Palladium-mediated reactions: Development and use of hypervalent silicon compounds in organic synthesis.

摘要

Over the past thirty years, there have been significant advances in the formation of carbon-carbon bonds by palladium-catalyzed cross-coupling reactions. These cross-coupling reactions are extremely important in organic synthesis because these processes are usually the most general and direct methods for assembling complex molecules from relatively simple starting materials. A novel Pd-catalyzed cross-coupling methodology has been developed in our laboratory employing hypervalent silicon reagents as the nucleophilic species in these reactions. This technology has been extended to utilize both preformed hypervalent silicon reagents such as tetrabutylammonium triphenyldifluorosilicate (TBAT), as well as hypervalent silicon reagents formed in situ by using a tetravalent siloxane and an appropriate activating agent such as tetrabutylammoniurn fluoride (TBAF). We have developed a high yielding method for the preparation of unsymmetrical biaryls using TBAT as the hypervalent silicon reagent with cross-couplings to aryl halides and aryl triflates. TBAT has also been used in the arylation of cyclohexenyl substrates and we have demonstrated that in comparison to Stille reactions conditions, TBAT gives not only higher yields, but increased regioselectivity as well. We have also developed an efficient and high yielding Pd-catalyzed cross-coupling for the transfer of aryl-, allyl- and vinyl groups to aryl halides using tetravalent siloxanes made hypervalent in situ by adding tetrabutylammonium fluoride (TBAF) as the activating agent. These siloxanes also arylate allylic benzoates with high stereo- and regiocontrol.