2102 J. Am. Chem. Soc., Vol. 121, No. 10, 1999
Adam et al.
with the optically active (S)-1h. The striking differences between
the two oxidation systems, MTO/85% H2O2 and MTO/UHP,
point to the essential role played by the urea additive in the
catalytic Si-H insertion.
The question arises what the possible causes are for the higher
reactivity and improved selectivity in the presence of urea,
although urea is insoluble in the reaction mixture. What
influence do the oxygen source, the catalyst, and the urea
additive have in this heterogeneous process? The water in the
85% H2O2 cannot be the culprit, because the conversion
remained low when a water-free solution of H2O2 in ethyl ether
was used. Additionally, the silanol/disiloxane selectivity was
shifted even further toward the disiloxane product under these
anhydrous reaction conditions.
also fit inside the urea channels, whereas larger silanes are
oxidized at the entrance of the urea channels by the rhenium
oxidant inside the channels. However, because of spatial
constraints in the urea channels, the condensation of the silanols
is prevented and excellent selectivities of silanol versus disi-
loxane are observed for all silane oxidations with the hetero-
geneous MTO/UHP but not with the homogeneous MTO/H2O2
oxidant. That the ordered structure of the undissolved urea
inclusion compound is responsible is evident from the additional
fact that the insoluble 1,3-diphenylurea, for which no inclusion
complexes have been reported, leads to both low conversion
(21%) and poor selectivity (8:92). Furthermore, the CH2Cl2-
soluble tetramethylurea (Table 2, entry 10) also results in low
conversion (only 20%) like the MTO/H2O2 combination, albeit
with high selectivity (96:4) like the MTO/UHP one. The reason
for the selectivity with tetramethylurea differs from that of the
MTO/UHP case. Because the tetramethylurea complexes with
the rhenium metal center,32 the reactivity is reduced (low
conversion), but the Lewis acidity is buffered, and thus, a high
selectivity results. In the case of the insoluble urea, no such
coordination takes place. Additional support that the Si-H
insertion takes place inside the urea channels and not on the
outer surface was provided by using silica gel as additive, which
is commonly used by zeolite chemists for similar control
experiments.33 In this heterogeneous system, no effect on either
the conversion of silane 1d or the product ratio 2d/3d was
observed with MTO/85% H2O2 as oxidant (Table 2, entries 1
and 14). Silica gel does not form inclusion complexes, and thus,
the oxidation takes place at the surface and/or in solution.
Alternatively, amylose, which is made up of glycopyranose
channels with 6-7 glucose units per turn of the helix,34 is known
to make inclusion complexes and should serve as a matrix like
urea. The inner diameter of these helical structures is 4.5-7.0
Å and, thus, in the same range as that of the urea channels.
Indeed, amylose has previously been used as a ‘reaction vessel’
for photochemical reactions.29b,35 This hunch proved to be
correct in that amylose served as a host for the MTO-catalyzed
silane oxidation;36 unfortunately, the silane conversions were
lower than those with urea (Table 2, entries 11-13). Presum-
ably, this is due to the more polar environment and the
conformationally fixed structure of the amylose compared to
the urea channels. The higher polarity of the amylose may be
counterproductive for the absorption of the relatively nonpolar
silanes. Nevertheless, in analogy to urea, the silanol (2d)/
disiloxane (3d) ratio was improved with increasing amounts of
the amylose additive.
Whereas the amount of H2O2 has no significant effect on the
conversion of 1d and the product distribution 2d/3d (Table 2,
entries 1 and 2), the concentration of the rhenium catalyst shows
a marked influence. Thus, with MTO/85% H2O2, for the test
substrate 1d, more catalyst led as expected to higher conversion
(Table 2, entries 1 and 3). In contrast, for MTO/UHP, the use
of more catalyst resulted in decreasing conversions of silane
1d and a deterioration of the silanol selectivity (entries 4-6).
