Stereoconvergent Generation of a Contrasteric syn ‐Bicyclopropylidene (= syn ‐Cyclopropylidenecyclopropane) by Stille ‐Like Coupling

Stereoisomerically pure endo‐ and exo‐ 7‐halo‐7‐(trimethylstannyl)benzonorcar‐3‐enes (= endo ‐ and exo ‐(1‐halo‐1a,2,7,7a‐tetrahydro‐1 H ‐cyclopropa[ b ]naphthalen‐1‐yl)trimethylstannane) 4 and 6 were selectively obtained by lithiumtin or magnesiumtin transmetalation in good yields ( Scheme 2 and 3 ). The reaction of these compounds with copper(I) thiophene‐2‐carboxylate (CuTC) produced in both cases the corresponding C S ‐symmetric bicyclopropylidene (=cyclopropylidenecyclopropane) syn ‐ 1 , a single diastereoisomer ( Schemes 5 and 6 ). The structure of syn ‐ 1 was undoubtedly elucidated by X‐ray single crystal diffraction. The coupling mechanism of the carbenoid cyclopropane is discussed ( Scheme 7 ).