Thermal rearrangement of 2,3-diaryl-1-phthalimidoaziridines

Article abstract of DOI:10.1016/j.tetlet.2015.07.093

2,3-Diaryl-1-phthalimidoaziridines and 2,3-diaryl-1-phthalimidoaziridine-2-carbonitriles were found to readily undergo thermal rearrangement into imines via 1,2-migration of the phthalimido group and accompanying C-C bond cleavage. Isomerization proceeds regioselectively with preferable migration to the electron-deficient carbon atom. Interestingly, this reaction was found to predominate even in the presence of dipolarophiles.

Full text of DOI:10.1016/j.tetlet.2015.07.093

Accepted Manuscript
Thermal rearrangement of 2,3-diaryl-1-phthalimidoaziridines
Alena S. Pankova, Mariia V. Sorokina, Mikhail A. Kuznetsov
PII:
S0040-4039(15)01269-1
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.07.093
TETL 46583
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
14 March 2015
1 July 2015
29 July 2015
Please cite this article as: Pankova, A.S., Sorokina, M.V., Kuznetsov, M.A., Thermal rearrangement of 2,3-diaryl-1-
phthalimidoaziridines, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.07.093
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Thermal rearrangement of 2,3-diaryl-1-phthalimidoaziridines
*
Alena S. Pankova, Mariia V. Sorokina, Mikhail A. Kuznetsov
Institute of Chemistry, Saint Petersburg State University, Universitetsky pr. 26, 198504
Saint Petersburg, Russia
*Corresponding author. Tel.: +78124286779; fax: +78124286733.
E-mail address: m.kuznetsov@spbu.ru (M.A. Kuznetsov).
Abstract
2,3-Diaryl-1-phthalimidoaziridines and 2,3-diaryl-1-phthalimidoaziridine-2-carbonitriles were
found to readily undergo thermal rearrangement into imines via 1,2-migration of the phthalimido
group and accompanying C–C bond cleavage. Isomerization proceeds regioselectively with
preferable migration to the electron-deficient carbon atom. Interestingly, this reaction was found
to predominate even in the presence of dipolarophiles.
Keywords: aziridines, rearrangement, phthalimide, stilbenes, imines
The utilization of aziridines in organic chemistry is popular due to the various transformations
that they undergo.1-3 Upon treatment with various nucleophiles4-6 or in the presence of Lewis
acids7,8 ring-opening via C–N bond cleavage usually occurs, giving highly functionalized amines
and the products of their subsequent transformations. On the other hand, upon thermolysis or
irradiation, C–C bond cleavage occurs, proceeding as a concerted process which obeys the
orbital symmetry conservation rules to afford octet-stabilized 1,3-dipoles – azomethine ylides.9
These reactive intermediates can be involved in 1,3-dipolar cycloaddition reactions with various
unsaturated bonds in both an intermolecular or intramolecular manner, allowing the formation of
five-membered heterocycles in one synthetic step.10-12
The N-phthalimidoaziridine motif is particularly interesting as it offers the advantage of a
masked amino group on the nitrogen atom, while the phthalimido group also acts as an electron-
withdrawing substituent which governs new transformations. We have previously demonstrated
the utility of N-phthalimidoaziridines in the synthesis of a wide range of monocyclic,13-15
polycyclic condensed16 and spiro17 nitrogen heterocycles via the 1,3-dipolar cycloaddition
reaction of the corresponding thermally generated N-phthalimidoazomethine ylides.
Additionally, we have reported that acyl substituted N-phthalimidoaziridines could be converted
into 1,3-oxazoles with simultaneous loss of a phthalimide group, that was explored as a useful
synthetic procedure.18,19 Another general thermal reaction is isomerization to give imines, which
proceeds via the 1,2-migration of the phthalimide fragment along with C–C bond cleavage and
has been reported for di-,20 tri- and tetrasubstituted21 N-phthalimidoaziridines. Whereas for the
majority of aziridines studied this rearrangement was observed as a side reaction or even
suppressed during the 1,3-dipolar cycloaddition reaction, in the case of 2,3-diphenyl-1-
phthalimidoaziridine, the corresponding imine – [(1E)-benzylidene][phenyl(phthalimid-2-
yl)methyl]amine – was formed as the major product even in the presence of active dipolarophiles
such as N-phenylmaleimide.20 While this is unusual because examples of ‘normal’ cycloaddition
reactions are known for N-alkyl-,22-26 N-aryl-,26-28 N-aroyl-,29 and N-alkoxycarbonyl-2,3-
diphenylaziridines,24 at the same time, certain N-benzyl-,30 N-cyclohexyl-31 and N-(1,2-
1

