Catalytic oxaziridinium-mediated epoxidation of olefins by Oxone. A convenient catalyst excluding common side reactions

Article abstract of DOI:10.1016/S0040-4039(01)02276-6

The nicely crystalline, easily prepared and handled, 3,3-dimethyl-3,4-dihydroisoquinolinium salt 6, is a convenient catalyst for the oxaziridinium-mediated epoxidation of alkenes by Oxone.

Full text of DOI:10.1016/S0040-4039(01)02276-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 803–805
Catalytic oxaziridinium-mediated epoxidation of olefins by
®
Oxone . A convenient catalyst excluding common side reactions
Luis Boh e´ * and Majed Kammoun
Institut de Chimie des Substances Naturelles, CNRS, Avenue de la Terrasse, 91198 Gif sur Yvette, France
Received 17 September 2001; revised 28 November 2001; accepted 29 November 2001
Abstract—The nicely crystalline, easily prepared and handled, 3,3-dimethyl-3,4-dihydroisoquinolinium salt 6, is a convenient
®
catalyst for the oxaziridinium-mediated epoxidation of alkenes by Oxone . © 2002 Elsevier Science Ltd. All rights reserved.
Oxaziridinium chemistry was introduced in the middle
seventies and it was early established that oxaziridinium
salts are electrophilic oxygen atom transfer agents
promising method has deserved growing interest.
8,9 10–12
Accordingly, some other cyclic
and acyclic
iminium salts have been tested for their potential as
asymmetric or racemic epoxidation catalysts.
1
towards nucleophiles. Subsequently the oxygenation of
2
3
4
alkenes, thioethers and N-nucleophiles with the iso-
Two factors lowering the catalytic efficiency of the
epoxidation process have been outlined:
5
lated oxaziridinium salt 1 has been described. Con-
cerning olefins, epoxides are produced in good yields by
–
the hydrolysis of the iminium salt in the reaction
the action of 1 equiv. of the isolated oxaziridinium salt
medium. This (directly) catalyst-consuming side reac-
tion affect principally the catalytic oxaziridinium-
mediated epoxidations involving acyclic iminium
2
1
(Scheme 1).
10–12
Owing to their high reactivity, oxaziridinium salts gen-
erally are not easily isolable compounds but it was
shown that they may be conveniently generated from
the corresponding iminium salt in the presence of an
olefinic substrate. As the oxygen transfer onto the
olefinic bond regenerates the iminium salt, X. Lusinchi
et al. have developed an oxaziridinium-mediated cata-
lytic epoxidation method using the iminium salt 2 as
salts as catalysts.
– the loss of active oxygen from the intermediate
oxaziridinium, in a reaction which does not regener-
ate the iminium, i.e. (Scheme 2) the irreversible base-
catalyzed isomerization of oxaziridinium salts having
protons a to the nitrogen atom (a), into a-hydroxy-
iminium salts (carbinol–iminium salts, b) which
evolve further according to their structure. In partic-
ular, the carbinol iminium salts resulting from the
isomerization of oxaziridiniums of the 3,4-dihy-
droisoquinolinium family lead to the corresponding
®
6
catalyst and Oxone as the oxygen source (Scheme 1).
The suitability of the catalytic method for performing
asymmetric epoxidation was then established using an
7
2,6
enantiomerically pure iminium salt. Thereafter, this
isoquinoliniun salts through a dehydration process.
Scheme 1.
*
0
Corresponding author. E-mail: bohe@icsn.cnrs-gif.fr
Scheme 2.
040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02276-6

