Novel binaphthalene-amine catalysts for the asymmetric epoxidation of alkenes

Article abstract of DOI:10.1055/s-2008-1072592

We have prepared a range of binaphthalene-derived azepine derivatives containing alcohol functionality, and used them as catalysts in the asymmetric epoxidation of alkenes by Oxone, giving ee values of up to 81%. We have obtained evidence that points to the involvement of iminium ions in the reaction pathway, suggesting that the oxidizing species in the epoxidation reactions are in fact the corresponding oxaziridinium ions. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1072592

LETTER
1381
Novel Binaphthalene–Amine Catalysts for the Asymmetric Epoxidation
of Alkenes
Asymmetric
E
h
poxidation
o
i
f
A
lken
l
es ip C. Bulman Page,*a Mohamed M. Farah,b Benjamin R. Buckley,b A. John Blacker,c Jérôme Lacourd
a
School of Chemical Sciences & Pharmacy, University of East Anglia, Norwich, Norfolk NR4 7TJ, UK
Fax +44(1603)592003; E-mail: p.page@uea.ac.uk
b
c
d
Department of Chemistry, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK
NPIL Pharmaceuticals (UK) Ltd, P.O. Box 521, Leeds Road, Huddersfield HD2 1GA, UK
Department of Chemistry, University of Geneva, 1211 Geneva 4, Switzerland
Received 27 February 2008
Abstract: We have prepared a range of binaphthalene-derived
azepine derivatives containing alcohol functionality, and used them
as catalysts in the asymmetric epoxidation of alkenes by Oxone,
giving ee values of up to 81%. We have obtained evidence that
points to the involvement of iminium ions in the reaction pathway,
suggesting that the oxidizing species in the epoxidation reactions
are in fact the corresponding oxaziridinium ions.
Ph
F
Ph
Ph
NH
Ph
N
N
H
H
1
2
3
Key words: asymmetric, Oxone, epoxidation, alkene, epoxide
Figure 1
chiral-binaphthalene-derived secondary amines have
been reported to date.
Nonracemic epoxides are important and highly versatile
building blocks in asymmetric synthesis,1 and the epoxide
functionality is itself a part of the structure of many natu-
ral products and biologically active compounds.2 Despite
the development over the past few years of several meth-
ods for asymmetric epoxidation, access to chiral epoxides
with high ee remains an important objective, perhaps be-
cause no single method is appropriate for all epoxide
structures. Organocatalysis, that is, reactions carried out
with substoichiometric quantities of organic molecules as
catalysts, represents one of the most active areas of re-
search in chemistry today.3 Organic-molecule-catalysed
reactions offer several notable advantages over the more
traditional transition-metal-mediated processes; for ex-
ample, reactions can often be carried out under wet and
aerobic conditions, the catalysts are generally inexpensive
and there are no associated toxicity problems. Currently,
the most utilized types of chiral organocatalyst for the
asymmetric epoxidation of alkenes are chiral dioxiranes,
oxaziridines, and oxaziridinium salts.4–6 More recently,
amines have been shown to be effective mediators for the
epoxidation of unfunctionalized alkenes.7
With this in mind, we envisaged that the known chiral di-
hydroazepine 3 might prove to be an effective epoxidation
catalyst due to its conformational rigidity; 3 has indeed
been widely used in developing chiral species for asym-
metric synthesis.10 These include Hawkin’s asymmetric
stereoselective carbon–nitrogen bond formation,11
Cram’s asymmetric addition of organolithium reagents to
aldehydes,12 and chiral phase-transfer catalysts.13
Treatment of the known dibromo compound 411 with
allylamine and triethylamine as a base led to allyldihy-
droazepine 5 in good yield. Subsequent N-deallyllation of
5 using Pd(OAc)2, Ph3P, and N,N¢-dimethylbarbituric acid
(NDMBA) in dichloromethane, followed by treatment
with concentrated hydrochloric acid, furnished the dihy-
droazepinium hydrochloride
(Scheme 1).
