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

Abstract α,α-Diarylalkenes are successfully hydrocarboxylated with Et 3 SiH and CO 2 with the aid of Et 3 SiB(C 6 F 5 ) 4 or EtAlCl 2 /Ph 3 SiCl to give carboxylic acids with a carboxy group at the β-position to the aryl groups. The EtAlCl 2 /Ph 3 SiCl-mediated reaction is also applicable to various α-arylalkenes. 1 H NMR analysis of a mixture of EtAlCl 2 , Ph 3 SiCl, and Et 3 SiH strongly suggests the formation of a μ -H complex, [Ph 3 Si–H–SiEt 3 ] + AlEtCl 3 − , which is an equivalent of R 3 Si + ions, while Et 3 SiB(C 6 F 5 ) 4 is an ion pair with a Et 3 Si + ion. Therefore, in these reaction systems, a siloxycarbonylium, R 3 Si O C O + , is considered to be a common electrophile, the addition of which to the substrate, followed by trapping of the resulting cationic species with Et 3 SiH seems to afford the desired acid after aqueous workup.