Homotropylium ions

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 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... 

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]). 

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... 

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). 

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... 

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... 

Oyclooctatetraene Mo(00)3 reacts with HOI to form a homotropylium ion complex ... 

Further examples of monohomoaromatic cations are the 27i-homoaromatic 11 -methyltricyclo[4.4.1.0 ]undec-l 1-yl cation 162 and the bir-homotropylium ion 163 of Cj symmetry generated and studied by Pettit et aL and Winstein et al. i... 

Although aromatic rings are normally planar, with uninterrupted conjugation, that is not always the case. When cyclooctatetraene is treated with acid, the homotropylium ion is formed. It has six electrons in a seven-membered ring that has a CHj group lying in a perpendicular plane, yet is aromatic. 

The generation of various substituted homotropylium ions is described 616-623) among them that of monobenzohomotropylium and dibenzohomo-... 

It is interesting to compare the properties of the above homotropylium ions, in which the breaking of the cyclopropane ring results in a homoaromatic system, with those of the bicyclo[3,l,0]hexenyl cations in these a similar breaking would result in a 4 71-electron antihomoaromatic system similar to the cyclopentadienyl cation (cf. >). 

Described subsequently were heptajnethyl- , hexamethyl-penta-methyl- , tetramethyl-and hydroxy-substituted 2-bicyclo[3,l,0]hexenylic cations In 1971 Berson generated the unsubstituted cation 491. In this ion the difference in the shifts of the methylene protons 5e <,o — turned out to be +0.30 ppm which is sharply different from A8 for the homotropylium ion 455... 

Hehre predicted by quantum chemical calculations that in the 8,8-dimethyl-homotropylium ion, contrary to the unsubstituted ion 455, the circumambulatory rearrangement is feasible. In 1978 Childs succeeded in generating 8,8-dimethyl-substituted ions and made the bridged atom migrate in them > ... 

The addition of bromine to cyclooctatetraene at —55° generates a stable, nonclassical homotropylium ion which is subsequently captured by Br stereo-specifically (39). The seemingly disconcerting fact is that both bromine atoms approach the olefin from the more hindered face yielding the less stable product in an a,a -diastereogenic reaction,... 

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