Activation of silanes by Grubbs' carbene complex Cl2(PCy3)2Ru=CHPh: Dehydrogenative condensation of alcohols and hydrosilylation of carbonyls

Article abstract of DOI:10.1016/S0040-4039(02)01385-0

This manuscript describes two catalytic methods for silyl ether synthesis using Grubbs' catalyst Cl₂(PCy₃)₂Ru=CHPh. Silyl ethers are obtained from the reaction of a variety of silanes with alcohols by dehydrogenative conde

Full text of DOI:10.1016/S0040-4039(02)01385-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 6363–6366
Activation of silanes by Grubbs’ carbene complex
Cl2(PCy3)2RuꢀCHPh: dehydrogenative condensation of alcohols
and hydrosilylation of carbonyls
Sarah V. Maifeld, Reagan L. Miller and Daesung Lee*
Department of Chemistry, University of Wisconsin, Madison, WI 53706, USA
Received 27 June 2002; accepted 5 July 2002
Abstract—This manuscript describes two catalytic methods for silyl ether synthesis using Grubbs’ catalyst Cl2(PCy3)2RuꢀCHPh.
Silyl ethers are obtained from the reaction of a variety of silanes with alcohols by dehydrogenative condensation and by the
hydrosilylation of carbonyl compounds. Both reactions occur under neat conditions. © 2002 Elsevier Science Ltd. All rights
reserved.
In recent years, olefin metathesis chemistry has emerged
as a powerful synthetic tool. The tremendous growth in
this area can primarily be attributed to the development
of new catalysts such as Grubbs’ ruthenium–carbene
complex Cl2(PCy3)2RuꢀCHPh (1) and its congeners.1
This catalyst has demonstrated remarkable efficiency
promoting ring opening, ring closing as well as cross
metathesis reactions while displaying tolerance toward
a wide variety of common functional groups. Further-
more, a growing number of newly discovered catalytic
processes mediated by this complex broaden its syn-
thetic utility beyond olefin metathesis.
Silyl ethers comprise one of the most widely used
classes of protecting groups in synthetic chemistry due
to their ease of formation and selective removal in the
presence of other protecting groups.7 In addition, silyl
ethers function as temporary tethers in organic synthe-
sis, adapting many intermolecular reactions as more
favorable intramolecular processes.8 Although the con-
ventional silylation procedure is straightforward and
dependable, its reliance on environmentally undesirable
solvents such as CH2Cl2 and DMF as well as excess
amine bases makes the synthesis of silyl ethers an
excellent target for greener methodology.
In our effort to develop a convenient and environmen-
tally benign route to silyl ethers, we investigated the
activation of silicon–hydrogen bonds by a variety of
transition metal complexes.9,10 Our initial efforts found
that ruthenium–carbene complex 1 is an effective cata-
lyst for the condensation of alcohols and silanes, show-
ing no indication of competing olefin metathesis even
with a terminal alkene (Table 1). In the presence of 0.5
mol% of 1, these reactions efficiently give complete
conversion of the substrate alcohols to the correspond-
ing silyl ethers in nearly quantitative yield.11 Generally,
dialkyl aryl (Me2PhSiH) and alkyl diaryl (Ph2MeSiH)
silanes react more efficiently than either trialkyl
(Et3SiH, tert-BuMe2SiH) or trialkoxy silanes
Complex 1 has been shown to catalyze the Kharasch
addition of chloroform across alkenes2 as well as the
removal of allyl groups from amines.3 Noels and
co-workers demonstrated that 1 promotes the atom
transfer radical polymerization (ATRP) of methyl
methacrylate,4 while Grubbs et al. built a difunctional
catalyst retaining ATRP activity and used it for the
synthesis of diblock copolymers.5 Complex 1 has been
also shown to act as an effective pre-catalyst for hydro-
genation of olefins, transfer hydrogenation of ketones,
and dehydrogenative oxidations of alcohols.6 Adding to
this list of synthetically useful transformations, our
studies show a new twofold use of complex 1 to activate
a variety of silanes in the presence of alcohols or
carbonyl compounds, yielding the corresponding silyl
ethers (Scheme 1).
25 - 55 °C
R
OH
PCy3
R
OSiR'3
Cl
Cl
Ru
neat
R'3SiH
Ph
O
OSiR'3
R
PCy3
1
R
R
R
50 - 80 °C
* Corresponding author. Tel.: (608) 265-8431; fax: (608) 265-4534;
e-mail: dlee@chem.wisc.edu
Scheme 1.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01385-0

