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)5CrC[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 6E showed low reactivity. When 1,2,3,4,5-pentamethylcyclopentadiene (7) was employed, the cycloadditions with α-alkoxyvinyl(ethoxy)carbene complexes 3−6E 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.