Samantha Strazzari

Radical chain reactions of α-azido ketones with tributyltin hydride: reduction vs nitrogen insertion and 1,2-hydrogen shift in the intermediate N-stannylaminyl radicals

Abstract The radical chain reactions of a variety of acyclic and cyclic α-azido ketones with tributyltin hydride have been investigated. The derived N-(tributylstannyl)aminyl radicals normally undergo H-abstraction reaction yielding corresponding amines, and thence symmetrical pyrazines by subsequent self-condensation, in competition with 1,2-H-migration from the α-carbon to nitrogen leading to α-imino ketone decomposition products with loss of the chain-carrying tributyltin radical. The noteworthy occurrence of a quite uncommon radical 1,2-hydrogen-atom shift is considered to be largely due to consequent formation of a highly stable, captodative carbon-centred radical. In contrast with our previous N-stannylaminyl radicals produced from α-azido-β-keto esters, the present aminyl congeners give poor amounts (or even none) of nitrogen-inserted amides/lactams, which are envisaged to arise from intramolecular three-membered cyclisation onto the ketone moiety followed by β-scission of the resultant alkoxyl radical. It is inferred that adequate stabilisation of the eventual ring-opened carbon radical be a major factor for the successful outcome of the regiospecific nitrogen insertion process. Evidence is also presented that chemoselective attack of tris(trimethylsilyl)silyl radical to the ketone oxygen of an α-azido ketone gives rise to deazidation as a likely consequence of β-elimination of azidyl radical by the ensuing α-silyloxyalkyl radical. X-Ray crystal structure analyses of the bromo ketone 5a , the azido ketone 5b , the caprolactam 22 , and the pyrazine 26 have been performed.

The Chemistry of Peroxynitrite: Involvement of an ET Process in the Radical Nitration of Unsaturated and Aromatic Systems

Reactions of peroxynitrous acid, HPN, with styrene under acidic conditions lead to the oxime 1, the nitrate 2, benzaldehyde (3), and α-nitroacetophenone (4) in overall yields that depend strongly on the pH value and with a product distribution that depends on the dioxygen concentration. The results are rationalized by assuming that HPN undergoes acid-catalyzed decomposition to give nitrous anhydride, or its synthetic equivalent, which is responsible for the regioselective nitration of the styrene double bond by an ET process. The resulting β-nitrobenzyl radical 6 can, depending on the reaction conditions, undergo reversible coupling with nitric oxide to afford the nitroso derivative 7 and then the tautomeric oxime 1, or trapping by dioxygen, eventually leading to products 2, 3, and 4 through the intermediacy of the peroxynitrite derivative 8. Oxime 1 and nitrate 2 are also obtained by treating styrene with nitrous anhydride under protic conditions, the latter being produced in situ from nitric oxide/dioxygen. Similarly to styrene, 1,4-diphenylbutadiene (14) gives radicals 22 and 21 by competitive trapping at the side chain and at the aromatic ring. In turn, radicals 22 and 21 undergo β-fragmentation reactions or trapping by dioxygen with eventual formation of nitrates 16 and 17, cinnamic aldehyde (18), and the diol 15. Finally, the HPN-promoted reaction of p-cresol (27) leads to the 2-nitro derivative 28 through an initial electron-transfer process followed by in cage recombination of the resulting radical ion pair.

S‐Nitrosothiol and Disulfide Formation through Peroxynitrite‐Promoted Oxidation of Thiols

Peroxynitrite reacts with thiols 1 at acidic pH to give the corresponding S-nitrosothiols 2 and disulfides 3. The formation of nitrosothiols 2, the yield of which is strongly pH-dependent, can be rationalized in terms of acid-catalyzed decomposition of the undissociated HPN, probably through the intermediacy of the protonated form H2PN+, leading to a species, X−NO, capable of nitrosating the thiol function. In contrast, the formation of disulfides 3 occurs in a manner independent of the pH, without the intermediacy of sulfanyl radicals. Under basic conditions, the peroxynitrite anion (PN) oxidizes the thiolate ion to sulfanyl radicals, eventually leading to disulfide 3, or undergoes a thiol-catalyzed decomposition. The former is the exclusive reaction exhibited by peroxynitrite at pH > 13.

Oxiranylmethyl radicals: EPR detection by spin trapping

The EPR detection of oxiranylmethyl radicals, formed via a 3-exo-trig process from the corresponding allyloxyl radical, is performed with Bu t ONO as a spin trap.

AROMATIC RADICAL ANIONS AS POSSIBLE INTERMEDIATES IN THE NUCLEOPHILIC AROMATIC SUBSTITUTION (SNAR) : AN EPR STUDY

The reactions among halonitrobenzenes or polynitrobenzenes and alkoxides, thiolates or tertiary amines have provided the evidence that in a SNAr reaction type a single electron transfer from the nucleophile to the aromatic substrate, to generate two radical species within the solvent cage, can take place to some extent. The detection of radical intermediates by EPR spectroscopy, in several SNAr reactions, is reported.