Synthesis and reactivity of trifluoromethyl substituted oxaziridines This paper is dedicated to the memory of Professor Ludovico Ronzini for its precious contribution to the organic chemistry teaching

Article abstract of DOI:10.1016/j.tet.2013.03.048

This article reports a simple and efficient synthesis of methyl(trifluoromethyl)oxaziridines, a new family of organic oxidizing agents. A detailed study on their oxygen transfer capability with respect to styrene, thioanisole and benzyl alcohol as model substrates for the synthesis of epoxides, sulfoxides and aldehydes is described. Moreover, an oxaziridine auto-oxidation at nitrogen and/or N-benzylic carbon is also reported.

Full text of DOI:10.1016/j.tet.2013.03.048

Tetrahedron xxx (2013) 1e7
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis and reactivity of trifluoromethyl substituted oxaziridines
,
,
Serena Perrone a, Francesca Rosato a, Antonio Salomone a b, Luigino Troisi a
*
a Dipartimento di Scienze e Tecnologie Biologiche ed Ambientali, University of Salento, Via Prov.le Lecce-Monteroni, 73100 Lecce, Italy
Dipartimento di Farmacia e Scienze del Farmaco, Universita degli Studi di Bari “A. Moro”, Consorzio C.I.N.M.P.I.S., Via E. Orabona, 4, 70125 Bari, Italy
b
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
This article reports a simple and efficient synthesis of methyl(trifluoromethyl)oxaziridines, a new family
of organic oxidizing agents. A detailed study on their oxygen transfer capability with respect to styrene,
thioanisole and benzyl alcohol as model substrates for the synthesis of epoxides, sulfoxides and
aldehydes is described. Moreover, an oxaziridine auto-oxidation at nitrogen and/or N-benzylic carbon is
also reported.
Received 20 November 2012
Received in revised form 19 February 2013
Accepted 11 March 2013
Available online xxx
Ó 2013 Elsevier Ltd. All rights reserved.
This paper is dedicated to the memory of
Professor Ludovico Ronzini for its precious
contribution to the organic chemistry
teaching
Keywords:
Oxaziridine
Trifluoromethyl group
Oxidation
Epoxide
Sulfoxide
1. Introduction
usually aprotic, neutral species capable of transferring electrophilic
oxygen or nitrogen to a plethora of nucleophiles (Scheme 2).7 The
Dioxiranes
I
and oxaziridines II are small heterocyclic
predominance of one process over another can be affected by
varying the substitution pattern on nitrogen. In general, oxazir-
idines with small groups on nitrogen (H, Me) act as aminating
agents,8 whereas those N-substituted with bulky or electron-
withdrawing groups preferentially transfer the oxygen atom.9
compounds that can be prepared by oxidation of ketones and im-
ines with peroxomonosulfate1 and meta-chloroperbenzoic acid2
(m-CPBA), respectively (Scheme 1). Due to the strained three
membered ring, both heterocycles are versatile reagents in organic
synthesis and their uses have rapidly grown in the past several
years.
Scheme 1. Synthesis of dioxiranes and oxaziridines.
Scheme 2. Oxaziridines as heteroatom transfer reagents.
Dioxiranes are powerful oxygen atom donors.3 Their reactivity
ranges from the transfer of an oxygen atom to ‘unactivated’ CeH
bonds of saturated hydrocarbons,3 to epoxidations,4 to the oxida-
tion of heteroatoms containing a lone pair, such as amines5 and
sulfides.2b,6 Oxaziridines play a central role in organic chemistry as
Electron-deficient oxaziridines, such as N-sulfonyl oxaziridines,7
have been widely used in the oxidation of sulfides to sulfoxides, in
the epoxidation of alkenes and in the hydroxylation of enolates;
amines, enamines and organometallic reagents can be oxygenated
as well.
The versatility of oxaziridines extends far beyond their use as
heteroatom transfer reagents. Indeed, oxaziridines have been
* Corresponding author. Fax: þ39 0832 298732; e-mail address: luigino.troisi@
unisalento.it (L. Troisi).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.03.048
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S. Perrone et al. / Tetrahedron xxx (2013) 1e7
shown also to undergo cycloaddition reactions with a variety of
heterocumulenes, such as ketenes, ketenimines, isocyanates
and isothiocyanates, to afford a diverse set of five-membered
heterocycles.10
Moreover, recently, a highly efficient [3þ2] cycloaddition re-
action of 2-alkyl-3-aryloxaziridines with alkenes,11,12 alkynes13 and
nitriles,14 leading to isoxazolidines 1, 4-isoxazolines 2 and 2,3-
dihydro-1,2,4-oxadiazoles 3, respectively, was described (Scheme 3).
and/or b-aminoketones 5a, cef (Table 1). The distribution of
products 4e5 depends on the stereoelectronic nature of the start-
ing amine.
When benzyl-amines, such as benzylamine and 1-
phenylethanamine, are reacted with 1,1,1-trifluoropropanone in
the reaction conditions described above, they form the corre-
sponding Schiff base and a trace amount of the
b-aminoketone
(Table 1, entries 1 and 3). Purification by chromatography on silica
gel of the crude mixture afforded the (E)-methyl(trifluoromethyl)
imines 4a, c and b-aminoketones 5a, c (Table 1, entries 1 and 3). The
imine 4b was instead the only product obtained in the reaction
between C,C-diphenyl-methylamine and 1,1,1-trifluoropropanone
(Table 1, entry 2). An important characteristic of the imines 4 is
that they exist only as E isomers (verified by NOESY).17
A similar trend was found in the reaction carried out on n-
butylamine as primary amine; after a 2 h period, the imine (E)-N-
(1,1,1-trifluoropropan-2-ylidene)butan-1-amine 4d together with
small quantities of the b-aminoketone 5d were identified in the
reaction mixture, and subsequently isolated by chromatography in
96% total yield (Table 1, entry 4).
