Investigation of the asymmetric heck reaction between 2, 3-dihydrofuran and phenyl triflate using deuterium labelling, time studies, and 1h and 31p NMR studies
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AbstractThe asymmetric intermolecular Heck reaction should prove to be very useful in organic synthesis, as the Heck reaction itself is a carbon-carbon bond-forming reaction, and tolerates a variety of functional groups. To make such an intermolecular reaction asymmetric would eliminate the need for classical resolution techniques, and a variety of achiral substrates could be combined to make chiral products. Much synthetic work has been devoted to the preparation of novel chiral ligands for the asymmetric intermolecular Heck reaction between 2,3-dihydrofuran and a variety of aryl triflates. A wide variety in the regioselectivity and enantioselectivity of the reaction has been seen, due to the use of different palladium sources, ligands, bases, solvents, and temperatures. Some researchers have proposed mechanisms to account for the asymmetric intermolecular Heck reaction, but these mechanisms are found to be contradictory in some respects. One set of Heck reaction conditions has been selected, and the mechanism of the Heck reaction under those conditions has been investigated more thoroughly, through the use of deuterium labelling, time studies, and NMR studies. Four deuterated dihydrofurans have been synthesized, each requiring a different synthetic pathway to be developed. The products from the Heck reaction with each of the four deuterated dihydrofurans have been characterized. Based on the structures of the products, it is possible to deduce the mechanisms of their formation under the Heck reaction conditions. Once the mechanism of product formation was known, the Heck reaction has been followed over time, to monitor the enantiopurity of the products. One of the products of the Heck reaction has been found capable of binding to the palladium (0) catalyst, causing an alteration in the ligand set around the metal, and thereby affecting the regioselectivity and enantioselectivity of the reaction. The results of all these studies are thought to be consistent with the data found in the literature, and give a more complete understanding of the mechanism of the asymmetric Heck reaction.
Bibliography: p. 405-415