Trishomocyclopropenyl cation

The parent trishomocyclopropenyl cation [47], first proposed by Winstein et al. (1959), has been invoked as an intermediate in various solvolysis studies, observed under stable ion conditions by NMR spectroscopy, and studied theoretically (see Story and Clark, 1972 Paquette, 1978). Similarly, the related ions [48]—[51] have all been advanced as trishomoaromatics (see Story and Clark, 1972 Paquette, 1978). 

Olah et al. (1979) reinvestigated the preparation of [47] (and some alkyl and aryl substituted analogues) from the superacid treatment of alcohol [54] or chloride [55], In contrast with earlier studies, the alcohol [54] clearly gave the desired trishomocyclopropenyl cation [47] and not the previously observed cyclopentyl ion [56], Olah et al. (1979) attributed this difference to the highly purified (freshly distilled) SbF5 used in their experiments. These new data and the 13C NMR spectrum of [47] provide strong support for its classification as a trishomoaromatic. 

Prakash et al. (1985) used the deuterium isotope effect on the l3C NMR spectrum of [47] to provide further evidence for the symmetrical, homoaromatic nature of this ion. They prepared the specifically deuterated trishomocyclopropenyl cation [57] by superacid treatment of the corresponding alcohol [58]. The 13C NMR spectrum of [57] displayed a triplet for the deuterated methine only 0.2 ppm to higher field than the undeuterated methine, indicating only an isotopic perturbation of resonance and not a rapidly equilibrating classical ion system (see Siehl, 1987). 

The initial report by Winstein and coworkers " in 1959 that cis-3-bicyclo[3.1.0]hexyl tosylate (108) undergoes solvolysis with cyclopropyl participation to give a homoaromatic trishomocyclopropenyl cation (109), opened up a large new area of... 

The proposal that cyclopropyl participation in solvolysis of the cis-3-bicyclo[3.1.0]hexyl tosylate (108) proceeds via a trishomocyclopropenyl cation intermediate (109) has been firmly established by product, deuterium scrambling and salt effect studies However, because the cyclopropyl participation in this system requires an unfavorable conformation, it is not apparent from simple kinetic studies. Thus, the cis-3-bicyclo[3.1.0]hexyl tosylate (108) has an acetolysis rate which is only nine times faster than that of the non-participating trans isomer, 110. However, when the 3-bicyclo[3.1.0]hexyl moiety is incorporated into a tricyclic or tetracyclic structure which locks it into a chair form, dramatically large rate enhancements are found. Some examples from the literature which are provided in the review by Haywood-Farmer are given below. These... 

Since the publication of Haywood-Farmer s comprehensive review which covered the literature in detail up through 1972 and in part into 1973, the solvolyses of a number of additional cyclopropane-containing systems have been studied in which trishomocyclopropenyl cations were implicated or discussed. Thus, Masamune and coworkers and Gassman and Creary " " studied the solvolyses of the exo- and endo-anti-tricyclo[3.1.1.0 ]heptan-6-yl p-nitrobenzoate and tosylate (111 and 112) (Scheme 16). For these two compounds, the exo.endo rate ratio after correcting for different leaving groups and solvents was 56 1. From looking at the products and from consideration of the relative rates of the non-cyclopropanated materials = 10 ) it was proposed... 

Kuezynski and Walkowicz have described an azide synthesis in which the trishomocyclopropenyl cation (19) is formed in the rate-limiting step of an 51,1 reaction of the cw-/ -toluenesulphonate (20). In the presence of azide ion the product was a mixture of the cis and tram azides, 21 and 22, and 3-azido-m-2,2-dimethylbicyclo[3,l,0] hexane (23), which were identified after reduction to the amines. The franj-/>-toluenesulphonate (24), however, afforded only the inverted azide (21) (equation 25), which seems surprising in view of the... 

The trishomocyclopropenyl cation derivative 229 has been investigated by both solvolytic and stable ion studies. - The and NMR data could be best explained with the intermediacy of the bishomopyramidal ion 230. Although no conclusive distinction could be made between a rapidly equilibrating system, structure 230 was preferred based on related MINDO/2 calculations. This has recently been confirmed on the basis of high-level computational studies. It was found that cation 229 is not an energy minimum on the potential energy surface and converges upon optimization into structure 230 (MP2/cc-pVTZ level). 

