Self-terminating radical reactions cyclizations

Since the computational studies support the general perception of good or poor leaving groups in self-terminating radical reactions, but clearly contradict the experimental findings, radical cyclization cascades initiated by N-centered radical addition to alkynes are not terminated by homolytic p-fragmentation. 

Finally, cerium(IV) ammonium nitrate can serve as the radical source by itself, generating NO3 radicals If by photolysis. The addition of such radicals to cyclo-alkynes 20 initiates an interesting tandem reaction [12]. Transannular hydrogen atom abstraction by the vinyl radical 21 affords the intermediate 22, which undergoes a 5-exo cyclization to the radical 23. In the last step, the ketone 24 is formed by elimination of NO2 in moderate yield thus, the overall sequence can be described as a self-terminating radical reaction (Scheme 7). 

On addition of S04 to the triple bond in the lO-member cycloalkyne 24 and cyclo-aUcynone 27, a nonchain, and anionic, self-terminating radical cyclization cascade is induced. In the former reaction (equation 22) the bicyclic ketones 25 and 26 are formed, and in the latter reaction (equation 23) the a,/3-epoxy ketones 28 and 29 are formed in good yields. Because of the difficulty of oxidizing isolated triple bonds, 804 does not react as an electron-transfer reagent in these reactions but acts as a donor of atomic oxygen. 

In this sequence, NO3 can formally be regarded as a synthon for O atoms in solution. Because the released NOa is a comparatively stable radical that does not initiate a radical chain process under the applied reaction conditions, this sequence is termed a self-terminating oxidative radical cyclization —the cascade-initiating radical contains the unreactive leaving group that is released in the final step of the reaction. In contrast to reactions performed under classical radical chain conditions and that are nearly always associated with a loss in overall functionality of the molecule, in self-terminating radical cyclizations the net amount of functional groups is retained in the system. 

Self-terminating radical oxygenations are not restricted to cyclic alkynes. Electro-and photochemically generated NO3 can be used for the oxidative cyclization of cycloalkyl-clamped alkynes 67 (Scheme 2.12). This reaction leads to formation of anelated tetrahydrofurans (68 with X = or pyrrolidines (68 with X = NTs,... 

Self-terminating radical cyclizations have been explored with both aminium and amidyl radicals using the reaction with cyclodecyne (52) as a model system. However, instead of N-containing compounds, the bicyclic ketones 53 and 54 were exclusively obtained. Their formation could be explained by the mechanism that is exemplary shown for reaction of the A-benzyl acetamidyl radical, AcBnN" (Scheme 2.19). ... 

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