Silanes
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) 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.
Tris(trimethylsilyl)silane
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.
Recent Literature
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.
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.
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,
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.
Selective conjugate reductions of α,β-unsaturated aldehydes were achieved in the
presence of rhodium(bisoxazolinylphenyl) complexes as catalysts and
alkoxyhydrosilanes as reducing agents.
Y. Kanazawa, H. Nishiyama, Synlett, 2006,
3343-3345.
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.
Treatment of β,β-disubstituted-α,β-unsaturated ketones bearing a ketone residue
with in situ generated, catalytic CuH ligated by a nonracemic ligand leads to
cyclic aldol products with three newly created adjacent chiral centers.
Excellent diastereoselectivities and enantioselectivities are obtained for
several examples studied.
B. H. Lipshutz, B. Amorelli, J. B. Unger, J. Am. Chem. Soc., 2008,
130, 14378-14379.
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.
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.
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.
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.
