LiAlH4-Induced Thia-Aza-Payne Rearrangement of Functionalized 2-(Thiocyanatomethyl)aziridines into 2-(Aminomethyl)thiiranes as an Entry to 5-(Chloromethyl)thiazolidin-2-ones

Article abstract of DOI:10.1002/ejoc.201700549

Nonactivated 2-(thiocyanatomethyl)aziridines with diverse substitution patterns were deployed as substrates to effect a LiAlH₄-promoted thia-aza-Payne rearrangement to provide access to functionalized 2-(aminomethyl)thiiranes in good to excelle

Full text of DOI:10.1002/ejoc.201700549

DOI: 10.1002/ejoc.201700549
Communication
Rearrangement
LiAlH4-Induced Thia-Aza-Payne Rearrangement of Functionalized
2-(Thiocyanatomethyl)aziridines into 2-(Aminomethyl)thiiranes as
an Entry to 5-(Chloromethyl)thiazolidin-2-ones
Jeroen Dolfen,[a] Kristof Van Hecke,[b] and Matthias D'hooghe*[a]
Abstract: Nonactivated 2-(thiocyanatomethyl)aziridines with hydride reduction of the thiocyanato moiety followed by intra-
diverse substitution patterns were deployed as substrates to molecular aziridine ring opening. Subsequent exposure of the
effect a LiAlH4-promoted thia-aza-Payne rearrangement to pro- obtained 2-(aminomethyl)episulfide intermediates to triphos-
vide access to functionalized 2-(aminomethyl)thiiranes in good gene resulted in the formation of 5-(chloromethyl)thiazolidin-2-
to excellent yields (78–94 %). The developed strategy involved ones.
Introduction
In continuation of our research efforts concerning the LiAlH4-
induced regioselective ring rearrangement of 2-(cyanoethyl)-
aziridines into either 2-(aminomethyl)pyrrolidines or 3-amino-
piperidines,[6] and in light of the growing interest in sulfur-
containing heterocycles,[7] the deployment of 2-(thiocyanato-
methyl)aziridines as substrates for a hydride-promoted thia-aza-
Payne rearrangement was envisaged. The feasibility of the
premised aziridine-to-thiirane interconversion was assessed
starting from nonactivated 2-(thiocyanatomethyl)aziridines 1.
These aziridines, bearing an electron-donating group on the
nitrogen atom and differing in substitution patterns, can be
prepared from corresponding 2-monosubstituted 2-(bromo-
methyl)aziridines 2, 2,2-disubstituted aziridines 3, or 2,3-disub-
stituted aziridines 4, which have amply proven to be versatile
precursors for further synthetic elaboration (Figure 1).[6,8]
Since Payne's comprehensive research on the reorganization of
2,3-epoxy alcohols into their isomeric counterparts in 1962,[1]
the “Payne rearrangement” has evolved into a powerful reac-
tion in organic chemistry. Moreover, owing to its broad applica-
bility, this elegant interconversion has become a widely used
method in natural product synthesis.[2] Although the involved
intramolecular ring-opening reactions occur in a stereospecific
SN2 fashion with inversion of configuration at the more-substi-
tuted carbon atom, the reversible character of the isomerization
process still represents a significant drawback.
The “aza-Payne rearrangement”, however, implying the con-
version of a 2-(hydroxymethyl)aziridine into its isomeric oxirane
or vice versa,[3] and the “thia-Payne rearrangement”, referring
to the equilibrium between a 2-(hydroxymethyl)thiirane and an
epoxide,[4] can be tuned and controlled to a certain extent de-
pending on the applied reaction conditions. Despite numerous
papers reporting epoxide–aziridine and/or epoxide–thiirane mi-
grations, only one article dealing with an aziridine-to-thiirane
rearrangement has been published so far.[5] Moreover, the
transformations in that particular study appeared to induce the
formation of side products as well, as treatment of a variety
of polysubstituted 1-tosyl-2-(tosyloxymethyl)aziridines with an
excess amount of benzyltriethylammonium tetrathiomolybdate
([BnEt3N]2MoS4) in CH3CN afforded the corresponding thiiranes
as the major products (75–80 %) and cyclic disulfides as the
minor compounds (20–25 %).
