Alkyl radicals from xanthates

In Nicolaou s synthesis of aspidosphytine, the quaternary spiro center at the indole 3 position was elaborated from an alcohol, which was converted into the alkyl radical precursor xanthate 53." Upon heating in the presence of TBTH and AIBN, 53 underwent a deoxygenation and 5-exo-trig radical cyclization leading to 54 after ring closure... 

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

Addition of carbon radicals to carbon-carbon double bonds is an important reaction that can be carried out under hydrogen transfer conditions [27], Peroxides are usually used as radical precursors and an application of this chemistry is presented in Scheme 3 (Eq. 3.1). More recently, reduction of alkyl radical by C-H hydrogen donor has been examined in order to find an environmentally friendly alternative to tin hydride. Zard has reported a simple and cheap alternative to tin hydride for Barton-McCombie deoxygenation reactions [80]. Heating of xanthates derived from carbohydrates in 2-propanol in the presence of dilauroyl peroxide affords the de-oxygenated products in good yields (Scheme 17, Eq. 17.1). 2-Propanol functions as... 

Along with tin hydride-mediated arylations, electrochemical [61], photochemical [62] and iodine transfer methods [63] have each been successfully used for the generation of alkyl radicals in homolyhc aromahc subshtuhons. Moreover, nucleophilic a-oxyalkyl radicals, prepared from the corresponding aldehydes by treatment with either samarium diiodide [64] or stannyl radicals [65], reacted with arenes to afford the homolyhc subshtuhon products. In a series of reports, Zard highlighted the potenhal of xanthates as alkyl radical precursors in homolytic aromatic subshtuhons [66]. 

It has also been demonstrated that trialkylsilanes, which are poor radical-based reducing agents due to their low hydrogen donation abilities [79], can reduce alkyl halides and xanthates (equation (36)) to the corresponding hydrocarbons in the presence of a catalytic amount of an alkanethiol [80]. The reaction consists of a chain process in which the key propagation step is the hydrogen transfer from the silane to the alkanethiyl radical. This approach has also been used for the hydrosilylation of chiral alkenes by Ph2SiH2 [81]. 

A radical initiator, such as azobisisobutyronitrile (AIBN, 7) decomposes homolytically under reaction conditions to generate 8 which abstracts a hydrogen from tributyltin hydride (9) creating a tin-centered radical 10. The tin radical attacks at the sulphur atom of the thiocarbonyl derivative 2, generating intermediate 11 which collapses leaving tributyltin xanthate 12 (which can further decompose to 0=C=S and 13) and alkyl radical 14. Intermediate 14 can abstract another atom of hydrogen from tributyltin hydride to generate the desired alkane product 3, and replenish the pool of tin radical 10. 

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] ... 

A range of 0-alkyl thiazolethiones such as (44) have been prepared from 4-chloroacetophenone. Bromination of 4-chloroacetophenone and reaction with hydroxylamine hydrochloride afforded the bromo oxime (42). (42) is then treated with potassium ethyl xanthate and oximino dithiocarbonate, followed by zinc chloride to afford the thiohydroxamic acid (43), which is then alkylated to give (44). These derivatives are a useful source of alkoxy radicals eg, photolysis of (44) leads to the alkoxy radical (45) <97SL848>. 

A novel radical chain reaction based on O-alkyl tin dithiocarbonate reagents which allows transformation of sugar xanthates to duosugar dithiocarbonates with retention of configuration has been developed. Its application to the preparation of a thionucleoside derivative is shown in Scheme 2. The tribenzyl ether 7 of benzyl 1,4-dithio-D-ribofuranoside has been obtained from L-... 

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