1
02
W. B. Jennings et al. / Tetrahedron Letters 42 (2001) 101–103
1
3
pounds is a quaternary C signal in the region l 82–83
typical of an oxaziridine ring carbon but split into a
doublet of quartets by coupling to both phosphorus
aziridine oxygen, with synchronous cleavage of the
4
NꢀO and CꢀO bonds. The imine H CꢁNH is effec-
2
tively the leaving group. There is considerable lengthen-
ing of the NꢀO and CꢀO bonds in the calculated
transition state. The present results appear to be consis-
3
2
and fluorine ( J 5.0±0.5 Hz; J 38.0±0.5 Hz).
CP
CF
tent with this mechanism as a CF group on the ring
carbon could inductively enhance the electrophilicity of
the adjacent oxygen and aid cleavage of the CꢀO bond.
3
Table 1. Second order rate constants (k) for the oxidation
a
of methyl phenyl sulphoxide by oxaziridines at 35.3°C
Compd R1
R2
k (l mol−1 s−1
)
Relative rate
5
N-Sulphonyloxaziridines, introduced by Davis, are
−
−
−
−
−
−
3
4
4
6
6
5
1
1
1
2
2
3
a
b
c
a
b
CF3
CF3
CH3
4-FC H
3.1×10
9.0×10
6.7×10
3.8×10
816
237
176
1
0.58
6.9
finding application in organic synthesis as mild aprotic
electrophilic oxidants. Compounds 1, especially 1a, may
offer a more reactive option, readily available from
inexpensive starting materials. A comparison of the
reactivity of N-phosphinoyloxaziridine 2a with the cor-
6
4
CF3
C H
6
5
CH3 CH3
CH3 1-Naphthyl 2.2×10
2.6×10
CH3
CH3
6
responding N-sulphonyloxaziridine 3 indicates that the
a
1
latter is somewhat more reactive by a factor of about
Reactions were monitored by H NMR using 0.143 M solutions of
7
:1 (Table 1); however, the new trifluoromethyl substi-
methyl phenyl sulphoxide in CDCl3.
tuted N-phosphinoyloxaziridines 1a–c are much more
7
reactive than 3.
The oxidising power of these new fluorine-containing
N-phosphinoyloxaziridines relative to the 3,3-dimethyl
and 3-methyl-3-naphthyl oxaziridines 2a and 2b was
assessed by kinetic studies of the representative oxida-
tion of methyl phenyl sulphoxide to the sulphone
(
Table 1). The results in Table 1 show that the trifl-
uoromethyl substituted oxaziridines 1a–c are much
more active oxidants than the standard N-phosphinoyl-
oxaziridines 2a and 2b. The most reactive compound 1a
derived from trifluoroacetone shows a remarkable rate
enhancement of about three orders of magnitude over
the non-fluorinated analogues 2a and 2b. A comparison
of oxaziridine 2a with 2b, and 1a with 1b or 1c,
indicates that the replacement of a 3-methyl group by a
Investigations are underway to explore the synthetic
applications of these new N-phosphinoyl-oxaziridines.
Initial results using the most reactive oxaziridine 1a
indicate that in addition to converting sulphides to
sulphoxides and sulphoxides to sulphones, alkenes can
be epoxidised and some alcohols can even be oxidised
8
to ketones. These applications will be reported in due
course.
3
-aryl group causes a modest reduction in the rate of
oxygen transfer.
Acknowledgements
We thank Enterprise Ireland for supporting this work
in the form of a research grant (no. SC/97/502).
The marked effect of the 3-trifluoromethyl group para-
llels the enhanced oxidising ability of methyl(tri-
fluoromethyl)dioxirane over dimethyldioxirane. Com-
3
References
pounds 1a–c are still much weaker oxidants than dioxi-
ranes, but they have the advantage of being reasonably
stable off-the-shelf crystalline reagents which can be
readily prepared from simple commercially available
precursors. The initial by-product of oxidations using
oxaziridines 1a and 2a is the N-diphenylphosphinoyl
imine which can be hydrolysed to the highly volatile
ketone and the relatively insoluble diphenylphosphinic
amide, Ph P(O)NH .
1. Cook, S. D.; Hamor, T. A.; Jennings, W. B.; Tebbutt, A.
A.; Watson, S. P.; Boyd, D. R. J. Chem. Soc., Perkin
Trans. 2 1991, 1281.
2. The experimental procedure followed was similar to that
reported in Ref. 1. Typically chlorodiphenylphosphine
(2.21 g, 10.0 mmol) in dry dichloromethane (30 ml) was
added to a solution of the oxime (10.0 mol) and triethy-
lamine (10.1 mmol) in dry hexane (90 ml) cooled to ca.
2
2
−
50°C and under nitrogen. The temperature was then
The similar reactivity of comparable N-phosphinoyl-
and N-sulphonyl-oxaziridines suggests a similar mecha-
nism of oxygen transfer. Model ab initio MO calcula-
allowed to slowly rise to ca 5°C over 1–2 h. The solution
containing the imine was then rapidly filtered to remove
the precipitated triethylamine hydrochloride, and immedi-
ately added to a stirred suspension of the 2:1 KF/MCPBA
complex, prepared from activated anhydrous KF (4.7 g)
tions on the hypothetical oxidation of sulphoxide H SO
2
by the unsubstituted oxaziridine H C[O]NH indicate
2
that the oxygen transfer process can be regarded as a
nucleophilic attack by sulphur on the electrophilic ox-
and a MgSO dried solution of commercial 70% MCPBA
4
(10.0 g), in 75 ml dichloromethane. (Camps, F.; Coll, J.;
Jennings, W.Brian
