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 (¼endo- and exo-(1-halo-1a,2,7,7a-tetrahydro-1H-cyclopropa[b]naphthalen-1-yl)trimethylstannane) 4 and 6 were selectively obtained by lithium-tin or magnesium-tin 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 CS-symmetric bicyclopropylidene (¼cyclopropylidenecyclopropane) 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).