| Alkylations of alkoxy-substituted alkenenitriles exert an unusual diastereoselectivity. A series of alkylations were computed with alkyl halides, alkyl benzoate derivatives, and alkyl carbonates. Diastereoselectivity depends on the nature of the electrophile, but the trend is difficult to correlate with specific structural features. Density functional theory (DFT) has been applied to compute transition structures using Gaussian 09 to address the reasons for the observed diastereoselectivities. Stereoselective ratios can be explained by comparing the transition structures energies, and it is found that the &agr;-carbon of the metalated nitrile experiences torsional strain with the stereocenter beta-carbon. Torsional strain is a result of a gauche orientation of two alkyl groups. The methoxy oxygen atom and the electrophile alkyl cation, as well as the alkyl cation clashing with the carbon backbone of the nucleophile influence stereoselectivity. This thesis work allows for an improved understanding and synthetic methodology for the alkylations of acyclic metalated nitriles. |
