Luisa Benati

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.

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.

Boron trifluoride promoted reaction of benzenesulphenanilides with alkenes-arylaminosulphenylation of alkenes.

Abstract The addition of benzenesulphenanilides to alkenes in the presence of boron trifluoride provides a practicable synthetic procedure for the arylaminosulphenylation of alkenes.

Synthetic utility of 4'-nitrobenzenesulfenanilide in the functionalization of carbon-carbon double and triple bonds : its use in the bromosulfenylation of alkenes and alkynes

Abstract The reaction of 4′-nitrobenzenesulfenanilide (NBSA) with hydrobromic acid, suitably carried out at room temperature in the presence of cyclohexene, trans -hex-3-ene, hex-1-ene and 3,3-dimethylbut-1-ene, results in quantitative isolation of corresponding 2-bromoalkyl phenyl sulfides which occur with trans -stereospecificity and anti-Markovnikov regiospecificity through electrophilic addition of initially-formed benzenesulfenyl bromide to the alkene double bond. Similar reaction in the presence of mono- and di- substituted alkyl-and phenyl-acetylenes generally affords (E)-2-bromovinyl phenyl sulfides in good yields, which become lower with decreasing nucleophilic power of the alkyne employed. However, in the presence of parent acetylene, no virtual formation of the corresponding sulfide adduct occurs, but almost exclusive formation of diphenyl disulfide essentially ascribable to preferred decomposition of the highly unstable benzenesulfenyl bromide intermediate. The present additions of benzenesulfenyl bromide to alkenes and alkynes are believed to involve the initial intermediacy of thiiranium- and thiirenium-like ions, respectively, by analogy with related AdE reactions of sulfenyl chlorides.

4′-Nitroarenesulphenanilides: Their use in the synthesis of unsymmetrical disulphides

Abstract The reaction of 4′-nitroarenesulphenanilides with thiols in the presence of boron trifluoride etherate can provide an effective route to unsymmetrical disulphides.

Diazo transfer reaction of 2-(trimethylsilyl)-1,3-dithiane with tosyl azide. Carbenic reactivity of transient 2-diazo-1,3-dithiane

Abstract A novel diazo transfer reaction of 2-lithiated 2-(trimethylsilyl)-1,3-dithiane with tosyl azide in a 1:20 HMPA-THF mixture furnishes 2-diazo-1,3-dithiane 2, which decomposes at about 0°C to give in fairly high yield bis(1,3-dithianylidene) 5 through formal dimerization of derived carbene 4. In the presence of dimethyl fumarate, dimethyl maleate, trans- and cis-1,2-bis(benzenesulfonyl)ethylene, the diazodithiane affords only trans-cyclopropane adducts in a stereoselective, but non stereospecific fashion. With dimethyl acetylenedicarboxylate or 1-tosyl-2-(trimethylsilyl)acetylene, no corresponding cyclopropene adducts are observed, but instead bis(ketene-dithioacetal) products, likely arising from further reaction of cyclopropene intermediates with diazodithiane 2 and/or its carbene 4. A related ring-opened dithioacetal product is observed upon treatment of diazodithiane with dimethyl 3,3-dimethylcyclopropene-1,2-dicarboxylate. The above reactions with electrophilic alkenes and alkynes are preferentially explained in terms of primary intervention of somewhat nucleophilic dithiocarbene 4 rather than its diazo precursor. The outcome of the usual reaction of 2-lithio-2-(trimethylsilyl)-1,3-dithiane with tosyl azide, in the presence of dimethyl acetylenedicarboxylate, is profoundly changed when performed in the absence of HMPA cosolvent. Under these circumstances the occurrence of bis(ketene-dithioacetal) product (and the dimer 5) is essentially suppressed in favor of significant formation of the silylated maleate 12; this is believed to arise from preferential trapping by the alkyne of the triazenyl anion 3, the presumable progenitor of diazodithiane. An X-ray crystallographic analysis of maleate 12 is reported.

A convenient synthesis of β-keto phenyl sulphides from alkynes.

