Norbornyl compounds, intermediates solvolysis

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. 

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... 

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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See also in sourсe #XX -- [ , ]

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