Organostannanes formation

The Stille reaction has developed as a popular protocol for the formation of C-C bonds due to the air- and moisture-stability as well as functional group compatibility of organotin compounds. Together with the Suzuki-Miyaura coupling it is one of the most powerful methods for the synthesis of molecules containing unsymmetrical biaryl moieties. However, despite its efficiency, this versatile reaction has slowly been displaced by other procedures that avoid the use of highly toxic organostannanes. 

The investigation of the mechanism of organoepoxypolystannane formation shows that radical hydrostannylation of epoxides proceeds mainly during the addition of organostannanes to the C=C bond according to the Former rule, the oxirane ring remaining unaffected 100>102). 

In 1998, the rhodium-catalyzed conjugate addition of organostannanes to a,/9-unsatu-rated ketones and esters was reported by Oi [29]. In the reaction, which was carried out in the presence of [Rh(COD)(MeCN)2]BF4 in THF at 60°C, a variety of a,/9-unsatu-rated ketones and esters were transformed into the corresponding conjugate addition products. It is noted that the yield was highly dependent on the substitution pattern of the substrates. For example, /9-substituted enone 40 afforded an 86% yield of 41, whereas methyl vinyl ketone 42 gave only an 18% yield of 43 (Scheme 3.14). The addition of water to the reaction mixture improved the reactivity for the formation of 41 (98%) and 43 (80%) [30]. 

Ionomers have been synthesized from reaction of tin II and tin IV metal halides and organostannane halides through reaction with an ethylene-acrylic acid copolymer. Mass spectral, infrared spectral, and elemental analysis results are consistent with the formation of tin-containing ionomers. The products all exhibit "ionomer-like" properties and soften below 150 C, many softening below 50 C. 

Matyjaszewski and coworkers reported the Wurtz-type synthesis of organostannane-organosilane copolymers under the influence of ultrasound starting from PhMeSiCl2 and Ph2SnCl2. This approach resulted in the formation of low molecular weight copolymers with a broad molecular weight distribution (M = 4 x 10, = 3.7). The NMR... 

The Stille coupling between organostannanes (aryl and vinyl) and an electrophile (such as aryl halide) in the presence of palladium complex is a versatile method for the formation of carbon-carbon bonds. 

The carbon-metal systems considered appropriate for this section (and not covered elsewhere in this series) are carbon-tin, carbon-mercury and, to a restricted extent, carbon-palladium bonds. Although or-ganomercurials and organostannanes have been studied for over a century, the former continue to attract attention because of their controlled formation by the Markovnikov oxymercuration reaction (OM) and its variants, and the latter because of newer and regiospecific methods for forming C—Sn bonds and oxidatively cleaving them. In both cases, signiticant examples of synthetically useful oxidations have... 

The imexpected formation of the methoxy ether (69b) in the reaction of lanosterol 5-methyl dithiocarbonate (69a) with BuaSnH has been reported. Studies using BuaSnD indicated that a hydrogen within the methoxy group originated from the organostannane reagent. A possible mechanism (Scheme 28) was postulated to explain these observations. ... 

Yamamoto has reported the desymmetrizing intra-molecular cyclization of the 1,3-diketone 97 for diastereoselective formation of cis-cis 98 vs. cis-trans 99. Optimized conditions with SnCU led to the jS-chelation in the synclinal arrangement 100 to give 98, whereas the use of TiCU facilitated rapid transmetalation of the organostannane yielding 99 (Scheme 5.2.22).22... 

Because tin-lifhium exchange and anionic cyclization are known to occur with retention of configuration at the metal-bearing carbon center [126], enantiomerically enriched organostannanes can serve as promising precursors for access to chiral heterocycles. On fhe basis of fhis idea Coldham prepared fhe optically active hexahydro-IH-pyrrolizine alkaloid (-i-)-pseudohehotridane with complete diastereoselectivity and enantiospecificity (Scheme 12.56) [127]. In sharp contrast with this success, six-membered ring formation by the same strategy resulted in a diastereomeric mixture of racemates (Scheme 12.57) [128]. 

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