Preparation of C 2-Symmetric Biaryl Bisiminium Salts and Their Use as Organocatalysts for Asymmetric Epoxidation

Article abstract of DOI:10.1055/s-0035-1560540

Two C ₂-symmetric bisiminium salt species containing biphenylazepinium units and derived from two chiral diamines were prepared and tested as organocatalysts for asymmetric epoxidation.

Full text of DOI:10.1055/s-0035-1560540

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© Georg Thieme Verlag Stuttgart · New York
2016, 27, 126–130
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Letter
Synlett
Preparation of C2-Symmetric Biaryl Bisiminium Salts and Their Use
as Organocatalysts for Asymmetric Epoxidation
Philip C. Bulman Page*a
Mohamed M. Farahb
Benjamin R. Buckleyb
N
N
Yohan Chana
A. John Blackerc
O
2 BPh4
a School of Chemistry, University of East Anglia, Norwich
Research Park, Norwich, Norfolk NR4 7TJ, UK
p.page@uea.ac.uk
b Department of Chemistry, Loughborough University,
Loughborough, Leicestershire LE11 3TU, UK
c School of Chemistry, University of Leeds, Leeds LS2 9JT,
UK
iminium salt (5 mol%)
Oxone (2 equiv)
NaHCO3 (5 equiv)
MeCN–H2O (10:1)
0 °C, 2 h
up to 55% ee
Received: 27.09.2015
Ph
Accepted after revision: 01.11.2015
Published online: 16.11.2015
DOI: 10.1055/s-0035-1560540; Art ID: st-2015-d0770-l
Me
Me
Me
N
Cl
N
ClO4
BF4
Abstract Two C2-symmetric bisiminium salt species containing biphe-
nylazepinium units and derived from two chiral diamines were pre-
pared and tested as organocatalysts for asymmetric epoxidation.
1
2
BPh4
BPh4
O
R
Key words iminium, diamine, organocatalysis, epoxidation, biaryl
N
O
N
Oxaziridinium salts generated in situ from iminium
salts with Oxone were first shown by Lusinchi to be effec-
tive electrophilic oxidants for epoxidation of alkenes, ren-
dering the development of a catalytic process possible.1 The
first enantiomerically pure iminium salt to be used in this
way was 1, with the controlling asymmetric centres sited in
the saturated ring of a dihydroisoquinolinium salt, and it
was shown to catalyse epoxidation of alkenes with enantio-
meric excesses of up to about 40% (Figure 1).2 Even exocy-
clic iminium salts such as 2, derived from condensation of
enantiopure pyrrolidine moieties with aromatic aldehydes,
can afford moderate enantiomeric excesses (up to 22%)
with relatively high catalyst loadings (up to 100 mol%), nec-
essary perhaps due to in situ hydrolysis of the iminium
units.3 Our own contribution has been to design chiral
iminium salts that contain asymmetric centres in the exo-
cyclic nitrogen substituent, based upon the reasoning that
such designs would bring the enantiocontrolling asymmet-
ric centres closer to the site of oxygen transfer, and hence
potentially increase enantioselectivity. We have shown that
iminium salts such as 3–7 provide enantiomeric excesses of
up to 99% in epoxidation under our standard conditions us-
ing Oxone as stoicheiometric oxidant in aqueous acetoni-
trile,4 or using tetraphenylphosphonium monoperoxysul-
fate (TPPP)5 under nonaqueous conditions,6 as well as other
oxidants.7
O
O
Ph
R
4
3
BPh4
O
BPh4
O
R
R
N
N
O
O
Ph
Ph
5
6
BPh4
O
R
O
O
H2N
N
O
Ph
Ph
7
8
Figure 1 Examples of chiral iminium salts
Catalysts incorporating an azepinium structure are
highly reactive, inducing complete epoxidation of alkenes
within 1–10 minutes in aqueous acetonitrile at 0 °C; they
are the most reactive iminium salt epoxidation catalysts
discovered to date, and among the most reactive known or-
ganocatalysts, effective at loadings of as low as 0.1 mol%.
We have applied the chemistry to the synthesis of several
natural products including lomatin8 and scuteflorin A.9
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Related chiral secondary amines have also been shown
to catalyse the asymmetric epoxidation of alkenes, giving
up to 66% ee.10 We have reported that amines correspond-
ing to a number of our iminium salts such as 4 and 6
(R = H), are indeed effective epoxidation catalysts under our
standard conditions, giving yields and enantiomeric excess-
es very similar to the corresponding iminium species. We
have postulated that the amine catalysts are oxidized in situ
to the corresponding iminium salts, which in turn epoxi-
dize the alkenes via the oxaziridinium salts.11 Yang has ex-
amined a range of chiral secondary amines for catalytic ac-
tivity;12 these studies revealed the presence of an electron-
withdrawing substituent (such as hydroxyl or fluorine
groups) at the β-position to the amine group to be benefi-
cial for catalytic activity.
