6-Oxo-1,1,4,4-tetramethyl-1,4-diazepinium Salts. A New Class of Catalysts for Efficient Epoxidation of Olefins with Oxone

Full text of DOI:10.1021/jo980342b

2
810
J . Org. Chem. 1998, 63, 2810-2811
6
-Oxo-1,1,4,4-tetr a m eth yl-1,4-d ia zep in iu m
modification of this framework, we designed the asymmetric
oxoammonium salts 1c3b and 1d but found that they were
ineffective as promoters of epoxidation. Only with the added
activation provided by an R-fluorine substituent (1e) was
6a
Sa lts. A New Cla ss of Ca ta lysts for Efficien t
Ep oxid a tion of Olefin s w ith Oxon e†
6
suitable catalytic efficiency for epoxidation found. The low
Scott E. Denmark* and Zhicai Wu
reactivities of these potential dioxirane precursors most
likely result from the unavoidable steric congestion caused
by the substitution required to create an asymmetric envi-
ronment adjacent to the ketone moiety. Clearly, greater
activation of the carbonyl carbon is needed to increase its
electrophilicity for the generation of an efficient asymmetric
epoxidation catalyst.
Roger Adams Laboratory, Department of Chemistry, University
of Illinois, Urbana, Illinois 61801
Received February 26, 1998
Dioxiranes are now among the most useful oxidants for
epoxidation of alkenes under mild conditions.1 These re-
agents are generated from potassium peroxomonosulfate
To enhance the electrophilicity of the carbonyl group,
maintain water solubility, and provide a foundation for
asymmetric modification, we envisaged the use of R,R′-bis-
(Oxone) and the parent ketones and are used either in situ
2
or in distilled solution as isolated species. In recent years,
as less volatile ketones have been employed, the convenient
in situ methods have become the protocol of choice, in which
(ammonium) ketones and formulated as point of entry the
oxodiazepinium salt 2, Chart 1. Compared to the monoam-
monium ketones 1a and 1b, the carbonyl carbon in 2 should
be much more electrophilically activated by the proximity
of two ammonium groups. Thus, we expected that the rate
of formation and subsequent reaction of the corresponding
dioxirane would be greatly increased. Moreover, the sym-
metrical disposition of the carbonyl group between two
cationic units ensures that 2 will be highly resistant to
degradation by Baeyer-Villiger oxidation, unlike the simple
the epoxidation can be performed either in a biphasic solvent
system2
a-d,3
or in a homogeneous aqueous organic solution.4
It is now well-recognized that oxygen atom transfer from
the dioxirane to the substrate can in principle be catalytic
and that, with suitably designed chiral ketones, asymmetric
epoxidation of olefins is possible. Indeed, high enantiose-
lectivities have been reported in recent years for the epoxi-
dation of unfunctionalized olefins with various ketonic
promoters.5 Nevertheless, a truly general and efficient
catalyst remains an elusive target. Toward that end, there
is considerable interest in understanding those structural
features that will lead to powerful catalysts and provide new
3
-oxopiperidinium salts examined previously.3 On the basis
of the greatly enhanced reactivity, stability, and water
solubility, ketone 2 was expected to be a promising catalyst
for epoxidation with Oxone.
3
,6,7
templates for the design of asymmetric reagents.
cently, we developed two efficient families of catalysts for
Re-
Ch a r t 1
3
epoxidation; 4-oxopiperidinium salts and simple fluoro
ketones.6 In continuation of our effort to develop novel
frameworks for general catalysts, we describe a new class
of agents, R,R′-bis(ammonium) ketones, for catalytic epoxi-
dation with Oxone under monophasic conditions.
a
Our previous studies revealed that 4-oxopiperidinium
salts could be customized to function as excellent catalysts
3
under either biphasic conditions (methyl, dodcecyl (1a )) or
8
in homogeneous medium (dimethyl (1b)) by simply altering
the lipophilicity of the ammonium group, Chart 1. By
The synthesis of 2 follows logically by a simple albeit
remarkable two-step route outlined in Scheme 1. Double
alkylative cyclization of tetramethyl ethylenediamine
†
Catalytic Epoxidation of Alkenes with Oxone. 3.
(TMEDA) with 1,3-dibromoacetone in acetone at room
(1) For reviews of dioxirane chemistry see: (a) Adam, W.; Curci, R.;
Edwards, J . O. Acc. Chem. Res. 1989, 22, 205. (b) Murray, R. W. Chem.
