Norbomyl-norbornyl

Fig. 5.12. Crystal structures of substituted norbomyl cations. (A) 1,2,4,7-Tetramethylnorbomyl cation (reproduced from Ref. 154 by permission of Wiley-VCH). (B) 2-Methoxy-l,7,7-trimethyl-norbornyl cation (reproduced from Ref 155 by permission of the American Chemical Society).

The stable 2-norbornyl cation has recently been shown to be a non-classical, unusually stabilized species. Olah et al. (1970) proved spectroscopically 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... 

The value for K was determined to be 10 litre mole at 20°C, which is of the same order of magnitude as those for tertiary alkyl cations ((0-07 — 2) X 10 litre mole . Section II) and dramatically different from those for secondary alkyl cations (about 10 ° litre mole , calculated from Figs. 2 and 3). These data show that the 2-norbomyl ion is only 1-6 kcal mole less stabilized than, for example, the tertiary butyl cation and about 8 kcal mole" more stabilized than secondary alkyl cations. Another thermodynamic argument for the high stability of the 2-norbornyl ion in solution is found in the work of Amett and Larsen (1968)... 

These authors also investigated 2-norbornyl lithium and found a protodelithiation enthalpy of —222 5 kJmoP. They asserted that the norbomyl lithium was present in an exo/endo ratio of essentially 1 1. Equating Gibbs energy and enthalpy differences vida infra), we will assume there is a negligible difference between the enthalpies of formation of the isomeric norbomyl lithiums. From equation 10 with n-butyl lithium as... 

Besides the work done on solvolysis of 2-norbomyl compounds, the 2-norbornyl cation... 

Kirmse and Sollenbohmer69 studied the trifluoroethanolysis of the norbomyl p-nitrobenzoate 57, and observed the formation of the intermediate 58, followed by a 6,2 migration of silicon and Wagner-Meerwein shifts to give a mixture of products. The solvolysis of the norbornyl system 57 is accelerated by a factor of 3 x 104 compared to unsubstituted 2-norbornyl p-nitrobenzoate. 

Due to computational savings, the open and closed forms of the 6,6-dimethyl-6-sila-2-norbornyl cation, 42 and 43, the closed 5,6,6-trimethyl-6-sila-2-norbomyl cation (44) and their corresponding benzene complexes 45, 46 and 47, respectively, were optimized... 

The solvolysis of 252 is one of the rare examples for a norbornyl-norpinyl rearrangement. While the ew-trimethylsilyl brosylate 251 yields mainly substitution and elimination products 253-255 with an intact norbomyl framework, 252 gives nearly 86% of norpinene 256 (equation 39). The bis-(trimethylsilyl)substituted compound 257 gives almost exclusively the norpinene derivative 258 (equation 40). While the trimethylsi-lyl group(s) in 252 and 257 exert no kinetic effect on the reaction rate, the /3-effect on the intermediate carbocations 252A and 259, respectively, determines the product distribution99. 

The methods that were developed in the early 1960s to generate and observe stable carbocations in low-nucleophilicity solutions18 were successfully applied to direct observation of the norbomyl cation (C7H114"). Preparation of the ion by the c route from 2-norbornyl halides, by the n route from 4-(2-haloethyl)-cyclopentenes, and by the protonation of nortricyclene ( bent o route ) all led to the same 2-norbomyl cation. 

Subsequently, Grunthaner reexamined the ESCA spectrum of the 2-norbornyl cation on a higher-resolution X-ray photoelectron spectrometer using highly efficient vacuum techniques.884 The spectrum closely matches the previously published spectra. Furthermore, the reported ESCA spectral results are consistent with the theoretical studies of Allen and co-workers885 on the classical and nonclassical norbomyl cation at the STO-3G and STO-4.31G levels. Using the parameters obtained by Allen and co-workers, Clark and co-workers were able to carry out a detailed... 

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). 

The 7-Norbornyl Cation. 7-Norbomyl derivatives were found to be extremely unreactive in solvolysis studies and product formation was shown to occur with predominant retention of configuration.917 920 These observations led to the suggestion by Winstein et al.917 that the cationic intermediate is a nonclassical ion. Attempts to isolate the 7-norbomyl cation under stable ion conditions in superacid... 

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-norbornyl 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 re-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... 

