Alkynylcyclohexanol chairs and twist-boats: Co(2)(CO)(6) as a conformational switch.

Abstract

This study examines how complexation with dicobalt octacarbonyl influences the ring conformation of alkynylcyclohexanol derivatives. When the bulky cobalt-alkyne cluster forms on an axially positioned ethynyl group, a conformational ring flip occurs to place the cluster in the equatorial position. A 4-tert-butyl substituent prevents this inversion, locking the coordinated alkynyl group in its original orientation. For the trans-diaxial diol, complexation drives a chair-to-chair inversion placing both cluster units equatorially. The cis-diol isomers instead adopt twist-boat conformations: one exhibiting intramolecular hydrogen bonding between hydroxyl groups, the other forming intermolecular hydrogen-bonded networks depending on the silyl substituent. Eight cobalt cluster complexes were structurally confirmed by X-ray crystallography, and the synthetic utility of twist-boat conformers is discussed.

Mechanism

The steric bulk of the dicobalt-alkyne cluster drives equatorial preference in cyclohexane ring conformations; cis-diol substrates instead adopt twist-boat geometries that facilitate intramolecular or intermolecular hydrogen bonding between hydroxyl groups.