Alkene from xanthates

Scheme 19 Formation of alkenes from vicinal nitro xanthates...

Alkenes from -hydroxy sulfides. This elimination was first observed on attempted reduction of the xanthate ester group of 1 with tri- -butyltin hydride the product, however, was the alkene 2. This is a general reaction of /J-hydroxy and jS-chloro sulfides. It has wide application since -hydroxy sulfides are readily... 

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>. 

The primary products obtained from 2-butanol are of mechanistic. significance and may be compared with other eliminations in the sec-butyl system 87). The direction of elimination does not follow the Hofmann rule 88) nor is it governed by statistical factors. The latter would predict 60% 1-butene and 40% 2-butene. The greater amount of 2-alkene and especially the unusual predominance of the cis-olefin over the trans isomer rules out a concerted cis elimination, in which steric factors invariably hinder the formation of cis-olefin. For example, the following ratios oicisjtrans 2-butene are obtained on pyrolysis of 2-butyl compounds acetate, 0.53 89, 90) xanthate, 0.45 (S7) and amine oxide, 0.57 86) whereas dehydration of 2-butanol over the alkali-free alumina (P) gave a cisjtrans ratio of 4.3 (Fig. 3). 

Radicals formed by fragmentation of xanthate and related thiono esters can also be trapped by reactive alkenes.217 The mechanism of radical generation from thiono esters was discussed in connection with the Barton deoxygenation method in Section 5.4. 

Deoxygenation of allyttc alcohols.3 A method for conversion of allylic alcohols to l-alkenes is outlined in equation (I). The first step is an allylic rearrangement of an Oallyl xanthate to 2. The second step is an allyl transfer from sulfur to tin with tri-n-butyltin hydride to give the allylic stannane (3). The last step, destannylation, is a well-known mule to terminal alkcnes.4... 

The potassium xanthate salts of some tertiary alcohols on pyrolysis yield alkenes. The overall yield in this case is often markedly better than those obtained from the corresponding 5-methyl xanthates (the classical Chugaev procedure). 

Barton and coworkers developed a very useful procedure for the deoxygenation of alcohols, which involves conversion of the alcohol to the corresponding xanthate derivative followed by reaction with BuaSnH. When applied to the bis(xanthates) derived from v/ c-diols the reaction gives alkenes, as illustrated in equation (17). Once again alkene formation is independent of the stereochemistry of the starting diols. 

A comparative study of radical reactions between xanthates and alkenes was performed with Wang resin and the aforementioned soluble resins. To both resins, either the xanthate or the alkene was bound. The reaction proved to be cleaner when the alkene (680) was immobilized. It was found that a large excess of xanthates (681) in solution could protect the desired compound (682) from further... 

Use of Tin Hydrides. From the mechanistic point of view, the reductive elimination of methyl xanthate derivatives of (3-hydroxy sulfones entails an interesting variation of the Julia reaction since an initial fragmentation of the C-0 bond through a Barton-McCombie-type radical deoxygenation takes place. Final aryl sulfonyl radical elimination affords the corresponding alkene (Eq. 52).98,99... 

More recently, 5-phthalimidomethyl xanthates derived from various amino acids have been added to various unactivated alkenes [12]. In particular, ornithine- and phenylalanine-derived xanthates, such as 9, added effectively to 3-cyanoindole and f-butyl 3-indolecarboxylate (8) in the presence of stoichiometric amounts of dOauroyl peroxide (DLP) to give the disubstituted indole 10 in 87% yield. Miranda also utilized this methodology recently to S5mthesize C-2-alkylated tryptamine derivatives en route to azepino[4,5-6]indoles [13]. 

An important group of alkene-forming reactions, some of which are useful in synthesis, are pyrolytic eliminations. Included in this group are the pyrolyses of carboxylic esters and xanthates, of amine oxides, sulfoxides and selenoxides. These reactions take place in a concerted manner, by way of a cyclic transition state and therefore proceed with syn stereochemistry, such that the hydrogen atom and the leaving group depart from the same side of the incipient double bond (in contrast to the eliminations discussed in Section 2.1) (2.14). 

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]. 

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