Metallopinacol intermediates

For most of the reactions in this study, a stoichiometry of two ketones per uranium has been used (Table 13.1). This is in accord with the postulated stoichiometry of active uranium (U ) producing UO2. When only a twofold excess of benzophenone is used, however, the combined yields of coupled products is 94%. This is in marked contrast to the titanium systems where each titanium is only capable of reacting with 1 equiv of ketone [82]. In the titanium systems, optimal yields of alkene are obtained with excess titanium, whereas we find that excess ketone produces the highest yields of alkene. This difference in M O stoichiometry is suggestive of a different type of metallopinacol intermediate (Figure 13.1). Further evidence which suggests the presence of this type of intermediate is the fact that 3 reduces the a-diketone benzyl to diphenyla-cetylene, whereas active forms of titanium have been shown by two authors not to effect this transformation [40,88]. 

LiAlH4 [3] they also proposed that metallopinacols were intermediates in this reaction, since pinacols could be isolated as by-products in many cases. 

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