Figure 1. Diverse aziridine substrate classes.
Results and Discussion
At the outset of this study, 2-(bromomethyl)aziridine 2a was
converted into 2-(thiocyanatomethyl)aziridine 5a in 90 % yield
upon treatment with KSCN (2 equiv.) in DMF at 70 °C,[8a] and
then 5a was used as a model substrate for the premised thia-
aza-Payne rearrangement (Table 1). In a first attempt, aziridine
5a was treated with LiAlH4 (2 equiv.) and indium(III) trifluoro-
methanesulfonate [In(OTf)3, 0.3 equiv.] at reflux temperature,[6]
but these reaction conditions resulted in intermolecular dimer
formation instead of intramolecular ring rearrangement
(Table 1, entry 1). Next, an equimolar amount of LiAlH4 and
[a] SynBioC Research Group, Department of Sustainable Organic Chemistry
and Technology, Faculty of Bioscience Engineering, Ghent University,
Coupure Links 653, 9000 Ghent, Belgium
E-mail: Matthias.Dhooghe@UGent.be
http://www.synbioc.ugent.be
[b] XStruct, Department of Inorganic and Physical Chemistry,
Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201700549.
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Communication
In(OTf)3 was used, which led to complete conversion into de- thia-aza-Payne rearrangement. In a first approach, monosubsti-
sired thiirane 6a (Table 1, entry 2). However, upon standing tuted 2-(thiocyanatomethyl)aziridines 5b–e (R2 = H) were syn-
for a few hours, the reaction product appeared to be unstable, thesized from corresponding 2-(bromomethyl)aziridines 2b–e
probably as a result of the presence of residual In(OTf)3. Lower- (2 equiv. KSCN, DMF, 70 °C, 17 h), which were subsequently
ing the reaction temperature from reflux temperature to 0 °C confronted with LiAlH4 (1.7 equiv.) at –78 °C under an argon
in the absence of In(OTf)3 again resulted in dimer formation atmosphere to furnish 2-(aminomethyl)thiiranes 6b–e in good
(Table 1, entry 3), whereas increasing the amount of LiAlH4 to excellent yields (79–92 %; Table 2, entries 2–5). Owing to the
(1.7 equiv.) in combination with a reaction temperature of unstable nature of obtained thiiranes 6b–e upon storing and
–78 °C afforded desired thiirane 6a in an acceptable yield of during purification on silica gel, these intermediates were im-
78 % (Table 1, entry 4). As isolated 2-(aminomethyl)thiirane 6a mediately treated with triphosgene (1 equiv.) in THF to produce
still appeared to be unstable upon standing for a few days, it stable 5-(chloromethyl)thiazolidin-2-ones 7b–e in 81–99 %
was trapped through treatment with triphosgene (1 equiv.) in yield.
THF to furnish 5-(chloromethyl)thiazolidin-2-one 7a in 85 %
Table 2. Scope of the thia-aza-Payne rearrangement of 2-(thiocyanato-
yield. Mechanistically, the thia-aza-Payne rearrangement of 2-
methyl)aziridines 5 and subsequent ring transformation with the use of tri-
phosgene.
(thiocyanatomethyl)aziridine 5a can be rationalized by hydride
addition across the thiocyanato moiety with concomitant re-
lease of HCN. The in situ formed sulfide anion effects intra-
molecular aziridine ring opening, and this results in 2-(amino-
methyl)thiirane 6a after aqueous workup. Although nonacti-
vated aziridines generally require activation of the ring system
prior to ring opening (in contrast to activated aziridines),[9] the
ring opening of 1-benzylaziridine 5a can be attributed to the
Lewis acid activity of LiAlH4 (through coordination of aluminum
with nitrogen).[10] Subsequent treatment of obtained 2-(amino-
methyl)thiirane 6a with triphosgene resulted in N-acylation to
give carbamate 8a, and this was followed by regioselective
chloride-induced ring opening of the thiirane core at the less-
substituted carbon atom[7d,11] and ring transformation into cor-
Entry
R1
R2
Product (yield[a] [%])
responding 5-(chloromethyl)thiazolidin-2-one 7a. Initial attack
of the amino group in 6a across triphosgene was corroborated
by reaction of thiirane 6a with methyl chloroformate and Boc2O
(Boc = tert-butoxycarbonyl) on an analytical scale, which af-
forded the N-acylated products without thiirane ring opening.
