RING SIZE LIMITS FOR THE DIRECT OBSERVATION OF HYDRIDO-BRIDGED SECONDARY CYCLOALKYL CATIONS - CORRESPONDENCE WITH CHEMICAL-REACTIVITY STUDIES

The direct observation of stable hydrido-bridged cycloalkyl cations, in rings containing 8-11 carbon atoms, corresponds exactly with those cycloalkyl rings where direct transannular hydrogen shifts have been observed in reactions involving a presumed carbocation intermediate. Within this observable ion series, there are interesting variations in both the ion reactivity and in the NMR properties. For rings of 8, 9, and 11 carbons, the corresponding 1-methyl 7-, 8-, and 10-carbon tertiary ion is formed, whereas the 10-membered hydrido-bridged ion eliminates hydrogen and forms the decalyl cation. The NMR results show that decreases in the chemical shift of the two terminal hydrogens are matched by a corresponding increase in the chemical shifts of the bridging hydrogen. This behavior is consistent with theoretical models which crudely picture the bonding as a hydride ion centered between two cationic centers. The negative charge on the bridging hydrogen increases as the separation of the two bridged carbons is increased. Correspondingly, the terminal protons experience an increased positive charge. © 1979, American Chemical Society. All rights reserved.