Generation and thermal bond relocation of the cyclononatetraenyl cation

Abstract

Sutlzmary 9-Chloro-cis4-cyclononatetraene undergoes rapid solvolytic rearrangement to exo-l-chloro-cis-8,9-dihydroindene, via the cyclononatetraenyl cation. THE cis4-cyclononatetraenyl cation(1) is an interesting 877 monocycle, since its dimensions are such as to allow planar as well as puckered geometries. Recently Schleyer, Boche, and their co-workers' presented evidence, based on rate data and deuterium labelling experiments, that thermolysis of anti-9-chlorobicyclo [6,1,0]nona-2,4,6-triene in aqueous acetone proceeds by what appears to be (1).We now report on the ready solvolytic generation of (1) from 9-chloro-cis4-cyclononatetraene (2a) .l 9-Chloro-cis4-cyclononatetraene (2a) rearranges cleanly and irreversibly to exo-l-chloro-cis-8,9-dihydroindene (3a) on mild heating in CDCl,, the conversion being characterized by t)(Oo) ca. 70min (AF: ca. 21 kcalmol-'). The same bond relocation (2a) -+ (3a)f also obtains in SO, although far more rapidly, e.g., t i ( -66') < 10 min, (AF* < 15 kcal mol-l).: The greatly enhanced rate of this conversion in the more ionizing medium (SO,) raised the question of whether rearrangement here might not be chiefly activated by initial ionization of (2). In order to test this possibility it was necessary to assess the thermal fate of appropriately labelled (2). We therefore prepared the monodeuterio-compound (2b) by exposing anti-9-chloro-7 Significantly, we find anti-9-chlorobicyclo[6,1,0]nona-2,4,6-triene to be inert in SO, a t -20".

Cp*3Al5I6: An Intermediate in Reactions Leading to Elemental Aluminum and AlIII Species?

“…Ninefold repetition of the enantiomerization results in complete distribution of a deuterium label in 4. In conclusion, the experimental findings reported nearly three decades ago [4,5] are explained by assuming that the (CH) 9 intermediates were 4n-electron Möbius aromatic systems. Our prediction that 4 is the most stable monocyclic (CH) 9 cation might be verified by applying modern experimental techniques such as laser flash photolysis, which has been employed to observe short-lived carbocations.…”

“…[4,5] In summary, the geometric and magnetic criteria (NICS and L) exhibited by 4 now reveal this species to be the first Möbius aromatic system in the Heilbronner sense, for which there is experimental evidence. [4,5] Without such evidence, the nature of 4 was not recognized originally. Furthermore, early speculations were incorrect: While conformation 6 is avoided, transition state 7 is not high in energy.…”

“…This cation reacts to give the observed cis-8,9-dihydroindenes 3 (X OH or Cl). [4,5] In summary, the geometric and magnetic criteria (NICS and L) exhibited by 4 now reveal this species to be the first Möbius aromatic system in the Heilbronner sense, for which there is experimental evidence. [4,5] Without such evidence, the nature of 4 was not recognized originally.…”

“…Crystals of 1 thus obtained contain toluene such that the formula may be written as 1´0.5 C 7 H 8 . The X-ray structure analysis [5] at À 73 8C shows that 1 has a cagelike framework comprising five Al and two I atoms, in which the aluminum atom Al(1) is coordinated in a tetrahedral fashion by two iodine atoms (Al(1) ± I(1) 257, Al(1) ± I(2) 254 pm) and two Cp*-substituted aluminum atoms (Al(2), Al(3); Al ± Al 252 ± 253, Al ± C 214 ± 224 pm) (Figure 1 a). The iodine atoms I(3) and I(4) lie at a considerably larger distance from Al (1) [6] In agreement with the average oxidation number 1.8 for aluminum, the Al ± Al and Al ± I distances in 1 resemble those in donor-stabilized aluminum(ii) diiodides.…”

Möbius Aromaticity and Delocalization

“…A nine-membered ring appears to be the first realistically sized system that can accommodate both a transoid alkene and the induced ring strain without orthogonalizing the 2p overlaps. In 1972, Anastassiou and Yakali had reported their speculations on why the charged 4 n −π nine-membered ring annulene 2 forms and scrambles 2 H so easily with an explanation couched in terms of a nonplanar C 2 symmetric conformation avoiding the destabilizing effects of anti-aromaticity. This argument was in 1998 replaced by a proposal for a stabilizing effect by, as it happens, the original discoverers of this cation.…”