Homotropylium

Homoaromatic Compounds. When cyclooctatetraene is dissolved in concentrated H2SO4, a proton adds to one of the double bonds to form the homotropylium ion (107). In this species an aromatic sextet is spread over seven carbons, as in the tropylium ion. The eighth carbon is an sp carbon and so cannot take part in the aromaticity. The NMR spectra show the presence of a diatropic ring current H/, is found at 5= - 0.3 at 5.1 6 Hj and H7 at... 

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... 

The crystallographic data for [16] are in excellent agreement with the analogous interatomic distances and angles calculated (Haddon, 1979b) for the parent homotropylium ion [12] and fully support the homoaromatic... 

Another measure of the homoaromatic stabilization of the homotropylium systems can be gained from the heats of protonation of the ketones [17]—[20] (Childs et al., 1983). The difference in heats of protonation between [18] and [19] (A//I8 i9) is significantly larger than the corresponding difference between [17] and [18] (AHl7 ls). This increase in stabilization is associated with the homoaromaticity of [21]. Similarly, there is a large discontinuity between A//17 18 and A//18 2o which is associated with the aromaticity of the tropylium ion [22]. 

The foregoing briefly summarizes the extensive literature on homotropylium ions up to the time of Childs (1984) review. There follows a selective review of the literature in this area since that time. 

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). 

Homoaromatic compounds. When cyclooctatetraene is dissolved in concentrated H2S04, a proton adds to one of the double bonds to form the homotropylium ion 94.247 In this species an aromatic sextet is spread over seven carbons, as in the tropylium ion. The... 

Childs and Winstein also showed that irradiation of 59 led to a clean photoisomerization and the formation of a photostationary state consisting of 58 and 59139 l41. Thus, in contrast to the homotropylium or homocyclopropenium ion systems, the open and closed forms of these 4 n systems are intercon verted in the first excited rather than ground state. 

Other noteworthy achievements of solid-state 13C CP/MAS NMR in the context of carbonium ions are that (1) the sec-butylcarbonium ion can be identified at low temperatures in a sec-butyl chloride/antimony pentafluoride matrix in the temperature range 80-190 K (2) the norbornyl carbonium ion has been characterized (356,396) at temperatures down to 5 K, there being a strong (but not yet incontrovertible) indication that the controversial non-classical ion (398) exists and (3) the homotropylium ion is best represented (399) by the completely delocalized (homoaromatic) seven-membered state (a below) rather than the incompletely delocalized state (b). 

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. 

Attempts to observe circumambulatory rearrangement in the 2,6-unri-tricyclo [5.1.0.03,5]octane-2,6-diyl dication 178 have been unsuccessful.397 The dication 178 would appear to rearrange instantaneously to the homotropylium ion 179 by proton elimination. However, substituted dications of type 178 (e.g., 180) are quite stable they are static, and a substantial part of the charge is delocalized into the cyclopropane rings. 

The highly reactive bridged homoazulene could be easily protonated to yield the homotropylium ion 221 [Eq. (3.53)].465... 

Since protonation of cyclooctatetraene is known to yield the homotropylium ion (see Section 3.5.3.1), Schroder and co-workers474 reasoned that the homo[15]annu-lenyl cation 229 can be formed by the protonation of the [16]annulene (Scheme 3.7). 

Other examples of monohomoaromatic cations are the 6ji homotropylium ion 179 of Pettit1020-1022 and Winstein1023 1025 and the previously discussed 2jt homoaromatic ll-methyltricyclo[4.4.1.01,6]undec-ll-yl cation 46.163... 

A DFT study of the 2 + 2/4 + 2-cycloaddition of benzyne with thiophene to produce 1-naphthyl- and 2-naphthyl phenyl sulfides led to a modification of the originally proposed reaction pathway.1 The reaction of an E-Z mixture of 7-benzylidenecycloocta-1,3,5-triene (1) with TCNE yields a mixture of 2 + 2- [(2) and (3)], 4 + 2- [(4) and (5)], and 8 + 2- (6) cycloadducts. The presence of pentadienyl and homotropylium zwitterions accounts for the products isolated (Scheme l).2... 

It is possible to detect thermally induced circumambulations in certain substituted homotropenylium ions. Hehre in his theoretical study (HF/STO-3G) of 11 suggested that the placement of methyl substituent at C(8) of the homotropenylium ions should reduce the barrier to the thermally induced circumambulation" . The correctness of this prediction was confirmed by Childs and Varadarajan who reported the generation of the deuterium labelled cation 67 and the measurement of the rate of deuterium scrambling associated with circumambulation of C(8) around the seven-membered ring (Scheme 24) 58 barrier for this circumambulation was found to be 14.5 kcalmoT. Scott and Brunsvold have also reported the occurrence of a circumambulation in a bridged homotropylium ion, 68 (Scheme 25). ... 

Another possible structure for [407] has been proposed by Leone et al. (1973) and is represented by the homotropylium ion structure [411]. Dewar... 

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