Summary β-Keto sulphides can be conveniently prepared by BF 3 -promoted reaction of 4′-nitrobenzenesulphenanilide in acetonitrile or acetic acid and subsequent hydrolysis of the resulting β-acetamidino- or β-acetoxy-vinyl phenyl sulphides.

Free-radical reactions of benzenethiol and diphenyl disulphide with alkynes. Chemical reactivity of intermediate 2-(phenylthio)vinyl radicals

Reaction of benzenethiol at 100 °C with neat alkyl- and dialkyl-acetylenes leads to virtually quantitative formation of isomeric mixtures of (E)- and (Z)-vinyl sulphide adducts in ratios which depend largely upon both the extent and the nature of alkyl substitution. Results are explained in terms of rapidly interconverting sp2-hybridized (E)- and (Z)-1-alkyl-2-(phenylthio)vinyl radical intermediates which can undergo hydrogen transfer from benzenethiol to an extent which is essentially dependent upon the steric hindrance of their cis-2-substituent. Consistent results are provided by related radical reactions of diphenyl disulphide with alkyl-substituted alkynes to afford varying amounts of 1,2-bis(phenylthio)ethylene adducts ascribable to SH2 reaction of the resulting 1-alkyl-2-(phenylthio)vinyl radicals with the disulphide present. Under analogous conditions benzenethiol and diphenyl disulphide react with phenylacetylenes to give vinyl sulphide or bissulphide adducts in a trans-stereoselective fashion. The findings are interpreted by suggesting, for the intermediate sp-hybridized 1-phenyl-2-(phenylthio)vinyl radicals, the occurrence of significant bonding interaction between the unpaired electron and the adjacent sulphur, which would essentially prevent attack of radical scavenger on the side cis to PhS. Evidence is also presented that 1-phenyl-2-(phenylthio)vinyl radicals can exhibit homolytic intramolecular cyclization reactions, leading to benzothiophene products to a comparatively much greater extent than the 1-alkyl-2-substituted analogues; however, 1-tert-butyl-2-(phenylthio)vinyl radical would represent a special case.

REACTION OF 4'-NITROBENZENESULFENANILIDE (NBSA) WITH LEWIS ACIDS. A STUDY OF ITS APPLICATION IN SULFENOCYCLIZATION OF ALKENES AND ALKYNES

Abstract Phenylsulfenocyclization of a number of alkenes and alkynes possessing internal hydroxyl, carboxyl or vinyl functionality has been investigated with NBSA. Thioetherification of 4-penten-1-ol 4 as well as thiolactonization of 4-penten-1-oic acid 7 and, to a modest extent, 4-pentyn-1-oic acid 8 can be successfully achieved with NBSA in the presence of boron trifluoride. On the other hand, under analogous conditions 3-buten-1-ol 3 and 3-buten-1-oic acid 6 fail to undergo thiocyclization and give instead oxa- and/or aza-sulfenylation 1,2-adducts. Similar failure is observed with 4-pentyn-1-ol 5, in which case diphenyl disulfide and 4-nitroaniline are the exclusive products. Hexa-1,5-diene 9, to some extent, affords a cyclized arylaminosulfide, i.e. 23, ascribable to formal loss of a methylene unit from the initial (phenylthiomethyl)- cyclopentyl cation 24. The reaction products are discussed in terms of intermediate thiiranium and thiirenium ions whose decomposition mode is strictly dependent upon their structural features. Novel evidence is also presented that aluminium chloride and bromide can promote reaction of NBSA with alkenes and alkynes to afford chloro- and bromo-sulfenylation adducts in varying yields.

Boron trifluoride-promoted reaction of 4′-nitrobenzenesulphenanilide and its N-methyl derivative with alkynes

4′-Nitrobenzenesulphenanilide and its N-methyl derivative react with alkynes in the presence of boron trifluoride–diethyl ether presumably affording thiirenium ion intermediates, which are trapped by acetonitrile to give eventually β-acetamidinovinyl sulphides.