6
in excellent yield
Ammonium salt 6 (5 mol%) was subsequently used as cat-
alyst in the epoxidation of 1-phenylcyclohexene in the
presence of Oxone (2 equiv), NaHCO3 (5 equiv) in
MeCN–H2O (10:1), the conditions reported by Yang.9
Disappointingly, 6 was found to be relatively unreactive
and a poor chiral inducer, giving only 28% conversion
into the 1S,2S-epoxide with 7% ee after two hours. In-
creasing the catalyst loading to 10 mol% failed to increase
the rate of conversion.
Chiral-amine-mediated epoxidation, reported indepen-
dently by Aggarwal8 and Yang,9 has provided moderate
enantioselectivities of up to 61% ee. These studies
showed secondary amines to be better catalysts than the
corresponding primary and tertiary amines. Chiral pyrro-
lidine analogues, such as 1 or 2, were found to be effective
mediators for the enantioselective epoxidation of alkene
substrates (Figure 1). Surprisingly, no studies using
Recently, we have shown that the amine analogues 7–9 of
our previously reported iminium salt catalysts also catal-
yse the epoxidation of unfunctionalized alkenes
(Figure 2).14
SYNLETT 2008, No. 9, pp 1381–1385
Advanced online publication: 16.04.2008
DOI: 10.1055/s-2008-1072592; Art ID: D06108ST
© Georg Thieme Verlag Stuttgart · New York
x
x.
x
x.
2
0
0
8
The poor reactivity of 6 may stem from the lack of a het-
eroatomic (e.g., O or F) group capable of stabilizing the
active oxidizing species either inductively or through hy-

1382
P. C. Bulman Page et al.
LETTER
Curiously, all but amines 12 and 14 failed to show any cat-
alytic activity. Catalyst 14 gave only 16% conversion into
the (–)-1S,2S-epoxide, with 40% ee, but catalyst 12
proved to be more reactive and enantioselective, giving
88% conversion into the (–)-1S,2S-epoxide, with 45% ee.
The 1H NMR spectroscopic analysis of the crude mixture
revealed that the amines decomposed under these reaction
conditions. We were puzzled by the difference in reactiv-
ity of these structurally similar amines, and so embarked
on an investigation of the decomposition pathways of
these amines, which inhibit their catalytic activities.
Br
i
N
Br
4
5
ii, iii
Cl–
NH2
We discovered that amine 14 is converted into the diaste-
reomerically pure oxazolidine 15 when submitted to the
epoxidation reaction conditions [Oxone (2 equiv),
NaHCO3 (5 equiv), MeCN–H2O (10:1)]. The same oxazo-
lidine 15 is also partially formed when a chloroform solu-
tion of amine 14 is allowed to stand at room temperature
overnight (Scheme 3).
6
Scheme 1 Reagents and conditions: i: allylamine (1.7 equiv), Et3N
(3 equiv), THF, 55 °C, 3 h, 68%; ii: Pd(OAc)2 (2 mol%), Ph3P (0.1
equiv), NDMBA (1.5 equiv), CH2Cl2, 35 °C, 6 h; iii: HCl (1.1 equiv),
90%.
O
O
O
O
i
N
N
N
N
HO
O
Ph
Ph
H
7
8
14
15
Scheme 3 Reagents and conditions: i: Oxone (2 equiv), NaHCO3 (5
equiv), MeCN–H2O (10:1), 5 h.
O
O
N
Ph
Oxazolidine formation was also observed when using
amines 12 and 13, but not amines 10 or 11. At this stage,
we rationalize that the formation of the oxazolidine prod-
uct 15 arises from cyclization of an iminium salt formed
under the reaction conditions. We postulate that, under
our reaction conditions, amine 14 is initially oxidized to
the corresponding N-oxide 16.15 Subsequent loss of hy-
doxide leads to the iminium salt 17. This iminium salt is
presumably in equilibrium with the oxazolidine 15 under
the reaction conditions (Scheme 4). The stereochemistry
of 15 was established by NOE experiments.