We obtained enantiopure (1S,2S)-DACH (10) from the
resolution of racemic trans-DACH (10) with D-tartaric acid
and subsequent liberation of the monotartrate salt 11 using
aqueous sodium hydroxide (Scheme 1).20 Other methodolo-
gies have been developed to resolve the DACH enantiomers,
including using the corresponding citrate salts,21 and high
pressure.22
NH2
NH2
NH2
NH2
i
ii
H3N
O2C
3NH
CO2
HO
OH
(
)-10
11
(1S,2S)-10
Scheme 1 Reagents and conditions: i) D-tartaric acid (1.0 equiv), H2O,
AcOH, r.t., 2 h, 40%; ii) 4 M NaOH, 10 min.
The design of chiral iminium salt catalysts in our group
has often centred on the use of (+)-(S,S)-L-acetonamine 8
(Figure 1) and its derivatives as chiral appendages, which
are effective presumably due to the electronic and steric
roles played by the 1,3-dioxane ring and the phenyl group.
We have confirmed that the aromatic C4 substituent in the
1,3-dioxane is vital to obtain high enantioselectivities. Oth-
er chiral appendages used for the synthesis of biphenyl-de-
rived iminium salt catalysts, for example N-(–)-isopino-
campheylamine (IPC), gave lower enantioselectivities.
In the past 20 years, optically active trans-1,2-diamino-
cyclohexane (DACH) has been extensively used as a chiral
ligand and as a chiral appendage in catalysts for use in di-
verse asymmetric reactions.13 For example, DACH has been
used in the synthesis of highly efficient salen ligands used
in the asymmetric epoxidation of alkenes,14 chiral thioureas
used as hydrogen-bonding catalysts,15 and ligands used in
palladium-catalysed allylic alkylations.16 Crystallization of
DACH–BINOL diastereoisomeric complexes have allowed
the resolution of racemic BINOL.17 Derivatives of DACH have
also been used as drugs: notably, complexes with platinum
have been developed as anticancer drugs,18 and water-solu-
ble salen ligands have been used as antibiotics.19
Using the procedure developed by Gawroński and
Kaik,23 monophthaloylation of (1S,2S)-10 was achieved in
excellent yield by heating a solution of (1S,2S)-10, phthalic
anhydride, and PTSA in xylenes under reflux for one hour.
The resulting salt was liberated using saturated aqueous so-
dium hydrogen carbonate overnight to give compound 12.
Eschweiler–Clark methylation of 12 using para-formalde-
hyde and formic acid as the hydrogen donor afforded the
bismethylated product 13 in 85% yield. Subsequent depro-
tection of the phthaloyl protecting group using hydrazine
monohydrate gave the desired compound 14 in 50–60%
yield (Scheme 2).
NH2
NH2
O
O
N
i, ii
iii
iv
N
NH2
N
NMe2
O
O
NH2
12
13
14
(1S,2S)-10
Scheme 2 Reagents and conditions: i) PTSA (1 equiv), phthalic anhy-
We envisaged that a catalyst incorporating DACH or its
derivatives might be a potent chiral inducer with the poten-
tial to enhance enantioselectivities induced by iminium salt
catalysts. With this in mind, we initially aimed to prepare
iminium salt catalyst 9, depicted in Figure 2.
dride (1 equiv), xylenes, reflux, 2 h, 95%; ii) sat. NaHCO3, CH2Cl2, r.t., 16
h, 80%; iii) CH2O (2.5 equiv), HCOOH, reflux, 16 h, 85%; iv) NH2NH2·H2O
(3 equiv), EtOH, reflux, 1 h, 50–60%.
The biphenyl portion was conveniently prepared from
2,2-biphenyl dimethanol (15), which, after treatment with
aqueous hydrobromic acid (24%) at 100 °C for 40 minutes,
gave the oxepine 16. Subsequent treatment of 16 with mo-
lecular bromine in carbon tetrachloride under reflux for
one hour led to carboxaldehyde 17 in 60% yield. Cyclocon-
densation of amine 14 with bromoaldehyde 17 followed by
cation exchange, however, furnished quaternary ammoni-
um salt 18 in 70% yield, rather than iminium salt 9 (Scheme
3). We assume that the formation of iminium salt 9 is fol-
lowed by an intramolecular addition of the nitrogen lone
pair at the iminium carbon atom, leading to the formation
BPh4
N
N
9
Figure 2 An iminium salt catalyst with a DACH derivative chiral ap-
pendage
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P. C. Bulman Page et al.
Letter
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of the thermodynamically more stable product 18. We have
recently used a similar process to prepare tetracyclic oxazo-
lidines.24
Table 1 Asymmetric Epoxidation of Alkenes Using Bisiminium Salts 19
and 20a
Catalyst
Alkene
19
20
Conv. ee
Conf.d
Conv.
