Silanes serve, depending upon the type of the silane, as a radical H-donor or as a hydride donor. The range reaches from simple alkylsilanes (Et3SiH), alkylsiloxanes (PMHS, DEMS), over different phenylsilanes (such as PhSiH3) and halosilanes (such as trichlorosilane) up to tris(trimethylsilyl)silane, which is due to its structure an outstanding radical reducing agent.
Silanes are often used as an alternative to toxic reducing agents, e.g. Bu3SnH. But they offer their own chemistry due to the outstanding affinity from silicon to oxygen and fluorine.
A highly efficient silver-catalyzed chemoselective method enables the reduction of aldehydes to their corresponding alcohols in water by using hydrosilanes as reducing agents. Ketones remained essentially inert under the same reaction conditions.
Z. Jia, M. Liu, X. Li, A. S. C. Chan, C.-J. Li, Synlett, 2013, 24, 2049-2056.
A direct reduction of alcohols to the corresponding alkanes using chlorodiphenylsilane as hydride source in the presence of a catalytic amount of InCl3 showed high chemoselectivity for benzylic alcohols, secondary alcohols and tertiary alcohols while not reducing primary alcohols and functional groups that are readily reduced by standard methods such as esters, chloro, bromo, and nitro groups.
M. Yasuda, Y. Onishi, M. Ueba, T. Miyai, A. Baba, J. Org. Chem., 2001, 7741-7744.
Various benzaldimines and ketimines can be hydrosilated efficiently with PhMe2SiH employing B(C6F5)3 as a catalyst. Spectral evidence supports the intermediacy of a silyliminium cation with a hydridoborate counterion formed via abstraction of a hydride from PhMe2SiH by B(C6F5)3 in the presence of imines.
J. M. Blackwell, E. R. Sonmor, T. Scoccitti, W. E. Piers, Org. Lett., 2000, 2, 3921-3923.
A zinc-catalyzed reduction of tertiary amides shows remarkable chemoselectivity and substrate scope tolerating ester, ether, nitro, cyano, azo, and keto substituents.
S. Das, D. Addis, S. Zhou, K. Junge, M. Beller, J. Am. Chem. Soc., 2010, 132, 1770-1771.
Reduction of secondary amides to imines and secondary amines has been achieved using low catalyst loadings of readily available iridium catalysts such as [Ir(COE)2Cl]2 with diethylsilane as reductant. The stepwise reduction to secondary amine proceeds through an imine intermediate that can be isolated when only 2 equiv of silane is used. This system shows high efficiency and an appreciable level of functional group tolerance.
C. Cheng, M. Brookhart, J. Am. Chem. Soc., 2012, 134, 11304-11307.
Silylative reduction of nitriles under transition metal-free conditions converts alkyl and (hetero)aryl nitriles efficiently to primary amines under mild conditions. The use of sterically bulky silanes enabled a partial reduction leading to N-silylimines.
N. Gandhamsetty, J. Jeong, Y. Park, S. Park, S. Chang, J. Org. Chem., 2015, 80, 7281-7287.
An experimentally simple Microwave-assisted reductive alkylation of methyl carbamate with a range of aldehydes provides, after basic work-up, structurally diverse primary amines. This method is particularly amenable to high-throughput synthesis.
F. Lehmann, M. Scobie, Synthesis, 2008, 1679-1681.
In an operationally convenient, mechanistically unique protocol, Lewis base activation of silyl acetals generates putative pentacoordinate silicate acetals, which fragment into aldehydes, silanes, and alkoxides in situ. Subsequent deprotonative metalation of phosphonate esters followed by HWE with aldehydes furnishes enoates.
U. S. Dakarapu, A. Bokka, P. Asgari, G. Trog, Y. Hua, H. H. Nguyen, N. Rahman, J. Jeon, Org. Lett., 2015, 17, 5792-5795.
A transition-metal-free catalytic hydrosilylation based on t-BuOK (5 mol %) and (MeO)3SiH or (EtO)3SiH allows the reduction of tertiary amides to their corresponding enamines with high selectivity in very good yields.
A. Volkov, F. Tinnis, H. Adolfsson, Org. Lett., 2014, 16, 680-683.
Activation of diphenylsilane in the presence of a catalytic amount of an N-heterocyclic carbene (NHC) enables hydrosilylation of carbonyl derivatives under mild conditions. Presumably, a hypervalent silicon intermediate featuring strong Lewis acid character allows dual activation of both the carbonyl moiety and the hydride at the silicon center. Some interesting selectivities have been encountered.