Scheme 3. [3þ2] cycloaddition reactions between oxaziridines and dipolarophiles.
In the reaction with 1,1,1-trifluoropropanone, the aromatic
amines, much less basic than aliphatic ones, provided only the
Schiff bases 4gei in 95e97% total yield (Table 1, entries 7e9).
In contrast, isopropylamine and tert-butylamine, characterized
by a high steric hindrance and thus by a poorer nucleophilicity
compared with the other aliphatic amines, led to the exclusive
It is well known that methyl(trifluoromethyl)dioxirane (TFDO)
is a more effective oxidizing agent than dimethyldioxirane (DDO).
The substitution of a methyl- with a trifluoromethyl group results
in a greater propensity of TFDO to act as an electrophilic oxidant
with respect to DDO.15 The introduction of a trifluoromethyl group
in position 3 of the oxaziridine ring could lead to similar results
increasing the reactivity of these compounds. For this reason,
several novel methyl(trifluoromethyl)oxaziridines were synthe-
sized and their reactivity as oxygen transfer reagents was studied.
Herein, we report our findings.
formation of b-aminoketones 5e and 5f, in 95% and 98% yield, re-
spectively (Table 1, entries 5e6).
A possible mechanism for the formation of the
5a, cef is proposed in Scheme 4.
b-aminoketones
2. Results and discussion
The Schiff bases 4aed, gei were readily synthesized by the di-
rect condensation of 1,1,1-trifluoropropanone and an appropriate
primary amine (Table 1).
Table 1
(1,1,1-Trifluoropropanylidene)amines synthesis
Scheme 4. Suggested mechanism for b-aminoketones formation.
We hypothesize that, among the aliphatic amines, the poor
nucleophilicity of isopropylamine and tert-butylamine (Table 1,
entries 5e6), determine the conditions for the aldol condensation
of 1,1,1-trifluoropropanone to produce a
dol product could dehydrate rather easily under the reaction con-
ditions (presence of molecular sieves) to give a -unsaturated
carbonyl compound. Due to the presence a trifluoromethyl group
on the -carbon, this conjugated alkene is a reactive Michael ac-
ceptor, characterized by a very electrophilic conjugated C]C bond.
For this reason, nucleophiles, such as the primary amine used in the
reaction, could undergo 1,4-conjugate addition (Michael addition)
b-hydroxyketone. The al-
Entry
R
Product distribution (%)a
Yield (%)b
4:5
a,b
1
2
3
4
5
6
7
8
9
CH2Ph
4a (97)
4b (100)
4c (95)
4d (96)
4e (d)
5a (3)
5b (d)
5c (5)
98
97
98
96
95
98
96
97
95
CH(Ph)2
CH(CH3)Ph
n-Butyl
i-Propyl
t-Butyl
b
5d (4)
5e (100)
5f (100)
5g (d)
5h (d)
5i (d)
4f (d)
Ph
4g (100)
4h (100)
4i (100)
with the
a,b-unsaturated carbonyl compounds to give the b-ami-
4-Cl(C6H4)
4-CH3(C6H4)
noketones 5a, cef (Scheme 4).
By monitoring the reaction over time, no trace of imine was
detected, also in the reaction carried out with an excess of amine.
For the other more nucleophilic aliphatic amines (Table 1, en-
tries 1e4) and less basic aromatic amines (Table 1, entries 7e9)
a prevalent nucleophilic attack to the ketone, could favour the
imine formation instead of promoting the aldol condensation.
Novel methyl(trifluoromethyl)oxaziridines (6aed, gei) have
been prepared from the corresponding imines according to
a method previously reported.2
a
Measured by 1H NMR spectroscopic analysis.
Isolated yields.
b
In particular, an excess of 1,1,1-trifluoropropanone (2.0 equiv)
was added to an ice cooled solution of a primary amine (1.0 equiv)
in CH2Cl2, in the presence of molecular sieves, according to Tagu-
chi’s protocol.16 The reaction mixture was stirred for 2e3 h at
0e5 ꢀC and monitored by gas chromatography. When the reaction
was complete, the molecular sieves were filtered off and the solvent
evaporated obtaining methyl(trifluoromethyl)imines 4aed, gei
In particular, when m-CPBA (1.1 equiv) was added to a solution
of 4a in 20 mL of CH2Cl2 at 0 ꢀC, after a 3 h period, GC and GCeMS
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S. Perrone et al. / Tetrahedron xxx (2013) 1e7
3
showed the total conversion of the starting imine into a new
product.
The reaction work-up was performed in a non aqueous way,
according to a modified Emmons’s procedure:2 the crude mixture
was stirred with excess powdered anhydrous sodium carbonate
with neutralization of the acid reaction mixture until carbon di-
oxide evolution ceased. After filtration, the solvent was evaporated
in vacuo at room temperature affording the oxaziridine 6a as a pure
compound (yield¼97%; Table 2, entry 1). NOESY revealed the
configuration of the oxaziridine as trans-isomer.
Scheme 5. Reactivity of oxaziridine 6a in the presence of a nucleophile, such as methyl
phenyl sulfide or styrene. No oxidation of the nucleophile was observed.
Table 2
Methyl(trifluoromethyl)oxaziridines synthesis
trifluoromethyl-oxaziridine 8 and 1,1,1-trifluoropropanone were
not identified in the mixture by 1H NMR spectroscopy, presumably
due to their high volatility.
Recently, a copper-catalysed transformation of oxaziridines to
allylic and benzylic alcohols via intramolecular CeH bond oxidation
has been described.18
Slightly different results were obtained in a reaction carried out
on oxaziridine 6b and styrene or methyl phenyl sulfide as the
electron-rich substrates. In both cases, after a 10 h period, 1H NMR
spectroscopic analysis of the mixture revealed no trace of epoxide
or sulfoxide and different molecules, such as diphenyl-methanone
oxime (10b),19 benzophenone and C,C-diphenyl-methylamine were
detected as the only reaction products (Scheme 6).