This decrease in both conversion and product distribution (2d/
3d) indicates that an optimal catalyst/urea additive ratio is
necessary to obtain the silanol 2d selectively in high yield, a
conclusion which may also be reached when the amount of the
urea additive is varied (entries 7-9). The fact that both
conversion and product selectivity depend on the amount of
urea employed is all the more astonishing if one considers that
the urea additive is insoluble in dichloromethane as reaction
medium. Because the urea is not in solution but nevertheless
favorably assists the catalytic Si-H insertion, this heterogeneous
process presumably takes place at the urea-solvent interface,
possibly within the urea interior.
It is known that urea can form channel-like structures, which
are made up of helical chains of hydrogen-bonded urea
molecules.25a These hexagonal channels are generated spontane-
ously when urea forms inclusion complexes. Even when the
urea is not dissolved, its crystal lattice changes from tetragonal
to hexagonal without passing through the dissolved state.25b
Indeed, a broad range of guest compounds, e.g., alkanes, olefins,
alcohols, ethers, ketones, acids, esters, amines, nitriles, halo-
genated compounds, and even silanes,25d form inclusion com-
plexes with urea. Moreover, suspensions of urea inclusion
complexes were recently shown to exchange guest molecules
with the liquid phase.28 Nothing appears to be known in the
literature about metal-catalyzed reactions inside urea channels.
In view of these facts, we suggest that the MTO-catalyzed
Si-H insertions take place within the urea channels. The
effective channel diameter in urea inclusion complexes is
reported to be ca. 5.5-5.8 Å;25c however, in analogy to
amylose,29 it is suggested that the lattice possesses a degree of
flexibility. Thus, the channels are large enough to absorb the
oxygen source H2O2, the catalyst MTO, and the rhenium peroxo
complexes A and B, and the absorption should be especially
advantageous for polar guest molecules as a result of hydrogen
bonding with the urea.30 Not too sterically encumbered silanes31
The dependence of the reactivity and selectivity for the MTO/
UHP oxidant on the catalyst/urea ratio is also indicative of host-
guest chemistry. Thus, with higher amounts of the catalyst
(Table 2, entries 4-6) or less urea (Table 2, entries 7-9), the
Si-H oxidation takes place only partially inside the urea matrix
and a significant portion occurs in solution, as displayed by the
lower conversions and worse silanol/disiloxane (2d/3d) selec-
(32) The analogous complexation of hexamethylphosphoramide (HMPA)
to the yellow diperoxo complex CH3Re(O)(O2)2‚H2O (by ligand ex-
change with the coordinated water) leads to the orange-reddish complex
CH3Re(O)(O2)2‚HMPA: Herrmann, W. A.; Correia, J. D. G.; Artus, G. R.
J.; Fischer, R. W.; Roma˜o, C. C. J. Organomet. Chem. 1996, 520, 139-
142.
(28) Mahdyarfar, A.; Harris, K. D. M. J. Chem. Soc., Chem. Commun.
1993, 51-53.
(33) Camblor, M. A.; Corma, A.; Garc´ıa, H.; Semmer-Herle´dan, V.;
Valencia, S. J. Catal. 1988, 177, 267-272.
(29) (a) Mikus, F. F.; Hixon, R. M.; Rundle, R. E. J. Am. Chem. Soc.
1946, 68, 1115-1123. (b) Allen, M. T.; Miola, L.; Suddaby, B. R.; Whitten,
D. G. Tetrahedron 1987, 43, 1477-1484.
(34) Hui, Y.; Russell, J. C.; Whitten, D. G. J. Am. Chem. Soc. 1983,
105, 1374-1376.
(30) Hollingsworth, M. D.; Santarsiero, B. D.; Harris, K. D. M. Angew.
Chem., Int. Ed. Engl. 1994, 33, 649-652.
(35) Suddaby, B. R.; Dominey, R. N.; Hui, Y.; Whitten, D. G. Can. J.
Chem. 1985, 63, 1315-1319.
(31) Force-field calculations reveal that the silane 1d fits inside the urea
channels.
(36) In some cases, the amylose additive turned yellow in the reaction
mixture, which is due to the presence of the rhenium diperoxo complex B.