Constraining Asymmetric Organometallic Catalysts within Mesoporous Supports Boosts Their Enantioselectivity

Article abstract of DOI:10.1021/ja030381r

By constraining tethered asymmetric organometallic catalysts within the nanopores of silica supports so as to increase the interaction between the pore wall and the active center (and hence to restrict access of the reactant to the catalyst), a significan

Full text of DOI:10.1021/ja030381r

Published on Web 11/15/2003
Constraining Asymmetric Organometallic Catalysts within Mesoporous
Supports Boosts Their Enantioselectivity
Robert Raja,*,‡ John Meurig Thomas,*,†,§ Matthew D. Jones, Brian F. G. Johnson, and
‡
‡
|
David E. W. Vaughan
DaVy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street,
London, U.K. W1S 4BS, Department of Chemistry, UniVersity of Cambridge, Lensfield Road,
Cambridge, U.K. CB2 1EW, Department of Materials Science and Metallurgy, UniVersity of Cambridge,
Cambridge, U.K. CB2 3QZ, and Materials Research Laboratory, PennsylVania State UniVersity,
UniVersity Park, PennsylVania, 16802
Received June 25, 2003; E-mail: robert@ri.ac.uk
The availability of surfactant-templated mesoporous siliceous
solids with sharply defined diameters that fall within the range of
2
0-250 Å has opened up new possibilities in heterogeneous
1
catalysis. Such large pores enable quite sizable metal complexes
to be tethered on to their inner walls so that appreciable (surface)
concentrations of accessible, well-definedsby in situ XAFS, FTIR,
2,3
and multinuclear CP/MAS NMR sactive centers may be as-
sembled in a spatially uniform fashion within these high area (ca.
2
-1
3
00-800 m g ) siliceous supports. It has previously been shown
4
that highly active epoxidation, as well as a variety of selective
hydroxylation1 hydrogenation, and allylic amination catalysts
c,3
5a,6
6
Figure 1. (A) Pore size distribution curve for the mesoporous sample
(Davison 923), which has a value of 38 Å mean pore diameter. (B)
Molecular structure of the Rh(I) complex to the chiral ligand PMP and
COD {(S)-(+)-1-(2-pyrrolidinylmethyl)-pyrrolidine and cyclo-octadiene}.
CCDC ref 212362; selected distances (Å) and angles (deg); Rh(1)-N(1)
yielding important organic products, may be designed in this
manner.
It has also been argued2b,6,7 that, by constraining the tethered
asymmetric organometallic catalyst so as to increase the interaction
between the pore wall and the active center and hence to restrict
access of reactant to the catalyst, an improvement in the enanti-
oselectivity ensues. In essence, the spatial constraint imposed upon
the chiral catalyst by the walls of the pores of the support enhances
its asymmetry by predisposing access of the prochiral reactant to
the active center (Supporting Information).8
)
2.1193(18), Rh(1)-N(2) ) 2.1738(19), Rh(1)-C(14) ) 2.113(2), Rh-
(1)-C(10) ) 2.126(2), Rh(1)-C(15) ) 2.143(2), Rh(1)-C(11) ) 2.152-
(2), N(1)-Rh(1)-N(2) ) 82.16(7). The ellipsoids are drawn at the 50%
probability level, and the H-atoms and triflate cations have been omitted
for clarity.
diameters13 of the pores of these silicas were, respectively, 38, 60,
and 250 Å, and their respective surface areas are 700, 500, and
Because we believe that this approach constitutes a potentially
