Rino Leardini

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.

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.

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.

Synthesis and X-ray characterisation of a new polycondensed heterocycle obtained by a novel Mn(III)-mediated cascade reaction of 2-cyanophenyl isothiocyanate

Abstract 2-Cyanophenyl isothiocyanate reacted with Mn(III) acetate in acetic acid or acetonitrile to give fair yields of a new polycondensed heterocycle arising from the joining together of two molecules of the starting isothiocyanate with loss of a CS moiety. The yields were close to 90% when the reaction was carried out in the presence of diethyl malonate. This compound was unambiguously identified by X-ray crystallography. Under the same conditions, 2-(methoxycarbonyl)phenyl isothiocyanate gave a quinazolinimine derivative instead which is likely to arise from cyclisation of an intermediate N,N′-diarylthiourea. The mechanism of formation of the former compound probably involves formation of a N,N′-bis(2-cyanophenyl)thiourea, followed by rearrangement and radical tandem ring closure of the corresponding cyclic imine derivative. This hypothesis is also supported by semiempirical calculations.

The reaction of (2-alkylidenaminophenyl)sulfides with di-iso-propyl peroxydicarbonate: radical versus non-radical pathway

Abstract The reaction of 2-methylthio- or 2-phenylthioarylimines with di-iso-propyl peroxydicarbonate (DPDC) is described. The phenylthio-derivatives undergo cyclization to benzothiazoles via a free-radical mechanism, whereas the methylthio-analogs afford arylthiomethyl iso-propyl carbonates, together with benzothiazoles as well; this last result could be accounted for with the intervention of non-radical mechanism operating side by side with the radical one.

N-(ortho-Aryloxyphenyl)arylimidoyl radicals: Novel 1,5-aryl radical translocation from oxygen to carbon

Abstract Imidoyl radicals 4a-d , obtained by hydrogen abstraction from imines 1a-d , give 7-membered cyclisation leading to oxazapines 2a-d . The intermediate spirocyclohexadienyl radicals of the competitive 6-membered ring closure ( 6a-d ) rearrange to aryloxy radicals, giving benzophenones 3a-d : the whole process entails a novel 1,5-aryl radical translocation from an oxygen to a carbon atom.

INTRAMOLECULAR ADDITION OF ARYL RADICALS TO CARBON-NITROGEN DOUBLE BONDS

Abstract Cyclisation of radicals 6a,b is highly regioselective towards a 5-exo process; 6-endo ring closure is a minor route and their ratio depends on the substituents. No ring expansion of the five-membered radical intermediates 7a,b was observed. Radicals 27a,b give rise to 5-exo cyclisation regiospecifically. A competitive 1,5-hydrogen shift leading to imidoyl radicals was noticed. An analogous behaviour is also exhibited by vinyl radicals when allowed to add to carbon-nitrogen double bonds.