Hydrostannation mechanism

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... 

Hydrostannation of conjugated esters and nitriles leads to /J-stannylatcd derivatives by a free radical mechanism (equation 26)58. The additions are highly stereoselective, but the relative stereochemistry of the adducts was not determined. Since both (E)- and (Z)-alkenoates gave rise to the same stereoisomeric adduct, it can be concluded that a stepwise process is involved. 

Hydrostannation can be catalyzed by Lewis acids. If a mixture of an alkyne, tributyltin hydride, and Z1CI4 or HfCl4 is stirred in hexane, the product of ////-addition is formed (Equation (19)) 98 the hydride can be generated in situ from Bu3SnCl and EtjSiH, and B(C6F5)3 is an alternative Lewis acid.99 100 The mechanism of these reactions is still a matter of speculation. 

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-. 

Hydrostannation of carbonyl compounds with tributyltin hydride is promoted by radical initiation and Lewis or protic acid catalysis.The activation of the carbonyl group by the acidic species allows the weakly nucleophilic tin hydride to react via a polar mechanism. Silica gel was a suitable catalyst allowing chemoselective reduction of carbonyl groups under conditions that left many functional groups unchanged. Tributyltin triflate generated in situ from the tin hydride and triflic acid was a particularly efficient catalyst for the reduction of aldehydes and ketones with tributyltin hydride in benzene or 1,2-di-chloromethane at room temperature. Esters and ketals were not affected under these conditions and certain aldehydes were reduced selectively in preference to ketones. 

This reaction constitutes a special type of process in which a hydrogen atom and a nucleophile are added across the diene with fonnation of a carbon-hydrogen bond in the 1-position and a carbon-Nu bond in the 4-position. Some examples of such reactions are hydrosilylation [12-18], hydrostannation [19,20] amination [21,22], and addition of active methylene compounds [21 a,23,24], These reactions are initiated by an oxidative addition of H-Nu to the palladium(0) catalyst, which produces a palladium hydride species 1 where the nucleophile is coordinated to the metal (Scheme 8-1). The mechanism commonly accepted for these reactions involves insertion of the double bond into the palladium-hydride bond (hydride addition to the diene), which gives a (jr-allyl)palladium intermediate. Now depending on the nature of the nucleophile (Nu) the attack on the jr-allyl complex may occur either by external trans-aVtBck (path A) or via a cw-migration from palladium to carbon (path B). 

Palladium-catalyzed hydrostannation of isoprene was used for in-situ generation of allylstannane 9, which was trapped by an aldehyde to give alcohol 10 [Eq.(4)] [20]. It was suggested that an intermediate HPdSn(OAc)Cl2 is formed. The authors proposed two mechanisms for the hydrostannation, one according to Scheme 8-1 where Nu = Sn(OAc)Cl2 and another where insertion of the double bond into the palladium-tin bond is followed by reductive elimination from a (jr-allyl)palladium hydride. 

Hydrostannation of highly polarized a,)8-unsaturated nitriles leads to the formation of N-stannylketenimines 31, 33) in what is considered to be a polar 1,4-addition. Thus, polar mechanisms are ascribed to reactions leading to either 1-cyanoalkyl or ketenimine adducts. 

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). 

In the presence of ZrCLt or HfCLt, alkynes react with Bu3SnH or Bu2SnH2 at 0 °C by anti addition.81 Plausible mechanisms involve direct electrophilic hydrostannation via a tin hydride-Lewis acid complex, or initial electrophilic hydrozirconation followed by transmetallation. 

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.

The general principles of hydrostannation are described in Section 4.4. The most common tin hydride to be used is tributyltin hydride, which is commercially available. Al-kynes are more reactive than alkenes. With non-polar alkynes, the reaction follows a radical chain mechanism, and is typically carried out at 60-80 °C, often with AIBN as a radical initiator, but reactions can be carried out at room temperature or below with initiation with UV light or sonication,8 or with a trialkylborane in the presence of a trace of air.9... 

The hydrostannation of carbonyl compounds to give alkoxystannanes (equation 14-5) can follow a homolytic or heterolytic mechanism depending on the structure of the reactants and on the reaction conditions (Section 20.1.3). This existence of alternative mechanisms is elegantly demonstrated in the reduction of cyclopropyl methyl ketone by tributyltin hydride. In methanol, 1-cyclopropylethanol is formed in a polar process, but, with irradiation with UV light, the main product is pentan-2-one, which is formed by ring opening of the cyclopropylmethyl radical by P-scission (Scheme 14-1)14... 

On the other hand, a homolytic mechanism is involved in the hydrostannation of methyl vinyl ketone in the presence of CuCl,110 and in the reaction of tributyltin hydride or allyltributyltin with cyclopropyl ketones in the presence of AIBN.14-111... 

Scheme 15-2 Homolytic chain mechanisms of hydrostannolysis and hydrostannation.

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). 

Hydrostannation can also be catalyzed by palladium compounds. Other tin hydrides may react by a polar rather than a radical mechanism. A series of PVC stabilisers (the Estertins) have been developed by the AKZO company, and which are prepared by the reaction of HCl with metallic tin or SnCla, to give the solvated chlorotin hydrides H2SnCl2 and HSnCls, respectively, which add, in a heterolytic Michael fashion to acrylic or propargylic esters [e.g. Equations (1.7) and (1.8)] the chlorides are then converted by anion exchange into the thioglycollates. 

It has been shown447 that hydrostannation of terminal and non-terminal olefins and of acetylenes occurs by a radical and not by a polar mechanism. 

Similar mechanisms through CPC-Pd intermediates have been proposed for the hydrometalation and bismetalation of MCPs. For example, hydrostannation... 

The chemistry of metal-palladium complexes has received particular attention in relation to the mechanism of Pd-catalyzed reactions such as hydrosilylation, bis-silylation, or hydrostannation. 

A precise proof for the radical mechanism by using galvinoxyl, as effected in the case of hydrostannation 189, 203), cannot be given here, because galvinoxyl is reduced by organolead hydride even at 0° C, see Table IX. 

The same facts are known for hydrostannation (165a). The polar mechanism is very greatly favored in the strongly polar systems of alkylidene-malonodinitriles (IX) and cyanoacetic esters (X) which are discussed in Section III,D,4 (195). 

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