di(methoxycarbonyl)vinyl)32-substituted aziridines have been reported to undergo similar
thermal rearrangements to the corresponding imines. It is important to note that each of the afore
mentioned compounds possess at least one aryl group at the aziridine carbon atoms. To explore
the generality of this rearrangement for N-phthalimidoaziridines and the role of the substituents,
we have examined the thermal behavior of 2,3-diaryl-1-phthalimidoaziridines with electron-
withdrawing and electron-donating substituents on the aryl rings and report our results herein.
trans-Stilbenes 1a-h were prepared by known methods using the Horner-Wadsworth-
Emmons reaction (Table 1).33-35 Oxidative addition of N-aminophthalimide gave stable,
crystalline aziridines 2a-h in good yields. Although the reactions were carried out at 0 °C, better
yields were achieved by warming the reaction to room temperature. According to their 1H NMR
spectra at 25 °C, aziridines 2b-e,g,h exist as mixtures of two invertomers in ratios ranging from
~1:0.23 for 2h to ~1:0.85 for 2c. The slow inversion on the NMR timescale of the endo-cyclic
nitrogen atom is a common feature of N-phthalimidoaziridines,36 and the high content of the
minor invertomers could be explained by the similar sizes of both aromatic substituents.
Symmetric aziridines 2a,f exist as single invertomers due to degenerate inversion, but their aryl
substituents showed two signal sets in the NMR spectra. The values of vicinal coupling constants
between the aziridine protons (3J = 5.5-6.0 Hz) confirmed the trans-orientation of the
substituents on the aziridines 2a-h.
Table 1. Preparation of aziridines 2a-h and their isomerization into imines 3 and 4
Entry Stilbene Ar1
Ar2
Aziridine Isolated yield (%) Time (h) Imines Ratio 3:4a
1
2
3
4
5
6
Ph
Ph
Ph
Ph
Ph
83
60
56
84
60
65
6
–
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
3a
3b/4b 1:1.5
3c/4c 1:1
3d/4d 1:0.8
3e/4e
3f
4-MeOC6H4
4-ClC6H4
4-O2NC6H4
7.5
6.5
6
5
8
4-O2NC6H4 4-MeOC6H4
4-O2NC6H4 4-O2NC6H4
1:2.5
–
7
Ph
62
1
3g/4g 1:2
1g
2g
8
Ph
84
2
traces
–
1h
2h
a According to the 1H NMR spectra of the crude product.
Thermolysis of trans-2,3-diaryl-1-phthalimidoaziridines 2b-h was performed in a sealed glass
1
reactor in absolute benzene at 120 °C.20 According to the H NMR spectra of the reaction
mixtures, in all cases the rearrangement occurred in nearly quantitative conversion, giving a
mixture of two products for unsymmetrical aziridines 2b-e,g and a single product for symmetric
aziridines 2a,f. The reaction of pyrazolylaziridine 2h resulted only in a mixture of decomposition
2

