Norbornyl compounds, intermediates

As discussed by Zollinger, 1995 (Sec. 7.5) this hypothesis of a detour around intermediates of very low stability is also useful for the differentiation of classical and nonclassical ion intermediates in nucleophilic substitutions of 2-norbornyl and related compounds. 

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. 

Only low yields of (l-norborny jHg are obtained from controlled-potential electrolysis of 1-iodo- and 1-bromonorbornane at Hg-pool electrodes in DMF containing [R4N][C104] as current carrier, the major product being norbornane . The 1-halonorbornane reductions are two-electron processes, whereas only one electron is transferred in 1-halodecane reductions. The intermediate 1-norbornyl radical is reduced electrolytically (or accepts a hydrogen atom from the solvent), whereas the 1-decyl radical is absorbed onto Hg. The electrochemical reaction of I(CF2)4.I in DMF provides [H(CF2)4]2Hg via the intermediacy of I(CF2)4H and H(CF2)4HgI. The compounds [CH3(CH2) -i]2Hg n = 4-7 or 12, are obtained from the controlled-potential electroreduction of Br(CH2)nBr. 

For many years, a lively controversy centered over the actual existence of nonclassical carbocalions. " The focus of argument was whether nonclassical cations, such as the norbornyl cation, are bona fide delocalized bridged intermediates or merely transition states of rapidly equilibrating carbenium ions. Considerable experimental and theoretical effort has been directed toward resolving this problem. Finally, unequivocal experimental evidence, notably from solution and solid-state C NMR spectroscopy and electron spectroscopy for chemical analysis (ESCA), and even X-ray crystallography, has been obtained supporting the nonclassical carbocation structures that are now recognized as hypercoordinate ions. In the context of hypercarbon compounds, these ions will be reviewed. 

In the context of the historical development of our knowledge of the structure of the 2-norbornyl cation since Winstein and Trifan s postulate in 1949, this comparison with the 1,2-dimethyl derivative is very important for aspects provided by the theory of scientific discoveries. As discussed in our book on aromatic diazo compounds (Zollinger, 1994, Chap. 9), verifications are never definitive, in science proofs are not possible (the term proof should only be used in mathematics) falsifications, however, can be definitive. It was shown that molecular orbital calculations and their application to experimental data on the IR and NMR spectra for the 2-norbornyl cation are consistent with a symmetrical, nonclassical cationic intermediate, but not with a rapid equilibrium between two classical intermediates. The... 

Brown has questioned whether the spatial structure of the solvolysis products of 2-exo-norbornyl brosylate is a proof of the intermediate nonclassical ion He believes the classical 2-norbornyl cation to yield, when attacked by the nucleophile, only exo compounds for two resons ... 

The reactions studied have been the first examples of solvolysis of 2-norbornyl aryl sulphonates without exclusive formation of exo products. But the comparison of solvolyses between compounds 113,114 and 63 is complicated by different hybridization of the C atom and by the possible formation of an intermediate hemiketal with 113 and 114. These complications are avoided by acetolyzing the isomeric dimethyl ketals 118 and 119 having the sp -hybridized C atom The solvolysis of compound... 

The results of the oxidation of other unsubstituted cycloaliphatic compounds are summarized in Table 4, which indicates that cyclopentane and cyclohexane are oxidized in high yield and selectivity. The medium-sized rings, however, afford only low yields of the trifluoroacetates in a complex product mixture. This lack of selectivity is probably due to extensive rearrangments of the intermediates and easy follow-up oxidations of the products. In dichloromethane/acetic acid the same non-selectivity is encountered. The electrolysis of bicyclo[2.2.1]heptane (3) leads in dichloromethane/trifluoroacetic acid to exo-bicyclo[2.2.1]heptan-2-ol (4) in 83% yield as single product after hydrolysis of the trifluoroacetate (equation 6). The stereochemistry of 4 indicates that a nonclassical norbornyl cation is involved as intermediate. In dichloromethane/20% acetic acid/0.05 M TBABF4 a slightly lower yield (61%) of exn-2-norbornyI acetate was obtained. 

The synthesis of tosylate 19 was specifically carried out to test the kinetic importance of bridging in the parent norbornyl system. Fusion of the trimethylene fragment to a norbornyl skeleton works against ionization to a nonclassical car-bonium ion because bridging would lead to a large increase in strain energy. No increase in strain would be predicted in ionization to a classical carbonium ion. Compound 19 underwent acetolysis only 8 times faster than its cndn-isomer 20, and was 280 times less reactive than cjco-norbornyl tosylate under the same conditions. It was concluded that exo-norbornyl systems do exhibit a rate enhancement that is best understood as an electronic stabilization of the cationic intermediate. 

The Leuckart reaction (reductive amination of carbonyl compounds with formamide and formic acid) of 2-norbornanone and (lR)-(V-(3,3-dimethyl-2-oxo-l-norbornyl) acetamide furnishes the expected A-(2-norbornyl)formamides. (l/()-AI-(7,7-Dimethyl-2-0X0-l-norbornyl)acetamide, on the other hand, gives a product resulting from a Wagner-Meerwein rearrangement of the usual cationic intermediate followed by an unprecedented transamination, which affords an A,A-diacylammonium ion. Hydrolysis gives (15 )-A-(3,3-dimethyl-2-oxo-l-norbornyl)acetamide, which then undergoes the normal Leuckart reaction. 

As should be the case with the establishment of any new principle, the concept of o--bridged nonclassical intermediates was subjected to a searching analysis. An alternative explanation, based on classical carbocations, was put forth by H. C. Brown of Purdue University. Brown pointed out that the evidence in support of the nonclassical formulation for norbornyl cation consisted of (a) rapid solvolysis of ejco-norbornyl substrates relative to model compounds, (b) high exoj endo rate ratios, and (c) predominant (99.99%) capture of the cation from the exo direction. 

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