Daniele Nanni

From Azides to Nitrogen‐Centered Radicals: Applications of Azide Radical Chemistry to Organic Synthesis

Over the last thirty years organic azides have drawn a great deal of attention as radical traps for carbon- and heteroatom-centered radicals, both in intra- and in intermolecular processes. The resulting intermediates (nitrogen-centered radicals such as triazenyls, aminyls, or even iminyls) can be conveniently employed in the synthesis of a variety of cyclic and acyclic nitrogen-containing compounds.

Radical annulations and cyclisations with isonitriles: the fate of the intermediate imidoyl and cyclohexadienyl radicals

Abstract The reaction of 4-methoxyphenylisonitrile with phenylacetylene and AIBN produces a novel cyclopenta-fused quinoxaline through addition of 2-cyanoprop-2-yl radical to the alkyne; the resulting vinyl radical attacks isonitrile to afford an imidoyl radical, which gives rise to a tandem 5-exo, 6-endo cyclisation. The whole process entails a new example of a rare 4 + 1 radical annulation. The cyanopropyl radical can also attack isonitrile to yield small amounts of quinolines deriving from 4 + 2 and 3 + 2 annulation between the resulting imidoyl radicals and phenylacetylene. The oxidation step leading to the final aromatic products involves the starting isonitrile, which is converted to an α-unsubstituted imidoyl radical and affords 2-unsubstituted quinolines. This behaviour was also found in cyclisations of biphenyl-2-ylisonitrile under various radical conditions. Finally, the title reaction gives small amounts of an α,β-unsaturated nitrile, which can arise from a spirocyclohexadienyl radical through fragmentation and subsequent β-scission of the resulting iminyl. This could be the first, direct evidence of the intermediacy of iminyl radicals in the rearrangements of the spirocyclohexadienyls obtained by 3 + 2 annulation between imidoyl radicals and alkynes.

An Insight into the Radical Thiol/Yne Coupling: The Emergence of Arylalkyne-Tagged Sugars for the Direct Photoinduced Glycosylation of Cysteine-Containing Peptides

An explorative study of the Thiol-Yne Coupling (TYC) reaction has been carried out using an aliphatic (1-octyne) and an aromatic alkyne (phenylacetylene) and two alkanethiols (methyl thioglycolate and N-acetyl-L-cysteine methyl ester). The outcomes of the TYC reactions strongly depend on the experimental conditions (e.g., temperature, solvent, and alkyne/thiol ratio), but these can be properly adjusted to achieve selective production of either mono- or bis-coupling products. With respect to 1-octyne, phenylacetylene undergoes notably easier radical hydrothiolation, further showing a notably higher aptitude for monohydrothiolation exclusive of bis-hydrothiolation. The overall findings were exploited in glycosylation of cysteine derivatives as well as of cysteine-containing peptides. A sugar featuring an arylacetylene moiety gave rise to a true click-reaction, that is, glycosylation of the tripeptide glutathione in its native form, by means of virtually equimolar amounts of reagents. This reaction was successfully applied, under physiological conditions, to a cysteine-containing nonapeptide with marked advantages over the analogous Thiol-Ene Coupling (TEC) derivatization. A TYC/TEC sequence affording bis-armed cysteine derivatives through dual functionalization of an alkynyl sugar was additionally devised.

Synthesis and some reactions of the first chiral tin hydride containing a C2-symmetric binaphthyl substituent

Abstract Starting from pure (S)-2,2′-bis(bromomethyl)-1,1′-binaphthyl, the synthesis of a new chiral, enantiomerically pure dinaphthostannepin is described. Preliminary reactions of this hydride with a racemic organic halide under radical conditions gave the reduced product in up to 41 % enantiomeric excess, depending on the experimental conditions.

DDQ-mediated formation of carboncarbon bonds: Oxidation of imines

Abstract The reaction of imines with alkynes and alkenes, in the presence of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), to give quinoline derivatives is described. The mechanism of the annulation is discussed, and evidence supporting a non-concerted pathway, at least when the alkene is butyl vinyl ether, is reported. Preliminary information is also given about solid adducts of imines with DDQ, which do not seem to be involved in the reaction path leading to quinolines, and should account for the dependence of product yields on the position of substituents on the starting imines.

