Tellurium tetrachloride reaction with alkenes

These results clearly indicate that in the tellurium tetrachloride addition to alkenes the main reaction pathway does not occur via a telluronium ion intermediate a radical addition pathway could explain the poor selectivity observed in many of these additions (see Tables 15 and 16). The strong effect of 4-benzoquinone on the syn/anti ratio supports the involvement of a radical pathway in chloro-telluration with tellurium tetrachloride. A possible radical chain reaction is shown87. 

TetralkylteUnriiims, prepared in situ by the reaction of tellurium tetrachloride with 4 equiv of alkyllithiums, react with arylacetylenes to afford dialkyl tellurides A, alkylation products B, an alkene C and minor amounts of a vinyl telluride D. ... 

Reaction of the appropriate alkene with 1 equiv of tellurium tetrachloride in chloroform or acetonitrile gives a 2-chloroalkyltellurium trichloride adduct in a high yield (see Section 3.5.1.2). Reduction of this adduct with aqueous sodium sulphide results in the instantaneous separation of elemental tellurium and formation of the starting alkenes which, in most cases, exhibit an inverted configuration (with relevant variations depending on the solvent and olefin used). The proposed mechanism involves an intermediate epitelluride. 

Electrophilic addition reactions of tetravalent tellurium compounds have been reviewed.64 2-Naph-thyltellurium trichloride (ArTeCb) adds to alkenes in an anti stereospecific manner (equation 11), whereas tellurium tetrachloride gives mixtures of 2 1 adducts with both syn and anti addition.72 A one-pot alkene inversion procedure has been developed, based upon TeCU addition to alkenes followed by treatment of the (3-chloroalkyltellurium trichloride adduct with aqueous Na2S (Scheme 37).73 Tellurium compounds such as tellurinyl acetates, ArTe(0)0Ac, prepared in situ through reaction of tellurinic acid anhydrides with acetic acid, can be employed in oxytelluration and aminotelluration procedures (Schemes 38 and 39).74 In the oxytelluration reaction intermediate triacetates of the type RCH2Te(OAc)2Ph are reduced with hydrazine to the corresponding tellurides. 

Ketones react with 16 probably via the enol form to give a-trichlorotellurium ketones.61,62 Reaction of alkenes with 16 in an inert solvent gives chloroalkyltellurium trichlorides (e.g., 18).63 When an alcohol is present in the reaction medium, an oxyalkyltellurium trichloride (e.g., 19) is formed64 (Scheme 5). In some cases, tellurium tetrachloride is generated in situ from TeC>2 and HC1.65... 

Equimolar quantities of tellurium tetrachloride and alkenes produce chloroalkyl tellurium trichlorides (Table 6, p. 302). The reaction of tellurium tetrachloride in chloroform with alkenes such as butenes, 1-decenes, cycloalkenes, and 3-phenoxypropenes usually gives mixtures of products arising from syn- and anti-1,2-addition. Catalytic amounts of p-benzoquinone, a radical inhibitor, and acetonitrile as solvent promoted sy -addition. In chloroform in the absence of a radical scavenger, the regiospecific, concerted syn-addition competes with the radical pathway15. 

Selenium and tellurium reagents have been used for stereoselective halogenations of alkenes. For example, trans addition of benzeneselenenyl chloride to alkenes followed by the displacement of the seleno moie with chloride can lead to dr-1,2-dichlorides (equation 25). The addition of 2-naph-thyltellurium trichloride proceeds in an anti stereospecific manner (equation 26), whereas tellurium tetra-diloride gives a mixture of syn and anti adducts. The reaction of allyl esters with tellurium tetrachloride accompanies acyl migration to give the l-(trichloiottlluro)-3-chk>io adduct (54 equation 27). ... 

It has been rationalized that these reactions proceed via a cyclic telluronium ion intermediate which undergoes a nucleophilic attack by the chloride ion giving the products with trans configuration. The addition of 2-naphthyltellurium trichloride to a variety of acyclic and cyclic alkenes is completely anti or trans stereoselective, but tellurium tetrachloride usually gives mixtures arising from syn (cis) and anti (trans) addition representative results are presented in Tables 15 and 16. 

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