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

Abstract

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).