Categories: O-Si Bond Formation >
Synthesis of silyl ethers
Inexpensive NaOH as the catalyst enables a cross-dehydrogenative coupling of an alcohol and hydrosilane to directly generate the corresponding silyl ether under mild conditions and without the production of stoichiometric salt byproducts. The scope of both coupling partners is excellent.
A. A. Toutov, K. N. Betz, M. C. Haibach, A. M. Romine, R. H. Grubbs, Org. Lett., 2016, 18, 5776-5779.
N-Heterocyclic olefins (NHOs) are promising organocatalysts with strong nucleophilicity and Brønsted basicity. NHOs are efficient promoters for a direct dehydrogenative silylation of alcohols or hydrosilylation of carbonyl compounds. Preliminary results of an asymmetric dehydrogenative silylation are also discussed.
U. Kaya, U. P. N. Tran, D. Enders, J. Ho, T. V. Nguyen, Org. Lett., 2017, 19, 1398-1401.
An iron catalyst promotes a transfer hydrosilylation of various alcohols with silyl formates. This protocol offers mild reaction conditions and the release of gases as the only byproducts (H2 and CO2).
T. Godou, C. Chauvier, P. Thuéry. T. Cantat, Synlett, 2017, 28, 2473-2477.
A commercially available proazaphosphatrane is an efficient and mild catalyst for the silylation of a wide variety of alcohols and phenols, including acid-sensitive, base-sensitive, and hindered substrates, using tert-butyldimethylsilyl chloride (TBDMSCl). The reactions are carried out in acetonitrile from 24 to 40°C and on rare occasions in DMF from 24 to 80°C. Although representative primary alcohols, secondary alcohols, and phenols were silylated using the more sterically hindered reagent tert-butyldiphenylsilyl chloride (TBDPSCl), tertiary alcohols were recovered unchanged.
B. A. D'Sa, J. G. Verkade, J. Am. Chem. Soc., 1996, 118, 12832-12833.
Reactions of alcohols with silyl chlorides in the presence of N-methylimidazole were significantly accelerated by addition of iodine. A general and high yielding method for efficient silylation of primary, secondary, and tertiary alcohols was developed.
A. Bartoszewicz, M. Kalek, J. Nilsson, R. Hiresova, J. Stawinski, Synlett, 2008, 37-40.
Tris(pentafluorophenyl)borane, B(C6F5)3, is an effective catalyst for a mild and efficient dehydrogenative silation of alcohols using a variety of silanes. Only the most bulky silanes (Bn3SiH and iPr3SiH) were not reactive under these conditions. Generally, the reactions are clean and high yielding, with dihydrogen as the only byproduct.
J. M. Blackwell, K. L. Foster, V. H. Beck, W. E. Piers, J. Org. Chem., 1999, 64, 4887-4892.
Two methods are described for the regioselective displacement of the primary hydroxy group in methyl glycosides with iodide. Products of the first method employing triphenylphosphine and iodine need purification on a reverse phase column. A one-pot procedure via sulfonates and subsequent substitution with iodide and methods for the protection of the iodoglycosides are also described.
P. R. Skaanderup, C. S. Poulsen, L. Hyldtoft, M. R. Jørgensen, R. Madsen, Synthesis, 2002, 1721-1727.
A one-pot alkylation-silylation reaction of various epoxides with R3Al-R'3SiOTf occurs stereospecifically to give the corresponding alkylation-silylation products in excellent yields.
P. Shanmugam, M. Miyashita, Org. Lett., 2003, 3265-3268.
A key intermediate in a highly efficient rhodium-catalyzed O-silylation of alcohols is chlorosilane, generated from vinylsilane and HCl, which is regenerated in the catalytic cycle. Various alcohols and vinylsilanes were applied to the preparation of silyl ethers with this catalyst system.
J.-W. Park, C.-H. Jun, Org. Lett., 2007, 9, 4073-4076.
Different types of unsymmetrically protected diols are available from the methylene acetal in a one-pot procedure. Monoprotected 1,2-diols with a silyl group at the less hindered hydroxy group as well as with a MOM group at the more hindered can be isolated in good yields. The reaction conditions are mild without affecting other functional groups.
