y Preparation of [Ru3]@Si-EGcap and [Ru3]@Si-BPcap: to a solution
of (m3,Z2,Z3,Z5-acenaphthylene)Ru3(CO)7 (1) (6.5 mg, 0.01 mmol) and
PMHS [Mw = 1500–1900; n = 25.6 (average); Si–H = 4 mmol] in
tetrahydropyran (0.7 mL) was added 1 mmol of either ethylene glycol
(EG) or 2,20-biphenol (BP), and then added 2 mmol of 2-methoxy-
ethanol and the mixture was heated at 50 1C. Dehydrogenative
silylation proceeded smoothly and [Ru3]@Si-EGcap and -BPcap were
formed as a wet gel after 2 h. The wet gel was washed with ether and
then dried under reduced pressure to afford the corresponding dry gel.
The Si–H groups remaining in the siloxane gels were less than 1%
(determined by IR). ICP-MS analysis of the ether solution revealed
that it contained only 0.4 mg of ruthenium (o0.04% of charged [Ru3]
species); 499.96% of [Ru3] species were immobilized in the polysil-
Scheme 3 Isomerization followed by alkylative Claisen rearrange-
ment.
or [Ru3]@Si-BPcap.z Allyl, crotyl and methallyl ethers were
completely converted to the corresponding vinyl ethers. In
contrast, the reaction of cinnamyl ether 2e gave an equilibrium
mixture of 2e and vinyl ether 3e, and no isomerization took
place in the reaction of sterically hindered prenyl ether 2f
(entries 6 and 7). Homoallyl ether 2g underwent isomerization
to give 3c (entry 8). The double bond migration of (R)-2h
(499.9% ee) proceeded without loss of its optical purity
(entry 10). In all of the above cases, treatment of the formed
vinyl ethers with hydrochloric acid in methanol gave the
corresponding alcohols in 71–93% isolated yields. In other
words, double bond isomerization of allyl or homoallyl ethers
catalyzed by [Ru3]@Si-EGcap or [Ru3]@Si-BPcap followed by
hydrolysis provides a practical method for the deprotection of
allyl and homoallyl ethers.6 As an application, the allyl-
selective deprotection of m-acetoxyphenyl allyl ether 2i was
achieved to give m-acetoxyphenol (4i) (entry 11).
oxane networks. IR (KBr) n 1272 (Si–Me), 1030 (Si–O) cmꢁ1
.
z A solution of allyl ether (1 mmol) in n-hexane (3 mL) was added to
the dry [Ru3]@Si catalyst ([Ru3] = 1 mol%), and it was stood at 50 1C
without stirring. After the reaction was complete, the vinyl ether
formed was obtained by filtration and washing the gel with hexane
(10 mL). Treatment of the vinyl ether with aq. HCl in MeOH at room
temperature for 2 h gave the deallylated alcohol, which was purified by
silica gel column chromatography.
8 The facile access to the catalyst immobilization is comparable to the
‘‘Polymer Incarcerated’’ method, which is a physical entrapment of
catalysts in polystyrene derivatives, see ref. 9.
** The diffusion velocity of organic molecules through the gel mem-
brane is very fast. For example, dry [Ru3]@Si-EGcap (ca. 500 mg)
absorbed 1.5 mL of hexane. Addition of DME (1.5 mL) to the
resultant gel gave two phases, liquid phase and swelled gel. At this
time, the liquid phase contains only DME. After 5 min, the ratio of
DME : hexane in the liquid phase attained equilibrium in a ratio of
46 : 54. See ESI.w
Another interesting application of this isomerization is the
selective preparation of a substrate for Claisen rearrangement.
Allyl prenyl ether (5) was selectively converted to ruthenium-
free prenyl 1-propenyl ether (6: E : Z = 40 : 60). A crude
sample of 6 was then subjected to the alkylative Claisen
rearrangement by treatment with Me3Al7 to give bishomoallyl
alcohol 7 in 72% yield (Scheme 3).
1 (a) J. A. Gladysz, Pure Appl. Chem., 2001, 73, 1319; (b) C. A.
McNamara, M. J. Dixon and M. Bradley, Chem. Rev., 2002, 102,
3275; (c) D. J. Cole-Hamilton, Science, 2003, 299, 1702; (d) P.
McMorn and G. J. Hutchings, Chem. Soc. Rev., 2004, 33, 108; (e)
Catalyst Separation, Recovery and Recycling, ed. D. Colo-
Hamilton and R. Tooze, Catalysis by Metal Complexes Book
Series, No. 30, Springer, The Netherlands, 2006.
