Verlaufen 1,3-Eliminierungen konzertiert oder nicht ?

PrCHBrCH2CHPrR (R = Br, F, or I) were converted into 1,2-dipropylcyclopropane by various metallic or organometallic reducing agents. Normally, cyclization occurred in a stereoconvergent manner, i.e. diastereomeric dihalides usually led to the same mixt. of cis-trans isomeric reaction products, indicating a nonconcerted cleavage of the 2 halogen atoms. The isomer ratios depended very strongly on the nature of the reducing agents and on possible salt additives, probably typical for reactions of short-lived intermediates and compatible with the assumed one-electron transfer mechanism. The \"endo tendency\" was obsd. again: the cis-disubstituted cyclopropane though thermodynamically less stable than the trans isomer was formed predominantly in most cases. [on SciFinder (R)