R.A. Feldman, J.M. Fraile / Catalysis Communications 83 (2016) 74–77
75
ligands (Fig. 1), noting that the nature of both the central bridge
isopropylidene or methylimino) and the substituent in the oxazoline
(
ring (phenyl, isopropyl, tert-butyl) have an influence on the stability
of the resulting copper complex [21].
The supported chiral catalysts were first tested in the
cyclopropanation of styrene (Scheme 1, R = H) with ethyl diazoacetate,
using an excess of styrene as solvent at 90 °C, which are the best condi-
tions found in our previous work [18]. At 90 °C in the homogeneous
phase, cationic polymerization of styrene rapidly occurs and so the ho-
mogeneous reactions were carried out at room temperature. This differ-
ence in temperature needs to be taken into consideration when
comparing the results, which are summarized in Fig. 2.
Fig. 1. Ligands used in this work.
In all cases the yield of cyclopropanes (Fig. 2A, bars) was better than
in the homogeneous phase, at least from the second run. An overall
trend is that the yield is consistently lower in the first run than in the
second run, and this may be attributed to the need for the reduction
of Cu(II) to Cu(I) with diazoacetate to build up the catalytically active
sites. Remarkably, the reduced Cu(I) sites seem to be stable upon
recycling, which can be inferred from the similar yields that were in
the range of 65–80%, in runs 2–5. The only exception to this behaviour
Cu-Al-MCM41 (500 mg, 0.1 mmol Cu) was added to a solution of the
corresponding chiral ligand (0.2 mmol) in 600 μL of dichloromethane,
the container was shaken to ensure that the ligand was evenly distribut-
ed, and this supported complex was then immediately used as a
catalyst.
2
2
.2. Catalytic tests
t
is the catalyst modified with box Bu, which reaches a maximum yield
.2.1. Homogeneous cyclopropanation
in the third run and is then deactivated in the successive reactions.
The diastereoselectivity to the trans cyclopropane is also consistently
lower in the case of the heterogeneous catalysts (Fig. 2A, dots). The
trans/cis ratio in solution varies from 68:32 to 76:24, but the modified
Cu-Al-MCM41 catalysts lead to mixtures with ratios close to 50:50.
Interpreting this behaviour would require one to be able to delineate
whether this is due to a confinement effect imparted by the support
or it is due to the higher reaction temperature.
At room temperature, EDA (2.5 mmol, 285 mg) dissolved in dichlo-
romethane (0.5 mL) was added over the course of 2 h with a syringe
pump to a solution containing the copper catalyst (0.025 mmol), n-dec-
ane (100 mg) and alkene (styrene or α-methyl styrene, 2 mL). After the
addition, the reaction was left to stir for 30 min and then analyzed for
the reaction yield and selectivity by CG.
2
.2.2. Heterogeneous cyclopropanation
The supported catalyst (100 mg, 0.02 mmol Cu) was suspended in
Enantioselectivities (Fig. 2B) are consistently lower for the heteroge-
neous catalysts, compared to the analogous homogeneous phase cata-
lysts, and again, one could attribute this to either a confinement or
temperature effect. Moreover, there are two separate trends of behav-
iour depending on the chiral ligand used. With the three
the alkene (styrene or α-methyl styrene, 2 mL) with n-decane
100 mg) and the mixture was heated at 90 °C. EDA (2.5 mmol,
85 mg) dissolved in dichloromethane (0.5 mL) was added over the
(
2
i
course of 15 min with a syringe pump. After the addition, the catalyst
was separated by centrifugation, the products were analyzed by GC,
and the next reaction was performed by immediately re-suspending
the catalyst in the alkene.
azabis(oxazolines) and box Pr the enantioselectivity values are moder-
ate and stable upon recycling, with values around 60% e.e. occurring
t
for aza Bu, as the best result. Even in the case of azaPh the enantiomeric
excess increases until it reaches a stable value of 47% e.e. in the fourth
run, and this begs the question as to whether some kind of re-distribu-
i
3
. Results and discussion
tion of the ligand occurs during the initial reactions. The Pr-substituted
ligands lead to enantioselectivities around 40 and 30% e.e. for aza and
t
The Cu-Al-MCM41 catalyst was prepared following the conditions
bis(oxazoline), respectively. On the contrary, with boxPh and box Bu
described in our most recent paper [18]. The chosen support was an
Al-MCM41 with a Si/Al molar ratio of 10, and with expanded porosity
obtained by introduction of 1,3,5-trimethylbenzene as co-template, to
yield a solid characterized to have an internal volume of 1.2 mL/g, BET
surface area of 628 m /g, and BJH pore diameter of 92 Å. Copper was in-
2
troduced by incipient wetness impregnation of Cu(OTf) , and the solid
was calcined at 450 °C under an air flow with the view to produce a
thermodynamically more stable interaction with the support and keep-
ing the oxidation state +2 in well isolated copper centres, as shown by
SEM-EDX. The copper may then be modified by incipient wetness im-
pregnation with different chiral bis(oxazoline) and azabis(oxazoline)
the enantioselectivities are already low in the first run and decreases
upon reuse. It seems plausible that these differences in behaviour can
be attributed to the different stabilities of the ligand-Cu complexes,
which are higher for azabox as demonstrated by theoretical calculations
[21] and the better performance and recoverability when immobilized
by electrostatic interactions on clays and nafion-like solids [22], in
ionic liquid phases [23] or by covalent grafting to organic polymers
[24]. Even this higher stability has been pointed out as the reason for
the better performance of azabis(oxazoline)-metal complexes as cata-
lysts in other reactions, such as the enantioselective reduction of unsat-
urated esters with cobalt catalysts, both in homogeneous phase [25] and
2
Scheme 1. Cyclopropanation of styrene and α-methylstyrene with ethyl diazoacetate.
Feldman, Robert A.
