Nitrogen-centered radical, reactions

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... 

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). 

In contrast to their oxygen- and nitrogen-centered analogs [reactions (6) and (25)], 1,2-H-shift reactions of thiyl radicals are not only slow but the equilibrium lies practically fully on the side of the thiyl radical [reaction (36) Zhang et al. 1994 Naumov and von Sonntag 2005]. 

In basic solution, Ura and Thy undergo a series of reactions as depicted in reactions (72)—(76) for Ura as an example (Fujita and Steenken 1981). Ura dissociates athighpH [equilibrium (72) for pfCa values see Table 10.11], Its OH-adductscan also be deprotonated at nitrogen leading to an oxidizing heteroatom-centered radical [reaction (76)]. 

The electronic nature of a nitrogen centered radical, dictated by reaction conditions and/or the radical precursor employed, is crucial to the mode of reaction, to the ability to undergo efficient intramolecular cyclizations or intermolecular additions, and to the products isolated from the radical reaction. The types of radicals discussed in this review include neutral aminyl radicals, protonated aminyl radicals (aminium cation radicals), metal complexed aminyl radicals, and amidyl radicals. Sulfonamidyl and urethanyl radicals are known (71S1 78T3241), but they are not within the scope of this chapter. 

The PTOC protocol for the preparation of nitrogen centered radicals is also compatible with a variety of Lewis acids that apparently complex with the aminyl radicals to give reactive, electrophilic species. Lewis acids offer potentially milder reaction conditions than protic acids for sensitive compounds. Efficient intermolecular addition reactions have... 

HYDROGEN-ATOM ABSTRACTION BY NITROGEN-CENTERED RADICALS THE HOFMANN-LOFFLER-FREYTAG REACTION... 

Since the Barton reaction and the Hofmann-Lofifler-Freytag reaction generate very reactive oxygen-centered and nitrogen-centered radicals respectively, the next 1,5- and 1,6-hydrogen atom abstraction reaction readily happens. However, 1,5-H shift does not proceed effectively by carbon-centered radicals, because there is not so much energy difference between the C-H bond before and after 1,5-H shift. So the reactions are quite limited. Eq. 6.21 shows iodine transfer from reactive 1-iodoheptyl phenyl sulfone (40) to a mixture of 5-iodoheptyl phenyl sulfone (41a) and 6-iodoheptyl phenyl sulfone (41b) initiated by benzoyl peroxide, through 1,5-H shift by an sp3 carbon-centered radical [56-58]. 

Azides are highly valuable radical acceptors that form nitrogen-centered radicals after addition onto them, as in the case of the transformation of 31 into 32. This reaction opens new possibilities for making pyrrolidines. Murphy, for instance, disclosed the synthesis of ( )-horsfihne and ( )-coerulescine by tandem cyclization of iodoaryl alkenyl azides such as 29 [42]. By the same strategy and using precursor 33, formal syntheses of ( )-vindohne [43] and ( )-aspidospermidine [44] have been rendered possible (Scheme 10). 

Whereas nitrogen-centered radical cations are prone to C-H, C-C bond-cleavage reactions, RsP and R2S + suffer predominantly nucleophilic attack [322, 323] few exceptions have been reported to date. This reaction plays an important part in the formation of oxygenated derivatives (P [324], S [325]), C-S [326], S-S, and Se-Se bond formation [327], and in removing sulfur-protecting groups [328-330]. 

Nitrogen centered radicals have received considerable attention in recent years. In particular, amidyl radicals have been shown to enter into cascade reactions to form pyrrolizidinone and indolizidinone derivatives. Thus, heating the (9-benzoyl hydro-xamic acid derivative 512 with Bu3SnH in the presence of AIBN produced 513 as a 3 2-mixture of diastereomers (Scheme 88) (99SL441). Separation of compound 513 from the tin residues was difficult, and the isolated yield (17%) was consequently low. When 514 was subjected to identical conditions, a 2 1-mixture of indolizidine 515 and pyrrolizidine 516 was isolated in 42% yield, along with the monocyclic... 

An electrophilic metal-carbene complex intermediate has been proposed in the cyclopropanation reaction, whereas a paramagnetic copper nitrene species, which behaves as an electrophilic, nitrogen-centered radical, is proposed as the intermediate for the aziridination reaction.45 [Cu(Tp GF3 2)(C2H4)] is a good aziridination catalyst, readily converting a variety of olefins into the corresponding A-tosyl aziridines.46... 

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