Tributyltin Hydride (Bu₃SnH)

Biographical Sketches

A. Jayanthi has completed her B.Sc. in Chemistry at the Government Arts College for Women, Pudukkottai, Tamil Nadu, India and her M.Sc. in Chemistry at the Bharathidasan University, Tiruchirappalli, Tamil Nadu, India. She has recently obtained her Ph.D. in the field of β-lactam chemistry at the National Chemical Laboratory, Pune, India, under the supervision of Dr. A. R. A. S. Deshmukh. She is currently working as a post-doctoral fellow with Dr. A. S. Gopalan at the New Mexico State University, USA.

Introduction

<P>The realization of compatibility of radical methods with a range of functional groups without further protection has led to an increased interest in the use of radicals in organic synthesis. Bu₃SnH is a well-known radical reagent because of its relatively weak and nonionic bond interaction between tin and hydrogen that can be cleaved homolytically. [1] Thus it has been explored extensively in reductive cleavage, [2a] radical dehalogenation, deoxygenation [2b] and intramolecular radical cyclization [2c] as well as in the synthesis of various heterocyclic compounds, natural products and many pharmaceutically important drugs. [2d-j] </P><P>Bu₃SnH is commercially available as a colourless liquid and can be stored under refrigeration. It can be prepared via reduction of tributyltin oxide with hydrosiloxane. [3a] It can also be generated easily in situ by the action of NaBH₄ or Et₃SiH on Bu₃SnCl. [1] [3b] </P>

<P>The problems associated with this reagent are that it is highly toxic and removal of tin impurities from the product is often very difficult. This has been overcome recently by using a mixture of KF and silica gel as the stationary phase in column chromatography to reduce the level of tin impurities in the product to 30 ppm. [3c] </P>

Abstracts

<TD VALIGN="TOP"> A) Bu₃SnH has been used in the preparation of β-hydroxy ketones in a single step via hydrostannylation of α,β-unsaturated ketones in the presence of CuCl followed by aldol reaction of the resulting tin enolates with aldehydes. [4] </TD><TD VALIGN="TOP">

</TD><TD VALIGN="TOP"> B) Bu₃SnH was explored for the reduction of the diazo group of α-diazocarbonyl compounds to the corresponding methylene group in the presence of Cu(acac)₂. It is an efficient methodology to transform a carboxylic acid into a methyl or an ethyl ketone ­under mild conditions. [5] </TD><TD VALIGN="TOP">

</TD><TD VALIGN="TOP"> C) Radical cyclization of unsaturated ethers bearing an aldehyde or an α,β-unsaturated ketone moiety has been effected using Bu₃SnH to synthesize tetrahydrofurans, chromanols and butyrolactones in good yields. The reaction proceeded through the addition of the tributyltin radical to the carbonyl double bond followed by intra­molecular addition of the resultant O-stannyl ketyl radical to ­electron-rich double bonds. [6] </TD><TD VALIGN="TOP">

</TD><TD VALIGN="TOP"> D) A combination of Bu₃SnH with Bu₃SnI·PPh₃O has been used as a mild reagent for the reduction of functionalized epoxides to the corresponding alcohols in high chemo- and regioselectivities via nucleophilic attack of the Sn-I bond on the epoxy ring. [7] </TD><TD VALIGN="TOP">

</TD><TD VALIGN="TOP"> E) Bu₃SnH has been used to deprotect highly stable N-sulfon­amides to the respective amides under mild reaction conditions in very good yields and with good chemoselectivity. N-Benzyl and phenyl derivatives showed excellent reactivity, whereas N-acetyl amides were non-reactive under the same conditions. [8] </TD><TD VALIGN="TOP">

</TD><TD VALIGN="TOP"> F) Bu₃SnH has also been utilized in the intramolecular radical ­cyclization of N-propargyl-substituted azetidin-2-ones. The ­cyclization is highly stereospecific and gives either the 6-exo-dig or the 7-endo-dig cyclized compound. [9] </TD><TD VALIGN="TOP">

</TD><TD VALIGN="TOP"> G) Reduction of imines generated in situ from aldehydes and anilines was achieved with Bu₃SnH on silica gel under solvent-free conditions to provide the corresponding amines in good yield. This methodology is unsuccessful on aliphatic amines since they are stronger bases compared to anilines and thus reduce the availability of reaction sites on silica gel. [10] </TD><TD VALIGN="TOP">

</TD><TD VALIGN="TOP"> H) Bu₃SnH-mediated stereoselective radical cyclization of Baylis-Hillman adducts provides pharmaceutically important tri- and ­tetra-substituted oxepanes in a short route. [11] </TD><TD VALIGN="TOP">

</TD>

References

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