Facial Stereoselectivity in the Diels−Alder Reactions of (Arylethynyl)ethoxycarbenes of Group 6 with 1,2,3,4,5-Pentamethylcyclopenta-1,3-diene and Unusual Rearrangement of Strained Norbornadienyl Fischer Carbene Complexes of Chromium(0)

A study of reactivity and facial selectivity in Diels−Alder reactions of the Fischer carbene complexes (CO)₅MC(C⋮CC₆H₄R)OCH₂CH₃ (1a, M = Cr, R = H; 1b, M = Cr, R = p-OCH₃; 1c, M = Cr, R = p-CH₃; 2, M = W, R = H; 3, M = Mo, R = H), with 1,2,3,4,5-pentamethylcyclopenta-1,3-diene (4) was carried out. The cycloadditions were found to be highly π-facial stereoselective, favoring the anti adduct. Among these complexes, 3 was the most reactive dienophile. The stereochemical assignment of the adducts 5−7 was supported by NOE measurements; structures of the anti cycloadducts 5a and 6 were established by single-crystal X-ray diffraction. A [4 + 2] cycloaddition/benzannulation/rearrangement cascade reaction was observed for the chromium carbene complexes 1a−c with 4 under thermal conditions, resulting in the naphthofurandione derivatives 9−11.