Theoretical Studies of Nucleophilic Additions to Alkynes and Stabilities of Anions.

Abstract

Nucleophilic additions to alkynes were investigated using ab initio molecular orbital theory. Analyses of the energetic consequences of certain molecule distortions showed that the anti selectivity of nucleophilic additions to simple alkyl, aryl, and halogenated alkynes was consistent with trans bending of acetylene. This premise was verified by considering the addition of the hydride ion to acetylene which gave a single transition state resulting from anti addition. Comparing the activation energies for hydride addition to ethylene and acetylene showed that addition to acetylene improves relative to addition to ethylene with increasing basis set sophistication. Nucleophilic addition of ammonia to cyanoacetylene was also investigated to determine the variable stereochemical results experimentally found for these reactions. This study showed that syn and anti transition states and zwitterions existed for the addition of ammonia to cyanoacetylene. Anti addition has a smaller activation energies than syn addition. However, the syn zwitterion was more stable than the anti zwitterion. Thus, a kinetic product and a thermodynamic product are produced from this reaction. A series of (beta)-substituted vinyl anions were also considered theoretically for structural details and qualitative understanding of the types of interactions which control isomer stability. This study showed that (beta)-substituents stabilize the vinyl anion more than ethylene and lower the isomerization barriers for cis-trans isomerizations. Heteroallyl anions were also studied computationally. This studies showed that heteroallyl anions possess an intrinsic preference for the syn configuration. This preference was consistent with electrostatic interactions between the nitrogen lone-pair and the (pi)-charge on the carbon terminus. Lithiation and subsequent solvation using water as the solvent were also studied. These latter investigation showed that (pi) lithiation favored the anti configuration. While (sigma) lithiation and solvation reinstated the syn preference.