Trialkylstannyl radicals

Allylic stannanes are an important class of compounds that undergo substitution reactions with alkyl radicals. The chain is propagated by elimination of the trialkyl -stannyl radical.315 The radical source must have some functional group that can be abstracted by trialkylstannyl radicals. In addition to halides, both thiono esters316 and selenides317 are reactive. 

Alkenyl radicals generated by addition of trialkylstannyl radicals to terminal alkynes can undergo cyclization with a nearby double bond. 

Fig. 10.3. Chain mechanism for radical addition reactions mediated by trialkylstannyl radicals.

Another class of compounds which undergo addition reactions with alkyl radicals are allylstannanes. The chain is propagated by elimination of the trialkylstannyl radical.229 230 231... 

Hydrostannylation of 3,3-disubstituted 1,6-dienes shows cix selectivity6. The reaction is initiated by addition of the trialkylstannyl radical to the diene affording the radical intermediate which undergoes 5-exo ring closure through a chairlike transition state with the trialkylstannyl-rnethyl substituent positioned equatorially. 

Crystal structures of ethylmagnesium bromide Crystal structure of tetrameric phenyllithium etherate Representation of tt bonding in alkene-transition-metal complexes Mechanisms for addition of singlet and triplet carbenes to alkenes Frontier orbital interpretation of radical substituent effects Chain mechanism for radical addition reactions mediated by trialkylstannyl radicals... 

New sources of RaSn- radicals that have been developed include the reversible thermal dissociation of bis(trialkylstannyl)pinacols (290-292), the )3-scission of /3-stannylalkyl radicals (293), and the photolysis of cyclopentadienyltin compounds (294). 

Trialkylstannyl methacrylates (TASM) readily undergo free-radical copolymerization with maleic anhydride 2,19), styrene871, and vinyl chloride24,88>. 

From the r, and r2 values it may be inferred that trialkylstannyl electron-accepting groups suppress almost completely the addition of TASM to its own radical. 

It is known that the polymerization of MMA. .. SnCl4 and (MMA)2. .. SnCl complexes (MMA = methyl methacrylate) yields a polymer which predominantly exhibits an isotactic structure90). From the analogy between these complexes and those discovered by the author of this article (MA. .. SnR3), it can be suggested that free-radical copolymerization of MA with trialkylstannyl methacrylates yields copolymers mainly exhibiting configuration19). 

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. 

It was anticipated that a C4 /1-trialkylstannyl group would intercept the C4 radical and result in loss of antimalarial activity. Indeed, 31 and 32 were inactive and iron(II) degradation afforded two major products in which the stannyl group had been eliminated (Scheme 6). They proposed that the deoxo derivative 33 was produced by elimination of a... 

For example, vinyl stannanes with carbonyl activation in the form of / -trialkylstannyl-a,/ -unsaturated esters react with bromoacetals producing good yields via a radical induced pathway (equation 115)715,716. 

The acyltrialkylstannanes are readily hydrolysed to the parent aldehydes, and are oxidised in the air to give the corresponding carboxylates, R3SnOCOR 63 in a radical chain reaction which presumably is similar to that of the autoxidation of an aldehyde, and involves as a key step the Sh2 reaction of an acylperoxyl radical at tin rather that at hydrogen (equation 6-26). The trialkylstannyl peroxyester which is formed then reacts with the parent stannyl ketone to give the trialkyltin carboxylate. 

A number of other p-stannylalkyl radicals have been used as sources of stannyl radicals. Thus isobutyltrimethyltin can be used as a photolytic source of trimethylstannyl radicals (equation 20-13),44 and bis(O-trialkylstannyl) derivatives of benzpinacol can be used as a thermal source (equation 20-14).42 45 47... 

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