Winstein suggested 1959 that the acetolysis of cis-bicyclo[3,l,0]hexyl tosylate SS9 involves the intermediate formation of a so-called trishomocyclopropenyl cation 700-702) equatorial isomer was found to solvolyse 50 times as fast as the axial 540 the quantitative product of the reaction is only cis-acetate 541 with about... 

Other examples of solvolysed 3-substituted cis- and trans-bicyclo[3,l,0]hexanes corroborate the intermediate formation of nonclassical trishomocyclopropenyl cations... 

The latter assumption is rejected by an experiment with a deuterated sample of tosylate 555- 2. The former assumption accounts for the racemization, but does not explain the exclusive formation of the thermodynamically unstable endo isomer resulting from a nucleophilic attack from a sterically unfavourable side. Thus the trishomocyclopropenyl cation 562 remains the most probable intermediate product. 

In 1972 Olah attempted to observe directly a trishomocyclopropenyl cation postulated in all the works described above. However, the ionization of cis-3-bicyclo[3,l,0]hexanol in FSOjH—SbFj—SOjFCl resulted only in a 3-methyl-cyclo-pentene-2-yl cation. The primarily formed bicyclic cation seems to be very unstable. 

Masamune et al. generated another stable trishomocyclopropenyl cation 611 with an ethane bridge the latter is obtained from 8-chlorotricyclo[3,2,l,0 ]octane 612 or from 4-chlorotricyclo[3,3,0,0 ]octane 613 in SbFj—SOjFCl. The data of the H and NMR spectra of ion 611 considering the peculiarities noted for ion 520 are used by the authors to confirm its nonclassical structure some contri-... 

Coates and Fretz have noted that tosylate 539 solvolyzes 100 times more slowly than the structurally similar tosylate cis-2-hydroxytetracyclo[4,3,0,0 , 0 ]-nonane 617a. The solvolysis of the latter, just as that of tosylate 559, is expected to involve the intermediate formation of the trishomocyclopropenyl cation. The authors have shown, howeverthat the acetolysis of the monodeuterated derivative 617b results in acetate 618 with the label so distributed as to suggest the intermediate formation of either a rapid equilibrium of two trishomocyclopropenyl cations or an ion of pyramidal structure. 

Pentacyclononyl and trishomocyclopropenyl cations and pyramidal dications The nmr spectrum of the 9-pentacyclo[4.3.0. 0 ]nonyl cation [118] (Coates and Fretz, 1975) shows only three peaks for the averaged C-6, 7, 9, C-1, 5, 8 and C-2, 3, 4 carbons, independent of temperature. Deuterium at C-9 is scrambled only among the three carbons C-6, 7, 9. This is consistent only with a static trishomocyclopropyl cation type structure. In the spectrum of the 9-deuteriated cation, the peak corresponding to C-6, 7, 9 is shifted less than 0.1 ppm, confirming the static symmetrical structure of the cation (Saunders and Kates, 1980). 

The isotope effect in the trishomocyclopropenyl cation [119] has been investigated (Olah et al 1985b). The virtual identity of the C nmr spectrum of 3-deuteriobicyclo[3.1.0]hexyl cation D,-[119] with that of the nonlabelled cation, except for a small (0.2 ppm — 80°C) upheld shift of the deuteriated methine triplet, is in accord with the symmetrical single minimum hypercoordinated structure of cation [119]. 

Analysis of the kinetics of cyclopropyl-assisted solvolysis of (133), using far i.r. and microwave information, indicates that reaction occurs in a highly excited vibrational state in which the molecule briefly has a chair-like conformation, possibly by way of the intermediate trishomocyclopropenyl cation (134). The /Cchair value agrees well with the observed rates of cyclopropyl-assisted solvolysis in rigid systems, for example (135) and (136). 

Buffered hydrolysis of the tosylate ester of the alcohol (137) is first order and affords only the em/o-alcohol (137). The intermediacy of the trishomocyclopropenyl cation (138) is suggested, which is in accord with the observed stereospecific solvent capture and the results obtained with the analogous 2-deuterio-tosylate ester. Hydrolysis of the exo-tosylate (139) under similar conditions proceeds more slowly and yields... 

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