Rh-Catalyzed Aziridine Ring Expansions to Dehydropiperazines

Article abstract of DOI:10.1021/acs.orglett.0c01124

Piperazines are prevalent in pharmaceuticals and natural products, but traditional methods do not typically introduce stereochemical complexity into the ring. To expand access to these scaffolds, we report Rh-catalyzed ring expansions of aziridines and N-sulfonyl-1,2,3-triazoles to furnish dehydropiperazines with excellent diastereocontrol. Productive ring expansion proceeds via a pseudo-1,4-sigmatropic rearrangement of an aziridinium ylide species. However, the structural features of the carbene precursor are important, as pyridotriazoles undergo competing cheletropic extrusion to furnish ketimines.

Full text of DOI:10.1021/acs.orglett.0c01124

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Rh-Catalyzed Aziridine Ring Expansions to Dehydropiperazines
§
§
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Hillary J. Dequina, Josephine Eshon, William T. Raskopf, Israel Fernandez,*
and Jennifer M. Schomaker*
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ABSTRACT: Piperazines are prevalent in pharmaceuticals and
natural products, but traditional methods do not typically
introduce stereochemical complexity into the ring. To expand
access to these scaffolds, we report Rh-catalyzed ring expansions of
aziridines and N-sulfonyl-1,2,3-triazoles to furnish dehydropiper-
azines with excellent diastereocontrol. Productive ring expansion
proceeds via a pseudo-1,4-sigmatropic rearrangement of an
aziridinium ylide species. However, the structural features of the carbene precursor are important, as pyridotriazoles undergo
competing cheletropic extrusion to furnish ketimines.
lides are unique molecules with a negatively charged
carbon atom directly attached to a positively charged
ylide counterparts and are generated in situ; however, they
display an array of interesting reactivities. Common N-ylides
(Figure 1B) include ammonium,1,11 azomethine,14 pyridi-
nium,15 and triazolium ylides.16 While ammonium ylides have
been widely employed, versions generated specifically from
aziridines, termed “aziridinium ylides”, are underexplored.
Aziridinium ylides are conveniently generated from the
reaction of aziridines with metal-supported carbenes; the
potential to harness the reactivity of these intermediates to
furnish diverse N-heterocycle scaffolds inspired the studies
described in this communication.
Y
heteroatom.1,2 The first example, a phosphorus ylide, was
reported in 1894 (Figure 1A);3 however, their full synthetic
One reason aziridinium ylides have not been extensively
investigated is the difficulty in controlling the ultimate fate of
the intermediate. In 1972, Watanabe attempted to convert an
aziridinium ylide to an azetidine via Cu-catalyzed addition of
an electron-rich aziridine to a diazoester.17 Instead of ring
expansion, ethylene and an α-imino ester were observed,
suggesting cheletropic extrusion competes (Scheme 1A).18 In
2004, Rowlands suppressed cheletropic extrusion in favor of a
[2,3]-Stevens rearrangement of an aziridinium ylide generated
by adding a vinyl aziridine to a Cu-supported carbene (Scheme
1A); however, a competing [1,5]-H shift resulted in only a
21% yield of the heterocycle.19 In 2017, we reported
aziridinium ylides generated from strained aziridines, and Rh-
supported carbenes undergo concerted [2,3]-Stevens rear-
rangement to give methyleneazetidines in good yields and dr
with broad scope.20,21 More recently, we also reported an
intermolecular carbene transfer between Rh-bound vinyl
Figure 1. Ylide development and types of N-ylides.
utility was not recognized until 1953, when Wittig utilized
them to prepare alkenes from aldehydes and ketones.4−6 Since
then, phosphorus ylide chemistry has provided powerful tools
to construct C−C and C−heteroatom bonds.7,8 Sulfur ylides
are popular for the synthesis of cyclopropanes, epoxides, and
aziridines.9−11 Other Group 5 and 6 ylides based on O,11 As,12
Se,10b,13 and Te10,13 are known; however, these are less
synthetically useful.
