Mechanism and Unusual Fragmentation Selectivities of Aryltrialkylstannane Cation Radicals

Mechanism and Selectivity of Aryltrimethylgermane Cation Radical Fragmentations

Aryltrimethylgermane cation radicals were generated by nanosecond transient absorption spectroscopy. Transient kinetics experiments show that the aryltrimethylgermane cation radicals react with added nucleophiles in reactions that are first-order in both the cation radicals and the nucleophiles. Preparative photo-oxidation experiments demonstrate that the intermediate cation radicals react with nucleophiles, resulting in aryl-Ge or Me-Ge nucleophile-assisted fragmentations. The aryltrimethylgermane cation radicals were found to react more slowly than analogous stannane cation radicals; however, loss of the thermodynamically disfavored aryl radicals remains competitive with methyl radical loss.

Stereochemistry of Nucleophilic Substitutions on Benzyl-silane and -germane Cation Radicals: Application of the Endocyclic Restriction Test

Benzyltrialkylgermane cation radicals were generated and spectroscopically characterized by nanosecond transient absorption spectroscopy. The germane cation radicals were found to rapidly react with nucleophiles (e.g., alcohols) in reactions that were first-order in cation radical and first-order in nucleophile. The geometries of the transition states for nucleophilic substitutions on benzyl-silane and -germane cation radicals were investigated by using the endocyclic restriction test. Cation radicals containing tethered nucleophiles that required endocyclic transition states with small angles between the bond being formed to the nucleophilic atom and the bond to the leaving group reacted ∼250 times more slowly than cation radicals with tethered nucleophiles where a large bond angle can be accommodated. The results are consistent with the nucleophile-assisted fragmentations proceeding through an inversion transition state.

Aryltrimethylstannane Cation Radical Fragmentation Selectivities That Depend on Codonor: Evidence for Reactions from Heterodimer Cation Radicals

The aryl/methyl fragmentation selectivities for the photooxidations of phenyltrimethylstannane and (4-methylphenyl)trimethylstannane by 1,2,4,5-tetracyanobenzene in acetonitrile were found to depend on the codonor used to generate the stannane cation radical intermediates. The aryl/methyl fragmentation selectivities for phenyltrimethylstannane and (4-methylphenyl)trimethylstannane varied by factors of 26 and 5.6, respectively, depending on the structures of the codonors. The fragmentation selectivities could be correlated with the oxidation potentials of the codonors and their steric bulk. The results can be interpreted by the intermediacy of heterodimer cation radicals formed between the stannane cation radicals and the neutral codonors, which thereby affect the fragmentation selectivities.