Debromination reactions with telluride

The order of reactivity observed for debromination of substrates (17a-d) by dihexyl telluride (b > c > a > d) is that expected if carbocation/bromonium ion character is developed in the transition state (Scheme 1) and differs from that for reaction with the much more nucleophilic BU4NI (b > a > d > c), which is believed to proceed by the 2-like process (Scheme 2). ... 

As we have seen, the study of the stereochemistry of the debromination reaction is the key choosing between the two mechanistic pathways. Both proposals could justify the kinetic data (rate law, nucleophilicity of the telluride, effects of solvent polarity) however, only Mechanism 2 could satisfactorily explain the stereoselectivity in all cases. The intermediacy of a bromonium ion and the role of the telluride as a scavenger of the Br seem to be the best option with all the data in hand. 

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. ... 

In contrast to the dichotomy of reaction pathways observed for the debromination of 27, debromination of 1,2-dibromodecane (29) with either di-n-hexyltelluride or tetra-M-butylammonium iodide gave identical (within experimental error) activation parameters (Table 3).44 The large negative values of (—108 37 and — 87 12 J K-1 mol-1, respectively) are consistent with a highly-ordered transition state, but the bromonium ion formed from this dibromide would not be particularly favored. The data suggest that both the telluride and the iodide proceed via the E2-like mechanism of Fig. 14 with anti-periplanar transition state 34. 

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