Hydrostannation of alkenes

Trialkyltin radicals are important intermediates in the reduction of alkyl halides, and in the hydrostannation of alkenes (1, 287). 

It is noteworthy that hydrostannation of alkenes and conjugated dienes128 can be performed either at high pressures, or under intense UV irradiation, or by using initiators like AIBN, as in the case of the hydrostannation of phenylcyclohexene 82 ... 

Hydrostannation of alkenes and alkynes (Equation (14)) can involve nucleophilic (R3Sn H+), electrophilic (R3Sn+H ), or homolytic (R3Sm) tin, or the hydride as a metal complex (R3SnMH). 

The hydrostannation of alkenes and alkynes can be catalyzed by a number of transition metals (Ni, Pd, Pt, Mo, etc.), but most studies have involved palladium.106 The mechanism that is most commonly accepted is shown for an alkyne in Scheme 1 the model for an alkene is similar. This mechanism accounts for the observations that the reactions normally involve cir-addition, that the R3Sn group enters so as to avoid steric hindrance by the largest substituent group on the alkene or alkyne, and that the regioselectivity depends on the polarization of the palladium-hydrogen bond in the sense Pd -H15-. 

The principal methods for forming the carbon-tin bond involve the reaction of organo-metallic reagents with tin compounds (equation 4-1), the reaction of stannylmetallic compounds with organic halides (equation 4-2), the reaction of tin or tin(II) compounds with alkyl halides (equation 4-3), the hydrostannation of alkenes or alkynes (equation 4-4), the reaction of acidic hydrocarbons with Sn-0 and Sn-N bonded compounds (equation 4-5), and carbonyl-forming eliminations (equation 4-6) the symbol sn represents 4Sn. 

This section deals mainly with the hydrostannation of alkenes, and further details can be found in the appropriate subsequent sections on, for example, stannyl radicals (Section 20.1.3), and stannacycloalkanes (Chapter 10). The hydrostannation of alkynes, allyl compounds, and allenes, is dealt with in Sections 8.1.1 and 10.1. which cover the vinyl-and allyl-tin compounds which are produced. 

Scheme 4-3 Mechanism of homolytic hydrostannation of alkenes and alkynes.

There are a few reports of the catalysis of the hydrostannation of active alkenes by soluble palladium catalysts,105- 106 but, with less active alkenes, the major product is the hexaalkyldistannane. Good yields of hydrostannation products, however, can be obtained with heterogeneous catalysts, and Table 4-2 shows examples of the hydrostannation of alkenes in the presence of a PdjOHVC catalyst.107 A recent, thorough, survey in Chemical Reviews is available.108 The mechanism of the palladium-catalysed hydrostannation is not known in detail, but presumably it involves oxidative addition and insertion of the alkene by stannylmetallation (or hydrometallation) as shown, followed by reductive elimination (Scheme 4-4). 

Scheme 4-4 Mechanism of palladium-catalysed hydrostannation of alkenes.

SnH Adducts are probably intermediates in the platinum-catalysed hydrostannation of alkenes and alkynes,92 and this reaction has been demonstrated for the hydride 19-15.93... 

The principal alternative route into the organotin(IV) manifold is based on the reaction of tin hydrides or stannylmetallic compounds, as shown in Scheme 1.1.2. The most common (radical) mechanism of hydrostannation of alkenes and alkynes with organotin hydrides is outlined, for alkenes, in Equations (1.1.5) and (1.1.6). 

The hydrostannation of alkenes, which has been known for nearly 30 years, follows a polar or a free radical way depending on substituents and conditions ". Under free-radical conditions, the stannyl radical generally adds reversibly either to double or triple bonds and this property has been used to control the vinyltin configuration depending on experimental conditions (equation 18). Therefore, with an excess either of acetylenic compound or of tin hydride, it is possible to obtain mainly the (Z)-isomer (kinetic control) or the (F)-isomer (thermodynamic control). ... 

The tin hydrides find important applications as reducing agents. Many of their reactions (particularly the reduction of alkyl halides and the hydrostannation of simple alkenes and alkynes) arc known to proceed through RaSn- intermediates, and this aspect of their chemistry is referred to in Section II,G. 

