Homotropylium cations

The cyclobutenyl cation (92) and the homotropylium cation, CgHcf 93 are both prototypes of homoaromatic systems. 

The cyclobutenyl cation 92 is one of the first examples demonstrating that electron correlation is required both for geometry optimization and NMR chemical shift calculations.14 The IGLO/6-31G(d,p) calculated 13C NMR chemical shifts of the planar form of a homotropylium cation 94 clearly deviate from the experimental values (mean deviation A = 45.6 ppm) alternating in the seven-membered ring between 122 and 194 ppm, whereas those of the non-planar structure for the homotropylium cation 93 are in good agreement with experiment (mean deviation A = 6.2 ppm).108... 

Following on directly from the suggestion of Childs et al. (1986a) that the 1-ethoxy-substituted homotropylium ion is not homoaromatic, Scott et al. (1986) presented NMR evidence for the diminution in homoaromaticity through a series of hydroxy-substituted homotropylium cations. They correlate the observed decrease in chemical shift difference between the H(8) (exo) and H(8) (endo) protons (AS) with a diminishing ring current and hence reduced homoaromaticity. The parent homotropylium cation [12] is considered to be the most homoaromatic and the order of homoaromaticity for monohydroxy substitution, based on A8 values, is [12] 4-OH > 2-OH > 1-OH > 3-OH (where 1-OH, 2-OH, etc., refers to the position of hydroxyl substitution on [12]). 

There follows a brief and very selective review of some of the more important theoretical studies on the homotropylium cation. 

One further theoretical method that merits consideration at this point is the topological theory of molecular structure exemplified by Bader (1985, 1990). In this method a topological description of the total electron density in the molecule is used. A major advantage of this method is that it allows the total interaction between various centres to be probed. Cremer et al. (1983) used the Bader method to examine the homotropylium cation [12] and concluded that it was indeed homoaromatic. 

There has been much debate over the nature of the bicyclo[5.4.1]dodecapentaenylium cation is it best represented as a benz-annelated homotropylium cation [65a] or a perturbed [1 l]annulenium cation [65b] The X-ray structure was determined (Destro et al., 1976), and from these data it was concluded that the perturbed annulenium cation picture [65b] provided the most accurate representation of the system. 

By analogy with the homoaromatic homotropylium cation, a prototype anionic system would be the homocyclopentadienyl system [138a]. However, Olah et al. (1978) demonstrated by H and 13C NMR that this species is not the homoaromatic [138a] but exists in the form of the planar [138b]. Theoretical MINDO/3 (Olah et al., 1978) and STO-3G RHF (Birch et al., 1980) results are in agreement with the experimental results. 

The presence or absence of a homoaromatic interaction is often based solely on the distance between the non-bonded atoms. Distances greatly over 2.0 A are thought to lead to a p-p overlap that is too small to make any significant contribution. This simplistic approach is not necessarily reliable as shown by Cremer et al. (1991). Their calculations on the homotropylium cation [12] indicate a double-minimum potential energy surface with respect to variations of the C(l)-C(7) distance at the Hartree-Fock level of theory. At the MP4(SDQ) level of theory, only a single-minimum curve was found with the minimum at 2.03 A. The calculated potential energy curves are quite flat in this region. 

All attempts of preparing the parent secondary dication (63), a t -tiicyclo(5.1.0.0 ) octa-2,6-diyl dication, were unsuccessful. The dication was originally anticipated to undergo the circumambulatory rearrangement, as in cyclopropylmethyl cation rearrangements However, it spontaneously rearranged into the thermodynamically more stable homotropylium cation (64). 

The product of this synthesis is an especially useful, highly functionalized hydroazulene that is not available commercially. We have used it as a synthetic precursor to homoazulene,5 and to a variety of homoazulene derivatives,6 bridged homotropylium cations,7 and azulene quinones.8 It could undoubtedly serve as a precursor to numerous natural products. The cyclization reaction tolerates electron-donating substituents3 9 but not halogens10 on the aromatic ring. 

In the course of an unsuccessful search for the tris-Paquette studied several biscyclopropylcarbinyl systems around a seven-membered ring. Some of their relative rate data for 3,5-dinitrobenzoate solvolyses in 80 %... 

Homoaromatic A compound that contains one or more sp hybridised atoms, usually carbon, in an otherwise conjugated cyclic system, e g. the homotropylium cation, C8H9+, in which there is one... 

This method allows generation and comparison of free homotropylium cations, e.g. 24, with the corresponding bicyclo[5.1.0]octadienylium-tricarbonyliron complexes, e.g. 25. ... 

The homotropylium cation, easily available to protonation of cycloocta-tetraene (121), has attracted considerable attention, particularly due to its non-classical homoaromatic structure (1). Two pathways can be discerned for the mutual interconversion of endo- into exo-80-8-d (122) (i) a conformational ring inversion passing through a planar classical cyclooctatrienyl cation, (ii) a walk rearrangement of the bicyclo[5.1.0] octadienyl cation formed as an intermediate, proceeding with retention at the migrating carbon atom C-8 (sr process) as postulated for an orbital symmetry controlled process (4). 

In the effort to prepare l,5-methano[10]annulene, elimination of HI from iodotetraene 83 was found to give the rearranged pentaene 84 (126). A reasonable explanation for its formation is a walk rearrangement of the intermediate 1,7-bridged bicyclo[5.1.0]octadienyl cation. In this case the walk may be favored by the steric effect of the three-atom linkage, which probably constrains the homotropylium cation to the closest bicyclo[5.1.0] octadienyl form. 

The influence of methyl substituents on the rates of the rearrangement predicted by ab initio MO calculations agrees with the experimental observations (114). Surprisingly, the calculations suggest the experimentally hitherto unknown stereochemical course of the thermal homotropylium walk to depend on the substitution pattern at C-8. As postulated by orbital symmetry considerations, the walk rearrangement in the parent homotropylium cation is predicted... 

Based on ab initio SCF calculations (114), the difference in the activation energies required for the two walk rearrangements might result entirely from these ground state differences between the bicyclo-[3.1.0]hexenyl and homotropylium cations. 

Treatment of 7,7-dimethoxycyclo-octa-l,3,5-triene with phosphorus pentachloride or thionyl chloride gave 7-chloro-7-methoxycyclo-octa-l,3,5-triene, which was unstable under the reaction conditions. When the reaction was carried out in liquid sulphur dioxide, the 1-methoxycyclo-octatrienylium cation (333), a homotropylium cation, was observed by n.m.r. ... 

Allylmethyl-Cyclopropylmethyl-Cyclobutyl Systems. The cyclopropylmethyl cation has been the subject of semi-empirical natural orbital analysis, and the bonding analysis supports the suggestion that there is delocalization of charge throughout the species. MINDO/3 geometries have been obtained for both planar and non-planar homotropylium cation, and the results of the study show the ion to be best represented as an open species and not as a delocalized bicyclo[5,l,0]octadienyl cation, as previously suggested. ... 

Scheme 5.1 Structure of the homotropylium cation, with resonance formulae...

Scheme 5.2 Homotropylium cation, with atom numbering scheme...

Table 5.5 Bond lengths, d, DTs, 5, and SF% values for symmetry-independent Cl-Cll (l,6-methano[10]annulene, CnHio) and C3-C2 (homotropylium cation, CgHg ) allylic bonds...

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