Entry
Starting material
R
Product yield (%)a
1
2
3
4
5
6
7
4a
4b
4c
4d
4g
4h
4i
CH2Ph
6a (97)
6b (96)
6c (94)
6d (93)
6g (73)
6h (77)
6i (79)
CH(Ph)2
CH(CH3)Ph
n-Butyl
Ph
4-Cl(C6H4)
4-CH3(C6H4)
a
Isolated yields.
The oxidation of the imines 4bed produced similar results
giving the methyl(trifluoromethyl)oxaziridines 6bed, in a trans-
configuration, in high yields (Table 2, entries 2e4).
Moreover, the same reactions performed with the Schiff bases
4g, 4h and 4i led to an almost complete disappearance of the
starting imine giving the trans-methyl(trifluoromethyl)oxaziridines
6g, 6h and 6i, respectively, and small quantities of different by-
products. The crude mixture was then purified by chromatogra-
phy on silica gel to obtain the (trifluoromethyl)heterocycles 6gei as
pure compounds (yield¼73e79%; Table 2, entries 5e7).
All the reactions proceeded with a high stereoselectivity and,
from each imine, only the trans-oxaziridine was generated. The
observed trans-stereoselectivity (trans:cis¼100:0) may be due to
a synchronous oxygen transfer from the m-CPBA to the imine of E-
structure.
Subsequently, the reactivity of the electron-deficient oxazir-
idines 6aed, gei towards electron-rich compounds, such as styrene
and methyl phenyl sulfide, was investigated.
Scheme 6. Reactivity of oxaziridine 6b. Auto-oxidation at the N-benzylic position
(pathway I) and at the nitrogen atom of the heterocycle (pathway II) was observed.
Oxaziridine 6a (1.0 equiv) was dissolved in CHCl3 and heated at
reflux in the presence of methyl phenyl sulfide as nucleophile
(1.0 equiv). The reaction was monitored by GC that showed the pro-
gressive decrease of 6a that, after a 10 h period, reacted completely.
Then a portion of the reaction mixture (1 mL) was concentrated
under vacuum and CDCl3 added before 1H NMR spectroscopic
analysis was performed; the spectrum showed that oxaziridine 6a
did not oxidize the sulfide, but benzaldehyde and benzylamine
(ratio 1:1), were the only products detected in the reaction mixture.
In similar experiments, performed by using styrene as nucleo-
phile or in the absence of a nucleophile, oxaziridine 6a behaved
similarly.
The production of benzaldehyde and benzylamine, identified
also by GCeMS, probably occurs via 6a auto-oxidation: oxaziridine
6a could transfer oxygen to another oxaziridine molecule at the
benzylic position to produce an unstable compound 7a and the
starting Schiff base 4a. The intermediate 7a subsequently rear-
ranges to form benzaldehyde and, presumably, 3-methyl-3-
trifluoromethyl-oxaziridine 8; imine 4a hydrolyzes generating the
benzylamine and 1,1,1-trifluoropropanone (Scheme 5). 3-Methyl-3-
The same result was achieved in a similar reaction performed
without nucleophiles.
The formation of these compounds, identified also by GCeMS,
could be explained through two different ways of 6b auto-
oxidation. Oxaziridine 6b could transfer oxygen to another oxazir-
idine molecule, at the benzylic position, to produce benzophenone
and the starting C,C-diphenylmethylamine, through a similar
mechanism described for compound 6a (Scheme 6, pathway I).
Moreover, 6b could transfer oxygen at the heterocyclic nitrogen
atom of another oxaziridine molecule to generate N-oxide 9b, as
a transient intermediate, which subsequently rearranges to the ox-
ime 10b, and the Schiff base 4b. The latter hydrolyzes giving the C,C-
diphenylmethylamine and the volatile 1,1,1-trifluoropropanone
(Scheme 6, pathway II).
Previously, the oxidation at the oxaziridine nitrogen by a peroxy
acid to give C-nitroso compounds and/or their tautomeric oximes
was described.20
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S. Perrone et al. / Tetrahedron xxx (2013) 1e7
By assessing the relative quantities of the products benzophe-
none and oxime 10b via 1H NMR spectroscopy and GC, a 2/1 ratio
between pathway I/II has been estimated.
Furthermore, the ability of 6b to transfer the electrophilic oxy-
gen to the benzyl position of other substrates was evaluated. In
particular, in the reaction with benzyl alcohol (1.0 equiv), the
oxaziridine 6b (1.0 equiv), treated under the same conditions, led to
benzaldehyde (20%) and the aforementioned products benzophe-
none, C,C-diphenyl-methylamine and oxime 10b (Scheme 7).
Scheme 9. Oxidation of methyl phenyl sulfoxide or styrene oxide by oxaziridine 6d
using CH3CN as reaction solvent.
butyraldehyde oxime 10d (Scheme 9). Probably due to its volatility,
1-butanamine was not identified by GCeMS and 1H NMR spec-
troscopic analysis of the mixture.
Oxime 10d was the only product observed when a solution of
(trifluoromethyl)oxaziridine 6d in CH3CN, without nucleophiles,
was heated at reflux.
Finally, a high thermal stability was instead observed for the
oxaziridines 6gei characterized by an aromatic group at the ni-
trogen atom. Oxaziridines 6gei heated at reflux in CHCl3, or in
CH3CN, both in the presence and in the absence of nucleophiles,
retained their structure and after a 24 h period of reflux they were
fully recovered.
Scheme 7. Oxidation of benzyl alcohol by 6b.
A solution of (trifluoromethyl)oxaziridine 6c (1.0 equiv) and
methyl phenyl sulfide or styrene (1.0 equiv) in CHCl3 was also
heated at reflux.
