Methylenecyclopropanes reductive coupling with

Methyl 2-bromo-2-cyclopropylideneacetate (11a) has never been tested in these reactions, but has been used as a starting material for the stepwise construction of 1,6-heptadienes with methylenecyclopropane units for intramolecular Heck reactions. Thus, bromo ester 11a, after reduction, subsequent conversion of the resulting alcohol to the bromide and coupling with enolates of substituted malonates, was transformed into dienes of the type 254 (Scheme 73) - versatile synthetic blocks for the preparation of functionally substituted spirocyclopropanated bicyclo[4.3.0]nonenes 255a-d by a domino Heck-Diels-Alder reaction [122a]. 

Indeed, both kinds of cycloaddition products (Type A and Type B) can be obtained in the presence of Ni(0) catalysts while Pd(0) catalysts exclusively lead to Type A codimers. The real course of these reactions however is somewhat more complicated. While distal ring-opening via Route a really leads to cycloaddition products of Type A, proximal ring-opening via Route b results only in an isomerization of methylenecyclopropane. Cycloaddition products of Type B are obtained indirectly via oxidative coupling of two alkene units with low-valent nickel followed by a cyclo-propylmethyl/3-butenyl rearrangement22,148b). Reductive elimination terminates the catalytic cycle (Eq. 78). 

A further support for the mechanism outlined in Eq. 118 is that with Ni(0) catalysts a second type of [3+2]-cycloaddition can occur which involves the oxidative coupling of two alkenes coordinated at the nickel (one must be methylenecyclopropane). The initially formed nickelacyclopentane derivative may collapse to give a spiro[2.3]cyclohexane derivate or rearrange into a 4-methylenenickelacyclohexane derivate, which at the end of this catalytic cycle gives methylenecyclopentanes with a new substitution pattern by reductive elimination (see Eq. 78 and Scheme 8). 

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