Rev. 1989, 89, 1187. (c) Curci, R. In Advances in Oxygenated Processes;
Baumstark, A. L., Ed.; J AI Press: Greenwich, 1990; Vol. 2, Chapter 1. (d)
Adam, W.; Hadjiarapoglou, L.; Curci, R.; Mello, R. In Organic Peroxide;
Ando, W., Ed.; J . Wiley & Sons: New York, 1992; Chapter 4. (e) Adam, W.;
Hadjiarapoglou, L. P. In Topics in Current Chemistry; Springer-Verlag:
Berlin, 1993; Vol. 164, p 45. (f) Curci, R.; Dinoi, A.; Rubino, M. F. Pure
Appl. Chem. 1995, 67, 811.
temperature afforded the dibromide 3, which was directly
converted to the ditriflate 2 by ion exchange with silver
triflate. The bis(ammonium) salt 2 is a stable (mp 204-
205 °C), highly crystalline solid that exists as a molecular
hydrate.
(2) (a) Curci, R.; Fiorentino, M.; Troisi, L.; Edwards, J . O.; Pater, R. H.
Sch em e 1
J . Org. Chem. 1980, 45, 4758. (b) Edwards, J . O.; Pater, R. H.; Curci, R.; Di
Furia, F. Photochem. Photobiol. 1979, 30, 63. (c) Gallopo, A. R.; Edwards,
J . O. J . Org. Chem. 1981, 46, 1684. (d) Cicala, G.; Curci, R.; Fiorentino, M.;
Laricchiuta, O. J . Org. Chem. 1982, 47, 2670. (e) Murray, R. W.; J eyaraman,
R. J . Org. Chem. 1985, 50, 2847.
(3) (a) Denmark, S. E.; Forbes, D. C.; Hays, D. S.; Depue, J . S.; Wilde, R.
G. J . Org. Chem. 1995, 60, 1391. (b) Forbes, D. C. Ph.D. Thesis, University
of Illinois at Urbana-Champaign, 1996.
(
4) Yang, D.; Wong, M.-K.; Yip, Y.-C. J . Org. Chem. 1995, 60, 3887.
(5) (a) Yang, D.; Yip, Y.-C.; Tang, M.-W.; Wong, M.-K.; Zheng, J .-H.;
Cheung, K.-K. J . Am. Chem. Soc. 1996, 118, 491. (b) Yang, D.; Wang, X.-
C.; Wong, M.-K.; Yip, Y.-C.; Tang, M.-W. J . Am. Chem. Soc. 1996, 118,
The potential for any ketone to serve as a promoter for
Oxone-based epoxidation involves two critical features, the
efficiency of dioxirane formation and the ability to transfer
an oxygen atom to the substrate. To evaluate the ability of
2 to be converted to a dioxirane, we measured the rate of
Oxone consumption of in the presence of a catalytic amount
of 2 and compared that to the rate with other ketones.
Dioxiranes are known to be the intermediates responsible
1
(
1311. (c) Tu, Y.; Wang, Z.-X.; Shi, Y. J . Am. Chem. Soc. 1996, 118, 9806.
d) Wang, Z.-X.; Tu, Y.; Frohn, M.; Shi, Y. J . Org. Chem. 1997, 62, 2328.
6) (a) Denmark, S. E.; Wu, Z.; Crudden, C. M.; Matsuhashi, H. J . Org.
(
Chem. 1997, 62, 8288. (b) See also: Armstrong, A.; Hayter, B. R. J . Chem.
Soc., Chem. Commun. 1998, 621.
(
7) (a) Song, C. E.; Kim, Y. H.; Lee, K. C.; Lee, S.-G.; J in, B. W.
Tetrahedron: Asymmetry 1997, 8, 2921. (b) Adam, W.; Zhao, C.-G. Tetra-
hedron: Asymmetry 1997, 8, 3995. (c) Wang, Z.-X.; Shi, Y. J . Org. Chem.
1
997, 62, 8622.
8) Denmark, S. E.; Wu, Z. Unpblished results.
(
S0022-3263(98)00342-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/09/1998

Communications
J . Org. Chem., Vol. 63, No. 9, 1998 2811
Ta ble 1. Ca ta lytic Ep oxid a tion of Selected Olefin sa
b
c
entry
alkene
time, h conversion, % yield, %
1
2
3
4
5
6
7
8
(E)-2-methylstyrene
(Z)-2-methylstyrened
indene
1-phenylcyclohexene
(E)-cinnamyl alcohol
geraniol
geraniol
1-octene
(E)-5
(E)-5
8
6
6
4
11
6
100
100e
100
100
100
100f
83
78
85
93
94
90
89
74
89
91
33
g
2.4
96
11
11
22
22
98
96
97
39
9
0
1
h
1
1
stilbene
a
Reactions done in CH3CN/H2O (pH 7.0) with 0.1 equiv of 2
b
and 10 equiv of Oxone (10 h addition) at 0 °C. GC analysis.
Chromatographically homogeneous material. (Z)-olefin/(E)-ole-
c
d
e
f
g
fin, 93/7. cis-epoxide/trans-epoxide, 93/7. Diepoxide. 6,7-Mono-
F igu r e 1. Rate of Oxone consumption by ketones.
epoxide/diepoxide, 77/19. h Reaction done with 0.05 equiv of 2 and
2
0 equiv of Oxone (20 h addition).