9
Figure 2
drogen bonding, as reported previously by Yang.9 In order
to investigate this structural motif further, we subsequent-
ly prepared a range of chiral binaphthalene-derived amino
alcohols 10–14 from dibromo compound 4 (Scheme 2).
With amines 10–14 in hand, we tested them as catalysts
(10 mol%) in the epoxidation of E-methylstilbene as sub-
strate, again using the conditions of Yang [Oxone (2
equiv), NaHCO3 (5 equiv), MeCN–H2O (10:1), 0 °C, 5 h].
N
HO
Br
R
i
N
Br
Oxone, NaHCO3
HO
10 R = (S)-Me, 98%
11a R = (S)-Ph, 95%
11b R = (R)-Ph, 96%
12 R = (S)-i-Pr, 97%
13 R = (S)-i-Bu, 98%
14 R = (S)-t-Bu, 97%
N
N
H
N
–
O
OH
OH
O
H
16
17
15
Scheme 2 Reagents and conditions: i: amine (1.1 equiv), K2CO3 (3
equiv), MeCN, reflux, 16 h.
Scheme 4
Synlett 2008, No. 9, 1381–1385 © Thieme Stuttgart · New York

LETTER
Asymmetric Epoxidation of Alkenes
1383
A potential explanation for the difference in reactivity ex-
hibited within the group of amines 10–14 towards the ep-
oxidation of alkenes lies in the competition between the
oxidation of the iminium salt to the oxaziridinium species
(the active oxidant) and the oxazolidine formation.
BPh4
N
With amine 12 being the most reactive in this series, a
number of other alkenes were next subjected to epoxida-
tion mediated by this catalyst (Table 1).
18
Figure 3
Table 1 Asymmetric Epoxidation of Alkenes with Amine 12a
An interesting observation during the course of the epoxi-
dation was that the reaction changed from colourless to in-
tense yellow, conceivably indicating the formation of the
corresponding iminium salt, which is generally a yellow
solid. After the completion of the epoxidation reaction,
the yellow residue was triturated in diethyl ether, and the
sample analysed by 1H NMR spectroscopy and mass spec-
trometry. We observed the iminium signal at around d =
10 ppm (CDCl3), while the mass spectrum revealed al-
most a 100% molecular ion corresponding to the iminium
salt, supporting our postulation that iminium salts are gen-
erated in situ and are the active catalysts mediating the ep-
oxidation reactions.
Alkene
Conversion (%) ee (%)
Configurationb–d
Ph
90 (10)e
81
80
(–)-1S,2S
Ph
62
(+)-1R,2Sg
Ph
88
47
45
28
(–)-1S,2Sf
(+)-Sf
Ph
Ph
Me
Ph
Ph
With the aim of improving the reactivity of our amines
and inhibiting the oxazolidine formation, we next pre-
pared the fluorine-containing amines 19 and 20 in one
step from amines 12 and 14. A wide variety of fluorinat-
ing reagents is available;17 diethylaminosulfur trifluoride
(DAST) is widely used to mediate the direct conversion of
alcohols into fluorides in high yields.18 Treatment of
amines 12 and 14 in dichloromethane with DAST at room
temperature for five hours afforded fluorinated products
19 and 20 respectively, albeit in low yields (28–30%),
along with other unidentified byproducts. Improved
yields were obtained when bis(2-methoxyethyl)aminosul-
fur trifluoride (Deoxofluor) was used as the fluorinating
reagent (Scheme 5).
