Studies on the oxidative addition reaction of 1,1-dibromo-1-alkenes, α-dehalopalladation, and the intramolecular bis(carbopalladation) reaction of alkenes.: An efficient entry to fused bicycles

Twenty-three examples of 1,1-dihalo-1-alkenes were synthesized by the conventional alkylation methods. The oxidative addition reactions of 1,1-dibromo-2,2-diphenylethene or 1,1-dibromo-2-phenylpropene with a stoichiometric amount of Pd(PPh3)(4) afforded 1,2-diphenylacetylene and 1-phenylpropyne, respectively;, indicating that alpha -dehalopalladation reaction occurred to afford vinylic carbene intermediates. However, alpha -dehalopalladation reaction was not observed in all 1,1-dihalo-1-alkenes containing an extra C = C bond suitable for cyclic carbopalladation under the current reaction conditions probably due to the fast cyclic carbopalladation reaction of 40A-type of palladium intermediates; A series of bicycles, i.e., fused 5,6-, 6,6-, 6,7-, and 7,7-bicyclic compounds, were prepared efficiently via this bicyclic carbopalladation protocol. Under condition A, within 0.5 h, 10 afforded the monocyclic product 37 in 79%. With prolonged reaction time, 37 was converted to bicycle 36. Even with isolated 37 the corresponding reaction under condition A afforded 36 in 92% NMR yield, indicating a stepwise oxidative addition-cyclic carbopalladation-beta -elimination mechanism for this bicyclization reaction.