products. Imines 3 and 4 were found to be stable in solution, but could not be isolated by column
1
chromatography or crystallization. Nevertheless, the H and 13C NMR spectra of the reaction
mixtures showed only signals for imines 3 and 4 and were clear enough to assign all protons.
The NMR spectra of imines 3 and 4 all had similar features and were comparable to the spectra
of compound 3a, of which the structure was previously established by our group.20 The aromatic
and phthalimide fragments of the initial aziridines were retained in the products, and the most
characteristic signals for imines 3a-f and 4b-e were assigned as the NCHN (δH 6.83-6.99, δC
72.0-74.1 ppm) and CH=N (δH 8.36-8.50, δC 159.9-163.4 ppm) groups (the positions of these
signals for the thienyl substituted imines 3,4g were slightly shifted, see ESI for details). The
HRMS data for the imines were the same as the corresponding aziridines indicating that they had
undergone structural isomerization.
1
The ratio of products 3/4b-e,g in the reaction mixtures was determined by H NMR
spectroscopy. The assignment of the proton signals to the isomeric imines was achieved by
sequential comparison of their positions, intensities and spin-spin coupling constants. For the
aromatic rings, the change of the ArCH=N position to NCH(Ar)N shifted the signals of the
ortho-protons to a higher field by 0.25-0.30 ppm, whereas the meta-protons remain almost
unchanged. Additionally, the structures of imines 3e and 4e were confirmed from the 2D
NOESY H-H spectrum of the mixture. For the major isomer, cross-peaks between CH=N (8.36
ppm) and NCHN (6.84 ppm) proton signals were clearly observed and similar NOE cross-peaks
were observed for the ortho-protons of 4-O2NC6H4 (7.77 ppm) / NCHN and the ortho-protons of
4-MeOC6H4 (7.83 ppm) / CH=N pairs which confirmed the assigned structure 3e. The
corresponding correlations were also found for the minor isomer 4e (see ESI).
Thus, for aziridines 2b-e,g the rearrangement proceeds in a regioselective manner with
preferred migration of the PhthN group to the more electron-deficient carbon atom (Table 1).
However, these results contradict the work of Person et al.21 who reported that the rearrangement
of tri- and tetrasubstituted N-phthalimidoaziridines with at least one cyano group occurred
quantitatively, even at room temperature in benzene or chloroform, to give a mixture of two
isomeric imines with preferred migration of the phthalimide moiety to the carbon atom without
the electron-withdrawing cyano group.21 However, this information regarding the isomer ratios
may be unreliable as these data21 were opposite to those reported in a previous article37 by the
same authors (Table 2). In the case of aziridine 6b (vide infra), a yield of 100% was indicated for
imine 8b whereas the characterization data were given for isomer 7b.21 In other cases, the NMR
spectroscopic data were specified only for one of the two isomers.
We then elected to reproduce several of the reactions described in the literature.21
2-Arylidenephenylacetonitriles 5a-c were prepared from substituted benzaldehydes and
phenylacetonitrile using
a
literature procedure38 and examined in the oxidative
aminoaziridination reaction (Scheme 1). According to our standard procedure, using K2CO3 as
base, we achieved an improved yield of aziridine 6a (80% instead of 61%21). Although the
decomposition point was lower (113-115 °C vs. 149 °C21), accurate HRMS data were obtained,
and the chemical shift of the aziridine proton (δ 5.71 ppm) corresponded to that reported
earlier.21 The high instability of aziridine 6a in CDCl3 solutions impeded obtaining a well-
1
resolved 13C NMR spectrum. Aziridine 6c possessed an analogous H NMR spectrum and
additionally appeared to be slightly more stable, although far less soluble in CDCl3; nevertheless
all characteristic signals were clearly observed in the 13C NMR spectrum, confirming the
assigned structure. Formation of aziridine 6b was observed by TLC but it could not be isolated
due to instability.
3