After a reaction time of 10 h the oxaziridine totally reacted and
GCeMS and 1H NMR spectroscopic analysis provides us with the ev-
idence for the formation of styrene oxide (conversion¼23%by1HNMR
spectroscopy) together with other compounds (1-phenylethanamine
and acetophenone oxime 10c21) resulting from auto-oxidation to 6c
N-oxide and subsequent ring-opening. According to the suggested
mechanism, oxaziridine 6c acts as an oxygenating agent both for
nucleophilic substrates and for itself at the heterocyclic nitrogen to
provide the detected compounds (Scheme 8).
3. Conclusions
In conclusion, this work shows that CF3-substituted oxaziridines
can be easily prepared and constitute a new family of organic ox-
idizing agents; some of them are able to oxidize, in mild conditions,
styrene, thioanisole and benzyl alcohol obtaining useful synthons
like styrene oxide, methyl phenyl sulfoxide and benzaldehyde. Due
to their reactivity in neutral medium, they could be used for the
oxygenation of acid-sensitive substrates. Moreover, we found a new
behaviour of oxaziridines that can undergo auto-oxidation re-
actions at the nitrogen of the ring and/or N-substituent (benzylic
carbon). Finally, their easy preparation as well as a more pro-
nounced oxidizing capability, with respect to non-fluorinated
oxaziridines, renders CF3-substituted oxaziridines promising re-
agents for asymmetric oxidation.
Scheme 8. Oxidation of styrene and methyl phenyl sulfide by oxaziridine 6c.
4. Experimental section
4.1. General methods
Similarly, by using methyl phenyl sulfide as nucleophile, methyl
phenyl sulfoxide (conversion¼44% by 1H NMR spectroscopy), 1-
phenylethanamine and oxime 10c were obtained. Oxaziridine 6c
selectively oxidize methyl phenyl sulfide to the corresponding
sulfoxide without over-oxidation to sulfone (Scheme 8).
All reactions involving air-sensitive reagents were performed
under an atmosphere of nitrogen in oven-dried glassware by using
syringe/septum cap techniques. CH2Cl2 was distilled from calcium
hydride before use. Petroleum ether refers to the 40e60 ꢀC boiling
fraction. 1,1,1-Trifluoropropanone, primary amines (phenylmethan-
amine, diphenylmethanamine, 1-phenyl-ethanamine, butan-1-
amine, propan-2-amine, 2-methylpropan-2-amine, aniline, 4-
chloroaniline, p-toluidine) m-chloroperbenzoic acid (m-CPBA), sty-
rene, thioanisole, acetonitrile, diethyl ether and chloroform were of
commercial grade (Aldrich) and used without further purification.
The 1H and the 13C NMR spectra were recorded with a Bruker
Avance 400 apparatus (400.13 and 100.62 MHz, for 1H and 13C, re-
spectively) with CDCl3 as the solvent and TMS as an internal stan-
No oxygen transfer at the benzylic position of 6c with formation
of acetophenone was observed. An increase of the steric hindrance
is believed to prevent the oxidation of 6c at the benzylic carbon.
Moreover, according to our results, a decreased ability to auto-
oxidation corresponds to an increase in the ability of the methyl(-
trifluoromethyl)oxaziridines to oxidize electron-rich substrates.
Furthermore, in a reaction performed without nucleophiles,
oxaziridine 6c, treated under the same conditions described above,
led to 1-phenylethanamine and acetophenone oxime 7c as the only
detected products (conversion>90% by 1H NMR spectroscopy).
No reaction product was instead found in a reaction carried out
between 2-butyl-3-methyl-3-trifluoromethyl-1,2-oxaziridine 6d
(1.0 equiv) and methyl phenyl sulfide or styrene (1.0 equiv) in CHCl3
at reflux. However, as shown in Scheme 9, the same reaction per-
formed in CH3CN as solvent, at reflux, after 10 h, resulted in the
oxidation of the nucleophile and in a 6d auto-oxidation at the
heterocyclic nitrogen. In particular, 1H NMR spectroscopic analysis
of the reaction mixture showed the presence of methyl phenyl
sulfoxide or styrene oxide in 59% or 26% yield, respectively, and
dard (
d
¼7.26 ppm for 1H spectra;
d
¼77.0 ppm for 13C spectra). 19
F
spectra were recorded in CDCl3 on Bruker Avance 600 spectrometer
(564.68 MHz) using trichlorofluoromethane as an internal standard.
The IR spectra were recorded with an FTIR spectrophotometer
Digilab Scimitar Series FTS 2000. Gas chromatography (GC) was
conducted on an Rtx-5 30-m fused silica capillary column (split ratio
100:1). The following program was used: method A¼initial tem-
perature of 100 ꢀC for 0.0 min, ramp 10 ꢀC/min to 280 ꢀC, and hold
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S. Perrone et al. / Tetrahedron xxx (2013) 1e7
5
for 15 min; the standard operating conditions were 300 ꢀC injector
temperature and 290 ꢀC detector temperature. GCeMS analyzes,
conducted using method A temperature programme , were per-
formed with an Agilent Technologies 6850 series II gas chromato-
graph (5% phenylpolymethylsiloxane capillary column, 30 m,
0.25 mm i.d.), equipped with a 5973 Network mass-selective de-
tector operating at 70 eV. The electrospray ionization [HRMS (ESI)]
experiments were carried out in a hybrid QqTOF mass spectrometer
(PE SCIEXeQSTAR) equipped with an ion-spray ionisation source.
1645, 1212 cmꢁ1. HRMS (ESI): calcd for C7H13F3N: (168.1001)
[MþH]þ; found: (168.1003).
