C O M M U N I C A T I O N S
(i) Mg2+ site, which is of Lewis-acid type, (ii) O2- site, which is
of Lewis-base type, (iii) lattice-bound hydroxyls, (iv) isolated
hydroxyls, and (v) anionic and cationic vacancies.10 NAP-MgO has
single-crystallite polyhedral structure, which has high surface
concentrations of edge/corner and various exposed crystal planes
(such as 002, 001, 111), which leads to inherently high surface
reactivity per unit area. Thus, NAP-MgO indeed display the highest
activity compared to that of NA-MgO and CM-MgO. CSC and
epoxidation of deactivated olefins are naturally driven by base
catalysts, and accordingly, the surface -OH and O2- of these oxide
crystals are expected to trigger these reactions. To examine the role
of -OH, the Sil-NA-MgO and Sil-NAP-MgO,10c devoid of free
-OH, are tested in CSC and epoxidation reactions. It was found
that the silylated MgO samples had longer reaction times than the
corresponding MgO samples in CSC and there was essentially no
epoxidation reaction. These results indicate that Brønsted hydroxyls
are sole contributors for the epoxidation reaction, while they add
on to the CSC, which is largely driven by Lewis-basic O2-sites.When
protected hydroxyls of DET, (+)-diethyl-2,3-O-isopropyledene-R,R-
tartrate (Table 3, SI), was used in place of DET in the AE reaction,
no ee was observed, which establishes that the hydrogen-bond
interactions between the -OH groups of DET and MgO are
essential for the induction of enantioselectivity. Although both the
NAP-MgO and NA-MgO possess defined shapes and the same
average concentrations of surface -OH groups, a possible rationale
for the display of higher ee by the NAP-MgO is that the -OH
groups present on edge and corner sites on the NAP-MgO are more
isolated and accessible for the DET, whereas on NA-MgO relatively
large portions of the -OH’s are situated on flat planes in closer
proximity with each other and thus are hindered.6b Conversely,
CM-MgO, which showed no ee, has assorted crystals.
XPS spectrum of the TBHP treated-NAP-MgO for the O 1s
exhibit two lines at 530.3 and 532.0, which can be attributed to
lattice oxygen in MgO and dioxygen of magnesium peroxide,11
respectively (Figure 2, SI). This provides evidence that peroxide
is formed on interaction with the Lewis-acidic site of Mg+ of
NAP-MgO. An endotherm at 310 °C that gives off a fragment (m/z
) 89 amu) corresponding to tert-butyl peroxide in DTA-TGA-MS
of the TBHP-treated NAP-MgO (Figure 3, SI) further reiterates
the formation of surface tert-butyl peroxide. When the TBHP-treated
NA-MgO and CM-MgO are subjected to DTA-TGA-MS, no such
endotherm is visible that corresponds to magnesium peroxide. This
is due to presence of higher concentrations of Mg+ ion (20%) in
NAP-MgO. This also reinforces the argument in favor of a higher
activity of NAP-MgO.
Scheme 2. Proposed Mechanism for Asymmetric Epoxidation of
Unsaturated Ketones Using TBHP as an Oxidant
The chiral epoxy ketones obtained in the present AE reaction
catalyzed by NAP-MgO show comparable ee’s with an absolute
configuration similar to that of the homogeneous system comprising
dibutylmagnesium, DET, and TBHP. Thus, the nanocrystalline
NAP-MgO with its defined shape, size, and accessible -OH groups
allows the chemisorption of TBHP, DET, and olefin on its surface
to evolve successfully the single-site chiral catalyst by the successful
transfer of molecular chemistry to surface metal-organic chemistry
to provide the optimum ee’s.
Acknowledgment. K.V.S.R thanks the CSIR India for the award
of SRF. Nanocrystalline MgO catalysts were obtained from
NanoScale Materials Inc., Manhattan, Kansas, U.S.A.
Supporting Information Available: Characterization of the catalyst
surface intermediates and chiral epoxy ketones; experimental procedures
(PDF). This material is available free of charge via the Internet at http://
pubs.acs.org
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In the AE, the Mg ion of MgO reacts with TBHP to produce
magnesium peroxide (ROO-) and also interacts with the oxygen
of the carbonyl function. The DET bound to the hydroxyls of
NAP-MgO directs the delivery of nucleophilic oxygen of the
peroxide12 stereoselectively to give the chiral epoxy ketones
(Scheme 2). The presence of electron-withdrawing groups on either
of the two aromatic rings of the chalcone facilitate the formation
of resonance-stabilized oxyanion, which might account for the
higher ee’s.
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