Selective Solvation and Solute-Solvent Binding

Besides solvation, a solvent can also participate in entrainment of ion-radical transformations. The reaction between tertiary aliphatic nitro compounds and the sodium derivative of nitromethane, NaCH2N02, is an example (Kornblum and Erickson 1981). To prepare NaCHjNOj, nitromethane is treated with sodium hydride. Then a tertiary aliphatic nitro compound is introduced into the solution formed. Several organic solvents were probed and CHjSOjCHj (DMSO) turned out to be the most effective. Kornblum and Erickson (1981) attributed this result to the formation of small amounts of NaCH2SOCH3 (sodium dimsyl) that was produced from DMSO as a result of its reaction with sodium hydride. Sodium dimsyl acts as a powerful one-electron reducer that induces the following chain anion-radical process  

Workentin et al. (1994) described another interesting solvent effect on the competition between electron transfer and the addition reaction between organic cation-radicals and azides. TEE and AN were compared as solvents. In TEE, the cation-radicals of 4-methoxystyrene (R =R =H), P-methyl-4-methoxystyrene (R =Me, R =H), or p,p-dimethyl-4-methoxystyrene (R =R =Me) react with the azide ion according to the following equation  

One additional example involves alkylation of the lithium salt of the anthracene anion-radical by 2-octyl fluoride (Herbert et al. 1985). The akylation does not occur in DME (which strongly solvates Li+), whereas it is facilitated and leads to the quantitative yield in diethyl ether. Diethyl ether is much less active at solvation of the lithium cation. That makes CH-F Li+ coordination possible and assists the formation of octylanthracene. 

It is worth mentioning that there are some solvents that combine good solvency power with coordinating properties. The most salient example is 1,2-dimethoxyethane as a solvent in the reduction of organic acceptors by alkali metals. The acceptors transform into anion-radicals, and alkali metals into their cations. These cations are bound in chelates with 1,2-dimethoxyethane, and the one-electron reduction of the acceptors becomes more energy advantageous. 

Another point of such coordination activity is the specific interaction of the solvents containing heteroatoms with cation-radicals having a suspended unpaired electron (cf Chapter 3). A pertinent example in this context is the interaction between the dialkylsulfide cation-radicals and the oxygen belonging to the water molecule. Such interaction enhances stability of the coordinated cation-radical  

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