Abstract
Stereoselectivity of Diastereogenic CC‐Linking Reactions II. A Carbene/Carbenoid‐Cascade and «Hypermechanisms»In a systematic study four pairs of dihalocarbene precursors were treated with methyllithium in the presence of 2‐methyl‐2‐pentene. Each pair was made up of a tetrahalomethane and a trihalomethane, namely CCl4 vs. CHCl3, CCl3F vs. CHCl2F, CCl2F2 vs. CHClF2 and CClF3 vs. CHF3. Since all the tetrahalomethanes reacted exclusively by chlorine/lithium exchange and all the trihalomethanes by hydrogen/lithium exchange, each pair gave rise (at least formally) to an identical ‘carbenoid’. Two different carbenoids may always undergo elimination of LiCl or LiF and thus collapse generating the same carbene.A comparison of the product composition (ratios of straight‐forward cycloaddition vs. consecutive substitution reactions, ratios of diastereoisomers) reveals more or less marked, but always significant discrepancies in the outcome of ‘carbene‐convergent’ as well as ‘carbenoid‐convergent’ reactions.
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