Stannane ligand synthesis

The coupling of benzyl halides with stannanes has been occasionally used in synthesis.One illustrative example is the total syntheses of piericidin A1 (119) and B1 (120).i2o in these syntheses a heterobenzylic Stille cross-coupling reaction of 116 with alkenylstannane 117 proceeded in the presence of a Pd(0) complex bearing bulky PrBuj as the ligand to give key intermediate 118 (Scheme 5.4.23). 

Pyridyl stannanes have seen use in the transformation of 250 into 251. These dinucleating phenanthroline-based ligands have found utility in the synthesis of bimetallic complexes [91]. 

Gennari and his group reported a more sophisticated palladium-catalyzed carbonylative coupling of vinyl stannanes with electron-poor enol triflates [61]. The effect of ligands and additives was tested, and a combination of Pd2dba3, AsPh3, LiCl, Cul and NMP proved to be the optimal system. Later, this methodology was applied to the total synthesis of sarcodictyins [62]. 

Bu SnH has been used in the synthesis of bicallcene, tetradent-ate organosulphur ligands, and to give the allylic stannane... 

In contrast, the Stille reaction was one of the first cross-coupling reactions performed on solid support. For the palladium-catalyzed coupling of various aryl, vinyl, or alkynyl stannanes with aryl or alkenyl bromides, iodides, or triflates, the reaction conditions employed were chosen in analogy to the liquid phase procedures and often feature an arsine or trifurylphosphine as an added ligand. Due to the tedious separation of tin and organo tin reagents or by-products in solution phase, this carbon-carbon bondforming reaction is particularly suitable for solid-phase synthesis. ... 

Both electron-deficient and electron-rich aryltins are suitable partners for the Stille reaction. The only limitation of the reaction was an ortho substitution on the organotin and the substrate until the recent use of electron-rich and sterically demanding ligands such as P(t-Bu)3, which permitted the synthesis of tetra-ortho-substituted biaryls. A plethora of heterocyclic stannanes [36] were used in Stille couplings. Their major applications concern the synthesis of pharmacologically interesting compounds, as illustrated by the synthesis of an endothelin antagonist [37] (Scheme 6.5), as well the preparation of supramolecular compounds. 

Ni(COD)2-catalyzed tandem carbostannylation of alkynes and allenes with alky-nylstannes in the presence of a phosphine ligand results in highly chemoselective synthesis of dienyne stannanes. The stereoselectivity of the reaction depends strongly on the chelation nature of the ligands [237]. For example, when tris(4-tri-fluoromethylphenyl)phosphine is used, the D/E ratio is 98 2. On the other hand, when a bidentate ligand (e.g. [o-(dimethylamino)phenyl)diphenylphosphine] is employed, the D/E ratio becomes 6 94. 

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