Use of trichloroacetonitrile as a hydrogen chloride generator for ring-opening reactions of aziridines

Article abstract of DOI:10.1039/C9OB00602H

Regioselective ring-opening reactions of 2-aryl-N-tosylaziridines are described, in which hydrogen chloride is generated by photodegradation of trichloroacetonitrile. HCl adducts are obtained in high yields in 1,4-dioxane, whereas methanol adducts are pre

Full text of DOI:10.1039/C9OB00602H

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Use of Trichloroacetonitrile as a Hydrogen Chloride Generator for
Ring-Opening Reactions of Aziridines
Received 00th January 20xx,
Accepted 00th January 20xx
Yasunori Toda,* Riki Matsuda, Shuto Gomyou and Hiroyuki Suga*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Regioselective ring-opening reactions of 2-aryl-N-tosylaziridines
are described, in which hydrogen chloride is generated by
photodegradation of trichloroacetonitrile. HCl adducts are
obtained in high yields in 1,4-dioxane, whereas methanol adducts
are predominantly obtained in methanol. Trichloroacetonitrile can
serve as a photoresponsive molecular storage generator for
hydrogen chloride.
(a) Tsuda et al. (ref 2a)
NH2
NH3Cl
>98%
OMe
CHCl3
& O
hv
2
OMe
OH
HCl, Cl2, COCl2
90%
Cl
photochemical
molecular storage
O
O
Halogenated alkanes, the most well-known of which is
chloroform, are often used as solvents and reagents in organic
synthesis. These compounds are known to undergo
degradation to produce hydrogen halides by external factors
such as light.1 Thus, it may be possible to use halogenated
alkanes as precursors to provide a facile method for the
generation of hydrogen halides on demand. In 2012, Tsuda
and co-workers reported a photochemical molecular storage
strategy using CHCl3 with ultraviolet (UV) irradiation under
oxygen atmosphere, wherein oxidative photodecomposition of
chloroform produced hydrogen chloride (HCl), chlorine (Cl2),
and phosgene (COCl2) (Fig. 1a).2 They applied a low-pressure
mercury lamp (20 W) that generated UV with wavelengths of
185 and 254 nm. Since treatment of highly hazardous
chemicals can be avoided, this type of application would be
very attractive, but has remained underdeveloped. We
therefore sought to explore other reagents as a potential
storage means for HCl that conveniently respond to
photoirradiation. Trichloroacetonitrile with the formula CCl3CN
has been employed in versatile organic reactions, e.g., the
Overman rearrangement, preparation of benzylating or t-
butylating agents, and glycosylation reactions.3 Taking
advantage of the strong electron-withdrawing character of the
cyano group, trichloroacetonitrile serves as an alternative to
carbon tetrachloride (CCl4) in substitution reactions with
triphenylphosphine.4 Moreover, the cyano group is able to
O
>98%
(b) Working hypothesis
Cl
Cl
Cl
hv
O2
NC
C
Cl
NC C•
Cl
NC
C
O O•
Cl
Cl
•
•
HCl
Cl
Cl
H-abstraction
(c) This work
NTs
Cl
Ar
OMe
CCl3CN
or
NHTs
NHTs
Ar
Ar
hv & air
(in 1,4-dioxane)
(in methanol)
Fig 1 Trichloroacetonitrile as photoresponsive molecular storage generator
of hydrogen chloride.
delocalize a carbon radical at the -position, which lowers the
energy of activation to enhance the rate of the overall
reaction.5 Thus, it is expected that trichloroacetonitrile would
photodecompose under mild conditions to release a chlorine
radical (Fig. 1b), which in turn, would abstract a hydrogen
atom to generate HCl.6 However, the established usage of
trichloroacetonitrile for the purpose of HCl generation is still
elusive.
