1,3 -dicyclopropyl-1,2-propadiene

Similarly, the Pd-catalyzed arylation of l,3-dicyclopropyl-l,2-propadiene 6/1-122 with iodobenzene in the presence of dimethyl maleate led to the diastereomeric cyclopropane derivatives 6/1-124 and 6/1-125 via 6/1-123 in 86% yield as a 4 l-mix-ture [65] (Scheme 6/1.33). Several other aryl halides and dienophiles have been used in this reaction. 

Knoke and de Meijere [60] recently developed a highly flexible domino Heck-Diels-Alder reaction of a symmetrically substituted cumulene 125, which also involves cross-couplings of an allene at the central position. Both aryl and hetaryl halides react efficiently with l,3-dicyclopropyl-l,2-propadiene (125) and furnish 1,3,5-hexatriene derivatives 126 as intermediates, which are usually trapped by acceptor-substituted olefins in a subsequent cycloaddition, providing adducts 127a/b in moderate to good overall yields (Scheme 14.30). 

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]. 

Knoke, M. and de Meijere, A. (2003) A versatile access to l-cyclopropyl-2-aryl-1,3,5-hexatrienes - domino Heck-Diels-Alder reactions of l,3-dicyclopropyl-l,2-propadiene. Synlett, 195-8. 

The a-allylpalladium intermediate 135a, which must be formed on coupling of l,3-dicyclopropyl-l,2-propadiene (134), with, for example, iodobenzene under palladium catalysis rapidly undergoes rearrangement to the homoaUylpalladium species 135b and subsequent P-hydride elimination to yield the 1,3,5-hexatriene 136 [152g,h]. This in turn undergoes [4 + 2] cycloaddition with an added dienophile to furnish the 3-(2-cyclopropyl-l-phenyl)cyclohexene derivatives 138 as a mixture of trans,trans- and ds,trows-diastereomers (Scheme 8.31). 

Scheme 8.31 A three-component reaction involving l,3-dicyclopropyl-l,2-propadiene, an aryl...

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