Organotin hydride

In recent years, the organotin hydrides have found extensive applications in organic syntheses which involve radical chain reactions in which the stannyl radical R3Sn is a chain-carrying intermediate. Tributyltin hydride has been used most widely, and is commercially available. 

The organotin hydrides are usually prepared by reducing an organotin halide or other derivative R SnX4 with a metal hydride. 

The exchange of the ligands H and X (X = halide, carboxylate etc.) between organotin compounds can be regarded as a special case of these reductions. 

Two alternative routes, which have been little exploited, are the thermal decomposition of an organotin formate, or the hydrolysis of a stannylmetallic compound, and a promising different approach involves the alkylation of a lithium tin hydride. 

Despite repeated attempts to isolate the green and purple intermediates, their instability has prevented identification. 

Fellmann, D. K. Huggins, H. D. Kaesz, abstracts of papers presented at the 8th International Conference on Coordination Chemistry, September, 1964, pp. 255-257. 

Organotin hydrides have found much interest in recent years both as synthetic reagents and as the subjects of numerous theoretical investigations. 

Kraus and Greer1 originally prepared a few organotin hydrides with considerable difficulty from the reaction between an organotin sodium compound and ammonium bromide or ammonium chloride in liquid ammonia. Their procedure remained the only synthetic route to these compounds for some 25 years until 1947 when Finholt, Bond, Wilzbach, and Schlesinger2 obtained the methyltin hydrides in a more facile manner by reducing a mixture of the methyltin chlorides with lithium aluminum hydride (lithium tetrahydroaluminate). 

It should be noted that the diborane (a highly toxic gas, spontaneously flammable in air) expected from the stoichiometric equation 

Et and n = 2, 3) were slowly decomposed at room temperature and easily oxidized by air However, Me3SnH and Me2SnH2 are little changed 

Since 1957 the triorganylstannanes Bu3SnH and Ph3SnH attracted scientists attention as effective reducing reagents. They easily reduced alkyl- , alkynyl- and aryl 

On treatment of an aldehyde with Bu3SnH and PhSeSiMe, in the presence of catalytic amount of AIBN as radical initiator hydrosilylation gave rise to the corre- 

When subjected to organotin hydride reduction mediated by AIBN, an a,y3- 

Reduction of an enone, in preference to a formyl group, with Bu2SnIH was re- 

In sharp contrast with these results, when efhenylenesulfones [227] and selenides 

Although organotin hydrides have been widely used as powerful reagents for or- 

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Unsymmetrical functional tetraorganotins are generally prepared by tin hydride addition (hydrostaimation) to functional unsaturated organic compounds (88) (see Hydroboration). The realization that organotin hydrides readily add to atiphatic carbon—carbon double and triple bonds forming tin—carbon bonds led to a synthetic method which does not rely on reactive organometatiic reagents for tin—carbon bond formation and, thus, allows the synthesis of... 

E. J. Kupchik in A. K. Sawyer, Organotin Hydrides, Organotin Compounds, Marcel Dekker Inc., New York... 

H.G. Kuivii.a, Reduction of Organic Compounds by Organotin Hydrides, Synthesis 1970, 499. 

By the addition of organotin hydrides to norbomene and norboma-diene, and subsequent reactions of the products, a variety of nor-bomyl-, norbornenyl-, and nortricyclyl-tin compounds has been isolated, and identified (67-69). 

For reviews of organotin hydrides, see Neumann, W.R Synthesis, 1987,665 Kuivila, H.G. Synthesis, 1970,499, Acc. Chem. Res., 1968,1,299. Tributyltin hydride also reduces vinyl halides in the prescence of a palladium catalyst. See Uenishi, J. Kawahama, R. Shiga, Y Yonemitsu, O. Tsuji, J. Tetrahedron Lett., 1996, 37, 6759. 

Organotin Hydrides, Reactions with Organic Compounds, 1, 47... 

Clive and coworkers have developed a new domino radical cyclization, by making use of a silicon radical as an intermediate to prepare silicon-containing bicyclic or polycyclic compounds such as 3-271 and 3-272 (Scheme 3.69) [109], After formation of the first radical 3-267 from 3-266, a 5-exo-dig cyclization takes place followed by an intramolecular 1,5-transfer of hydrogen from silicon to carbon, providing a silicon-centered radical 3-269 via 3-268. Once formed, this has the option to undergo another cyclization to afford the radical 3-270, which can yield a stable product either by a reductive interception with the present organotin hydride species to obtain compounds of type 3-271. On the other hand, when the terminal alkyne carries a trimethylstannyl group, expulsion of a trimethylstannyl radical takes place to afford vinyl silanes such as 3-272. 