Ph
65
44
17
47
(–)-1S,2Sf
(+)-1R,2Sg
Ph
a Epoxidation conditions: amine (10 mol%), Oxone (2 equiv),
NaHCO3 (5 equiv), MeCN–H2O (10:1), 0 °C, 2 h, unless otherwise
indicated.
b Conversions were evaluated from the 1H NMR spectra by integra-
tion of alkene, dio, and epoxide signals.
c Enantiomeric excesses were determined by chiral HPLC on a Chiral-
cel OD column, or by chiral GC on a Chiraldex B-DM column.
d The absolute configurations of the major enantiomers were deter-
mined by comparison with literature values except where indicated.
e The numbers in parentheses refers to the percentage conversion into
diol.
R
R
i
N
N
f Reaction time: 5 h.
g Epoxidation conditions: iminium salt (5 mol%), Oxone (2 equiv),
Na2CO3 (4 equiv), MeCN–H2O (1:1), 0 °C, 5 h.
HO
F
12 R = i-Pr
14 R = t-Bu
19 R = i-Pr, 66%
20 R = t-Bu, 60%
As illustrated in Table 1, amine 12 is an effective catalyst
for the enantioselective epoxidation of a range of sub-
strates. While the conversions into epoxides are moderate
to good (44–100%), enantioselectivities of up to 81%
were obtained with 1-phenylcyclohexene oxide. This
amine catalyst, while being less reactive, is comparable in
enantioselectivity to our previously reported N-isopropyl-
substituted iminium salt catalyst 18 (Figure 3), which af-
forded up to 83% ee.16 It is noteworthy that amine 12 in-
duced epoxidation of 1,2-dihydronaphthalene with higher
ee (47%) than all other reported binaphthalene-derived
amines and iminium salts to date.
Scheme 5 Reagents and conditions: i: Deoxofluor (1.05 equiv),
CH2Cl2, 0 °C to r.t., 24 h.
We subsequently tested amines 19 and 20 (10 mol%) in
the epoxidation of trans-a-methylstilbene. Surprisingly,
no conversion into epoxide was observed using these
amines after five hours. Changing the reaction conditions
to the biphasic dichloromethane/water (3:2) conditions in
the presence of 18-crown-6, developed by Lacour, also
failed.14
Recently, Lacour reported a range of enantiopure doubly
bridged biphenyl azepines and azepinium salts as cata-
Synlett 2008, No. 9, 1381–1385 © Thieme Stuttgart · New York

1384
P. C. Bulman Page et al.
LETTER
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were effective catalysts, while others showed no catalytic
activity at all, and subsequently identified a decomposi-
tion pathway involving N-oxide formation and Cope elim-
ination that presumably inhibited the catalytic activities of
some of the amines.19 The catalytic activity of one of these
amines could be restored by the addition of NBS (5 mol%)
to the amine prior to addition of the substrates and other
reagents, presumably by in situ formation of the iminium
species. In our hands, addition of a catalytic amounts of
NBS (10 mol%) to amines 19 and 20 in dichloromethane
(1 mL) for ten minutes prior to the addition of water (0.5
mL), the substrate, 18-crown-6, Oxone, and NaHCO3,
failed to give any epoxide after a reaction time of five
hours.
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In conclusion, we have prepared a range of binaphthalene-
derived azepines containing alcohol functionality and
used them as catalysts in the asymmetric epoxidation of
alkenes. Amine 12 exhibited the best reactivity and enan-
tioselectivity profile, giving ee values of up to 81%. We
have also identified a reaction pathway involving the for-
mation of oxazolidines, which presumably retards the cat-
alytic activity of these amines. We have obtained limited
spectroscopic evidence that points to the involvement of
iminium ions in the reaction pathway, suggesting that the
oxidizing species in the epoxidation reactions are in fact
the corresponding oxaziridinium ions.
Acknowledgment
This investigation has enjoyed the support of NPIL Pharmaceuticals
(UK) Ltd and Loughborough University. We are indebted to the
Royal Society for an Industry Fellowship (to P.C.B.P.), and to the
EPSRC Mass Spectrometry Unit at The University of Wales, Swan-
sea.
(w) Tian, H.; She, X.; Yu, H.; Shu, L.; Shi, Y. J. Org. Chem.
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