[4 + 2]-Cycloadditionen mit Übergangsmetall-koordinierten Heteroalkenen: Einfluß der Koordination auf die Stereoselektivität der Cycloaddition

[4 + 21 Cycloadditions with Transition-Metal-Coordinated Heteroalkenes: The Influence of the Coordination on the Stereoselectivity of the Cycloadditon Pentacarbonyl(thiobenzaldehyde), -(selenobenzaldehyde), and -(diphenyl selenoketone) complexes (CO)5M[X = C(Ph)R] [R = H: M = W, X = S (1a), Se (1b); M = Cr, X = S (1c), Se (1d); R = Ph: M = W, X = Se (1e)] react with cyclopentadiene (2) and 1,3-cyclohexadiene (4), respectively, by [4 + 2] cycloaddition to give metal-coordinated this- and selenacycles 3 and 5. The additions of 2 and 4 to 1a-d are stereoselective. In contrast to the endo selectivity observed in the reactions of 2 and 4 with uncomplexed thio- and selenoaldehydes, the additions of 2 and 4 to 1a-d are exo selective. However, the reactions of pentamethylcyclopentadiene (6) with 1a,b to form 7 are endo selective. The structures of the main isomer of the addition of 4 to 1c (5c-exo) and of 6 to 1 b (7b-endo) were confirmed by X-ray analyses.