Angewandte
Chemie
Table 1: Hydrosilylation catalyzed by the [silica]-SMAP-Rh system.[a]
strongly s-donating SMAP ligand may also have some
influence on the catalytic properties of the [silica]-SMAP-
Rh system.
In summary, a functionalized silica gel, [silica]-SMAP,
consisting of a compact, bridged bicyclic trialkylphosphane
was developed. The monocoordinating character and useful-
ness as a ligand for heterogeneous catalysts were demon-
strated in the Rh-catalyzed hydrosilylation, which showed
unprecedented high tolerance toward hindered ketones and
silanes. More detailed analysis of the surface structure, use of
other silica supports such as mesoporous silica, and the
application to other synthetically useful reactions will be the
subjects of future work.
Entry
Ketone
Hydrosilane
t[b]
Yield [%]
1
2
3
PhMe2SiH
Et3SiH
tBuMe2SiH
<5 min
<5 min
30 min
93[c]
100[c]
100[c]
4
5
6
PhMe2SiH
Et3SiH
tBuMe2SiH
1 h
4 h
99[c]
99[c]
10 h
100[c]
7
8
9
PhMe2SiH
Et3SiH
tBuMe2SiH
96 h
12 h
30 h
92[c]
96[d]
97[d]
Received: March 3, 2007
[a] Reagents and conditions: 4 (1 mmol), R3SiH (1.2 mmol), [{RhCl-
(C2H4)2}2] (0.005 mmol), 1 (0.010 mmol), benzene (1 mL). Conversion of
4 is 100% in all entries. [b] Approximate time required for 100%
conversion. [c] Yield based on GC. [d] Yield of isolated product.
Keywords: hydrosilylation · immobilization · phosphanes ·
phosphorus heterocycles · rhodium
.
[1] Ph2P(CH2)n-functionalized silicas were used for the Rh-catalyzed
hydrosilylation of 1-hexene. The activity with the Ph2PCH2 group
was tenfold higher than those with the longer-chain analogues:
catalyst could be recovered by simple filtration and be reused
for the same transformation (100% conv. of 4b with 1.2 equiv
of tBuMe2SiH, 10 h, 238C, six times, total TON = 700).
The impact of the immobilization on the tolerance toward
the hindered substrates is overwhelming. The catalysts
(1 mol% Rh) prepared from the homogeneous Ph-SMAP
(2) ligand or other various conventional phosphanes including
PPh3, PtBu3, and 1,2-bis(diphenylphosphanyl)ethane (dppe)
showed almost no activity for the transformation correspond-
ing to entry 6 in Table 1 (4b, tBuMe2SiH, 1 mol% Rh, RT,
10 h, < 2% conv.), irrespective of the Rh/P ratio ranging from
1:1 to 1:2. The most likely reason for the lower activity of the
homogeneous catalysts for Rh/P 1:1 as compared with that of
the [silica]-SMAP-Rh system would be the lack of a mono-
phosphane–rhodium species in the former.[8] In fact, the
strong tendency of compact phosphane 2 to congregate on the
rhodium center was confirmed in the 31P NMR spectra for a
mixture of [{RhCl(C2H4)2}2] and Ph-SMAP (2) (P/Rh 1:1, 2:1,
3:1, 4:1) in C6D6 in ethylene atmosphere (see the Supporting
Information).
As reasonably expected from the aforementioned results
of the titration of [silica]-SMAP (1) with [{RhCl(cod)}2], the
use of excess 1 caused no change in the rate of the
hydrosilylations. This result is in sharp contrast to the results
with a homogeneous catalyst system involving the soluble
phosphane Ph-SMAP (2) in the hydrosilylation with reactive
substrate couples such as cyclohexanone/PhMe2SiH. The
reaction proceeded smoothly with a P/Rh ratio of 1:1
(1 mol% Rh, 1 h, 100% conv.; even faster than with a BSP
ligand; BSP = bowl-shaped phosphane), but the second
equivalent of 2 drastically inhibited the reaction (see the
Supporting Information).
ˇ
a) M. Czakovµ, M. Capka, J.Mol.Catal. 1981, 11, 313; b) Z. M.
ˇ
Michalska, M. Capka, J. Stoch, J.Mol.Catal. 1981, 11, 323.
[2] For a related study, in which a bulky phosphane attached to a rigid
polymer support was used for the Suzuki–Miyaura coupling, see:
Q.-S. Hu, Y. Lu, Z.-Y. Tang, H.-B. Yu, J.Am.Chem.Soc. 2003,
125, 2856.
[3] SMAP = silicon-constrained monodentate alkylphosphane. a) A.
Ochida, K. Hara, H. Ito, M. Sawamura, Org.Lett. 2003, 5, 2671;
b) A. Ochida, S. Ito, T. Miyahara, H. Ito, M. Sawamura, Chem.
Lett. 2006, 35, 294.
[4] The approximate SMAP/Me3Si ratio for 1 was 1:4 according to
the 29Si CP/MAS NMR spectrum.
[5] a) O. Niyomura, M. Tokunaga, Y. Obora, T. Iwasawa, Y. Tsuji,
Angew.Chem. 2003, 115, 1325; Angew.Chem.Int.Ed. 2003, 42,
1287; b) O. Niyomura, T. Iwasawa, N. Sawada, M. Tokunaga, Y.
Obora, Y. Tsuji, Organometallics 2005, 24, 3468; see also: c) A.
Ochida, M. Sawamura, Chem.Asian J. , 2007, 2, 609; d) H. Ito, T.
Kato, M. Sawamura, Chem.Lett. 2006, 35, 1038.
[6] Lipshutz et al. reported that some Cu–bisphosphane catalysts are
effective for the ketone hydrosilylation with tBuMe2SiH, but their
reactivity toward highly hindered ketones has not extensively
been studied: B. H. Lipshutz, C. C. Caires, P. Kuipers, W. Chris-
man, Org.Lett. 2003, 5, 3085.
[7] Nolan and co-workers reported on Cu–NHC catalysts that are
effective for the hydrosilylation of hindered ketones with Et3SiH.
The Cu–NHC-catalyzed reaction, however, demands high catalyst
loading (3 mol%) and high temperature (55–808C); furthermore,
its applicability using hindered silanes, such as tBuMe2SiH, has
yet to be demonstrated: a) S. Díez-Gonzµlez, H. Kaur, F. K. Zinn,
E. D. Stevens, S. P. Nolan, J.Org.Chem. 2005, 70, 4784; b) S. Díez-
Gonzµlez, N. M. Scott, S. P. Nolan, Organometallics 2006, 25,
2355.
[8] DFTcalculations of the P-donor power of MeO-SMAP, a model
compound for 1, indicated that the electronic effect of the support
is negligible; see reference [3a].
[9] The reaction of 1 with [{RhCl(C2H4)2}2] should form [silica]-
[RhCl(C2H4)2(smap)]. This catalyst precursor should release the
two ethylene ligands upon rapid hydrosilylation prior to entering
a catalytic cycle. [silica]-[RhCl(cod)(smap)] can also be used but
needs to be aged with a hydrosilane before the addition of a
ketone.
It is presumed that the monocoordination of the compact
ligand creates an exceptionally sparse catalytic environment[9]
and induces a unique reaction mechanism that is indispens-
able for the activity toward the sterically demanding sub-
strates. Besides the major effects of the P coordination
number and the compactness, the electronic effect of the
Angew. Chem. Int. Ed. 2007, 46, 5381 –5383
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