5
6
7
1
2
3
4
5
6
7
8
Ph
H
H
H
H
5a (90)
5b (87)
5c (95)
5d (83)
5e (81)
5f (86)
5g (96)
5h (91)
6a (78)
6b (82)
6c (79)
6d (92)
6e (86)
6f (94)
6g (91)
6h (93)
7a (85)
7b (89)
7c (81)
7d (85)
7e (99)
7f (97)
7g (99)
7h (92)
4-MeC6H4
4-ClC6H4
iPr
cyclohexyl
Ph
H
Table 1. Optimization of the reaction conditions for the thia-aza-Payne rear-
rangement of 1-benzyl-2-(thiocyanatomethyl)aziridine (5a).
Me
Me
Me
4-MeC6H4
4-MeOC6H4
[a] Yield of isolated product.
The effect of an additional substituent at the aziridine C2
position on the thia-aza-Payne rearrangement was investigated
next. In that respect, 2-methyl-2-(thiocyanatomethyl)aziridines
5f–h (R2 = Me) were synthesized in excellent yields (86–96 %)
starting from 2-bromomethyl-2-methylaziridines 3a–c upon
treatment with an equimolar amount of KSCN in DMF at 65 °C
(Table 2, entries 6–8).[8d] Subsequent addition of an excess
amount of LiAlH4 (1.7 equiv.) in THF at –78 °C induced the de-
sired aziridine-to-thiirane reorganization, and 2-(amino-
methyl)thiiranes 6f–h were isolated in high yields (91–94 %).
As a consequence, it can be concluded that the presence of a
quaternary carbon center in aziridines 5 does not have a nega-
tive impact on the thia-aza-Payne rearrangement. On the con-
trary, 2-aminomethyl-2-methylthiiranes 6f–h were isolated in
slightly higher yields (91–94 %) than 2-(aminomethyl)thiiranes
6a–e (78–92 %) and appeared to be more stable upon pro-
Entry
LiAlH4
[equiv.]
In(OTf)3
[equiv.]
Temp.
[°C]
Yield [%] of 6a
[c]
1
2
3
4
2
1
1.2
1.7
0.3
1
0
reflux
reflux
0
–
45[d]
[c]
–
0
–78
78
[a] Reaction conditions: KSCN (2 equiv.), DMF, 70 °C, 17 h.[8a] [b] Reac-
tion conditions: Triphosgene (1 equiv.), THF, r.t., 17 h. [c] Dimer formation.
[d] Thiirane 6a appeared to be unstable in the presence of residual In(OTf)3.
Having the optimal reaction conditions for the conversion of longed storage at 4 °C and during purification on silica gel.
aziridine 5a into thiirane 6a in hand, other 2-(thiocyanato- Subsequently, obtained 2,2-disubstituted thiiranes 6f–h were
methyl)aziridines were prepared next to trigger the observed treated with triphosgene (1 equiv.) in THF at room temperature
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to afford corresponding 5-chloromethyl-5-methylthiazolidin-2- intensive efforts, structural isomers 10b and 11b could not be
ones 7f–h in almost quantitative yields (92–99 %; Table 2, separated, and – as a consequence – this mixture was used as
entries 6–8). Notably, also in the case of gem-disubstituted such in the next step. In addition to the unexpected influence
thiiranes 6f–h, ring opening of the thiirane core by chloride of the 2-aryl substituent on the thiocyanate-induced tosyloxy
proceeded regioselectively at the less-substituted carbon displacement, the effect of the N-substituent in vic-disubsti-
atom.[12]
For aziridine substrate class 4 (Figure 1) featuring a vic-disub-
tuted aziridines 4 was also studied by tosylation and subse-
quent treatment with KSCN of 1-isopropyl-2-phenylaziridine 4c.