Received: March 30, 2020
The importance of nitrogen in bioactive molecules has
stimulated significant interest in the chemistry of nitrogen
ylides. N-Ylides are typically less stable than their S- and P-
https://dx.doi.org/10.1021/acs.orglett.0c01124
© XXXX American Chemical Society
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a
Scheme 1. Divergent Reactivities of Aziridinium Ylides
Scheme 2. Cheletropic Extrusion with Pyridotriazole 2
a
Computed reaction profile with relative free energies (ΔG,
computed at 298.15 K and 1 M) and bond distances in kcal/mol
and Å, respectively. All data are computed at the SMD-B3LYP-D3/
def2-SVP level.
carbenes and aziridines via a pseudo-[1,4]-sigmatropic
rearrangement to furnish dehydropiperidines in excellent
yields and dr.22
Inspired by the lack of structural diversity in piperazines and
related heterocycles generated from known methods, we first
attempted to prepare fused piperazines. Pyridotriazole 2
(Scheme 2A) is known to form α-imino Rh-supported
carbenes upon heating;23 unfortunately, reaction between 1a
and 2 gave ketimine 3 instead of the desired 4. The preference
for the formation of 3 over 4 was computationally explored
(Scheme 2B). Focus was placed on the fate of the metal-free
ylide INT2-pyr, formed upon nucleophilic addition of the
aziridine to the dirhodium carbene, followed by dissociation of
the Rh2 catalyst. Ylide INT2-pyr can evolve into alkene 3′ (the
Et group in 3 was replaced by a Me in the calculations) via
TS2-pyr in a highly exergonic transformation (ΔGR = −47.5
kcal/mol). This saddle point resembles that located for the
cheletropic extrusion pathway involving ylide TS2 (Figure 2,
vide infra) and is associated with the concerted rupture of both
aziridine C−N bonds. Interestingly, calculations indicate that
the subsequent aza-Diels−Alder reaction is unfeasible in view
of the high barrier computed for this cycloaddition (ΔG⧧ > 45
kcal/mol). This can be mainly ascribed to the loss of
aromaticity of the pyridine moiety, which is also reflected in
the computed endergonicity of the process (ΔGR = +1.1 kcal/
mol), despite the formation of two new C−N bonds.
alternative pathway is not competitive due to the higher barrier
required to reach TS2′-pyr (and TS3′-pyr), as compared to
the chelotropic extrusion pathway via TS2-pyr.
Interestingly, the barrier computed for INT2-pyr →TS2′-
pyr is much higher than that computed for the analogous
process involving INT2 (Figure 2, vide infra), which can be, at
least in part, ascribed to the loss of aromaticity of the pyridine
ring. This is supported by markedly different C···N bond
distances in the corresponding transition states TS2′ (Figure
2) and TS2′-pyr. While in the former saddle point the
computed C···N distance is 2.741 Å, a much longer distance of
2.898 Å is computed in the latter species. This indicates TS2′-
pyr does not benefit from a significant C···N interaction,
although TS2′ does. As a result, the INT2-pyr → TS2′-pyr
reaction not only is kinetically less favored but also proceeds in
a stepwise fashion.
We hypothesized the use of N-sulfonyl-1,2,3-triazoles might
alter the ultimate fate of the aziridinium ylide, as the
nucleophilicity of the α-imino group of metallocarbenes
derived from these precursors could bias the reaction toward
ring expansion (Scheme 1B). Differences in the electro-
philicities of Rh-supported carbenes formed from N-sulfonyl-
1,2,3-triazoles vs pyridotriazoles, as well as varying steric
congestion and ability of the ylide to delocalize charge, could
also play roles in dictating the outcome. In addition, the
requirement for slow addition of typical diazoesters might be
overcome by the use of more robust N-sulfonyl-1,2,3-triazole
carbene precursors. Ideally, an intermediate α-imino rhodium
carbene would be generated, followed by nucleophilic addition
An alternative sigmatropic rearrangement pathway involving
NMs-ylide INT2-pyr was located. Instead of a direct pathway
leading to 3′, computations showed a stepwise process which
first transforms INT2-pyr into the zwitterion INT4-pyr (via
TS2′-pyr), followed by a ring-closure reaction (through TS3′-
pyr). From the data in Scheme 2B, it is apparent that this
B
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Figure 2. Computed reaction profile for the process involving 1a′ and dirhodium-bound carbene 5-Rh2. Relative free energies (ΔG, computed at
298.15 K and 1 M) and bond distances are given in kcal/mol and angstroms, respectively. All data have been computed at the SMD-B3LYP-D3/
def2-SVP level.
