Telluronium intermediates reactions

A study of debrominations of vtc-dibromides promoted by diaryl tellurides and din-hexyl telluride has established several key features of the elimination process the highly stereoselective reactions of e/7f/tro-dibromides are much more rapid than for fhreo-dibromides, to form trans- and cw-alkenes, respectively the reaction is accelerated in a more polar solvent, and by electron-donating substituents on the diaryl telluride or carbocation stabilizing substituents on the carbons bearing bromine. Alternative mechanistic interpretations of the reaction, which is of first-order dependence on both telluride and vtc-dibromide, have been considered. These have included involvement of TeAr2 in nucleophilic attack on carbon (with displacement of Br and formation of a telluronium intermediate), nucleophilic attack on bromine (concerted E2- k debromination) and abstraction of Br+ from an intermediate carbocation. These alternatives have been discounted in favour of a bromonium ion model (Scheme 9) in which the role of TeArs is to abstract Br+ in competition with reversal of the preequilibrium bromonium ion formation. The insensitivity of reaction rate to added LiBr suggests that the bromonium ion is tightly paired with Br. ... 

The reaction is regioselective and sterospecific the tellurium moiety is added exclusively to the terminal carbon of terminal aUcenes, and only frany-adducts are obtained with cyclohexene. The reaction may proceed through the tellurenylation of the olefin (in a rapid equilibrium as demonstrated in separate experiments), forming a telluronium intermediate which is then methanolysed. 

Dimethyl and methyl phenyl 4,4-dimethyl-2,6-dioxocyclohexylidene tellurium compounds transferred a methyl group to triethylamine, triphenylphosphane, tris[di-methylamino]phosphane, and triphenylarsane. The methyl onium cations were isolated as lelraphenylborates. The alkylidene organo telluronium intermediate was identified by 31P-NMR spectroscopy in the reaction of the ylide with tris[dimethylamino]phosphanc . 

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. 

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. 

We add an investigation of Chinese chemists to this section, although it is not related to carbenoid reagents that we have discussed above. Zhou et al. (1993) studied reactions of dimethyl diazomalonate and ethyl diazoacetate with carbonyl compounds mediated by diorganyl tellurides and catalytic amounts of cuprous iodide (8-69). Dibutyl telluride (8.155) yields the dimethyl l-arylethene-2,2-dicarboxylate 8.157 with 4-chlorobenzaldehyde in 95<7b yield at 100 °C. It is assumed that the reaction passes the telluronium ylide 8.156 as intermediate. If so, the process is clearly different from the carbenoid transformations discussed in this section. The originally diazo-substituted C-atom has nucleophilic character in 8.156 and is not electrophilic, as in 8.104. 

Di-, tri-, and tetra-substituted 2-azadienes (64) carrying electron-withdrawing groups have been synthesised by the aza-Wittig reaction. A one-pot synthesis of olefins from diorganyltelluride, diazo compounds, and carbonyl compounds in the presence of a Cu(I) catalyst is suggested to involve a Wittig-type reaction of an intermediate telluronium ylide. ... 

TeCl4 has been found to display reactivity toward alkynes similar to that of p-methoxyphenyltellurium trichloride, whereas it reacts with aromatic and 3-hydro-xyalkynes by different mechanisms as shown by the characteristic stereochemistries of the products. The complete anti-stereospedfidty of the additions of TeCU to all propargyl alcohols studied is consistent with a cyclic chelated telluronium ion intermediate 87 in this reaction (Figure 3.8). Using ESI-MS and ESI-MS(/MS),... 

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