Radicals nitrogen-centered radical

The volume is divided grossly into sections dealing with individual types of free radicals such as carbon-centered radicals, nitrogen-centered radicals, nitroxyl radicals, oxygen-centered radicals and radicals, centered on other heteroatoms. These sections deal mainly with irreversible reactions. In addition, there are sections on reversible electron and proton transfer processes and their equilibria and a chapter on biradicals. An index of radicals formulae will facilitate data retrieval. 

A few studies have appeared on systems based on persistent nitrogen-centered radicals. Yamada et al.2"1 examined the synthesis of block polymers of S and MMA initiated by derivatives of the triphenylverdazyl radical 115. Klapper and coworkers243 have reported on the use of triazolinyl radicals (e.g. 116 and 117). The triazolinyl radicals have been used to control S, methacrylate and acrylate polymerization and for the synthesis of block copolymers based on these monomers [S,243 245 tBA,243 MMA,243 245 BMA,245 DMAEMA,24 5 TMSEMA,247 (DMAEMA-Wbc/fc-MMA),246 (DMAEMA-Woc -S)246 and (TMSEMA-6/ocfc-S)247]. Reaction conditions in these experiments were similar to those used for NMP. The triazolinyl radicals show no tendency to give disproportionation with methacrylate propagating radicals. Dispcrsitics reported arc typically in the range 1.4-1.8.2"43 246... 

Table 5 Radical stabilization energies (in kj/mol) of nitrogen-centered radicals at 0 K according to Eq. 3... 

The 5-dig-mode of cyclization has been applied in the synthesis of N-heterocycles. For example, treatment of the /i-allenyl dithiosemicarbazide 37 with Bu3SnH and AIBN in hot benzene furnishes the substituted 3H-pyrrole 38 in 41% yield and the isomeric heterocycle 39 in 30% yield (Scheme 11.13) [68], Iminyl radical 40 is formed via Bu3Sn addition to the thiocarbonyl group of the radical precursor 37 and fragmentation of the adduct (not shown). Nitrogen-centered radical 40 adds 5-dig-selectively to provide substituted allyl radical 41. The latter intermediate is trapped by Bu3SnH to furnish preferentially product 38 with an endocydic double bond. 

The method used by Bawn and Mellish relies on the presence of a radical trap in the reaction mixture, that is, a compound that reacts very fast with the acyl radicals produced, thus preventing their recombination. This substance was the vivid colored 2,2-diphenyl-1-picrylhydrazyl radical (figure 15.1). When these nitrogen-centered radicals, herein abbreviated by P, react with an acyl radical (reaction 15.6), the solution color change can be monitored with a spectrophotometer. 

The kinetic data for reactions of nitrogen-centered radicals with silicon hydrides is limited to rate constants for piperidinyl radical 18 (Table IV) by using ESR spectroscopy.61 The two remarkable features of the data are... 

Rate Constants for Reactions of Nitrogen-Centered Radicals with Silicon Hydrides... 

The limited kinetic data for reactions of tin hydride with nitrogen-centered radicals apparently demonstrates the combined effects of the enthalpies of the reactions and polarization in the transition states for H-atom transfer. The aminyl and iminyl radicals are electron-rich, and the N-H bonds formed are relatively weak these radicals react relatively slowly with tin hydride. On the other hand, the electrophilic amidyl and aminium cation radicals form strong N-H bonds and react rapidly with the tin hydride reagents. 

It is noteworthy that the dimethylamino radical reaction with 02 is about a factor of 106-107 slower than its reactions with NO and N02. For example, Lindley et al. (1979) measured the ratio of rate constants /c81//c82 = 1.5 X 10-6 and /c81//c83a = 3.9 X 10-7. Thus, at 10 ppb NOx, reactions of the nitrogen-centered radical with NO and N02, in addition to 02, become important. This is perhaps not surprising, given that NH2 radicals also react extremely slowly with 02, with an upper limit of 6 X 10-21 cm3 molecule-1 s-1 (Tyndall et al., 1991). 

Figure 7.83 Possible routes for the metabolic activation of hydralazine. The oxidation of the hydrazine group may also involve the formation of a nitrogen-centered radical, which could also give rise to phthalazine with loss of nitrogen.

To facilitate the cyclizations of nitrogen-centered radicals, one need only increase the electronegativity at nitrogen. This makes the radical more electrophilic (which accelerates the cyclization), and strengthens the forming bonds (which shifts the equilibrium to the cyclic product). Amminium radical cations are more well-behaved in cyclizations than aminyl radicals, and these radical cations can either be formed directly from ammonium salts, or by protonation of aminyl radicals (which are relatively weak bases and require strong acids for protonation). Metal-complexed aminyl radicals are highly reactive, and amidyl radicals are also useful.178... 

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