Chromium-Catalyzed syn-Selective Ring-Opening Aryl-/Alkylation of 7-Oxabenzonorbornadiene Derivatives with Grignard Reagents

Chromium(III) trichloride in combination with aryl/alkyl Grignard reagents under ligand-free conditions exhibited extremely high catalytic activity (turnover number of up to 2.4 × 104) toward the syn-selective ring-opening aryl/alkylation of 7-oxabenzonorbornadiene derivatives, producing syn-2-aryl/alkyl-1,2-dihydronaphthalen-1-ols in high yields. The superior catalytic activity of the chromium catalyst under the ring-opening arylation conditions enabled us to use unstable aryl Grignard reagents with esters and N-protecting groups synthesized from the corresponding aryl iodides and turbo-Grignard reagent. Control experiments to elucidate the reactivity of arylchromium species such as neutral triarylchromium(III) and anionic tetraarylchromate(III) suggested that in situ formed diarylchromate(I) from tetraarylchromate(III) served as the active species in this catalytic reaction. The magnetic susceptibility of the in situ generated chromium species from CrCl3(thf)3 and excess amounts of p-tolylmagnesium bromide was reasonable for the low-spin Cr(I) species (μ = 1.93 μB).