Stannanes transmetallation, allylstannane

Two approaches for the synthesis of allyl(alkyl)- and allyl(aryl)tin halides are thermolysis of halo(alkyl)tin ethers derived from tertiary homoallylic alcohols, and transmetalation of other allylstannanes. For example, dibutyl(-2-propenyl)tin chloride has been prepared by healing dibutyl(di-2-propenyl)stannane with dibutyltin dichloride42, and by thermolysis of mixtures of 2,3-dimethyl-5-hexen-3-ol or 2-methyl-4-penten-2-ol and tetrabutyl-l,3-dichlorodistannox-ane39. Alternatively dibutyltin dichloride and (dibutyl)(dimethoxy)tin were mixed to provide (dibutyl)(methoxy)tin chloride which was heated with 2,2,3-trimethyl-5-hexen-3-ol40. 

Both allylstannane transmetalation and thermolysis of homoallyl stannoxanes have been used to prepare 2-butenyltin halides as (E)j(Z) mixtures44-45. The reaction between 2-butenyl-(tributyl)stannane and dibutyltin dichloride initially provides dibutyl(l-methyl-2-propenyl)tin chloride as the kinetic product by an SE2 process, but this isomerizes under the reaction conditions to give a mixture containing the (Z)- and (E)-2-butenyl isomers46. 

SnCU is effective in promoting the addition of nucleophiles to simple aldehydes. Among the most synthetically useful additions are allylstannane and -silane additions. The product distribution in the stannane reactions can be influenced by the order of addition, stoichiometry, and reaction temperature. The anti geometry of the tin-aldehyde complex is favored because of steric interactions. Furthermore, the six-coordinate 2 1 complex is most probably the reactive intermediate in these systems. The use of crotylstaimanes provides evidence for competing transmetalation pathways (Eq. 35) [60]. TiCU results in superior selectivity. 

The efficient transmetalation of allylic stannanes to allylboron reagents has generated an attractive methodology for asymmetric allylation. Corey and coworkers first described the use of enantiomers of bromoborane 228 (Scheme 5.2.51) for mild and quantitative transmetalation of allylstannane to yield the allylboron reagent 229. i The asymmetry in the bis-toluenesulfonamide of 228 is derived from l,2-diamino-l,2-diphenylethane, and both antipodes are readily available in high optical purity, by resolution of the starting diamines producing (R,R)- and (5, 5 )- Stein chiral auxiliaries in transmetalation product 229. 

Stannanes and silanes may act as carbanion derivatives that can be activated with suitable Lewis adds. Thus, as shown by Cossy et al. [440], allylstannanes may be transmetalated with InCU to generate allylindium derivatives in situ. The latter can react with aldehydes to give mixtures of syn and anti adducts (with aromatic aldehydes and 2-buten-l-al). With aliphatic aldehydes, only the anti adducts are observed. The authors concluded that product mixtures were partly caused by the kinetic formation of a mixture of indium (Z)- and ( )-allyl esters prior to the condensation. They then allowed the indium species to equilibrate prior to the aldehyde addition and obtained mainly anti adducts (627, 628) (Scheme 130). 

Oxidative addition of certain stannanes to Pd(0) complejKS is also possible. Thus, aUcynylstannanes have been shown to react with Pd(0) complexes [207, 208). In addition, the Pd(0)-catalyzed reaction of allylstannanes with alkynes has been found to afford aUylstannylation products 31 (Scheme 1.23) [209]. A likely mechanism involves oxidative addition of the allylstannanes to Pd(0) to give (ri -allyl) palladium complexes 32 (L = alkyne) (Scheme 1.23). In this transformation, the usually nucleophilic allylstannanes behave as electrophiles. Complexes of type 32 are probably formed by transmetallation of (T) -aUyl)palladium complexes with hexamethylditin [210]. An oxidative addition to form complexes 32 has been proposed in the Pd(0)-catalyzed carboxylation of allylstannanes with COj [211]. Although compleres 32 have not been isolated as stable species, work on the intramolecular reachon of allylstannanes with alkynes and theorehcal calculations give support to the formation of these complexes by the oxidahve addition of allylstarmanes to Pd(0) [212]. 

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