Hydrostannation alkenes

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). 

The hydrostannation reaction can proceed either by a free-radical mechanism, or, with polar-substituted alkenes or alkynes, by a polar mechanism, respectively resulting in anti-Markownikoff or Markow-nikoff orientation. Both t3rpes of reaction are particularly suitable for preparing functionally substituted, organotin compounds. 

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. 

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

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 ... 

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-. 

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... 

If the acceptor molecule contains both an alkene and an alkyne group, hydrostannation occurs at the alkyne moiety. 1,6-Enynes undergo ring closure initiated by addition to the alkyne, as shown by the fate of the deuterium in Equation (26).108... 

Unactivated alkenes, however, can be hydrostannated in good yields, with Pd(OH)2/C as a heterogeneous catalyst (Equations (27) and (28)).109... 

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. 

In 1962, Kuivila showed that the reaction of trialkyltin hydrides with alkyl halides (hydrostannolysis) (equation 1-6) was a radical chain reaction involving short-lived trialkyltin radicals, R3Sn, 12 and in 1964, Neumann showed that the reaction with non-polar alkenes and alkynes (hydrostannation) (equation 1-7) followed a similar mechanism,13, 14 and these reactions now provide the basis of a number of important organic synthetic methods. 

With a metal hydride as the nucleophile, the organotin hydrides, R SnH4. are formed, which, by addition to an alkene or alkyne (hydrostannation), usually by a radical chain mechanism involving stannyl radicals, RaSn", provide the second way of generating the tin-carbon bond (Scheme 1-2). 

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. 

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. 

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. 

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... 

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

---