Radical reagents

The carbon atoms of azole rings can be attacked by nucleophilic (Section 4.02.1.6 electrophilic (Section 4.02.1.4) and free radical reagents (Section 4.02.1.8.2). Some system for example the thiazole, imidazole and pyrazole nuclei, show a high degree of aromati character and usually revert to type if the aromatic sextet is involved in a reaction. Othei such as the isoxazole and oxazole nuclei are less aromatic, and hence more prone to additio reactions. 

In their thermal stability the diaziridines approximate to the oxaziranes. As with most oxaziranes, they are stable at 100° C for short periods they are decomposed by heating at 200°C 1,2-di-n-butyl-3-ri-propyldiaziridine thus eliminates butylamine. The thermal decomposition has not yet been investigated in detail. Similarly no information is available on the reaction of radical reagents on diaziridines. 

Whereas bromine radicals (133) and succinimidyl radicals (134) react by the Sh2 mechanism at the tin center in tetraalkyltins, but not in alkyltin halides, alkoxyl radicals (135) and ketone triplets (136) react with alkyltin halides, but not tetraalkyltins this may reflect the conflicting, electronic demands of the radical reagents which, as electrophilic species, should be more reactive towards tetraalkyltins than alkyltin halides, but which would also tend to make use of a 5d orbital... 

The Hammett equation has also been shown to apply to many physical measurements, including IR frequencies and NMR chemical shifts. The treatment is reasonably successful whether the substrates are attacked by electrophilic, nucleophilic, or free-radical reagents, the important thing being that the mechanism be the same within a given reaction series. 

Finally, conjugated enoximes can be synthesized by the reactions of radical reagents with specially synthesized BENA containing a system of conjugated double bonds (527) (see Scheme 3.280, for more details, see Section 3.5.4.2.3). The advantage of this method is that it does not afford unstable intermediate a-nitrosoalkenes. 

To conclude, silylation of AN and their derivatives made it possible to substantially extend the reactivity of AN. The reactions of AN with nucleophilic, electrophilic, and radical reagents at both the a- and 3-carbon atoms are summarized in Chart 3.24. 

Norbornadiene is also known to add electrophilic or free-radical reagents across the 2- and 6-positions with formation of nortricyclene derivatives (85). Neither type of adduct, 84 nor 85, has yet been obtained from 7-azabicyclo[2.2.1]heptadiene derivatives, but aromatiza-tion of the latter is induced by some electrophilic reagents (see Section II,E). 

As described in Chapter III, morusin (3) has been found to be anti-tumor promoter in a two-stage carcinogenesis experiment with teleocidin. Considering the similarity of the structures between morusin (3) and artonin E (7), artonin E (7) was expected to be an anti-tumor promoter. Furthermore we found a novel photo-oxidative cyclization of artonin E (7) as follow photo-reaction of artonin E (7) in CHCI3 containing 4% ethanol solution with high-pressure mercury lamp produced artobiloxanthone (8) and cycloartobiloxanthone (9), and the treatment of artonin E (7) with radical reagent (2,2-diphenyl-1-picrylhydrazyl DPPH) resulted in the same products, Fig. (15), [84]. 

Fig. (15). Photoreaction of artonin E (7) and the reaction with radical reagent.

For these reasons the reactivity of pseudoazulenes is higher than that of the carbocyclic analogs, the azulenes. Thus, a generally lower stability toward electrophilic, nucleophilic, and radical reagents results. This explains why the general stability of some pseudoazulenes is so low (see Section IV,A). 

The classical and the most useful laboratory method for the preparation of quinones is the oxidation of monohydric phenols with the radical reagent, potassium nitrosodisulphonate [(K03S)2NO] (Fremy s salt) (the Teuber reaction).5 Details for the conversion of 3,4-dimethylphenol into 3,4-dimethyl-1,2-benzoquinone may be regarded as typical55 the probable mechanistic pathway is formulated below. 

If the hydrocarbon radical cation has a definitive structure, proton loss occurs from one particular, well-defined position and these transformations are more selective than the alternative C-H abstractions from alkanes with radical reagents (Eq. 2). For example, C-H substitutions of the adamantane cage with radical reagents always give mixtures of 1 and 2-substituted adamantanes [2], As the adamantane radical cation (4) has one single structure, proton transfer from the radical cation to the solvent occurs highly selectively. Scheme 2 shows the geometry of 4 and the structure of the complex of the adamantane radical cation with acetonitrile (S) where the tertiary C-H bond is already half-broken. 

Ceric (IV) ammonium nitrate (CAN) works as the same oxidative radical reagent as Mn(OAc)3. Diethyl 3-furylphosphonates was prepared in good yield under mild conditions by a simple two-step procedure using CAN-promoted oxidative addition of p-ketophosphonate (139) to vinyl acetate (140), followed by acid-catalyzed cyclization as shown in eq. 4.49. 

Bu3SnH cannot generally be used for radical alkylation of heteroaromatics, since it is a reductive radical reagent. However, O-acyl esters (2) are not a reductive reagent. 

In the previous chapters, Bu3SnH has been used as a typical and useful radical reagent in a benzene solvent. Generally, radical reactions with Bu3SnH initiated by AIBN, proceed effectively in benzene, which bears a conjugated Tr-system. Probably, the formed radicals are somewhat stabilized through the SOMO-LUMO or SOMO-HOMO interaction between the radicals and benzene. 

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