Importance of Ligand Exchanges in Pd(II)-Br酶nsted Acid Cooperative Catalytic Approach to Spirocyclic Rings

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• 作者：Garima Jindal ; Raghavan B. Sunoj
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：November 12, 2014
• 年：2014
• 卷：136
• 期：45
• 页码：15998-16008
• 全文大小：773K
• ISSN：1520-5126

文摘

Increasing number of reports in the most recent literature convey the use of palladium and Br酶nsted acids as cooperative catalytic partners. However, the mechanistic understanding of several such cooperative catalytic reactions and the origin of cooperativity continue to remain limited. In transition metal catalysis, it is typically assumed that the native ligands, such as the acetates in palladium acetate, are retained throughout the catalytic cycle. Herein, we convey the significance of invoking ligand exchanges in transition metal catalysis by using the mechanism of a representative cooperative dual-catalytic reaction. Density functional theory (M06 and B3LYP) computations have been employed to decipher the mechanism of Pd(II)-Br酶nsted acid catalyzed migratory ring expansion reaction of an indenyl cyclobutanol to a spirocyclic indene bearing a quaternary carbon. The molecular role of water, benzoquinone and phosphoric acid has been probed by computing the energetics using several combinations of all these as ligands on palladium. Of the two key mechanistic possibilities examined, a Wacker-type pathway (involving a semipinacol ring expansion of cyclobutanol followed by a reductive elimination) is found to be energetically more preferred over an allylic pathway wherein the ring expansion in a Pd-蟺-allyl intermediate occurs subsequent to the initial allylic C鈥揌 activation. The Gibbs free energies of the transition states with the native palladium acetate are much higher than a Pd-bis-phosphate species generated through ligand exchanges.