High Stereoselectivity and Facial Selectivity in Diels−Alder Cycloadditions of Novel Captodative Olefins:  α-Alkoxyvinyl(ethoxy)carbene Chromium Complexes with Cyclopentadienes

Abstract

A study of reactivity and selectivity of the new Fischer carbene complexes (CO)5CrC[C(CHR‘)(OR)]OCH2CH3 (3, R‘ = H, R = CH2CH3; 4, R‘ = H, R = CH2CH2CH3; 5, R‘ = H, R = CH2CH2CH2CH3; 6, R‘ = CH3 (E and Z), R = CH2CH3) was carried out in Diels−Alder cycloadditions with cyclopentadiene (1) and 1,2,3,4,5-pentamethylcyclopentadiene (7). The cycloadditions between the α-alkoxyvinyl(ethoxy)carbene complexes 3, 4, and 5 with 1 were found to be highly stereoselective, favoring the endo adduct at a much higher level than that observed for the analogous α-alkoxy-α,β-unsaturated esters, whereas complex 6 E showed low reactivity. When 1,2,3,4,5-pentamethylcyclopentadiene (7) was employed, the cycloadditions with α-alkoxyvinyl(ethoxy)carbene complexes 3−6 E were highly anti/exo selective. The exo adducts form new alkoxy-chelated tetracarbonyl carbene complexes. The stereochemical assignment of the cycloadducts was supported by NOE measurements, and the chelated exo cycloadduct 21 was further characterized by single-crystal X-ray diffraction.

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)

“…Recently, we have reported the Diels−Alder reaction of (α-alkoxyvinyl)ethoxycarbene complexes ( 8 ) with pentamethylcyclopentadiene ( 4 ), which gave only the anti - exo diastereoisomer . The exo selectivity was probably due to the strong repulsive interactions between the metal moiety and the methyl group of the diene at the endo transition state.…”

“…Recently, we reported the reactivity and selectivity in the reaction of (α-alkoxyvinyl)ethoxycarbene complexes with 1,2,3,4,5-pentamethyl-1,3-cyclopentadiene, which produced new alkoxy-chelated tetracarbonyl carbene complexes with a high anti / exo selectivity . This study has now been extended to the reaction of the (arylethynyl)ethoxycarbene complexes 1 − 3 in Diels−Alder cycloadditions.…”

Microwave-Assisted Reactivity of a Fischer Alkynyl Carbene Complex with Benzylidene Anilines

“…To initiate the reactivity study, benzylidene aniline 2a [43] and Fischer carbene complex 1 were utilized as model reagents [57]. Four anhydrous solvents (toluene, benzene, methyl t-butyl ether (MTBE), and THF) were tested to avoid the hydrolysis of 2a (Table 1, entries 1-4).…”

Fischer Carbene Complexes: A Glance at Two Decades of Research on Higher‐Order Cycloaddition Reactions

“…FCCs containing group VI transition metals [29,31–33] are very versatile in cyclization and cycloaddition reactions with an α,β‐unsaturated system under different conditions, [29] resulting in carbocycles, [34–39] heterocycles [29,40–43] and a diversity of other compounds. Chromium carbene complexes have shown particularly suitable reactivity, compared to the greater stability of tungsten and the extreme reactivity of molybdenum complexes [44–47] .…”