4042 J. Am. Chem. Soc., Vol. 122, No. 17, 2000
Yang et al.
trans/cis
Table 5. Diastereoselective Epoxidation Reactions of
1,3-Dimethylcyclohexene (16)a
time convn yield
reaction system
(h)
(%)
(%) epoxide ratio
Cb
Cb/BTEACc
60
24
54
27
40
89
81
83
2.8:1
4.3:1
4.3:1
Figure 2. 1H NMR spectrum (300 MHz) of the mixture of Chloram-
ine-M (0.11 mmol), benzaldehyde (0.4 mmol), and BTEAC (0.1 mmol)
in CD3CN (2 mL) at room temperature after 15 h.
Chloramine-M/PhCHO/BTEACd 24
a Reactions were carried out with 0.3 mmol of olefin in 6 mL of
anhydrous acetonitrile at room temperature. All the experimental data
were obtained by GC analyses. b 1.1 equiv. c 1 equiv. d 1.1 equiv of
Chloramine-M, 4 equiv of PhCHO, and 1 equiv of BTEAC.
the latter two reaction systems provided identical diastereose-
lectivity in epoxidation of 1,3-dimethylcyclohexene, suggesting
that they may share the same epoxidizing intermediate despite
the lower conversion in the C/BTEAC system. trans-N-
Sulfonyloxaziridine C gave a lower diastereoselectivitiy, which
further supports the conclusion that C is not the oxidizing agent
in our in situ epoxidation system.
On the basis of these results, we reasoned that BTEAC may
have two functions. As a phase transfer reagent, BTEAC
improves the solubility of Chloramine-M in CH3CN. More
importantly, chloride anion introduced by BTEAC plays a
crucial role in the equilibrium depicted in Scheme 2. Chloride
anion can attack the trans-N-sulfonyloxaziridine C at the
nitrogen site, yielding conformer A.15 By the pyramidal inver-
sion of the nitrogen atom of A, conformer B is formed, although
it is less favored than A. After intramolecular SN2 displacement,
cis-N-sulfonyloxaziridine D is generated. D may have higher
reactivity than the corresponding trans-isomer C due to the
relatively large steric interactions between the phenyl group and
the methylsulfonyl group. Therefore, according to the Curtin-
Hammett principle,16 the reaction may proceed through the
minor intermediate D if only D can epoxidize trans-stilbene.17
There are several examples in the literature that cis-2-alkyl-3-
phenyloxaziridines showed greater reactivity than the corre-
sponding trans-isomers when utilized to oxidize nucleophilic
reagents such as amines, sulfides, PhSH, PhSeH, Ph3P, and Ph3-
As.18
Figure 3. 1H NMR spectrum (300 MHz) of the reaction mixture of
trans-stilbene (0.1 mmol), trans-N-sulfonyloxaziridine C (1.1 equiv),
and BTEAC (1 equiv) in CD3CN (2 mL) at room temperature after
72 h.
temperature (Figure 2). However, N-sulfonyloxaziridines C and
1
D were not detected by H NMR since no peak was found in
the range of 5-6 ppm, which is the range at which the signals
for benzylic protons of 3-phenyl-N-sulfonyloxaziridines usually
appear.
We then tried to detect the reaction intermediate by using
both electrospray ionization (ESI) and atmospheric pressure
chemical ionization (APCI) mass spectrometry techniques in
the negative-ion mode. The mass spectra patterns (peaks at m/z
198 and 199) for the reaction mixture of Chloramine-M (0.11
mmol) and benzaldehyde (0.4 mmol) in 2 mL of CH3CN, with
or without BTEAC (0.1 mmol), were quite similar to that of
trans-N-sulfonyloxaziridine C (molecular weight 199).9 This
suggests that N-sulfonyloxaziridines C and D could be the
reaction intermediates.
Epoxidation of trans-stilbene using 1.1 equiv of trans-N-
sulfonyloxaziridine C12 was then examined in CD3CN at room
temperature.9 trans-N-Sulfonyloxaziridine C was not destroyed
after 72 h, and the yield of trans-stilbene oxide was negligible
(less than 2% by 1H NMR). Therefore, the possibility of trans-
N-sulfonyloxaziridine C as the oxidant for in situ epoxidation
of trans-stilbene is excluded.
To the best of our knowledge, there is no literature report on
the isolation of cis-N-sulfonyloxaziridine D and its reactivity.
Our attempt to isolate D also met with failure, which prompted
us to investigate the energy difference between C and D.
Theoretical calculations using the Gaussian 94 program19
(15) It is also possible that chloride ion attacks the N-sulfonyloxaziridines
generated in situ at the oxygen site, yielding hypochlorite ion and
N-sulfonimine (MeSO2NdCHPh). This could not be the major pathway
since ClO- has been excluded as the epoxidizing agent (vide supra) and
no sulfonimine was detected in Figure 2.
(16) Carey, F. A.; Sundberg, R. J. AdVanced Organic Chemistry;
Plenum: New York, 1993; Part A, Chapter 4, pp 215-216.
Most interestingly, trans-N-sulfonyloxaziridine C (1.1 equiv)
together with BTEAC (1 equiv) did epoxidize trans-stilbene at
room temperature in CD3CN (Figure 3), and trans-stilbene oxide
was isolated in 10% yield with 13% conversion after 72 h. The
low conversion is probably because trans-C could oxidize
chloride ion into hypochlorite ion and N-sulfonimine (MeSO2Nd
CHPh) as a side reaction. This is supported by the fact that the
amount of N-sulfonimine was higher than that of trans-stilbene
oxide in Figure 3. It was also found that, even in the absence
of trans-stilbene, trans-N-sulfonyloxaziridine C was completely
destroyed by BTEAC within 25 min and benzaldehyde together
(17) In principle, D could also be decomposed by BTEAC, similar to
the case of C. Thus, there are two competing pathways for D: reaction
with Cl- and epoxidation. The former pathway mainly gives back B but
the latter provides the epoxide products. As long as the equilibrium between
C and D is set up and D is kinetically competent, according to the Curtin-
Hammett principle, epoxide products can be formed. Under our in situ
reaction conditions, the concentrations of C and D at equilibrium were low,
which explains why epoxidation reactions were generally slow.
(18) (a) Hata, Y.; Watanabe, M. J. Org. Chem. 1981, 46, 610-614. (b)
Hata, Y.; Watanabe, M. J. Am. Chem. Soc. 1979, 101, 6671-6676.
(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Peterson, G.
A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,
V. G.; Oriz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-
Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94; Gaussian, Inc.:
Pittsburgh, PA, 1995.
1
with N-sulfonimine (CH3SO2NdCHPh) were detected by H
NMR.9
To further probe the reactive intermediate, diastereoselective
epoxidation reactions of 1,3-dimethylcyclohexene were con-
ducted with trans-N-sulfonyloxaziridine C alone, trans-N-
sulfonyloxaziridine C together with BTEAC, and our in situ
epoxidation system, respectively (Table 5). It was evident that