Q. Zhao, D. P. Curran, M. Malacria, L. Fensterbank, J.-P. Goddard, E. Lacôte, Synlett, 2012, 23, 433-437.
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.
An indium triiodide catalyst promoted the Mukaiyama Aldol Reaction of silyl enolates with esters to form β-hydroxycarbonyl compounds in the presence of hydrosilanes. Various esters were applicable, and the high chemoselectivity of this system brings compatibility to many functional groups, such as alkenyl, alkynyl, chloro, and hydroxy.
Y. Inamoto, Y. Nishimoto, M. Yasuda, A. Baba, Org. Lett., 2012, 14, 1168-1171.
A mild, enantioselective hydrosilylation of 3-oxo-3-arylpropionic acid methyl or ethyl esters using axially chiral BINAM N-heterocyclic carbene (NHC)-Rh(III) complexes as catalysts gave 3-hydroxy-3-arylpropionic acid methyl or ethyl esters in good yields with good to excellent enantioselectivities under mild conditions.
Q. Xu, X. Gu, S. Liu, Q. Duo, M. Shi, J. Org. Chem., 2007, 72, 2240-2242.
Asymmetric ligand-accelerated catalysis by copper hydride allows the synthesis of valued nonracemic allylic alcohols in very good yields.
R. Moser, Ž. V. Bošković, C. S. Crowe, B. H. Lipshutz, J. Am. Chem. Soc., 2010, 132, 7852-7853.
Palladium-catalyzed hydrosilylation of α,β-unsaturated ketones and cyclopropyl ketones with hydrosilanes gives (Z)-silyl enolates in good yields.
Y. Sumida, H. Yorimitsu, K. Oshima, J. Org. Chem., 2009, 74, 7986-7989.
3-Methyl-1-phenylphospholane-1-oxide as precatalyst and an organosilane reducing agent are the key components in a Wittig reaction catalytic in phosphine. Various heteroaryl, aryl, and alkyl adehydes could be efficiently converted to the corresponding alkenes in good yield using this precatalyst. The protocol also functions well on larger scale.
C. J. O'Brien, J. L. Tellez, Z. S. Nixon, L. J. Kang, A. L. Carter, S. R. Kunkel, K. C. Przeworski, G. A. Chass, Angew. Chem. Int. Ed., 2008, 48, 6836-6839.
A Friedel-Crafts acylation of arenes with esters has been achieved in the presence of dimethylchlorosilane and 10 mol % of indium tribromide . The key intermediate RCOOSi(Cl)Me2 is generated from alkoxy esters with the evolution of the corresponding alkanes. The scope of the alkoxy ester moiety was wide: tert-butyl, benzyl, allyl, and isopropyl esters were successful.
Y. Nishimoto, S. A. Babu, M. Yasuda, A. Baba, J. Org. Chem., 2008, 73, 9465-9468.
A CuH-catalyzed hydroamination of alkenes using an amine transfer reagent and a silane provides chiral amines with high efficiency and stereoselectivity. However, the current technology has been limited to dialkylamine transfer reagents (R2NOBz). A modified type of monoalkylamine transfer enabled the synthesis of chiral secondary amines, including those derived from amino acid esters, carbohydrates, and steroids.
D. Niu, S. L. Buchwald, J. Am. Chem. Soc., 2015, 137, 9716-9721.
A catalytic reductive cleavage of C(sp2)- and C(sp3)-SMe bonds under ligandless conditions offers a wide scope and high chemoselectivity profile including challenging substrate combinations, allowing the design of orthogonal and site-selectivity approaches.
N. Barbero, R. Martin, Org. Lett., 2012, 14, 796-799.
A new, mild protocol for deoxygenation of various phosphine oxides with retention of configuration is described. Mechanistic studies regarding the oxygen transfer between the starting phosphine oxide and triphenylphosphine are also presented.
H.-C. Wu, J.-Q. Yu, J. B. Spencer, Org. Lett., 2004, 6, 4675-4678.
A copper hydride-catalyzed, enantioselective, intramolecular hydroalkylation of halide-tethered styrenes enables the synthesis of enantioenriched cyclobutanes, cyclopentanes, indanes, and six-membered N- and O-heterocycles.