Table 1. Reaction Optimization for Aziridine Expansion
entry
cat (mol %)
T (°C)
yield (%)
entry
cat (mol %)
T (°C)
yield (%)
a
1
2
3
4
5
Rh2(OAc)4 (6)
Rh2(OAc)4 (6)
Rh2(oct)4 (6)
Rh2(TPA)4 (6)
Rh2(esp)2 (6)
25
70
70
70
70
0
6
7
8
9
Rh2(esp)2 (3)
Rh2(esp)2 (1)
Rh2(esp)2 (0.5)
Rh2(esp)2 (0.1)
70
70
70
70
78
79
79
72
a
33
51
44
79 (75)
a
a
a
a
1H NMR yield using mesitylene. All other yields are isolated yields.
of a bicyclic aziridine to the electrophilic carbene center to
furnish the aziridinium ylide. Ring expansion then yields the
dehydropiperazine.
of >19:1. Increasing branching in the aziridine substituent of
1b furnished 6bb in good yield, suggesting this system tolerates
steric pressure. Heteroatoms in the aziridine scaffold were well-
tolerated, including an alkyl chloride in 1d and a silyl-protected
alcohol in 1e to deliver 6db and 6eb in good yields as single
diastereomers. Substitution on the aziridine precursor is not
necessary, as 6fb was produced in good yield and dr.
The scope of the N-sulfonyl-1,2,3-triazoles was examined
with 1a (Scheme 3, bottom). Mesyl- and tosyl-protected
triazoles 5a and 5b furnished 6aa and 6ab in good yields. Due
to easier removal of the N-tosyl group, a series of phenyl-
substituted N-tosylated triazoles were explored to understand
how the electronics and sterics of the triazole impact the
reaction outcome. Triazoles 5h−j and 5g, substituted with
electron-donating substituents, delivered 6ah−j and 6ag in
good yield. The trifluoromethyl-substituted triazole 5c gave
6ac in a 48% yield, suggesting that electron-poor carbene
precursors are not as effective for ring expansion, although an
aryl bromide was tolerated to deliver 6ad. Finally, carbene
transfer with triazole 5i was successful to furnish 6ai.
Demonstration of the scalability of the ring expansion was
carried out on a 3.54 mmol scale using 1a and 5b to give a 75%
yield of 6ab.
In initial attempts, treatment of cis-1a with 5a and
Rh2(OAc)4 at room temperature gave no conversion to the
desired 6aa (Table 1, entry 1). However, a 33% NMR yield of
6aa was obtained when the reaction was heated to 70 °C
(entry 2). Other commercially available rhodium catalysts
known to promote carbene transfer were also tested (entries
3−5). Increased yields were observed in moving to the bulkier
catalysts Rh2(oct)4, Rh2(tpa)4, and Rh2(esp)2. Rh2(esp)2 was
selected as the optimal catalyst, and loadings gradually
decreased to identify a good balance between yield and
reaction time (entries 5−9). Although the loading of Rh2(esp)2
could be dropped to 0.1 mol % for 6aa, other substrates
required longer reaction times; thus, scope studies were carried
out using 0.5 mol % of Rh2(esp)2.
With the optimized reaction conditions in hand, the
aziridine scope was explored (Scheme 3, top). Silver-catalyzed
nitrene transfer conditions developed in our group were used
to prepare aziridines 1a−f from the corresponding homoallylic
carbamates.24 Linear alkyl-substituted aziridines 1a and 1c gave
dehydropiperazines 6ab and 6cb in good yield and excellent dr
C
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us in the processes involving related aziridines21,22 and by
others in the fates of related transition metal ylides.25
Intermediate INT2 may undergo a cheletropic extrusion
similar to that observed initially by Watanabe (see Scheme
1A) to produce the alkene intermediate INT3. This highly
exergonic reaction (ΔGR = −20.6 kcal/mol) proceeds with a
relatively low activation barrier (ΔG⧧ = 10.5 kcal/mol) via
TS2, a saddle point associated with the rupture of both
aziridine C−N bonds in a concerted manner. INT3 is ideally
suited to undergo [4 + 2] cycloaddition to produce the
corresponding dehydropiperazine 6aa′. This final aza-Diels−
Alder reaction is also highly exergonic (ΔGR = −32.6 kcal/
mol) and occurs in a concerted fashion through TS3 with a
barrier of 23.5 kcal/mol, which is fully compatible with the
temperatures (70 °C) used in this reaction. Despite this, an
alternative reaction pathway was identified which directly
produces the dehydropiperazine 6aa′ from ylide INT2. As
shown in Figure 2, INT2 undergoes a facile (ΔG⧧ = 4.5 kcal/
mol) sigmatropic rearrangement via TS2′ which involves the
concomitant, yet highly asynchronous, breaking of the aziridine
C−N bond and formation of the new C−N bond involving the
NM moiety. Therefore, although both pathways are feasible
within the experimental reaction conditions, our calculations
suggest that the direct path involving TS2′ is kinetically
preferred over the stepwise mechanism involving TS2 and
TS3.
