Carbon-centered radicals azidation

Carbon-centered radicals can cyclize at N-N multi bonds such as an azo group or azide group via 5-exo-trig or 6-exo-trig manner to give pyrrolidine or piperidine derivatives as shown in eq. 3.25 [91, 92]. Aminosugars such as nojirimicins can be prepared by this method. 

Magnus reported the direct a- and )9-azido functionalization of triisopropylsilyl enol ethers using trimethylsilylazide and iodosylbenzene. In this mechanistically complex reaction sequence, it is believed that azidation of a carbon centered radical is occurring [78],... 

While phenyliodine(ni) diacetate and TMSN3 are known to form PhI(N3)2 in solution and homolyticaUy cleave to the azide radical (see this section, below), the combination is sensitive to decomposition at elevated tanperatures. A more stable azide source was developed by Zhdankin and cowoikers. Azidoiodinane 11 is thermally stable and in fact needs a radical initiator to affect the radical reaction. Following radical initiation, hydrogen-atom abstraction leads to a carbon-centered radical... 

The Antonchick group demonstrated the oxidative coupling of nitrogen heterocycles with alkanes. °° Combination of phenyliodine(lll) bis(trifluoroacetate) with TMSNj is a known method of generating azide radicals. The azide radical abstracts a hydrogen atom from an alkane to form a carbon-centered radical. The carbon-centered... 

The hydrohydrazination represented a general solution for the amination of alkenes, but the protected hydrazines obtained are sometimes difficult to transform to the free amines. At this point, we turned to sulfonyl azides as nitrogen sources, based on then-capacity to react both with enolates and carbon-centered radicals. Mechanistic investigations of the hydrohydrazination reaction had suggested a radical character for the formed organocobalt intermediate. " We were pleased to see that the Cobalt-catalyst 4 was able to promote the hydroazidation of 4-phenylbut-l-ene (3) with ethanesulfonyl azide (7), giving the product derived from the formal Markovnikov addition of hydrazoic acid onto the C-C double bond exclusively, albeit in moderate yields (50%). 

The intermolecular addition of carbon-centered radicals to unactivated alkenes followed by azidation (a formal carboazidation of alkenes) has been reported. A one-pot procedure similar to the one used for intramolecular reactions gives good results (Scheme 8.28). Slow addition of benzenesulfonyl azide is not necessary because this electrophilic reagent does not react with the initial electrophilic or ambiphilic radicals. Excellent results are obtained with a-iodo and -xanthate esters. a-Bromoacetates give also satisfactory results. The carboazidation process allows to prepare pyrroUdinone derivatives in a straightforward manner (Scheme 8.28, bottom example). A tin-free version of this reaction using triethylborane instead of hexabutylditin has also been reported. ... 

Moreover, Bols et al. developed another methodology for the synthesis of carbamoyl azides from aldehydes by treatment with iodine azide at reflux in acetonitrile [41]. The carbamoyl azides are obtained in 70-97 % yield from the aliphatic and aromatic aldehydes (Scheme 5.4). When the reaction of phenyl-propanal with IN3 at 25 °C was performed in the presence of the radical trap, no acyl azide was observed, which was taken as support for a radical reaction mechanism. The mechanism shown in Scheme 5.6 is proposed for the reaction. Iodine radicals are formed by homolysis of the weak iodine-azide bond, abstracting the aldehyde hydrogen atom. The resulting carbon-centered radical reacts with iodine azide to produce an acyl azide. The following Cuitius rearrangement provides carbamoyl azides. 

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