Again, a mixture of isomers 10c and 11c was obtained in a ratio
of 30:70, although in favor of 2-[phenyl(thiocyanato)methyl]-
aziridine 11c in this case. Subsequent purification of the reac-
tion mixture by column chromatography (silica gel) allowed the
isolation of major isomer 11c in 44 % yield. Notably, analysis
of intermediates 10a and 10b by NMR spectroscopy (CDCl3)
appeared to be impossible owing to unclear resolution of the
corresponding signals. On the basis of the obtained experimen-
tal results, the addition of KSCN to in situ formed 3-(tosyloxy-
methyl)aziridines 9 seems to provoke a competition between
stitution pattern, the preparation of the corresponding 3-(thio-
cyanatomethyl)aziridines appeared to be highly dependent on
the nature of the substituents of the involved aziridines. Tosyl-
ation of 2-(4-chlorophenyl)-3-(hydroxymethyl)aziridine 4a with
4-toluenesulfonyl chloride (TsCl, 1.05 equiv.) in the presence of
4-(dimethylamino)pyridine (DMAP, 0.1 equiv.) and Et3N
(1.1 equiv.) in CH2Cl2, followed by nucleophilic substitution
upon the addition of KSCN (1 equiv.) in DMF at 65 °C, afforded
3-(thiocyanatomethyl)aziridine 10a in 52 % yield as a single re-
action product (Table 3). Surprisingly, by applying the same re-
action conditions to 2-phenyl-3-(hydroxymethyl)aziridine 4b, 3- ring opening at the benzylic position (route a) and the
(thiocyanatomethyl)aziridine 10b and 2-[phenyl(thiocyanato)- expected direct tosyloxy group displacement (route b)
methyl]aziridine 11b were obtained in a 55:45 ratio. Despite (Table 3).[8c] Remarkably, aziridines 4a–c gave rise to a different
Table 3. Thia-aza-Payne rearrangement of (thiocyanatomethyl)aziridines 10 and 11, followed by ring transformation upon treatment with triphosgene.
Conversion of 3-aryl-2-(hydroxymethyl)aziridines 4 into (thiocyanatomethyl)aziridines 10 and 11
Substrate
R
Ar
Ratio (10/11)[a]
Product (yield[b] [%])
4a
4b
4c
Bn
Bn
iPr
4-ClC6H4
Ph
Ph
10a/11a (100:0)
10b/11b (55:45)
10c/11c (30:70)
10a (52)
10b (–)[d]
10c (0)
11a (–)[c]
11b (–)[d]
11c (44)
Conversion of (thiocyanatomethyl)aziridines 10 and 11 into 2-(aminomethyl)thiiranes 12 and 13
Substrate(s)
R
Ar
Ratio (12/13)[a]
Product (yield[b] [%])
10a
10b + 11b
11c
Bn
Bn
iPr
4-ClC6H4
Ph
Ph
–
12a (88)
12b (–)[e]
12c (–)[c]
13a (–)[c]
13b (–)[e]
13c (90)
12b/13b (55:45)
–
Conversion of 2-(aminomethyl)thiiranes 12 and 13 into thiazolidin-2-ones 14 and 15
Substrate(s)
R
Ar
Ratio (14/15)[a]
Product (yield[b] [%])
12a
12b + 13b
13c
Bn
Bn
iPr
4-ClC6H4
Ph
Ph
–
14a (71)
14b (33)
14c (–)[c]
15a (–)[c]
15b (22)
15c (95)
14b/15b (53:47)
–
[a] Determined by analysis of the crude reaction mixture by 1H NMR spectroscopy (CDCl3). [b] Yield of isolated product. [c] Not applicable. [d] Aziridines 10b
and 11b were isolated as a mixture in a combined yield of 28 %, and this mixture was used as such in the next step. [e] Thiiranes 12b and 13b were isolated
as a mixture in a combined yield of 93 %, and this mixture was used as such in the next step.
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Communication
reactivity profile, which pointed to the fact that the observed wave irradiation resulted in the formation of substitution prod-
thiocyanate-induced nucleophilic attack across 1-alkyl-2-aryl-3- ucts 17 and 18, both in 87 % yield. The use of benzylamine,
(tosyloxymethyl)aziridines 9 is governed by a subtle interplay NaOAc, and KCN as nucleophiles, however, appeared to be less
between the different substituents present at the aziridine scaf- straightforward and resulted in more complex reaction mix-
fold.