Extensive studies have been conducted on ring-opening
reactions of aziridines with various nucleophiles,7 because the
transformations afford synthetically valuable products
including 1,2-aminoalcohol derivatives.8 Although several
methods, especially catalytic processes with Lewis acids, have
a. Department of Materials Chemistry, Faculty of Engineering, 4-17-1 Wakasato,
Nagano 380-8553.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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been developed to date, the regioselectivity of the adducts can
be problematic depending on the nucleophiles or the
substituents of the aziridines.9,10 We envisaged that the
photochemical molecular storage strategy might be suitable to
provide practical access to aziridine ring-opening reactions,
because the electrophilic activation of a substrate by a
Brønsted acid is one of the most common approaches to
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DOI: 10.1039/C9OB00602H
Table 1 Optimization of reaction conditionsa
CCl3CN (3.0 equiv)
UV lamps (365 nm)
Cl
NHTs
NTs
1a
NHTs
Cl
Ph
Ph
Ph
1,4-dioxane (0.1 M), air
30 °C, 3 h
2a
2a'
2a (%)b
>98 (95)
<5
promote
a reaction. Herein, we demonstrate HCl and
entry
variation from standard conditions
none
methanol addition to 2-aryl-N-tosylaziridines by employing a
fragment of photodecomposed trichloroacetonitrile, leading to
1
2
no light
the
synthesis
of
both
-chloroamines
and
-
3
4
5
6
CHCl3 instead of CCl3CN
CCl4 instead of CCl3CN
HCl instead of CCl3CN
under Ar
<5
<5
98
<5
methoxyethylamines (Fig. 1c).
At the outset of our study, we tested the HCl addition to 2-
phenyl-N-tosylaziridine (1a) with the use of 3.0 equiv of
trichloroacetonitrile (Table 1). The initial experiment was
performed in 0.1 M 1,4-dioxane with 365 nm UV irradiation11
under air atmosphere at ambient temperature for 3 h (Table 1,
entry 1). To our delight, the desired -chloroamine 2a was
obtained in high yield, and no regioisomer 2a’ was observed. A
control experiment under dark conditions resulted in no
consumption of starting material 1a (Table 1, entry 2). Notably,
the privileged reactivity of trichloroacetonitrile was proven by
comparison of chloroalkanes CHCl3 and CCl4, neither of which
provided 2a (Table 1, entries 3 and 4). As expected, the use of
HCl solution in 1,4-dioxane led to a similar result as entry 1
(Table 1, entry 5). Interestingly, air atmosphere was important
for the efficient conversion, which was revealed by reactions
under argon and oxygen atmospheres (Table 1, entries 6 and
7). Although oxygen is necessary on the basis of the reported
mechanism (cf. Fig. 1b),1m,2a a high concentration of oxygen
seems to suppress the free radical reaction. Furthermore,
solvent screening showed an intriguing tendency (Table 1,
entries 8-16). In all cases of ether-type solvents, complete
conversion of aziridine 1a was observed. The reaction in
tetrahydrofuran afforded 2a in moderate yield, but significant
amounts of by-products were observed, probably due to
decomposition of tetrahydrofuran (Table 1, entry 8). Switching
to 2-methyltetrahydrofuran improved the yield of 2a up to
83%, and the reaction in diethyl ether achieved a yield
comparable to that in 1,4-dioxane (Table 1, entries 9 and 10).
Use of toluene, dichloromethane, acetonitrile, and dimethyl
sulfoxide resulted in recovery of 1a (Table 1, entries 11-14).
Regioisomer 2a’ was obtained, albeit with low conversion, in
high polar solvents such as dimethylformamide and
dimethylacetamide (Table 1, entries 15 and 16).
7
8
9
10
11
12
13
14
15
16
under O2
in THF
in 2-MeTHF
in Et2O
in toluene
in CH2Cl2
in CH3CN
in DMSO
in DMF
26c
54d
83d
>98
<5
<5
<5
<5
<5e
13e
in DMA
aUnless otherwise noted, all reactions were carried out on a 0.10 mmol
scale using aziridine 1a and 3.0 equiv of trichloroacetonitrile with 365
nm UV lamps under air atmosphere at 30 °C for 3 h. bDetermined by 1H
NMR analysis using 1,1,2,2-tetrachloroethane as an internal standard
(isolated yield is shown in parentheses). cRecovery of 1a; entry 6: 91%;
entry 7: 70%. dComplete conversion of 1a. eYield of 2a’ (%); entry 15:
trace; entry 16: 17%.