2 (a) P. Bernfeld and J. Wan, Science, 1963, 142, 678; (b) E. Kokufuta,
S. Matsukawa, T. Ebihara and K. Matsuda, in ACS Symposium
Series, No. 548, ACS, Washington, DC, 1993, ch. 39, p. 507.
3 (a) R. F. Parton, I. F. J. Vankelecom, D. Tas, K. B. M. Janssen,
P.-P. Knops-Gerrits and P. A. Jacobs, J. Mol. Catal. A: Chem., 1996,
113, 283; (b) A. Wolfson, S. Janssens, I. Vankelecom, S. Geresh, M.
Gottlieb and M. Herskowitz, Chem. Commun., 2002, 388; (c) I. F. J.
Vankelecom, D. Tas, R. F. Parton, V. Van de Vyver and P. A.
Jacobs, Angew. Chem., Int. Ed. Engl., 1996, 35, 1346; (d) D. F. C.
Guedes, T. C. O. Leod, M. C. A. F. Gotardo, M. A. Schiavon, I. V.
P. Yoshida, K. J. Ciuffi and M. D. Assis, Appl. Catal., A, 2005, 296,
120; (e) M. T. Mwangi, M. B. Runge and N. B. Bowden,
J. Am. Chem. Soc., 2006, 128, 14434, and references cited therein.
4 Y. Motoyama, K. Mitsui, T. Ishida and H. Nagashima, J. Am.
Chem. Soc., 2005, 127, 13150.
5 For similar stabilization effects, see: (a) B. P. S. Chauhan, J. S.
Rathore and T. Bandoo, J. Am. Chem. Soc., 2004, 126, 8493; (b) B.
P. S. Chauhan and J. S. Rathore, J. Am. Chem. Soc., 2005, 127,
5790. Also see ref. 3.
6 Reviews: (a) F. Guibe, Tetrahedron, 1997, 53, 13509; (b) T. W.
Green and P. G. M. Wuts, Protective Groups in Organic Synthesis,
John Wiley & Sons, New York, 3rd edn, 1999. Recent representa-
tive papers: (c) C. Cabot, P. I. Dalko and J. Cossy, Tetrahedron
Lett., 2002, 43, 1839; (d) S. Tanaka, H. Saburi and M. Kitamura,
Adv. Synth. Catal., 2006, 348, 375.
In summary, the siloxane gels containing organoruthenium
species, [Ru3]@Si-EGcap and [Ru3]@Si-BPcap, have been de-
monstrated for the first time to be a new type of heterogenized
molecular catalyst. They are easily available by simply mixing
the catalyst, cross-linker, capping reagent, and PMHS. This is
in sharp contrast to tedious procedures for conventional cata-
lysts which are prepared by anchoring a ligand to a support
followed by complexation with transition metals.88 It is im-
portant that success of the present catalytic isomerization
without catalyst leaching and facile separation of the product
is first achieved by the special feature of [Ru3]@Si-EGcap
;
substrates, products, and solvents rapidly permeate through a
gel membrane,** whereas the [Ru3] species is immobilized in the
polymer networks throughout the reaction. We now envision
construction of inner structures of the siloxane gels by the
appropriate choice of both cross-linker and capping reagents.
Investigations in this line and application of the resulting gels to
chemo- and stereoselective reactions are underway.
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
7 (a) K. Takai, I. Mori, K. Oshima and H. Nozaki, Tetrahedron
Lett., 1981, 22, 3985; (b) Y. Yamamoto, R. Fujikawa and N.
Miyaura, Synth. Commun., 2000, 30, 2383.
8 Representative papers: (a) R. Yoshida, T. Takahashi, T. Yamaguchi
and H. Ichijo, J. Am. Chem. Soc., 1996, 118, 5134; (b) F. Keller, H.
Weinmann and V. Schurig, Chem. Ber., 1997, 130, 879; (c) Y.-S. Fu
and S. J. Yu, Angew. Chem., Int. Ed., 2001, 40, 437; (d) A. Hu, H. L.
Ngo and W. Lin, J. Am. Chem. Soc., 2003, 125, 11490.
9 (a) R. Akiyama and S. Kobayashi, J. Am. Chem. Soc., 2003, 125,
3412; (b) S. Kobayashi, H. Miyamura, R. Akiyama and T. Ishida,
J. Am. Chem. Soc., 2005, 127, 9251, and references therein.
Notes and references
z Occluded homogeneous catalysts in polydimethylsiloxane (PDMS)
derivatives were reported: zeolite-filled PDMS membranes,3a silica-
filled PDMS films,3b elastomeric type PDMS membranes,3c hybrid
siloxane films,3d and PDMS slabs.3e
ꢀc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 5321–5323 | 5323