Sch em e 2
a pH stat) whereby 100% conversion of (E)-4 to 5 was
obtained after 11 h with a catalytic amount (10 mol %) of 2.
Interestingly, epoxidation at pH 6.0 proceeded almost as well
as at pH 7.0. Presumably, the increased electrophilicity of
the carbonyl center allows the reaction to proceed under
slightly acidic conditions.
for the ketone-catalyzed decomposition of Oxone.2b There-
fore, the ability of a ketone to catalyze the consumption of
Oxone would reflect its ability to form a dioxirane. The rate
of consumption of 15 mmol of Oxone at pH 7.0 and 0 °C in
the presence of 0.1 mmol of each of four ketones (1b, 2
To illustrate the generality of the epoxidation procedure,
various olefin types were examined. The results collected
in Table 1 are 1 mmol scale oxidations with 10 mol % of 2
at pH 7.0 under otherwise the same conditions as above.
All of the alkenes were efficiently oxidized (96-100%
conversion) to afford the epoxides in very good yield (74-
cyclohexanone and cycloheptanone) is graphically presented
in Figure 1.9 Whereas Oxone is unstable at pH 7.0, we first
9
4%). Stilbene is the only exception (entry 11, 39% conver-
measured a background rate (s). Under these conditions,
sion, 33% yield) because of its poor solubility. The important
reactivity trends for this epoxidation mirror those for 1b.
As expected, the epoxidation was stereospecific (entries 1
and 2). The rate of oxidation increased with increasing
olefin substitution. An allylic hydroxyl group decreased the
epoxidation rate (entries 1 and 5), while highly electron-
deficient olefins (methyl cinnamate) failed to react at an
appreciable rate. Thus, excellent regioselectivity can be
achieved for the epoxidation of geraniol (entry 7, no 2,3-
monoepoxide detected during the reaction). Moreover, the
extreme reactivity and oxidative stability of 2 allows for the
catalyst loading to be further reduced. With 5 mol % of 2,
even the unreactive olefin (E)-5 was epoxidized to 97%
conversion afforded a 91% yield of 5 in 22 h (entry 10).
4
5% of the active oxidant was consumed in ca. 8 h. The
dramatic acceleration of this decomposition by the 4-oxopi-
1
0
peridinium salt 1b (first noted by Montgomery ) is appar-
ent. However, the rate of consumption of Oxone catalyzed
by the bis(ammonium) salt 2 dwarfs that of 1b reaching
completion in less than 30 min. To illustrate the effect of
the ammonium moieties within the cyclic ketones, the data
for cyclohexanone and cycloheptanone are provided. Clearly,
these ketones do not efficiently form dioxiranes under these
conditions. The rapid consumption of Oxone in the presence
of 2 indicated its facility toward dioxirane formation and
presaged its potential as an epoxidation catalyst.
The second stage in the catalytic process was examined
by optimizing the ability of 2 to epoxidize a test olefin (E)-
4
6
in monophasic solution (acetonitrile/water) at 0 °C,
In summary, we have demonstrated that the readily
available bis(ammonium) ketone 2 is a general and efficient
catalyst for epoxidation of a wide variety of olefins with
Oxone under mild conditions. In addition, the availability
of many chiral nonracemic diamines of significant structural
diversity bodes well for the development of asymmetric
catalysts with enhanced catalytic activity and enantioselec-
tivity.
Scheme 2. Since Oxone decomposition by the dioxirane from
was extremely fast, it was necessary to keep the concen-
2
tration of Oxone in the reaction mixture as low as possible
so that oxygen atom transfer to the substrate could be
maximized. Accordingly, for efficient use of oxidant, 10
equiv of aqueous Oxone solution (0.56 M) was added over
1
0 h by a syringe pump. Because the volume of water
increased during the reaction, the initial acetonitrile/
phosphate buffer (pH 7.0) ratio was set at 3/1 to avoid oiling
out of the olefin substrate. The optimum pH for the
epoxidation was found to be 7.0 (maintained throughout by
Ack n ow led gm en t. We are grateful to the Pharmacia
and Upjohn Co. for generous financial support. Z.W. thanks
the University of Illinois for a Graduate Fellowship.
(
9) Experimentally, the rate was determined by monitoring the volume
Su p p or tin g In for m a tion Ava ila ble: The preparation and
characterization of 2 along with epoxidation procedures are
provided (5 pages).
of aqueous potassium hydroxide solution added to neutralize the acid
generated to maintain pH 7.0. See the Supporting Information for details.
(10) Montgomery, R. E. J . Am. Chem. Soc. 1974, 96, 7820. Montgomery
estimated that 1b was more efficient than acetone in this process by a factor
of 1300.
J O980342B