Novel synthesis of phenol derivatives by palladium-catalyzed cyclocarbonylation of 2,4-pentadienyl acetates

Abstract

Phenyl acetates were selectively obtained in good yields by cyclocarbonylation of 2,4-pentadienyl acetates in the presence of NEt 3 , Ac 2 O, and a catalytic amount of palladium complexes such as PdCl 2 (PPh 3 ) 2 at 120-140 o C under 50 atm of CO. No five-membered ring products were observed. A platinum complex PtCl 2 (PPh 3 ) 2 was also effective as a catalyst. The reaction of 5-phenyl-2,4-pentadienyl bromide with M(CO)(PPh 3 ) 3 (M=Pd or Pt) under CO gave the corresponding 6-phenyl-3,5-hexadienoyl complexes in a high yield, which in turn afforded 2-acetoxybiphenyl in 41-51% yield on heating to 160 o C under 50 atm of CO in the presence of NEt 3 and Ac 2 O. Similar 3,5-hexadienoyl complexes ere proposed to be intermediates in the catalytic cyclocarbonylation of 2,4-pentadienyl acatates. On the other hand, PdCl 2 (PPh 3 ) 2 -catalyzed carbonylation of o-(bromomethyl)(1-alkenyl)benzenes in the presence of NEt 3 and Ac 2 O gave 2-naphtyl acetates in moderate yields, while the reaction in the absence of Ac 2 O gave five-membered ring products such as 2-indanones or a tricyclic lactone by incorporation of one or two CO molecules, respectively