## 3 Results
### Carbocupration of the Alkyne Bond  
Addition of the CuCH\({}_{3}\) to the ynamide triple bond may subsequently occur in two distinct ways, depending on the orientation of the Cu atom relative to the ynamide carbonyl (Figure 3). In conformation 1, no interaction between copper and the ynamide oxygen exists, whereas in conformation 2, the metal atom initially interacts with oxygen through a 2.72 A bond. Conformation 1 is only slightly more stable (by 2.3 kcal-mol\({}^{-1}\)) than conformation 2. Methyl transfer from CuCH\({}_{3}\) to the alkyne may occur from either of these conformations. In both cases, the transition state is reached when the methyl group lies 2.10 A from the receiving carbon atom. Distances between the methyl group and the Cu atom hardly differ (by 0.01 A) in the transition states arising from each conformation. Activation energies are, in both cases, extremely high (Figure 3C) and are incompatible with an experimentally viable mechanism, since they translate to reaction rates between 3.5 \(\times\) 10\({}^{-12}\) and 2.1 \(\times\) 10\({}^{-16}\) s\({}^{-1}\) at 298 K. The addition of the methyl group to the ynamide must therefore follow a different mechanism. We therefore analyzed the pathways arising from direct complexation of the ynamide substrate with THF-solvated CuI or MgCH\({}_{3}\)Br. Analysis of the solvation energetics  
Figure 3: Carbocupration of the ynamide with THF-solvated CuCH\({}_{3}\). (**A**) transition state (conformation 1); (**B**) transition state (conformation 2); (**C**) potential energy surface of the carbocupration reaction arising from conformation 1 (blue) and conformation 2 (red). Several important bond distances (in Å) are highlighted. Figure 2: Formation of CuCH\({}_{3}\) from CuI and MgCH\({}_{3}\)Br. (**A**) reactant state; (**B**) transition state; (**C**) separated products. Several important bond distances (in Å) are highlighted. shows that the free energy change upon solvation of CuI by a single THF is favorable by 16.3 kcal-mol\({}^{-1}\), and that the addition of a second THF molecule to the complex is strongly disfavored because the small decrease in electronic energy afforded by the formation of the new Cu-O bond is more than offset by the decrease of system entropy caused by the decrease of molecularity of the system (Figure 4A). CH\({}_{3}\)MgBr, in contrast, exists as a complex with two solvent molecules due to the comparable electronic energy decreases entailed by the formation of each of the Mg-O bonds, which are in both cases more than enough to offset the entropic losses. The complexation of MgCH\({}_{3}\)Br:2 THF with the substrate carbonyl (Figure 4B) proceeds through the ejection of one of the metal-solvating THF molecules and therefore entails an increase in the entropy of the system. The reaction is, however, slightly disfavored, by 4.8 kcal-mol\({}^{-1}\), showing that the interaction of MgCH\({}_{3}\)Br with the ynamide carbonyl is weaker than its interaction with the THF molecule that has been released. The \(\pi\)-complexation of Cu\({}^{+}\) by the ynamine alkyne bond (Figure 4C), in contrast, is exergonic by 10.0 kcal-mol\({}^{-1}\) and proceeds without a barrier. It is therefore much more likely for CuI to complex the ynamide than to undergo the metathesis described above to yield CuCH\({}_{3}\). Subsequent addition of MgCH\({}_{3}\)Br to the Cu\({}^{+}\)-complexed ynamide (Figure 4D) then becomes exergonic (by 0.5 kcal-mol\({}^{-1}\) for MgCH\({}_{3}\)Br:2 THF and 7.1 kcal-mol\({}^{-1}\) for MgCH\({}_{3}\)Br:THF) and yields the reactive ynamide: (CuI:THF):(MgCH\({}_{3}\)Br:THF) complex where direct transfer of the methyl group from the organomagnesim to the alkyne bond can occur.