Stannanes radical substitution reactions

Radical substitution reactions involving allylic tin derivatives could be accompanied by a photoinduced 1,3-rearrangement54,55. A photostationary mixture of cinnamyl(tri-phenyl)stannane with its regioisomer l-phenylprop-2-enyl(triphenyl)stannane has been shown to form in the photolysis of ( )-cinnamyl(triphenyl)stannane in benzene under aerobic conditions, or in the presence of halogenated organic compounds or radicaltrapping reagents (equation 21). 

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. 

A novel aromatic substitution reaction with electron-deficient radicals, which avoids the use of stannanes, is promoted by the addition of tetra-n-butylammonium bromide [54]. Iodoacetonitrile and iodoacetic esters react with pyrroles and indoles in good to high yield upon photolysis in the presence of 2-methyloxirane and sodium thiosulphate (Scheme 6.34). 

Phenylmenthyl esters are also suitable chiral groups for inducing stereoselectivity in radical addition reactions, as shown in the allylation of phenylmenthyloxycarbonyl-substituted xanthates. The photoinitiated reaction of the radical precursor with tributyl(2-propenyl)stannane at — 78 =C affords only one diastereomer4. The absolute configuration of (— )-8-phenylmenthyl 2-methyl-2-phenyl-4-pentenoate (5) is not known. 

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]. 

The success of such reactions depends on the intramolecular hydrogen transfer being faster than hydrogen atom abstraction from the stannane reagent. In the example shown, hydrogen transfer is favored by the thermodynamic driving force of radical stabilization, by the intramolecular nature of the hydrogen transfer, and by the steric effects of the central quaternary carbon. This substitution pattern often favors intramolecular reactions as a result of conformational effects. 

Substitution at the terminal position of the allylstannane, as in crotonyltributyl stannane, however, is not tolerated, because hydrogen abstraction from the allylic position is a competing reaction [21], An extension of the method involves the coupling of the anomeric radical precursors 28 with the allyltributyltin reagent 29 [14], In the reagent 29 the double bond is activated toward addition of nucleophilic radicals by the electron-withdrawing t-butoxy carbonyl group. The obtained product 30 has been useful en route to 3-deoxy-D-marmo-2-octulosonic acid (KDO). 

Dussault and coworkers described the preparation of allylstannanes (116, 117) as part of their synthetic studies (equation 93)731. It is interesting to note the preferred geometries of the products which appear to be dependent on the nature of the stannane employed. In this last example, Yu and Oberdorfer reported the use of free-radical hydrostannylation in their preparation of (tributylstannyl)vinyl-substituted 2-deoxyuridine derivatives (e.g. 118) for use in halogenation and radiohalogenation reactions (equation 94)733. 

Photolysis of [Rh(tfacac)3] (tfacac is the unsymmetrically substituted 1,1,1-trifluoromethyl-acac) reveals the existence of two photoinduced reaction paths the relative efficiency of the two paths is dramatically solvent dependent.1140 In cyclohexane, mer- cis isomerization is the only observed photoreaction, but if ethanol or 2-propanol is added to the solvent, the photoisomerization efficiency decreases, and photodecomposition occurs. The nature of the photodecomposition products is not specified, but the enhanced photoreactivity in the presence of tri(n-butyl)stannane, a hydrogen atom donor, and flash and continuous photolysis studies in mixed-solvent systems strongly implicate hydrogen atom abstraction from the solvent as a key step in the photodecomposition of wer-[Rh(tfacac)3] and suggests that the photo reactive states have considerable radical character .1140 Analysis of quantum efficiencies implies that at least two distinct photoproduced excited states must be involved. 

Work carried out in our laboratories over the past three years and in conjunction with the research group of Daktemieks at Deakin University has been directed toward the development of novel enantiomerically pure stannanes for use in free-radical reduction chemistry. To that end Dunn prepared a series of menthyl-substituted stannanes 18 -20 and some others derived from aromatic amines (eg. 21, 22).Perchyonok tested these reagents against a series of substrates while Henry modelled the reactions in question through the use of ab initio molecular orbital theory. 

---