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.
- Carbon dioxide
- Lewis acid