New highly efficient three-component domino Heck-Diels-Alder reaction with bicyclopropylidene: Rapid access to spiro[2.5]oct-4-ene derivatives

Bicyclopropylidene (1) was found to surpass even methyl acrylate (17a) in its rate of undergoing carbopalladation with aryl- or alkenylpalladium species. leading to substituted allylidenecyclopropanes 5, 7 and 10, mostly in high yields (37 - 78%). These dienes and cross-conjugated trienes react in a Diels-Alder mode with dienophiles to give spiro[2.5]octenes 18a-Ph, 18b-Ph and 18a-Vin, respectively, in good yields (89, 69 and 65%). The overall transformation can be achieved as a one-pot three-component reaction with a variety of dienophiles to furnish the domino Heck-Diels-Alder products 18 regioselectively in most cases in good to very high yields (49-100%). The reaction of I with iodobenzene (2-Ph) and 17a gave 18a-Ph in virtually quantitative yield-also on a gram scale-using, only 1 mol% of catalyst, and even bromobenzene (22) gave 18a-Ph in 59% yield. Bicyclopropylidene (1). in the presence of palladium acetate/triphenylphosphane underwent rearrangement to allylidenecyclopropane (5-H), which in turn dimerized (73%) in the absence of other reaction partners, or could be trapped by diethyl fumarate (17c) to give the Diels-Alder adduct 18c-H in 45% yield. The coupling of oligoiodobenzenes with I and subsequent cycloaddition could he extended to a multicomponent reaction. In this way, 1.4-diiodobenzene (37). 1 and an alkyl acrylate gave the products 38 of a twofold Heck-Diels-Alder reaction in up to 87% yield, 1,3,5-triiodobenzene (39) reacted in up to 72% yield and ultimately 1,2,4,5-tetraiodobenzene (41) gave the fourfold domino Heck-Diels-Alder product 42 in 47% isolated yield, in a single operation in which 12 new carbon-carbon bonds were formed.