1.4- Hydrostannation

Reaction of isoprene with tributyltin hydride in the presence of catalytic amounts of Pd(PPh3)4 gave 1,4-hydrostannation product 8 in high stereo- and regioselectivity [Eq.(3)] [19]. The Z-stereochemistry can be explained as for the hydrosilylation [cf. Eq.(2)]. 

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Pd-catalyzed hydrostannation of dienes with HSnBu3 affords the (Z)-2-alke-nylstannane 89 with high regio- and stereoselectivities[84], Dimerization-dou-... 

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. 

Hydrostannation of chiral menthyl esters of substituted acrylic acids proceeds stereoselectiveiy, providing a route to optically active alkyl-... 

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). 

Sequential hydrostannation-palladium catalysed cyclisation of the allene 56 leads to formation of the vinyltetrahydobenzoxazepine 57 <96T13441>. 

In addition, Wu and Li recently have developed an efficient rhodium-catalyzed cascade hydrostannation/conjugate addition of terminal alkynes and unsaturated carbonyl compounds in water stereoselectively (Scheme 4.5).88... 

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 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. 

Hydrostannation, applied to alkynes with Pd as catalyst, produces vinylstannanes129,131 as mixtures of two regioisomers, a and b ... 

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 ... 

The preference for the /3-silyl isomer product complements methods available for hydrostannation of alkynes, for which the a-stannyl regioisomer is formed preferentially.70 7011 70c In addition, the /3-silyl products serve as the platform for a tertiary alcohol synthesis (Scheme 15). Upon treatment of vinylsilanes such as B with tetrabutylam-monium fluoride (TBAF) in DMF at 0 °C, a 1,2 carbon-to-silicon migration occurs, affording the tertiary heterosilane E. Oxidation of the C-Si bond then provides the tertiary alcohol. Good 1,2-diastereocontrol has been demonstrated for y-alkoxy substrates, as in the example shown. The studies suggest that the oxidation of the sterically demanding silane intermediate is facilitated by the intramolecular formation of a silyl hemiketal or silyllactone for ketone or ester substrates, respectively.71... 

The Stille adduct of 2-bromothiophene and l-ethoxy-2-tributyl-n-stannylethene or 1-ethoxy-l-tri-n-butylstannylethene is a masked thienyl aldehyde or a masked ketone, respectively [77-80]. Vinylstannane 93, derived exclusively as the -isomer from hydrostannation of bis(trimethyl-silyl)propargyl amine (92), was coupled with 2-bromothiophene to form ( )-cinnamyl amine 94 upon acidic hydrolysis [81, 82], In another case, stereoisomerically pure phenyl ( )-2-tributylstannyl-2-alkenoate 95, arising from Pd-mediated hydrostannation of phenyl ( )-2-alkynoate, was joined with 2-iodothiophene to deliver the stereodefmed trisubstituted a,p-unsaturated ester 96 [83, 84],... 

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). 

Homolytic hydrostannation can also be initiated at room temperature by thiophenols, when a trace of oxygen may oxidize the thiol to the ArS radical, which abstracts hydrogen from the stannane to give the stannyl radical (Equation (15)).93 With azoisobutyronitrile (AIBN) initiation, alkynes undergo only monohydrostannation, but with the thiol, simple alkynes show bis(hydrostannation) or bis(stannation). 

Geminal bis(hydrostannation) is attributed to quenching of the bis-2-tributylstannylethyl radical by the thiol, with polarity reversal catalysis (Equation (16)). 

X-ray contrast agents, have been prepared by reactions, one of which involves a 12-fold homolytic hydrostannation (Equation (18)).96 97... 

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 principal developments in hydrostannation during the past decade are in the use of hydrides R2SnXH, where X is an electronegative group such as halide, carboxylate, or sulfonate, and in the use of the hydrides complexed to a transition metal, particularly palladium. 

Most attention to date has been paid to the hydrostannation of propargyl alcohols and ethers where interaction of the Lewis-basic oxygen enhances the regio- and stereoselectivity and gives high yields of the Z-/3-adducts (e.g., Equations (20) and (21)), whereas hydrostannation with Bu3SnH gives only the Z-cr-adducts. 

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... 

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