Alkylcopper-magnesium

Two different alkyl groups can be added to a terminal alkyne " in one laboratory step by treatment with an alkylcopper-magnesium bromide reagent (called Normant... 

The third example in Table 6.8 is particularly interesting since the product shown is an intermediate in a general method for the synthesis of various trisubsti-tuted alkenes. Thus, the alkylcopper-magnesium bromide species (the Nonnant reagent) adds to a primary alkyne so that the alkyl and metal addends are Z- (or syn-, or cis- or suprafacial) with the metal at C-1. In a second (and thus separate) step and with stereochemical retention (i), a second alkyl group can be added—as an alkyl iodide, for example, iodoethane (ethyl iodide, CH3CH2I)— which then replaces the metal (ii) a carboxylic add can be formed by treatment of the copper reagent with carbon dioxide (CO2) or (iii) iodine (I2) itself can be used so that a vinyl iodide results (Scheme 6.74). 

Rao and Knochel have reported the addition of copper reagents prepared by transmetallation from polyfunctional organozinc iodides to reactive alkynes in a stereochemically well-defined syn-addition. Intramolecular carbocupration of functionalized alkynyl-substituted alkylcopper species such as 74 obtained from the alkyl iodide 75 allows the preparation of highly substituted five-membered car-bocylcles giving stereochemically pure exo-alkylidenecyclopentane derivatives such as 76. The lower reactivity compared to the lithium- or magnesium-copper reagents does not allow the reaction with unactivated terminal alkynes but tolerates ester, nitrile or chloride functions (Scheme 9.23) [34]. 

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