Diels-Alder reactions cascade carbopalladation

Grigg and Xu have developed a variety of so-called queuing cascades involving allenes. The intra-intermolecular carbopalladation sequence of the <9-iodo-A-methyl-A -(methylallyl)aniline 142 and 1,1-dimethylallene 143 with subsequent / -dehydropalladation leads to the 1,3-dienyl-substituted indole derivative 144, which is immediately trapped by an added dienophile (e.g., A-methylmaleimide) in a Diels-Alder reaction to yield 145 (Scheme 37)7 ... 

When the carbopalladation of the bicyclopropylidene is performed in the presence of methyl acrylate, the reaction takes a different course (Scheme 8.34) [79]. The 1,3-diene intermediate 75 reacts in situ with the dienophile to give the spiro[2.5]octane derivative 76. An extension of this cascade Heck-Diels-Alder reaction involving l,3-dicyclopropyl-l,2-propadiene as the alkene partner, an alkenyl or aryl halide and a dienophile has been reported [80]. 

In their pioneering work on the catalytic carbopalladation reaction of 1,2-heptadiene with phenyl iodide in the presence of a suitable base, Shimizu and Tsuji observed the formation of the corresponding substituted 1,3-dienes 62 via a / -hydride elimination from the 7z>allyl intermediate 61 [61]. Based on these observations, a three-component Heck-Diels-Alder cascade process has been developed by Grigg and co-workers [73]. A wide variety of aryl and heteroaryl iodides were used for the intermolecular reaction with dimethylallene to afford the corresponding 1,3-dienes. These subsequently react in situ with N-methylmaleimide to give the bicyclic adducts 63 (Scheme 8.30). 

Double, triple, and even quadruple Heck-Diels-Alder cascade reactions involving 1,4-diiodo-, 1,3,5-triiodo-, or 1,2,4,5-tetraiodobenzene, respectively, andbicyclo-propylidene (16) have been accomplished (cf. Scheme 8.5) [121b]. The efficiency of these sequences, in which each carbopalladation across the highly strained alkene is followed by a cyclopropylmethyl to homoallyl rearrangement with concomitant 3-hydride elimination to yield an allylidenecyclopropane, which subsequently undergoes a smooth [4 + 2] cycloaddition (Scheme 8.5), is quite remarkable [121bj. 

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