Photon Factory Activity Report 2004 #22 Part B (2005)
Chemistry

9C,12C/2004G081

The Structures of Surface-Functionalized Chiral Cu-BOX Complexes
for Asymmetric Diels-Alder Reaction
Satoka TANAKA, Mizuki TADA, and Yasuhiro IWASAWA*
The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Introduction
The design of asymmetric heterogeneous catalysts isstill a challenging topic in catalytic chemistry. We havedesigned and prepared surface-functionalized SiO2supported Cu-bis(oxazoline) (BOX) complexes forasymmetric Diels-Alder reaction and found that thesurface functionalization with an achiral silane-coupling
reagent
3-methacryloxypropyltrimethoxysilane
remarkably amplified enantioselectivity for the reaction.
The structures of active Cu complexes were characterizedby Cu K-edge XAFS.
Experimental
Preparation: (S)-t-Bu-BOX modified with twotriethoxysilyl groups was immobilized on a SiO2 surface.
The SiO2-supported Cu-BOX complex (Scheme 1 (b))
was prepared by the coordination of Cu(ClO4)2·6H2O toThe surface
the SiO2-supported BOX ligand (a).
supporting the BOX ligand was functionalized withmethacryloxypropyltrimethoxysilane, followed by thecoordination of the Cu precursor (c).
EXAFS measurements and analysis: Cu K-edge
EXAFS spectra were measured at 298 K in a transmission
and a fluorescence mode at BL-9C and 12C stations.
3Background was subtracted by AUTOBK and then k weighted EXAFS functions were Fourier-transformed
into R space. FT spectra were fitted in R space by
FEFFIT program. Backscattering amplitudes and phaseshifts were calculated by FEFF8 code.
Results and discussion
Similar Cu K-edge XANES spectra were observed forhomogeneous Cu(BOX)(ClO4)2, (b), and (c), indicatingbivalent Cu species. Table 1 shows the curve-fitting
results of EXAFS functions at Cu K-edge forCu(BOX)(ClO4)2, (b), and (c). UV/VIS and ESR spectrasuggested that both supported Cu complexes (b) and (c)possessed two Cu-N bonds. The curve-fitting analysis forthree Cu-BOX complexes was carried out with Cu-N shell,
whose bond distance and CN were 1.98 Å and 4,
respectively, for all Cu-BOX complexes (Table 1). Fittingwith Cu-O shell was suggested similar 4-coordinated Cucomplexes. Therefore, the supported complexes (b) and(c) had two Cu-O bonds and two Cu-N coordination
bonds, which were similar to Cu(BOX) (ClO4)2.
According to g-values calculated from ESR spectra, both(b) and (c) complexes have square-planar conformationon the surface. The determined structures of the
supported Cu-BOX catalysts are presented in Scheme 1.

34

Users' Report

Asymmetric Diels-Alder reactions were carried out on(b) and (c). While the supported complex (b) showed low
enantioselectivity (15 ee%), the achiral methacrylfunctionalizedcomplex(c)amplifiedtheenantioselectivity up to 65 ee%. The EXAFS analysis
showed that there was no change in the local coordinationof the Cu-BOX complexes (b) and (c). Thus, it issuggested that the oxygen of methacryl moiety may
interact with the NH group of the chiral BOX ligand byhydrogen bonding and an asymmetrically-assembled
structure may be created, resulting in the large increase inenantioselectivity on the surface.
Table 1: Curve-fitting results of Cu K-edge EXAFS
spectra for homogeneous Cu(BOX)(ClO4)2, supported CuBOX complex (b), and surface-functionalized Cu-BOXcomplex (c) measured at 293 KShell
SampleCNR /Å
σ2 / Å2
Cu(BOX)(ClO )Cu-N 4.5±0.6 1.98±0.01 (4±1)x10-3
Cu-N 4.4±0.5 1.99±0.01 (2±1)x10-3(b)Cu-N 4.1±0.5 1.98±0.01 (4±1)x10-3(c)k = 3-10 Å-1, R = 1.1-2.0 Å.
4 2

Scheme 1: Preparation of supported Cu-BOX complex (b)and surface-functionalized Cu-BOX complex (c) on SiO2.
* iwasawa@chem.s.u-tokyo.ac.jp