Trimethyltin radicals

Radical homologation. This tin pinacolate is known to generate trimethyltin radicals at 60° and appears to be superior to tributyltin hydride as a source of stannyl radicals for addition of alkyl halides to O-benzylformaldoxime (equation I).1 Iodides, bromides, and selenides can be used as radical precursors. The same... 

The corresponding reaction with PlnSnM proceeds with complete inversion regardless of addition mode (equation 21). In this case, radical reactions appear unimportant. Displacement of the tosylate by a trimethyltin cuprate was also found to take place by inversion. 

Finally, Podoplelov and coworkers41 have prepared trimethyltin hydride and benzyltrimethyltin labelled with tin-117 from a tin sample enriched to 92% in 117Sn. The photochemical reaction between dibenzyl ketone and trimethyltin-117 hydride was used to investigate the chemically induced dynamic nuclear polarization of tin-containing radicals. 

Trimethyltin hydride adds mainly 1,2 to piperylene, but a variety of other organotin compounds are also formed. This suggests that the reaction may be a radical addition (IS). 

Hojo Y, Kobayashi T, Shigemitsu Y, et al. 1998. Aluminum(3)-induced brain toxicity and hydroxyl radical generation Comparison with trimethyltin. Eisei Kagaku 44 10. 

The addition of trimethyltin hydride to (- )-menthyl (ii)-2,3-diphenylpropenoate under free radical conditions leads to a mixture of all four possible diastereomers, two of them being in higher proportion. One of the major diastereomers, after isolation by column chromatography, has been reduced to chiral 2,3-diphenyl-3-trimethylstannylpropanol22. 

A clean separation between the homolytic and heterolytic processes can be achieved with trimethyltin hydride and methyl propiolate.87 In the dark at 50 °C, with acetonitrile as solvent, and galvinoxyl present as a radical inhibitor, only methyl 2-trimethyl-stannylacrylate is formed, but at 60 °C in cyclohexane, with AIBN as a radical initiator, a 4 1 mixture of methyl Z- and E-2-trimethylstannylacrylate is obtained (equation 4-49). 

It is interesting to note that Ashby s investigations of the reactions of alkyl halides with (trimethyltin)sodium and dialkylcuprates by means of the l-methyl-5-hexenyl probe which led to the conclusion that only radicals cyclize 98), have been questioned by Lee and San Fillipo in 1983 " . These authors correctly state that the cyclization alone does not prove a radical pathway. The cis/trans ratios from reactions of the halides 136 with (trimethyltin)sodium (revised 4.8 at 0 °C)96 , however, are consistent with 100% radical cyclization. 

The live-ring closure of prostereogenic radicals constructs two new stereogenic centers with good diastereoselectivity, as demonstrated by the trimethyltin hydride mediated cyclization of an ester-substituted diene4. The reaction proceeds via regioselective addition of the stannyl radical to the terminal end of the less substituted alkene and subsequent cyclization. 

With this information in hand, it seemed reasonable to attempt to use force field methods to model the transition states of more complex, chiral systems. To that end, transition state.s for the delivery of hydrogen atom from stannanes 69 71 derived from cholic acid to the 2.2,.3-trimethy 1-3-pentyl radical 72 (which was chosen as the prototypical prochiral alkyl radical) were modeled in a similar manner to that published for intramolecular free-radical addition reactions (Beckwith-Schicsscr model) and that for intramolecular homolytic substitution at selenium [32]. The array of reacting centers in each transition state 73 75 was fixed at the geometry of the transition state determined by ah initio (MP2/DZP) molecular orbital calculations for the attack of methyl radical at trimethyltin hydride (viz. rsn-n = 1 Si A rc-H = i -69 A 6 sn-H-C = 180°) [33]. The remainder of each structure 73-75 was optimized using molecular mechanics (MM2) in the usual way. In all, three transition state conformations were considered for each mode of attack (re or ) in structures 73-75 (Scheme 14). In general, the force field method described overestimates experimentally determined enantioseleclivities (Scheme 15), and the development of a flexible model is now being considered [33]. 

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