Organotin hydrides R SnI L- , where R represents an alkyl or aryl group, are generally prepared quite easily by reduction of the corresponding organotin halides using either LiAlFLt or NaBFL in ether or dioxane ... 

Several less general methods for preparing organotin hydrides are also used, though often with less satisfactory yields. However, reduction of N, A-diethylaminostannanes by dire -butylaluminum hydride or diborane occurs with high yields224 ... 

A new organotin hydride with a polar tail, di-w-butyl(4,7,10-trioxaundecyl)stannane, useful for various syntheses in organic chemistry, is prepared by the following sequence of reaction237 (Scheme 23). Bu2HSnCl is prepared in ether from LiAlH4 and Bu2SnCl2 at low temperatures. 

As for organogermanium hydrides, a simple organotin hydride, like tri-w-butyltin hydride, can be used for the reduction of structurally more complicated diorganotin dichlorides228 ... 

An application of the reduction method of organotin halides by simple organotin hydrides is the preparation of l, >-bis(hydridodimethylstannyl)alkanes235 ... 

The propensity of organotin hydrides for SET reactions has been utilized to initiate radical chain reactions. Anodically promoted oxidation of Ph3SnH to [Ph3Sn] at 0.80 V (vs SCE) initiates the cyclization of several haloalkyne and haloalkene ethers as well as of some fi-lactam derivatives. The catalytic cycle shown in Scheme 1 is based on... 

The most commonly used method for synthesizing organotin hydrides with a hydrogen isotope bonded to the tin atom is to reduce the appropriate chlorostannanes with labelled hydride reagents, such as lithium aluminium deuteride or sodium borodeuteride. For example, tributylchlorostannane can be reduced with lithium aluminium deuteride42-45 or deuterated or tritiated sodium borohydride46 to give tributyltin deuteride and tritiated tributyltin hydride, respectively (equations 40 and 41). 

Many other groups can be selectively reduced by organotin hydrides. Although these reactions have not been used to date to form labelled compounds, the reactions are included here using tributyltin deuteride as the reducing agent to illustrate the synthetic possibilities for this reagent. 

Although these groups can be reduced with organotin hydrides, John and coworkers78 found that cyclic products accompanied the reduced (hydrocarbon) products in some reactions see Schemes 16 and 17 (vide supra). 

Finally, some important functional groups, notably the keto group and the carboxylic acid group, cannot be reduced with organotin hydrides. However, these groups can be reduced provided they are converted into a more reactive group. 

All of these syntheses begin with the addition of an organotin hydride to a carbon-carbon it bond. Therefore, it is important to understand the scope of these addition reactions. Organotin hydrides add without initiators to olefinic bonds, which are activated with an electron-withdrawing group. The hydride of the organotin hydride always adds to the more positive carbon in the addition reaction (equation 87). 

Organotin hydrides can also be added to unactivated olefinic (without an electron-withdrawing group on one carbon) bonds provided the reaction is initiated by UV radiation, AIBN or y-rays113-116. In one instance, an organotin hydride has even been added to an unactivated olefin without an initiator. This unexpected addition reaction occurred at high pressure117. 

Organotin hydrides add across carbon-carbon triple bonds even more easily than they add across carbon-carbon double bonds. For example, the addition to the unactivated C=C bond of phenylacetylene proceeds spontaneously at 20 °C without an initiator (equation 88), whereas the addition to an unactivated olefin requires the presence of an initiator118. 

The trialkylstannyl intermediates required in this synthetic sceme to prepare labelled compounds can be obtained in several ways. One method is the addition of the organotin hydride to the carbon-carbon triple bond of an alkyne (equation 93). These reactions have already been discussed in detail above. A second approach is to add a trialkylstan-nylvinyllithium to a ketone (equation 95), and a third method involves adding trialkylstan-nyllithium to a /J-halo, a, /J-unsaturated ester (equation 96). Although this last reaction gives a suitable trialkylstannane, these stannanes have proven to be inert in the destanny-lation reaction and, therefore, have not been used extensively to prepare radiolabelled compounds. 

Hannon and Traylor158 used a specifically labelled organotin hydride, l/iiw-3-deutero-2-trimethylstannylbutane, to determine the mechanism and stereochemistry of the hydride abstraction from an organostannane by a carbocation (equation 101). 

The dehalogenation of organic halides by organotin hydrides takes place in most cases with a free-radical mechanism [1, 84, 85], The stereospecific reduction of 1,1-dibromo-l-alkenes with Bu3SnH discovered by Uenishi and coworkers [86-89], however, did not occur in the absence of palladium complexes and did not involve radicals. For the synthesis of (Z)-l-bromo-l-alkenes, [(PPh3)4Pd] proved to be the most effective catalyst which could also be generated in situ. The reaction in Eq. (7) proceeded at room temperature and a wide range of solvents could be used. 

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