4.2.3. (E)-4-Chloro-N-(1,1,1-Trifluoropropan-2-ylidene)aniline
(4h). Yield: 214 mg (97%), pale yellow oil. 1H NMR (400.13 MHz,
CDCl3)
d
¼2.02 (3H, s, CH3), 6.73 (2H, d, J 8.5 Hz, Ph), 7.35 (2H, d, J
8.5 Hz, Ph) ppm. 13C NMR (100.62 MHz, CDCl3)
d
¼14.4 (CH3), 119.7
(q, J 278.5 Hz, CF3),118.2,129.3,130.7,146.1,158.1 (q, J 34.7 Hz, C]N)
ppm. 19F NMR (564.68 MHz, CDCl3)¼ꢁ74.6 (s) ppm. GCeMS
(70 eV): m/z (%)¼221 (36) [M]þ, 152 (100), 111 (90), 75 (62). FTIR
(CHCl3)¼3030, 2960, 2860, 1640, 1213 cm-1. HRMS (ESI): calcd for
C9H385ClF3N: (222.0297) [MþH]þ; found: (222.0295).
MS (þ) spectra were acquired by direct infusion (5
m
L minꢁ1) of
a solution containing the appropriate sample (10 pmol
mL
ꢁ1) dis-
solved in a solution 0.1% acetic acid, methanol/water (50:50) at the
optimum ion voltage of 4800 V. The nitrogen gas flow was set at
30 psi (pounds per square inch) and the potentials of the orifice, the
focussing ring and the skimmer were kept at 30, 50 and 25 V relative
to ground, respectively. Melting points were determined with an
Electrothermal melting point apparatus and are uncorrected. TLC
was performed on Merck silica gel plates with F-254 indicator;
viewing was by UV light (254 nm) or p-anisaldehyde and phos-
phomolybdic acid staining solutions. Column chromatographies
were performed on silica gel (63e200 mm) using petroleum ether/
diethyl ether (Et2O) mixture as eluent.
4.2.4. 4-(Benzylamino)-1,1,1,5,5,5-hexafluoro-4-methylpentan-2-one
(5a). Yield: 9 mg (3%), pale yellow oil. 1H NMR (400.13 MHz, CDCl3)
d
¼1.38 (3H, s, CeCH3), 2.77 (1H, d, J 17.5 Hz, CHH), 3.18 (1H, d, J
17.5 Hz, CHH), 3.50 (1H, s, broad, NH). 4.52 (2H, s, CH2ePh), 7.22e7.33
(5H, m, Ph) ppm.13C NMR (100.62 MHz, CDCl3)
¼20.0 (CH3eC), 40.1
d
(CH2ePh), 46.2 (CH2eCO), 69.9 (q, J 30.2 Hz, CeCF3), 115.1 (q, J
292.7 Hz, CeCF3), 125.1 (q, J 286.3 Hz, OCeCF3), 127.5, 128.5, 129.0,
140.3, 189.1 (q, J 37.5 Hz, C]O) ppm. 19F NMR (564.68 MHz,
CDCl3)¼ꢁ81.1 (s), ꢁ85.1 (s) ppm. FTIR (CHCl3)¼3332, 3060, 3030,
2964, 2930, 2875, 1765, 1600, 1589, 1272, 1224, 1170 cmꢁ1. HRMS
(ESI): calcd for C13H14F6NO: (314.0979) [MþH]þ; found: (314.0978).
4.2. General procedure for the synthesis of methyl(tri-
fluoromethyl)imines 4aed, gei and b-aminoketones 5a, cef
4.2.5. 1,1,1,5,5,5-Hexafluoro-4-methyl-4-[(1-phenylethyl)amino]-
pentan-2-one (5c). Yield: 16 mg (5%), pale yellow oil. 1H NMR
An excess of 1,1,1-trifluoropropanone (2.0 mmol) was added to
an ice cooled of the primary amine (1.0 mmol) in anhydrous CH2Cl2
(400.13 MHz, CDCl3)
d
¼1.38 (3H, d, J 8.2 Hz, CHeCH3), 1.51 (3H, s,
ꢁ
(20 mL) in the presence of 7 g of molecular sieves (4 A, 1.6 mm
CeCH3), 2.80 (1H, d, J 17.5 Hz, CHH), 3.22 (1H, d, J 17.5 Hz, CHH), 3.40
(1H, s, broad, NH), 4.08 (1H, q, J 8.2 Hz, CHeCH3), 7.22e7.35 (5H, m, Ph)
pellets) according to Taguchi’s method.16 The reaction was taken
under magnetic stirring and cooling (0e5 ꢀC) for 2e3 h and mon-
itored by GC. After this time, the molecular sieves were filtered and
the solvent was removed in vacuo to give a crude material.
ppm. 13C NMR (100.62 MHz, CDCl3)
d¼20.9 (CH3eC), 21.2 (CHeCH3),
40.0 (CHeCH3), 51.5 (CH2eCO), 72.4 (q, J 30.1 Hz, CeCF3), 114.0 (q, J
289.7.7 Hz, CeCF3), 125.0 (q, J 288.3 Hz, OCeCF3), 127.8, 128.3, 128.4,
143.7, 193.6 (q, J 37.4 Hz, C]O) ppm. 19F NMR (564.68 MHz, CDCl3)¼ꢁ
81.2 (s), ꢁ84.9 (s) ppm. FTIR (CHCl3)¼3331, 3062, 3030, 2965, 2933,
2876, 1763, 1600, 1589, 1271, 1224, 1172 cmꢁ1. HRMS (ESI): calcd for
By using this procedure, the imines 4b, gei and the b-amino-
ketones 5eef were obtained directly as pure products. For the re-
actions shown in the entries 1, 3e4 of Table 1, the crude material
was than purified by flash chromatography (silica gel, eluent pe-
troleum ether/diethyl ether 70:30) to afford, for each reaction, the
C
14H16F6NO: (328.1136) [MþH]þ; found: (328.1138).
corresponding imine 4a, ced and the
b
-aminoketone 5a, ced as
4.2.6. 4-(Butylamino)-1,1,1,5,5,5-hexafluoro-4-methylpentan-2-one
(5d). Yield: 11 mg (4%), pale yellow oil 1H NMR (400.13 MHz,
pure compounds.
