Organotin hydrostannation

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 hydrostannation of olefinic compounds in the presence of azobisisobutyronitrile AIBN is quite a well known reaction in organotin chemistry. Its mechanism is given below... 

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

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 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... 

In recent years, these reactions have found an important place in organic synthesis. We will consider in this chapter only the reactions of radicals with tin hydrides, and the basic hydrostannolysis processes that depend on these reactions. Hydrostannation reactions are covered in Section 4.4, and the reactions of stannyl radicals with substrates R X in Section 20.1.3. The further use of organotin hydrides in organic synthesis, which depends largely on transformations of the radicals R " in hydrostannolysis reactions, or of the radicals R3SnXY" in hydrostannation reactions, are beyond the scope of this book, but have been extensively reviewed.2 64 66 104 107... 

Organotin compounds are used in many synthetic procedures as stoichiometric reagents (hydrostannolysis, hydrostannation, allylation, Stille coupling, Sn/Li transmetallation, transesterification, etc), and this presents the problem of removing the organotin residues from the required product. This is particularly important in the preparation of pharmaceuticals as these organotin compounds are to some degree toxic. 

An alternative solution is to use organotin reagents supported on an insoluble, but porous, polymer at the end of the reaction, the polymer can be filtered off, and the catalyst can be regenerated.72 Most of the work in this field has involved polymer-bound tin hydrides (for hydrogenolysis of halides, thiocarbonyl compounds, or hydrostannation of ketones),73-76 but also tin chlorides (for use as Lewis acids),77 allylstannanes (for allyla-... 

Reduction of the halides with a metal hydride such as lithium aluminium hydride, sodium borohydride, or poly(methylhydrosiloxane) gives the corresponding organotin hydrides These have an important place in organic synthesis for the reduction of halides to hydrides (hydrostannolysis) and the addition to alkenes and alkynes (hydrostannation), by radical chain reactions. Further reactions may intervene between the pairs of reactions shown in Equations (1.1.3) and (1.1.4), and (1.1.4) and (1.1.5), and these reactions are particularly useful for inducing ring-closure reactions. 

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). 

This access to a , y-bis(tricyclohexyltin)alkanes proves to be very convenient when the corresponding organic dihalides are available. When this is not the case, and when a , y-dienes are more easily accessible, a double hydrostannation of these unsaturated compounds can be used. As indicated above, this radical addition has to be conducted in less mild conditions than for the addition of tri-n-butyltin hydride, for instance, because of the bulkiness of the organotin center and also of the more nucleophilic character of the tricyclohexyltin radical. Nevertheless, the treatment of 4,4 -di(butenyl)biphenyl or 4,4 -bis(but-3-enyloxymethyl)biphenyl with tricyclohexyltin hydride at 130 °C over seven days affords the corresponding adducts 72 and 75 in good yields (Scheme 3.7.11). Due to its rapid decomposition at this temperature, the AIBN initiator has to be added in small portions for the duration of the reaction. 

As is true for hydrostannation, acetylenes are also more reactive than the olefins toward hydroplumbation. Acetylenedicarboxylic acid dimethyl ester and ethyl propiolate 152) add Bu3PbH at — 50° C (used as a pure substance, or made in situ from organotin hydrides) spontaneously and without side reactions. Me3PbH adds in the same smooth manner to cyanoacetylene and methyl propiolate 165). Phenylacetylene reacts under the same conditions but in a side reaction Pb is precipitated and H2 evolves. After a short while starting materials disappear if excess of lead hydride is used. The IR spectrum shows absorption at 990 cm , which is in accordance with a trans adduct 152,194). 

Organotin phosphates catalyse epoxide polymerisation, and triflates the hydrostannation of aldehydes and ketones,while triorganotin enolates, which result from the alkoxide and alkenyl acetates, are used to couple ciC-haloketones (to 1,4-diketones and... 

Hydrostannation reactions provided adducts with the organotin moiety at the P-carbon [25], while mercaptans readily yielded various P-alkylthioboronic esters [21]. This reaction has been exploited for the synthesis of several water-soluble boronic acids as potential agents in a Boron Neutron Capture Therapy strategy [26]. More recently, this approach was used to prepare new efficient arginase inhibitors (Scheme 9.10) [27]. 

Pd(0)-catalysed hydrostannation of alkynes HC=CCMc20H has been reported to proceed cleanly with the ionic liquid-supported organotin reagents RSnBu2Cl... 

This reaction, which can be called hydrostannation, is perhaps the most valuable one among those undergone by organotin hydrides. It provides a... 

Detailed studies on the mechanisms of addition of organotin hydrides to oleflns and acetylenes remain to be made. Certain inferences can be drawn, however, on the basis of information now available. The catalysis by free radical sources demonstrates that the reaction can proceed by a free radical chain mechanism, since only a few mole per cent of initiator is needed in many cases for complete reaction. An attractive reaction sequence is shown in Eqs. (25-28), in which R is a carbon free radical and Sn is a trisubsti-tuted tin radical. Reaction (26) is the chain initiation step and reactions (27) and (28) comprise the propagation steps. Most of the characteristics of the hydrostannation reaction can be accommodated in such a scheme. For example, free radicals attack terminal oleflns at the terminal carbon atom because the resulting secondary or tertiary radical is more stable than the... 

II Hydrostannation reaction of unsaturated nitriles with organotin hydrides RsSnH and R SnE ... 

The increase in activity in the field of organotin hydrides is mostly due to the facility of addition of the tin-hydrogen bond to unsaturated systems and to the multiple mechanistic investigations of the hydrostannation reaction. Hev is a number of organometallic reactions, i.e., addition to transition metal complexes7 condensation with metal alkyls yielding tin-metal derivatives and with metal amides for synthesis of compounds with longer tin chains. 

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