Carbocations bornyl

In the conversion of a-pinene (2.8) into bornyl chloride (2.9) endo isomer), the rearrangement to a 2° carbocation is favoured by relief of small-ring strain (Scheme 2.9). In a similar manner the conversion of camphene hydrochloride (2.10) into isobornyl chloride (2.11) involves rearrangement known as the Wagner-Meerwein rearrangement (Scheme 2.10). 

Similarly, the bornyl carbocation (2.17) can react with water to give borneol and this can be oxidised to camphor, the characteristic odorant of camphor wood. The isocamphane skeleton is formed by a Wagner-Meerwein rearrangement of the bornyl carbocation. 

The rearrangement in Figure 18.12, giving Mo-bornyl chloride, has been known for over a century. The reaction mechanism can be written in classical terms the right answer is achieved, but should cause us some worries. The migration involves forming a secondary carbocation from a tertiary one, and a single stereoisomer of the product is obtained, for no reason that is readily obvious. There is clearly more to this process than was immediately obvious when it was discovered. 

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