Radical anion formation transition state

In an attempt to resolve this dilemma, the reaction rates were measured for a series of organic bromides with sodium anthracene (Table I) and correlated with two model systems. The model for the first possibility, a one-step process with bond dissociation, is the tri-n-butyltin hydride reaction with these same halides (6 ). Correlation of the reaction rates would indicate that the transition state for anthracene radical anion reduction is similar to the transition state for radical formation by tri-n-butyltin radical. On the other hand, the model for the second possibility, the organic halide reduction potential, is a measure of organic halide radical anion formation (7). 

The effect of protic additives was rationalized by the formation of hydrogen-bonded adducts, such as that between benzoic acid and the carbonyl group of the coordinated benzoate, as observed by 1H NMR. Such an adduct would be expected to facilitate the formation of a polar transition state. The reaction rate also increases upon modification of the ligand with anionic groups and in polar solvents. Radical initiators have no effect. 

One of the main advantages of the anionic cyclizations is their regioespecificity and stereoselectivity when compared with radical or other types of reactions leading to cyclic systems. This is usually due to the formation of complexes involving the lithiated alkyl, vinyl or aryl substrate and an unsaturated, double or triple, C—C bond. In some cases, a heteroatom is involved in stabilizing the transition state for the reaction. In other cases, the stereoselectivity of the cyclization is determined by the presence of several functional groups in the substrate. 

As with other aromatic substitutions, the substitution step itself can be considered to involve an approximately sps hybridization at the carbon atom under attack (10). In the idealized substitution process shown in Eq. (16), 10 may constitute either an intermediate or a transition state. If proton loss ensues directly, the process is properly called a substitution. In other situations the intermediate 10 may become allied with a radical or an anion, leading thereby to a covalent adduct 11. The final substituted product 12 may then be formed either by the elimination of H—Z (first H, then Z) or by the reversal to 10, followed by proton loss. The first case is a classical example of an addition-elimination halogenation, where the adduct is an essential species in the process. In the second case, structure 10 is a common intermediate for both the substitution and the addition reactions. Being merely a diversion of 10, the addition product is not essential to the substitution. In consequence of this, the isolation of adduct 11 may not mean that addition-elimination is the principal pathway of substitution reversal to 10 may be faster than the elimination of H—Z ( 2, k3>ki). On the other hand, the mere failure to detect adduct 11 does not rule out an addition-elimination process, for dehydrohalogenation of adduct 11 may be much faster than its formation (ki>klt k2). 

Decamethylferrocene has also been used as a donor for the formation of CT complexes with inorganic acceptors. Such acceptors are mainly composed of late transition metal complexes containing planar ligands. Some of these acceptors are illustrated in Scheme 8-3. As for their organic counterparts, the inorganic partners have at least two reversibly accessible oxidation states. The reduced form present in the CT complex is usually a radical anion. 

Reduction in dry aprotic solvents allows the formation of ion pairs between the radical anion and a countercation. Where the cation can interact simultaneously with two radical species, dimerization favors the ( )-product because there are fewer steric interactions, in the corresponding transition state, between groups attached to the reacting radicals. Solvent and supporting electrolyte combinations that give a high yield of ( )-pinacol are acetonitrile with tetraethylammonium bromide [82], dimethylformamide with sodium perchlorate [83], and dimethylformamide with europium(III) chloride. Europium(II) is formed in the last... 

Since the reaction of the formaldehyde radical anion with methyl chloride involves overcoming a barrier over time scales much larger than a picosecond, Yamataka et al. (1999) could not directly simulate the entire reaction process. Instead, they started at a transition-state structure, known from static quantum mechanical calculations chose random initial velocities and performed nine simulations at 298 K. Tliree of these simulations resulted in the formation of the reactants, three resulted in the formation of products via an electron-transfer reaction, and three resulted in the formation of products via a carbon-substituted Sn2 reaction. These three sets of resulting structures are shown in Fig. 4. 

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