Dirhodium tetracarboxylates as catalysts for selective intermolecular C-H functionalization.

C-H Functionalization has become widely recognized as an exciting new strategy for the synthesis of complex molecular targets. Instead of relying on functional groups as the controlling elements of how molecules are assembled, it offers a totally different logic for organic synthesis. For this type of strategy to be successful, reagents and catalysts need to be developed that generate intermediates that are sufficiently reactive to functionalize C-H bonds but still capable of distinguishing between the different C-H bonds and other functional groups present in a molecule. The most well-established approaches have tended to use substrates that have inherently a favored site for C-H functionalization or rely on intramolecular reactions to control where the reaction will occur. A challenging but potentially more versatile approach would be to use catalysts to control the site-selectivity without requiring the influence of any directing group. One example that is capable of achieving such transformations is the C-H insertion chemistry of transient metal carbenes. Dirhodium tetracarboxylates have been shown to be especially effective catalysts for these reactions. This review will highlight the development of these dirhodium catalysts and illustrate their effectiveness to control both site-selective and stereoselective C-H functionalization of a wide variety of substrates.