Regioselective, Stereoselective, and Conformationally Controlled Synthesis of (η4-Tetraarylcyclobutadiene)(η5-carbomethoxycyclopentadienyl)cobalt Metallocenes

The Friedel–Crafts reaction of (η4-tetraphenylcyclobutadiene)(η5-carbomethoxycyclopentadienyl)cobalt with acid chlorides/aluminum chloride resulted exclusively in para-phenyl acylation. Both monoacylated (1.1 equiv of RCOCl/AlCl3) and tetraacylated products (>4 equiv of RCOCl/AlCl3) were synthesized. Reaction of PhCC(o-RC6H4) (R = Me, i-Pr) with Na(C5H4CO2Me) and CoCl(PPh3)3 gave predominantly (η4-1,3-diaryl-2,4-diphenylcyclobutadiene)(η5-carbomethoxycyclopentadienyl)cobalt metallocenes (1,3-[trans] vs 1,2-[cis] selectivity up to 6:1). Conformational control of Friedel–Crafts reactions on the major isomers gave exclusively para-acylation of the unsubstituted phenyl groups.