7-Norbomadienyl cation

The 7-sila-7-norbomadienylcation congener, 4, has a bent structure similar to the 7-norbomadienyl cation (Fig. 19). The silicon chemical shift is strongly shielded in the equilibrium C, structure, but it is deshielded in the Qv transition state 5 (while the chemical shifts of transition structures cannot be observed, the calculated values provide useful interpretive information [49]). The relative stability of 4 vs the 7-norbomadienyl cation is 15.1 kcal mof at MP2-FC/6-31G according to Eq. 3. 

However, two compounds which are potentially within this definition have been prepared and their spectra obtained (80 and 81, see Table 16). The 7-norbomadienyl cation, 80 220,227), definitely prefers not to avail itself of bicycloaromatic stabilization of the bridge carbonium ion by "both double bonds, as the expected C211 symmetry is ruled out by the n.m.r. spectrum which in fact resembles that of the 7-norbomenyl cation, 71. Thus, bishomoconjugation only is observed, and the barrier to bridge flipping is estimated to be >19.6 kcal/mole 228). 

The high reactivity of the 7-chloronorbomadiene 237 indicates high stabilization of the transition stete. The high activity of chloride 237 as compared with chloride 239 may be due to the bicycloaromaticity of the 7-norbomadienyl cation which can be regarded as an aromatic Mdbius structure (4rt-electrons) ... 

As noted earlier, in exceptionally weakly nucleophilic media the NMR method is used to observe directly many nonclassical ions — intermediates postulated in explaining unusual rates, products and stereochemistry of the above solvolysis reactions. This enables research under stable-ion conditions may result in dis-coverii new, earlier unknown kinds of carbocation rearrangements illustrated by the 7-norbomenyl and 7-norbomadienyl cations. 

To determine the value of the energetic barrier of the degenerate rearrangement of the bridge-flipping type Winstein generated the 7-norbomadienyl cation from... 

The most stable of all the methyl-substituted norbomadienyl cations is the 2-methyl 440, the least stable the 1-methyl ion 444. As for the 7- and 5-methyl ions 439 and 445, their relative stability is still unclear. 

The quantum chemical calculations for the 7-norbomadienyl ion have confirmed its unsymmetrical bishomoaromatic structure. The first calculations for this ion were made as early as 1963 but this was done by using the method LCAO MO in the n-electron approximation at the same time the nondassical electron delocalization in the 7-norbomadienyl cation does not obviously have the 7c-character, but is intermediate between jt- and a-delocalizations. More reliable, therefore, are the Jesuits obtained in 1971-1972 by the CNDO/2 and the MINDO/2 method with all the valent electrons taken into account. According to these calculations, the positve charge is mainly concentrated on C, C, but not on C and C ... 

Summary The synthesis of intramolecularely Jt-stabilized silylium ions such as 7-sila-norbomadienyl- and 2-silanorbomyl cations was attempted. While benzo-7-sila-norbomadienyl cation 7 could not be detected at room temperature in hydrocarbon solvents, its donor-acceptor complex 8 was identified by NMR spectroscopy. In contrast, persistent 2-silanorbomyl cations could be synthesized and characterized by NMR spectroscopy and modem quantum chemical methods. Silanorbomyl cations 5 and 25 are stable at room temperature and have a bridged stmcture in which the silicon adopts a [3+2] coordination. 

In 1960, Winstein reported a record-breaking anchimerically assisted solvolysis rate acceleration 7-norbomadienyl precursors reacted lO times faster than their saturated 7-norbomyl analogs [47]. The cation favors a C, rather than a Qv structure, [47, 48] since interaction between the cationic center and... 

The carbonium centers in the norbomyl cation, 7-norbornenyl cation and 7-norbomadienyl cation10,207 are similar and related to the ethenemethonium ion,... 

Chelation v. cyclometalation in a cationic dipyridylpyridazine-Rh(I) complex. Milstein and co-workers reported the results of a study of the interaction of rhodium norbomadiene tetrafluoroborate (Rh(NBD)2) with the tetradentate ligand 3,6-bis-(2-pyridyl)pyridazine (DPPN) (241). Mononuclear complexes, 242, were obtained quantitatively. Treatment of the complex of DPPN Rh(NBD)2BF4 with a second equivalent of the metal precursor (Rh(NBD)(CH3CN)2)BF4 led to a dinuclear complex, 243, that involves a unique rearrangement of the norbomadienyl moiety. 

For the 7-norbornadienyl cation 236, various formulae were suggested from chemical data. However, the first PMR study of 7-norbomadienyl fluoroborate obtained from 7-chloronorbornadiene showed this salt to be unsymmetrical because the PMR signals showed four types of protons (2 2 2 1). Such a spectrum corresponds either to formula 428 or to 226. 

The a- and P-protons of a-cyclopropylcarbinyl ions have close chemical shifts (t 7). The signal of H (x 3.19) for ion 440 is shifted 3.4 ppm to a lower field than that of H and 3 ppm to a lower field than the a-proton of cyclopropylcarbinyl ions Thus, one can add the spectral data to the above-discussed chemical and stereochemical reasons against considering norbomadienyl ions as rapid equilibria of tricyclic cations. 

Subsequently Winstein directly observed 1-methyl-444 and 5-methyl-substituted 445 norbomadienyl ions, to elucidate the rearrangement mechanism of ion 439 into ion 440. At —105 °C cation 446 is rearranged into a mixture (55 45) of 1-methyl-444 and 5-methyl-cation 445. At —75 °C ion 444 is rearranged irreversibly into ion 445 which at —45 °C is decomposed but not converted into ion 440. Ion 445 was obtained from another precursor — quadricyclanoi 447 with intermediate rearrangement of the 7-methyl-7-quadricyclyl ion. TTie structure of ions 444 and 445 is established by PMR analysis and by conversion of ion 445 into syn-methyl ether 448 which is different from the anti-isomer 441 formed from ion 440. 

Nishikubo, T., Kameyama, A., Kishi, K., Kawashima, T., Fujiwara, T., and Hijikata, C., Synthesis of new photoresponsive polymers bearing norbomadiene moieties by selective cationic polymerization of 2- [ [ (3-phenyl-2,5-norbomadienyl)-2-carbonyl] oxy] ethyl vinyl ether and photochemical reaction of the resulting polymers. Macromolecules, 25,4469-4475,1992. 

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