[(p-Cymene) RuCl2]2: An Efficient Catalyst for Highly Regioselective Allylic Alkylations of Chelated Amino Acid Ester Enolates

Chelated amino acid ester enolates are excellent nucleophiles for ruthenium-catalyzed allylic alkylations. Although [Cp*Ru(MeCN)(3)]PF6 was found to be the most reactive catalyst investigated, with the resulting allyl complexes reacting at temperatures as low as -78 degrees C, unfortunately the process took place with only moderate regio- and diastereoselectivity. In contrast, [(p-cymene)RuCl2](2) allowed allylations to be performed with a high degree of regioretention. Secondary allyl carboxylates with a terminal double bond were found to be the most reactive substrates, giving rise to the branched amino acids with perfect regioretention and chirality transfer. In this case, no isomerization of the Ru-allyl complex formed in situ was observed, in contrast to the analogues palladium complexes. This isomerization-free protocol can also be used for the synthesis of (Z)-configured ,-unsaturated amino acid derivatives, starting from (Z)-allylic substrates. Here, the more reactive phosphates were found to be superior to the carboxylates, providing the required amino acids in almost quantitative yield with perfect regio- and stereoretention. Therefore, the Ru-catalyzed allylation reactions are well positioned to overcome the drawbacks of Pd-catalyzed processes.