NEW CYCLOPROPYL BUILDING BLOCKS FOR ORGANIC SYNTHESIS .9. NUCLEOPHILIC SUBSTITUTIONS OF 1-ALKENYLCYCLOPROPYL ESTERS AND 1-ALKYNYLCYCLOPROPYL CHLORIDES CATALYZED BY PALLADIUM(O)

The 1-ethenylcyclopropylsulfonates 2e,f and 2-cyclopropylideneethyl esters 10b,c, readily available from cyclopropanone hemiacetal 1, undergo regioselective Pd(0) catalyzed nucleophilic substitution via the unsymmetric 1,1-dimethylene-pi-allyl complex 23. With stabilized anions (enolates of malonic ester, beta-dicarbonyl compounds, beta-sulfonyl ester, and Schiff bases as well as acetate anion, sulfonamide anion, etc.) the nucleophilic substitution occurs at the terminal vinylic position exclusively, providing cyclopropylidencethyl derivatives as building blocks of high synthetic potential. Competition experiments have disclosed that 1-ethenylcyclopropyl tosylate (2e) and cyclopropylideneethyl acetate (10b) are more reactive than dimethylallyl acetates 19 and 22, respectively. Use of chiral phosphines as ligands in the palladium catalyst can provide optically active methylenecyclopropane derivatives. With phenyl-, methyl-, and even n-butylzinc chloride as nucleophiles, the reaction apparently proceeds with initial transfer of the organic residue to palladium, followed by reductive elimination entailing tertiary substitution on the cyclopropane ring exclusively; the same type of product is obtained with azide and bis(trimethylsilyl)amide. But the site of hydride attack to yield reduction products depends on the hydride source. 1-Alkynylcyclopropyl chlorides 12, 13, and 14 react only with organozinc chlorides (nonstabilized nucleophiles) to provide mixtures of ethenylidenecyclopropanes 65 and alkynylcyclopropanes 66, via the sigma-palladium complexes 69 and 70, while chloride 15 undergoes mainly reduction. Other transition metal catalysts (Ni, Mo) also induce substitutions, but with poorer regioselectivity.