## 3 Results
### Homologation of Intermediate 1 with ICH\({}_{2}\)ZnCH\({}_{2}\)I
#### 3.4.1 Attack through the Direction Opposite the Oxazolidinone Carbonyl  
The pre-reactional complex arising from the attack through the face opposite the oxazolidinone carbonyl (Figure 6B) is appreciably more stable (by 10.5 kcal-mol\({}^{-1}\)) than the infinitely-separated reactants due to the electrostatic attraction between the positively-charged Zn\({}^{2+}\) and the anionic intermediate 1. Surprisingly, our exploration of the subsequent potential energy surface showed that the CH\({}_{2}\) transfer from the zinc carbenoid to the C=C bond in this case takes place through the direct intervention of Cu\({}^{+}\) (Figure 6C). This preference can be rationalized by examination of the average local ionization energies (Figure 7A) of the Int2 intermediate, which feature an absolute minimum (i.e., highest nucleophilic character) on the Cu\({}^{+}\), which will therefore be the location with the highest propensity to attack the relatively electrophilic CH\({}_{2}\) group (Figure 7B) on ICH\({}_{2}\)ZnCH\({}_{2}\)I. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Reactant State** & **Transition State** & **Product** \\ \hline Mg moiety & 0.6 & 5.8 & \(-\)2.0 \\ \hline Cul moiety & 0.0 & \(-\)1.0 & 2.2 \\ \hline Mg-organic moiety & \(-\)2.8 & \(-\)21.6 & \(-\)4.8 \\ \hline Cul-organic moiety & \(-\)0.2 & \(-\)7.1 & \(-\)10.5 \\ \hline organic moiety & \(-\)0.1 & 1.7 & \(-\)10.1 \\ \hline Full complex & \(-\)3.0 & \(-\)17.7 & \(-\)10.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Electronic energy (kcal-mol\({}^{-1}\)) differences (including solvation effects in THF) between the individual fragments of linear and branched isomers of intermediate 1, computed with the PBEPW91 density-functional. Computations of the energies of the individual fragments have been performed at the same geometry as that in the full molecule. Negative values denote preference for the ”linear” over the “branched” isomer. Figure 5: Regioselecttivity of addition of THF-solvated MgCH\({}_{3}\)Br to Cul:vynamide. (**A**) transition state (TS1) of the formation of the linear isomer; (**B**) transition state (TS1) of the formation of the branched isomer; (**C**) potential energy surface of the formation of each regiosomer. Several important bond distances (in Å) are highlighted. In the transition state, located only 10.7 kcal-mol\({}^{-1}\) above the pre-reactional complex, the bond order of the C-I bond has decreased to 0.45, and a half-bond has formed between the CH\({}_{2}\) and Cu\({}^{+}\) group. The collapse of this transition state is then effected through complete transfer of the loosely-bound iodine to the Zn\({}^{2+}\) and formation of a new C-C bond between CH\({}_{2}\) and the hetero-substituted carbon in the C=C double bond. This is accompanied by a partial migration of Cu\({}^{+}\) to the other carbon of the double bond and by reduction of 0.5 in the total bond orders of each fragment. Analysis of Lowdin charge variations shows that the ZnICH\({}_{2}\)I moiety present in the product now contains 0.69 electrons more than in the beginning, of which 0.46 electrons originate in the Int2 moiety and 0.23 have come from the transferred CH\({}_{2}\). Examination of the evolution of the bond orders along the reaction coordinate leading from the transition state to Int2 (Figure 8) shows that this rearrangement occurs in three stages: first the bond order between CH\({}_{2}\) and I decreases, concurrently with a simultaneous increase of the bond orders of the Cu-CH\({}_{2}\) and Zn-I bonds. Then, as the CH\({}_{2}\)-I bond completely disappears, the bond orders of both the Cu-CH\({}_{2}\) bond and the bond between Cu and the heterosubstituted cyanide carbon decrease in similar amounts to enable the formation of the bond between CH\({}_{2}\) and the heterosubstituted carbon. This stage is accompanied by a decrease in the double-bond character of the formal C=C bond in ynamide.