Table 2 Optimization of reaction conditionsa
CCl3CN (3.0 equiv)
UV lamps (365 nm)
OMe
Cl
NTs
NHTs
NHTs
Ph
Ph
Ph
methanol (0.1 M), air
30 °C, 0.5 h
1a
3a
2a
entry
variation from standard conditions
none
3a (%)b
>98 (98)
<5
1
2
3
4
5
6
7
no light
no CCl3CN
0.05 equiv of CCl3CN (24 h)
HCl instead of CCl3CN
CHCl3 instead of CCl3CN
CCl4 instead of CCl3CN
<5
97
70c
<5d
41d
aUnless otherwise noted, all reactions were carried out on a 0.10 mmol
scale using aziridine 1a and 3.0 equiv of trichloroacetonitrile with 365
nm UV lamps in a 0.1 M of methanol under air atmosphere at 30 °C for
0.5 h. bDetermined by 1H NMR analysis using 1,1,2,2-tetrachloroethane
as an internal standard (isolated yield is shown in parentheses). cHCl
adduct 2a was obtained in 26% yield. dRecovery of 1a; entry 6: 97%;
entry 7: 56%.
Subsequently, we attempted the methanol addition to
aziridine 1a, and identified the optimal conditions shown in
entry 1 of Table 2. The reaction of 1a with methanol under air
afforded the corresponding adduct 3a in 98% isolated yield
within 30 min (standard conditions). Almost no conversion of
1a was observed without light or without trichloroacetonitrile
(Table 2, entries 2 and 3). Even a catalytic amount (5 mol %) of
trichloroacetonitrile provided 3a in high yield, although 24 h
was needed for the consumption of starting material 1a (Table
2, entry 4). The reaction using a HCl solution in methanol led to
decreased yield due to formation of HCl adduct 2a (Table 2,
entry 5). CHCl3 and CCl4 were less effective, revealing the merit
of trichloroacetonitrile photodecomposition (Table 2, entries 6
and 7). Lastly, the amount of HCl in the reaction mixture was
estimated by Mohr’s method (see ESI for details). It should be
noted that the HCl generation in 1,4-dioxane (ca. 0.3 equiv of
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9OB00602H
X
CCl3CN (3.0 equiv)
(a) condition A:
in 1,4-dioxane, 6 h
NTs
CCl3CN (3.0 equiv)
UV lamps (365 nm)
NHTs
Cl
Ar
Ar
NTs
UV lamps (365 nm)
solvent (0.1 M)
air, 30 °C
NHTs
(a)
(b)
(c)
(b) condition B:
in methanol, 0.5 h
1
2
: X = Cl
: X = OMe
Ph
Ph
1,4-dioxane (0.1 M), air
30 °C, 3 h
3
1a
2a
: 92%, 7% ee
(R)- : 99% ee
(a) Scope of HCl addition
Cl
CCl3CN (3.0 equiv)
UV lamps (365 nm)
OMe
NTs
Ph
Me Cl
Cl
NHTs
Ph
Me
NHTs
NHTs
NHTs
methanol (0.1 M), air
30 °C, 0.5 h
1a
3a
(R)- : 99% ee
(S)- : 92%, 96% ee
2b
2e
2h
2c
2d
2g
: 91%
: 91%
: 93%
Me
Cl
CCl3CN (3.0 equiv)
UV lamps (365 nm)
OMe
Cl
Cl
Cl
Cl
Br
Cl
O
OH
Ph
Cl
NHTs
NHTs
NHTs
Ph
methanol (0.1 M), air
30 °C, 0.5 h
5
6
(R)- : 95%, 82% ee
(S)- : 99% ee
a
2f
4
: 95%
: 93%
: 98%
MeO
NHTs
NHTs
Scheme 2 Reactions of optically active aziridine 1a and epoxide 5.
b
: 91%
: 94%
trichloroacetonitrile (0.3 equiv) enabled selective formation of
3f. 1,1-Disubstituted aziridine 1i was cleanly converted to the
product 3i, but a 1,2-disubstitution pattern was challenging.