The compounds 4a,22,23 4c,17 4g,24 4i,24 are known and their
CDCl3)
d¼0.95 (3H, t, J 7.3 Hz, CH2eCH3), 1.38 (2H, sext, J 7.3 Hz,
characterization data are in agreement with those reported in lit-
CH2eCH3), 1.55 (3H, s, CeCH3), 1.67 (2H, quintet, J 7.3 Hz,
CH2eCH2eCH2), 2.91 (1H, d, J 17.6 Hz CHH), 3.27 (1H, d, J 17.6 Hz
CHH), 3.44 (2H, t, J 7.3 Hz, CH2eN) ppm. 13C NMR (100.62 MHz,
erature. The imines 4b, 4d, 4h and
b-aminoketones 5a, cef are
unknown and their characterization is reported below.
CDCl3)
d
¼13.1 (CH2eCH3), 14.2 (CH3eC), 20.5 (CH2eCH3), 31.0
4.2.1. (E)-1,1-Diphenyl-N-(1,1,1-trifluoropropan-2-ylidene)methan-
amine (4b). Yield: 268 mg (97%), white solid with mp 64.1e65.5 ꢀC.
(CH2eCH2eCH2), 40.1 (CH2eCO), 51.4 (CH2eN), 72.1 (q, J 30.2 Hz,
CeCF3), 117.1 (q, J 290.8 Hz, CeCF3), 127.0 (q, J 289.5 Hz, OCeCF3),
190.1 (q, J 39.4 Hz, C]O) ppm. 19F NMR (564.68 MHz, CDCl3)¼ꢁ81.0
(s), ꢁ85.1 (s) ppm. FTIR (CHCl3)¼3332, 2964, 2930, 2872,1765,1587,
1H NMR (400.13 MHz, CDCl3)
d
¼1.98 (3H, s, CH3), 5.69 (1H, s, CH), 7.18
(2H, t, J 7.7 Hz, Ph), 7.26 (4H, t, J 7.7 Hz, Ph), 7.35 (4H, d, J 7.7 Hz, Ph)
ppm.13C NMR (100.62 MHz, CDCl3)
d
¼13.0 (CH3), 68.0 (CH),120.0 (q, J
1466, 1381, 1270, 1226, 1170 cmꢁ1
. HRMS (ESI): calcd for
278.6 Hz, CF3),127.1,127.2,128.6,142.6,155.8 (q, J 33.3 Hz, C]N) ppm.
19F NMR (564.68 MHz, CDCl3)¼ꢁ74.5 (s) ppm. GCeMS (70 eV): m/z
(%)¼277 (5) [M]þ, 198 (8), 167 (100), 152 (28). FTIR (CHCl3)¼3332,
2920, 2850, 1640, 1210 cmꢁ1. HRMS (ESI): calcd for C16H15F3N:
(278.1156) [MþH]þ; found: (278.1154).
C
10H16F6NO: (280.1136) [MþH]þ; found: (280.1134).
4.2.7. 1,1,1,5,5,5-Hexafluoro-4-(isopropylamino)-4-methylpentan-2-
one (5e). Yield: 252 mg (95%), pale yellow oil. 1H NMR (400.13 MHz,
CDCl3)
(1H, d, J 17.5 Hz, CHH), 3.26 (1H, d, J 17.5 Hz, CHH), 3.78 (1H, septet, J
6.3 Hz, CHe(CH3)2) ppm. 13C NMR (100.62 MHz, CDCl3)
(CH3eC), 22.6 (CHe(CH3)2), 40.0 (CH2), 51.5 (CHe(CH3)2), 72.7 (q, J
30.1 Hz, CeCF3), 114.8 (q, J 294.7 Hz, CeCF3), 125.1 (q, J 285.3 Hz,
OCeCF3), 188.8 (q, J 38.6 Hz, C]O) ppm. 19F NMR (564.68 MHz,
CDCl3)¼ꢁ81.0 (s), ꢁ85.0 (s) ppm. FTIR (CHCl3)¼3330, 2965, 2930,
2873, 1764, 1589, 1466, 1381, 1272, 1224, 1170 cmꢁ1. HRMS (ESI):
calcd for C9H14F6NO: (266.0979) [MþH]þ; found: (266.0978).
