Host-Guest Complexation. 52. Bridged and Chiral Hemispherands

John A. Tucker, Carolyn B. Knobler, Israel Goldberg, Donald J. Cram*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

The synthesis, binding properties, and configurational stability of 10 new chiral hemispherand hosts containing the quaterphenyloxy framework are reported, as well as the crystal structures of free hosts 1, 3, 4, and 10 (Chart I) and six complexes of 1. Hosts 1 and 2, although chiral and strong binders, are configurationally unstable due to the relatively low barriers to ring inversions of their CH3OC6H3and EtOC6H3parts at ordinary working temperatures. Host 3 is configurationally stable due to the inability of the PhCH2OC6H3parts to undergo ring inversion. This host is a relatively poor binder in CDCl3at 25 °C of Li+, Na+, K+, Rb+, Cs+, NH4+, CH3NH3+, and (CH3)3CNH3+picrates, in spite of the fact that its crystal structure shows it to be beautifully organized with all oxygens turned inward and all benzyls turned outward. Hosts 4-6 contain extra bridges in addition to the parent 18-membered macrorings. They are configurationally stable and are relatively strong binders. The presence of CH3groups in the four positions para to the quaterphenyloxy moiety in 5 produced an increase in binding compared to 4 without the CH3groups from 0.4 to 1.8 kcal mol-1(Δ(-ΔG°) values), depending on the guest. A crystal structure of 4 showed all of its oxygens' unshared electron pairs to be turned inward. Host 5 was obtained in an enantiomerically pure state. Hosts 5 and 6, whose oxygen-bridging units are CH2CH2CH2and 1,2-CH2C6H4CH2, respectively, provided similar -ΔG° values for binding the eight cations. Host 7, containing three chiral elements, was found to undergo ring inversion of its four EtOC6H3groups at working temperatures, whereas 8, containing four C6H5CH2OC6H3groups, was configurationally stable. As with 3, the presence of the C6H5C-H2OC6H3groups markedly depressed its binding properties. Without exception, peak binding for the 18-membered ring hosts (1-8) involved NaPic as guest. Hosts 9-12 all have 21-membered macrorings incorporating a quateraryloxy and either a cis-2,5-tetrahydrofuran or a 2,6-pyridine ring bridging unit to give seven heteroatom binding sites. The crystal structure of free 10 shows it is not highly preorganized for binding. Compounds 11 and 12 are diastereomers, which combine the chiral elements of the cis-2,5-tetrahydrofuran unit and an additional 1,2-CH2C6H4CH2oxygen-bridging unit. Hosts 9,11, and 12 show peak binding with KPic and 10 with CsPic. The highest Ka(Na+)/Ka(K+) ratio (150) was observed for 7, and the highest Ka(K+)/Ka(Na+) ratio (2500) was observed for 8 (Kaare association constants, M-1). These hemispherands are better binders of cations than the corands but poorer than the spherands, cryptands, or cryptahemispherands.

Original languageEnglish
Pages (from-to)5460-5482
Number of pages23
JournalJournal of Organic Chemistry
Volume54
Issue number23
DOIs
StatePublished - 1 Nov 1989
Externally publishedYes

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