In a next step, obtained (thiocyanatomethyl)aziridines 10
and 11 were treated with LiAlH4 (1.7 equiv.) in THF at –78 °C,
which evoked thia-aza-Payne rearrangement to afford
tures.
a
thiirane(s) 12 and/or 13 in excellent yield(s) (88–93 %, Table 3).
The relative trans stereochemistry of thiiranes 13b and 13c was
confirmed by the vicinal coupling constants between the 2H
and 3H protons on the thiirane ring (Jtrans = 5.2–5.4 Hz), which
is in accordance with the literature.[13] Separation of thiiranes
12b and 13b, obtained from aziridine mixture 10b and 11b,
appeared to be inconvenient, and as a consequence, the mix-
ture was used as such in the ring-transformation reaction with
triphosgene (1 equiv.) in THF. After heating at reflux tempera-
ture for 4 h, corresponding thiazolidin-2-ones 14b and 15b
were produced, and they could eventually be separated and
isolated by means of preparative TLC (silica gel) in yields of 33
and 22 %, respectively. Aminomethylated thiiranes 12a and 13c
(obtained from aziridines 10a and 11c, respectively) were also
treated with triphosgene (1 equiv.) in THF under reflux condi-
tions, and they afforded 5-(chloromethyl)thiazolidin-2-ones 14a
and 15c in yields of 71 and 95 %, respectively. The molecular
identity of thiazolidin-2-one 15c was unequivocally established
by means of single-crystal X-ray analysis (see the Supporting
Information), which provided clear evidence for the regioselect-
ive chloride-induced ring opening of thiiranes 13b and 13c at
the benzylic position.[11b,11c]
Scheme 1. Reactivity of thiazolidin-2-one 7c with respect to KOtBu, KSCN,
and NaI.
Conclusions
In conclusion, an efficient and reliable thia-aza-Payne rearrange-
ment of nonactivated 2-(thiocyanatomethyl)aziridines toward 2-
(aminomethyl)thiiranes was developed. The deployment of dif-
ferent classes of aziridine substrates showed that diverse substi-
tution patterns did not impose any restrictions on the desired
aziridine-to-thiirane migrations. In addition, the obtained 2-
(aminomethyl)thiiranes were easily converted into 5-(chloro-
methyl)thiazolidin-2-one building blocks, which points to a re-
gioselective thiirane ring opening by chloride.
CCDC 1536450 (for 15c) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
From the above-described results, it is clear that nonacti-
vated 2-(thiocyanatomethyl)aziridines 1, derived from corre- charge from The Cambridge Crystallographic Data Centre.
sponding aziridines 2–4, represent valuable substrates for an
unprecedented and efficient thia-aza-Payne rearrangement, as
shown by the synthesis and characterization of 12 2-(amino-
Acknowledgments
The authors are indebted to Ghent University - Belgium (BOF).
The input of N. Piens is gratefully acknowledged. K. V. H. thanks
the Hercules Foundation (project AUGE/11/029 “3D-SPACE: 3D
Structural Platform Aiming for Chemical Excellence”) and the
Research Foundation Flanders (FWO) for funding.
methyl)thiiranes. Furthermore, the involved experiments show
that the aziridine-to-thiirane migrations are irreversible and oc-
cur with inversion at the stereogenic center. Moreover, subse-
quent treatment of the obtained 2-(aminomethyl)thiiranes with
triphosgene resulted in the formation of chloromethyl-substi-
tuted thiazolidin-2-ones by regioselective thiirane ring opening
by chloride at the less-substituted or benzylic position, which
is in accordance with the literature concerning the ring opening
of thiiranes.[7d,11,12] Notably, this report discloses the first
method for an aziridine-to-thiirane conversion in a selective
and straightforward manner, and it should therefore be consid-
ered as a powerful strategy in modern organic chemistry.
Keywords: Heterocycles · Rearrangement · Reduction ·
Regioselectivity · Small ring systems
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Received: April 20, 2017
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