Addition of Chiral Allenylzinc Reagent to Acetaldehyde: Diastereotopic Cyclic Transition States with a Tetrahedral Zinc Atom Located by ab Initio and Density Functional Theory
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Abstract
Transition-state structures for the reaction between the chiral allenylzinc reagent (M)-2 and acetaldehyde are located at the B3LYP/6-31G* and HF/6-31++G** levels of theory. A tetrahedrally coordinated zinc atom in the transition state is found to give energy differences that are most consistent with the observed results. The tetrahedral zinc atom with the coordination of a solvent molecule also allows the staggering of the bonding sp2 carbons, which avoids the eclipsing of the aldehyde and the allenyl substituents. A fluoride ligand is used to mimic the methanesulfonate anion present in the reaction, and a methyl ether molecule is used to explicitly mimic the solvent molecule THF. The allenyl carbons in the transition states are slightly bent, despite the fact that both the starting allenylzinc reagent and the propargylic product are linear. The transition state for allenylzinc addition to acetaldehyde is “late” in comparison to cyclic transition states of similar type.
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