d
¼1.19 (6H, d, J 6.3 Hz, CHe(CH3)2), 1.55 (3H, s, CeCH3), 2.90
4.2.2. (E)-N-(1,1,1-Trifluoropropan-2-ylidene)-butan-1-amine
(4d). Yield: 154 mg (92%), colourless oil. 1H NMR (400.13 MHz,
d¼20.9
CDCl3)
CH2eCH3), 1.67 (2H, quintet, J 7.3 Hz, CH2eCH2eCH2), 2.0 (3H, s,
CH3eC]N), 3.44 (2H, t,
7.3 Hz, CH2eN) ppm. 13C NMR
(100.62 MHz, CDCl3)
¼12.3 (CH3eC]N), 13.7 (CH3eCH2), 20.5
d¼0.95 (3H, t, J 7.3 Hz, CH2eCH3), 1.38 (2H, sext, J 7.3 Hz,
J
d
(CH3eCH2), 31.9 (CH2eCH2eCH2), 51.4 (CH2eN),120.0 (q, J 279.0 Hz,
CF3), 156.0 (q, J 34.0 Hz, C]N) ppm. 19F NMR (564.68 MHz,
CDCl3)¼ꢁ79.1 (s) ppm. GCeMS (70 eV): m/z (%)¼167 (3) [M]þ, 152
(13), 138 (7), 124 (92), 98 (100). FTIR (CHCl3)¼2960, 2930, 2860,
4.2.8. 4-(tert-Butylamino)-1,1,1,5,5,5-hexafluoro-4-methylpentan-2-
one (5f). Yield: 273 mg (98%), colourless oil. 1H NMR (400.13 MHz,
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6
S. Perrone et al. / Tetrahedron xxx (2013) 1e7
CDCl3)
d
¼1.22 (9H, s, Ce(CH3)3), 1.55 (3H, s, CeCH3), 2.90 (1H, d, J
CH2eCH3), 1.65 (2H, quintet, J 7.3 Hz, CH2eCH2eCH2), 1.67 (3H, s,
CH3eCN), 2.74e2.81 (1H, m, NCeHH), 2.85e2.92 (1H, m, NCeCHH)
17.5 Hz, CHH), 3.26 (1H, d, J 17.5 Hz, CHH) ppm. 13C NMR
(100.62 MHz, CDCl3)
d¼20.6 (CH3), 30.5 (Ce(CH3)3), 40.2 (CH2), 49.0
ppm. 13C NMR (100.62 MHz, CDCl3)
d¼9.6 (CH3eCN), 13.7
(Ce(CH3)3), 72.5 (q, J 30.2 Hz, CeCF3), 114.9 (q, J 292.0 Hz, CeCF3),
125.2 (q, J 285.0 Hz, OCeCF3), 188.8 (q, J 36.7 Hz, C]O) ppm. 19F
NMR (564.68 MHz, CDCl3)¼ꢁ81.1 (s), ꢁ85.1 (s) ppm. FTIR (CHCl3)¼
(CH3eCH2), 20.3 (CH3eCH2), 29.9 (CH2eCH2eCH2), 53.9 (CH2eN),
78.8 (q, J 36.5 Hz, CeO), 122.2 (q, J 276.8 Hz, CF3) ppm. 19F NMR
(564.68 MHz, CDCl3)¼ꢁ82.5 (s) ppm. FTIR (CHCl3)¼2962, 2934,
2875, 1335, 1209, 1166, 1110, 880 cmꢁ1. HRMS (ESI): calcd for
C7H13F3NO: (184.0949) [MþH]þ; found: (184.0947).
3337, 2965, 2873, 1765, 1588, 1466, 1380, 1273, 1224, 1172 cmꢁ1
.
HRMS (ESI): calcd for C10H16F6NO: (280.1136) [MþH]þ; found:
(280.1138).
4.3.5. 3-Methyl-2-phenyl-3-(trifluoromethyl)-1,2-oxaziridine
(6g). Yield: 148 mg (73%), yellow oil. 1H NMR (400.13 MHz, CDCl3)
4.3. General procedure for the synthesis of oxaziridines 6aed,
gei
d
¼3.35 (3H, s, CH3), 7.24e7.27 (2H, m, Ph), 7.41e7.45 (3H, m, Ph)
ppm. 13C NMR (100.62 MHz, CDCl3)
¼17.9 (CH3), 96.1 (q, J 32.1 Hz,
CeO), 110.0 (q, J 286.0 Hz, CF3), 127.3, 129.0, 129.5, 140.6 ppm. 19
d
A small excess of m-CPBA (270 mg, 1.1 mmol, 70%) was added to
an ice cooled solution of the methyl(trifluoromethyl)-imines 4aed,
gei (1.0 mmol) in anhydrous CH2Cl2 (20 mL). The reaction was
taken under magnetic stirring and cooling (0e5 ꢀC) for about 3 h
and monitored by GC and GCeMS. When the reaction was com-
plete, the solution was stirred at 0e5 ꢀC with excess powdered
anhydrous sodium carbonate until carbon dioxide evolution
ceased. After filtration, the mixture was concentrated in vacuo
affording the methyl(trifluoromethyl)oxaziridines 6aed as pure
compounds; in particular, oxaziridine 6c was obtained as only one
diastereoisomer.
For the oxaziridines 6gei, after filtration and evaporation of the
solvent in vacuo, the residue was purified by flash chromatography
(silica gel, partly deactivated with triethyl-amine, eluent petroleum
ether/diethyl ether 95:5 for 6g and 6i; petroleum ether/diethyl
ether 90:10 for 6h).
F
NMR (564.68 MHz, CDCl3)¼ꢁ72.1 (s) ppm. GCeMS (70 eV): m/z
(%)¼203 (100) [M]þ, 134 (48), 106 (75), 77 (90). FTIR (CHCl3)¼3027,
2955, 2927, 2850, 1490, 1210, 1161, 887 cmꢁ1. HRMS (ESI): calcd for
C9H9F3NO: (204.0636) [MþH]þ; found: (204.0639).
4.3.6. 2-(4-Chlorophenyl)-3-methyl-3-(trifluoromethyl)-1,2-
oxaziridine (6h). Yield: 182 mg (77%), yellow oil. 1H NMR
(400.13 MHz, CDCl3)
d
¼3.34 (3H, s, CH3), 7.19 (2H, d, J 8.5 Hz, Ph),
7.41 (2H, d, J 8.5 Hz, Ph) ppm. 13C NMR (100.62 MHz, CDCl3)
d¼20.3
(CH3), 97.4 (q, J 31.9 Hz, CeO), 116.0 (q, J 285.9 Hz, CF3), 128.8, 129.8,
135.0, 139.1 ppm. 19F NMR (564.68 MHz, CDCl3)¼ꢁ67.5 (s) ppm.
GCeMS (70 eV): m/z (%)¼237 (90) [M]þ, 168 (25), 140 (75), 110
(100). FTIR (CHCl3)¼3027, 2958, 2927, 2853, 1489, 1211, 1161,
888 cmꢁ1. HRMS (ESI): calcd for C9H835ClF3NO: (238.0246) [MþH]þ;
found: (238.0245).
