Large rings radical attack

Some interesting photo-NOCAS-type reaction (photochemical nucleophile-olefin combination, aromatic substitution) have been reported by three groups. Arnold has developed the photo-NOCAS reaction as three components photoaddition.Xu et al., reported the intramolecular photocyclization of A-(co-hydroxyalkyl)-tetrachlorophthalimide (138, 141) with alkenes to give medium- and large-ring heterocycles (140,143). These photoreactions proceeded via 1, n-biradicals generated from the nucleophilic attack of alcohols to alkenes between the radical anions of phthalimides and the radical cations of alkenes. 

Surprisingly little is known about the attack of radicals on small and large heterocycles. Hydrogen abstraction from the heteroatom of small rings leads to ring opening, and in the... 

The thermolysis and photolysis of mesoionic phenyl-1,3,2-oxathiazolones were extensively investigated during the period 1980-1990 <1996CHEC-II(4)433>. Electrophilic and nucleophilic attack at ring atoms of 1,3,2-dithiazoles (nitrogen, carbon, and sulfur) occurs quite rarely and little attention was paid to it in CHEC-II(1996). A large part of the section was devoted to the transformation of 1,3,2-dithiazolium radicals to corresponding cations and vice versa. 

For the compounds obeying the Hammett equation, the distribution pattern for the OH attack can be estimated by assuming that the position on the ring with the maximum C7 value has the least probability, while the one with the minimum has the greatest probability. In our recent work > on cinnamate derivatives where the addition of the OH radical to the olefmic double bond is an additional pathway, a value of-0.3 was estimated from the Hammett plot (Fig. 5). This value is in agreement with those reported earlier for several substituted benzenes (p" = -0.52 to -0.4). However, the Hammett treatment for reactions with rate constants close to diffusion-controlled ones may not be satisfactory due to limited variation in kinetic data and large experimental uncertainty. In contrast, better correlation is expected in the corresponding gas phase reactions. 

The stereoselectivity in hydrogen-abstraction reactions of carbohydrate radicals differs from the reactions of mono- or disubstituted carbocyclic radicals in some characteristic ways. Generally, the ring oxygen atom leads to pronounced higher axial attack if located next to the radical center. At the same time, it has been observed that some of these carbohydrate C-l radicals prefer a twist boat to a chair conformation44 4b. Furthermore, the large number of substituents present simultaneously make the conformational analysis of axial and equatorial transition states more difficult. 

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