Scheme 1. Preparation of aziridines 6a-c
Although aziridines 6a,c appeared as stable colorless crystalline substances, they were found
to decompose readily in solutions of chloroform, acetone, and benzene at room temperature. A
1,2-shift of the PhthN group appeared to be the main process and two imines were detected using
NMR spectroscopy and TLC (Table 2). Pure samples of imines 7a,b and 8a,b could be isolated
by column chromatography on silica while the mixture of imines 7c/8c was inseparable.
Table 2. Rearrangement of aziridines 6 into imines 7 and 8
Entry Aziridine Ar
Previously reported Ratio 7:8
Isolated yield (%)
ratio 7:821
(total yield, solvent)a
7
8
20b
1
Ph
1:1.08 (chloroform)
1:0.75 (81%, chloroform) 63b
6a
1:0.44 (79%, benzene)
1:0.35 (82%, acetone)
2
3
4-MeOC6H4 0:1 (dichloromethane) 1:0.43 (dichloromethane)c 51
1:0.77 (81%, chloroform)
15
6b
6c
73 (1:0.55)b
4-O2NC6H4
–
1:0.62 (90%, benzene)
a According to the 1H NMR spectra.
b Reaction in benzene.
c NMR yield is not indicated since aziridine 6b was not isolated in pure form.
Signal assignment in the NMR spectra of the isomeric imines was based on similarities
previously discussed for the disubstituted analogs. The aliphatic proton showed a singlet at δ
8.80-8.97 ppm for the imine position of 7a-c and at δ 7.20-7.29 ppm for the amine position of
8a-c, which corresponded well with the values for imines 3 and 4. The ortho-protons of the
phenyl ring near the cyano group in imines 8a-c gave the most downfield signal (δ ~8.12 ppm)
13
amongst all of the aromatic protons of both imines. In the C NMR spectra, the signals of the
carbon atom bearing the PhthN group were found at nearly the same position for both isomers (δ
76.9-77.1 ppm for imines 7a-c and δ 71.1-72.7 ppm for imines 8a-c) whereas the chemical shift
of the C=N carbon atom was more characteristic: 161.6-164.0 ppm for imines 7a-c and 143.6-
144.0 ppm for imines 8a-c.
Compared to the disubstituted N-phthalimidoaziridines 2, trisubstituted compounds 6 were
found to be much less stable which was in agreement with the greater reactivity of tri- and
tetrasubstituted aziridines.15 However, imines 7 and 8 could be isolated from reaction mixtures as
pure solid substances whereas imines 3 and 4 were very unstable.
The ratios of 7:8 demonstrated that the thermal isomerization of 2,3-diaryl-1-
4

phthalimidoaziridine-2-carbonitriles proceeded with the same regioselectivity as we had
established for the analogous 2,3-diarylaziridines. The PhthN group preferably migrated to the
carbon atom with the more electron-withdrawing substituent.
Finally, we examined the ability of selected 2,3-diaryl-1-phthalimidoaziridines to take part in
a 1,3-dipolar cycloaddition reaction (Table 3). As we had previously reported,20 aziridine 2a
does not react, even with an active dipolarophile – N-phenylmaleimide. Thermolysis of
aziridines 2d,e and 6a,c in the presence of dimethyl acetylenedicarboxylate (9) or N-
phenylmaleimide (10) (equimolar ratio of reagents) gave the corresponding imines as the major
components of the reaction mixtures, and only trace signals of cycloaddition products were
found in the crude 1H NMR spectra, clearly indicating that rearrangement of these aziridines into
imines predominates over the 1,3-dipolar cycloaddition reaction.
Table 3. Transformations of aziridines 2d,e and 6a,c in the presence of dipolarophiles 9,10
Aziridine
Ar1
Ph
4-O2NC6H4
Ph
4-O2NC6H4
Ar2
4-O2NC6H4
4-MeOC6H4
Ph
R
H
H
CN
CN
Dipolarophile T (°C)
Imines
3d/4d
3e/4e
7a/8a
7c/8c
120
120
25
2d
2e
6a
6c
9
10
10
10
Ph
25
Taking into account the above mentioned examples30-32 of 1,2-migration of the N-substituent,
we propose that such thermal rearrangement is inherent to aziridines possessing at least one aryl
group at the carbon atoms. Earlier we reported that thermolysis of a cinnamonitrile derived
N-phthalimidoaziridine gave an imine with the PhthN group at the carbon atom containing the
electron-withdrawing cyano substituent.20 However, this aziridine successfully underwent the
1,3-dipolar cycloaddition reaction and isomerization into the imine was not the major
transformation even in the absence of a dipolarophile (imine yield 23%).20 However, for
aziridines 2a-g and 6a-c the cumulative effect of the two aryl groups was dramatic.
On one hand, the presence of the two aryl rings together with the phthalimide group may
provide a large steric hindrance for the W- or U-azomethine ylide configuration, which are
usually proposed as intermediates in the thermal transformations of trans-substituted aziridines
(Scheme 2). In this case, a cis-2,3-diarylaziridine can give a different result due to thermally
allowed conrotatory ring-opening of the C-C bond to stereospecifically generate the less
sterically crowded S-azomethine ylides.
5

