Constraining of small-ring cyclic ether triads by stereodefined spiroannulation to an inositol orthoformate platform. Solution- and gas-phase alkali metal binding affinities for three- to five-membered ring structural combinations.
The structural features most conducive to complexation of the alkali metal ions Li(+), Na(+), and K(+) in a series of constrained inositol orthoformate derivatives have been probed in solution, in the solid state, and in the gas phase by electrospray ionization mass spectrometry. The eight spirotricyclic polyethers differ in the size of the rings containing the potentially ligating oxygen atoms. Although the ring sizes have been limited to three to five atoms inclusively, the combinations of oxirane, oxetane, and tetrahydrofuran are rather extensive and consist of many options. The overall trend for lithium ion affinity is [5.5.5] > [ 5.5.4] > [4.4.4] > [5.5.3] > [5.4.3] > [4.4.3] > [1.1.1] > [3.3.1], an ordering that correlates with the differing polarizabilities of the oxygen atoms, ease of alignment of the nonbonded electron pairs, and the overall size of the ligand as gauged by nonbonded O......O distances.