powerful means of effecting highly enantioselective syntheses and
2
-1
3
00 m g
.
Rather than tethering the cationic Rh(I) complex containing the
9
10
other organic processes, we summarize here a systematic study
chiral diamino ligand (and cyclooctadiene, COD) using our
customary2 covalent procedure (involving 3-bromopropyl-trichlo-
rosilane to link up with a surface silanol group), we have instead
employed the noncovalent immobilization approach recently de-
of a range of porous silicas in each of which there is a very narrow
spread of pore diameter and on to the inner walls of which one of
four distinct cationic Rh(I) or Pd(II) complexes containing chiral
bidentate ligands has been anchored. Partly because of their ease
of synthesis,11 diamino ligands (of three kinds; see below), rather
than diphosphino ligands, attached to the Rh(I) or Pd(II) center,
were used. Rather than employing as porous supports the popular
,9
14
scribed by de Rege et al. In this method, a surface-bound triflate
-
(
(
(
CF SO
3 3
) counterion securely anchors the cationic Rh(I)(COD)
14,15
Figure 1B) or the Pd(allyl) diamino complex to the inner wall.
This straightforward method circumvents the need for ligand
(
organic-template-derived) MCM types (which have nonintersecting
12
modification to secure covalent tethering, and its advantages are
pores) and related silicas such as SBA-15, we have used a set of
commercially available desiccant silicas having narrow pore size
distributions (Figure 1 A) (designated Davison 923, 634, and 654).
These are made by reacting sodium silicate with a strong mineral
described fully elsewhere.1
The four constrained chiral catalysts were: [Rh(COD)(S)-(+)-
-(2-pyrrolidinylmethyl)-pyrrolidine]-CF SO , [Pd(allyl) (S)-(+)-
-(2-pyrrolidinylmethyl)-pyrrolidine] CF SO , [Rh(COD) (S)-(-)-
SO , and [Rh(COD) (1R,2R)-
+)-1,2-diphenyl-ethylenediamine] CF SO , which we abbreviate
5b
)
1
1
3
3
3
3
acid (usually sulfuric acid), with the pore size being controlled by
gel time, final pH, temperature, concentration of reactants, etc.12
2-aminomethyl-1-ethyl-pyrrolidine] CF
3
3
(
3
3
Compared to MCM-41-type silicas, they are much lower in cost,
more thermally and mechanically stable, less susceptible to
structural collapse, and available in a range of granularities. They
also have some intersecting pores that facilitate the diffusion of
the reactant species to the immobilized catalyst. The average
to Rh(COD)PMP, Pd(allyl)PMP, Rh(COD)AEP, and Rh(COD)-
DED, respectively. The test reaction (see Supporting Information
for experimental conditions) was the asymmetric hydrogenation of
methyl benzoylformate to its corresponding methyl mandelate.
In their homogeneous form, only the Rh(COD)PMP and
Pd(allyl)PMP exhibit any significant enantioselectivity (ee) under
the reaction conditions (see Table 1) employed by us, whereas the
other two homogeneous catalysts (namely, Rh(COD)AEP and
‡
Department of Chemistry, University of Cambridge.
†
Davy Faraday Research Laboratory.
§
Department of Materials Science and Metallurgy, University of Cambridge.
Pennsylvania State University.
|
14982
9
J. AM. CHEM. SOC. 2003, 125, 14982-14983
10.1021/ja030381r CCC: $25.00 © 2003 American Chemical Society