Scheme 2. Thermally allowed conrotatory ring-opening of trans- and cis-aziridines into
azomethine ylides
Therefore, we prepared cis-2,3-diphenyl-1-phthalimidoaziridine39 (2i) from cis-stilbene (1i)
(Scheme 3). Thermolysis in the same manner as the trans-isomer aziridine 2a gave only imine
3a, thus steric factors could not be considered as the main reason for this chemoselectivity.
Scheme 3. Preparation and thermolysis of aziridine 2i
Person21 proposed a concerted 1,2-shift of the phthalimide moiety from the nitrogen atom to
the carbon atom in azomethine ylides as a mechanism for this process, similar to the Stevens
rearrangement of tertiary ammonium salts.40 This ignores the fact that N–N bond lies in a -
system nodal plane of the octet-stabilized azomethine ylide; therefore a concerted shift in such
ylide is restricted.
We presume that the 1,2-shift of the PhthN group occurs simultaneously with (or not later
than) the C–C bond cleavage without real formation of the azomethine ylide (the sluggish 1,3-
dipolar cycloaddition can be considered as an additional argument) and may be facilitated by a
weak N–N bond. This hypothesis can also explain the regioselectivity of the migration. If
heterolytic cleavage (evidence against a radical mechanism have been discussed)21 of the N–N
bond provides a negatively charged phthalimido moiety (stabilized by two C=O groups), it
should preferably migrate to the carbon atom containing the electron-withdrawing substituent as
we have established. Assuming this migration in azomethine ylides, the regioselectivity should
be opposite because the electron-deficient end of 1,3-dipole is better stabilized with a donating
substituent.
Scheme 4. Plausible mechanism for the thermal rearrangement of N-phthalimidoaziridines into
imines
In summary, the rearrangement into imines is a general reaction pathway in the thermal
transformation of 2,3-diaryl-1-phthalimidoaziridines. The reaction includes a 1,2-shift of a
6

phthalimide fragment, preferably to the carbon atom with a more electron-withdrawing
substituent, along with C–C bond cleavage. The presence of an aryl group(s) at the carbon atom
and a PhthN group on the nitrogen atom plays a key role, whereas the configuration of the initial
aziridine had no impact. This process takes place even in the presence of dipolarophiles making
participation of these aziridines in synthetically useful intermolecular reactions (e.g., 1,3-dipolar
cycloaddition) unfavorable. Similar behavior can be anticipated for other 2,3-diarylaziridines and
therefore should be taken into account in synthetic planning.
Acknowledgements
The authors thank Russian Scientific Fund for financial support (grant № 14-13-00126).
NMR and HRMS analyses were performed at the Saint Petersburg State University Center for
Magnetic Resonance and Center for Chemical Analysis and Materials Research, respectively.
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Graphical Abstract
Thermal rearrangement of 2,3-diaryl-1-
phthalimidoaziridines
Leave this area blank for abstract info.
Alena S. Pankova, Mariia V. Sorokina, Mikhail A. Kuznetsov
9