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Internal Lewis Acid Activation of Ureas
S. S. So, J. A. Burkett, A. E. Mattson*
The Ohio State University, Columbus, USA
18 March 2011 (online)
The authors reported the synthesis and application of boronate ureas, such as 1, for the activation of nitrostyrenes towards indole additions. With this type of catalyst in hand, an up to ten-fold rate enhancement compared to Schreiner-type ureas was observed (see for example: P. R. Schreiner, A. Wittkopp Org. Lett. 2002, 4, 217). The high activity results from a strong Lewis acid mediated acidification of the urea functionality. However, it was also demonstrated that further pK a enhancement by additional electron-withdrawing substitution in the same benzene ring is counterproductive.