1048642,
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Keywords: nitrene • silver • aziridines • asymmetric • amines
[
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Angewandte Chemie International Edition
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COMMUNICATION
Ju, M.; Weatherly, C.D.; Guzei, I.A.;
Schomaker, J.M.*
Page No. – Page No.
Chemo- and Enantioselective Silver-
catalyzed Aziridinations
Asymmetric nitrene transfer reactions are
a powerful tool for preparing
enantioenriched amines. We have developed chemo- and enantioselective silver-
catalyzed aminations that transform di- and trisubstituted homoallylic carbamates
into aziridines in good yields and ee of up to 92%. Stereochemical models are
proposed to rationalize the observed absolute stereochemistry of the products,
which undergo nucleophilic ring-opening to yield enantioenriched amines with no
This article is protected by copyright. All rights reserved.