A Computational Study of Cyclic Alkynes and the Search for Selectivity in the Cycloadditions of Substituted Cyclohexynes
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In this work, the geometric and electronic properties of five- to twelve-membered cyclic alkynes have been studied computationally. The data reveal two regimes that have a transition point at a ring size of eight. The data suggest that as the ring size decreases, the occupancy of the π orbitals contributing to the triple bond decreases while the occupancy of the σ bond increases. The difficulties in organic synthesis with extremely reactive molecules such as cyclohexyne are twofold. They readily engage in unwanted side reactions, and substitution on the molecule imparts little selectivity in its reactions. A major side reaction with cyclohexyne is its tetramerization, and density functional calculations indicate that this occurs through a reaction of cyclohexyne with an anionic starting material. The regiochemistry of [4+2]-cycloadditions of many substituted cycloalkynes has been studied. While in most instances selectivity would be negligible, the calculations have identified selective reactions involving 3,3-dimethylcycloalkynes and 2-tert-butylfuran.