Organotin with radicals

B. Giese, Syntheses with radicals. Carbon-carbon coupling via organotin and -mercury compounds, Angew. Chem. Int. Ed. Engl. 24 553 (1985). 

Syntheses with Radicals C-C Bond Formation via Organotin and Organomercury Compounds"... 

Replacement of Labile Chlorines. When PVC is manufactured, competing reactions to the normal head-to-tail free-radical polymerization can sometimes take place. These side reactions are few ia number yet their presence ia the finished resin can be devastating. These abnormal stmctures have weakened carbon—chlorine bonds and are more susceptible to certain displacement reactions than are the normal PVC carbon—chlorine bonds. Carboxylate and mercaptide salts of certain metals, particularly organotin, zinc, cadmium, and antimony, attack these labile chlorine sites and replace them with a more thermally stable C—O or C—S bound ligand. These electrophilic metal centers can readily coordinate with the electronegative polarized chlorine atoms found at sites similar to stmctures (3—6). 

Although toxic agents have the potential to cause necrosis, some of them can interfere with intracellular signaling pathways and induce apoptosis instead of necrosis. It seems that organotin(IV) compounds exert their toxic effects involving all these processes. The precise balance of these actions and their outcomes may differ radically from one cell type to another and among different organisms. 

The hydrostannation reaction can proceed either by a free-radical mechanism, or, with polar-substituted alkenes or alkynes, by a polar mechanism, respectively resulting in anti-Markownikoff or Markow-nikoff orientation. Both t3rpes of reaction are particularly suitable for preparing functionally substituted, organotin compounds. 

The versatility, predictability and functional-group tolerance of free radical methodology has led to the gradual emergence of homolytic reactions in the armory of synthetic chemistry. Tin hydrides have been successfully employed in radical chemistry for the last 40 years however, there are drawbacks associated with tin-based chemistry. Organotin residues are notoriously difficult to remove from desired end products, and this, coupled with the fact that many organotin compounds are neurotoxins, makes techniques using tin inappro-... 

The readily available organotin compounds include tin hydrides (stannanes) and the corresponding chlorides, with the tri-n-butyl compounds being the most common. Trialkylstannanes can be added to carbon-carbon double and triple bonds. The reaction is usually carried out by a radical chain process,137 and the addition is facilitated by the presence of radical-stabilizing substituents. 

Our research in this field is mainly carried out on a) synthesis and conversion of oligoorganoepoxystannanes b) free-radical copolymerization of organotin monomers with various vinyl monomers and c) cross-linking of organotin macromolecules and development of protective polymeric coatings with specific properties on their basis. 

Free-radical copolymerization of trimethyl- or tributylvinyltin with styrene or methyl methacrylate results in low ( 10%) yield of copolymer. Moreover, both the reaction rate and viscosity decrease considerably with higher vinyltin content in the starting mixture 49). These findings imply that organotin monomers tend to inhibit free-radical copolymerization. 

By comparing the constants of the copolymerization of MA with organotin methacrylates with the known values for the copolymerization of the MA — MMA system = 0.03 and r2 = 3.5)89) where almost no complexation takes place, the following conclusion on the effect of the electron-accepting groups SnR3 on free-radical copolymerization and be reached. 

It is likely that the observed coordination interaction between individual segments of the macroradicals and monomer units determines the stereoorientation of free-radical copolymerization of organotin methacrylates with MA. 

Binary free-radical copolymerizations of organotin derivatives of unsaturated acids (tri-n-butylstannyl methacrylate, bis-triethylstannyl maleate (TESM) and P-phenyl-tri-n-butylstannyl methacrylate (PBSM)) with certain vinyl monomers such as styrene (St) 87) and vinyl chloride (VC) have been studied 24,25,92). 

Free-radical copolymerization of organotin derivatives of unsaturated acids such as bis-TESM, TBSM and PBSA with vinyl chloride has been carried out 24,25,92). 

The resulting values point to the fact that organotin monomer units enter the macromolecular chain. To reveal the contribution of trialkylstannyl groups to radical copolymerization, the copolymerization of their organic analogs (BMA and MA) with VC was investigated. 

Clive and coworkers have developed a new domino radical cyclization, by making use of a silicon radical as an intermediate to prepare silicon-containing bicyclic or polycyclic compounds such as 3-271 and 3-272 (Scheme 3.69) [109], After formation of the first radical 3-267 from 3-266, a 5-exo-dig cyclization takes place followed by an intramolecular 1,5-transfer of hydrogen from silicon to carbon, providing a silicon-centered radical 3-269 via 3-268. Once formed, this has the option to undergo another cyclization to afford the radical 3-270, which can yield a stable product either by a reductive interception with the present organotin hydride species to obtain compounds of type 3-271. On the other hand, when the terminal alkyne carries a trimethylstannyl group, expulsion of a trimethylstannyl radical takes place to afford vinyl silanes such as 3-272. 

The dehalogenation of organic halides by organotin hydrides takes place in most cases with a free-radical mechanism [1, 84, 85], The stereospecific reduction of 1,1-dibromo-l-alkenes with Bu3SnH discovered by Uenishi and coworkers [86-89], however, did not occur in the absence of palladium complexes and did not involve radicals. For the synthesis of (Z)-l-bromo-l-alkenes, [(PPh3)4Pd] proved to be the most effective catalyst which could also be generated in situ. The reaction in Eq. (7) proceeded at room temperature and a wide range of solvents could be used. 

Tin-based reagents are not always snitable owing to the toxicity of organotin derivatives and the difficulties often encountered in removing tin residues from the final product. Therefore, the same authors have carried out additional experiments with 17d and several different alkyl halides under tin-free conditions. The treatment of 16d with tert-butyldiphenylsilyl chloride (TBDPSCl) and triethylamine in the presence of silver triflate in CH2CI2 affords the bis(silyloxy)enamine 17d in 92% yield (Scheme 17). When the radical reaction was carried out with ethyl iodoacetate in the presence of 2,2 -azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70) as the initiator in CH2CI2, the oxime ether 19 was obtained in 83% yield (Scheme 17). 

Recent advances in radical reactions have greatly benefited from the efficiency of organotin reagents as mediators. Radical reaction of alkyl iodides with trifluoromethyl phenylsulfonyl oxime ether 69 and hexamethylditin at 300 nm in benzene afforded the corresponding trifluoromethyl oxime ethers 70 in high yields (Scheme 37) . 

As with the silanes, some of the most useful synthetic procedures involve electrophilic attack on alkenyl and allylic stannanes. The stannanes are considerably more reactive than the corresponding silanes because there is more anionic character on carbon in the C—Sn bond and it is a weaker bond.103 104 There are also useful synthetic procedures in which organotin compounds act as carbanion donors in palladium-catalyzed reactions, as discussed in Section 8.2.3 Organotin compounds are also very important in free-radical reactions, which will be discussed in Chapter 10. 

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