Y.-M. Wang, N. C. Bruno, A. L. Placeres, S. Zhu, S. L. Buchwald, J. Am. Chem. Soc., 2015, 137, 10524-10527.
Cu-catalyzed asymmetric conjugate reduction of β-substituted ketones leads to enantiomerically enriched diphenylsilyl enol ethers, which are utilized in a diastereoselective Pd-catalyzed α-arylation of various aryl bromides to yield disubstituted cycloalkanones with excellent levels of enantiomeric and diastereomeric purity. The procedure can be carried out in one-pot.
J. Chae, J. Yun, S. L. Buchwald, Org. Lett., 2004, 6, 4809-4812.
An ester enolate Claisen rearrangement is catalyzed by [(cod)RhCl]2 and MeDuPhos with good yields and diastereocontrol. The mild reaction conditions tolerate base-sensitive functionalities.
S. P. Miller, J. P. Morken, Org. Lett., 2002, 4, 2743-2745.
An indium(III) hydroxide-catalyzed reaction of carbonyls and chlorodimethylsilane afforded the corresponding deoxygenative chlorination products. Ester, nitro, cyano, or halogen groups were not affected during the reaction course. Typical Lewis acids such as TiCl4, AlCl3, and BF3·OEt2 showed no catalytic activity. The reaction mechanism is discussed.
Y. Onishi, D. Ogawa, M. Yasuda, A. Baba, J. Am. Chem. Soc., 2002, 124, 13690-13691.
An efficient rhodium-catalyzed method allows the preparation of aryltriethoxysilanes from arenediazonium tosylate salts. A new method for hydrodediazoniation has also been explored.
Z. Y. Tang, Y. Zhang, T. Wang, W. Wang, Synlett, 2010, 804-808.
A copper-catalyzed hydroalkylation of terminal alkynes using alkyl triflates as coupling partners and (Me2HSi)2O as a hydride donor proceeds with excellent anti-Markovnikov regioselectivity and provides exclusively (E)-alkenes. Both alkyl- and aryl-substituted alkynes can be used as substrates, together with 1° alkyl and benzylic triflates. Finally, the transformation can be accomplished in the presence of a wide range of functional groups.
M. R. Uehling, A. M. Suess, G. Lalic, J. Am. Chem. Soc., 2015, 137, 1424-1427.
A catalytic anti-Markovnikov hydrobromination of aryl- and alkyl-substituted terminal alkynes affords terminal E-alkenyl bromides in high yield and with excellent regio- and diastereoselectivity. The reaction conditions are compatible with a wide range of functional groups, including esters, nitriles, epoxides, aryl boronic esters, terminal alkenes, silyl ethers, aryl halides, and alkyl halides.
M. R. Uehling, R. P. Rucker, G. Lalic, J. Am. Chem. Soc., 2014, 136, 8799-8803.
The complementary use of small cyclopropenylidene carbene ligands or highly hindered N-heterocyclic carbene ligands allows the regiochemical reversal in aldehyde-alkyne reductive couplings with unbiased internal alkynes, aromatic internal alkynes, conjugated enynes, or terminal alkynes.
H. A. Malik, G. J. Sormunen, J. Montgomery, J. Am. Chem. Soc., 2010, 132, 6304-6305.
A nickel(0) N-heterocyclic carbene complex-catalyzed coupling of α-silyloxy aldehydes and alkynylsilanes provides an effective entry to various anti-1,2-diols with excellent diastereoselectivity.
K. Sa-ei, J. Montgomery, Org. Lett., 2006, 8, 4441-4443.
An attractive catalytic hydrofluorination of olefins using a cobalt catalyst offers exclusive Markovnikov selectivity, functional group tolerance, and scalability. A preliminary mechanistic experiment showed the involvement of a radical intermediate.
H. Shigehisa, E. Nishi, M. Fujisawa, K. Hiroya, Org. Lett., 2013, 15, 5158-5161.
The use of unsupported nanoporous gold (AuNPore) as a catalyst and organosilane with water as a hydrogen source enables a highly efficient and regioselective hydrogenation of quinoline derivatives to 1,2,3,4-tetrahydroquinolines. The AuNPore catalyst can be readily recovered and reused without any loss of catalytic activity.
M. Yan, T. Jin, Q. Chen, H. E. Ho, T. Fujita, L.-Y. Chen, M. Bao, M.-W. Chen, N. Asao, Y. Yamamoto, Org. Lett., 2013, 15, 1484-1487.