Comparison of the cyclic systems in Table 17 leads to the opposite conclusion, however the destabilization of the 1-norbornyl radical relative to the 1-adamantyl is less for the azo decompositions. Perhaps the mechanism of the azo decompositions of the more unreactive systems is different from that of, for example, the f-butyl azo compound (i.e. the rate determining step of the 1-norbomyl azo compound may be a one bond homolysis rather than the synchronous two bond fission of the f-butyl system312, 315)). Also, the smaller 1-norbornyl/1-adamantyl rate ratio for the f-butyl perester decompositions may be due to a greater influence of polar effects in these reactions 309a). This problem is under active investigation 309a). 

Scheme 2.6 Poly(2-norbornyl methacrylate) (P2NBM), and poly(3-methyl-2-norbomyl methacrylate) (P3M2NBM). (From ref. [35])...

Besides the work done on solvolysis of 2-norbomyl compounds, the 2-norbornyl cation has also been extensively smdied at low temperatures there is much evidence that under these conditions the ion is definitely nonclassical. Olah and co-workers have prepared the 2-norbomyl cation in stable solutions at temperamres below 150°C in SbFs—SO2 and FSO3H SbF5 S02, where the stmcmre is static and hydride shifts are absent Studies by proton and NMR, as well as by laser Raman spectra and X-ray electron spectroscopy, led to the conclusion that under these conditions the ion is nonclassical. A similar result has been reported for the 2-norbomyl cation in the sohd state where at 77 and even 5 K, NMR spectra gave no evidence of the freezing out of a single classical ion. ... 

The marked similarity in the Goering-Schewene diagrams for secondary 2-norbornyl (Fig. 2) and tertiary 2-norbornyl (Fig. 3) is persuasive of the interpretation that similar physical origins must be involved in both systems. However, when the question was posed, Is it reasonable to propose two very different explanations for phenomena which appear so similar , Professor Paul von R. Schleyer answered, Yes, it certainly is. Indeed, over the years a number of workers have taken this position that the origins of the high exo endo rate ratios in secondary and tertiary 2-norbomyl derivatives are different. It is appropriate then to consider the various proposals. 

Proposal No. 2. It was next proposed that the solvolysis of entfo-norbomyl tosylate is enhanced by large solvent participation comparable in magnitude to carbon participation in the exo isomer. Thus, one of the two interpretations considered, referring to the 2-Me/2-H reactivity ratios, was that, These = 10s values can be rationalized by the postulation of anchimeric assistance in the exo and solvent assistance in the endo secondary cases 40. This position was adopted and fully discussed by J. M. Harris and S. P. McManus in their interesting attempt to extrapolate from tertiary to secondary 2-norbornyl rates41. ... 

The recent determination of the calorimetric heats of ionization of representative alkyl chlorides in S02C1F fails to reveal stabilization of this magnitude in the 2-norbornyl system46. For example, the calorimetric heat of ionization of 2-propyl chloride (-15.3 0.9 kcal mol"1) is less than that of tert-butyl chloride (-25.4 0.8) by some 10.1 kcal mol-1. On the other hand, the value for exo-norbornyl chloride (-23.6 0.8) is less than that of 2-methyl-ev -norbomyl chloride (-3.15 1.5) by some 7.4 kcal mol-1. Similarly, the difference between 2-propyl chloride and 2-phenyl-2-propyl chloride (—30.3 0.3) is 15.0 1.2 kcal mol-1, whereas the... 

It is also instructful to discuss studies on 2-methyl- and 2-phenyl-2-norbomyl catons, 167 and 168, both of which have been investigated by proton10,210 and carbon-13 211 NMR spectroscopy. The 2-methyl-2-norbornyl cation can be characterized as a partially a-bond (i. e., C.1-C.6) delocalized carbenium ion, while the 2-phenyl-2-norbomyl cation exhibits the properties of a classical carbenium ion with no significant a delocalization (of course the positive charge is extensively delocali-... 

For the norbomyl cation (22), on the other hand, the average 13C chemical shift for C—1,2 was found at 126.1 ppm (at -150°C where all other rearrangements except for the degenerate alkyl shift are frozen out )55)- Thus, C-1,2 absorb by ca. 80 ppm further upfield that the corresponding carbons of the equilibrating ion (25). A similar discrepancy (85 ppm) is estimated on the basis of isopropyl cation and C—1 of norbomane as a model. Some doubt, however, remains whether these acyclic ions are good approximations of the hypothetical 2-norbornyl cation. 

Many rearrangements of bicyclic cations in superacids have been reported which are not observable under solvolytic or acidcatalyzed conditions. The equilibration of the arylbicyclooctyl cations (45)—(47)12 and the rearrangement of the rapidly equilibrating 2,3-dimethyl-2-norbornyl cation (48) into the more stable 1,2-dimethyl-2-norbomyl cation (49)73 may serve as examples. These transformations are thought... 

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