6364
S. V. Maifeld et al. / Tetrahedron Letters 43 (2002) 6363–6366
Table 1. Dehydrogenative condensation of alcohols and silanes catalyzed by 1a
((EtO)3SiH). In particular, diphenylsilane (Ph2SiH2)
reacts vigorously with (1R,2S,5R)-menthol yielding the
silyl ether almost immediately at ambient temperature
(entry 7). Although trialkyl silanes may react at 25°C
(entry 4), most reactions involving trialkyl silanesre-
quired gentle heating (ca. 55°C) to complete triethylsilyl
(TES) or tert-butyldimethyl silyl (TBS) ether formation
in a reasonable timeframe (entries 2, 7, 9, 14 and 15).
Triethoxysilane ((EtO)3SiH) is even less reactive, requir-
ing longer reaction times (entries 3 and 10). A less
reactive aromatic alcohol, m-cresol, reacted with
Et3SiH to form the TES ether efficiently but also
required a higher temperature and a longer reaction
time (50°C, 8 h) (entry 15). Solid alcohols, if suitably
miscible with silanes, provided the respective silyl ethers
without any use of solvent (entry 11); immiscible alco-
hols required a minimal amount of solvent to partially
wet the reaction affording smooth silylation (entries 14
and 15). Since 1 has been shown to effect the hydro-
genation of unsaturated systems,6a it is not surprising
that some olefinic alcohols undergo hydrogenation by 1
with the hydrogen gas generated in situ from the con-
densation between alcohols and silanes. The terminal
double bond of 4-penten-1-ol was hydrogenated com-

S. V. Maifeld et al. / Tetrahedron Letters 43 (2002) 6363–6366
Table 2. Hydrosilylation of carbonyl compounds catalyzed by 1a
6365
yield (%)b
>95
entry
1
substrate
O
silane
temp (°C) / h
silyl ether
OSilyl
Me2PhSiH
50 / 3
H
Silyl = SiMe2Ph
Silyl = SiEt3
Et3SiH
2
3
50 / 1
>95
>95
Silyl = SiMe2t-Bu
t-BuMe2SiH
80 / 0.5
O
OSilyl
Et3SiH
4
80 / 1
>85
Silyl = SiEt3
Silyl = SiPh2Me
Ph2MeSiH
SiMe2H
80 / 0.5
50 / 0.5
>85
>95
5
6
SiMe2
O
H
O
H
O
OSiPh2H
Ph2SiH2
Ph2SiH2
7
8
50 / 2
>95
>80c
85
50 / 3
80 / 6
80 / 6
H
O
OSiPh2H
O
OSilyl
Et3SiH
9
Silyl = SiEt3
t-BuMe2SiH
Silyl = SiMe2t-Bu
10
42
a1.0 mol% 1, 1 mmol silane, 1.1 mmol carbonyl
bYields were reported on the basis of 1H NMR of the crude reaction mixture
c3:1 mixture of axial and equatorial silyl ethers
pletely (entry 4) while a trans-disubstituted alkene
showed less than 50% hydrogenation (entry 5). The
extent of hydrogenation of tri-substituted alkenes
depends on reaction temperature and silane structure.
No loss of unsaturation was detected when b-(S)-cit-
ronellol was reacted at 25°C with Me2PhSiH (entry 6);
however, at 45°C with less reactive tert-BuMe2SiH,
partial hydrogenation was observed (entry 7). Similarly,
at 35°C, the cyclic tri-substituted double bond in (R)-
myrtenol was completely retained in reactions with
Et3SiH but underwent slight hydrogenation (5–10%)
with (EtO)3SiH (entries 9 and 10).
1 mol% of 1 promoted the hydrosilylation of aldehydes
and ketones with a variety of silanes in good yields
(Table 2).
In contrast to dehydrogenative condensation between
alcohols and silanes, hydrosilylation reactions mediated
by 1 required higher temperatures (>50°C), which gen-
erated a slightly increased amount of silyl byproduct.
This impurity could be avoided by employing a substo-
ichiometric amount of silane.
In general, silanes in both hydrosilylation and dehydro-
genative condensation reactions showed similar reactiv-
ity profiles. Diphenylsilane, an extremely reactive silane
in previous dehydrogenative condensation reactions,
underwent hydrosilylation at 50°C with ketones (entry
7, 8) while the hydrosilylation of ketones by trialkylsi-
lanes was sluggish even at elevated temperatures and in
After demonstrating the generality of 1 as a catalyst for
dehydrogenative condensation between alcohols and
silanes, we sought to extend 1’s utility by investigating
its potential as a catalyst for the hydrosilylation of
aldehydes and ketones.12 In most cases, we found that

