Lewis acid-mediated β-selective hydrocarboxylation of α,α-diaryl- and α-arylalkenes with R3SiH and CO2

Article abstract of DOI:10.1016/j.tetlet.2015.04.090

α,α-Diarylalkenes are successfully hydrocarboxylated with Et₃SiH and CO₂ with the aid of Et₃SiB(C₆F₅)₄ or EtAlCl₂/Ph₃SiCl to give carboxylic acids with a carboxy group at the β-position to the aryl groups. The EtAlCl₂/Ph₃SiCl-mediated reaction is also applicable to various α-arylalkenes. ¹H NMR analysis of a mixture of EtAlCl₂, Ph₃SiCl, and Et₃SiH strongly suggests the formation of a μ-H complex, [Ph₃Si-H-SiEt₃]⁺ AlEtCl₃^-, which is an equivalent of R₃Si⁺ ions, while Et₃SiB(C₆F₅)₄ is an ion pair with a Et₃Si⁺ ion. Therefore, in these reaction systems, a siloxycarbonylium, R₃SiOCO⁺, is considered to be a common electrophile, the addition of which to the substrate, followed by trapping of the resulting cationic species with Et₃SiH seems to afford the desired acid after aqueous workup.