Indene-derived 1j formed not only 3j (71%) but also its
regioisomer (18%).
To expand the applicability of our methodology, optically
active aziridine (R)-1a was submitted to the optimized reaction
conditions. Mechanistically, it is of interest that contrasting
results were obtained as shown in Scheme 2a and 2b. Serious
stereochemical erosion occurred during the course of the HCl
addition (7% ee), indicating an SN1-like pathway that involved
(b) Scope of methanol addition
OMe
Me
Cl
OMe
OMe
OMe
Me
NHTs
NHTs
NHTs
3b
3c
3d
: 90%
: 96%
: 96%
Me
Cl
OMe
OMe
Cl
NHTs
NHTs
NHTs
c
3e
3f
3g
: 94%
: 80%
: 94%
OMe
OMe
OMe
Me
MeO
NHTs
NHTs
NHTs
3j
a
carbocationic intermediate. On the other hand, the
3h
3i
: 98%
: 97%
: 71%
methanol addition to (R)-1a yielded enantio-enriched and
stereoinverted product (S)-3a with 96% ee, wherein an SN2-
type ring-opening could proceed. Furthermore, styrene oxide
(5) was tested as an analogue of 3-membered heterocycles. To
our delight, enantiopure (S)-5 was smoothly converted to the
methanol adduct (R)-6 in high yield with perfect
regioselectivity, but partial loss of enantiomeric excess was
observed (Scheme 2c).
a
2f 2f'
12 h (
:
= 96:4). bCBr4 was used. c0.3 equiv of CCl3CN.
Scheme 1 Substrate scope of the ring-opening reactions of aziridines with
photodecomposed trichloroacetonitrile.
HCl upon aziridine 1) was more than four times as fast as that
in methanol (ca. 0.07 equiv). In addition, less than 120 mol %
of HCl based on trichloroacetonitrile was observed even at a
longer irradiation time of 12 h in 1,4-dioxane.
In summary, we have demonstrated regioselective ring-
opening reactions of 2-aryl-N-tosylaziridines 1 for the synthesis
of -chloroamines
2 and -methoxyethylamines 3. HCl
The scope of substrates is summarized in Scheme 1. The
HCl addition to 1 was first examined by varying the aziridine
component (Scheme 1a, upper half). Steric effects of the aryl
group of 1 were investigated, but there was no significant
difference between meta-, ortho-, and para-tolyl groups (2b:
91%; 2c: 91%; 2d: 93%). Aziridines bearing electron-deficient
and electron-rich aryl groups afforded 2e, 2g, and 2h in high
yields (91-98%). We observed less than 5% of regioisomer 2f’
in the case of ortho-chloro-substituted 1f. CBr4 was also
applicable to give the HBr adduct 4 in 94% yield. Next, the
scope of the methanol addition was investigated (Scheme 1b,
lower half). A series of aziridines 1b–1h furnished the
corresponding methanol adducts 3b–3h in good to high yields
(80-97%) under the optimal reaction conditions except for
ortho-chlorophenyl 1f. Although a competitive HCl addition
took place to form 2f when 1f was employed with 3.0 equiv of
addition to 1 afforded 2 in a solution of 1,4-dioxane, whereas
switching the solvent to methanol efficiently formed the
corresponding adduct 3. In this strategy, HCl is generated by
photodegradation of trichloroacetonitrile that functions as a
UV-light responsive molecular storage generator of HCl.
Further applications of this methodology will be performed in
due course.
This work was partially supported by the Japan Society for
the Promotion of Science (JSPS) through a Grant-in-Aid for
Young Scientists (B) (Grant No. JP17K14483). We gratefully
acknowledge the Nippon Shokubai Award in Synthetic Organic
Chemistry, Japan and the alumni association “Wakasatokai” of
Faculty of Engineering, Shinshu University. We are highly
thankful to Kousuke Hashimoto for preparation of the
aziridines.
trichloroacetonitrile,
a
decreased
amount
of
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There are no conflicts to declare.
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