Cyclopentyne Explained

Cyclopentyne is a cycloalkyne containing five carbon atoms in the ring. Due to the ideal bond angle of 180° at each atom of the alkyne but the structural requirement that the bonds form a ring, this chemical is a highly strained structure, and the triple bond is highly reactive. The triple bond easily undergoes both [2+2] and [4+2] cycloaddition reactions.^[1] Unlike benzyne, which undergoes a [2+2] addition with loss of stereochemistry at the alkene partner, cyclopentyne reacts with alkenes with retention of geometry of the partner,^[2] an example of the relevance of orbital symmetry even for highly reactive structures.^[3] The structure can also form a π complex with lithium cations, which affects the cycloaddition reactivity.^[1] It can even interact strongly enough with copper species to form a novel type of metallacycle.^[4]

Notes and References

1. Competitive Intermolecular Pericyclic Reactions of Free and Complexed Cyclopentyne . John C. . Gilbert . Duen-Ren . Hou . J. Org. Chem. . 2003 . 68 . 10067–10072 . 10.1021/jo0352880 .
2. Competing Pathways in the [2+2] Cycloadditions of Cyclopentyne and Benzyne. A DFT and ab Initio Study . Ilker . Ozkan . Armagan . Kinal . J. Org. Chem. . 2004 . 69 . 16 . 5390–5394 . 10.1021/jo049542f .
3. Stereochemistry of the [2+4] cycloaddition of cyclopentyne . Gilbert . John C. . Hou . Duen-Ren . Tetrahedron . 2004 . 60 . 2 . 469–474 . 10.1016/j.tet.2003.11.003.
4. Complexes between cyclopentene and cyclopentyne derivatives with HCu and FCu. The importance of cyclization effects . International Journal of Quantum Chemistry . Arslancan . Serra . Lamsabhi . Al Mokhtar . Mó . Otilia . Yáñez . Manuel . 2017 . 10.1002/qua.25489 .