On the Regioselectivity of Palladium(0)-Catalyzed Substitutions on Cyclopropyl Group Containing Allyl Esters¹

Palladium-catalyzed substitutions of 1-vinylcyclopropyl Tosylates 1a and mesylates 1b with nucleophiles such as phenylzinc chloride and sodium azide lead to 1-vinylcyclopropyl derivatives 16 and 23, but with potassium acetate, potassium phthalimide or sodium 3-phenylallyloxide give cyclopropylideneethyl compounds 3c, 17 and 20, respectively, in high yields. This observation is interpreted in terms of a π-allylpalladium complex with a highly unsymmetrical charge distribution. The reaction of a (dimethylvinyl)cyclopropyl derivative 11 with the test nucleophile dimethyl 2-sodiomalonate surprisingly yields a mixture of both regioisomers. An application of this Pd(0)-catalyzed reaction makes methylenecyclopropylglycine 27 (2-amino-4-cyclopropylidenebutanoic acid), an interesting isomer of hypoglycine A 28 readily available.