Radicals xanthates

Aryl-l,2-dihydro-3-nitro[l,8]naphthyridines have been obtained by the 6jt-thermal electrocyclization of l-(2-arylide-neamino-3-pyridyl)-2-nitroethylenes, obtained in situ from aromatic aldehydes and l-(2-amino-3-pyridyl)-2-nitroethylene in xylene <2002SC747>. 2-Chlorotetrahydro[l,8]naphthyridines have also been obtained from 2,6-dichloropyridines using a free radical xanthate-mediated cyclization <20040L3671>. 

The salts of alkyl xanthates, A/,A/ -di-substituted dithio-carbamates and dialkyidithiophosphates [26] are effective peroxide decomposers. Since no active hydrogen is present in these compounds, an electron-transfer mechanism was suggested. The peroxide radical is capable of abstracting an electron from the electron-rich sulfur atom and is converted into a peroxy anion as illustrated below for zinc dialkyl dithiocarbamate [27] ... 

Certain, Y, Y-dialkyl dithioearbamates [e,g. benzyl A)/V-diethyl dithiocarbamate (14)] and xanthates have been used as photoinitiators. Photodissociation of the C-S bond of these compounds yields a reactive alkyl radical (to initiate polymerization) and a less reactive sulfur-centered radical (to undergo primary-radical termination) as shown in Scheme 9.9.30 41 4 ... 

Living radical polymerization using thiocarbonylthio RAFT agents (including dithioesters, trithiocarbonates and xanthates) was first described in a patent published in 1998.40S The first paper describing the process also appeared in 1998.1R Other patents and papers soon followed. Papers on this method, along with NMP and ATRP, now dominate the literature on radical polymerization. 

The trend in relative effectiveness of RAFT agents with varying Z is rationalized in terms of interaction of Z with the C=S double bond to activate or deactivate that group towards free radical addition. Substituents that facilitate addition generally retard fragmentation. O-Alkyl xanthates (Z=0-alkyl, Table... 

O-Alkyl xanthates and A -ary l-A -alkyl dithiocarbamates are effective with vinyl acetate. 97 Dithioesters and trithiocarbonates give severe retardation or even inhibition which is attributed to slow fragmentation of the adduct radical. 

Substitution at the SiH moiety has been carried out with alkylthio groups, such as MeS and i-PrS. Tn s(alkylthio)silanes, (RSlsSiH, are radical-based reducing agents which can effect the reduction of bromides, iodides, xanthates, phenylselenides, and isocyanides in toluene, using AIBN as the initiator at 85... 

The precise reaction conditions for optimal yields depend upon the specific reagents and both thermal290 and photochemical291 conditions have been developed. Phenyl thionocarbonates are easily prepared and are useful in radical generating reactions.292 A variety of other thiono esters, including xanthates and imidazolyl thiocarbonates also can be used.293... 

Phenacyl radicals can be generated from the corresponding xanthates and add in good yield to various substituted propenes. The products of the reaction can then be cyclized to tetralones using an equivalent of a peroxide.313... 

Scheme 10.17 illustrates allylation by reaction of radical intermediates with allyl stannanes. The first entry uses a carbohydrate-derived xanthate as the radical source. The addition in this case is highly stereoselective because the shape of the bicyclic ring system provides a steric bias. In Entry 2, a primary phenylthiocar-bonate ester is used as the radical source. In Entry 3, the allyl group is introduced at a rather congested carbon. The reaction is completely stereoselective, presumably because of steric features of the tricyclic system. In Entry 4, a primary selenide serves as the radical source. Entry 5 involves a tandem alkylation-allylation with triethylboron generating the ethyl radical that initiates the reaction. This reaction was done in the presence of a Lewis acid, but lanthanide salts also give good results. 

Quiclet-Sire B, Zard SZ (2006) The Degenerative Radical Transfer of Xanthates and Related Derivatives An Unusually Powerful Tool for the Creation of Carbon-Carbon Bonds. 264 201-236... 

