Hydrostannation, and

Metallostannation often gives a better regio- and stereoselectivity than does hydrostannation, and the vinylmetal group that is formed can often react further with suitable electrophiles such as halogens, acids, alkyl halides, acyl halides, metal halides, aldehydes, or epoxides.110 ... 

Sonochemistry has been applied to acceleration of the Reformatsky reaction, Diels-Alder reactions, the arylation of active methylene compounds nucleophilic aromatic substitution of haloarenes, and to hydrostannation and tin hydride reduction. " Other sonochemical applications involve the reaction of benzyl chloride and nitrobenzene, a Sr I reaction in liquid ammonia at room temperature, and Knoevenagel condensation of aromatic aldehydes. lodination of aliphatic hydrocarbons can be accelerated, and oxyallyl cations have been prepared from ot,ot -diiodoketones using sonochemistry. Sonochemistry has been applied to the preparation of carbohydrate compounds.When sonochemistry is an important feature of a chemical reaction, this fact will be noted in the reactions presented in Chapters 10-19. 

Concurrent hydrostannation and addition of fhe resulting vinylic radical to a carbonyl have also been exploited in fhe synfhesis of (-i-)-quercitol derivatives [274[ and the carbocychc core of CP-263,114 [275]. 

Allenyltins are conveniently prepared from the terminal alkynes which have a suitable leaving group. An example is shown in equation 19. A two-step one-pot procedure for the synthesis of allene by hydrostannation of alkynes has recently been reported starting from propargylic alcohols a hydrostannation and subsequent deoxystannylation generates the allenes 6 as shown in equation 20. A chiral version of this procedure has also been described in the paper. 

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... 

Since the first work reported in 1987, more than a dozen papers on Pd-catalyzed hydrostannation of alkynes have been reported. As detailed below, Pd catalysts significantly accelerate hydrostannation and hydrogermation of alkynes. Thus, the reaction now proceeds at or below room temperatures. However, the selectivity aspects remain unpredictable and often problematical, with three isomers—a, p-trans, and 13-cis— being formed in a capricious manner. The isomer profiles for Pd-catalyzed hydrostannation are often substantially different from those observed under either uncatalyzed or radical-initiated conditions. When all of the seemingly random results are sorted out, however, the following potentially useful generalizations may be presented as a set of guidelines. 

TABLE 1. Pd-Catalyzed Hydrostannation and Hydrogermation of Nonheterofnnctional Allies... 

Similarly, the reactions of olefins with metal hydrides, such as silicon hydrides, germanium hydrides, tin hydrides and lead hydrides, were also found. These reactions are called hydrosilylation, hydrogeimanation, hydrostannation and hydroplumbylation, respectively. These reactions easily proceed even if the R in eq. (2.28) are functional groups. Then these reactions are conveniently used for syntheses of organometallic compounds having a functional group by a one-stage reaction. 

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

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). 

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... 

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, 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 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). 

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... 

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