4.3.1. 2-Benzyl-3-methyl-3-(trifluoromethyl)-1,2-oxaziridine
(6a). Yield: 210 mg (97%), yellow oil. 1H NMR (400.13 MHz, CDCl3)
4.3.7. 3-Methyl-2-(p-tolyl)-3-(trifluoromethyl)-1,2-oxaziridine
(6i). Yield: 171 mg (79%), orange oil. 1H NMR (400.13 MHz, CDCl3)
d
¼1.79 (3H, s, CH3), 3.96 (2H, s, CH2ePh), 7.28e7.38 (5H, m, Ph)
d
¼2.37 (3H, s, CH3ePh), 3.33 (3H, s, CH3eCO), 7.10 (2H, d, J 8.2 Hz,
ppm. 13C NMR (100.62 MHz, CDCl3)
d¼9.8 (CH3), 57.9 (CH2), 79.1 (q,
Ph), 7.20 (2H, d, J 8.2 Hz, Ph) ppm. 13C NMR (100.62 MHz, CDCl3)
J 36.6 Hz, CeO). 122.2 (q, J 280.9 Hz, CF3), 128.0, 129.3, 128.7,
134.8 ppm. GCeMS (70 eV): m/z (%)¼217 (45) [M]þ, 202 (15), 148
(21), 91 (100). 19F NMR (564.68 MHz, CDCl3)¼ꢁ80.2 (s) ppm. FTIR
(CHCl3)¼3027, 2962, 2930, 2870, 1335, 1212, 1160 cmꢁ1. HRMS
(ESI): calcd for C10H11F3NO: (218.0792) [MþH]þ; found: (218.0793).
d
¼15.2 (CH3eCO), 18.0 (CH3ePh), 95.5 (q, J 32.1 Hz, CeO), 108.1 (q, J
286.1 Hz, CF3), 127.1, 130.1, 132.4, 139.0 ppm. 19F NMR (564.68 MHz,
CDCl3)¼ꢁ74.2 (s) ppm. GCeMS (70 eV): m/z (%)¼217 (50) [M]þ, 198
(5), 148 (100). FTIR (CHCl3)¼3028, 2960, 2927, 2855, 1490, 1212,
1161, 890 cmꢁ1. HRMS (ESI): calcd for C10H11F3NO: (218.0792)
[MþH]þ; found: (218.0794).
4.3.2. 2-Benzhydryl-3-methyl-3-(trifluoromethyl)-1,2-oxaziridine
(6b). Yield: 281 mg (96%), orange oil. 1H NMR (400.13 MHz, CDCl3)
4.4. General procedure for the oxidation reactions using ox-
aziridines 6aed, gei
d
¼1.68 (3H, s, CH3), 4.64 (1H, s, CH), 7.20e7.35 (8H, m, Ph), 7.44 (2H,
d, J 7.7 Hz, Ph) ppm. 13C NMR (100.62 MHz, CDCl3)
d¼10.3 (CH3),
70.7 (CH), 79.7 (q, J 36.8 Hz, CeO). 122.2 (q, J 281.2 Hz, CF3), 128.2,
128.5, 128.9, 138.4 ppm. 19F NMR (564.68 MHz, CDCl3)¼ꢁ81.3 (s)
ppm. GCeMS (70 eV): m/z (%)¼293 (27) [M]þ, 292 (35), 180 (100),
165 (60). FTIR (CHCl3)¼3027, 2960, 2935, 2868, 1335, 1210,
1162 cmꢁ1. HRMS (ESI): calcd for C16H15F3NO: (294.1105) [MþH]þ;
found: (294.1103).
A solution of the methyl(trifluoromethyl)oxaziridine (1.0 mmol)
in CHCl3 (20 mL) was heated at reflux in the presence of a nucleo-
phile (1.0 mmol), such as styrene or thioanisole. The reaction was
taken under magnetic stirring and heating for a 10 h period and
monitored by TLC and GC. When it was complete, reaction products
were identified by GCeMS and 1H NMR spectroscopic analysis by
comparison with literature characterization data.
Auto-oxidation reactions were performed by heating a solution
of the oxaziridine (1.0 equiv) in CHCl3 (20 mL) at reflux for 10 h.
The reactions involving oxaziridines 6d, gei were carried out
using also CH3CN (20 mL) as reaction solvent.
4.3.3. 3-Methyl-2-(1-phenylethyl)-3-(trifluoromethyl)-1,2-
oxaziridine (6c). Yield: 217 mg (94%), yellow oil. 1H NMR
(400.13 MHz, CDCl3)
CH3eCO), 3.63 (1H, q, J 6.4 Hz, CHeCH3), 7.27e7.36 (5H, m, Ph)
ppm. 13C NMR (100.62 MHz, CDCl3)
d¼1.60 (3H, d, J 6.4 Hz, CH3eCH), 1.63 (3H, s,
d¼9.9 (CH3eCO), 23.1
(CH3eCH), 63.1 (CHeCH3), 79.6 (q, J 36.3 Hz, CH3CeO). 122.1 (q, J
281.0 Hz, CF3), 126.6, 128.1, 128.9, 139.6 ppm. 19F NMR (564.68 MHz,
CDCl3)¼ꢁ82.1 (s) ppm. GCeMS (70 eV): m/z (%)¼231 (<1) [M]þ,
120 (25), 105 (100), 77 (70). FTIR (CHCl3)¼3027, 2959, 2930, 2871,
Acknowledgements
Thanks are due to the University of Salento and C.I.N.M.P.I.S.
(Consorzio Interuniversitario Nazionale Metodologie e Processi
Innovativi di Sintesi) for financial support.
1334, 1210, 1160 cmꢁ1
. HRMS (ESI): calcd for C11H13F3NO:
(232.0949) [MþH]þ; found: (232.0948).
References and notes
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Please cite this article in press as: Perrone, S.; et al., Tetrahedron (2013), http://dx.doi.org/10.1016/j.tet.2013.03.048