2- Norbomyl cation stability

The problem of norbomyl cation stabilities vs. solvolysis rate discrepancies in the norbomyl system has been addressed in an important paper.159 The classical and non-classical norbomyl cations do not resemble the 2-endo- and 2-exo -norbomyl solvolysis transition states very closely. The authors conclude that Brown was wrong, but that Winstein was not entirely right either.159 A substituent in the benzene ring has little effect upon the kinetics of the acid-catalysed hydrolysis of 2-exo-norbomyl phenyl ether.160 The FTIR spectra of matrix-isolated 2-methylbenzonorbomen-2-yl cations have been examined at —196 °C the structure can best be represented as (108), rather like a phenonium cation, but at higher temperatures a transition takes place to a structure that is more nearly represented as (109), with some 71-bridging.161 The stereoselectivities of some 7-methyl-7-norbom(en)yl cations have been investigated (110) has a classical structure and reacts in a stereo-random manner, whereas (111) is... 

The description of the nonclassical norbomyl cation developed by Wnstein implies that the nonclassical ion is stabilized, relative to a secondary ion, by C—C a bond delocalization. H. C. Brown of Purdue University put forward an alternative interpreta-tioiL He argued that all the available data were consistent with describing the intermediate as a rapidly equilibrating classical ion. The 1,2-shift that interconverts the two ions was presumed to be rapid relative to capture of the nucleophile. Such a rapid rearrangement would account for the isolation of racemic product, and Brown proposed that die rapid migration would lead to preferential approach of the nucleophile fiom the exo direction. 

These results, which pertain to stable-ion conditions, provide strong evidence that foe most stable structure for foe norbomyl cation is foe symmetrically bridged nonclassical ion. How much stabilization does foe a bridging provide An estimate based on molecular mechanics calculations and a foermodynamic cycle suggests a stabilization of about 6 1 kcal/mol. An experimental value based on mass-spectrometric measurements is 11 kcal/mol. Gas-phase Itydride affinity and chloride affinity data also show foe norbomyl cation to be especially stable. ... 

X-r crystal structure determinations have been completed on two salts containing bicyclo[2.2.1]heptyl cations (Fig. 5.12). Both are more stable than the 2-norbomyl cation itself 18 is tertiary whereas 19 contains a stabilizing methoxy group. The crystal structure of 18 shows an extremely long (1.74 A) C—C bond between C-1 and C-6. The C(1)—C(2) bond is shortened to 1.44 A. The distance between C-2 and C-6 is shortened from 2.5 A in norbomane to 2.09 AThese structural changes can be depicted as a partially bridged structure. 

No such stabilization is present in the 1,2-dimethyl-2-norbomyl cation and the proton spectrum does not permit a choice because the exo- and enrfo-6-protons exhibit a common peak. The methyl groups also afford a single non-resolvable b[Pg.219]

Proton and C-nmr, ESCA, and Raman studies provide a wealth of information which unfortunately is not subject to a unique interpretation. The main conclusion to be drawn therefore is that the structure of the solvent stabilized cation is still unproven. Gas phase estimates of the heat of formation of the norbomyl cation imply a rather marked stability of the stmcture relative to other secondary ions (Kaplan et al., 1970). When combined with other estimates of the heat of formation of the t-butyl cation, however, these data suggest that hydride transfer from isobutane to the norbomyl ion will be endothermic by 6 to 15 kcal mole . This is contrary to experience in the liquid phase behaviour of the ion, and the author s conclusion that their observation of enhanced stability is evidence of stabilization by bridging deserves further scmtiny. 

In a subsequent calculational study [HF and MP2 levels of theory with 6-31G(d) basis set and MP4(SDQ)], Szabo and Cremer849 explored the he C7Hn+ potential energy surface. Cation 487 (tricyclo[4.1,0.01,3]heptyl cation), the protonated ethano-bridged derivative of spirocyclopentyl cation, was considered to be the missing link between the bicyclo[3.2.0]hept-3-yl cation 488 and the 7-norbomyl cation 489. It is a kinetically stabilized species separated from cations 488 and 489 by 18.9 and 15.9 kcal mol-1, respectively. 

If the classical structure were correct, the 2-norbornyl cation would be a usual secondary carbocation with no additional stabilization provided by c-delocalization (such as the cyclopentyl cation). The facts, however, seem to be to the contrary. Direct experimental evidence for the unusual stability of the secondary 2-norbomyl cation comes from the low-temperature solution calorimetric studies of Arnett and Petro.75 In a series of investigations, Arnett and Hofelich76 determined the heats of ionization (AHi) of secondary and tertiary chlorides in SbF5-SC>2ClF [Eq. (3.131)] and subsequently alcohols in HS03F-SbF5-SC>2ClF solutions [Eq. (3.132)]. 