Radical annulations with nitriles: Novel cascade reactions of cyano-substituted alkyl and sulfanyl radicals with isonitriles

Abstract New radical annulation reactions are described involving addition of cyano-substituted alkyl and sulfanyl radicals to aromatic isonitriles. The tandem cyclisation of the resulting imidoyl radicals onto the cyano group affords cyclopenta- and thienoquinoxalines, respectively. The intervention of the isonitriles in the aromatisation process of the cyclohexadienyl radicals is discussed, as well as the regiochemistry of the cyclisation of the iminyl radicals obtained by addition of the imidoyls to the nitrile moiety. The hypothesis of an exclusive 6-membered ring closure onto the aromatic ring is also supported by the results of semiempirical AM1 calculations.

Reduction of azides to amines by samarium diiodide

Abstract Amines can be prepared by reduction with samarium diiodide under mild conditions and in good yields.

Iminyl Radicals from α-Azido o-Iodoanilides via 1,5-H Transfer Reactions of Aryl Radicals: New Transformation of α-Azido Acids to Decarboxylated Nitriles

The radical reaction of tributyltin hydride with o-iodo- N-methylanilides derived from alpha-azido acids provides an excellent access to alpha-(aminocarbonyl)iminyl radicals through 1,5-hydrogen transfer reaction of initially formed aryl radicals followed by beta-elimination of dinitrogen from ensuing alpha-azido-alpha-(aminocarbonyl)alkyl radicals. The outcoming iminyls display a peculiar tendency to form corresponding nitriles by beta-elimination of aminocarbonyl radicals.

PEROXYDICARBONATE-MEDIATED OXIDATION OF N-(ORTHO-ARYLOXYPHENYL) AND N-(ORTHO-ARYLAMINOPHENYL)ALDIMINES

Abstract Imidoyl radicals 5 , obtained from imines 1 by hydrogen abstraction with di- iso -propyl peroxydicarbonate (DPDC), give dibenzoxazepines through 7-membered ring closure. A competitive 6-membered cyclisation leads to intermediate spirocyclohexadienyl radicals that rearrange to aryloxy radicals; this process entails a novel 1,5-aryl radical translocation from an oxygen to a carbon atom and leads to benzophenones, benzoxazoles, and biphenyls. The possibility that the oxazepines arise from rearrangement of the 6-membered-ring-closure intermediates is discussed. With imine 1e , the formation of 5e occurs to a minor extent owing to a side-reaction of the iso -propoxycarbonyloxy radicals, which give rise to an intermolecular aromatic ipso -substitution on the benzenic ring linked to the two oxygen atoms. The 1,5-aryl migration can also be observed with imidoyl radicals generated by radical addition to 2-phenoxyisocyanobenzene. In contrast, the reactions of imines 2 with DPDC do not afford imidoyl radicals, as abstraction of the iminic hydrogen is slower than oxidation of the methyl group: this process entails the formation of carbamoyl radicals, which cyclise onto the carbon-nitrogen double bond, furnishing quinoxalinone derivatives, or loose carbon monoxide to yield benzimidazoles through ring closure of aminyl radicals. A novel cyclisation of a nitrogen-centred radical onto a formamido group could account for the formation of a benzimidazolinone derivative.

Tin-free generation of alkyl radicals from alkyl 4-pentynyl sulfides via homolytic substitution at the sulfur atom.

Homolytic substitution at the sulfur atom of beta-(phenylsulfanyl)vinyl radicals, obtained by radical reaction of benzenethiol with easily accessible alkyl 4-pentynyl sulfides, is a mild, effective, tin-free route for the generation of all types of alkyl radicals. This protocol can be employed in reductive defunctionalizations as well as cyclizations onto both electron-rich and electron-poor C-C double bonds.