C O MMU N I C A T I O N S
Table 1. Asymmetric Hydrogenation of Methyl Benzoylformatea
a
Methyl benzoylformate ≡ 0.5 g; solvent (methanol) ≡ 30 mL; catalyst (homogeneous) ≈ 10 mg; (heterogeneous) ≡ 50 mg; H2 ≡ 20 bar; T ) 313 K.
b
c
Conversion % ) [(moles of initial substrate - moles of residual substrate)/(moles of initial substrate)] × 100. Enantiomeric excess values (for the
product methyl mandelate) were calculated from the peak areas of the enantiomers using the following formula: ee ) ([R] - [S]) × 100/([R] + [S]). ee’s
17 d
for methyl mandelate and conversions were determined at Bayer AG, Business Unit Chemicals, Leverkusen Germany.
TOF ) [(molsubstrate-converted)(molcomplex(Rh
-
1
-1
or Pd)/silica)
h ].
(
2) (a) Maschmeyer, T.; Oldroyd, R. D.; Sankar, G.; Thomas, J. M.; Shannon,
I. J.; Kleptko, J.; Masters, A. F.; Catlow, C. R. A. Angew. Chem., Int. Ed.
Engl. 1997, 36, 1639. (b) Thomas, J. M. Angew. Chem., Int. Ed. 1999,
38, 3588.
(3) Bell, A. T. Science 2003, 299, 1688.
(
4) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature 1995, 378,
159.
Rh(COD)DED) did not display any significant ee. This is probably
because the bulkiness of PMP in comparison to AEP and DED
exerts further spatial congestion in the vicinity of the active center.
Table 1 summarizes the results with all four chiral catalysts and
shows that, as expected from arguments given above, chiral
restriction does indeed boost the ee values in a manner that logically
reflects the declining influence of spatial constraint in proceeding
from the 38 Å to the 60 Å to the 250 Å pore diameter silica (see
Supporting Information). For the heterogeneous catalysts, the trend
with Rh(COD)PMP is mirrored by both AEP and DED ligands,
and it is clear that even when some of the asymmetric catalysts
exhibit significant ee’s under homogeneous conditions, their
performance is much enhanced when immobilized in a constrained
environment. It is also noteworthy that the noncovalent method of
anchoring the organometallic catalyst does not lead to facile
leaching when the catalyst is recycled. Further experimental details
are given in two recently filed patents.16
(
(
(
5) (a) Thomas, J. M.; Johnson, B. F. G.; Raja, R.; Sankar, G.; Midgley, P.
A. Acc. Chem. Res. 2003, 36, 20 and references therein. (b) Bae, S. J.;
Kim, S.-W.; Hyeon, T.; Kim, B. M. Chem. Commun. 2000, 31.
6) Johnson, B. F. G.; Raynor, S. A.; Shephard, D. S.; Maschmeyer, T.;
Thomas, J. M.; Sankar, G.; Bromley, S. T.; Oldroyd, R. D.; Gladden, L.
G.; Mantle, M. D. Chem. Commun. 1999, 1167.
7) (a) Thomas, J. M. Faraday Discuss. 1995, 100, C9. (b) Thomas, J. M.;
Maschmeyer, T.; Johnson, B. F. G.; Shephard, D. S. J. Mol. Catal. A
1999, 141, 139.
(8) Maschmeyer, T. Oral report to the Davy Faraday Laboratory, March 1995.
Unpublished work. See also refs 7(a) and 7(b).
(9) Jones, M. D.; Raja, R.; Thomas, J. M.; Johnson, B. F. G.; Lewis, D. W.;
Rouzard, J.; Harris, K. D. M. Angew. Chem., Int. Ed. 2003, 42, 4326.
(10) (a) Haller, G. L. J. Catal. 2003, 216, 12. (b) Derouane, E. G.; Andre, J.
M.; Lucas, A. A. J. Catal. 1988, 110, 1998. In all these studies, however,
there was no reference to enantioselectivity, and the range of pore
diameters explored was very narrow (e.g., 25-36 Å in ref 10b).
(
11) Tommasino, M. L.; Thomazeau, C.; Touchard, F.; Lemaire, M. Tetrahe-
dron: Asymmetry 1999, 10, 1813.
(12) Acker, A. G. J. Colloid Surf. Sci. 1970, 32, 41.
(
13) These values were obtained from the dV/d log(D) plots pertaining to the
Supporting Information Available: Experimental details, analyti-
desorption isotherm of N on the silicas.
2
cal methods employed, and single-crystal structure details are given
(14) de Rege, F. M.; Morita, D. K.; Ott, K. C.; Tumas, W.; Broene, R. D.
(PDF, CIF). This material is available free of charge via the Internet at
Chem. Commun. 2000, 1797.
http://pubs.acs.org.
(15) (a) Bianchini, C.; Burnaby, D. G.; Evans, J.; Frediani, P.; Meli, A.;
Oberhauser, W.; Psaro, R.; Sordelli, L.; Vizza, F. J. Am. Chem. Soc. 1999,
1
21, 5961. (b) Rouzard, J.; Jones, M. D.; Raja, R.; Johnson, B. F. G.;
References
Thomas, J. M.; Duer, M. J. HelV. Chim. Acta 2003, 86, 1753.
(
1) For selected examples, see: (a) Thomas, J. M. Nature 1994, 368, 289.
(16) Raja, R.; Thomas, J. M.; Jones, M. D.; Johnson, B. F. G. German Patents
DE 10305946 and DE 10305943, filed Feb 12, 2003.
(
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(
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