Gas-phase mass spectrometric studies891-894 also indicate exceptional stability of the 2-norbomyl cation relative to other potentially related secondary cations. A study by Kebarle and co-workers895 also suggests that the 2-norbornyl cation is more stable than the tert-butyl cation in the gas phase (based on hydride transfer equilibria from their respective hydrocarbons). 

MP2-FC/6-31G calculations reveal the symmetrically bridged 6-sila-2-norbomyl cation 1 not only to be a local minimum (Fig. 18), but also to be 17.2 kcal mof more stable than the 2-norbomyl cation (Eq. 3) at MP2-FC/6-31G + AZPE(SCF/6-31G 0.89). However, the inherently greater stability of silyl cations contributes to this difference. The Si NMR chemical shift of the bridging silicon atom, ca 1 ppm vs TMS (IGL0/H //MP2-FC/6-31G ), is very strongly shielded in comparison with ca 300 ppm expected for a free RSiH2 species [38]. Thus, the sila congener of the 2-norbomyl carbocation also possesses a nonclassical stracture which is reflected by its stracture as well as its NMR properties. 

Calculations by the MlNDO/3 method showed the relative stability of the classical and nonclassical 2-norbomyl cations consequently the classical ion is more stable than the nonclassical one by 2 kcal/mole. The application of this method in calculating the norbomane formation heat led to an error of 20.8 kcal/mole ... 

Several publiscations 279) support the stability of the 2-norbomyl cation in the... 

In 1980 Arnett determined the heat of isomerization of the secondary 4-methyl-2-norbornyl cation to the tertiary 2-methyl-2-norbomyl ion in SbFj— SOjFQ in such an experiment the stabilities of neutral molecules are of no significance. R rrangement of a 4-methyl-2-norbomyl cation into a 2-methyl-2-norbomyl releases 6.57 0.41 kcal/mole in contrast, rearrangement of sec-butyl to tert-butyl ion releases 14.20 + 0.60 kcal/mole. Thus, the secondary 2-norbomyl ion is more stable than the usual s ondary ions by 7.5 kcal/mole the 4-methyl group is assumed to exert an insignificant effect on the charge at C. Also the ionization heats (AHj) of 2-exo-norbomyl chloride and 4-methyl-2-exo-norbornyl chloride into the respective secondary ions are very close to each other —23.16 + 0.43 and —22.20 0.49 kcal/mole at —100 °C. All these data indicate a specific stabilizing effect in the secondary 2-norbomyl ions. 

In 1981 Farcasiu applied the molecular mechanics technique to calculate the relative stability of secondary methyl-2-norbomyl cations and showed the most stable isomers to be those with methyl in the bridge-head (cf. Therefore he, as well as Schleyer substituted Arnett s models by others deprived of secondary effects ... 

The stable 2-norbomyl cation has recently been shown to be a non-classical, unusually stabilized species. Olah et at. (1970) proved spectax>-scopically that this ion is a comer-protonated nortricyclene with a pentavalent carbon atom. The value for the carbonylation-decarbonyla-tion equilibrium constant K (= of the 2-norbomyl ion illustrates... 

Muchall and Werstiuk performed an extensive AIM study on the structure of the l,2,4,7-anft-tetramethyl-2-norbomyl cation. The topology confirms a hyper-conjugatively stabilized 2-norbomyl cation, which is not bridged and does not show a 3c-2e bond as had been reported. This study clearly shows that it is necessary to consider the full information available from the charge density if the goal is to obtain information about the structure of a molecule. 

X-ray crystal stmcture determinations have been completed on two salts containing bicyclo[2.2.1]heptyl cations (Fig. 5.9). Both are more stable than would be the 2-norbomyl cation itself 18 is tertiary whereas 19 contains a stabilizing methoxy... 

Decades of extensive research followed, fueling controversy on the subject. In 1983, Olah, Saunders, and Schleyer were able to identify the intermediate as indeed the methylene-bridged nonclassical carbonium form of the norbomyl cation. This was demonstrated through extensive spectroscopic analysis, including H- and C-NMR, Raman, ESCA, and further physical and kinetic studies. Molecular mechanics and thermodynamic cycles exhibited a stabilization energy of 6 1 kcal/mol for the a bridging feature of the nonclassical ion. 

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