Silyl effect on the regioselective synthesis of silylated alkylidenecyclopropanes

Nucleophilic substitution of pi-3-trialkylsilyl-1,1-dimethyleneallyl pallad ium complexes 2a,b occurred on the carbon bearing the silyl group whatever the hard (hydride) or soft (malonate anion) nature of the nucleophiles. Con versely, when located on the cyclopropane ring, i.e., in beta of the pi-all yl palladium complex 12, the silyl group appeared able to overcome the ring strain effect and to direct the substitution on the three-membered ring. T his effect was however reversed by the simultaneous presence of these two t rialkylsilyl substituents on the pi-allyl palladium complex 27. Electrophil ic substitution of 1,1-dimethyleneallylsilanes 4a,b by benzaldehyde occurre d regioselectively on the cyclopropane ring.