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Metal Triflate-Promoted Allylic Substitution Reactions of Cinnamyl Alcohol in the Presence of Orthoesters and Acetals.

ABSTRACT: The product distribution of ethers formed from the reaction of cinnamyl alcohol with orthoesters in the presence of indium (III) triflate (InOTf)₃ is dependent on both the reaction temperature and catalyst loading. Carrying out the reaction at room temperature under low loadings of the catalyst leads to a facile reaction generating the unexpected secondary allyl ether as the major product. In contrast, carrying out the reaction under higher catalyst loadings at elevated temperatures provides the expected primary linear ether in high yield and with excellent selectivity. The etherification reaction is also effective in the presence of acetals and ketals in place of orthoesters and allows for the development of the procedure to encompass a telescoped etherification protocol in which the acetal is generated in situ.

SUBMITTER: Chaffey DR

PROVIDER: S-EPMC6776978 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:In depth mechanistic studies of iridium catalyzed regioselective and enantioselective allylic substitution reactions are presented. A series of cyclometalated allyliridium complexes that are kinetically and chemically competent to be intermediates in the allylic substitution reactions was prepared and characterized by 1D and 2D NMR spectroscopies and single-crystal X-ray difraction. The rates of epimerization of the less thermodynamically stable diastereomeric allyliridium complexes to the thermodynamically more stable allyliridium stereoisomers were measured. The rates of nucleophilic attack by aniline and by N-methylaniline on the isolated allyliridium complexes were also measured. Attack on the thermodynamically less stable allyliridium complex was found to be orders of magnitude faster than attack on the thermodynamically more stable complex, yet the major enantiomer of the catalytic reaction is formed from the more stable diastereomer. Comparison of the rates of nucleophilic attack to the rates of epimerization of the diastereomeric allyliridium complexes containing a weakly coordinating counterion showed that nucleophilic attack on the less stable allyliridium species is much faster than conversion of the less stable isomer to the more stable isomer. These observations imply that Curtin-Hammett conditions are not met during iridium catalyzed allylic substitution reactions by η(3)-η(1)-η(3) interconversion. Rather, these data imply that when these conditions exist for this reaction, they are created by reversible oxidative addition, and the high selectivity of this oxidative addition step to form the more stable diastereomeric allyl complex leads to the high enantioselectivity. The stereochemical outcome of the individual steps of allylic substitution was assessed by reactions of deuterium-labeled substrates. The allylic substitution was shown to occur by oxidative addition with inversion of configuration, followed by an outer sphere nucleophilic attack that leads to a second inversion of configuration. This result contrasts the changes in configuration that occur during reactions of molybdenum complexes studied with these substrates previously. In short, these studies show that the factors that control the enantioselectivity of iridium-catalyzed allylic substitution are distinct from those that control enantioselectivity during allylic substitution catalyzed by palladium or molybdenum complexes and lead to the unique combination of high regioselectivity, enantioselectivity, and scope of reactive nucleophile.

Dataset Information

Metal Triflate-Promoted Allylic Substitution Reactions of Cinnamyl Alcohol in the Presence of Orthoesters and Acetals.

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