8
04
L. Boh e´ , M. Kammoun / Tetrahedron Letters 43 (2002) 803–805
We considered that the efficiency of the catalytic oxy-
gen-transfer process should be improved by preventing
this (indirectly) catalyst-consuming aromatization reac-
tion to occur. In consequence we decided to prepare a
both systems are indicated in Scheme 4. The epoxida-
tion of trans stilbene in the 3,3-disubstituted iminium’s
system is notably faster than in the reference system
involving salt 1. Since the electrophilicities of both
intermediate oxaziridinium species (7 and 2, respec-
tively) are most probably very similar, the exclusion of
the aromatization path (1“3, Scheme 4) by the disub-
stitution of the endocyclic a position allows to improve
the catalyst turnover and consequently enhances the
efficiency of the oxaziridinium-mediated catalytic epoxi-
dation systems operating with dihydroisoquinolinium-
derived iminium salts as catalysts.
3
,3-disubstituted-dihydroisoquinolinium salt in order to
evaluate its catalytic capabilities, with the unsubstituted
iminium salt 2 as standard.
Thus, we have prepared the 3,3-dimethyl-dihydroiso-
quino-linium salt 6 and we report herein that it is a
convenient catalyst with regard to the oxaziridinium-
mediated epoxidation system.
1
3
The results of the oxaziridinium-mediated oxidation of
The iminium salt 6, an easily handled crystalline solid,
®
a variety of olefinic substrates (Fig. 1) by Oxone using
was synthesized from the commercially available ter-
the iminium salt 6 as catalyst are given in Table 1.
Under the conditions used for trans stilbene 8, di- and
trisubstituted alkenes 9–16 are quantitatively converted
into the corresponding epoxides, which may be isolated
in good yields (entries 1–8), while incomplete epoxida-
tion was observed for the less reactive terminal double
tiary alcohol 4 as indicated in Scheme 3. The for-
1
4
mamide from step (a) was cyclized, following a
16
1
5
three-step sequence, into the dihydroisoquinoline 5
which was finally alkylated with the Meerwein’s salt
trimethyloxonium tetrafluoroborate.
bond of the undecylenic methyl ester 17 (entry 9). As
With oxaziridinium 7, derived from iminium 6, as the
2
expected, the conjugated double bond of
L
-(−)-carvone
active oxidizing agent in the epoxidation system
1
5 (entry 7) was not affected and the epoxidation
(
Scheme 4), an aromatization pathway as the one
occurred selectively on the exocyclic double bond
observed in the iminium 2 catalyzed system is not
possible. In order to estimate the effect of the (endo-
cyclic) a-disubstitution on the catalytic efficiency, we
performed the epoxidation of trans stilbene 8 (widely
used as model substrate for these epoxidation reactions)
under the same conditions as previously described with
1
7
(
affording a 1:1 mixture of diastereoisomers). The
epoxidation of olefin 12 (entry 4) produced a mixture of
syn- and anti-epoxides in the molar ratio syn:anti
18
1
:3.5. Similar anti-selectivity, arising from preferential
oxygen approach from the less hindered face, also
occurred in the peracidic epoxidation of the analogous
6
the unsubstituted iminium salt 1. The results from
19
methyl ester. Finally, in the epoxidation of cholesterol
(
entry 8) a mixture of a- and b-epoxides in the molar
20
ratio a:b 1:3.5
was produced. Interestingly, the
oxaziridinium 7-mediated epoxidation of cholesterol
displays b-selectivity in contrast to the well-known a-
selectivity of the usual peracidic epoxidation of this
5
21
D -steroid.
Scheme 3. (a) KCN, SO H –AcOH, rt; (b) oxalyl chloride,
4
2
+
Cl CH ; (c) FeCl ; (d) MeOH, SO H , reflux; (e) Me O
2
2
3
4
2
3
−
F B , Cl CH ; rt.
4
2
2
Scheme 4.
Figure 1.