a
Scheme 3. Scope of the Aziridine Ring Expansion
In conclusion, we have shown that the fate of aziridinium
ylide intermediates depends on the structural features of the
carbene precursor. Our initial attempt to prepare piperazine
scaffold using a pyridotriazole carbene precursor gave an
aziridinium ylide that preferentially underwent cheletropic
extrusion to furnish a ketimine, as opposed to the desired ring
expansion. Computations show this is due to the loss of
aromaticity of the pyridine ring in the expansion pathway. By
changing the nature of the carbene precursor to N-sulfonyl-
1,2,3-triazoles, effective aziridine ring expansion provided
access to densely substituted dehydropiperazines in excellent
yields and diastereoselectivity. Computations suggest the
mechanism involves a [1,4]-sigmatropic rearrangement of the
key aziridinium ylide.
a
Conditions: 0.5 mol% of Rh2esp2, 0.05 M CHCl3, 70 °C.
Conducted on 0.5 g (3.5 mmol scale) of 1a, 75%.
b
The dehydropiperazine synthesis was streamlined via a one-
pot Cu-catalyzed azide−alkyne cycloaddition, followed by Rh-
mediated carbene transfer/ring expansion (Scheme 4). Under
Scheme 4. One-Pot Synthesis from p-TsN3
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01124.
CuTC-catalyzed conditions, treatment of p-TsN3 with phenyl-
acetylene gave full conversion to 5b. Addition of 1a and
Rh2(esp)2 to this mixture gave 6ab in 79% yield and >19:1 dr.
Similarly, p-methoxyphenylacetylene ultimately furnished 6ah
in 71% yield and excellent dr.
Experimental procedures, computational details, and
characterization data for all new compounds (PDF)
To gain a better understanding of the mechanism of the ring
expansion, computational studies were carried out (Figure 2).
The process involving cis-bicyclic aziridine 1a′ and dirhodium
carbene 5-Rh2, formed from 5a and Rh2(OAc)4, was explored.
Similar to transformations involving related bicyclic aziridines
(Scheme 1A),20−22 the process begins with the exergonic
(ΔGR = −10.8 kcal/mol) nucleophilic addition of the aziridine
nitrogen atom to the electrophilic carbene carbon atom of 5-
Rh2 via the transition state TS1 (ΔG⧧ = 8.1 kcal/mol). This
step forms ylide INT1, which evolves into the metal-free ylide
INT2 by the dissociation of the Rh2 catalyst (ΔGR = −1.8
kcal/mol). A similar barrierless Rh2 dissociation was found by
AUTHOR INFORMATION
■
Corresponding Authors
Jennifer M. Schomaker − Department of Chemistry, University
of Wisconsin, Madison, Wisconsin 53706, United States;
orcid.org/0000-0003-1329-950X; Email: schomakerj@
chem.wisc.edu
́
́
Israel Fernandez − Departamento de Organica I and Centro de
́
en Quımica Avazanda (ORFEO−CINQA), Facultad de
Ciencias, Universidad Complutense de Madrid, 28040 Madrid,
Spain; orcid.org/0000-0002-0186-9774; Email: israel@
quim.ucm.es
D
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Authors
pubs.acs.org/OrgLett
Letter
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Hillary J. Dequina − Department of Chemistry, University of
Wisconsin, Madison, Wisconsin 53706, United States
Josephine Eshon − Department of Chemistry, University of
Wisconsin, Madison, Wisconsin 53706, United States
William T. Raskopf − Department of Chemistry, University of
Wisconsin, Madison, Wisconsin 53706, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01124
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Author Contributions
§H.J.D. and J.E. contributed equally. The manuscript was
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ACKNOWLEDGMENTS
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J.M.S. thanks the NIH R01 GM11412 and the ACS-PRF No.
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by the National Science Foundation (NSF; CHE-9208463,
CHE-9629688) and the National Institutes of Health (NIH;
RR08389-01). The Q-Exactive mass spectrometer was
acquired with funds from an NIH-S10 award through the
National Institutes of Health (NIH-1S10OD020022-1). I.F.
acknowledges financial support from the Spanish MINECO-
FEDER (Grants CTQ2016-78205-P and CTQ2016-81797-
REDC).
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