Carbenoids 4,4,5,5-tetramethyl-2-

Du Bois originally used rhodium(n) acetate and rhodium triphenylacetate (tpa) as catalysts and found that regio-and diastereocontrol was influenced by the catalysts, but neither was particularly effective when low catalyst loadings were used. Inspired by the bridged dirhodium catalysts which have been developed for carbenoid chemistry,40,273,274 a second generation catalyst Rh2(esp)2 116 (esp = a,a,a, o -tetramethyl-l,3-benzenedipropionate) was designed which was capable of much higher turnover numbers (Scheme ll).275 Furthermore, this catalyst was effective in intermolecular reactions. 

The chelate formation in lithium complexes 17 or 20 contributes to stabilization. Enhancement of kinetic acidity arises from the formation of pre-complexes 16 and 19, respectively. Here, already a dipole is induced and, in addition, proton exchange can proceed intramolecularly via a five- or six-membered ring. Despite these favourable features, the acidity of alkyl carbamates 15 is lower than those of the 1-proton in butane n-BuLi does not lead to deprotonation. In order to suppress carbonyl attack, a branched amino residue NR2 such as diisopropylamino (in Cb) or 2,2,4,4-tetramethyl-l,3-oxazolidin-3-yl (in Cby) is essential. A study on the carbenoid nature of compounds 17 was undertaken by Boche and coworkers. ... 

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