Radicals containing heteroatoms

VI. CARBON-CENTERED ALKENYL RADICALS CONTAINING HETEROATOMS... 

Radicals containing heteroatoms (e.g. N, O) as it-centers are included when, in terms of valence bond resonance structures, the unpaired electron is not located at the heteroatom (e.g. 2-azaallyl, 2-oxaallyl). 

Addition ofN-, S- and P-Nucleophiles The reaction of nitrones with heteroatom centered nucleophiles has been little investigated and are mainly applied to the synthesis of new heterocyclic systems and stable nitroxyl radicals, containing a heteroatom at the a-carbon atom. 

Table 5.3 Kinetic data for the addition of silyl radicals to heteroatom-containing multiple bonds...

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 ... 

Cleavage is a common reaction pathway for substrates which, upon reduction to the intermediate radical-anion stage, may split off relatively stable anions. Often such substrates contain heteroatoms which stabilize the negative charge on the cleaved anion. 

Heteroatomic Coals, especially low-rank coals that are rich in oxygen, might be expected to contain heteroatomic free radicals. Yen and Sprang (29) showed that the g values of coals as well as those of other bituminous materials showed a systematic trend with their heteroatom content. In particular, they plotted the ESR g values as a function of where X refers to the atom fractions of oxygen, nitrogen, and sulfur in the coals and values are spin-orbit coupling constants of the respective atoms, i.e., 152 cm , 70 cm , and 382 cm . This... 

The combination of radicals can therefore be treated, like union, in terms of an interaction between a pair of orbitals, either NBMOs or AOs. In the case of hydrocarbons, the orbitals will be degenerate in the case of compounds containing heteroatoms, they will not be. However, the same situation will still hold, the relationship being similar to that between the homopolar molecule H2 and the heteropolar ion HHe" (Sections 1.5 and 1.7). 

The discussion of radical cations (see Fig. 7.7) indicates that the energy of union of an odd AH radical and an odd AH cation to form a radical cation is just half the energy of union of the two corresponding odd AH radicals to form a neutral AH. As long as the resulting AH is alternant, the rules for aromaticity for it and for the radical cation derived from it will be the same and it is easily shown that this is also true for isoconjugate molecules containing heteroatoms. The course of pericyclic reactions where the transition... 

A comparison of the magnitudes of the absolute rate constants of addition is also of interest. It would seem at first sight that the rapid rate of radical addition to the nitronyl function is unusual since unsaturated functions containing heteroatoms are not known to accept radicals readily. Radicals preferentially... 

Hydroxyindole (181) represents a well known example of a compound in which the hydroxyl group is to the ring heteroatom. The equilibrium mixture again contains mainly the carbonyl form (182), indoxyl. Deprotonation gives a reactive ambident anion which can be methylated either on oxygen or C-2 (Scheme 73). Indoxyl is easily oxidized to indigo (184), which may be formed by dimerization of the radical (183) produced by electron loss from the anion. 

Another example of resort to heteroatoms to obtain both oral potency and a split between androgenic and anabolic activities Ls tiomestrone (99). Trienone, 98, prepared in much the same way as 23, undergoes sequential 1,6 and 1,4 conjugate addition of thioacetic acid under either irradiation or free radical catalysis to afford the compound containing two sulfur atoms. 

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