Silastannation

Palladium-catalyzed addition of a silicon-tin linkage across a carbon-carbon triple bond was first reported in 1985 by the Mitchell group and the Ghenard group independently.251,252 Since then, the silastannation reaction of alkynes has been studied extensively (Table J) 25S 261... 

The palladium-isocyanide complex is effective for silastannation of ethoxyethyne, a labile alkyne, to produce (Z)-1-ethoxy-l-silyl-2-stannylethene with high regioselectivity (Equation (106)).253 The regioselectivity observed has also been studied by computation.263... 

Table 8 Palladium-catalyzed silastannation of alkoxy- and thioalkanes...

A thermodynamically stable (silyl)(stannyl)palladium(n) complex is synthesized by an oxidative addition of the Si-Sn linkage to palladium(O) (Scheme 63).267 The complex has the square-planar geometry with a m-arrangement of the silicon and tin atoms. An alkyne reacts with the complex to afford a silastannated product as a mixture of cisjtrans stereoisomers (10 1). 

The silastannation products serve as useful building blocks. Treatment of (Z)-a-stannyl-/3-silylstyrenes with HG1 in the presence of Et4NGl results in protodestannation with retention of stereochemistry to give the corresponding CE)-/ -silylstyrenes in high yield (Equation (108)).254... 

A silastannation product bearing an epoxide moiety undergoes titanium(m)-mediated cyclization with the vinyl-stannane moiety (Equation (109)).268... 

A similar silastannative cyclization reaction has been extensively studied with 1,6-enynes (Table 9).271-274 Several palladium catalysts including palladium(0)-DBA complex and even a heterogeneous catalyst are effective, whereas Pd(PPh3)4 gives an uncyclized simple adduct predominantly. 

Table 9 Palladium-catalyzed silastannative cyclization of enyne derived from diethyl malonate...

The regioselective silastannation is applicable to other 1,6-enynes in the presence of various palladium catalysts (Table 10).275... 

Norbornene also undergoes silastannation from the exo-side under identical conditions (Equation (112)). However, ordinary alkenes such as 1-hexene, styrene, and cyclohexene fail to react with silylstannanes. 

The palladium-isocyanide catalyst is also effective for the silastannation of cyclopropenes (Equation (115)).158 The reaction proceeds smoothly under mild conditions (RT, 10-30 min) with high face selectivity to give tetrasubstituted... 

Silastannative coupling of 1,3-diene and aldehydes is achieved both in inter- and intramolecular fashions (Equations (116) and (117)).277 Interestingly, the reaction is catalyzed by nickel(0) complexes, whereas a platinum complex is used for simple 1,4-silastannation of 1,3-diene.278... 

Silastannative cyclization of bis(diene) catalyzed by PdC cod) achieves formal 1,8-silastannative G-G bond formation (Equation (118)).125 Poor stereoselectivity at the allylic metal moieties (E/Z) is observed. 

Highly regio- and stereoselective silastannation is achieved by employing phosphine-free Pd(dba)2 as the catalyst (Equation (119)).2 The addition takes place at the less substituted allenic C=C bond to afford /3-silyl (E)-... 

The palladium-catalyzed silastannation of bis(allene)s gives trans-cyclized product stereoselectively (Equation (121)).162 This stereochemical outcome stands in contrast to the distannation of the bis(allene)s, which affords m-cyclized product. 

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... 

Silastannation similarly appears to involve intermediates based on Si-PdL -Sn complexes, the rate-determining step being the insertion of the alkyne into the Sn-Pd bond (Equation (32)). 

Palladium-phosphine catalysts have been used most commonly, but a PdfOAc Z-alkyl isocyanide123 or Pdfdba) P(OEt)3 (cheaper)124 catalyst is more effective, and will bring about silastannation at room temperature, for example, Equation (33). 

Kang et al. demonstrated that the palladium-catalyzed tandem cyclization—silastannation of the bis(allene) 72 with trimethylsilyltributyltin 70 produced the trans fused heterocycle 73 (Scheme 25),... 

Recently, Hada et al. [74] have reported a theoretical study on the reaction mechanism and regioselectivity of silastannation of acethylene with a palladium catalyst. Experimentally, terminal acetylenes react with silylstan-nanes to give highly regio- and stereoselective l-silyl-2-stannylalkenes with tetrakis(triphenylphosphine)palladium as a catalyst [75]. The products are always cis adducts tin adds to the internal position as follows ... 

Figure 24. Reaction scheme of silastannation adopted by Hada et al. [74].

The reaction of silastannation of acetylene catalyzed by a Pd complex involves the oxidative addition of the H3Si-SnH3 to the catalyst Pd(PR3)2, followed by coordination of acetylene to the catalyst accompanied with dissociation of one of the PR3 bondi and reductive elimination of the product. The rate-determining step of the reaction is the insertion of substituted acetylene RCCH into the Pd-Sn or Pd-Si bond, the activation energy of which increases in the order of R CN < OCH3 < H < CH3. 

---