Hydrostannation refers to the addition of triorganotin hydride to different types of alkenes or alkynes, in the presence of catalysts, making possible the preparation of various types of stannylalkanes or stannylalkenes130. 

Using triethylborane, hydrostannation of alkynes can also be performed to achieve the addition of the triphenylstannyl group to the terminal acetylenic carbon. The reaction is regioselective, giving a mixture of E- and Z-l-triphenylstannyl-l-alkenes ... 

There are few reports of the hydrostannation of simple alkenes with metal catalysts in homogeneous solution, but steric strain in the ring causes cyclopropenes to be reactive even at —78 °C, with addition of Sn-H to the less sterically hindered face (Equation (25)). Distannation with Me3SnSnMe3 and silastannation with Me3SiSnBu3 could similarly be achieved with Pd(OH)2 as catalyst.107... 

When the hydrostannation of l- -dodecyne is performed with Bu2SnIH systems, the sole use of Bu2SnIH gives a mixture of E/Z isomers. The tin hydride ate complex, Bu2SnIH-MgBr2-OEt2 shows the highest effect where an cr-stannylated alkene is obtained (Equation (36)).106 The employment of EtOAc as a solvent sharply increases the yield of the cr-stannylated adduct. 

The hydrostannation of an alkene or alkyne by an organotin hydride provides a versatile way of preparing alkyl- and vinyl-tin compounds. Variations on this general theme are illustrated in Scheme 4-2. 

Hydrostannation reactions of alkenes and alkynes which involve stannyl radicals have received much wider attention. They are often tolerant of a functional group, and can be used for preparing functionally-substituted organotin compounds.84... 

Some examples which illustrate the scope and conditions of the homolytic hydrostannation reactions of alkenes are shown in equations 4-56 to 4-59,99 4-60,100 4-61,94 4-62,101 4.63,88 and 4-64.102 The stannane can be formed in situ from the reduction of BusSnCl with I hSiH in the presence of a Lewis acid catalyst (equation 4-65).103 In the hydrostannation of an enone, the tin enolate which is formed can be reduced by PhSiH3 to regenerate the tin hydride (equation 4-66), and only a catalytic amount of the tin hydride is required.104... 

Hydrostannation with tricyclohexyltin hydride is only successful with unactivated unsaturated compounds when the reaction is conducted at high temperature (110 °C) with azobis(isobutyronitrile) added portion-wise, as an initiator. Under these conditions, it gives the expected adducts 29, 32, 34, 36, 38, 40, 43, 45, 48, and 50 in good yields with total regiospecificity, the tin being linked to the terminal carbon atom of the unsaturated compound (Scheme 3.7.3). Hydrostannation of unactivated alkenes with tricyclohexyltin hydride has been reported up to now as being unsuccessful, even under high pressure. ... 

This concept turned out to be useful for a variety of transformations, including reductions, epimerizations [14], hydrosilylations of alkenes, hydrostannations of al-kynes [15] and carbon-carbon bond formation [13]. As an example, thiols catalyze the cyclization of acyl radicals [16a] and masked acyl radicals [16b] these reactions have no preparative value in the absence of thiols because the nucleophilic radicals resulting from the cyclization step are not able to abstract efficiently a hydrogen atom from the starting material, whereas thiyl radicals can (Scheme 2). 

Hydrometallations of alkenes and acetylenes such as hydroboration, hydrosila-tion, hydroalumination, hydrostannation and hydrozirconation are useful synthetic reactions. Rhodium-phosphine complexes catalyze hydroboration of substituted styrenes with a catecolborane (Scheme 68). An extensive survey of the phosphine... 

This type of reaction is also observed in Main Group chemistry e.g. hydroboration (p. 66), hydroalumination (p. 80), hydrostannation (p. 108). It and its reversal are central to catalytic cycles such as those involved in the hydrogenation (p. 182) and hydroformylation (p. 387) of alkenes. Hydrozirconation (p. 290) provides another example of its use in stoichiometric organic synthesis. Its reverse... 

---