Chemistry of amidyl radicals produced from N-hydroxypyridine-2-thione imidate esters

A set of allene‐tethered benzoyloximes (5) has been treated with nBu3SnH. Depending on their substitution pattern, a wide range of compounds has been obtained. If the stannyl radical adds on the allene, the C‐centred radical thus formed undergoes either a 5‐exo ring closure to give the cyclopentene derivatives 7 or a 6‐endo ring closure onto the N atom to give the dihydropyridines 8. If the stannyl radical adds on the benzoyl moiety, an iminyl radical is formed which leads to the 3H‐pyrroles 9 and the alkylidene‐pyrrolines 10. Steric effects as well as polar effects are the factors governing the reaction course.

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“…[5] [8] In this area, Zard and co-workers have obtained pyrrolines using an iminyl radical, easily produced by the tributylstannane reduction of…”

Section: Resultsmentioning

confidence: 99%

Tin-Mediated Free-Radical Cyclization of β-Allenylbenzoyloximes

et al. 2000

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“…The 31 P NMR spectra (relative intensities of product signals) of HA 1a and LR mixtures for reactions performed in the dark and under UV light are identical. A series of experiments on hydroxamic acids with tethered internal functional groups, which would allow such intermediates as amidyl radicals or acyl nitrenes (4-pentenohydroxamic acid 1e, N-allylbenzohydroxamic acid 1f) to be trapped, and also in the presence of Ph 2 Se 2 , [20] have ruled out the formation of the corresponding cyclic products (Scheme 7). Similarly, the products of radical or nitrene addition to double bonds have not been observed in reaction mixtures to which exter- nal vinyl reagents, nucleophilic or electrophilic in character (vinyl acetate, methyl acrylate), were added.…”

Section: O-dithiophosphonylated Hydroxamic Acid As An Intermediate Anmentioning

confidence: 99%

Mechanism of the Reaction of Lawesson’s Reagent with N‐Alkylhydroxamic Acids

Przychodzeń

2005

Eur J Org Chem

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The mechanism of the reaction under discussion has been established by investigating the products of the reaction between 2,4‐bis(4‐methoxyphenyl)‐1,3,2,4‐dithiadiphosphetane 2,4‐disulfide (Lawesson’s reagent, LR) and N‐alkylhydroxamic acids HAs 1. The primary intermediate is an adduct, O‐dithiophosphonylated hydroxamic acid 19, which decomposes to yield metathiophosphonate (AnsPOS), a sulfur atom, and an amide. At the same time, owing to the co‐existence of 19 and metadithiophosphonate (AnsPSS) in equilibrium, the carbonyl group is thionated. It has also been established that monomeric AnsPOS formed in both reduction and thionation processes does not undergo oligomerisation to cyclic trimer 5 and linear oligomers, which is typical for amides reacting with LR. Since there is unreacted HA 1 in the reaction mixture, AnsPOS takes part in a controlled transformation to form a dimer, the corresponding pyrothiophosphonate 3, together with the intermediate O‐thiophosphonylated hydroxamic acid 2. A hydrolysed product of AnsPOS, namely (4‐methoxyphenyl)thiophosphonic acid 4, participates in the last reaction. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

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“…[1a,6Ϫ8] Previous work has shown that pyritered [5] radicals. However, alkoxyl radicals have served as dinethione-derived O-esters of cyclic thiohydroxamic acids useful intermediates in several selective transformations [e.g.…”

Section: Introductionmentioning

confidence: 99%

A New Generation of Alkoxyl Radical Precursors – Preparation and Properties ofN-(Alkoxy)-4-arylthiazole-2(3H)-thiones

Hartung¹,

Schwarz

Svoboda

et al. 1999

Eur. J. Org. Chem.

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N‐(Hydroxy)thiazole‐2(3H)‐thiones 6–10 have been prepared in a short and efficient synthesis from p‐substituted acetophenones. Alkylation of heterocycles 6–10 in the form of their potassium or tetraalkylammonium salts 11–15 affords N‐alkoxy‐4‐arylthiazole‐2(3H)‐thiones 16–20 in good to satisfactory yields. The hitherto unknown thiones 16–20 have been subjected to a detailed structural investigation (NMR spectroscopy and X‐ray crystallography) and furthermore to a mechanistic study in order to explore their utility as sources of oxygen‐centered radicals in solution. From the results of these studies, the following conclusions can be drawn: (i) X‐ray analyses of the p‐chlorophenyl‐substituted acid 9, of the O‐alkyl derivatives 19c, 19f, and of the O‐mixed anhydride 19k indicate short C–S bonds [C2–S2 = 1.637(5)–1.684(2) Å] and long N–O connectivities [N3–O1 = 1.369(3)–1.379(2) Å] in the thiohydroxamate functionalities. Furthermore, O‐alkyl‐ or O‐acyl substituents at O1 are twisted out of the thiazolethione plane by ca. 90°, which points to lone‐pair repulsion between nitrogen and oxygen atom as the underlying structural motif of the cyclic thiohydroxamate derivatives. (ii) Alkylation of ambidentate thiohydroxamate anions (salts 11–15; oxygen and sulfur nucleophiles) affords almost exclusively O‐esters 16–20 (alkylation at the oxygen atom). (iii) Based on the results of X‐ray diffraction studies and on the 1H‐ and 13C‐NMR spectra, guidelines for the characterization of N‐(alkoxy)thiazolethiones 16–20 and 2‐(alkylsulfanyl)thiazole N‐oxides 21–25, i.e. the products of S‐alkylation of thiohydroxamate salts 11–15, could be derived. (iv) Photolyses of substituted N‐(4‐pentenoxy)‐4‐arylthiazolethiones 16–20 in general and in particular of p‐chloro derivatives 19 were carried out in the presence of the hydrogen donor Bu3SnH, and afforded substituted tetrahydrofurans 31 or tetrahydropyrans 32 as major products in good yields. The observed stereo‐ and regioselectivities of ethers 31 and 32 point to alkoxyl radicals 30 as reactive intermediates, which add intramolecularly by selective 5‐exo‐trig or 6‐endo‐trig pathways to the olefinic double bonds. In terms of synthetic access and ease of handling of the radical precursors, the p‐chlorophenyl‐substituted thiazolethiones 9 and 19 exhibit significant advantages over all the other thiones used in this study and are considered as excellent substitutes for the pyridinethiones as efficient sources of free alkoxyl radicals. Consequently, the present compounds may be of use in both mechanistic and synthetic studies.

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Chemistry of amidyl radicals produced from N-hydroxypyridine-2-thione imidate esters

Cited by 80 publications

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Tin-Mediated Free-Radical Cyclization of β-Allenylbenzoyloximes

Tin-Mediated Free-Radical Cyclization of β-Allenylbenzoyloximes

Mechanism of the Reaction of Lawesson’s Reagent with N‐Alkylhydroxamic Acids

A New Generation of Alkoxyl Radical Precursors – Preparation and Properties ofN-(Alkoxy)-4-arylthiazole-2(3H)-thiones

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