Palladium-Catalyzed Bicyclization of 2-Bromo-1,6-dienes and -1,6-enynes to 5-Membered-Ring-Annelated Vinylcyclopropane Derivatives

Palladium-catalyzed cyclizations of the 2-bromo-1,6-diene 6 with an acetoxymethyl substituent on the bromoalkenyl moiety did not only lead to the expected 1-acetoxymethyl-1,3-diene 40 but also to the bicyclic vinylcyclopropane 35, the 1,3-diene 36 and the triene 37 (a dendralene). All these compounds result from an initial 5-exo-trig cyclization of the 2-bromo-1,6-diene. By proper choice of the reaction conditions (ligand, base, allylic leaving group) each of these compounds could be formed selectively. Small amounts (3–5%) of the isomerized 1-acetoxymethyl-1,3-diene 38 and the 6-endo product 39 were also isolated. With the acetoxymethyl substituent on the alkenyl moiety (compound 27) only the 1,4-diene 44 was formed. The related 1,6-enyne 57 with a methoxycarbonyloxymethylene substituent on the triple bond gave in 36% yield the 2,3-bis(bicyclo[3.1.0]hex-1-yl)-substituted 1,3-butadiene 63, which can be regarded as a dehydrodimer of the vinylcyclopropane 35. The presumption that the formation of 63 involves the alkenylpalladium species 65 was supported by its successful inter- as well as intramolecular trapping with formate as a hydride source to yield the vinylcyclopropane 35. The reaction pathways leading to the vinylcyclopropane derivatives 35 and 63 have in common that an alkylpalladium species rather undergoes a 3-exo-trig cyclization than an internal rotation followed by a β-hydride elimination.