Norbomyl system

Another line of evidence that bridging is important in the transition state for solvolysis has to do with substituent effects for groups placed at C-4, C-5, C-6, and C-7 on the norbomyl system. The solvolysis rate is most strongly affected by C-6 substituents, and the exo isomer is more sensitive to these substituents than is the endo isomer. This implies that the transition state for solvolysis is especially sensitive to C-6 substituents, as would be ejqiected if the C(l)—C(6) bond participates in solvolysis. ... 

Further evidence for the SnI mechanism is that reactions run under SnI conditions fail or proceed very slowly at the bridgehead positions of [2.2.1] (norbomyl) systems (e.g. 1-chloroapocamphane, 8). 

A vast amount of work has been done on solvolysis of the 2-norbomyl system in an effort to determine whether the 1,6 bond participates and whether 52 is an intermediate. Most, although not all, chemists now accept the intermediacy of 52. 

On balance, the solvolysis results just described indicate that there is little reason to require the norbomyl ion reached under solvolytic conditions to be non-classical. Before we turn to other aspects of the problem, it is interesting briefly to consider the results of deaimina-tion studies in the norbomyl system. 

An excellent review of the use of secondary a and isotope effects in the norbomyl system has recently been written by Scheppele (1972). We shall briefly summarize a few of the factors and conclusions arrived at because of their relevance to the interpretation of other solvolysis data, but the interested reader should turn to that article and the original sources quoted for a more extensive discussion. 

The norbomyl system has been subjected to an analogous series of experiments with the results shown in Table 4. When exo-2-chloro-norbornane was solvolysed in the presence of a hydride donor good yields of norbornane were obtained. The product had not acquired any protons from the acid and thus an edge-protonated intermediate had not formed before reaction with the hydride donor occurred. 

It has been argued that the energy required for the 3,2 shift is substantially higher than a 2-4 kcal mole estimate for 1,2-hydride shifts around the cyclopentyl cation, the 7-9 kcal mole difference being attributed to stabilization of the norbomyl system by the formation of the comer protonated non-classical stmcture (Olah et al., 1969). 

The "classical" - "non-classical" carbocation controversy concerned the Wagner-Meerwein rearrangement of norbomyl systems ... 

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... 

Reversible, random carbonium ion formation is not required to explain the rearrangements of both 2 and 37 to adamantane in the highly acidic media. Sequential 1,2 alkyl shifts coupled with the well documented 5316,2- and 3,2-hydride shifts of the norbomyl system permit a rearrangement pathway analogous to that discussed earlier as the most likely route for the Lewis acid catalyzed rearrangement of 2 to adamantane. 

A detailed search for nonclassical resonance energy by determining the Me/H rate ratios at positions 1- and 2- in the 2-norbomyl system likewise failed to uncover any evidence for its presence47. ... 

The NMR spectra of triads 1-4 were obtained at 500 MHz. The resonances were assigned by comparison with model compounds, known coupling constants for norbomyl systems (24), and two-dimensional (2D) COSY experiments. The model compounds included the corresponding dyads (5, 6, 7, and... 

In contrast to Brown s assertions and in accord with Winstein s and Trifan s assumption, the solvolysis of these secondary systems proceeds with anchimeric acceleration. This is concluded from the following facts a) the exo endo rate ratio for 2-norbomyl systems is 10 -10 as the reaction rate of the endo isomer is not anomalous (see above), hende the exo isomer reacts at an elevated rate b) the rate of solvolysis of exo isomers is 10 to 10 times as high as that calculated according to the semiempirical scheme from only steric effects c) the ratio of the reaction rate of secondary 2-exo-norbomyl systems to the solvolysis rate of secondary cyclopentyl analogues is 100 times as great as that of tert-2-exo-norbomyl derivatives and tert-cyclopentyl analogues since tert-2-norbomyl derivatives are solvolyzed without anchimeric assistance, the factor of 100 characterizes tentatively the amount of anchimeric assistance in the secondary 2-exo-norbornyl systems d) exo- and endo-6-substituents decrease the solvolysis rate of 2-exo-norbomyl tosylate this cannot be accounted for without participation of the electrons of the 1,6 bond in the transition state their participation increases the non-bonded interaction due to a decrease in the C -C distance. 

Another system, which has attracted much interest, is that of the cyclopropyl-methyl cation, for similar reasons to the 2-norbomyl systems namely, the great ease of solvolysis of cyclopropyhnethyl substrates accompanied by formation of rearrangement products. Of particular interest is that regardless of the method of generation of the cationic species the ratio of products is always very similar. Both hydrolysis of cyclopropylmethyl chloride under conditions favoring 1 substitution and diazotization of cyclopropylamine produce cyclobutanol and but-3-en-l-ol as well as cyclopropyhnethanol in approximately 47%, 5%, and 48% yields respectively. Moreover, diazotization of cyclobutylamine also produces the same three alcohols in the same ratios (Scheme 2.38). 

Further support for the nonclassical structure of the 2-norbomyl cation came from an application of NMR spectroscopy that is based on the difference between the total chemical shift of a carbocation and that of the corresponding alkane. Differences in total chemical shift of 350 ppm or more are associated with classical carbocations, while differences of less than 200 ppm are thought to indicate nonclassical, bridged carbonium ions. For example, the sum of the total NMR chemical shift of propane is 47 ppm, while the sum for the 2-propyl cation is 423 ppm. The difference, 376 ppm, indicates that the 2-propyl cation is a classical ion. For the 2-norbomyl system the total of the shifts is 408 ppm, while the total for norbomane is 233 ppm. The difference, 175 ppm, was taken as evidence for a nonclassical structure. ... 

The 2-Norbomyl System.—Evidence concerning the 2-norbomyl cation has been collated and critically analysed diametrically opposed conclusions have been drawn, namely finding in favour of the unsymmetrical localized ion, and the favouring of the symmetrical a-bridged cation. ... 

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