L. Boh e´ , M. Kammoun / Tetrahedron Letters 43 (2002) 803–805
805
Table 1. Epoxidation of olefins using iminium salt 6 as
4. Hanquet, G.; Lusinchi, X. Tetrahedron 1994, 50, 12185–
2200.
a
catalyst
1
5
. (a) Hanquet, G.; Lusinchi, X.; Milliet, P. Tetrahedron
Lett. 1987, 28, 6061–6064; (b) Hanquet, G.; Lusinchi, X.;
Milliet, P. Tetrahedron 1993, 49, 423–438.
Entry
Olefin
Time (h)b
Yield (%)c
1
2
3
4
5
6
7
8
9
9
10
11
12
13
14
15
16
17
12
12
8
24
9
10
12
8
81
72
82
6. Hanquet, G.; Lusinchi, X.; Milliet, P. C.R. Acad. Sci.
Paris, (II) 1991, 313, 625–628.
d
74
7. (a) Boh e´ , L.; Hanquet, G.; Lusinchi, M.; Lusinchi, X.
Tetrahedron Lett. 1993, 34, 7271–7274; (b) Boh e´ , L.;
Lusinchi, M.; Lusinchi, X. Tetrahedron 1999, 55, 141–
79
76
e
80
154.
92f
8. Aggarwal, V. K.; Wang, H. F. J. Chem. Soc., Chem.
g
24
28
Commun. 1996, 191–192.
a
9. (a) Page, P. C. B.; Rassias, G. A.; Bethell, D.; Schilling,
M. B. J. Org. Chem. 1998, 63, 2774–2777; (b) Page, P. C.
B.; Rassias, G. A.; Barros, D.; Bethell, D.; Schilling, M.
B. J. Chem. Soc., Perkin Trans. 1 2000, 3325–3334.
10. (a) Armstrong, A.; Ahmed, G.; Garnett, I.; Goacolou, K.
Synlett 1997, 1075–1076; (b) Armstrong, A.; Ahmed, G.;
Garnett, I.; Goacolou, K.; Wailes, J. S. Tetrahedron 1999,
Molar ratio olefin:iminium 6:KHSO :CO HNa=1:0.1:2:4, CH CN–
5
3
3
H O (3%), rt.
2
b
c
Reactions monitored by TLC.
Epoxides were purified by C.C. on silicagel (the yields are not
optimized) and gave satisfactory spectroscopic characterization.
d
e
f
1
Molar ratio syn:anti=2.5:7.5 determined by H NMR analysis of
the crude product.
Exclusively exocyclic epoxide, 1:1 mixture of diastereoisomers (ratio
determined by 1H NMR analysis of the crude product).
Epoxidation performed using CH CN/dioxane (1:1)-H O (3%) as
55, 2341–2352.
11. Minakata, S.; Takemiya, A.; Nakamura, K.; Ryu, I.;
Komatsu, M. Synlett 2000, 1810–1812.
12. Wong, M.-K.; Ho, L.-M.; Zheng, Y.-S.; Ho, C.-Y.;
Yang, D. Org. Lett. 2001, 3, 2587–2590.
3
2
1
solvent, molar ratio a:b=2.2:7.8 determined by H NMR analysis
of the crude product.
g
1
Conversion olefin“epoxide (40%) determined by H NMR analysis
of the crude product (60% of the substrate was unchanged).
13. N,3,3-Trimethyl-3,4-dihydroisoquinolinium tetrafluoro-
1
borate (6). mp: 89–90°C (Cl CH –Et O); H NMR
2
2
2
(
CDCl , 300 MHz): 1.50 (s, 6H); 2.71 (s, 2H); 3.76 (s,
3
In conclusion, we have prepared the new 3,3-dimethyl-
,4-dihydroisoquinolinium salt 6, a nicely crystalline
3
1
3
1
H); 7.34 (m, 1H); 7.44 (m, 1H); 7.69 (m, 1H); 7.88 (m,
3
13
H); 8.98 (s, 1H). C NMR (CDCl , 75 MHz): 23.82;
3
and easily handled iminium salt. Improved catalyst
turnover and consequently enhanced efficiency resulted
using this gem-disubstituted iminium salt as a conve-
nient alternative to the parent unsubstituted iminium 1
9.51; 42.51; 62.47; 128.31; 128.56; 134.31; 137.98; 124.28;
+
’
36.17; 167.82. MS (FAB): 174 [(M −tetrafluoroborate),
base peak]. Anal. calcd for C H NBF : C, 55.21; H,
6
12
16
4
.18; N, 5.37%. Found: C, 55.08; H, 6.12; N, 5.35%.
®
in the catalytic oxidation of olefins by Oxone .
1
1
4. Ritter, J.; Kalish, J. Org. Synth. 1964, 44, 44–47.
5. Larsen, R. D.; Reamer, R. A.; Corley, E. G.; Davis, P.;
Grabowski, E. J. J.; Reider, P. J.; Shinkai, I. J. Org.
Chem. 1991, 56, 6034–6038.
Acknowledgements
16. Seeger, E.; Engel, W.; Teufel, H.; Machleidt, H. Chem.
Ber. 1970, 103, 1674–1691.
7. Diastereoisomer ratio determined by H NMR (Cl CD)
We thank Dr. Pierre Potier for his interest and for kind
complementary financial support to one of us (M.K.).
1
1
3
analysis of the crude product, integrating the C9 methyls
at l 1.31 ppm (s, 3H) and l 1.33 ppm (m, 3H)
1
8. Kuhn, T.; Tamm, C.; Reisen, A.; Zedner, M. Tetrahedron
References
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9. syn:anti Molar ratio determined by H NMR (Cl CD)
1
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1
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1
2
0. Molar ratio a:b determined by H NMR (Cl CD) analy-
2
3
3
sis of the crude product, integrating the oxiranic protons
(
C6-H) signals at l 3.05 ppm (d, J 1.7 Hz, 1H, b-epoxide)
and l 2.86 ppm (d, J 4.4 Hz, 1H, a-epoxide).
5
21. Kirk, D. N.; Hartshorn, M. P. Steroid Reaction Mecha-
nisms; Elsevier: Amsterdam, 1968; pp. 69–77.