(%)b
ee (%)c Conf.d
BPh4
(%)b
(%)c
O
OH
OH
i
ii
iii
O
N
Ph
100
22
(–)-1S,2S
100
26
(–)-1S,2S
Br
N
15
16
17
18
94
55
4
(–)-1S,2Re
(–)-1S,2S
100
100
18
6
(–)-1S,2Re
(–)-1S,2Sf
Scheme 3 Reagents and conditions: i) HBr (24% in H2O), 100 °C, 40 min,
85%; ii) Br2 (1.1 equiv), CCl4, reflux, 1 h, 60%; iii) 14 (1 equiv), EtOH, r.t.,
16 h; then NaBPh4 (1.1 equiv), EtOH, MeCN, r.t., 5 min, 70%.
100
Ph
Ph
Ph
Ammonium salt 18 was subsequently tested in the ep-
oxidation of 1-phenylcyclohexene under standard condi-
tions [ammonium salt (5 mol%), Oxone (2 equiv), NaHCO3
(5 equiv), MeCN–H2O (10:1), 0 °C], but failed to mediate any
epoxidation of the alkene substrate at a catalyst loading of
10 mol%, perhaps suggesting that the potential equilibrium
of 18 with iminium salt 9 is unfavourable.
We conjectured that cyclocondensation of two equiva-
lents of bromoaldehyde 17 with the parent (1S,2S)-diami-
nocyclohexane might generate an interesting C2-symmetric
bisiminium salt species 19, which, we reasoned, might pro-
vide improved enantioselectivity through a more ordered
transition state resulting from higher steric crowding. We
were successful in preparing bisiminium catalysts 19 and
20, by condensation of bromoaldehyde 17 with (1S,2S)-10
and (1S,2S)-(–)-1,2-diphenylethylenediamine, respectively,
followed by treatment with sodium tetraphenylborate and
recrystallization (Scheme 4).25
Me
Ph
100
17
(+)-1R,2R
55
12
(+)-1R,2Rf
a Epoxidation conditions: iminium salt (5 mol%), Oxone (2 equiv), NaHCO3
(5 equiv), MeCN–H2O (10:1), 0 °C, 2 h.
b Conversions were evaluated from the 1H NMR spectra by integration of
alkene vs. epoxide signals.
c Enantiomeric excesses were determined by chiral HPLC on a Chiralcel OD
column, or by chiral GC on a Chiraldex B-DM column.
d The absolute configurations of the major enantiomers were determined
by comparison with literature values except where indicated.
e Epoxidation conditions: iminium salt (5 mol%), Oxone (2 equiv), Na2CO3
(4 equiv), MeCN–H2O (1:1), 0 °C, 4 h.
f 5 h.
lectivities for most substrates, except for 1,2-dihydronaph-
thalene, typically a challenging substrate for chemical
asymmetric epoxidation processes, with catalyst 20 giving
higher enantioselectivity than 19 (55% vs. 18% ee). The dif-
ference in enantioselectivities for this substrate may result
from changes in the transition-state geometry arising from
π stacking between the aromatic groups of catalyst 20 and
the alkene substrate. The reactions are remarkably clean,
with only the alkene substrate, epoxide, and catalyst evi-
dent in the 1H NMR spectra of the product mixtures.
In conclusion, C2-symmetric bisiminium salts derived
from (1S,2S)-diaminocyclohexane (10) are successful or-
ganocatalysts for asymmetric epoxidation, providing enan-
tioselectivities of up to 55%, in the epoxidation of 1,2-dihy-
dronaphthalene. A related ammonium salt 18, arising from
iminium salt 9 by in situ intramolecular conjugate addition
of the nitrogen atom to the iminium unit failed to mediate
epoxidation.
R
N
R
N
O
i, ii
Br
2 BPh4
17
19: R–R = (CH2)4
20: R = Ph
Scheme 4 Reagents and conditions: i) amine (0.5 equiv), EtOH, r.t., 48
h; ii) NaBPh4 (1.1 equiv), EtOH, MeCN, r.t., 5 min, 19, 55%; 20, 52%.
We subsequently utilized these catalysts in the epoxida-
tion of a number of prochiral alkenes (Table 1).26
Interestingly, catalyst 19 was more reactive than 20.
This catalyst provided 90–95% isolated yields for all four ep-
oxides; it induced complete epoxidation of trans-α-methyl-
stilbene and trans-stilbene in two hours, while catalyst 20
required prolonged reaction times of up to five hours to
achieve 100% conversion of trans-α-methylstilbene, and
gave a moderate 55% epoxide conversion with trans-stilbe-
ne in five hours. Both catalysts induced similar enantiose-
Acknowledgment
This investigation has enjoyed the support of NPIL Pharmaceuticals
(UK) Ltd, Loughborough University, and the University of East Anglia.
We are indebted to the Royal Society for an Industry Fellowship (to
PCBP), and to the EPSRC UK National Mass Spectrometry Facility at
Swansea University.
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, 126–130