H. Fujioka, K. Senami, O. Kubo, K. Yahata, Y. Minamitsuji, T. Maegawa, Org. Lett., 2009, 11, 5138-5141.
A new procedure for catalytic reductive coupling of aldehydes and alkynes uses Ni(COD)2 with an imidazolium carbene ligand as the catalyst and triethylsilane as the reducing agent.
G. M. Mahandru, G. Liu, J. Montgomery, J. Am. Chem. Soc., 2004, 126, 3698-3699.
In the presence of catalytic amounts of PtCl2 and metal iodides, β-substituted vinylsilanes reacted with aldehydes at the β-position to give allyl silyl ethers. Addition to aromatic aldehydes proceeded efficiently in the presence of LiI whereas MnI2 was found to be effective in addition to aliphatic aldehydes.
K. Miura, G. Inoue, H. Sasagawa, H. Kinoshita, J. Ichikawa, A. Hosomi, Org. Lett., 2009, 11, 5066-5069.
A C2-symmetric copper-bound N-heterocyclic carbene (NHC) exhibits excellent reactivity and enantioselectivity in the hydrosilylation of a variety of structurally diverse ketones including challenging substrates as 2-butanone and 3-hexanone. Even at low catalyst loading (2.0 mol %), the reactions occur in under an hour at room temperature and often do not require purification beyond catalyst and solvent removal.
A. Albright, R. E. Gawley, J. Am. Chem. Soc., 2011, 133, 19680-19683.
The rhenium-catalyzed hydrosilation of aldehydes and ketones under ambient temperature and atmosphere gave protected alcohol as silyl ether in good yields. The mechanism is discussed.
E. A. Ison, E. R. Trivedi, R. A. Corbin, M. M. Abu-Omar, J. Am. Chem. Soc., 2005, 127, 15374-15375.
Optimizations to generate CuH in situ have led to an efficient and inexpensive hydrosilylation method for dialkyl ketones.
B. H. Lipshutz, C. C. Caires, P. Kuipers, W. Chrisman, Org. Lett., 2003, 5, 3085-3088.
Aliphatic carboxyl derivatives (acids, acyl chlorides, esters) and aldehydes were efficiently reduced to the methyl group by HSiEt3 in the presence of catalytic amounts of B(C6F5)3. Aromatic carboxylic acids, as well as other carbonyl functional equivalents, underwent smooth partial reduction to the corresponding TES-protected benzylic alcohols in competition with a Friedel-Crafts-like alkylation decreasing the overall selectivity of the reduction process.
V. Gevorgyan, M. Rubin, J.-X. Liu, Y. Yamamoto, J. Org. Chem, 2000, 66, 1672-1675.
Re2(CO)10 efficiently catalyzes the direct reduction of various carboxylic acids under UV irradiation at ambient temperature. While aliphatic carboxylic acids were readily converted to the corresponding disilylacetals with low catalyst loading in the presence of Et3SiH, aromatic analogues required more drastic conditions.
D. Wei, R. Buhaibeh, Y. Canac, J.-B. Sortais, Org. Lett., 2019, 21, 7713-7716.
An intermolecular reductive coupling of ynoates and aldehydes in the presence of a silane using catalytic amounts of Ni(COD)2, an N-heterocyclic carbene ligand, and PPh3 delivers invaluable silyl-protected γ-hydroxy-α,β-enoates. This methodology provides a quick entry to many other 1,4-difunctional compounds and oxygen-containing five-membered rings. The intermediacy of metallacycles in the catalytic process has been established.
S. K. Rodrigo, H. Guan, J. Org. Chem., 2012, 77, 8303-8309.
A β-amino alcohol-Ti(Oi-Pr)4 complex efficiently catalyzes a mild, enantioselective cyanosilylation of aldehydes. Aromatic, conjugated, heteroaromatic, and aliphatic aldehydes were converted to their corresponding cyanohydrin trimethylsilyl ethers in excellent yields with high enantioselectivities.
Y. Li, B. He, B. Qin, X. Feng, G. Zhang, J. Org. Chem., 2004, 69, 7910-7913.
Combinations of N-oxides and Ti(OiPr)4 act as bifunctional catalysts in the cyanosilylation of ketones. The reaction is promoted by the dual action of these new titanium complexes via activation of the ketone by the titanium and of TMSCN by the N-oxide.
Y. Shen, X. Feng, Y. Li, G. Zhang, Y. Jiang, Tetrahedron, 2003, 59, 5667-6675.