Palladium(0)‐ and Nickel(0)‐Catalyzed [3 + 2] Co‐Cyclization Reactions of Bicyclopropylidene with Alkenes
Bicyclopropylidene (1) readily undergoes a palladium(0)-catalyzed [3 + 2] co-cyclization with electron deficient alkenes (methyl acrylate, methyl trans-crotonate, methyl cinnamate and diethyl fumarate) as well as with some strained alkenes (norbornene, norbornadiene) by distal ring cleavage of one of the three-membered rings of 1. All these co-cyclizations are regioselective with respect to 1 as well as regio- and stereoselective with respect to the alkenes to give the corresponding 4-methylenespiro[2.4]heptane-trans-6-carboxylates 2a−5 with the electron deficient alkenes and the cycloadducts 6 and 7 with the strained alkenes in acceptable to good yields (56 to 83%). In contrast to palladium(0) catalysts nickel(0) complexes catalyze both distal ring opening of 1 and oxidative coupling of the two double bonds when 1 is reacted with e.g. diethyl fumarate. The result is a mixture of the methylenecyclopentane derivative 5 with the [2 + 2] cycloadduct 8 and the cotrimer 9.