6366
S. V. Maifeld et al. / Tetrahedron Letters 43 (2002) 6363–6366
the presence of excess silane (entry 9, 10). Reaction
between an a,b-unsaturated aldehyde such as croton-
aldehyde with allyldimethylsilane yielded a 1.4:1 mix-
ture of trans and cis silyl enol ethers, respectively;
higher reaction temperatures gave rise to the allylic silyl
ether resulting from 1,2-hydrosilylation of the carbonyl.
ner, K. B. Macromolecules 2000, 33, 3196; (c) Drouin, S.
D.; Zamaniun, F.; Fogg, D. E. Organometallics 2001, 20,
5495.
7. (a) Green, W. T.; Wuts, P. G. M. Protecting Groups in
Organic Synthesis, 3rd ed.; John Wiley and Sons: New
York, 1999; (b) Kocienski, P. J. In Protective Groups;
Enders, D.; Noyori, R.; Trost, B. M., Eds.; Thieme:
Stuttgart, 1994.
8. (a) Bols, M.; Skrustrup, T. Chem. Rev. 1995, 95, 1253; (b)
Fensterbank, L.; Malacria, M.; Sieburth, S. McN. Syn-
thesis 1997, 813.
9. For examples of dehydrogenative condensations of alco-
hols and silanes, see: (a) Zhang, C.; Laine, R. M. J. Am.
Chem. Soc. 2000, 122, 6979; (b) Blackwell, J. M.; Foster,
K. L.; Beck, V. H.; Piers, W. E. J. Org. Chem. 1999, 64,
4887 and references cited therein; (c) Ojima, I.; Kogure,
T.; Nihonyangi, M.; Kono, H.; Inaba, S. Chem. Lett.
1973, 501.
In summary, we have demonstrated that Grubbs’ ruthe-
nium carbene complex 1 activates a variety of silanes
affording silyl ethers from dehydrogenative condensa-
tion with alcohols and hydrosilylation of carbonyls.
Current studies in our laboratory are aimed toward
developing this newly discovered reactivity for further
application.
Acknowledgements
10. For examples of carbonyl hydrosilylation, see: (a) Chris-
man, W.; Noson, K.; Lipshutz, B. H. J. Organomet.
Chem. 2001, 624, 367; (b) Parks, D. J.; Blackwell, J. M.;
Pier, W. E. J. Org. Chem. 2000, 65, 3090; (c) Bushell, S.
M.; Lawrence, N. J. Tetrahedron Lett. 2000, 4507; (d)
Lee, S.; Kim, T. Y.; Park, M. K.; Han, B. H. Bull.
Korean Chem. Soc. 1996, 17, 1082; (e) Parks, D. J.; Piers,
W. E. J. Am. Chem. Soc. 1996, 118, 9440; (f) Wang, D.;
Chan, T. H. Tetrahedron Lett. 1993, 34, 3095; (g) Cor-
nish, A. J.; Lappert, M. F.; Filatovs, G. L.; Nile, T. A. J.
Organomet. Chem. 1979, 172, 153.
11. In some cases, a small amount of silyl byproduct was
produced presumably from the dehydrocondensation of
two silanes or the formation of siloxanes. However, for
most cases, filtering the reaction through a short plug of
silica to remove catalyst provides pure silyl ethers.
12. Only a few reports of carbonyl hydrolsilylation catalyzed
by metal–carbene complexes exist in the literature. For
rhodium(I) complexes, see: (a) Hill, J. E.; Nile, T. A. J.
Organomet. Chem. 1977, 137, 293; (b) Herrmann, W. A.;
Goosen, L. J.; Kocher, C.; Artus, C. R. Angew. Chem.,
Int. Ed. 1996, 35, 2805; (c) Enders, D.; Gielen, H. J.
Organomet. Chem. 2001, 617–618, 70; (d) For rhodium(I)
and ruthenium(II), see: Haskell, R. K.; Lappert, M. F. J.
Organomet. Chem. 1984, 264, 217.
D.L. gratefully acknowledges UW-Madison and the
Dreyfus Foundation for financial support. R.L.M.
thanks NIH for the support via a Chemistry and Biol-
ogy Interface Training Grant (T32 GM08505). NSF
and NIH support for NMR and Mass Spectrometry
instrumentation are greatly acknowledged.
References
1. For recent reviews, see: (a) Grubbs, R. H.; Chang, S.
Tetrahedron 1998, 54, 4413; (b) Armstrong, S. K. J.
Chem. Soc., Perkin Trans. 1 1998, 371; (c) Blechert, S.
Pure Appl. Chem. 1999, 71, 1393; (d) Furstner, A. Angew.
Chem., Int. Ed. 2000, 39, 3013.
2. Tallarico, J. A.; Malnick, L. M.; Snapper, M. L. J. Org.
Chem. 1999, 64, 344.
3. Alcaide, B.; Almendros, P.; Alonson, J. M.; Aly, M. F.
Org. Lett. 2001, 3, 3781.
4. Simal, F.; Demonceau, A.; Noels, A. F. Angew. Chem.,
Int. Ed. 1999, 38, 538.
5. Bielawski, C. W.; Louie, J.; Grubbs, R. H. J. Am. Chem.
Soc. 2000, 122, 12872.
6. (a) Louie, J.; Bielawski, C. W.; Grubbs, R. H. J. Am.
Chem. Soc. 2001, 123, 11312; (b) Watson, M. D.; Wag-