Coupling of cyclobutenediones with Fischer carbene complexes: A one-step synthesis of cyclopentenediones and/or 5-alkylidenefuranones via net insertion of the carbene unit into a C-C bond

The reaction of Fischer carbene-chromium complexes with 3-cyclobutene-1,2-d iones has been investigated. In most cases, the major product of the reacti on is the C-C bond insertion product, a 2-alkoxy-4-cyclopentene-1,3-dione, accompanied by a minor amount of the partial deoxygenation product, a 4-cyc lopentene-1,3-dione. In some cases, 5-alkylidenefuranones are also formed. A mechanism involving oxidative. addition of the coordinatively unsaturated Fischer carbene complex followed by acyl migration and reductive eliminati on was proposed to account for cyclopentenedione formation. Furanone format ion was thought to arise via demetalation of the acyl migration product, fo llowed by O-acylation. An electronic dependence was noted for cyclopentened ione/alkylidene-furanone ratio, which was evaluated using the Hammett equat ion.