Xanthates serve as a reliable source of electrophilic radicals, and this was exploited by Zard and coworkers for a short synthesis of ( )-matrine (3-304), a naturally occurring alkaloid which has been claimed to have anti-ulcerogenic and anticancer properties [116]. Heating a mixture of xanthate 3-299 and the radical acceptor 3-300 (3 equiv.) in benzene in the presence of lauroyl peroxide as initiator, gave 3-301 in 30% yield and a 3 1 mixture of the tetracylic products 3-302 and 3-303 in 18% yield (Scheme 3.76) [117]. The three compounds could be converted into the... 

Epoxides will also participate in radical reactions and this usually results in ring opening of the epoxide. The addition of a radical derived from xanthate 38 to butadiene monoepoxide provides the addition product 39 in good yields as an E/Z mixture of olefins <06AG(I)6520>. This reaction presumably proceeds through the addition of the xanthate-derived radical to the olefin, which then opens the epoxide. 

A Sml2-induced reductive cyclization of (V-(alkylketo)pyrroles provided an entry into medium ring 1,2-annelated pyrroles <06EJO4989>. An oxidative radical alkylation of pyrroles with xanthates promoted by triethylborane provided access to a-(pyrrol-2-yl)carboxylic acid derivatives <06TL2517>. An oxidative coupling of pyrroles promoted by a hypervalent iodine(III) reagent provided bipyrroles directly <060L2007>. 

All reactions of benzotriazole derivatives of the type Bt-CR RbS discussed above are based on electrophilic or nucleophilic substitutions at the ot-carbon, but radical reactions are also possible. Thus, the first report on unsubstituted carbon-centered (benzotriazol-l-yl)methyl radical 841 involves derivatives of (benzotriazol-l-yl)methyl mercaptan. 3 -(Benzotriazol-l-yl)methyl-0-ethyl xanthate 840 is readily prepared in a reaction of l-(chloromethyl)-benzotriazole with commercially available potassium 0-ethyl xanthate. Upon treatment with radical initiators (lauroyl peroxide), the C-S bond is cleaved to generate radical 841 that can be trapped by alkenes to generate new radicals 842. By taking the xanthate moiety from the starting material, radicals 842 are converted to final products 843 with regeneration of radicals 841 allowing repetition of the process (Scheme 134). Maleinimides are also satisfactorily used as radical traps in these reactions <2001H(54)301>. 

Spiro tricyclic pyrrolizinone 171 was obtained with 65% yield (and almost poor stereoselectivity) by intramolecular radical cyclization of the xanthate 170 upon exposure of the latter to 2equiv of lauroyl peroxide, in a refluxing 3 1 mixture of methanol and 1,2-dichloromethane. The radical generated from the xanthate moiety cyclizes with the... 

Several, oxidatively coupled xanthates (64-66), compounds (also called xanthides) containing the photochemically reactive, sulfur-sulfur bond, have been studied.130 Homolytic cleavage of this reactive bond is the primary reaction for these compounds, although this process is normally masked by recombination of the radicals produced. This primary, light-initiated process becomes apparent when a mixture of the xanthide 64 and ethyl xanthide (67) is irradiated in cyclohexane, because an equilibrium between 64, 67, and the mixed xanthide 68 is rapidly established. 

A method of grafting vinyl monomers to substrates of cellulose xanthate was invented by Faessinger and Conte-. The initiation is a reaction of ferrated (12) or acidic (13) cellulose xanthate with hydrogen peroxide according to the following scheme (HO-OH and Fe + give HO- radicals) ... 

It is noteworthy that the absolute rate constants for the reaction of the benzophenone triplet with Et3SiH, n-C5HnSiH3, PhSiH3, and Cl3SiH have been measured by LFP,56 and comparison of the kinetic data with corresponding data for reactions of /-BuO radicals shows that these two transient species have a rather similar reactivity toward silanes. Furthermore, the xanthate and the p-methoxyacetophenone triplets were found to be more and less reactive, respectively, than the benzophenone triplet with Et3SiH.56 Similar behavior of excited states in reactions with tin hydrides is discussed in Section V. 

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